Lightweight Electric/Hybrid Vehicle Design

Lightweight Electric/ Hybrid Vehicle Design

Ron Hodkinson and John Fenton

OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 A division of Reed Educational and Professional Publishing Ltd

A member of the Reed Elsevier plc group First published 2001

© Reed Educational and Professional Publishing Ltd 2001

All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloguing in Publication Data

A catalogue record for this book is available from the Library of Congress

ISBN 0 7506 5092 3

Contents

Preface vii

About the authors ix

Introduction xi

Part 1 Electromotive Technology (Ron Hodkinson MSc MIEE) 1

1 Current EV design approaches 3

1.1 Introduction 3

1.2 Case for electric vehicles 3

1.3 Selecting EV motor type for particular vehicle application 15

1.4 Inverter technology 21

1.5 Electric vehicle drives: optimum solutions for motors, drives and batteries 24

2 Viable energy storage systems 29

2.1 Electronic battery 29

2.2 Battery performance: existing systems 29

2.3 Status of the aluminium battery 35

2.4 Advanced fuel-cell control systems 39

2.5 Waste heat recovery, key element in supercar efficiency 50

3 Electric motor and drive-controller design 56

3.1 Introduction 56

3.2 Electric truck motor considerations 56

3.3 Brushless DC motor design for a small car 58

3.4 Brushless motor design for a medium car 61

3.5 Brushless PM motor: design and FE analysis of a 150 kW machine 64

3.6 High frequency motor characteristics 68

3.7 Innovative drive scheme for DC series motors 73

4 Process engineering and control of fuel cells, prospects for EV packages 80

4.1 Introduction 80

4.2 Reforming and other hydrogen feedstocks 82

4.3 Characteristics, advantages and status of fuel cells 83

4.4 Thermodynamics of fuel cells 84

4.5 Process engineering of fuel cells 87

4.6 Steps towards the fuel-cell engine 89

4.7 Prospects for EV package design 93

4.8 Fuel-cell vehicles and infrastructure 96

4.9 The PNGV programme: impetus for change 98

Part 2 EV Design Packages/Design for Light Weight 103 (John Fenton MSc MIMechE)

5 Battery/fuel-cell EV design packages 105

5.1 Introduction 105

5.2 Electric batteries 105

5.3 Battery car conversion technology 115

5.4 EV development history 119

5.5 Contemporary electric car technology 122

5.6 Electric van and truck design 128

5.7 Fuel-cell powered vehicles 135

6 Hybrid vehicle design 141

6.1 Introduction 141

6.2 Hybrid drive prospects 143

6.3 Hybrid technology case studies 146

6.4 Production hybrid-drive cars 156

6.5 Hybrid passenger and goods vehicles 164

7 Lightweight construction materials and techniques 173

7.1 Introduction 173

7.2 The ‘composite’ approach 173

7.3 Plastic mouldings for open canopy shells 178

7.4 Materials for specialist EV structures 182

7.5 Ultra-lightweight construction case study 191

7.6 Weight reduction in metal structures 192

8 Design for optimum body-structural and running-gear performance efficiency 199

8.1 Introduction 199

8.2 Structural package and elements 200

8.3 ‘Punt’-type structures 209

8.4 Optimizing substructures and individual elements 211

8.5 Designing against fatigue 217

8.6 Finite-element analysis (FEA) 218

8.7 Case study of FEA for EVs and structural analysis assemblies 223

8.8 Running gear design for optimum performance and lightweight 223

8.9 Lightweight vehicle suspension 231

8.10 Handling and steering 232

8.11 Traction and braking systems 235

8.12 Lightweight shafting, CV jointing and road wheels 241

8.13 Rolling resistance 243

Index 251

Preface

The stage is now reached when the transition from low-volume to high-volume manufacture of fuel cells is imminent and after an intense period of value engineering, suppliers are moving towards affordable stacks for automotive propulsion purposes. Since this book went to press, the automotive application of fuel cells for pilot-production vehicles has proceeded apace, with Daewoo, as an example, investing $5.9 million in a fuel-cell powered vehicle based on the Rezzo minivan, for which it is developing a methanol reforming system. Honda has also made an important advance with version 3 of its FCX fuel-cell vehicle, using a Ballard cell-stack and an ultracapacitor to boost acceleration. Its electric motor now weighs 25% less and develops 25% more power and start-up time has been reduced from 10 minutes to 10 seconds. Ballard have introduced the Mk900 fuel cell now developing 75 kW (50% up on the preceding model). Weight has decreased and power density increased, each by 30%, while size has dropped by 50%. The factory is to produce this stack in much higher volumes than its predecessor. While GM are following the environmentally-unfriendly route of reformed gasoline for obtaining hydrogen fuel, Daimler Chrysler are plumping for the methanol route, with the future option of fuel production from renewables; they are now heading for a market entry with this technology, according to press reports.

A recent DaimlerChrysler press release describes the latest NECAR, with new Ballard Stack, which is described in its earlier Phase 4 form in Chapter 5, pp. 139–140. NECAR 5 has now become a methanol-powered fuel cell vehicle suitable for normal practical use. The environmentally friendly vehicle reaches speeds of more than 150 kilometres per hour and the entire fuel cell drive system – including the methanol reformer – has been installed in the underbody of a Mercedes-Benz A-Class for the very first time. The vehicle therefore provides about as much space as a conventional A-Class. Since the NECAR 3 phase, in 1997, the engineers have succeeded in reducing the size of the system by half and fitting it within the sandwich floor. At the same time, they have managed to reduce the weight of the system, and therefore the weight of the car, by about 300 kg. While NECAR 3 required two fuel cell stacks to generate 50 kW of electric power, a single stack now delivers 75 kW in NECAR 5. And although the NECAR 5 experimental vehicle is heavier than a conventional car, it utilizes energy from its fuel over 25% more efficiently. The development engineers have also used more economical materials, to lower production cost.

Methanol ‘fuel’ could be sold through a network of filling stations similar to the ones we use today. The exhaust emissions from ‘methanolized’ hydrogen fuel cell vehicles are very much lower than from even the best internal combustion engines. The use of methanol-powered fuel-cell vehicles could reduce carbon-dioxide emissions by about a third and smog-causing emissions to nearly zero. Methanol can either be produced as a renewable energy source from biomass or from natural gas, which is often burned off as a waste product of petroleum production and is still available in many regions around the world. To quote D-C board members, ‘there have already been two oil crises; we are obligated to prevent a third one,’ says Jürgen E. Schrempp, Chairman of the Board Of Management of DaimlerChrysler. ‘The fuel-cell offers a realistic opportunity to supplement the ‘petroleum monoculture’ over the long term.’ The company will invest about DM 2 billion (over $ 1 billion) to develop the new drive system from the first prototype to the point of mass production. In the past six years the company has already equipped and presented 16 passenger cars, vans and buses with fuel cell drives–more than the total of all its competitors worldwide. Professor Klaus-Dieter Vöhringer, member of the Board of Management with responsibility for research and technology, predicts the fuel cell will be introduced into vehicles in several stages ‘In 2002, the company will deliver the first city buses with fuel cells, followed in 2004 by the first passenger cars.’

The electric-drive vehicle has thus moved out of the ‘back-room’ of automotive research into a ‘design for production’ phase and already hybrid drive systems (IC engine plus electric drive) have entered series production from major Japanese manufacturers. In the USA, General Motors has also made very substantial investments with the same objective. There is also very considerable interest throughout the world by smaller high-technology companies who can use their knowledge base to successfully enter the automotive market with innovative and specialist-application solutions. This last group will have much benefit from this book, which covers automotive structure, and system design for ultra-light vehicles that can extend the range of electric propulsion, as well as electric-drive technology and EV layouts for its main-stream educational readership.

NECAR5 fuel-cell driven car.

About the authors

Electro-technology author Ron Hodkinson is very actively involved in the current value engineering of automotive fuel-cell drive systems through his company Fuel Cell Control Ltd and is particularly well placed to provide the basic electro-technology half of this work. He obtained his first degree in electrical engineering (power and telecommunications) from the Barking campus, of what is now the University of East London, on a four-year sandwich course with Plessey. At the end of the company’s TSR2 programme he moved on to Brentford Electric in Sussex where he was seconded on contract to CERN in Switzerland to work on particle-accelerator magnetic power supplies of up to 9 MW. He returned to England in 1972 to take a master’s degree at Sussex University, after which he became Head of R&D at Brentford Electric and began his long career in electric drive system design, being early into the development of transistorised inverter drives. In 1984 the company changed ownership and discontinued electronics developments, leading Ron to set up his own company, Motopak, also developing inverter drives for high performance machine tools used in aircraft construction. By 1989 his company was to be merged with Coercive Ltd who were active in EV drives and by 1993 Coercive had acquired Nelco, to become the largest UK producer of EV drives. In 1995 the company joined the Polaron Group and Ron became Group Technical Director. For the next four years he became involved in both machine tool drives and fuel cell controls. In 1999 the group discontinued fuel-cell system developments and Ron was able to acquire premises at Polaron’s Watford operation to set up his own family company Fuel Cell Control Ltd, of which he is managing director. He has been an active member of ISATA (International Society for Automotive Technology and Automation) presenting numerous papers there and to the annual meetings of the EVS (Electric Vehicle Seminar). He is also active in the Power Electronics and Control committees of the Institution of Electrical Engineers. Some of his major EV projects include the Rover Metro hybrid concept vehicle; IAD electric and hybrid vehicles; the SAIC fuel-cell bus operating in California and Zetec taxicabs and vans.

Co-author John Fenton is a technology journalist who has plotted the recent course in EV design and layout, including hybrid-drive vehicles, in the second half of the book, which also includes his chapters on structure and systems design from his earlier industrial experience. He is an engineering graduate of the Manchester University Science Faculty and became a member of the first year’s intake of Graduate Apprentices at General Motors’ UK Vauxhall subsidiary. He later worked as a chassis-systems layout draughtsman with the company before moving to automotive consultants ERA as a chassis-systems development engineer, helping to develop the innovative mobile tyre and suspension test rig devised by David Hodkin, and working on running-gear systems for the Project 378 car design project for BMC. With ERA’s subsequent specialization on engine systems, as a result of the Solex acquisition, he joined the Transport Division of Unilever, working with the Technical Manager on the development of monocoque sandwich-construction refrigerated container bodies and bulk carriers for ground-nut meal and shortening-fat. He was sponsored by the company on the first postgraduate automotive engineering degree course at Cranfield where lightweight sandwich-construction monocoque vehicle bodies was his thesis subject. He changed course to technology-journalism after graduating and joined the newly founded journal Automotive Design Engineering (ADE) as its first technical editor, and subsequently editor. A decade later he became a senior lecturer on the newly founded undergraduate Vehicle Engineering degree course at what is now Hertfordshire University and helped to set up the design teaching courses in body-structure and chassis-systems. He returned to industry for a short period, as a technology communicator, first Product Affairs Manager for Leyland Truck and Bus, then technical copywriter and sales engineer (special vehicle operations). With the merging of ADE with the Institution of Mechanical Engineers JAE journal he had the opportunity to move back to publishing and subsequently edited the combined journal Automotive Engineer, for fifteen years, prior to its recent transformation into an international auto-industry magazine.

Introduction

0.1 Preface

This book differs from other automotive engineering texts in that it covers a technology that is still very much in the emerging stages, and will be particularly valuable for design courses, and projects, within engineering degree studies. Whereas other works cover established automotive disciplines, this book focuses on the design stages, still in process for electric vehicles, and thus draws on a somewhat tentative source of references rather than a list of the known major works in the subject. The choice of design theory is also somewhat selective, coming from the considerable volume of works the disciplines of which are combining to make the production electric vehicle possible.

0.1.1 BIBLIOGRAPHIC SOURCES

Electrical propulsion systems date back virtually to the time of Faraday and a substantial body of literature exists in the library of the Institution of Electrical Engineers from which it is safe only to consider a small amount in relation to current road vehicle developments. Similarly a considerable quantity of works are available on aerospace structural design which can be found in the library of the Royal Aeronautical Society, and on automotive systems developments within the library of the Institution of Mechanical Engineers. With the massive recent step-changes in capital investment, first in the build-up to battery-electric vehicle development, then in the switch to hybrid drive engineering, and finally the move to fuel-cell development – it would be dangerous to predict an established EV technology at this stage.

A good deal of further reading has been added to the bibliographies of references at the ends of each chapter. This is intended to be a source of publications that might help readers look for wider background, while examining the changes of direction that EV designers are making at this formative stage of the industry. The final chapter also lists publications which seem to be likely sources of design calculations pertinent in designing for minimum weight and has a table of nomenclature for the principal parameters, with corresponding symbol notation used in the design calculations within the text of the chapters.

0.1.2 CONTEXT AND STRUCTURE

The current period of EV development could be seen as dating from a decade or so before the publication of Scott Cronk’s pivotal work published by the Society of Automotive Engineers in 1995, Building the E-motive Industry. As well as pulling together the various strings of earlier EV development, the book takes a very broad-brush view of the many different factors likely to affect the industry as it emerges. Readers seeking to keep abreast of developing trends in EV technology could do little better than to follow the bound volumes of proceedings on the subject which have appeared annually following the SAE Congresses in February/March, as well as studying the proceedings of the annual worldwide FISITA and EVS conferences. One of these factors, put forward by Cronk, is the need for a combination of electromotive technology with those which went into the USA Supercar programme, aimed at unusually low fuel consumption born out of low-drag and lightweight construction. This is the philosophy that the authors of Lightweight Electric/Hybrid Vehicle Design are trying to follow in a work which looks into the technologies in greater depth. The book is in two parts, dealing with (a) electromotive technology and (b) EV design packages, lightweight design/construction and running-gear performance.

Ron Hodkinson draws on long experience in electric traction systems in industrial vehicles and more recently into hybrid-drive cars and control systems for fuel-celled vehicles. His Part One contains the first four chapters on electric propulsion and storage systems and includes, within his last chapter, a contributed section by Roger Booth, an expert in fuel-cell development, alongside his own account of EV development history which puts into context the review material of the following chapters. In Part Two, John Fenton, in his first two chapters, uses his recent experience as a technology writer to review past and present EV design package trends, and in his second two chapters on body construction and body-structural/running-gear design, uses his earlier industrial experience in body and running-gear design, to try and raise interest in light-weighting and structural/functional performance evaluation.

0.2 Design theory and practice

For the automotive engineer with background experience of IC-engine prime-moving power sources, the electrical aspects associated with engine ignition, starting and powering auxiliary lighting and occupant comfort/convenience devices have often been the province of resident electrical engineering specialists within the automotive design office. With the electric vehicle (EV), usually associated with an energy source that is portable and electrochemical in nature, and tractive effort only supplied by prime-moving electric motor, the historic distinctions between mechanical and electrical engineering become blurred. One day the division of engineering into professional institutions and academic faculties defined by these distinctions will no doubt also be questioned. Older generation auto-engineers have much to gain from an understanding of electrotechnology and a revision of conventional attitudes towards automotive systems such as transmission, braking and steering which are moving towards electromagnetic power and electronic control, like the prime-moving power unit.

In terms of reducing vehicle weight, to gain greatest benefit in terms of range from electromotive power, there also needs to be some rethinking of traditional approaches. The conventional design approach of automotive engineers seems to involve an instinctive prioritizing of minimizing production costs, which will have been instilled into them over generations of Fordist mass-production. There is something in this ‘value-engineering’ approach which might sacrifice light weight in the interests of simplicity of assembly, or the paring down of piece price to the barest minimum. Aerospace designers perhaps have a different instinctive approach and think of lightweight and performance-efficiency first. Both automotive and aerospace design engineers now have the benefit of sophisticated finite-element structural analysis packages to help them trade off performance efficiency with minimum weight. In earlier times the automotive engineer probably relied on substantial ‘factors of safety’ in structural calculations, if indeed they were performed at all on body structures, which were invariably supported by stout chassis frames. This is not to mention the long development periods of track and road proving before vehicles reached the customer, which may have led engineers to be less conscious of the weight/performance trade-off in detail design. Individual parts could well be specified on the basis of subjective judgement, without the sobering discipline of the above trade-off analysis.

Not so, of course, for the early aeronautical design engineers whose prototypes either ‘flew or fell out of the sky’. Aircraft structural designers effectively pioneered techniques of thin-walled structural analysis to try to predict as far as possible the structural performance of parts ‘before they left the drawing board’, and in so doing usually economized on any surplus mass. These structural analysis techniques gave early warning of buckling collapse and provided a means of idealization that allowed load paths to be traced. In the dramatic weight reduction programmes called for by the ‘supercar’ design requirements, to be discussed in Chapters 4 and 6, these attitudes to design could again have great value.

Design calculations, using techniques for tracing loads and determining deflections and stresses in structures, many of which derive from pioneering aeronautical structural techniques, are also recommended for giving design engineers a ‘feel’ for the structures at the concept stage. The design engineer can thus make crucial styling and packaging decisions without the risk of weakening the structure or causing undue weight gain. While familiar to civil and aeronautical engineering graduates these ‘theory of structures’ techniques are usually absent from courses in mechanical and electrical engineering, which may be confined to the ‘mechanics of solids’ in their structures teaching. For students undertaking design courses, or projects, within their engineering degree studies, these days the norm rather than the exception, the timing of the book’s publication is within the useful period of intense decision making throughout the EV industry. It is thus valuable in focusing on the very broad range of other factors–economic, ergonomic, aesthetic and even political–which have to be examined alongside the engineering science ones, during the conceptual period of engineering design.

0.2.1 FARTHER-REACHING FACTORS OF ‘TOTAL DESIGN’

Since the electric vehicle has thus far, in marketing terms, been ‘driven’ by the state rather than the motoring public it behoves the stylist and product planner to shift the emphasis towards the consumer and show the potential owner the appeal of the vehicle. Some vehicle owners are also environmentalists, not because the two go together, but because car ownership is so wide that the non-driving ‘idealist’ is a rarity. The vast majority of people voting for local and national governments to enact antipollution regulation are vehicle owners and those who suffer urban traffic jams, either as pedestrians or motorists, and are swinging towards increased pollution control. The only publicized group who are against pollution control seem to be those industrialists who have tried to thwart the enactment of antipollution codes agreed at the international 1992 Earth Summit, fearful of their manufacturing costs rising and loss of international competitiveness. Several governments at the Summit agreed to hold 1990 levels of CO2 emissions by the year 2000 and so might still have to reduce emission of that gas by 35% to stabilize output if car numbers and traffic density increase as predicted.

Electric vehicles have appeal in urban situations where governments are prepared to help cover the cost premium over conventional vehicles. EVs have an appeal in traffic jams, even, as their motors need not run while the vehicles are stationary, the occupant enjoying less noise pollution, as well as the freedom from choking on exhaust fumes. There is lower noise too during vehicle cruising and acceleration, which is becoming increasingly desired by motorists, as confirmed by the considerable sums of money being invested by makers of conventional vehicles to raise ‘refinement’ levels. In the 1960s, despite the public appeals made by Ralph Nader and his supporters, car safety would not sell. As traffic densities and potential maximum speed levels have increased over the years, safety protection has come home to people in a way which the appalling accident statistics did not, and safety devices are now a key part of media advertising for cars. Traffic densities are also now high enough to make the problems of pollution strike home.

The price premium necessary for electric-drive vehicles is not an intrinsic one, merely the price one has to pay for goods of relatively low volume manufacture. However, the torque characteristics of electric motors potentially allow for less complex vehicles to be built, probably without change-speed gearboxes and possibly even without differential gearing, drive-shafting, clutch and final-drive gears, pending the availability of cheaper materials with the appropriate electromagnetic properties. Complex ignition and fuel-injection systems disappear with the conventional IC engine, together with the balancing problems of converting reciprocating motion to rotary motion within the piston engine. The exhaust system, with its complex pollution controllers, also disappears along with the difficult mounting problems of a fire-hazardous petrol tank.

As well as offering potential low cost, as volumes build up, these absences also offer great aesthetic design freedom to stylists. Obviating the need for firewall bulkheads, and thick acoustic insulation, should also allow greater scope in the occupant space. The stylist thus has greater possibility to make interiors particularly attractive to potential buyers. The public has demonstrated its wish for wider choice of bodywork and the lightweight ‘punt’ type structure suggested in the final chapter gives the stylist almost as much freedom as had the traditional body-builders who constructed custom designs on the vehicle manufacturers’ running chassis. The ability of the ‘punt’ structure, to hang its doors from the A- and C-posts without a centre pillar, provides considerable freedom of side access, and the ability to use seat rotation and possibly sliding to ease access promises a good sales point for a multi-stop urban vehicle. The resulting platform can also support a variety of body types, including open sports and sports utility, as no roof members need be involved in the overall structural integrity. Most important, though, is the freedom to mount almost any configuration of ‘non-structural’ plastic bodywork for maximum stylistic effect. Almost the only constraint on aesthetic design is the need for a floor level flush with the tops of the side sills and removable panels for battery access.

0.2.2 CHANGING PATTERNS OF PRODUCTION AND MARKETING

Some industry economists have argued that local body-builders might reappear in the market, even for ‘conventional’ cars as OEMs increasingly become platform system builders supplied by systems houses making power-unit and running-gear assemblies. Where monocoque structures are involved it has even been suggested that the systems houses could supply direct to the local body-builder who would become the specialist vehicle builder for his local market. The final chapter suggests the use of an alternative tubular monocoque for the sector of the market increasingly attracted by ‘wagon’ bodies on MPVs and minibuses. Here the stylist can use colour and texture variety to break up the plane surfaces of the tube and emphasize the integral structural glass. Although the suggested tubular shell would have a regular cross-section along the length of the passenger compartment, the stylist could do much to offer interior layout alternatives, along with a host of options for the passenger occupants, and for the driver too if ‘hands-off’ vehicle electronic guidance becomes the norm for certain stretches of motorway.

Somehow, too, the stylist and his marketing colleagues have to see that there is a realization among the public that only when a petrol engine runs at wide open-throttle at about 75% of its maximum rotational speed is it achieving its potential 25% efficiency, and this is of course only for relatively short durations in urban, or high density traffic, areas. It is suggested that a large engined car will average less that 3% efficiency over its life while a small engined car might reach 8%, one of the prices paid for using the IC engine as a variable speed and power source. This offsets the very high calorific value packed by a litre of petrol. An electric car has potential for very low cost per mile operation based on electrical recharge costs for the energy-storage batteries, and EVs are quite competitive even when the cost of battery replacement is included after the duration of charge/recharge cycles has been reached. It needs to be made apparent to the public that a change in batteries is akin to changing the cartridge in a photocopier–essentially the motive-force package is renewed while the remainder of the car platform (machine) has the much longer life associated with electric-driven than does the petrol-driven vehicle. In this sense batteries are amortizable capital items, to be related with the much longer replacement period for the vehicle platform which could well carry different style bodies during its overall lifetime.

The oversizing of petrol engines in conventional cars, referred to above, arises from several factors. Typical car masses, relative to the masses of the drivers they carry, mean that less than 2% of fuel energy is used in hauling the driver. Added to the specifying of engines that allow cars to travel at very large margins above the maximum speed limit is of course the conventional construction techniques and materials which make cars comparatively heavy. The weight itself grossly affects accelerative performance and gradient ability. Also some estimates consider six units of fuel are needed to deliver one unit of energy to the wheels: one-third wheel power being lost in acceleration (and heat in consequent braking), one-third in heating disturbed air as the vehicle pushes through the atmosphere and one-third in heating the tyre and road at the traction, braking and steering contact patch. This puts priorities on design for electric vehicles to cut tare weight, reduce aerodynamic drag and reduce tyre rolling resistance.

0.2.3 QUESTIONING THE INDUSTRY-STANDARD APPROACH

The design process in the main-line automotive industry is driven by the edicts of the car-makers’ styling departments who ultimately draw their inspiration from the advertising gurus of Madison Avenue, whose influence has, of course, spread worldwide. The global motor industry has been predominately US dominated since Henry Ford’s pioneering of systematic volume production and General Motors’ remarkable ability to appeal to widely different market sectors with quite modestly varied versions of a standard basic vehicle. Thus far the electric, or hybrid drive, vehicle had to conform to historically developed design norms with the cautious conservatism of marketing management defining the basic scantlings. Conventional automotive design must conform to the requirements of Mr and Mrs Average, analysed by countless focus groups, while meeting the necessities of mass-production equipment developed during the first century of the motor vehicle.

When bold attempts have been made to achieve substantial reductions in weight below that of the standard industry product, the limitations of these major constraints have usually moderated the design objectives, Fig. 0.1. The overruling necessity to ‘move metal’ at the scale of ten million vehicles per year from each of the world’s three main areas of motor manufacture makes radical design initiatives a scary business for ‘corporate bosses’. Advertising professionals, with their colleagues in public relations, have skilfully built up customer expectations for the conventional automobile, from which it is difficult for the designer to digress in the interests of structural efficiency and light weight. Expectations are all about spacious interiors with deep soft seats and wide easy-access door openings; exterior shape is about pleasing fantasies of aggressiveness, speed and ‘luxury’ appearance. Performance expectations relate to accelerative ability rather than fuel economy, as Mr Average Company Representative strains to be ‘first off the grid’.

Ecologists who seek the palliative effects of electric propulsion will need to face up to educating a market that will appreciate the technology as well as convincing motor industry management of the need for radical designs which will enable the best performance to be obtained from this propulsion technology. The massive sensitivity of the general public to unconventional vehicle configurations was made abundantly clear from the reaction to the otherwise ingenious and low cost Sinclair C5 electric vehicle. While clearly launched as a motorized tricycle, with a price appropriate to that vehicle category, the C5 was nearly always referred to by its media critics as an ‘electric car’ when operationally it was more appropriate for use on reserved cycleways of which, of course, there are hardly enough in existence to create a market. While the Sunracer Challenge in Australia has shown the remarkable possibilities even for solar-batteried electric vehicles, it is doubtful whether the wider public appreciate the radical design of structure and running gear that make transcontinental journeys under solar power a reality, albeit an extremely expensive one for a single seater. Electric cars are perceived as ‘coming to their own’ in urban environments where high traffic densities reduce average speeds and short-distance average journeys are the norm. There is also long-term potential for battery-powered vehicles to derive additional ‘long-distance’ energy from the underground inductive power lines which might be built into the inside lanes of future motorways. It is not hard to envisage that telematics technology for vehicle guidance could be enhanced by such systems and make possible electronically spaced ‘trains’ of road vehicles operating over stretches of motorway between the major urban and/or rural recreational centres.

0.2.4 MARKET SEGMENTATION

At the time of writing some customer-appealing production hybrid and electric drive vehicles have already come onto the market. The Toyota Prius hybrid-drive car, described in Chapter 6, is already proving to be well received in the Japanese market where imaginative government operational incentives are in place. A variety of conversions have been made to series production compact cars which allow short-range urban operation where adequate battery recharging infrastructure is available. However, GM surprised the world with the technically advanced prototype Impact medium-range electric car, but the market has reportedly not responded well to its production successor and generally speaking there is not yet an unreservedly positive response.

Like the existing market for passenger cars, that for electric-drive cars will also be segmented, in time, with niches for sedan, convertible, dual-purpose, sports, utility, limousine and ‘specialist’ vehicles. The early decades of development, at least, may also be noted for the participation of both high and low volume builders. The low volume specialist is usually the builder prepared to investigate radical solutions and in the, thus far, ‘difficult’ market for electric cars it would seem a likely sector for those EVs which are more than drive-system conversions of existing vehicles.

With the high volume builders, already under pressure from overcapacity, their main attention is likely to be focused on retaining markets for current design vehicles, without the ‘distraction’ of radical redesigns. The ambitious, imaginative and high technology specialist has thus much to gain from an informed innovative approach and could benefit from a reported longer-term trend when drive systems will be manufactured by huge global producers and vehicle manufacturing will tend towards a regional basis of skilled body shops catering for local markets.

0.2.5 EV AS PART OF A WIDER TRANSPORTATION SYSTEM

The ‘physical’ design package for an electric vehicle will result from a much larger ‘design package of affecting factors’ which encompasses vehicle operational category, manufacturing systems/ techniques, marketing and distribution. Packages for industrial trucks and specialist delivery vehicles are already established but those for passenger-car variants much less so. It has been suggested that the first substantial sales of electric cars might well be to electricity generating companies in the public utilities sector, who would rent them to railway operators for end-use by rail travellers. Such people would purchase their hire with return travel tickets to destination stations at which EVs would be parked in forecourts for the use of travellers. Other potential customers might be city-centre car hire fleets, taxicab operators in fossil-fuel exhaust-free zones or local authorities setting up city-centre car pools.

One of the most imaginative EV applications is the lightweight mini-tram, Fig. 0.2, as exhibited at the Birmingham ElectriCity event in 1993. This is a vehicle that runs on low cost tracks which can be laid on an ordinary road surface without further foundation. The vehicle can travel up to 50 km/h and is a flywheel-assist hybrid machine having its batteries recharged via low voltage conductor rails positioned at intervals around the track. Each car weighs just over 3 tonnes unladen and can carry 14 seated and 11 standing passengers. A 5 km route, including rails, can be constructed, to include five trams, ten stops and four charge points, at a cost of just £1 million. It seems an ideal solution to the problem of congested cities that have roadways that date back to pre-automobile days, with the mini-trams able to transport both passengers and goods in potential ‘pedestrian precincts’ that would be spoilt by the operation of conventional omnibuses and tramcars. The proposal serves well to illustrate the opportunities for electric vehicles, given some imaginative lateral thinking.

Since launch, larger vehicles have been produced and entered service. The one seen at Bristol Docks (Fig. 0.2, right) has a steel frame with GRP body panels and weighs 13 tonnes, compared with the smallest railcar which weighs 48 tonnes. There are four production variants on offer, carrying 30, 35 or 50 passengers, and a twin-car variant of the latter. Use of continuously variable transmission now ensures the flywheels run at constant speed; a third rail at stations is used for taking in electricity for ‘charging up’ the flywheel. A 2-minute recharge would be required for the flywheel to propel the vehicle its maximum distance of two miles; so more frequent stops are recommended to reduce recharge time, 0.5 km being the optimum. A hybrid version with additional LPG power was due for launch in Stourbridge, UK, as a railcar in early 2001.

Some of the above projects are all based on the proposition that the more conservative motor manufacturers may not follow the lead set by Toyota and Honda in offering hybrid-electric drive cars through conventional dealer networks. In the mid 1990s the US ‘big-three’ auto-makers were crying that there was little sales interest from their traditional customers for electric cars, after the disappointing performance of early low volume contenders from specialist builders. The major motor corporations are considered to operate on slender profit-margins after the dealers have taken their cut, but a change to supermarket selling might weaken the imperative from high volume products which could favour specialist EVs from the OEM’s SVO departments. That the corporations have also jibbed violently against California’s mandate for a fixed percentage of overall sales being EVs, and wanted to respond to market-led rather than government-led forces, suggests a present resistance to EVs.

A number of industrial players outside the conventional automotive industry are drawing comparisons between the computer industry and the possible future electric vehicle industry, saying that the high-tech nature of the product, and the rapid development of the technologies associated with it, might require the collaboration of companies in a variety of technical disciplines, together with banks and global trading companies, to share the risk of EV development and capitalize on quick-to-market strategies aimed at exploiting the continually improving technology, as has already been the case in personal computers. They even suggest that the conventional auto-industry is not adapting to post-Fordist economic and social conditions and is locking itself into the increasing high investment required of construction based on steel stampings, and ever more expensive emission control systems to make the IC engine meet future targets for noxious emissions. The automotive industry reacts with the view that its huge investment in existing manufacturing techniques gives them a impregnable defence against incomers and that its customers will not want to switch propulsion systems on the cars they purchase in future.

It may be that the US domestic market is more resistant to electric vehicles than the rest of the world because the cultural tradition of wide open spaces inaccessible to public transport, and the early history of local oilfields, must die hard in the North American market where petrol prices are maintained by government at the world’s lowest level, for the world’s richest consumers. Freedom of the automobile must not be far behind ‘gun law’ in the psyche of the American people. In Europe and the Far East where city-states have had a longer history, a mature urban population has existed for many centuries and the aversion to public transport is not so strong. Local authorities have long traditions of social provision and it may well be that the electric vehicle might well find a larger market outside America as an appendage to the various publicly provided rapid transit systems including the metro and pre-metro. And, according to a CARB contributor to Scott Cronk’s remarkable study of the potential EV industry1, with the control equipment in the most up-to-date power stations ‘urban emissions which result from charging an electric vehicle will be 50–100 times less than the tail-pipe emissions from (even) … ULEV’ vehicles, a very different story to that put out by IC-engined auto-makers’ PR departments.

It is also argued within Cronk’s collection of essays that fuel savings from ultra-lightweight vehicles might predate the impact of electric vehicles, on public acceptance, particularly within European and Far-Eastern markets where petrol prices are at a premium and usually bear heavy social taxes. Fuel savings by such a course could be very substantial and the customer might, as a second stage, be more ready to take the smaller step to a zero-emissions vehicle. This is when he/ she realizes that the cost of overnight battery charge, at off-peak rates from the utilities, could prove an irresistible economic incentive. The vehicles would be produced in a lean-production culture which would also help to pare the substantial overhead costs that are passed onto the customer in traditional auto-manufacture.

0.2.6 DIFFERENT CULTURE FROM THE PRIVATE I.C.E. CAR

The different performance package offered to the public by the EV involves disadvantages, such as comparatively low range and carrying capacity, which need to be offset in the customer’s mind by advantages such as low maintenance, noise and vibration, creating the need for a different form of marketing and distribution from that of the conventional private car. The lower volume production rates also involve a quite different set of component and system suppliers, for servicing a specialist manufacture of this nature. The need for a charging infrastructure different from petrol stations also serves to distinguish EVs as a separate culture. Purchase price will be higher and resale price probably lower due to obsolescence in the face of advancing technology. The notion of periodically billing the customer for an ongoing personal mobility is likely to be preferable to just selling a car. The customer is thus spared the hassle of bargaining with dealers, obtaining finance, insurance and registration as well as the bother of refuelling and making arrangements for periodic servicing. Periodic servicing is likely to be extended to 50 000 mile intervals for EVs, and systems for refurbishing high mileage vehicles with updated technology systems might well be ‘on the cards’. The interlinking of mobility providers by horizontal networks would obviously benefit the customer as he/she travels from one area to another, possibly using different transport modes. The provider might be a sort of cross between travel agent and customer liaison officer of a motoring organization, but principally the leaser of the EV, Fig. 0.3.

The need to perceive the EV as a function-specific addition to the family vehicle fleet is also important so that a town car for the school-run, shopping or commuting can complement the conventional car’s use for weekend and holiday outings of longer distance. The local mobility provider will need PR skills to be regularly contactable by clients, but will not need the high cost service station premises of the conventional car dealer. In manufacturing the EV a different a ‘partnership’ chain of long-term suppliers and appoint a project leader to coordinate design, development and production, leading a cross-company team. Such leadership would carry the authority for detailed cost investigations in any of the member firms. EV leasers would need to network with manufacturing project leaders and provide carefully researched hire schedules of potential lessees upon which series production could be planned. This is without need for large parks of finished vehicles which conventional OEMs use as a buffer between supply and demand, as well as their need to maintain excess idle production capacity in slack periods. Organizational innovation thus shares similar importance with technological innovation in EV production.

0.2.7 PUBLIC AUTHORITY INITIATIVES

National government programmes, such as the ARPA EV programme in the USA, can be used to unite heavy defence spending with value to civilian producers. As combat vehicles have very high auxiliary power demand they become almost hybrid in the sense of their power sources, albeit only one of them being conventionally the prime mover. Coupled with the need to operate tanks in silent mode during critical battle conditions, this makes the study of hybrid drive a reality for military as well as civilian operators. The idea of helping sustain civilian product development must be almost impossible to contemplate by British military hierarchies but if ever a cultural transformation could be brought about, the technological rewards might considerably improve on the efforts made by the military to sell technology to British industry. The USA has the tremendous built-in advantage of their military supremos caring deeply about maintaining the country’s industrial base not normally part of the culture of UK military commanders!

Regional government initiatives can also be valuable in kick-starting cooperative ventures between companies from different industries. Again the US example, in California, is noteworthy where aerospace supplying companies have been encouraged to support pilot EV programmes. Valuable inputs to EV construction have therefore been made by companies skilled in structural design, computer simulation, lightweight materials, aerodynamics, fibre-optic instrumentation, head-up displays and advanced joining/fabrication. Of course, regional governments inevitably help EVs in the execution of environmental policies and already city authorities in many countries around the world have banned many vehicle categories from their central areas. National governments are also contemplating the huge sums of money spent in defending their oil supplies and probably noting the decreases in oil usage by industries such as building, manufacturing and power generation while transport oil usage continues to rise. The burgeoning use of computer and other electronics systems is also demanding more reliable electricity generation, that can accommodate heavy peak loads. Power generators will be increasingly pleased to step up utilization of the expanded facilities in off-peak periods by overnight charging of EVs. In the longer term, governments might even appreciate the reskilling of the workforce that could follow the return to specialization in the post-Fordist economic era and see that helping to generate new technological enterprises, as EV development and build could help recivilize a society condemned for generations to the mindlessness of mass production and the severe and dehumanizing work routines which accompany it.

0.2.8 REFINING THE CONVENTIONAL CAR PACKAGE FOR THE EV

The American ‘supercar’ programme, discussed in Chapters 4 and 7, has been an invaluable indicator as to how lightweight construction can dramatically improve the efficiency of automotive propulsion. As only 4% of a conventional car’s engine is needed for city driving conditions, the oversizing of engines in multi-functional cars makes the reduction of exhaust pollution a particularly difficult task on IC-engined vehicles. Expert analysts maintain that half the engine efficiency gains made in the decade 1985–1995 were lost by making engines powerful enough, in the US, to drive at twice the speed limit on the open road. Obviously the situation is worsened if conventional heavyweight steel construction is used and the tare weight of cars rises with the increasing proliferation of on-board gadgetry. While ‘supercar’ construction has shown how structure weight can be reduced, advanced technology could also be used to reduce the 10% of engine power used in powering ‘accessories’ such as power steering, heating, lighting and in-car entertainment.

The imperative for power steering is removed by the ultra-light construction of the ‘supercar’, provided steering and handling dynamics are properly designed. In EV supercars, wheel motors might provide for ABS and ASR without further weight penalty. High intensity headlamp technology can considerable reduce power demand as can the use of fibre-optic systems which provide multiple illumination from a single light source. Light-emitting diode marker lamps can also save energy and experts believe that the energy consumption of air-conditioning systems could be reduced by 90%, if properly designed, and used in cars with sandwich panel roofs, heat-reflecting windows and solar-powered ventilation fans. But none of this compares with the savings made by high strength composite construction which has the potential to bring down average car weight from 3000 to 1000 lb. It is reported that many of the 2000 or so lightweight EVs operating in Switzerland already weigh only 575 lb without batteries.

The ability to achieve net shape and finish colour from the mould in polymer composite construction is important in offsetting the higher cost of high strength composites over steel. But also the cost of steel is only 15% of the conventional structure cost, the remainder being taken up in forming, fabrication and finishing. Around half the cost is taken up by painting. The cheaper tooling required for polymer composites is also important in making small-scale production a feasible proposition, alongside direct sales from the factory of ‘made-to-order’ cars. A number of these factors would help to remove the high mark-up to the customer of the factory price which is typical of conventional car sales and distribution.

0.3 lean production, enterprise structures and networking

Lean production has grown out of post-Fordist ‘flexible specialization’ which has led to growing specialization of products, with a new emphasis on style and/or quality. The differentiated products require shorter production runs and more flexible production units, according to Clarke2. The flexibility is made possible by new technologies, the emerging economic structure being based on computerization and other microchip hardware. Rapid gains in productivity are made through full automation and computerized stock control within a system that allows more efficient small batch production. Automatic machine tools can be reprogrammed very quickly to produce small quantities of much more specialized products for particular market niches. Economies are set to be no longer dominated by competition between hierarchically organized corporations and open to those dominated by cooperation between networks of small and interrelated companies.

Lean enterprises are seen as groups of individuals, functions, and legally separate but operationally synchronized companies that create, sell, and service a family of products, according to Womack et al3. This is similar to the Japanese ‘keiretsu’ concept of large, loose groupings of companies with shareholding connections. They cooperate both technically and in sharing market information and the result is an array of business units competing in vertically and horizontally links with other companies within a single project. A trading company with well-developed worldwide networks is usually at the centre of the operation and can feed back vital market trends to the production companies. Of almost equal importance is the involvement of international banking corporations who can provide a source of industrial finance. Changes in legislation are required by European countries to make a similar system of common shareholdings plus private ownership acceptable to company law.

Lean production is the approach pioneered by Toyota in which the elimination of unnecessary steps and aligning all steps in a continuous flow, involves recombining the labour force into cross-functional teams dedicated to a particular activity, such as reducing the weight of an EV platform. The system is also defined by the objective of continually seeking improvement so that companies can develop, produce and distribute products with halved human effort, space, tools, time, and, vital to the customer, at overall halved expense.

Enterprise structures aim to exploit business opportunities in globally emerging products and markets; to unite diverse skills and reapply them in long-term cooperative relationships; to allocate leadership to the member best positioned to serve the activity involved regardless of the size of company to which he/she belongs; and finally to integrate the internal creation of products with the external consequences of the product. In EVs this would involve ensuring an adequate operational infrastructure be provided by an electricity generating company, in combination with local authorities. The products involved are those, such as the electric vehicle, that no one member company on its own could design, manufacture and market. Partners in an EV enterprise might also lead it into additional businesses such as power electronics, lift motors, low cost boat-hull structures and energy storage systems for power station load levelling, for example. Internally the use of combined resources in computer software technology could be used to develop simulation packages that would allow EVs to be virtual tested against worldwide crashworthiness standards. Managing of product external consequences could be facilitated by forming partnerships with electricity generators, material recyclers and urban planners, finance, repair and auto-rental service suppliers as well as government agencies and consumer groups.

0.3.1 COOPERATIVE NETWORKS

Unlike the Japanese networks of vertically integrated companies, such as the supply chains serving Toyota, an interesting Italian experience is one of horizontal networking between practitioners in specialist industries. Groups of small companies around Florence, in such areas as food processing, furniture making, shoe manufacturing, have been unusually successful and, in the case of tile manufacture, have managed to win an astonishing 50% of the world market. Export associations have been formed on behalf of these small companies and at Modena even a finance network has been formed between companies in which the participants guarantee one another’s bank loans. The normal default rate of 7% for bank loans in this region has become just 0.15% for this industrial network, demonstrating the considerable pride built up by companies in meeting their repayment obligations. Commentators liken the degree of trust between participants as being akin to that between different branches of traditional farming families. Like the grandfathers of the farming families the ‘elders’ of the industrial networks offer their services for such tasks as teaching apprentices in local colleges. The secret, some say, is that these areas around Florence escaped the era of Fordism which affected northern Italy and many other industrial centres of Europe.

The approach to setting up such a network is to build on elements of consensus and commonalilty so as to create mutual facilities of benefit to groups of small companies wishing to compete successfully against the international giants. Generally a network has a coordinating structure of interlinked elements which are individuals, objects or events. The links can be in the form of friendship, dependence, subordination or communication. In a dense network everyone knows everyone else while some networks may, for example, comprise clusters of dense elements with ties between clusters perhaps only involving one individual in each. The specific definition of a network is the set of relations making up an interconnected chain for a given set of elements formed into a coordinating structure.

Analysts usually consider solidarity, altruism, reciprocity and trust when examining networks in general. Solidarity is largely brought about by sharing of common experience; so social class and economic position layers are sometimes seen as having solidarity as do family and ethnic groupings. With altruism, of course, people help each other without thought of gain. Because it is rare in most societies, rewards and penalties for actions tend to exist in its absence. Repeat commitment to a network is expressed as loyalty and individuals often react to disturbance either by ‘exit’, ‘voice’ (try and change things for better) or ‘loyalty’. The latter may be expressed as ‘symbolic relations’ in which an individual is prepared to do his duty and meet his obligations. ‘Voice’ is important in the organization of networks as it involves argument, debate and persuasion, which is often fundamental to the direction taken by small to medium sized groups. Another stabilizing coordination is the reciprocity with which symmetry is maintained between giving and receiving. Of all the attributes, trust plays a central organizing role; essential if not all members behave absolutely honestly. Individuals bet against the opportunistic behaviour of others according to their reputations. Networks are often ‘flat’ organizations in the sense of having equality of membership. There is an underlying tendency for individuals to become involved with cooperative solidarity, if only because of the higher cost of not cooperating. Generally trust is built up over a period of recognizing and evaluating signals from other actors and having opportunities to test interpretations, over a rule-learning period, which leads to eventual solidification of mutual interest.

A study of French subcontracting companies to the engineering sector in the Lyons area, between 1975 and 1985, has shown that network coordination has improved performance relative to larger firms during that period, often becoming dynamic investors in flexible CNC machine tools. Essentially small firms benefited from large forms farming out some of their activities because they could not run flexible machines long enough to amortize the capital cost. But this was only the trigger and the firms later found the network of cooperation brought them trading advantages way beyond those available in a classic market. Recent economics approaches have dealt with transaction costs as a means of examining social ties between traders and such analysis involves the organizational implications of the transaction cost. Trust can lubricate the friction behind such costs. In the French study the small subcontractors were mainly supplying large engineering companies in the capital goods sector involved in large, complex, customized and expensive products for which client firms were unable to forecast requirements beyond a period of six months. Employees of the subcontracting firms undergo periods of training in the assembly shops of the client and the client firm becomes an expert in the engineering processes of the subcontractor so that mutual understanding can be built. Each subcontractor takes orders from one client of not more than 10–15% of total sales and the clients put themselves in the position of the subcontractors in determining optimal level of orders. The relatively low percentage figure allows the client a degree of flexibility without undermining the viability of the subcontractor. A ‘partnership’ exists in that in exchange for improved performance on quality and delivery the client firm guarantees a level of work for the subcontractor. Any defection of a subcontractor is made known to the whole community of suppliers and the full penalty has to be made for non-delivery, so that trustworthiness is not just judged by reputation; the long-term message from the experience was that ‘trust is expedient’.

Other examples show that large companies often tend to divest themselves of activities to the extent that they become essentially ‘systems integrators’ among a specialized consortia of companies in the particular manufacturing environment. Quoted examples are Fiat, BMW and Volkswagen. This breaking up of vertical integration may involve affiliated organizations or separate suppliers, with many aspects of R&D and design being divested to systems suppliers. Relationships between sub-units are too delicate to be left to market-type arrangements in this ‘associationalist’ way of working.

0.. Electric-drive fundamentals

While battery-electric vehicles were almost as common as IC-engined ones, at the beginnings of the commercialization of the powered road vehicle, it was not until the interwar years that serious studies were taken into operating efficiency of such systems, as a precursor to their introduction in industrial trucks and special purpose vehicles such as milk floats. Figure 0.4 illustrates some of the fundamental EV traction considerations as the technology developed. For the Mercedes Electromobile of the early 1920s, for example, seen at (a), more sophisticated wheel drives were introduced, with motors formed in the wheels to eliminate transmission gear losses. An energy diagram for this drive is seen at (b). The basic definitions and relationships of electromagnetism are helpful in the appreciation of the efficiency factors involved.

0.4.1 ELECTROMAGNETIC BASICS

While the familiar magnetic line-of-force gives the direction of magnetic force at any point, its field strength H is the force in dynes which would act on a unit pole when placed in the field. For magnetic material such as soft iron placed in the field, the strength of field, or magnetic intensity B, inside the iron is greater than H, such that B = µH, where µ is the permeability of the material (which is unity for non-metallics). When the cross-section of the object, at right angles to the magnetic field, is denoted by a, the magnetic flux φ is the product Ba in maxwells. Since it is taken that at unity field strength there is one line of force per square centimetre, then magnetic induction is measured in lines per cm2 and flux is often spoken of as in ‘lines’.

Faraday’s law defined the induced EMF as rate of change of flux (-dφ/dt×10-8 volts) and Lenz’s law defined the direction of the induced EMF as such that the current set up by it tends to stop the motion producing it. The field strength of windings having length l, with N turns, carrying current I is

H = 4πIN/10l which can be rearranged as φ(l/ma) = 4πIN/10

where the flux corresponds to the current in an electrical circuit and the resistance in the magnetic circuit becomes the reluctance, the term on the right of the equation being the magneto-motive force. However, while in an electric circuit energy is expended as long as the current flows, in a magnetic circuit energy is expended only in creating the flux, not maintaining it. And while electrical resistance is independent of current strength, magnetic permeability is not independent of total flux. If H is increased from zero to a high value, and B plotted against H for a magnetic material, the relationship is initially linear but then falls off so there is very little increase in B for a large increase in H. Here the material is said to be saturated. When H is reduced from its high value a new BH curve lies above the original curve and when H is zero again the value of B is termed the retentivity. Likewise when H is increased in the negative direction, its value when B is zero again is the coercive force and as the procedure is repeated, (c), the familiar hysteresis loop is obtained.

In generating current electromagnetically, coils are rotated between the poles of a magnet, (d), and the current depends on both the strength of the magnetic field and the rate at which the coils rotate. Either AC or DC is obtained from the armature rotor on which the coils are mounted, depending on the arrangement of the slip-ring commutator. A greater number of coils, wound around an iron core, reduces DC current fluctuation. The magnetic field is produced by a number of poles projecting inwards from the circular yoke of the electromagnet. Laminated armature cores are used to prevent loss of energy by induced eddy currents. Armature coils may be lap-wound, with their ends connected to adjacent commutator segments, or wave-wound (series) when their ends are connected to segments diametrically opposite one another. The total EMF produced

Motor

Planet wheels

(a)

1200 1100

Efficiency per centTonque lb ft

B
A
orque
Efficiency T
A
RPM B
A
B

POLE SHOE

1000 100 400 900 90 360 800 80 320 700 70 280 600 60 240 500 50 200 400 40 160 300 30 120 200 20 80 100 10 40

RPM

40 80 120 160 200 240 280 320 360 400 Amps

(b) Electric-drive fundamentals

B

(c) (d)

is (φnZ × 10-8/60)P/K where for lap-winding K= P and for wave-winding K= 2. Z is the number of conductors in the armature and n is its rotational speed.

The armature-reaction effect is set up by the current in the armature windings affecting the magnetic field between the poles. In a simple 2 pole machine, armature current would produce transverse lines of force, and the resulting magnetic field would be as shown in the figure. Hence the brushes have to be moved forward so that they are in the neutral magnetic plane, at right angles to the resultant flux. Windings between AB and CD create a field opposed to that set up by the poles and are called demagnetizing turns while those above and below are called cross-magnetizing turns. Armature reaction can be reduced by using slotted pole pieces and by separate compensating field windings on the poles, in series with the armature. Also small subsidiary inter-poles, similarly wound, can be used.

When the machine runs as a motor, rather than generator, the armature rotates in the opposite direction and cuts field lines of force; an induced voltage known as a back-EMF is generated in the opposite direction to that of the supply and of the same value as that produced when the machine is generating. For current I, applied to the motor, and back-EMF Eb, the power developed is EbI. By substituting the expression for Eb, the torque transmitted in lb ft is (0.117IφZP/K) × 108.

The field current can be separately excited (with no dependence on armature current) or can come from series-wound coils, so taking the same current from shunt-wound coils – connected in parallel with the armature and having relatively high resistance, so taking only a fraction of armature current. Compound wound machines involve a combination of series and shunt. In examining the different configurations, a motor would typically be run at a constant input voltage and the speed/ torque curve (mechanical characteristic) examined. Since the torque of a motor is proportional to flux × armature current, and with a series wound machine flux itself varies with armature current, the torque is proportional to the square of current supplied. Starting torque is thus high and the machine attractive for traction purposes. Since the voltage applied to a motor in general remains constant, and back-EMF is proportional to φn which also remains constant, as the load increases, φ increases and therefore the speed decreases – an advantage for traction work since it prevents the motor from having to carry excessive loads.

The speed of a motor may be altered by varying either the brush voltage or the field flux. The first is altered by connecting a resistance in series with the armature, but power wastage is involved; the second, field control, is more economical – and, with a series motor, a shunt is placed across the field winding.

0.4.2 ELECTRIC TRANSMISSION

Electric transmission, Fig. 0.5, survived electric power sources in early vehicles and the engineers of the time established the parameters for optimizing the efficiency of the drive. In a 1920s paper by W. Burton4, the author points out that for a given throttle opening and engine speed, the output in watts is fixed as the familiar product of voltage V and current I in the electrical generator. The ideal power characteristic thus becomes a rectangular hyperbola with equation VI = a constant. The simplest electrical connection between generator and electric transmission motor is as at (a). Generator and motor have to fulfil the function of clutch and gearbox, in a conventional transmission, and closure of the switch in the appropriate position provides for either forward or reverse motion ‘clutching’. Below a nominal 300 rpm the generator provides insufficient power for vehicle motion and the engine idles in the normal way. The change speed function will depend on generator characteristic and a ‘drooping’ curve is required with generator voltage falling as load rises, to obtain near constant power – suggesting a shunt-wound machine. By adding a number of series turns the curve can be boosted to a near constant-power characteristic. These series windings also

Notation

H = Field strength B = Magnetic intensity µ = Permeability φ = Magnetic flux N = Number of field turns Z = Number of armature

turns = �urrent V = Voltage L = Length of windings n = Rotational speed Eb= Back-EMF

(b)

Fu dy namfo
ield
7.2W i19.8W in resisn resistance bo tance box x
Const ant wat t curve
132.8W 102.8W 72.8W 42.8W
1000 RPM

50 100 150 200 250 300 Amps

Volts

5000 4600 4200

(a)

3800 3400

300 3000

250 2600

200

(c)

2200

150 1800

Brushes againstrotation
BBrus rushes no rmal
hesforward
100 1400

50 1000

0

(d) (e) Amps at 250 volts

Fig. 0.5 Electric transmission basics: (a) ‘clutching’ of electric transmission; (b) high EMF at low loads; (c) horned interpoles; (d) brush movement effect; (e) motor characteristics.

help in rapid build-up of generator EMF. The resulting problem is heat build-up of these series windings under heavy vehicle-operating loads. Efforts to counteract this by reducing the length of the shunt coil creates the further difficulty of slow excitation after vehicle coasting. Since the brushes of the generator or motor short-circuit one or more sections of the armature winding, it is important that these sections are in the neutral zone between field magnets of opposite polarity at the moment they are shorted. To otherwise avoid destructive arcing under heavy load, the machine characteristic may be altered by moving the brushes either with or against the direction of armature rotation. This will provide more or less droop of the characteristic as shown at (b), but on interpole machines there is the added problem of the interpoles being prevented, under brush movement, of fulfilling their role of suppressing arcing.

Horned interpoles, (c), may be used to offset this effect. The shape of the horn is made such that the magnetic flux under the foot of the interpole is not altered but the additional shoe section is magnified sufficiently to act on a few turns of the armature, these turns providing sufficient induced EMF to give the required compounding effect for rapid excitation from standstill and under heavy loads. The view at (d) shows the performance characteristics by a machine of this type. While the curve for the full field (no series resistance) approximates to the constant power characteristic, its EMF rises at light loads. The effect of inserting resistance is also shown. However, for a given motor torque, speed is proportional to EMF applied so that if the engine speed is reduced, motor and thus vehicle speed will fall. To avoid this, the motor field windings have a diverter resistance connected in parallel to them, to weaken the motor field; the counter-EMF is reduced, and more current is taken from the generator, which increases motor speed again. Thus a wide speed ratio is provided. In earlier times resistance was altered by handles on the steering column; with modern electronics, auto-control would, of course, be the norm. Regenerative braking can be obtained by reversing the field coil connections of the motor which becomes a ‘gravity-driven’ series-wound generator, running on short-circuit through the generator armature. However, the currents involved would be too heavy and an alternative approach is required.

The theme is taken up by H.K. Whitehorne in a slightly later paper5, who pays especial tribute to Burton’s skewed horn interpole invention. He goes on to consider motor characteristics and favours the series-wound machine because its speed is approximately inversely proportional to the torque delivered, adjusting its current demand to the speed at which it runs and to the work it has to do. Characteristic curves of a motor running on a fixed voltage are shown at (e). Conditions are shown for full field, and for two stages of field diversion. Examination of the 50 kW line makes it apparent that the torque/amp curve is independent of voltage; speed is practically proportional to voltage and generally characteristics vary on the size of the motor, its windings and length of its core. However, on low voltage and heavy current, the efficiency falls rapidly which makes electric transmission a difficult option for steep gradients. There is considerable flexibility, though, as engine and generator running at 1500 rpm deliver 50 kW at 250 V, 200 A, the electric motor for this output being designed to run at 3800 rpm giving torque of 70 lb ft, for overdrive cruising, yet at 800 rpm giving 315 lb ft for gradients.

0.5 EV classification

EVs in common current use include handling trucks, golf carts, delivery vans/floats and airport people movers/baggage handlers. The more challenging on-road application is the subject of most of what follows in this book, where the categories include motor scooter, passenger car, passenger service vehicle, taxi and goods vehicle.

The smallest road-going EVs are probably the electric bicycles such as the Sinclair Zike and the Citibike product. Both these companies also produce bolt-on pedal assist systems for conventional bicycles. Electric motorcycles are less common than electric scooters, the BMW C1 being an example. Recent electric cars have divided between conversions of standard production models and a small number of purpose built vehicles. Japan’s flourishing microcar market of smaller and lighter cars is an important target group for electric conversion, for which acceleration and efficient stop-start driving is more important than range. Such city cars are distinct from longer-range inter-urban cars and the latter market currently attracts hybrid drive cars of either gasoline or diesel auxiliary engines, with series or parallel drive configurations. Fuel-cell cars for the inter-urban market are still mostly in the development stage of value engineering for volume production.

Commercial and passenger service vehicle applications, that section of the market where downtime has to be kept to a minimum, and where low maintenance costs are at a premium, are particularly attractive to EVs. Municipal vehicles operating in environmentally sensitive zones are other prime targets. In passenger service applications battery-electric minibuses are a common application in city centres and IC-electric hybrids are increasingly used for urban and suburban duties. Gas-turbine/electric hybrids have also been used in buses and fuel-cell powered drives.

Guided buses include kerb-guided and bus/tram hybrids, the former having the possibility for dual-mode operation as conventionally steered vehicles. Guided buses have been used in Essen since 1980. Trolleybus and tramway systems are also enjoying a comeback.

At this relatively early stage in development of new generation EVs tabular classification is difficult with probably the only major variant being traction battery technology. A useful comparison was provided in a Financial Times report6 on ‘The future of the electric vehicle’ as follows:

Battery Advantages Disadvantages Comments
Lead-acid Established technology; low cost and fairly long life (1000 cycles). Low energy and power density. Horizon and other high performance batteries greatly improve the suitability for EVs but must be made cheaper.
Nickel-cadmium Higher energy density and cycle life than lead-acid. �admium very toxic. Being used for second generation, purpose-built EVs.
Lithium High energy and power densities. Safety concerns overcome. Expensive. Research into scaling up to EV size will probably provide a mid-term battery.
Sodium-sulphur High efficiency and energy density. Thermal enclosure and thermal management is expensive. �orrosive components. Several technical issues to be resolved before this could become an option.
Sodium-nickel chloride High energy and power densities. Long life (over 1000 cycles). Thermal enclosure and thermal management are expensive. Promising mid-term option but currently over twice the cost of the USAB� target.
Nickel-metal hydride High power density, Long cycle life (over 2000 cycles). Twice the energy storage of lead-acid. Expensive. Promising mid-term option but currently over twice the cost of the USAB� target.
xxx Lightweight Electric/Hybrid Vehicle Design
Battery Advantages Disadvantages Comments Zinc-air High energy density. Infrastructural Interesting longer-term Rapid mechanical needs. option for rapid recharging (3 minutes). recharging. Nickel-iron High energy density. Hydrogen emitted Research to increase Long life (over 1000 -safety concerns. efficiency and deep charge/discharge Periodic topping overcome disadvantage cycles). up with water could lead to a long-needed. term EV battery. Nickel-High energy density. Fairly expensive Already used in hydrogen Robust and reliable, no (due to hand communications overcharge/overassembly). satellites. �ost discharge damage. competitive for high Very long life. cycle operations.

References

  1. Cronk, S., Building the E-motive industry, SAE paper, 1995
  2. Clarke, S., The crisis of Fordism or the crisis of social democracy, Telos, spring, No. 83, pp. 71–98, 1990
  3. Womack et al., From lean production to lean enterprise, Harvard Business Review, March–April, 1994
  4. Burton W., Proceedings of the Institute of Automobile Engineers, 1926–1927
  5. Whithorne, H., Proc. IAE, 1929–1930
  6. Harrop, G., The future of the electric vehicle, a viable market? Pearson Professional, 1995

J.F.

PART ONE ELECTROMOTIVE TECHNOLGY

1

Current EV design approaches

1.1 Introduction

The environmental arguments for electric propulsion become more compelling when they can be supported by an economic case that will appeal to the vehicle buyer. Here the current technology of electric and hybrid drive is reviewed in a way that shows the technical imperatives alongside the economic ones. After an analytical study of drive system comparisons for different vehicle categories, ‘clean-sheet-design’ integrated vehicle electric-drive systems are reviewed for small and medium cars and a concluding section encapsulates a procedure for optimizing motor, drive and batteries in the form of a power-pack solution.

A section on electric-drive fundamentals, establishing basic terminology, appears in the Introduction. In the preface to the case study chapters (5 and 6), contained in the second half of Chapter 4, the whole macro-economics of electric vehicles is discussed, with the wider aspects of the fuel infrastructure, as is a full analysis of competing electric-drive and energy-storage systems, for EVs.

1.2 Case for electric vehicles

1.2.1 ENVIRONMENTAL IMPERATIVE

The current world population of motor vehicles stands at 700 million, of which over 600 million are owned in G7 economies1. This number is set to increase to around 1000 million in the next ten years. The bulk of this growth is expected to occur in Second World countries where per capita income is reaching levels where car ownership is known to commence. This has two serious implications (Fig. 1.1): a large increase in the usage of hydrocarbon fuels and an increase in

80

5T

40

20 LOG ENERGY USAGE TONS/ANNUM COAL EQUIVALENT

Fig. 1.1 Life expectancy related to energy

LIFE EXPECTANCY YEARS

usage, as seen by the World Bank. ENERGY USAGE

pollution to globally unsustainable levels. Much has been heard of the so-called Greenhouse Effect. If carbon dioxide is on a scale of 1 as a greenhouse gas, methane is 25 and CFCs are 30 000– 50 000. Clearly the release of hydrocarbons and CFCs by man must be curtailed as soon as possible; CO2 is a different matter. If the quantity in the atmosphere was doubled from 20 to 40%, the temperature would increase by 5oC and the sea level would rise by 1 metre. However, the additional plant activity would eliminate famine for millions in Africa, the Middle East and Asia. In scientific circles, the ‘jury is still out’ on carbon dioxide.

The problem emissions are those of carbon monoxide, sulphur dioxide, nitrous oxide and lead, not to mention solid particles from the exhausts of diesels. In all of these, man is competing with nature. The problem is that man’s emissions are now set to reach levels which history shows have had dramatic consequences in nature. For example, in 1815, a volcano emitted 200 million tons of sulphur dioxide into the atmosphere. In 18l6 there was a cloud of sulphuric acid in the sky which blocked out the sun in the northern hemisphere for the whole of the summer. The temperature fell by 7°C and there were no crops. Every 2000 megawatt power station which runs on coal emits 150 000 tons of sulphur dioxide per annum. Acid rain destroys our forests and buildings in the northern hemisphere. Pollution on this scale in the southern hemisphere is unsustainable. Nitrous Oxide is emitted when nitrogen burns at 1500° C or above. This gas reaches high concentrations in cities and is converted by sunlight into photosynthesis smog, which is becoming a major health hazard worldwide. A change in the technology of motor transport could have the fastest impact on this problem as most vehicles are replaced every ten years.

1.2.2 ELECTRIC VEHICLES AS PRIMARY TRANSPORT

Consumers vote with their wallets! Electric vehicles will only have a healthy market based on a primary transport role using technology that achieves the performance of internal combustion engines. This means sources of energy other than batteries (Fig. 1.2). In reality we have a choice of IC engine, gas turbine and fuel cell, but how can we maintain performance whilst reducing pollution? The secret is to stop wasting the 72% of energy that currently goes out of the exhaust pipe or up from the radiator. The IC engine is currently operated with a fuel/air ratio of 14:1. This can be increased to 34:1 but the engine can no longer accelerate rapidly. Fortunately, this can be overcome by other means. The gas turbine is an efficient solution for large engines over 100 kW in commercial vehicles. Its performance is not as good as an IC engine’s at lower powers, however, and fuel-cell electrics offer the best promise. Fuel cells are the technology of the future. There are many sorts but only one type of any immediate relevance to vehicles and this is the proton exchange membrane (PEM) cell. Using the Carnot cycle, this has a conversion efficiency limit of 83%. Scientists can achieve 58% now and are predicting 70% within ten years. Fuel cells have many excellent qualities. Small units are efficient – especially at light load. New construction techniques are reducing costs all the time and £200/kW was already achievable in 1992 using a hydrogen/air mixture. The real problem is providing the fuel.

1.2.3 THE FUEL INFRASTRUCTURE

Current engines obtain their energy by burning hydrocarbons such as propane, methane, petrol, diesel and so on. However, hydrogen is the fuel of the future. What powers a Saturn 5 Moon Rocket? Coincidence, or sheer necessity? Liquid hydrogen has an energy density of 55 000 BTUs per pound compared to 19 000 BTUs per pound for petrol and 17 000 BTUs per pound for propane. The problem is obtaining large amounts of hydrogen efficiently from hydrocarbon fuels. The percentage of hydrogen directly contained in these fuels is small in energy terms. For example, methane (CH4) has 17.5% of its energy in carbon and 25% in hydrogen. However, there is now a solution to this problem, with a reforming process developed by Hydrogen Power Corporation/Engelhard called Thermal Catalytic Reforming. Put simply, it is the chemical process:

3Fe + 4HO = FeO+ 4Hand FeO + 2C = 3Fe + 2CO

2342 342

The first process takes place with a catalyst at 130° C. The hydrogen is stored in a hydride tank until required. The iron is returned to a central facility for reduction by the second process. The main points about this cycle are that a high proportion of hydrocarbon heat energy is converted into hydrogen and that 1 kg of iron provides enough hydrogen for a small car to travel 6 km on a fuel cell.

IC engines and gas turbines run well on most hydrocarbons and hydrogen. Fuel cells need hydrogen. Hydrogen has to be used and stored safely. This could be achieved by reforming it on demand at fuel stations – the waste heat would be used to generate electricity to be pumped back into the national grid. The primary fuel could be any hydrocarbon such as petrol, diesel, methanol, propane or methane. The only constraint is that the fuel source must have low sulphur content so as not to poison the catalyst. In the UK, we have a head start called the Natural Gas Grid. This is likely to become of critical importance for energy distribution, removing the need to distribute petrol and diesel by road. To satisfy future transport needs, we retain our ‘fuel’ stations as the means of distribution. This brings us to the problem of on-board hydrogen storage.

l. Petrol car: A journey of 68 miles each day consumes 2.5 gallons of fuel and takes 2 hours. Amount of energy in fuel 5.14 x 108 joules Thermal power 71.3 kW Mechanical power 20 kW average Efficiency 28%

  1. Battery electric car as secondary transport. Power station efficiency 40% Electric car efficiency 80% OVERALL 32% �ON�LUSION: Pollution is moved from car to power station. There is only an environmental return if the car's performance is sacrificed or the power station is non-thermal and range/ performance is limited.
  2. Hybrid car as primary transport. Hydrocarbon to electricity Via lean burn petrol engine 45% Electricity to mechanical power 90% OVERALL 40.5% �ON�LUSION: Pollution reduced by 55% and fuel consumption is 70% of petrol vehicle with performance/range as the petrol vehicle.
  3. Fuel-cell electric car as primary transport. Hydrocarbon to hydrogen conversion 80% Fuel-cell hydrogen to electricity 60% Electricity to mechanical power 90% OVERALL 43% (potential for 48% in 10 years) �ON�LUSION: Pollution reduced by 90%; fuel consumption is 66% of petrol vehicle and performance/range is as petrol vehicle.

Fig. 1. 2 Some crude comparisons for fuel related to pollution.

20 KW electricity to load

Iron titanium hydride has long been known as a storage medium but one would need 500 kg to store 10 litres of hydrogen, at a cost of £3000 in 1992. The gas is stored in a standard propane tank filled with this material. If the tank is ruptured, the gas is given off slowly because of its absorption in the hydride. In the USA experiments are also taking place with cryogenic storage which is potentially cheaper and lighter. The overall distribution scheme is illustrated in Fig. 1.3. To summarize, the benefits of a change to hybrid/fuel-cell electric vehicles are: (i) engineering is practical; (ii) performance is acceptable to the consumer; (iii) it reduces fuel consumption; (iv) it reduces pollution, especially Nitrous oxide; (v) it reduces dependence on imported oil; (vi) it can be achieved quickly; (vii) it can be achieved at sensible cost; (viii) it prevents increased demand for oil; (ix) it fits in with the existing fuel infrastructure and (x) it solves the pollution problem in relation to projected pollution levels, not existing ones – the prime cause of the catalytic converter being ineffective.

1.2.4 FUEL-CELL ELECTRIC VEHICLE

This vehicle category, Fig. 1.4, will use a fuel cell to provide the motive power for the average power requirement and utilize a booster battery to provide the peak power for acceleration. Hydrogen would be stored in a tank full of metal hydride powder, or cryogenically. This system provides enough waste heat for cabin heating purposes. The fuel cell can recharge the battery when the vehicle is not in use. If the vehicle has an AC drive, it is possible for it to generate electricity for supply to portable tools, a house, or injection into the national grid. Fuel cells should reduce emission levels by a factor of 10, compared with IC engines on 14:1 air:fuel mixture.

1.2.5 CHARACTERISTICS OF FUEL CELLS

What is a fuel cell? It is an electrochemical cell which converts fuel gas and oxidant into electricity and water plus waste heat (see Chapter 4). The PEM cell has graphite electrodes with a layer of membrane sandwiched in between, plus gas-tight seals. Each cell is about 6 mm thick and produces 1 V off-load and 0.7 V on-load, at a current of around 250 amps. Consequently a fuel cell for a 15

Waste heat

kW average power would produce about 60–70 V DC at 250 amps. In size it would be about 200 mm square and about 600 mm long. The cell operates at a temperature of 80°C. When cold, it can give 50% power instantly and full power after about 3 minutes. The units exhibit very long life. The problem until recently has been seal life when operated on air as opposed to oxygen. New materials have solved this problem. Output doubles when pure oxygen is used. Fuel cells do not like pollutants such as carbon monoxide in the source gases. Gas is normally injected at 0.66 atmospheres into the stack. The main challenge now is to refine the design so as to optimize the cost relative to performance. This will take time because the effort deployed at this time is small in relation to the effort put into batteries or other fuel-cell types. There is a very real case for a major multinational effort to train scientists and engineers in this technology in the short term, and to reduce the time to introduction on a large scale.

1.2.6 THE ROLE OF BATTERIES

Batteries have been with us for at least 150 years and have two main problems: they are heavy and they do not like repeated deep discharge. Batteries which are deep cycled, irrespective of the technology, deteriorate in performance with age. So the question must be asked ‘what can batteries do well?’. The answer is to provide limited performance in deep discharge, or alternatively, much better performance as a provider of peak power for hybrid and fuel-cell vehicles.

Much work is under way on high temperature cells. These are unlikely to meet cost or weight constraints of primary transport applications. The best high temperature batteries can offer 100 Wh/kg. Overall, fuel cells already give 300 Wh/kg and this can be improved with development. What is needed is a battery with different capabilities to normal car starter batteries, namely very low internal resistance, long life, excellent gas recombination, room temperature operation, totally sealed, compact construction, reasonable deep discharge life as well as being physically robust.

The battery which satisfies the above criteria is the lead–acid foil battery, as manufactured by Hawker Siddeley. This type of construction has replaced nickel–cadmium pocket batteries on many aircraft. In particular the lead–acid foil battery retains far more charge from regeneration than conventional designs and can be charged and discharged rapidly. However, there is a trick to achieving this. Most batteries are made up of ‘rectangular’ arrays of cells so it is no wonder that the temperature of the cells varies with position in the stack. To charge a battery quickly it is vital to keep the cells at an even temperature. Consequently it is necessary to liquid cool the cells so as to obtain best performance and long life. Other points worthy of note are that batteries work best when hot; 40°C is ideal for lead–acid. The battery electrolyte is just the place to dump waste heat from the motor/engine/fuel cell.

Nickel–cadmium batteries offer better performance than lead–acid but are double the cost per Wh of storage at present and sealed versions are limited to 10 Ah but larger units are under development. The best nickel–cadmium units available at present are the SAFT STM/STH series. Sealed lead–acid and aqueous nickel–cadmium cells have peak power in W/kg of 90 and 180, with Wh/kg values being 35 and 55 respectively.

In terms of safety, long series strings of aqueous batteries are not a good idea. The leakage from tracking is high and they are very dangerous to work on. Consequently batteries should be of sealed construction with no more than 110 V in a single string. Ideally, the maximum voltage should be 220 V DC, that is +/ 110 V to ground arranged as two separate strings with a centre tap, so that no more than 110 V appears on a connector, with respect to ground, Fig. 1.5.

There is an opening in the market place for a low cost 2 pole, 220 V, 300 A remote-control circuit breaker to act as battery isolator with 5 kA short-circuit capacity. However, there is a problem with earthing the centre tap of the battery as one may need an isolating transformer in the battery charger. Consequently, in many of the new schemes proposed in the USA, a different route is implemented which is used in trolley buses – the all-insulated system. In most of these schemes, large capacity batteries are used (15–30 kWh) at a typical nominal voltage of 300 V. This will vary from 250 V fully discharged to 375 V at the end of charging. The electrical system is fully insulated from earth. During charging, the mains supply can be either centre tap ground or one-end ground. In the centre tap ground (typical USA situation,

system Balanced voltage to earth

with 110/0/110) the potential of the vehicle electrics is balanced to earth. When one end is earth (typical European situation) the potential of the vehicle electrics will move up and down at the supply frequency with respect to ground and there is the prospect of earth leakage current through any capacitance to earth of the vehicle electrical system. However, this is very small, usually because the tyres isolate the vehicle. However, when charging it would be desirable to ground the vehicle body to prevent any shocks from people touching the vehicle and standing on a grounded surface.

1.2.7 ELECTRIC VEHICLE SPECIFICATIONS

From the previous considerations one can now start the task of specifying EV capability/ performance trade-offs. Polaron believe EVs will be partitioned as shown in Fig. 1.6. This does not pretend to be an exhaustive list but to show the range and scale of requirements to be provided for. The most interesting observation is that in the mass market, 30–150 kW, a solution is possible using just two sizes of drive, 45 and 75 kW. To complement the drives, motors are required of two speed ratings for each size, say 5000 rpm where compatibility with a prime mover is required, and 12 000 rpm for the direct drive series hybrid/pure electric case.

1.2.8 HYBRID VEHICLE EXAMPLES

It is now proposed to have a look at two cases (a) 45 kW parallel hybrid vehicle; (b) 90 kW series hybrid vehicle, as in (Fig. 1.7). The 45 kW parallel hybrid vehicle consists of, typically, a small engine driving through a motor directly into the differential gear and hence to the road wheels. Minimization of weight is the key issue on such a design along with low rolling resistance and low drag. At 60 mph a good design can expect to draw 8 kW to keep going on a flat level road. The vehicle would be fitted with an engine rated to supply about one-third of the peak requirement, that is 15 kW plus an allowance for air conditioning if relevant. The motor has to deliver up to 45 kW using energy stored in batteries. This can be done either by a constant torque motor operating via a gearbox or a constant power motor operating with only two gears or without a gearbox. The latter is rapidly becoming the standard for EVs using front wheel drive.

Fig. 1.6 Short-term battery electric and hybrid vehicles.

Power Rating GVW Engine Motor type Motor rating Turbo alternator Application
Below 40 kW Less than 2 tons None Brush D� Up to 40 KW None Straight battery electric van or car
40 kW 150 kW 150 kW to 2 ton 2 ton 3 ton 5 ton 7 ton 10 ton I GT I GT GT GT Brushless D� Brushless D� Brushless D� Brushless D� Brushless D� Switched reluctance motor 1 x 45 kW 5000 rpm 1 x 75 kW 12 000 rpm 1 x 75 kW 5000 rpm 2 x 45 kW 12 000 rpm 2 x 75 kW 12 000 rpm 1 x rating 5000 rpm None 1 x 100 kW 60 000 rpm None 1 x 100 kW 60 000 rpm 1 x 150 KW 50 000 rpm 1 x rating 50 000 rpm at 150 kW Parallel hybrid family car Parallel hybrid performance saloon Parallel hybrid 1 ton truck Series hybrid 2 ton truck Series hybrid single deck bus Heavy traction and road haulage
1 MW 40 tons 25 000 rpm at 1 MW Series hybrid configuration

The vehicle uses the battery to provide peak acceleration power for overtaking, hill climbing and so on. On the flat a 0–60 mph acceleration time of around 12 seconds would be typical for this class of vehicle and a top speed of perhaps 80 mph, where permitted; the engine is started when the road speed exceeds 20 mph and then clutched into the motor. The engine then charges the batteries as well as satisfying the average demand of the car. During acceleration the electric drive and the engine work together to provide peak acceleration. It is in the cruise condition that optimum efficiency is required. Consequently more sophisticated designs use 3 way clutch units so that the motor can be mechanically disconnected when the battery is fully charged and only switched back in for acceleration. In this condition attention must also be paid to the minimization of rolling resistance and windage losses (Figs 1.6 and 1.8(a)).

The series hybrid vehicle corresponds to a high performance sports saloon. A 0–60 mph time of 7 seconds and a top speed of 120 mph could be expected (Figs 1.7 and 1.8(b)). The main power source would be a gas turbine which would operate through a PWM inverter stage to feed 300– 500 V DC into the main bus. There are two separate drives, each driving a rear wheel of the vehicle. To reduce weight, the motors would be designed for 12 000 rpm and gearboxes employed to reduce the speed to the road wheels – about 1800 rpm at 120 mph. The gas turbine may operate over a 2:1 speed range to give good efficiency. Specific fuel consumption is doubled at 15 kW compared to 100 kW. However, overall consumption would still be that of a ‘Mini’, with emissions to match. The peak power for acceleration would come from batteries – probably nickel–cadmium in this case, where cost pressures are not so demanding.

Port

Starboard

1.2.9 ELECTRICAL SYSTEM DESIGN CHALLENGE

What are the design problems for the electrical system? The first one is cost. Unless the final product is attractive to the consumer, we do not have a market. Where are we now? For 1000 off systems at 45 kW, a brushless DC motor would cost £1000, a controller £2000, and a battery £2000 (lead–acid). These 1992 prices will reduce with mass production. The second design challenge is one of methodology. Electric vehicles have been traditionally built by placing motor and batteries then spreading the electrical system over the vehicle. This needs to change. Polaron would like to suggest a modular approach to the problem whereby sealed batteries and controller power electronics are in one unit and the motor is in fact the second. The third design challenge is one of compatibility. Low performance vehicles can be built with 110 V electrical systems. However, as the power increases this is not practical. But both fuel cells and batteries are low voltage heavy current devices – how can this conflict be addressed?

The solution is to use power conversion. In Fig. 1.9 a 100 stage fuel cell is integrated with a 216 V battery to give a stabilized 300 V DC rail. The motor and controller are then built at 300 V where the currents are significantly reduced on the 100 V system. As the power level rises, voltages up to 500 V DC can be anticipated. However, when the power conversion is switched off the highest voltage will be the battery voltage. This additional power conversion will be needed for another reason. If vehicles are equipped with small booster batteries for acceleration, the DC link voltage will change significantly according to load conditions. The power conversion provides a means of stabilizing for this variation.

1.2.10 MOTOR TYPES AND LOCATIONS (FIG. 1.10)

Which is the best type of motor? Answer – the cheapest. Which is the cheapest motor? Answer – the lightest. Which is the lightest motor? Answer – the most efficient. On this criteria, there is no doubt that a permanent magnet brushless DC motor would sweep the board. However, our

Fig. 1.9 Fuel-cell power conversion.

Fig. 1.10 Motor specifications.

Nelco electric 34 kW brush motor specification Voltage current 216 V 177 A Resistance inductance 45 milliohms 115 µH Field time constant 0.3 seconds Field volts/amps 22 amps Weight 80 kg/rating (10–60 mins) Efficiency 86% at 34 kW 3000 rpm Cooling Air forced Cost (1000 up) £600 (batch prod.)
Coercive systems 45 kW 5000 rpm BDC motor specification Voltage current frequency 230 V 130 A 666 Hz No. of poles 16 Weight 45 kg rating continuous Efficiency at a. 1500 rpm 45 kW 96% b. 3500 rpm 10 kW 95% Cooling Oil–2 litre/min Cost (1000 up) £1000 (batch prod.) £600 (mass prod.)
Coercive systems 45 kW 12 000 rpm BDC motor specification Voltage current frequency 220 V 130 A 800 Hz No. of poles 8 Weight 25 kg rating continuous Efficiency at a. 3600 rpm 45 kW 96% b. 5000 rpm 10 kW 95% Cooling Oil–2 litre/min Cost (1000 up) £600 (batch prod.)

enthusiasm must be tempered by two other considerations, cost of materials and controller costs. The factors affecting selection are covered in Section 1.3.

1.2.11 CHOPPER CONTROLLER FOR A 45 kW MOTOR

Figure 1.11 illustrates a typical pure battery electric vehicle scheme which could also be used in hybrid mode with an engine if required. The motor is a shunt field unit such as the Nelco Nexus 2 unit used in many industrial EVs. This machine is a 4 pole motor with interpoles and operates at a maximum voltage of 200 V DC. The field supply is typically 30 amps for maximum torque. The controller consists of a 2 quadrant chopper with a switch capacity of 400 amps. An electromechanical contactor shorts out the positive chopper switch in cruise mode for maximum efficiency. The chopper is fitted with input RF filtering and precharge to extend contactor life. The chopper switches at 16 kHz and the output contains a small L/C filter to remove the dv/dt from the machine armature. A Hall effect DCCT measures the armature current for the control system.

In the power supply area, there are four components: first is the battery charger, in this case a CUK converter, or a boost/buck chopper is also a possibility to make the mains current look like a sine wave for ensuring IEC555 compliance. Control of battery charging conditions is one of the most important considerations in extending battery life in deep discharge. For lead–acid batteries the level of float voltage is critical as well as maintaining cell temperature. The battery charger could incorporate a 20 kHz isolating transformer if costs permit. Experiments are under way with inductive power transfer which isolates the car and makes it necessary to plug in for charging. Another possibility is an automatic self-aligning connector which the car drives into when parking. The next consideration is the auxiliary 13.6 V battery supply. The vehicle seems likely to retain a separate 12 V battery for lighting and control functions. A 300 W DC/DC converter will satisfy this requirement. The third consideration is the control system power. This is a small (20 W) DC/DC converter which provides the control power for the chopper. It is likely to be incorporated with the main control PCB and could also be supplied from the 13.6 V battery. The final factor is the field controller. This is a 4 quadrant chopper which provides the motor field supply. It has to be able to reverse the current so that the motor can reverse without contactors in the armature circuit. If the motor has a tachometer fitted, this may be used for braking control and blending with electromechanical brakes. The important issue with this controller is that the power switching is contained in a single unit so that all the DC components are kept in one place. This is important for another reason to meet IEC555 RF interference legislation. Therefore all insulated systems will require an isolated conductive casing which can be connected to vehicle chassis.

1.2.12 CONTROLLER FOR A 45 kW AC MOTOR (BRUSHLESS DC OR INDUCTION)

This is illustrated in Fig. 1.12. The drive consists of a 3 phase PWM Drive which feeds the 3 phase motor. The beauty of this arrangement is that the motor can be disconnected and the mains fed to the inverter arms to give a high power battery charger, by phase locking the PWM to the mains.

An alternative to this arrangement is for the inverter to put power back into the mains. In case of fault, three alternistors provide current limit protection. In the brushless DC case, the motor permanent magnets provide 50% of the flux and the remainder comes from a 50 amp circulating current Id at right angles to the torque producing component Iq.

The inverter is constructed using 300 amp IBGT phase leg packages which minimize the inductance between transistors and associated bypass diodes. The inverter output is filtered by 6 x 10 µΗ capacitors plus 3 x 5 µΗ inductors. This reduces the 18 kHz carrier ripple current in the motor to about 20 AP/P. There is a real time digital signal processor (DSP) which performs vector control using state space techniques and this includes 3rd harmonic injection to maximize the inverter output voltage. Comprehensive overload protection is fitted. The inverter demand is a torque signal and a speed feedback is provided for the vehicle builder to close the speed loop. Both signals are PWM format (10–90%) on a 400 Hz carrier. The drive can be adapted for induction motor control but this is not so efficient, as explained in the motor section below.

1.2.13 TURBO ALTERNATOR SYSTEM FOR GAS TURBINES

Figure 1.13 illustrates a turbo alternator scheme for gas turbines. This scheme has two purposes: it starts the turbine, and provides a stabilized DC link voltage for a 2:1 change in turbine speed and changes in DC link current from no-load to full-load. The alternator itself is the result of many years’ development in high speed gas compressors. It is a 4 pole unit which allows iron losses to be kept low and in particular the tooth tip temperature reasonable whilst still using silicon steel laminations (2 pole permanent magnet alternators are potential fireballs!). The magnet material is samarium cobalt with a carbon fibre or Kevlar sleeve. At these speeds, one needs every bit of strength possible. The magnets are capable of operation at 150° C. The use of metallic magnets is not a problem here because the weight is small. Hall sensors are fitted for machine timing during starting and voltage control purposes. A small L/C filter limits the amplitude of the carrier ripple on the alternator windings.

1.2.14 MODULAR SYSTEMS

From the foregoing considerations, it will be apparent that the motor car of the future needs power electronics to be viable. Fortunately, we now have the technology to satisfy the most demanding applications. There may be some rivalry between different types of power switches but cost will be the final judge. A manufacturer who constructs the power electronics as an all-insulated system in a single module permits module exchange as the first means of maintenance. Liquid cooling also makes sense. It can cool the motor, warm/cool the sealed batteries and provide power steering at the same time. This concept will make it possible to convert existing chassis as well as develop new ones, thus enabling product to be brought to market quickly. Standard electronics packages are the only way to achieve the unit costs necessary for product acceptance in the market. Interchangeable batteries will make it possible for maximum vehicle utilization in intensive duty applications, such as taxis and delivery vehicles. This method of construction also opens the door to new methods of financing EVs; for example, the user buys vehicle then rents battery/power electronics.

1.3 Selecting EV motor type for particular vehicle application

1.3.1 INTRODUCTION

Motor and drive characteristics are selected here for three different applications: an electric scooter; a two-seater electric car and a heavy goods vehicle, from four motor technologies: brushed DC motor, induction motor, permanent-magnet brushless DC and switched reluctance motor2. Any of the four machines could satisfy any application. This is not a battle of ‘being able to do it’, it is a battle to do it in the most cost-effective manner. There are two schools of thought regarding EVs – group A believe they should create protected subsidized markets for environmental reasons and are not too concerned with cost. Group B realize that until this technology can compete with

Specification

Speed 60 000 rpm
Power 90 kW
Voltage 200 V
Frequency 2000 Hz
Weight 20 kg (housed)
Dimensions 150 mm OD x 175 mm long
Current 262 amps
Efficiency 99%
Resistance 14 milliohms
Inductance 15 microhenries
Cooling Liquid (oil or water)

Fig. 1.13 Turbo alternator.

piston engines in terms of performance and cost there will be no significant competition, hence no major market share. Polaron are putting their money on group B. What is clear is that the economics will come right at lower powers first, then work upwards. Another fact is that a market needs to be established before custom designs can be justified and the most immediate need is for conversion technology for existing vehicle platforms.

1.3.2 BRUSHED DC MOTOR

This consists of a stationary field system and rotating armature/brushgear commutation system. The field can be series or shunt wound depending on the required characteristics. The technology is well established with more than a century and half of development. The main problem is one of weight compared with alternative technologies, consequently Polaron believe DC is best at lower powers overall, due to the built-in commutation scheme. As the power level rises many problems become significant: commutation limited to 200 Hz for high speed operation; problems with commutator contamination; significant levels of RF interference; brush life limitations and cooling/ insulation life limitations. Polaron’s Nelco division has made these machines for many years and

A

TORQUE Nm

A

1000 2000 3000 4000 5000

Face commutator

1.

has introduced a new design to help overcome some of the problems. The so-called Gemini series consists of an armature with a face commutator at both ends of the armature. This permits two independent windings which may be connected in series or parallel. Improvements in the torque speed curve are seen in Fig. 1.14, while Fig. 1.15 shows a recently developed controller. While existing controllers have single quadrant choppers with contactors for reversing and braking, and field control is effected by a separate chopper unit, Polaron feel such a design gives limited overall performance and is better replaced by the arrangement shown. Brushed DC motors have a role in applications below 45 kW but, if power rises above this figure, mechanical considerations such as the removal of heat from the rotor become more important. There are also factors to take into account in terms of efficiency when partially loaded. In many of these respects, the use of brushless DC motors could provide a better alternative. These have a number of features acting in their favour, including high efficiency in the cruise mode and a readily adjustable field, plus the practical benefits of a more easily made rotor.

1.3.3 BRUSHLESS DC MOTOR

The term ‘brushless DC motor’, however, is a misnomer. More accurately it should be described as an AC synchronous motor with rotor position feedback providing the characteristics of a DC shunt motor when looking at the DC bus. It is mechanically different from the brushed DC motor in that there is no commutator and the rotor is made up of laminations with a series of discrete permanent magnets inserted into the periphery. In this type of machine, the field system is provided by the combined effects of the permanent magnets and armature reaction from vector control. Similar in principle to the synchronous motor, the rotor of this machine is fitted with permanent magnets which lock on to a rotating magnetic field produced by the stator. The rotating field has to be generated by an alternating current and in order to vary the speed, the frequency of the supply must be changed. This means that more complex controllers based on inverter technology have to be used.

Induction motors are used by many US battery-electric cars. The rotors are cooled with internal oil sprays which also lubricate the speed reducer. Operation at 12 000 rpm is common to minimize the torque and some designs operate under vacuum to reduce the noise. The one good point is that these motors are reasonably efficient under average cruise conditions (8000 rpm, 1/3 FLT). Polaron’s view is their use will be short lived. Induction motors always have lagging power factors which cause significant switching losses in the inverter, and vector control is complex.

1.3.4 SWITCHED RELUCTANCE MOTORS

SRMs, Fig. 1.16, use controlled magnetic attraction in the 6/4 arrangement to produce torque. Existing SR drives are unipolar, in that the voltages applied to windings are of only one polarity. This was done to avoid shoot through problems in the power devices of the inverter. The 6/4 machine has a torque/speed curve similar to a DC series motor with a 4:1 constant power operating region. Torque ripple can be serious at low speed (20%).

In an attempt to improve the SR drive, two groups have made significant contributions: SR drives have worked with ERA Drives Club in developing the 8/12 SR motor, with much smoother operation; a University of Newcastle upon Tyne company, Mecrow, have postulated a bipolar switched reluctance machine using wave windings. This doubles copper utilization and increases output torque. It also uses a standard 3 phase bridge converter. Existing SR motors are both heavier and less efficient than PM BDC machines, for example a 45 kW unit (3.5:1 constant power/5000 rpm) would weigh 65 kg and have an efficiency of 94%. The new bipolar design should give a motor which is close to PM BDC in terms of weight (45 kg). However, in terms of efficiency, the BDC has the edge, both in the machine and the inverter, because it operates with a leading power factor under constant power conditions. However, SR motors are excellent for use in hostile environments and it is Polaron’s expectation that they will be successful in heavy traction, where magnet cost may preclude brushless DC.

1.3.5 ELECTRIC MOTORCYCLE

An electric motorcycle is an interesting problem for electric drives. The ubiquitous ‘Honda 50’, an industry standard, is typical of personal transport in countries with large populations. The petrol machine weighs 70 kg and has an engine capable of about 5.5 bhp. Honda have developed an electric version where the engine is exchanged for an electric motor and lead– acid batteries. Honda’s solution weighs 110 kg and has a range of 60 km; it is offered in prototype quantities at £2500 ($3500), 1996 prices. Some elementary modelling shows that the key problem is battery weight – especially using lead–acid. To minimize this requires good efficiency for both motor and driveline. The standard driveline from engine to wheel is about 65% efficient. A better solution is to use a low speed motor with direct chain drive onto the rear wheel. This solution offers a driveline efficiency of 90%. However, we need a machine to give constant power from 700 to 1500 rpm. Cruising power equates to 1.5 bhp at 40 km/h and 5 bhp at 60 km/h. Vital in achieving good rolling resistance figures is to use large diameter tyres of, say, 24 inches.

CONSTANT T CONSTANT P

POWER (kW)

50 40 30 20 10

Fig. 1.16 Switched reluctance motor.

It is assumed that sealed batteries are to be used and consequently a battery voltage of 96 V was chosen to optimize the efficiency of motor and controller and particularly with an eye to controller cost. 200 V MOSFETS are near optimal at 100 V DC. A battery of 15 Ah 96 V weighs 40 kg (for comparison 24 V 60 Ah weighs 35 kg). In lead–acid 36 Wh/kg is achieved, while for comparison nickel hydride cells could offer 80 cells x 1.2 V x 25 Ah in a weight of 30 kg. The motor has to deliver a torque of about 40 Nm maximum and consequently a pancake-type design was chosen. Induction motors were rejected due to low efficiency and large mass for this duty. The four practical contenders are: permanent magnet brushless DC; permanent magnet DC brush pancake motor; DC series motor or switched reluctance motor. A tabulated comparison at Fig. 1.17(a) compares results. As can be seen, the permanent magnet brushless DC motor is the optimum performer at the two key cruise conditions. It has been estimated that with regenerative braking and flat terrain, a range of 70 km could be achieved with a 96 V 15 Ah lead–acid battery. The 25 Ah nickel hydride pack could give 120 km. However, 70 km is quite adequate for average daily use.

1.3.6 SMALL CAR

The small electric car is in the Mini or Fiat 500 class. Such a vehicle would weigh 750 kg and accelerate from 0 to 50 mph (80 km/h) in 12 seconds and have a range of 80 km with lead–acid batteries. The motor power would be 20 kW peak. As originally there were only aqueous batteries available, battery voltage was limited to 120 V DC by the tracking that took place across the terminals of the batteries due to electrolyte leakage. Two battery technologies were available: lead–acid and nickel–cadmium and vehicles were designed with efficiency = 25%, that is 188 kg of batteries if efficiency is expressed as battery mass/gross vehicle mass (for lead–acid 60 Ah 120 V 7.2 kWh and for nickel–cadmium 85 Ah 120 V 9.9 kWh).

Single quadrant MOSFET choppers were developed by Curtis and others to supply DC brushed series motors. The main advantage of this system was low cost (for example, lead–acid battery £900 in 1996; quadrant chopper £500; motor DC series £750). However, the apparent cheapness of this system is deceptive because: (a) fitting regeneration can raise the battery voltage to 150 V

an unsustainable level for some choppers – consequently friction braking was often used; (b) a separate battery charger was required. More recently sealed battery systems have become available and batteries of around 200 V are possible in two technologies, lead–acid foil and nickel hydride. These batteries are used with 600 V IGBT transistors which can operate at voltages up to 350 V DC. Battery capacity becomes limited if other services such as cabin temperature control/lighting/ battery thermal management are taken into consideration. A small engine driven generator transforms this problem and it is perhaps worth noting Honda have achieved full CARB approval for their small lean burn carburettor engines with the discovery that needle jet alignment is critical to emissions control and negates the need for catalytic converters.

All motor technologies are viable at 196 V; however, the practical consideration is that inverters are more costly than choppers which accounts for the popularity of DC brushed motors/choppers. To counteract the inverter cost premium, the electronically commutated machines have been designed for 12 000 rpm, to reduce the motor torque (DC brush machine 20 kW at 5000 rpm; other types 20 kW at 12 000 rpm). Another benefit of the higher transistor voltage capability is that the inverters/choppers can function as battery chargers direct off 220/240 V without additional equipment. High rate charging is possible where the supply permits. All electronically commutated machines provide regeneration. The motor comparison is tabulated at Fig. 1.17(b). All the machines deliver constant power (20 kW) over a 4:1 speed range, making gear changing unnecessary. The induction/brushless motors are assumed to use vector control.

20 Lightweight Electric/Hybrid Vehicle Design
(a)
PM BDC Brushed PM DC series Switched
pancake motor reluctance
Size (mm) 200 × 100 200 × 100 200 × 175 200 × 150
Weight (kg) 10 10 18 14
Rating 3 @ 750 3 @750 3 @750 3 @1500
(kW@rpm@V) 40 40 60 70
3 @1500 3 @1500 3 @1500
70 80 80
Efficiency 0.3/750 80% 3/750 75% 3/750 70% 3/750 80%
(motor 0.75/750 94% 750/750 80% 750/750 70% 750/750 85%
only) 3/1500 93% 3/1500 85% 3/1500 80% 3/1500 85%

(b)

Brushless Induction Switched Brushed
DC PM motor motor reluctance DC motor
Speed (rpm) 3000 3000 3000 1250
Torque (Nm) 64 64 154
rising to:
Speed (rpm) 12 000 12 000 5000
Torque (Nm) 16 16 38.5
Voltage (V) 150 A 150 75-150 192
�urrent (A) 126-81 A 164-106 A 180-90 D� 122 D�
Power (kW) 20 20 20 20
Frequency (Hz) 800 400 800 (equiv. 125 Hz)
Weight (kg) 12 25 20 50
Efficiency
% @ 3000 95 90 92 (1250) 80
% @ 12 000 97 92 94 (5000) 85
�ooling oil oil oil air

(c)

Fig. 1.17 Motor comparisons for three vehicle categories (the four motor types are also discussed in Chapter 4).

PM (DC) Induction Switched DC
brushless motor reluctance brush
Speed (rpm) 1000 1000 1000 1000
Torque (Nm) 2866 2866 2866 2866
at speed (rpm) 4000 4000 4000 4000
Torque (Nm) 716 716 716 716
Voltage (V) 380 380 190/380 500
�urrent (A) 753-486 980-630 1000/500 520
Power (kW) 300 300 300 300
Frequency (Hz) 1056 133 266 (133 equiv.)
Weight (kg) 300 600 500 1000
Efficiency
% @ 1000 95 93 94 85
% @ 4000 97 95 96 89
�ooling oil oil oil air
1.3.7 HGV

The heavy goods vehicle is an articulated truck which weighs 40 tonnes. Often omitted from clean air schemes on the grounds of low numbers they travel intercontinental distances every year and are major emitters of NOx and solid particles. Their presence is felt where there are congested urban motorways, and each one typically deposits a dustbin-full of carbon alone into the atmosphere every day, the industry declining to collect and dispose of this material! What is the solution? Use hybrid drivelines based on gas turbine technology; these vehicles would be series hybrids.

A gas turbine/alternator/transistor active rectifier, Fig. 1.18, provides a fixed DC link of 500 V. This is backed up by a battery plus DC/DC converter. A battery of 220 V (totally insulated) is used for safety. High quality thermal management would be vital to ensure long battery life; 2 tonnes of lead–acid units would be needed (144 × 6 V × 110 Ah) to be able to draw 400 bhp of peak power. It is likely that capital cost would be offset by fuel cost savings. Another benefit is that the gas turbine can be multifuel and operation from LNG could be especially beneficial. The drive wheels are typically 1 metre in diameter giving 683 rpm at 80 mph. Usually there are 3:1 hub reductions in the wheels and a 2:1 ratio in the rear axle, giving a motor top speed of 4000 rpm. Translated into torque speed this means 2866 Nm at 1000 rpm, falling to 716 Nm at 4000 rpm. All motors are viable at this power; however, two factors dominate: (a) low cost and (b) low maintenance. DC brushed motors with 3000 hour brush life are unlikely contenders! PM brushless DC is unlikely on cost grounds, requiring 36 kg of magnets for 2900 Nm of torque. Both induction motors and switched reluctance are viable contenders but switched reluctance wins on efficiency and weight. The contenders are tabulated at Fig. 1.17(c).

In the above review of four motor technologies for three vehicle categories, there is no clear winner under all situations but a range of technologies is evident which are optimal under specific conditions. Continuing development should improve the electronically commutated machines especially brushless DC and switched reluctance types. The relative success of these machines will be determined by improvements in magnet technology, especially plastic magnets, and cost reduction with volume of usage. On the device front, development is approaching a near ideal with 1/2 micron line width insulated gate bipolar transistors (40 kHz switching/l.5 V VCE saturated) but reduction in packaging cost must be the next major goal.

1.� Inverter technology

Inverters are one area where progress is being made in just about every area3: silicon, packaging, control, processors and transducers. The task is to find a way down the learning curve as quickly as possible. Polaron believe the lowest cost will come from packaging motor and inverter as a single unit. The major development this year is that of reliable wire bond packaging for high

Case (epoxy resin) Common (C.E.) Emitter (E) Cobector (C) Base and emitter (B. E. B. E)
Al wire
Epoxy Ceramic Cu plate Lead wire

substrate (AlO)

23Base electrode Collector electrode (Cu) (Cu) Molybdenum plate Silicon chip

SPEED
25°C ΔT 100°C
TEMP DIFFERENTIAL Fig. 1.19 Econopack 3 wire bonded package (left)
and typical lead frame packaging (right).

power silicon. New wire bond materials can offer a fatigue life of up to 10 million full current cycles with a Delta T of 25°C across the wire bond. The shorter pins on the package coupled with liquid cooling give best results. In Fig. 1.19, note that the temperature differential is the temperature difference between the connection pins and the baseplate in °C.

Traditional insulated packaging uses lead frame construction with wire bonding to the chip to give fuse protection. This technique has a guaranteed life of 25 000 full current cycles but package cost is high. Connections are by bolted joints. Wire bonded packaging uses a plastic pin frame which is wire bonded to the die. This construction technique is standard for low power six packs (complete 3 phase bridge on a chip as used in air conditioners). What is new is the capability to offer this packaging in a high power device. In USA designers seem to prefer MOS gated thyristor MCTs. In Europe and the Far East insulated gate bipolar transistors (IGBTs) are popular. In fact both devices are converging on a common specification of: (a) maximum volt amp product per unit area of silicon; (b) saturation voltage of 1.5 V at Ic max; (c) high frequency forced commutation capability.

Currently MCTs have the better saturation but IGBTs have better commutation. In the coming years makers will see better saturation figures for IGBTs and even lower switching energies. This is the result of smaller line widths and thinner silicon device structures. Currently a 1200 V, 100 A six pack can switch 600 V DC at up to 16 kHz (V Sat) 2.2 V/100 A, E 18 mJ E 14 mJ, cycle

ceonoff

32 mJ. The 600 V, 200 A six packs are now available as samples. Since the chips use non-punch

IM

WRF BOND LEAD FRAME IPM

Fig. 1.20 Silicon cost for 70 kW drive.

through (NPT) technology, they may be connected in parallel without matching due to the inherent equalization characteristics of the die. Many vendors offer IGBTs in lead frame packaging, but this construction is not cost effective for electric vehicles. Devices of 1000 A, 1200 V are available. Intelligent power modules are also available, for example Semikron, SKIpacks Fuji, Toshiba and Mitsubishi. These integrate gate control with the power devices and have protection integral with the device. The cost of this approach is high at present; it is wire bonded packaging that offers the lowest device costs. A 1200 V, 100 A six pack is around 100 dollars (1997 price), Fig. 1.20.

Fundamental to the cost equation is that inverter cost is proportional to motor current. Electric and hybrid vehicles are tending to use drives of 70 kW because the vehicles weigh 1500 kg. What Fig. 1.21 illustrates is that the induction machine requires almost 1.8 times the current capacity of the brushless DC inverter for 3.5:1 constant power speed range. Typical circuit diagrams are illustrated in Figs 1.22a and b. The view in (a) is a typical induction motor drive with just six switches. This drive will need 3-off 600 V, 200 A six packs in parallel. Under US conditions, cars seldom require 70 kW for more than 10 seconds during overtaking. With current designs of battery peak power falls to 55 kW at minimum battery voltage limited by internal resistance (typically

1.75 V/cell for lead–acid). The view in (b) is a brushless DC drive using a double chopper circuit. Essentially a 300 V battery is increased to 600 V link with a 460 V motor. This inverter can be built with just two 1200 V, 100 A six packs. With oil at 40°C the package can operate at 140 A continuous. It will operate at 96 A RMS, 136 A peak on a 50% duty cycle for short periods. The brake resistor in the circuit prevents battery overcharging during regeneration. If the battery is overcharged its life may be reduced. In flat terrain the friction brakes may fulfil this role; however, in steep terrain the energy per 1000 metres height is 14.7 million joules or about 2400°C on average family car disc brakes. Electric cars do not have engine braking.

There are many benefits of using the high voltage circuit. First the motor current is 100 A or less. This makes the motor easier to wind and permits the use of printed circuit technology in the inverter. Second there is a major control benefit. An optimum control strategy is to use current-source PWM at low speed and voltage-source square wave at high speed. If a 300 V battery is used the DC link voltage is kept low until the motor voltage exceeds the DC link and then increased as the speed and voltage rise. This strategy reduces PWM carrier losses and permits better efficiency along the no-load-line of the vehicle. Use of printed circuit technology not only assists automatic assembly but also reduces EMC. EMC compliance is not too difficult in steel body cars but is much more of a challenge in composite structure vehicles. Having considered the inverter core some thoughts concerning the peripheral components are needed. Clearly this configuration requires an L/C filter for the chopper and an output filter for the motor to limit dv/dt on the motor windings. The dimensions of the L/C filter are determined by two factors: permitted inductor current ripple and permitted capacitor current ripple.

Polaron prefer to split the inductor to give good common mode rejection with respect to the battery. A value of 100 microhenries is suitable with a capacitance of 1250 microfarads. The inductors are made as air-core units with 10 mm microbore copper pipe. The turns may be close spaced by insulating the outside of the copper with epoxy powder coat paint. The spacing can be reduced further by using X extrusion copper which permits bending in two planes. The capacitors are ripple current dominated. With 100 A of motor current a capacitor that can handle 100 A peaks (30 A RMS) at temperatures of up to 50°C so oil immersion is the requirement. Polaron Group have chosen electrolytic units of 470 microfarads, 385 V, arranged five in parallel in series with five more in parallel. The cans are of the solder mount type choosing five more pins for mechanical strength in 40 mm × 50 mm cans.

Fig. 1.21 Base speed/max speed operating points for induction and brushless DC motors.

Induction motor� Brushless�DC motor� Brushless�DC motor�
Motor speed 4000 rpm 13 500 rpm Power 300 V 120 V 361 A 220 V 197 A 70 kW 300 V 220 V 197 A 220 V 223 A 70 kW 600 V 460 V 94 A 460 V 96 A 70 kW

The inductors for the dv/dt units are more challenging. An inductance of 10/20 microhenries is needed but it is advantageous if the inductance is more at low current. Consequently this application favours a cored inductor with low permeability iron powder and oil immersed litz wire winding. The core needs to have a moulded bobbin to provide inter-turn insulation for the litz wire and as a casting mould for the core material. A final point is that if one were prepared to hand wind the motor Polaron believe it would be possible to eliminate the dv/dt inductors by the use of an insulation extrusion to control the ground capacitance of the winding – the capacitance/inductance characteristic as a uniform transmission line.

In summary, for motors, Polaron believe brushless DC will prove to be the dominant technology especially for hybrid vehicles where efficiency at peak power matters. Machines for 12 000 rpm are well established. Successful operation of 70 kW machines at 20 000 rpm has been demonstrated and 150 kW machines are in development. Currently, higher speeds present a number of technical/ cost obstacles (there are successful company designs operating to 150 000 rpm but not using low cost methods). Improvements in materials could radically change this in the next few years. In the inverter area, cost is proportional to current and the brushless DC motor requires 60% of the current of the induction motor to achieve a 3.5:1 constant power operating envelope. The double chopper circuit offers many benefits over the single bridge solution and is cheaper to construct. For 70 kW a 600 V DC link is best. The use of a controlled DC link becomes even more important in hybrid vehicles where smaller batteries lead to greater voltage variations between peak motoring and peak regeneration. The use of high voltage is not a safety hazard so long as the motor and inverter are contained in a single enclosure where the active components are not accessible. Oil immersed construction offers the lowest temperature rises and the best component reliability, especially for the silicon and filter capacitors. This method of construction permits complete subsystem testing before mounting in a vehicle.

1.5 Electric vehicle drives: optimum solutions for motors, drives and batteries

Optimum supply of voltage for the power electronics of EVs is around 300 V DC using the latest IGBT power transistors4. This also provides a sensible solution for the motor because in the power range of 30–150 kW the line currents are quite reasonable. A consequence of using a 300 V battery is that the rail voltage will vary from 250 to 400 V under different service conditions.

1.5.1 BATTERY CONSIDERATIONS

A good commercial battery for deep discharge work is the Trojan 220 Ah 6 V golf cart unit. This gives 75 A for 75 mins and weighs 65 lb, consequently a 108 V stack weighs 1170 lb and cost $1080 in 1991. It also requires considerable maintenance and occupies a projected area of 1342 square inches and is 10 5/8 inches high.

In comparison, sister company Nelco have available a sealed lead–acid battery of 12 V, 60 Ah and arranged into 18 cells to give 108 V. It occupies 720 square inches of plan area and weighs 697 lb. This arrangement can also provide 75 A for 75 minutes. The problem area is cost. This battery cost $2700 in 1991. If the vo1tage was increased to 312 V, with the same stored energy, the cost rises by 20% at 45 kW. Such 300 V battery systems require great attention to safety; 100 V batteries may be feasible at 45 kW but this ceases to be true at 150 kW. In fact, one can draw the graph in Fig. 1.23(a) to define minimum voltage for a given output power. Other areas worthy of comment are maintenance and battery life. High voltage strings of aqueous batteries are dangerous and should be banned by legislation. This is not so of sealed lead–acid batteries as there is no need for maintenance access. However, no voltage greater that 110 V should be present in a single string or an individual connector. Long series strings present a potential maintenance problem with respect to cell equalization. The problem may only be resolved by keeping all cells at the same temperature. A final problem is fast charging; this is ternperature limited to 60oC max cell temperature. The newer cells may be fast charged so long as the temperature is contained and the individual cell voltage is below 2.1 V.

1.5.2 FUEL-CELL CONSIDERATIONS

There is no doubt that the long-term power supply for electric vehicles will be some form of hydrogen fuel cell, the leading current technology being the PEM membrane system as manufactured by Vickers/Ballard. This is a complete system measuring 30 × 18 × 12 inches which produces about 5 kW at 45% efficiency.

The unit consists of 36 plates of 250 A rating and the fuel gases operate at 3 bar and the exhaust temperature is around 80oC. This arrangement leads to the relation in Fig. 1.23(b). Hence for a vehicle with a storage battery approximately one-third maximum power +10 kW is the peak fuel-cell

250 V

MINIMUM VOLTAGE

100 V

(a)

200 V

FUEL CELL VOLTS

Fig. 1.23 Voltage vs power relationship for (a)

(b)

POWER

lead–acid battery and (b) fuel cell.

load. Hence for 45 kW this amounts to five modules producing 100 V at 250 A. For a 150 kW system, vehicle builders will need ten modules giving 50 kW at 200 V. The voltage may not rise above 200 V due to problems relating to the hydrogen. Warm up takes about 5 minutes from cold with units producing 50% output at 20oC. Once hot, response is 1–2 seconds for load steps and endurance has been confirmed as greater than 20 000 hours. One of the more intriguing possibilities offered by fuel cells is to use the power converter to produce 50 Hz for powering lights and portable tools on site vehicles.

1.5.3 OVERALL SOLUTION

There is a basic incompatibility between the power source voltage and the motor voltage; so how can this problem be addressed?

The solution is to put a reversible chopper between the battery/fuel cell and the inverter (Fig. 1.24). This means that the supply to the inverter is stabilized under all conditions resulting in full performance during receding battery conditions and no overvoltage during battery charging mode. By using the inverter as the battery charger express charging can be performed, where mains supply permits, in approximately 3 hours.

1.5.4 MOTORPAK SAFETY CONSIDERATIONS

To charge and discharge the battery quickly whilst optimizing battery use requires perfect control of the battery temperature. Since the battery is sealed this is best achieved by immersing in silicon fluid. A circulating pump passes fluid to and from the motor. This keeps the batteries cool and at equal temperature during charging using the motor as a heatsink and, during discharge, the motor warms the batteries to give optimum performance. Hence the batteries are built into a tank and this prevents access by the operator.

The next concept is to make the battery module interchangeable. This permits refuelling either by recharging the battery or by exchanging the battery module.

1.5.5 MOTORPAK CONSTRUCTIONAL CONSIDERATIONS

If costs are to be optimized, it makes sense to locate the power controller close to the battery. In the above case, Nelco have taken the concept one stage further. The power controller is located in the base of the battery tank. We call this concept Motorpak, Fig. 1.25, and as can be seen the mechanical execution could not be made much simpler. The motor and PCU pack are mounted

+300 V

under the vehicle either in place of or in addition to the conventional power train. No gearbox is needed and the motor provides nearly 300 Nm of torque directly. The following specification applies for a 45 kW Motorpak:

Input 50–240 V AC, 40–65 Hz single or 3 phase up to 30 A; recharge time 3
hours
Output 0–220 V, 3 phase up to 750 Hz 60 kVA, 13.6 V DC 500 W
Batteries 18 off, 12 V, 60 Ah sealed lead–acid units, may be configured as 108
or 216 V unit
Weight 800 lb (362 kg)
Dimensions 30 in long, 27 in wide, 14 in high
Construction Weatherproof
Controls Function switch, accelerator pedal, voltmeter/ammeter/amp hour meter,
13.6 V for auxiliaries, 2 oil pipes to motor (4 litres/min)
Deep discharge 800 cycles to 80%
performance
Stored energy 10 kWh
Cost in 1991 £3000 at 1000 off ex batteries (£5000 with batteries), price includes
motor
Temp. range 20°C to + 40°C
45 kW traction motor
Type Brushless DC permanent magnet
Size 375 long × 250 diameter, weight 50 kg

EMOVE

Engine Motorpak unit Gearbox Motor Cooling system Vacuum pump for brake servo Fuel tankOptional air conditioner I heater

AMMETER I VOLTMETER

POWER TO DIFFERENTIAL

Fig. 1.25 Motorpak concept.

Torque 0–1500 rpm, 280 Nm falling to 70 Nm at 5000 rpm on 45 kW constant power curve

Construction Flange mount with double ended shaft and integral encoder

Cooling Silicon oil, 4 litres/min

Electrical rating 220 V, 130 A, 750 Hz

Power pack contains: batteries, power conversion unit, 12 V, 500 W supply for auxiliaries and hydraulic power steering supply/cooling for motor. This unit is interchangeable in seconds. A 45 kW unit weighs just 800 lb; the motor is oil cooled and weighs 130 lb.

1.5.6 ADVANTAGES OF THE SYSTEM DESCRIBED

If the conventional engine is replaced by a battery/motor the weight increases by approximately 300 lb for a 1 tonne vehicle. This means the system can be fitted to existing chassis designs or retrofitted to cars. The system can be used standalone or as a hybrid. The complete electrics pack is interchangeable for instant refuelling and the PCU works with any battery input supply for 100– 250 V. Batteries are rated for 800 deep discharge cycles to 80% depth of discharge. On a 45 kW unit, the battery can supply 75 A for 75 minutes at 108 V DC or 37.5 A for 75 minutes at 216 V.

Total safety is ensured by all electrical parts except the motor which is contained in a single totally insulated module with no parts distributed over the vehicle. Batteries are sealed to give best resistance to crash situations. Electrics are protected against short-circuits with both fuses and circuit breaker. The oil cooling system can supply the power steering if required. Minimized technical risk is ensured by a total package solution and if technology improves only one module has to be changed. The module approach makes many finance packages feasible, facilitating user acceptance; for example, the user buys the vehicle and motor but hires the battery and PCU. The battery pack can be recharged in 3 hours where mains supply permits. The PCU functions as a battery charger and the drive system can supply up to 45 kW of mains electricity for short periods

longer if used with a fuel-cell prime mover. The PCU makes use of portable power appliances viable which is particularly useful for the building industry. Finally, the concept makes conversion of existing vehicles possible.

References

  1. Hodkinson, R., 45 kW integrated vehicle drive, EVS 11, Florence, 1992
  2. Hodkinson, R., Machine and drive characteristics for hybrid and electric vehicles, ISATA 29, Stuttgart, 1996
  3. Hodkinson, R., Towards 4 dollars per kW, p. 4 et seq., EVS 14, Orlando, December 1997
  4. Hodkinson and Scarlett, Electric Vehicle Drives, Coercive Ltd report, December 1991
Further reading

Electric vehicle technology, bound volume of SAE papers, 1990 Electric and hybrid vehicle technology, bound volume of SAE papers, 1992 Electric and hybrid vehicle design studies, bound volume of SAE papers, 1997 Technology for electric and hybrid vehicles, bound volume of SAE papers, 1998 Strategies in electric and hybrid vehicle design, bound volume of SAE papers, 1996 Electric vehicle design and development, bound volume of SAE papers, 1991 Breaking paradigms, the seamless electro-mechanical vehicle, Convergence 96, SAE, 1996

2

Viable energy storage systems

2.1 Electronic battery

Electric vehicles are at a historical turning point – the point where technology permits the performance of electric vehicles to exceed the performance of thermal engines1. Currently quality battery technology is expensive and heavy. This favours hybrids with small peaking batteries – typically 10 Ah at 300 V, with a capability of 70 kW for 2 minutes. New battery geometries have been developed for this application in the form of high performance D cells with 6 mm thick end caps and M6 terminals. A single string of cells handles 100 amps for 10 seconds. The heat is transferred to the end caps then removed by forced air cooling. It is vital to maintain an even cell state of charge in long strings. This solution has two drawbacks at present: (1) Cost – a 300 V 6.5 Ah stack costs more than $10 000 in January 98; (2) reliability – with only one string a single high resistance cell disables the battery.

Using the best available nickel–cadmium cells, it is possible to build a reliable peak power stack and use electronic control to maintain equal currents in strings. A later section will consider the next generation, an all electric hybrid with aluminium battery and alkaline fuel cell; this last important energy storage system is also discussed in depth in Chapter 4. A review of different battery types and performances is given in Chapter 5.

2.2 Battery performance: existing systems

All battery technologies can offer some solution to the peak power problem but there is only one parameter which ultimately matters, and this is the internal resistance of the cell. This is much more related to cell geometry than cell chemistry, as we shall discover. When AA, C, D and F cells were originally designed, nobody was thinking of discharging them at hundreds of amps, so it is hardly surprising that they are not ideal for the purpose. This problem will become even more extreme as power density improves:

D cell characteristics

Lead Acid NiCad NiMh Lithium Aluminium

Amp Hr (20hr) 2.5 4.0 6.5 7.5 50 Cell voltage 2.0 1.2 1.2 3.6 3.0 Max C rate 40.0 25.0 11.0 40.0 Unknown Ah/25°C 0.5 3.0 5.0 6.0 Unknown

The first volume hybrid electrics in the market came from Toyota (Prius) and Nissan, Fig. 2.1. Toyota uses a 288 V string of NiMh D cells to give a peak power of 21 kW. The battery is mounted in horizontal strings of 6 cells in a matrix weighing 75 kg and is made by Matsushita Panasonic EV Energy. Nissan has worked with Sony who have developed the lithium ion battery, of AEA Technology (UK), into a high performance D cell, of 3.6 V 18 Ah, with a peak power of 1700 W/ kg. (This cell is 250 mm long × 32 mm OD and includes an electronic regulator.) Both solutions use advanced thermal management and both deliver the performance but at high cost. The task in hand is to reduce costs by 50%.

2.2.1 BATTERY PERFORMANCE: ALTERNATIVE APPROACH

Question: Which application gets the best peak power battery performance at present? Answers: Portable power tools and model aeroplanes; chemistry: nickel–cadmium; cell size: RR/C 1–2 ampere hour; peak discharge: 30° C; maximum current: 32 amps limited by connections (tags),

Toyota Prius battery pack Principal specifications Characteristics of nickel metal-hydride batteries for HEVs
Item Battery module Battery pack (example)
Battery construction 6 cells in series 40 modules in series
Nominal 7.2 V 288 V
voltage Nominal 6.5 Ah 6.5 Ah
capacity State of change (Ah)
Energy Power Regenerative power Weight 45 Wh 525 W (10 s) 500 W (10 s) 1.1 kg 1.8 kWh 21 kW (10 s) 20 kW (10 s) 75 kg
Modules

Example of battery system for HEVs

(a)
Fig. 2.1 Production hybrid battery technologies compared:
(a)
Toyota Prius; (b) Sony/Nissan.

Fig. 2.2. The performance issues of these cells are complex. Amazingly there are 14 different types of sealed Ni–Cad cells in four main product groups: (a) standard, (b) high discharge current,

(c) fast charge and (d) high temperature.

It is the double-sintered fast charge cells that are used in model aeroplanes and in the world championships the Sanyo Cadnica is specified using a 1.7 Ah cell. A Tamiya connector can deliver up to 32 amps. Battery packs of 7.2 V six packs are usually fitted. In the power tool area, Black and Decker have the Versapack, a three cell string. Two or four strings are used in series with larger cordless power tools. One problem is that sealed Ni–Cad cells cannot be connected in parallel. At the end of the charge cycle, the cell voltage falls, causing rapid charge/discharge between parallel cells. On the face of it, the packaging problem is daunting, with more than 2000 cells in a pack.

Sony/Nissan battery pack

Specifications of prototype cell

Weight 1.2 kg

Rated capacity 22 Ah
(4.0 V)
�ell shape �ylindrical
Dimensions, mm D50 x L250

Ni�Cad cells

BatteryType RR Sub D D

�ell capacity (Ah) 1.0 1.7 2.8 4.0 Int. resistance� (milliohms) 4.5 �.5 �.0 2.8

�ell dimensions, DxL (mm) 22x�4 22x56 �2x42 �2x56 Weight (gm) 42 65 1�0 200 Int. Resistance of 4 Ah milliohms 1.1255 1.6 2.5 2.8

�Internal Resistance is at 20� � and 50� state of charge.

Fig. 2.2 Typical Ni–Cad packages and capacities.

2.2.2 INTERNAL RESISTANCE PERFORMANCE OF FAST-CHARGE NI–CAD CELLS

The results discussed here are based on Sanyo products but the same trends are seen in Varta, Panasonic and Saft cells. See Fig. 2.2.

Why does internal resistance matter so much? This is because at 25° C discharge rate, the voltage drop on a 1.2 V cell is as tabulated below:

Voltage drop on a 1.2 V cell at 25° C discharge rate

RR C Sub DD

112.5 mV 160 mV 250 mV 280 mV

The 1.2 V cell is thus no longer a 1.2 V cell but closer to 1 V at 20° C. Why does the 1 Ah cell win? It is because it is short and fat – the others are long and thin. Cell geometry is the decisive factor for low internal resistance.

To test the performance of individual cells, Polaron built a string of six and charged/discharged at 24° C; that is 24 amps on 1 Ah cells. After five cycles, discharge time increased to 130 seconds, and temperature rise was about 10° C. It ran for several hundred cycles with virtually no change in characteristics. This is very severe compared to the true operating conditions, where the cells will have to supply 24 amps for perhaps 10 seconds under real world conditions, Fig. 2.3.

2.2.3 BUILDING A STRING OF CELLS

To achieve 70 kW for 2 minutes will require ten strings of 260 cells at l Ah. This is economical but not optimal. Three strings of 3.3 Ah would be optimal. This corresponds to using short D cells. The use of l Ah packages does have some benefits. Spot welded connections can handle the current without special packaging. The key problem is that of automatic assembly with so many cells. This is easily accomplished using welding robots. One technique is to use two parallel plates which each can hold two strings of 260 cells. The cells are sealed with O-rings so that the centre of each cell is oil cooled. The connections have to be in air, because the gas seal should not be immersed in oil as the seal may be damaged – and the oil contaminated with potassium hydroxide. At a link current of 25 A nickel-plated steel links seem to be quite adequate. A second interesting packaging concept would be to create a ten pack version of the Versapack concept. These could be mounted in horizontal strings and air cooled for cell equalization.

2.2.4 CELL CURRENT SHARING

Given ten parallel strings for 70 kW peak power, there is only one way to ensure equal currents in each string – active regulation. This seems a very expensive proposition but it fits in well with the structure of modern inverter drives. The present trend is to use a 300 V battery with a boost chopper and increase the battery voltage to give a DC bus of 600 V, as used in industrial drives. Normally one would parallel a number of transistors to give a current rating of 300 amps. Three times 100 amps would be optimal as this is a complete 3-phase pack of IGBTs. In this case it is necessary to use smaller packs of 30 amps where each leg has its own independent current regulator. This would not be an attractive proposition if it were not for the fact that at this current the required control circuitry is available economically. Ten such circuits may be connected to a common DC bus. This arrangement ensures excellent current sharing in both charge and discharge and prevents the situation where strings charge/discharge into one another. What we need now is some improvement in battery packaging. To operate three strings in parallel would be optimal from a cost versus reliability standpoint, as one would use the separate 100 amp phase legs in a six pack to control the current in the individual strings, Fig. 2.4.

2.2.5 BATTERY SYSTEM PROSPECTS

It has been shown that a matrix of small rechargeable cells can be made to give large peak powers on a repeated basis with excellent life performance. The new D cell designs in NiMh and lithium ion are very expensive and a cheaper alternative is to use l Ah Ni–Cad cells with ultra-low internal resistance. Using this technique it would be possible to buy the cells for a 21 kW/10 second pack for about $2000 in 1999. To this the packaging and control cost must be added. However, at present this is still significantly cheaper than the use of custom battery packages.

Cell geometry is the decisive factor in achieving low internal resistance and there is much room for improvement on existing cell packages. Short cells with minimum distance from foil to terminal give best results. The use of multiple strings of cells in parallel, with active current sharing, improves reliability and reduces cost since the currents in individual packages are modest compared to single strings. Temperature control of the strings helps to maintain even state of charge at high charge/discharge rates and keeps cells cool, while extending cell life.

CHOPPER CHOPPER

260

CELLS

I AH

Fig. 2.4 Cell current sharing: typical EV drive

TIMING SYNC

(top); current loop control

0 V

of PWM chopper (centre); multiple chopper

CURRENT

implementation (below).

DEMAND

Cell frame

Electrolyte

The development of advanced battery chemistries with increased power and energy density will place even greater demands on cell packaging in the future and a new family of optimum proportions needs to be designed for the job.

2.3 Status of the aluminium battery

In 1997, patents were filed in Finland for a new aluminium secondary battery. The inventor was Rainer Partanen of Europositron Corporation who claims major improvements in power density and energy density for the new cell based on a 1.5 V EMF2. The author is interested in this problem because it represents one of the last major barriers to be overcome before the widespread introduction of electric and hybrid vehicles. In recent years, significant effort has been directed at improving secondary battery performance and this effort is beginning to bear fruit. We can now see advanced lead acid, nickel metal hydride and Lithium Ion products out in the market place with performance of up to 100 Wh/kg and 200 W/kg.

Market requirements fall into two distinct categories: (a) small peak power batteries of 500 Wh (2 kWh for hybrids) and (b) 30–100 kWh for pure electric vehicles. Each of the cell types has its own distinctive attributes but none has so far succeeded in making the breakthrough required for mass market EV implementation. The fundamental problem is one of weight. At the factory gate, vehicle cost is almost proportional to mass, as is vehicle accelerative and gradient performance. Consequently it will take at least 300 Wh/kg and 600 W/kg to achieve the performance/weight ratio for long range electrics we really desire. This would make one type of hybrid particularly attractive – the small fuel cell running continuously together with a large battery.

If we consider a low loss platform for a passenger car at 850 kg with N = 0.3, we have 250 kg for the battery. At 300 Wh/kg we obtain 75 kWh, which would give a range of more than 250 miles after allowing for auxiliary losses. A low loss platform would consume 5 kW at 60 mph + 5 kW for auxiliary losses – total 10 kW – this equates to 7.5 hours at 60 mph = 450 miles steady state. This level of performance is an order of magnitude better than lead–acid at the current time. Clearly a new approach to the problem is required.

2.3.1 WHY ALUMINIUM?

In simple terms the answer involves (a) abundancy, (b) low cost and (c) high energy storage. If we consider the recent developments in batteries they all seem to use materials like nickel which are highly dense and limited in supply. Likewise in fuel cells using platinum catalysts material scarcity is implicit, the annual production being about 80 tons worldwide. Any mass market battery needs to use materials available in abundance. In the bumper year of 1985, 77 million tons of bauxite were mined worldwide; aluminium is one of the most plentiful materials available on Earth. In terms of 1999 costs, aluminium is $2000 per ton so 250 kg would cost $500 – an acceptable sum.

In terms of energy storage, aluminium has one of the highest electrical charge storage per unit weight except for the alkali metals:

Aluminium 0.11 coulombs per gram 2.98 Ah per gram
Lithium 0.14 coulombs per gram 3.86 Ah per gram
Beryllium 0.22 coulombs per gram 5.94 Ah per gram
Zinc 0.03 coulombs per gram 0.82 Ah per gram

Lithium and beryllium are alkali metals and are not suitable for use with liquid electrolytes, due to rapid corrosion, so are normally used with solid electrolytes.

Filter
Filter Backflush
Hydrargillite
2.3.2 DEVELOPMENT HISTORY OF ALUMINIUM BATTERIES

The first serious attempt to build an aluminium battery was made in 1960 by Solomon Zaromb3 working for the US Philco Company. In Zaromb’s concept for an aluminium air cell, the anode was aluminium, partnered with potassium hydroxide, and air was the cathode. This battery could store 15 times the energy of lead–acid, achieving 500 Wh/kg and a plate current density of 1 A/sq. cm. The main drawback was corrosion in the off-condition, which resulted in the production of a jelly of aluminium hydroxide and the evolution of hydrogen gas. To overcome this problem Zaromb developed polycyclic/aromatic inhibitors and had a space below the cell for the aluminium hydroxide to collect. The chemical reaction is

Al + 3H2O = Al(OH)3 + 3/2H2

In 1985 another attempt was made by DESPIC4, using a saline electrolyte. Additions of small quantities of trace elements such as tin, titanium, indium or gallium move the corrosion potential in the negative direction. DESPIC built this cell with wedge-shaped anodes which permitted mechanical recharging, using sea water as the electrolyte in some cases. The battery was developed by ALUPOWER commercially. The battery had limited peak power capability because of conductivity limitations of the electrolyte, but provided substantial watt-hour capacity.

Other attempts have involved aluminium chloride (chloroaluminate) which is a molten salt at room temperature, with chlorine held in a graphite electrode. This attempt in 1988 by Gifford and Palmisano5 gives limited capacity due to high ohmic resistance of the graphite. Equally significant is work by Gileadi and co-workers6 who have succeeded in depositing aluminium from organic solvents though the mechanisms of the reactions are not well understood at this time.

Between 1990 and 1995 Dr E. J. Rudd7 led a team at Eltech Research in Fairport Harbor, Ohio, USA, which built a mechanically recharged aluminium battery for the PNGV programme, Fig.

2.5. It had 280 cells and stored 190 kWh with a peak power of 55 kW, and weighed 195 kg. This battery used a pumped electrolyte system with a separate filter/precipitator to remove the aluminium hydroxide jelly, Fig. 2.6. Alupower8 built a 6 kW aluminium–air range-extender system under the same programme, Fig. 2.7.

Aluminium–air battery

Charger

Electrolyte storage tank Lead–acid battery pack

Controller Electric drive

2.3.3 NEW-CONCEPT ALUMINIUM BATTERY

The cell invented by Rainer Partanen, Fig. 2.8, is an attempt to defeat the disadvantages of the aluminium–air cell. It is a secondary battery which uses coated aluminium for the anode and pure aluminium for the cathode. The electrolyte is a mixture of two elements: (a) an anion/ cation solution currently consisting in proportion of 68 g of 25% ammonia water mixed with 208 g of aluminium hydroxide, and made up with water to give 1 litre of solution; (b) a semi-organic additive consisting of metal amines.

The exact formulation of the additive is a commercial secret. The inventor claims that this electrolyte achieves a large increase in charge carrier mobility and this results in figures of up to 1246 Wh/kg and 2100 Wh/litre, which have been achieved in many prototype cells that have been constructed. The figures relate to active materials, without casing. It is suggested that the technology is suitable for the construction of plate (wet cell) and foil (sealed) cells, with no limitations on capacity. The test cells have achieved a life of up to 3000 cycles, the main degradation mechanism being corrosion of the coating on the anode during recharging. One remaining hurdle to be overcome is the identification of a better coating material to reduce the corrosion.

The battery has some unusual characteristics in that it operates over a very wide temperature range, 40 to +70° C. This is in stark contrast to most batteries whose low temperature/high temperature performance is poor. The cell voltage is a nominal 1.5 V. Some interesting consequences arise if one assumes that the claims are true. The most significant is packaging. If we take the D cell which is 32 mm diameter × 58 mm long, as used by Panasonic/Toyota in the PRIUS battery pack, a battery with 150 g active mass stores 6.8 Ah and has a peak discharge current of around 100 amps. If we build a D Cell at a value of 1246 Wh/kg, this leads to a figure of 150 Ah. Polaron understand that very high levels of discharge current are possible – the inventor claims up to 20 times more power than existing cells in the market – but finding methods of supporting these currents in such a small space is a major challenge to achieve low terminal resistance, lead-outs and sealing, Fig. 2.9. It is claimed that the new technology uses environmentally safe materials which are fully recyclable.

Other developments which lend support to this invention9 are the emergence of ultracapacitors and electrolytic capacitors, both using aluminium electrodes with biological

�eclanch Alkaline �ead�acid Ni�Cad Ni�Mh �ithium�ion Aluminium
Amp. hrs (20) �ell voltage �ax �rate AH�25� present 4.5 18 1.5 1.5 2� 2� Not possible IR limited 2 2.5 2.0 40� 0.5 4.0 1.2 25� �.0 6.5 1.211 5.0 18 �.6 40� 14 150 (75 Dem) 1.5 �� Not possible at Package IR limits current to 500 A
Aluminium metal in anode�cathode Anion�cation reactant solution 486 g 11�� g 180 cm820 cm
Theoretical maximum energy and current capacity
2100 Wh�litre 1246 Wh�kg 1448 Ah�litre 85� Ah�kg

Practical cells in a package should achieve 70�80� of the above values when package mass is included.

Fig. 2.8 Characteristics of D cell (32 × 62 mm) against those of a 1 litre Partanen cell.

electrolytes. Very significantly, ultracapacitors operate well at low temperatures. In Russia 24 V modules, 150 mm diameter × 600 mm long store 20 000 joules and are used for starting diesel engines at 40° C.

2.3.4 PATENT PROTECTION

The technical background to the invention is the result of a remarkable discovery in the field of complex electrochemistry and is based on the composition of the solution for electrical analysis and catalysis, releasing the energy potential of aluminium. Patent protection is being applied in three areas:

(l)
The first is a solution which, under discharge, generates a reaction on the cathode side causing the energy potential of the aluminium to be released, and by ionization changes the molecular structure from metal to solution. Patent application Fi 954902 PCT/EPO (published).
(2)
The second is a solution which, under discharge, generates a decomposition reaction in the chemical reactant mass. This is in crystal form which dissolves into solution and produces electrical potential on the anode side. Patent application Fi 981229 PCT/EPO (registered).
(3)
The third component are the electrodes which have a dual role. They are formed of materials which enable them to act concurrently as non-ionized anode and ionized cathode. These electrodes are used in a multicell configuration as in existing battery technology. Patent application Fi 981379 PCT/EPO (registered).

The composition of these solutions, and the reactant mass, have the capability of producing an electrical current from the non-ionized anode and aluminium cathode when conductivity (resistance) is placed between them. When discharging, power is generated by the energy of the released aluminium, which reduces to about 35% of its original molecular density. When recharging, the reactant solution returns to its original form in solution and crystal mass and the aluminium atoms are deposited back onto the electrodes.

2.3.5 ALUMINIUM PROSPECTS

An aluminium secondary battery looks to be a very promising candidate for the storage of substantial energy. Whether the inventor Rainer Partanen has found the correct technique remains to be demonstrated. Although the claims for peak power and energy density seem very high, Sony have demonstrated 1800 watts per kg in lithium–ion recently and aluminium–air cells achieved 500 Wh/kg in 1964. The author considers aluminium to be a worthy contender for advanced battery construction and clearly this is an area which merits much greater investigation in the future. One point is clear – by making the active aluminium electrode the cathode, the parasitic reaction that is the big drawback of the aluminium–air cell is avoided, because the 1.5 V potential across the cell suppresses the reaction. Two questions that remain to be answered concern the levels of conductivity and mobility that will need to be exceptional to justify the claims made for the Partanen cell, also whether cell packaging will be a significant problem, requiring a new range of packages to be developed.

2.. Advanced fuel-cell control systems

This section considers the development of a fuel-cell controller and power converter for a vehicle weighing 2 tons, for operation in an urban environment10. The techniques employed can be used with either PEM membrane fuel cells or alkaline units. The main challenge is to re-engineer a high cost system into a volume-manufactured product but this is unlikely to be achieved‘overnight’. What is required is a new generation of components which are plastic as opposed to metal based.

The power electronics are practical, but need integrated packaging to reduce costs. Equally important is improvement in the fuel-cell stack specifications. This section considers the requirements and performance of a low pressure scheme at the current state of the art and predicts the measures needed to achieve significant cost reduction.

Modern hybrid cars are demonstrating major improvements in fuel consumption (3 litres/100 km) and emissions (ULEV limits) compared to conventional thermal engines. These designs use small peaking batteries which weigh less than 100 kg, for a family sedan, and store perhaps 2 kWh.

A new aluminium battery chemistry has been identified whereby it should be possible to store 50 kWh in a weight of 150 kg in perhaps 3/5 years from now. Nickel–metal hydride needs 500 kg with current technology to achieve 50 kWh. This makes a new type of hybrid an interesting long-term contender – the electric hybrid with a small fuel cell. In this vehicle a 2–5 kW fuel cell would charge the battery continuously. The only time the battery would

cells 76 each stack

Voltage/temp. electronics

Interface and vehicle through connectors

(mounted with AUV electronics)

Performance� Dimensions� Power 2.5 kW �ass �60 kg

�uel 25 kg aluminium anodes �xidant 22 kg oxygen at 4000 lb�in2 Non�dimensional performance� Buoyancy Neutral, including aluminium Volumetric energy density 265 Wh�l

hull section �ravimetric energy density 265 Wh�kg Time to refuel � h

Fig. 2.9 Aluminium/oxygen power system and its characteristics (courtesy Alupower).

become discharged would be if one travelled more than 400 km in one day. In this case the battery would be rapidly charged at a service station. Since the battery is light the cost is moderate and because it is not normally deep cycled a long life can be expected. Aluminium test cells have already demonstrated over 3000 deep discharge cycles and operation down to 80°C, as seen in the previous section.

At the present time we need to use larger fuel cells and smaller batteries similar to the hybrids with thermal engines. The vehicle which is going to be the development testbed is the new TX1 London taxi chassis made by LTI International, a division of Manganese Bronze in Coventry, shown in Fig. 2.10. This vehicle has been chosen because of growing air quality problems in London. The City of Westminster is now an Air Quality Improvement Area. This is mainly due to a large increase in diesel use which has resulted in unacceptable levels of PM10 emissions. Public Transport is a major contributor, with the concentration of large numbers of vehicles in the central zone.

Two types of fuel cell are attractive for use in vehicles – the PEM membrane and the alkaline types, as described in the following chapter. Both types have undergone a revolution in stack design in the last few years with the result that the stack (Fig. 2.11) is no longer the major cost item in small systems, it is the fuel-cell controller and the power converter. In this section we shall review the problems to be solved and offer some suggestions as to the likely course of development. As always the fundamental issue is to convert a high cost technology for mass production civilian use. Current (1998) fuel cells cost $1000 per kW and most of that cost lies in the control system and power conversion. Stacks will cost less than $100 per kW in mass production. The challenge is to reduce the control system cost. It is for this reason that most vehicle fuel-cell manufacturers are opting to supply the stacks, and leave the car industry to manufacture the controller, Fig. 2.12. This is an opportunity that Fuel Cell Control Ltd intends to take up by offering control systems commercially.

(a) (b)

(c) (d) Fig. 2.11 Developed PEM fuel cell: (a) plate; (b) stack; (c) anode; (d) cathode.

Intelligence: 80 I/O programmable logic controller at 24 V DC Control: Close loop: hydrogen 0–1/10 bar; air 0–45 cubic metres/hour proportional to demand Purge: Dry nitrogen loop

2.4.1 WHAT IS IN A FUEL-CELL SYSTEM?
Here is a typical specification:
Power: 7.2 kW max
Output voltage: 96 V DC no-load
64 V DC full-load
Output current: 110 amps
Operating temperature: 70°C
Fuel: Air 45 cubic metres per hour
Pure hydrogen 5 cubic metres per hour
Hydrogen storage: Cryogenic – 180°C
High pressure 200 bar
DC/DC converter 1
Input: 60–100 V DC

Output: 0–396 V at 2.45 V per cell, lead–acid - 18 A Current ripple less than 1 part in 10 000

Fuel-cell controller
Pumps: (l) Hydrogen 80 watts
(2) Air 320 watts
(3) KOH 85 watts
(4) Water 10 watts
Valves: 10 off electropneumatic control
Preheat: 2 kW – 312 V DC
DC/DC converter 2
Input: 200/400 V DC
Output: 27.6 V DC 600 W for control system
Fig. 2.12 Fuel-cell system specification.

Figure 2.13 shows the cell layout of the two fuel-cell types, alkaline and proton exchange membrane (PEM). In the alkaline type, the electrolyte is a liquid – potassium hydroxide or KOH. This is the same material used in alkaline batteries. The anode membrane is porous and has eight very small amounts of platinum catalyst (1 g would cover three football fields of plate area). The cathode side has a silver catalyst made by Hoechst. It is possible to use platinum but the cost is much greater.

In the PEM type the electrolyte is a solid, Nafion 115 sheet – a proprietary Du Pont product; there are competitors such as Dow Chemical and Ashai in Japan. The anode is similar to the alkaline type. The cathode membrane has a platinum catalyst and much research has been aimed at reducing the cathode loading which is why many PEM cells use the high pressure approach, since it helps to reduce the amount of catalyst for a given current density. Catalysts are the main cost in stack construction and optimizing their use is a major research area. Other differences between the two types are cooling and source gas purity.

In both systems about 40% of the fuel expended is given out as heat. In the PEM type, water cooling plates are used to remove this heat. In the alkaline type the electrolyte does this job and has the added advantage that it does not freeze at 0°C. Consequently both systems need a liquid cooling system.

In Europe, Esso is already committed to making hydrogen available at vehicle service stations. Hydrogen may also be used to power aircraft in the future. In America, petrol is effectively subsidized (see Chapter 4) which makes it very hard for other fuels to compete. One of the main interests there has been reforming petrol and methanol to produce hydrogen. If done in the vehicle this produces a hydrogen supply which contains a high concentration of carbon monoxide. PEM systems can be made to tolerate this impurity. To date alkaline stacks need pure hydrogen.

However, the whole business of on-board reforming is undesirable in terms of cost and complexity and is inefficient in terms of fuel consumption compared to using pure hydrogen made at a central

HYDROGEN GAS HYDROGEN GAS ANODE (NICKEL MESH)

ANODE (NICKEL MESH)

POROUS MEMBRANE (PLATINUM CATALYST)
LIQUID KOH (POTASSIUM HYDROXIDE)
POROUS MEMBRANE (SILVER CATALYST)
POROUS MEMBRANE (PLATINUM CATALYST)
SOLID NAFION POLYMER ELECTROLYTE
POROUS MEMBRANE (PLATINUM CATALYST)

CATHODE (NICKEL MESH)

CATHODE (NICKEL MESH) OXYGEN GAS OXYGEN GAS

Fig. 2.13 Alkali (left) and PEM cell layouts compared.

facility. There are two main ways hydrogen can be stored: gas or liquid. As a gas it is usually compressed to 200 bar and stored in steel tanks with man-made fibre reinforcement and carbon additives to assist in the absorption. This technique works for large vehicles where bottles can be roof mounted – buses, for example. As a liquid the energy density is three times that of petrol, being 57 000 BTUs per lb compared with 19 000 BTUs per lb for gasoline. Two gallons would be needed to travel 500 miles in a 3 litre/100 km (80/100 mpg) PNGV specification vehicle. The gas liquefies at 180°C and 20 bar. In modern super-insulated vehicle tanks, hydrogen can be kept liquid for 2 weeks without refrigeration. A 20 watt Sterling cycle refrigerator can keep it liquid indefinitely. This system is suitable for application where space is limited, such as aeroplanes and cars. Many people believe the compression process uses too much energy. In fact it is the LIND refrigeration cycle, which is used to take hydrogen down to 269°C from 180°C where hydrogen is liquid at atmospheric pressure, that is the heavy consumer of compressor energy.

Polaron believe the technique permits the early use of hydrogen because tank exchange is possible until the investment in on-board refuelling is possible. The tank for a car would only be the size of an outboard-engined boat fuel tank. Cryogenic storage is already well established in the natural gas industry where liquid natural gas (methane) at 160°C is used to fuel 1000 bhp heavy duty trucks in Europe and Japan.

Considering again fuel-cell stacks, in either system an anode or cathode plate is 2.5 mm thick, so a pair of plates give a 5 mm build-up. Each pair of plates gives 1 V at no-load and typically 0.66 V at full load. This means that the stack length is around 500 mm, plus manifolds, for a 64 V, 7.2 kW continuous rating stack. It should also be pointed out that stack power doubles, at least, if pure oxygen is used instead of air. This is unlikely, however, as on-board enrichment to 40% oxygen is promised in the near future, as are some significant improvements in stack chemistry, especially in the catalyst area.

The fuel-cell stack is controlled by regulating the hydrogen pressure in the range 0–30 millibars. A recirculation loop permits water vapour to be added as PEM fuel cells work best with wet hydrogen. The air pressure is regulated by changing the blower speed in conjunction with the fuel-cell current demand. This takes 10 seconds to rise in a low pressure system, but may fall rapidly. The DC/DC converter determines the load applied to the fuel cell.

2.4.1 FUEL-CELL OPERATING STRATEGIES

As we can now see in Fig. 2.14, a fuel cell is a complex system and the key problems are that the feedstock must be kept pure and power consumption minimized in the auxiliaries. There are two main fuel-cell operating strategies: high pressure, 1–3.5 bar, and low pressure, at 1/20 bar The benefit of high pressure systems is fast hydrogen diffusion in the membrane which results in fast response – less than 1 second. Consequently it is possible for the fuel cell to follow the vehicle load profile and operate without a battery. This strategy is spoilt by warm-up issues. The stack must be at 70°C to deliver rated power. Warm-up can take 15 minutes. Another problem is that the power to supply the compressed services is significant – perhaps 25% of output at peak power.

Low pressure systems have modest auxiliary power needs, perhaps 10% of rated output at full power and proportionally less at low power. The main consumers are the air compressor and the KOH pump. The price is slower response. It typically takes 10 seconds for the fuel cell to ramp up to full power, consequently a peaking battery is needed to provide power during acceleration. This means that generally a smaller fuel cell may be used.

Fuel cells are the opposite of most electrical devices in that peak efficiency occurs at minimum load. In a high pressure system this profile is ideal for a motorway express coach where most time

The hydrogen pump (shown left in Fig. 2.16) is a side channel blower and has to operate at 1/30 bar at 5 cubic metres/hour with wet hydrogen at 70°C, plus slight KOH contamination. The pressure criterion usually results in a choice of blower made by Gast and Rietschle. The standard unit is a 150 mm cube and weighs 5 kg. The operating point is 2800 rpm and power consumption by the pump is around 85 W, with an additional 36 W of copper loss in the motor. Fuel Cell Control Ltd rewound the 2 pole D56 induction motor as a 4 pole 20 V unit. Our second attempt will be an 8 pole design which should reduce the copper loss to about 8 watts. The low voltage is chosen for safety and the unit will be driven with a linear sine wave inverter (shown centre in Fig. 2.16). The dv/dt is kept low to avoid spontaneous ignition, in case hydrogen enters the motor chamber. The windings are potted, to avoid direct electrical contact and reduce the free volume in the motor chamber. All parts in the hydrogen contact area are nickel plated (zinc–copper–nickel). The blower is made of aluminium.

The air pump (shown right in Fig. 2.16) is about a 300 mm cube and weighs about 15 kg. The motor is a 1/2 hp D63 induction machine and has been rewound as 8 pole 20 V, 15 A, 325 W at 192 Hz (2900 rpm). The inverter is a switching unit to minimize losses, as the ignition risk is lower than with hydrogen. A 30 amp inverter provides good efficiency and the speed of this blower is adjusted with variations in power demand.

For the future, the company are working on a high speed channel blower, to operate at 10 000 rpm, using a brushless DC motor. In star-winding form, at 4000 rpm, it will satisfy 5 cubic metres per hour and at 10 000 rpm 45 cubic metres per hour. Thus a single design could do both jobs and it only weighs about l kg. However, silencing must be carefully considered.

The water and KOH pumps are standard 10 and 85 watt capacity permanent-magnet brushed DC driven pumps running at 3000 rpm on 28 V DC. The pumps are magnetically coupled with Talcum parts to resist aggressive fluids (such as potassium hydroxide).

2.4.3 FUEL-CELL CONTROL VALVES AND ACTUATION

Selecting suitable valves with such a diverse array of media and operating conditions has not been easy, Fig. 2.17. In fact the valves themselves are neither expensive nor heavy. The problem area is the actuators and it is intended to redesign these for the next version. Currently there is no safety legislation in place for hydrogen powered vehicles. The onus is on the supplier to demonstrate fitness for purpose and that all reasonable precautions have been taken. It is felt this will change once meaningful experience has been achieved. Clearly, declaring a vehicle to be a class 1 safety area would destroy all economic viability. Consequently, as with petrol and propane, safe techniques need to be established and demonstrated before regulations are enforced. Some are obvious, such as no fuel or fuel processes to be contained in the passenger compartment. Others require experience such as fuel storage and distribution. Storage in a closed building needs careful consideration.

The valves can be neatly split into two groups, high and low pressure. The high pressure units are standard metal valves with electric solenoid actuators and spring return; they operate with 28 V coils. The larger units were chosen as 2 and 3 way plastic ball valves, using polypropylene bodies and EPDM seals for KOH compatibility plus high temperature operation (70–80°C).

Polaron had a major problem with the actuators. Fail-safe operation with low power consumption was needed. Solenoid valves in larger sizes use the controlled medium as a pilot fluid and consequently do not operate reliably with pressures as low as 1/30 bar. The solenoids are direct acting, with no economy measures or permanent magnet biasing, and thus consume significant power. In the end nitrogen was used as a pilot fluid, with 4 watt pilot valves to control the opening. This approach works well but the valves use up a lot of space, especially the actuators.

The intention for the future is to design a plate with spring-loaded clutches operating from a common motor drive. This should cut down on the volume and permit a much lower cost solution. Actuation accounted for 70% of the cost and 70% of the volume of the valves. It was found to be a niche sector market where nobody has a comprehensive system of interchangeable valves, seals and actuators suitable for onerous conditions.

2.4.4 PROGRAMMABLE LOGIC CONTROLLER (PLC)

An 80 I/O PLC with interface modules cost $1500 in 1998. Quantity build could halve this price

but still nowhere near the objectives. A Mitsubishi F Series was chosen for development, Fig.

2.18. Production units are destined to use a custom-engineered microprocessor unit based on a Siemens/Thompson C167 CAN bus processor, which is becoming a standard in the European and American car industry. This unit must represent one of the toughest design challenges. To convert low voltage, heavy current into higher voltage with galvanic isolation, ultra-low current ripple and high efficiency. Many solutions have been analysed, although this one offers the best combination of characteristics. Figure 2.19 shows electronic system circuits.

Let us consider a square wave phase-shift chopper: at minimum volts input, we need full reinforcement to achieve 396 V output. However, at low load we have a 96 V DC link and perhaps 90 degrees phase shift between A and B. This is not too bad, except that we only draw output current for 50% of the time: this means that the DC link contains 100% current ripple at 2F switching frequency. Since the pulse width should always be 50% plus, the solution is to have three such choppers with 120° phase shift between them. This has the effect of overlapping the converters if the correct measures are taken. Consequently, the supply now only contains 30% ripple worst case at 6F, but when the current is largest we have maximum overlap; see Fig. 2.19 (bottom).

The first attempt is to build this converter with each stage operating at 3 kHz, with torroidal

0.08 mm silicon steel cores. This is both silent and efficient. The edges are deliberately softened to reduce dv/dt (capacitive ripple). At low frequency this does not cost much in losses, with 10 microsecond edges, but reduces the spikes when the diodes reverse recover. The design is adaptable to different output voltages by rewinding the output transformers and chokes. It is believed 90% efficiency can be achieved with this design at 60 V input, 7.2 kW. A double L/C Filter attenuates the current to the fuel-cell stack to ensure compliance with 0.01% ripple current rating. The reason for this is to prevent poisoning of the fuel-cell catalysts.

The cost of this unit is a problem. The silicon for the main switches are LAPT transistors, at 15 A and 200 V, using 2SA1302 and 2SC3276; the six switches cost $150 in parts (1998 prices). It is intended to have these parts integrated into a high power package. The chokes cost $150 and capacitors $60. To improve the costs higher frequency is needed; time will tell if this can be achieved without sacrificing efficiency and current ripple.

2.4.5 SENSORS AND TRANSDUCERS

This is perhaps the hardest area in which to reduce the cost. The devices operate under hostile conditions and are currently made to order. One success has been to reduce the cost of the 2 kW preheater from $1000 to $200 by a complete redesign. In other areas there has been success in combining functions such as the flow meter and the temperature controller in the KOH loop. A complete new family of low cost sensors is needed before production cost targets can be met.

2.4.6 FUEL-CELL FUTURE PROSPECTS

It is early days for vehicle fuel cells and the main challenge is better, lighter, cheaper, more convenient to use parts – preferably plastics. Insulated packaging of semiconductors is the main issue in the DC/DC converters at low voltage and heavy current. This in time will lead to control systems with lower parts count and greater reliability. As we get down the learning curve, and volumes increase, costs will fall. Major improvements in fuel storage, oxygen enrichment and stack materials should lead to continuing increases in current density and hence smaller stacks for the same power output.

2.� �aste heat recovery� key element in supercar efficiency

In the longer term vehicles will be electrically propelled using flywheel storage, hydrogen fuel cells or both. These systems potentially offer high cycle efficiencies, and low emissions vital to improving air quality in our big cities11.

Bill Clinton’s initiative for US family cars to achieve 100 miles per gallon by 2003 is a serious challenge to the engineering industry; see Chapter 4. Current solutions include lightweight structures, reduced running losses and small engines. However, the author believes that the target can be achieved using conventional vehicle platforms with low drag floorpans and instead attention is focused on high efficiency drivetrains. Small engines give poor acceleration so to achieve acceptable performance hybrid technology is required. The internal combustion engine car achieves 28% efficiency under motorway conditions and half that on an urban cycle. The key problem is how to convert more of that energy into useful work. The account below will investigate two schemes: (a) turbine recovery system and (b) thermoelectric recovery system.

In both schemes the energy produced is converted into electricity. This is because such an arrangement provides a plausible method for matching the power into the electrical drive. It is accepted that all mechanical solutions are also viable in a hybrid vehicle.

2.5.1 HYBRID ELECTRIC DRIVE

Figure 2.20 illustrates a parallel hybrid driveline using a Wankel engine and a brushless DC Motor. The package can produce 70 kW peak power and 20 kW average. This combination provides excellent acceleration using energy stored in a small flat plate lead-acid battery. Tests to date show this battery still delivers 100% peak power of 45 kW and 80% capacity after 22 000 cycles to 30% depth of discharge. Thermal management is vital to achieving these figures.

The engine operates on a two stroke cycle and produces high quality waste heat at the exhaust. The temperature of the gas is around 1000°C. This gas contains 72% of the energy in the fuel. If we can convert a third of this energy into electricity we can nearly double the NTG of the vehicle under motorway conditions.

Why is this important? Hybrid solutions are very effective at improving efficiency under urban cycle conditions but make no impact under motorway conditions. Here is a system that can begin to solve this problem. Supposing in the existing scheme we require 20 kW to operate a vehicle at

engine and brushless DC motor.

70 mph on a motorway. The engine produces 55 kW of waste heat. If a third is converted to electricity 18.33 kW is made available and perhaps 15 kW of this is additional mechanical power. Consequently the engine only has to produce 60% of the mechanical power to give the same output of 20 kW and the electrical system could be reduced to 10 kW. To summarize, the 20 kW requirement could be met by using 12 kW of engine power and 8 kW of waste heat recovery power. Future schemes could possibly improve on these figures.

2.5.2 THE TURBINE RECOVERY SCHEME

In this scheme the concept is to use a small turbine system in association with an electric generator. Figure 2.21 illustrates the conceptual realization of the idea. Some modelling of the turbine shows that, for reasonable efficiency, speeds of 150 000 rpm are necessary. If such speeds can be achieved the sizes of the components will be tiny. For example, for 10 kW, the rotor of the generator will be 25 mm diameter by 15 mm long. The generator needs to be kept separate from the turbine stages due to the high temperatures involved in the thermodynamic processes. The turbine bearings will be hydrodynamic gas bearings – the back of the turbine rotors and the shaft will be plated with a zig-zag pattern, microns thick, designed to created turbulence at high speeds. Dynamically the system will operate below the first critical speed.

The generator bearings will be angle contact bearings (6 mm) using ceramic balls (RHP-INA) and Kluber Isoflex Super LDS 18 grease. The rear bearing is preloaded and free to slide by means of a crinkle spring. The coupling between generator and motor can be a tongue and fork, permitting easy removal, as the torque is below 1 Nm.

The generator losses preheat the air entering the first compressor stage. The generator has laminations of 0.2 mm radiometal with powder coat insulation applied to ensure minimum eddy current losses. The rotor has a one-piece tubular magnet, 22 mm diameter and 15 mm long by

3.5 mm thick, of ‘one-five’ samarium cobalt. This is glued onto a stainless steel shaft of high

ENGINE EXHAUST WASTE HEAT

resistivity. The magnets are retained by a prestressed carbon fibre ring of 1.5 mm wall thickness. The generator has a 4 pole configuration and the machine winding is designed to give 165 V (RMS) at 150 000 rpm, resulting in a line current of 42 A at 10 kW. An advantage to this method of construction is that the generator may be built and tested separately from the turbine. The turbine rotors will be only 40 mm outside diameter and machined from aluminium. The rotors are held to the shaft by Loctited nuts and there is a hole down the centre to facilitate temperature measurement. One of the nuts contains the fork for the drive coupling.

The design of these stages with their casing and expanders is confidential. The heat exchanger is an air to air unit rated for 30 kW at a temperature of 600°C. The same unit also functions as an exhaust silencer for the engine and special construction techniques are required to resist the high temperature of the exhaust from a Wankel engine (typically 1000°C). Polaron envisage a battery of 216 V nominal varying between 180 V and 255 V. The speed of the turbine may vary over the entire range but meaningful output will only occur between 120 000 and 150 000 rpm.

The turbine, Fig. 2.22, is started by a transistor bridge connected across the diode bridge and as the compression of inlet air starts, and is expanded, the output turbine takes over supplying rotational power, and the transistor bridge is then used as a switching regulator, to match the generator voltage to the battery voltage. Sensorless timing techniques are possible but it is simpler to use three Hall sensors operating from the rotor field system.

AIR OUT

245 V DC

150 000 RPM

IGBT TRANSISTORS

5 KHZ

You might ask: Why not let the load line of the turbine generator intersect with that of the battery on an open loop basis? The problem is that in most cases one would not obtain the correct operating point. The turbine power is proportional to speed cubed. One obtains the correct operating point at just one speed for a given power, whereas the battery operates from 1.75 to 2.35 V per cell. Consequently it is necessary to have closed loop control of the power flow from generator to battery. But there is a second reason; this mode of control with the transistor bridge permits the turbine to be used as a brake – power flow is reversible between turbine and battery. This is very useful when negotiating long steep gradients, for example.

Overall it is believed that an efficiency of 30% is achievable with such a process and thus the system can make a major contribution to fuel utilization under motorway conditions.

2.5.3 THERMOELECTRIC GENERATOR

The turbine recuperator technique involves some very high technology mechanics to make the system work. It prompts the questions: Is there any other way of achieving the same objective? Is a solid state solution possible?

Thermoelectrical devices were invented in 1821 and are perhaps best known today for the small fridges we have on our cars and boats to cool food and drinks. An array of bismuth telluride chips 40 mm square can produce 60 watts of cooling with a temperature differential of 20°C. If we go back 60 years to the 1930s there were thermopiles which one placed into a fire and the pile provided the current for a vacuum tube radio. It is only very recently that here in the UK a group of engineers started to ask the question ‘Why are thermopiles so inefficient?’ What happens to the 96% of the energy consumed that does not appear at the output terminals? Why is the output voltage so small – typically microvolts per °C at top temperature?

At Southampton University Dr Harold Aspden soon identified the answer to the efficiency question. The energy was being consumed by circulating currents within the device. It was then realized that if a dielectric was placed between the thermopile layers, and the pile was oscillated mechanically, that an AC voltage could be obtained up to 50 times the amplitude of the original DC voltage, Fig. 2.23. This oscillation has been tested with frequencies from DC to RF and the process holds good across the spectrum. Dr Aspden has concentrated his efforts on producing thermopile arrays for use on the roof of a building, with temperature differentials of 20–40°C.

However, if we return to our waste heat recovery problem we are dealing with top temperatures of 600°C plus and consequently alternative materials will be required and the number of stages in series to produce a given voltage will be reduced. But, with a top temperature of 30°C existing, devices can convert 20 W of power with an efficiency of 25%. It should be emphasized that this work is at an early stage of development at this time.

The thermopile elements suitable are iron and constantin 40% nickel/60% copper (Type J thermocouple material); at 600°C, with mechanical excitation, a voltage of 300–500 mV per stage can be achieved, hence 500 cells in series would produce 216 V DC. The circulating current in each cell is proportional to the temperature difference but the output AC voltage may be controlled by adjusting the amplitude of the mechanical excitation. The most interesting point is that to give 10 kW a suitable unit could be very compact – our calculations suggest about 100 mm cube. We believe the mechanical excitation is best supplied by ultrasonic piezoelectric transducers driven by a HiFi amplifier. The power required is around 200 watts. One interesting point is that the unit offers reversible power flow. How? It can be converted from refrigerator to heater and act as a braking device.

JUNCTIONS COLD

CERAMIC DIELECTRIC

JUNCTIONS COLD

IRON

CONVENTIONAL THERMOPILE

TEMPERATURE GRADIENT CONSTANTIN JUNCTIONS HOT

CONSTANTIN

CONSTANTIN

JUNCTIONS SERIES CONNECTED WITH D.C. OUTPUT ASPDEN THERMOGENERATOR

MECHANICAL

EXCITATION

THERMAL

GRADIENT

References

  1. Hodkinson, R., The electronic battery, paper 98EL004, ISATA31
  2. Hodkinson, R., The aluminium battery – a status report, paper 99CPE012, ISATA 32, 1999
  3. Zaromb, S. and Faust, R. A., Journal of the Electrochemical Society, 109, p. 1191,1962
  4. Despic, A. and Parkhutik, V., Modern Aspects of Electrochemistry, No. 20, J. O. M. Bockrus, Plenum Press, New York
  5. Gifford, P. R. and Palmissano, J. B., Journal Electrochem. Soc., 135, p. 650, 1988
  6. Zagiel, A., Natishan, P. and Gileadi, E., Electrochim Acta, 35, p. 1019, 1990
  7. Rudd, E. J., Development of Aluminium/Air Batteries for Applications in Electric Vehicles, Eltech Research Corp. to Sandia Nat Labs, Contract AN091–7066, December 1990
  8. ALUPOWER INC, Internal ALUPOWER-Canada Report 1992
  1. Gibbons, D. W. and Rudd, E. J., The Development of Aluminium/Air Batteries for Propulsion Applications
  2. Hodkinson, R., Advanced fuel cell control system, EVS 15, Brussels, September 1998
  3. Hodkinson, R., Waste heat recovery – a key element in supercar efficiency, paper 94UL004, ISATA 27, 1994
Further reading

Proceedings 28th IECEC 1993 Rand et al., Batteries for electric vehicles, Research Studies Press/Wiley, 1998 Berndt, Maintenance-free batteries, Research Studies Press/Wiley, 1993

3

Electric motor and drive-controller design

3.1 Introduction

While Chapter 1 introduced the selection and specification of EV motors and control circuits, this chapter shows how system and detail design can in themselves produce very worthwhile improvements in efficiency which can define the viability of an EV project. The section opens with discussion of the recently introduced brushed DC motor, by Nelco Ltd, for electric industrial trucks, then considers three sizes of brushless DC machine for electric and hybrid drive cars, before examining the latest developments in motor controllers.

3.2 Electric truck motor considerations

EV motor makers Nelco say the requirements for traction motors can be summarized as light weight, wide speed range, high efficiency, maximum torque and long life. The company recently developed their diagonal frame Nexus II motor, for general electric truck operation. In this motor, Fig. 3.1, active iron and copper represent 50 and 30% respectively of the motor weight. Holes in the armature lamination, (a), have resulted in some weight reduction and the use of a faceplate commutator, (b), has also helped keep weight down – with only 30% of the copper required for a barrel-type commutator – because the riser forms part of the brush contact face. With use of aluminium alloy for the non-active parts, such as brush holders

(c) of the motor, weight of the 132L motor is held to 80 kg, a power to weight ratio of 450 watts/kg. Tolerance of high accelerations comes from perfection of the faceplate commutator to retain brush track surface stability. Usually the constraint on high power at high speeds, particularly when field strengths are reduced, is commutation ability, Nelco maintains.

The patented segmented frame of the Nexus, (d), makes the provision of interpoles quite an easy option – to optimize commutation at all current loadings, so reducing brush heating losses and compensating for interpole coil resistance losses. As output torque is a function of armature current, flux and the number of conductors, all these must be maximized. Short time high current densities, over the constant torque portion of the performance envelope, are possible given adequate cooling. Cost is held down by such measures as use of a segmented yoke/pole assembly, (e); extruded brush holders are also used, (f). Figure. 3.2 shows rating and efficiency curves for the N180L machine.

100

5000

3750 75

2500

1250 50

25

0 0 10 20 30 40 50 60
OUTPUT kw
Fig. 3.2 N180L motor characteristics.

3.3 Brushless DC motor design for a small car

In this case study of the design of a 45 kW motor1 commissioned for a small family hatchback – the Rover Metro Hermes – the unit was to give rated power from 3600–12 000 rpm at a terminal voltage of 150 V AC. The unit has been tested on a dynamometer over the full envelope of performance and methods for improving the accuracy of measurement are discussed below. The results presented show a machine with high load efficiency up to expectations and the factors considered are important in minimizing losses.

3.3.1 BRUSHLESS MOTOR FUNDAMENTALS

A key aspect of motor design for improved performance is vector control, which is the resolution of the stator current of the machine into two components of current at right angles. Id is the reactive component which controls the field and Iq is the real component which controls the power. Id and Iq are normally alternating currents. In this example, Fig. 3.3, the machines being considered are of the rare-earth surface-mounted magnet type with a conventional 3 phase stator and a rotor consisting of a magnetic flux return with a number of motor pole magnets mounted on it. The open loop characteristics of the machine are considered as follows: if the shaft of the motor is driven externally to 12 000 rpm a voltage of 260 V will be recorded, (a). In this condition with full field at maximum speed, iron losses will be high and the stator will heat up very quickly. At this operating point the motor could supply about 135 kW of power. However, this is not the purpose of the design, (b).

VOLTAGw

TO QUw VOLTAGw

260 V 1666 Hz

SPwwD

SPwwDSPwwD

12000 rpm 4600 5600

(a) (b)

Iq x q

Fig. 3.3 Example brushless motor characteristics: (a) no-load terminal voltage when machine is operated as a generator;

(b) variation of machine terminal voltage with torque and speed (left) with variation of power factor with torque and speed (right); (c) vector diagram (right) of PMB DC motor (left), in field weakening condition 12 000 rpm no-load.

The torque–speed requirement for a typical small vehicle is shown to be constant torque to base speed (around 3600 rpm) then constant power to 12 000 rpm. This assumes a fixed ratio design speed reducer. During the first region the voltage rises with speed. In the second region the voltage is held constant at 150 V by deliberately introducing a circulating current – Id which produces 152 V at 12 000 rpm to offset the 260 V produced by the machine, to leave 150 V at the machine terminals. The circulating current produces this voltage across the inductance of the machine winding. It also produces armature reaction which weakens the machine field; total field = armature reaction + permanent magnet field gives a lower air gap flux and lower iron losses. This mode of operation is known as vector control. What happens if we reverse the direction of Id? Theoretically we strengthen the field. However, with a surface mounted magnet motor the machine slows down due to the effect of the circulating current on the machine inductance. However, the torque per amp of Iq current remains constant.

If we supply the motor from a square wave inverter we observe some interesting phenomena when we vary the position of the rotor timing signals. In the correct position the stator current is very small. When the current lags the voltage the motor slows and produces current with sharp spikes and considerable torque ripple. When the current leads the voltage the motor runs faster and produces a near sine wave with smooth torque output. It is the field weakening mode we wish to use in our control strategy, (c).

3.3.2 MOTOR DESIGN: METHOD OF MEASUREMENT

In the following account details are given of the motor design, Fig. 3.4, and of the predicted and measured efficiency maps. The measured efficiency maps were carried out using a variable DC link voltage source inverter. Polaron conducted the trials with two waveforms: a square wave with conduction angle 180° and a square wave with harmonic reduction, conduction angle 150°, the purpose being to assess the effects of the harmonics on motor performance, (a).

Stack 00 220 mm Stator mass 14.1 kg Stack 0 142.5 mm Rotor mass 4.12 kg Length 80.5 mm Total mass 34 kg 0verall length 140.5 mm Rotor inertia 0.016 kg m2V Pole number 16 Thermal resistance 0.038°C/Watt Peak torque 200 Nm Thermal capacity 6000 joules/°C Motor constant km RMS 3.03 Nm/sqr (W) Rotor critical speed 21 000 rpm Motor constant km 0C2 89 Nm/sqr (W) Nominal speed 12 000 rpm Electrical time constant 10.4 millisecs Back EMF at 12 000 rpm = 260 V Mechanical time constant 1.9 millisecs Winding resistance 0.096 ohm Friction 0.171 Nm Winding inductance 100 microhenries

(a)
Motor torque constant 0.3 Nm/A Vector control voltage 150 V Winding star connected RMS line to line
a = 150° audible a = 180° audible SPEE0 V P noise V P noise 1000 29V 7.3A 75W 52dB 28V 12.4A 72W 54dB 2000 55V 8.1A 216W 54dB 55V 12.8A 216W 54dB 3000 82.6V 8.4A 396W 56dB 84V 13.2A 405W 55dB 4000 113V 9.12A 540W 56dB 110V 13.6A 630W 56dB 5000 138V 9.12A 765W 58dB 137V 13.8A 900W 57dB 6000 150V 25A 990W 59dB 150V 24A 1080W 58dB
(b)
8000 150V 87A 1440W 60dB 150V 84A 1800W 63dB 10000 150V 122A 2250W 67dB 150V 123A 2700W 69dB

Fig. 3.4 Motor design data: (a) XP1070 machine data; (b) no-load losses (machine only).

The measurement of electrical input power is accurately achieved using the ‘three wattmeter’ method. Measurement of mechanical power is more difficult. Polaron found it necessary to mount the motor into a swing frame with a separate load cell to obtain accurate results at low torque. Even so, other problems such as mechanical resonances and beating effects at 50 Hz harmonics require care in assessing results. The operating points were on the basis of maximum efficiency below 150 V AC terminal voltage.

Results are in the form of three efficiency maps which give predicted and measured performance on both waveforms. The losses in this type of motor are dominated by resistance at low speed and iron losses at high speed. What the results show is that low speed performance was accurately predicted but high speed performance was less efficient especially at light load. The reason for this is that the iron loss at 10 000 rpm, no-load, should be about 1000 W, sine wave, (b). With 150 V terminal voltage the measured figure was 2200 W. The following paragraphs discuss the factors affecting this result but it is believed that the main contributors are larger than expected hysteresis losses due to core steel not being annealed, and larger than expected eddy current losses because of lower than specified insulation between laminations.

Annealing causes oxidation of the surface of the steel, leading to improved interlayer insulation. Polaron subsequently coat the laminations with epoxy resin then clamp them in a fixture to form a solid core for winding.

3.3.3 MOTOR DESIGN FACTORS AFFECTING MACHINE EFFICIENCY

For the stator the important factors are: (i) shape of lamination – optimized lamination has a much larger window than 50 Hz induction motor lamination and a bigger rotor diameter relative to the stator diameter; (ii) use of high nickel steels is counteracted by poor thermal conductivity. Thin silicon steel with well-insulated laminations gives best results. Laminations should be annealed and not subjected to large mechanical stresses. The core can be a slide fit in casing at room temperature as expansion due to core heating soon closes the gap. Stator OD should be a ground surface; (iii) winding must be litz wire and vacuum impregnated to ensure good thermal conductivity. Varnish conducts 10 times the heat of air gap.

For the rotor the main ones are: (i) if magnets are thick (10 mm in this case) mild steel flux return is satisfactory; (ii) magnets are unevenly spaced to remove cogging torque; (iii) individual poles must not contain gaps between magnet blocks making up the pole. Such gaps lead to massive high frequency iron losses. This can be checked by rotating the machine at lower speed and observing the back-EMF pattern. If there are sharp spikes in the wave form the user will have problems with losses.

3.3.4 MOTOR CONTROL

Battery operated drives must make optimum use of the energy stored in the battery. To do this, the efficiency of both motor and driveline are critically important. This is especially true in vehicle cruise mode typically two-thirds speed one-third maximum torque, therefore Polaron proposed to build a drive with two control systems: (i) current source control in constant torque region and (ii) voltage source operation in constant power region. At 45 kW 6000 rpm we would expectIL 175 A, VAC 150 V; inverter switching loss 10 kHz, 1.8 kW; converter saturated loss 0.9 kW, using PWM on the windings and IBGT devices.

If, however, we use a square wave at the machine frequency, Fig. 3.5, and the machine operates with a leading power factor, the switching losses are greatly reduced for additional iron loss, of 225 W, at top speed. The inverter efficiency increases from 94% to 97%. In the low speed constant torque region there is no alternative to using PWM in some form.

3.4 Brushless motor design for a medium car

3.4.1 INTRODUCTION

Here the task is to optimize the 45/70 kW driveline for the family car of the future2. This involves improvements in fundamental principles but much more in materials and manufacturing technology. The introduction of hybrid vehicles places ever greater demands on motor performance.

It is the long-term aim of the US PNGV programme to reduce the cost of ‘core’ electric motor and drive elements to 4 dollars per kW from around 10 dollars charged in 1996 for introductory products supplied in volume. The price may be reduced to 6.5 dollars using new manufacturing methods to be reviewed below. Further savings may come from very high volume production. This will require significant investment which will not occur until there is confidence in the market place and technical maturity in a solution. In terms of design, we may increase speed from 12 000 to 20 000 rpm. For reasons to be explored, a further increase becomes counterproductive unless there is a breakthrough in materials. In the inverter area Polaron believe the best cost strategy is to use a double converter with 300 V battery, 600 V DC link and 260 V motor. This assumes power levels of 70 kW.

The motor can be induction type or brushless DC. Induction is satisfactory in flat landscape/ long highway conditions. For steeper terrain, and shorter highways as exists in Europe brushless DC is more suitable – especially for high performance vehicles and drivelines for acceleration/ braking assistance in hybrid vehicles. Excellent progress has been made in the silicon field. The introduction of high reliability wire bonded packaging in association with thin NPT chip technology for IGBTs is reducing prices and improving performance. Currently a 100 A 3 phase bridge costs around $100 in volume. The arrival of complete 3 phase bridge drivers in a single chip at low cost is a further improvement in this area. Individual driver chips provide better device protection and drive capability at this time.

Great progress has been made in batteries in recent years. However, the time has come for a change in emphasis. Previously the pure battery electric was seen as the desired solution. Even if the remaining technical issues can be addressed, we are still impeded by weight and cost of such a solution. Consequently Polaron believe they should focus on hybrid solutions and this needs batteries optimized for peak power not energy capacity. It requires batteries with geometries optimized for peak power – ultra-low internal resistance and perhaps high capacitance at the same time. It will certainly require new packaging. A capacity of 2 kWh at 2 minute rate would be adequate for the average family car. It will also require a low cost short-circuit device to bypass high resistance cells in long series strings.

There is now little doubt that brushless DC machines offer the best overall performance when used in vector control mode, with high voltage windings, Fig. 3.6. The reason is that the brushless DC motor offers the lowest winding current for the overall envelope of operation. An electric vehicle has to provide a non-linear torque/speed curve with constant power operation from base speed to maximum speed. In a brushless DC motor, the motor voltage may be held constant over this range using vector control. In an induction motor, the motor voltage must rise over the constant power speed range. If V and I are the voltage and current at maximum speed and power the values at base speed are V × (Base Speed/Max Speed)1/2, I × (Max Speed/Base Speed)1/2. If maximum speed / base speed = 3.5 times, the current at base speed is 1.87I. Consequently the induction motor inverter requires 1.87 times the current capacity of the brushless DC motor inverter.

The most significant improvement recently for brushless DC machines has been the development of the Daido magnet tube in Magnaquench material. This product offers the benefits of high energy magnet and containment tube. This leads to a third benefit which is not immediately obvious but very significant. Surface magnet motors usually employ a containment sleeve which adds several millimetres of air gap to the magnetic circuit. Since magnet tube does not require a sleeve if used within its speed capability, a thinner magnet tube is possible whilst maintaining

*N0TE: 35 kW continuous, 70 kW short time rated.

Power (3.5:1 CPSR) (kW) 45 70 70 70 150
Speed max 12 000 10 000 13 500 20 000 20 000
Stator 00 (mm) 218 200 220 200 225
Rotor 00 (mm) 141 113 141 113 145
Active length (mm) 80.5 190 97 110 160
0verall length (mm) 141 260 157 170 230
Stator voltage (V) 150 360 460 360 460
Max Efficiency 96% 96% 98% 96.5% 98.6%
Winding L (mH) 0.1 1.78 1.37 0.85 0.28
Winding R (mW) 9.6 66 116 38 13.4
Poles 16 8 8 8 8
Stator/rotor mass (kg) 19 40 21 24 44

Fig. 3.6 Current designs of vector controlled brushless DC machines.

the same air gap flux density. The benefit is reduced magnet weight for a given motor design. For example, 140 mm diameter Daido grade 3F material with a 5 mm wall will operate unsupported to 13 500 rpm.

The rotor of the machine, Fig. 3.7, is assembled with the magnet tube glued to the flux return tube, with the magnets de-energized. The pole pattern is applied with a capacitor discharge magnetizer from inside the flux return tube. The end plates and motor shafts are then fitted using a central bore for precise axial alignment. Use of a solid rotor is not practical unless a rotor material which does not saturate until 3 tesla is used. Since such material costs $50 per kg the hollow tube is the best alternative. The use of magnet tube makes complete automation of rotor construction possible achieving significant savings in labour costs, Fig. 3.7a.

Many designers are attracted by the possibility of running motors faster than the current 12 000 rpm. The objective is to reduce the peak torque requirement in an effort to reduce weight and cost of active materials. One obvious method is to compromise the constant power over the

3.5:1 speed:range requirement. Polaron’s own investigations into faster speed suggest any increase above 20 000 rpm will be counterproductive. There are many reasons for this:

(a)
The maximum frequency of operation is limited to 1500 Hz using Transil 315 in 0.08 mm thickness (3.15 W/kg at 50 Hz). Most designers are concerned with no load line losses and are endeavouring to optimize this.
(b)
Consequent on (a), as the speed rises above 20 000 rpm the pole count has to be reduced from 8 to 6 to 4 poles. This results in thicker magnets and longer flux return paths.
(c)
Optimum machine geometry is rotor OD = stator length. The Polaron 70 kW machine has rotor OD = 140 mm and rotor length of 95 mm which is close to optimal. The machine has 8 poles and gives 70 kW from 4000 to 13 500 rpm.
(d)
Machines that are below 100 mm rotor diameter are not easy to make as the windings cannot be inserted by automatic machinery. This is especially true of heavy current windings.
(e)
Machines with low pole count have poor rotor diameter to stator diameter ratio, which increases the mass of stator iron and results in large winding overhangs increasing copper losses.
(f)
Laminations for these machines should have a large number of teeth to reduce the thermal resistance from copper to water or oil jacket. The limitation is when the tooth achieves mechanical resonance in the operating frequency range of the machine. Typically it is the 6f component that causes excitation (6f = 6 times motor frequency). Silicon steel (Transil) has good thermal conductivity. High nickel steels such as radiometal exhibit poor thermal conductivity but lower

50 � 100 �

(a) (b) P

iron losses. Machines with a high peak torque requirement are better in Transil where the copper losses of peak torque can be safely dissipated.

(h)
If a better core material at a sensible price were available it would be a real boon. This is one area where there is much room for improvement. Polaron are aware of powder core technology using sintered materials but the tooth tip flux density is only 0.8 tesla. Ferrites are worse at 0.5 tesla.
(i)
If makers are prepared to use containment sleeves, a power–speed graph for high speed radial brushless DC machines would look like that in Fig. 3.7a (based on 3.5:1 constant power torque/speed curve). This is the maximum power achievable in consideration of dynamic stability requirements. This graph assumes two point suspension and that the first critical speed must be 20% higher than the top speed of operation (25 kW rotor from 25 000 to 80 000 rpm would be 57 mm OD × 100 mm long).
(j)
One problem with high speed machines is the increased kinetic energy stored in the rotor. This can place a severe strain on subsequent speed reducers unless torque limiting devices are provided.
(k)
Acoustic noise is often severe at high speed. For a reduction try: (i) impregnation of stator;
(ii)
removing sharp edges on outside of rotor; (iii) operating rotor at reduced pressure using magnetic seals or (iv) using machine with liquid cooling jacket.
(l)
Speed reduction is another difficult area at high speed. Since torques are low, friction speed reducers are quieter than gears by a factor of ten.
(m)
Bearings and mechanical stability are challenging problems at turbomachinery speeds.

Polaron believe the best cost/performance ratio can be achieved for 70 kW system by: (1) using a Transil 315 stack 0.08 mm thick made as a continuous helix using the punch and bend technique;

(2) using a rotor made from 5 mm magnet tube of surface mount structure mounted on 12.54 mm of 14/4 stainless steel; (3) magnetizing the rotor after assembly to flux density of 3 tesla for 2 millisecs for maximum flux density; (4) choosing a stator frequency of less than 1500 Hz, mean air gap flux density 0.6 tesla; (5) using a liquid cooled stator; (6) insulating the stator from earth with low capacitance coupling; (7) choosing stator of 215 mm OD with 48 teeth stack of 95 mm giving 70 kW from 4000 to 13 500 rpm. Alternatively a stator of 185 mm with 24 teeth and rotor of 110 mm OD × 140 mm long will give 70 kW from 6000 to 20 000 rpm; (8) winding the machine for 460 V in constant power region (460 V at 4000 rpm) with machine driven as a generator open circuit. This gives good efficiency and substantial winding inductance to minimize carrier ripple, Fig. 3.7b.

3.5 Brushless PM motor: design and FE analysis of a 150 kW machine

3.5.1 INTRODUCTION

High speed permanent magnet (PM) machines with rotor speed in the range from 5000 to 80 000 rpm have been developed3, applications of which include a gas turbine generator with possible application in hybrid electric vehicles. The motor considered below runs at infinitely variable speeds up to 2 kHz at full power and has been designed for different requirements at an output power of 150 kW. Machine parameters have been calculated from software package 141 developed at Nelco Systems Ltd.

The drive system of this design consisted of a brushless DC machine and an electronic inverter (a chopper and DC link) to provide the power. The performance parameters set out are aimed to producing a design specification of the machine shown in Fig. 3.8, Fig. 3.9 showing the machine controller.

In the initial stage, a detailed specification was set out for the peak torque performance of 150 Nm from 10 000 to 20 000 rpm, the no-load back-EMF at 20 000 rpm of 600 V(RMS); the total number of poles are 8 (1.33 kHz at 20 000 rpm), and the maximum total weight is 45 kg.

3.5.2 ROTOR AND STATOR CONFIGURATION

Main constraints were found to be weight and inductance; in the high speed application it is important to keep the weight to a minimum, therefore a ring design is the most suitable which means a sufficient number of poles is required on the rotor, 8 poles in this case. The main advantage with this configuration is that the return path for the magnetic circuit in the core and yoke is much smaller in cross-section area (the thickness of the ring has been considered within the customer’s shaft requirements). While 8 pole design was found to give the best solution, a 16 pole design was also considered which resulted in lower weight, but was rejected because the return path had to be increased, in that area, to give sufficient mechanical strength to the unit. In general a machine of high number of poles, at high frequency, produces high specific core loss and the reduction in the stator mass meant that the total core loss was a few watts more. To achieve the required winding inductance, careful attention had to be made to the shape of the stator lamination so as to reduce the slot leakage. The reduction of current density in the copper conductors has also been considered, but the slot shape and area have had an effect on the winding inductance. The final lamination design has been optimized for minimum slot leakage, to achieve the required performance.

3.5.3 MAGNETIC MATERIAL SELECTION

High energy-density rare earth magnets, of samarium cobalt, have been chosen in this design because of the material’s higher resistance to corrosion, and stability over a wide temperature range. Also it has a high resistance to demagnetization, allowing the magnetic length of the block to be relatively small. This shape of block lends itself to being fixed onto the outside diameter of the rotor hub, to produce the field in the d-axis, which gives advantages of a greater utilization of the magnet material with lower flux leakage, the low slot leakage resulting in low winding inductance. The magnets in this application have been fitted with a sleeve on the rotor outside diameter, for mechanical protection and to physically hold the magnets in place. A carbon-fibre sleeve was chosen for this application; it offers at least twice the strength of the steel sleeve in tension, so a much greater safety factor can be achieved. The sleeve on the rotor increases the effective air gap but an unloaded air gap flux density of 0.6 tesla was achieved from this high energy density rare earth magnet. The core loss in the stator, due to high frequency, is considered and must be kept to an acceptable level. The grade of material considerable is radiometal 4550. This alloy has a nominal 45% nickel content and combines excellent permeability with high saturation flux density.

3.5.4 MAGNETIC CIRCUITS

The magnetic circuit for this design was calculated using the Nelco software. The most important parameters in the design of the magnetic circuit were weight and to keep the core losses down to a minimum whilst reducing the slot leakage to minimize the winding inductance. This is achieved when a compromise has been reached in which the flux density in the teeth is 1.15 tesla, the density in the core is 0.8363 tesla, and the yoke flux density is 0.78 tesla.

3.5.5 BRUSHLESS MACHINE DRIVE

The machine drive consists of a polyphase, rotating field stator, a permanent magnet rotor, a rotor position sensor, and the electronic drive. During operation the electronic drive, according to the signals received from the rotor position sensor, routes the current in the stator windings to keep the stator field perpendicular to the rotor permanent field, and consequently generates a steady torque. Conceptually, the drive operates as the commutator of a DC machine where the brushes are eliminated. The main advantage here is that no current flow is needed in the rotor. As a result, rotor losses and overheating are minimal, the input power factor approaches unity and maximum efficiency is obtained. This is especially relevant in continuous duty applications, where the limiting factor of traditional induction drives is invariably the difficulty of removing rotor losses.

3.5.6 MOTOR DESIGN: FINITE-ELEMENT MODELLING

3D finite-element modelling (FEM) was not required, as the topology of the machine in x–y plane is the same along the axial length, except at each end where the end turns winding exists. However, a 2D finite-element model has been employed for the machine to calculate and analyse the flux distribution in it, Fig. 3.10. This is done to facilitate the rotor movement relative to the stator, so that the characteristics of interest such as the flux modulation due to slot ripple effect on the magnet and the rotor hub can be examined. To carry out this kind of analysis, several meshes have to be created, one for each rotor position, and then each solved in turn. The software program has a facility for coupling meshes, using Lagrange multipliers. This technique has been used to join the independent rotor and stator meshes at a suitable interface plane, a sliding Lagrange interface being placed in the middle of the air gap. The view at (a) shows a close-up view of the joined meshes for the machine, and in (b) is the rotor of the machine at 45° from base (half of the rotor mesh is missing for clarity).

The stator winding flux linkage waveforms of the machine have been calculated from the time transient solution, as the rotor speed is dynamically linked to the program, at 20 000 rpm. The experimental phase flux linkage has been deduced by integration of the phase EMF generated from the machine at no-load. These EMFs are shown to be within 8% difference, the value calculated by FEM being the higher. The flux in the air gap was measured using a search coil that is inserted on the stator side. From this search coil, a flux waveform was recorded and it is shown together within the flux calculated from FEM in (c). The flux plot, as contours and vectors at 0° rotor position for the machine, is shown at (d).

3.6 High fre�uency motor characteristics

In the 1970s motor designers were introduced to Bipolar Darlington transistors which permitted switching up to 2 kHz at mains voltage. In the 1980s insulated packaging was mastered and motor costs have been reduced. In the 1990s we have the IBGT which permits operation to 16 kHz for the first time at high power. This gives the designer a new freedom4. Hitherto the market sector has been dominated by 50 Hz machine designs. Now we can choose our operating point so the question must be asked: what is the optimum point and which is the best type of motor?

There is no simple answer to this question. We have several types of machine each with characteristics which are good in particular tasks. What is certain is that whatever type of machine is used, it can be made smaller than its 50 Hz counterpart by using a high frequency design. During the next ten years lies the challenge of the hydrogen economy with an increased demand for electric drives. IBGTs make new inverter topologies possible. The inverter on a chip in the back of the motor is now a reality.

3.6.1 HF MACHINE PROPERTIES

Motors designed for high frequency operation are of many types; however, they all share common design attributes. The 50 Hz motor designer will be used to the idea that at the full-load operating point copper loss = iron loss. This is not true for HF machines – iron loss dominates, accounting for up to 80% of the losses. Another factor is the power density which is in general 5–20 times greater. The use of HF windings means that the number of turns on a winding is reduced. So a high frequency motor can be expected to have much lower winding resistance and inductance than a 50 Hz machine.

For good loss management it is necessary to minimize the weight of core material. Generally, the flux density at the tooth is greater than in the main body of the core. It is common for all 50 Hz machines to use 2 or 4 pole windings; on HF machines, 8–32 poles are much more common.

Machines with a high pole number have a much smaller diameter build-up on the rotor; for a given stator OD the designer achieves a bigger rotor diameter which gives more torque and reduces stator mass. Machines with large numbers of poles are much easier to wind with only short winding overhangs. This is important because the overhangs contain the winding hot spots. See the example below.

Dl60 frame IM 380 V 50 Hz motor

1500 RPM 12 000 RPM
Power 11 kW 60 kW
Frequency Resistance Inductance (air cooled) 50 Hz 0.5 ΩL/L 2.5 mH (water cooled) 400 Hz 0.2 ΩL/L 400 mH
Stator flux density 1.5 tesla 0.75 tesla

Currently 500–1000 Hz represents the optimum operating point for stator iron. HF machines are very suitable for use in non-linear torque speed regimes because it is possible to operate at much higher flux densities at low speed. We therefore need to investigate vector control characteristics.

3.6.2 VECTOR CONTROL

Understanding of this subject has been delayed by years with torturous mathematical explanations of how it is achieved. In practice, vector control is a powerful technique because: (1) the full power of the stator controller can be brought to bear on the field system; (2) only a single winding set is involved. The stator current has two components: (a) field component Id and (b) real power component Iq. As these axes are at right angles, they may be independently controlled so long as the field is capable of supporting the demanded torque.

Vector control is nothing more than power factor control. The reactive element controls the field and the real power element controls the generated torque. In induction motors there is an added complication; there has to be slip between the rotor and stator to create rotor current for producing the field. This involves an axis transformation which makes for all the difficult mathematics. Synchronous motors are much easier; vector control only involves manipulation of phase shift.

Permanent magnet machines offer great flexibility because it is possible to manipulate the field with vector control currents. This has no damaging effect on the magnets so long as the material has a recoil permeability of unity (or a linear 2nd quadrant demagnetization curve) such as ferrite and samarium cobalt.

However, the level of ampere turns needed to control the field varies dramatically between different types of machines in accordance with the magnetic reluctance in the d-axis. It may be seen that this becomes more critical in HF machines which have smaller numbers of turns on the stator, for example a machine with four sets of windings per phase. If windings are arranged in star, Fig. 3.11a, generated back-EMF is 380 V at 2800 rpm, or by letting the circulating current at 100% field be 1 and rearranging the windings in parallel delta, as at (b), an alternative situation arises. Now 380 V is produced at 30 000 rpm and the current for 100% field increases to 4(3)1/2I or 6.82I. To give some idea, I is approximately 30 A for a 500 nm surface mounted PM magnet machine.

It may seem attractive to do away with the permanent magnets altogether. In practice this is not a good idea because the machine has a poor power factor and requires an oversize inverter. However, there is a variation on the concept which is possible, called the switched reluctance motor. This machine ignores Fleming’s LH rule and instead relies on the attraction forces between an electromagnet and soft iron. The problem is that the production of torque is not smooth; however, they are suitable for use in difficult environments.

3.6.3 OPEN LOOPS OR CLOSED LOOPS FOR INDUCTION MOTORS

In the early days of inverter drives, open loop operation of induction motors was the main objective. Generally this is satisfactory over a 10:1 speed range but is problematical at slow speed due to: harmonic torques; stability problems – especially with low load inertias; and lack of rotor cooling.

Vector control may be used to improve stability and can be applied on an open loop basis. To do this, estimates are used for the load inertia/rotor current and lead to errors where fast dynamics are involved. However, Jardin and Hajdu wrote one of the leading papers on this subject whilst developing the Budapest Tramcar drive system6.

As the motor frequency is increased, the low winding resistance makes eddy current losses, induced by DC circulating currents in the windings, a problem. Voltage source inverters without active balancing are unlikely to be satisfactory.

One machine which gives excellent performance on an open loop control is the buried PM motor developed by Brown Boveri/CEM/Isosyn and now also produced by GEC/Alsthom Parvex. Ken Binns at Liverpool University is a well-known authority in this area.

For fast dynamics and tight control there is no substitute for proper closed loop operation of a permanent magnet machine using vector control. Such an arrangement can give a constant power torque/speed range of 4:1 and this can be increased by using winding switching up to 70:1. Such systems are ideal for traction drives in vehicles with torque bandwidths of up to 1 kHz.

3.6.4 INDUCTION MOTORS

Of the various types of motor, Fig. 3.12, induction motors (IMs) are practical up to about 30 kW and 15 000 rpm. Beyond these limits exhausting the rotor losses is generally a problem (at 1500 rpm megawatt level machines are commonly constructed). At 15 kW IMs run satisfactorily up to 100 000 rpm but special motor construction techniques are needed to give strength to the cage. Water cooling is used at high power. A typical specification, for example, might be 36 kW, 12 000 rpm, PF 0.9 at 400 Hz, 36 kW – 4 pole: efficiency 0.9 at 400 Hz, 36 kW, 380 V, 68 A line current; slip (pure aluminium cage) 50 rpm cold, 70 rpm hot – torque 29 Nm (0.7 tesla), hot rotor diameter 6 in, active length 4 in, stator 9 in OD, peak torque at low speed 100 Nm (1.5 tesla), rotor cooling 8 CFM compressed air at 17 Psi, stator cooling 4 litres water/minute.

3.6.5 SURFACE MOUNTED MAGNET SYNCHRONOUS MACHINES

This design is first choice for high power drives. The rotor consists of a steel sleeve to which the magnets are glued and a containment band fitted on the outside. This fits inside a standard stator with water jacket. This design is practical up to 0.5 megawatts at up to 100 000 rpm and is used for traction drives.

Another benefit is that the output frequency is no longer related to shaft rpm and multipole designs/speeds over 3000 rpm may be considered. Using vector control, voltage and frequency may be separately controlled and much faster speed of response can be achieved. Many people are wary of PM designs because of concern about high temperature performance. The latest Nitromag alloys operate up to 250o C. These use nitrogen as the alloying element and are being investigated as part of the Joint European Action on Magnets Programme.

Most commercial motors use samarium cobalt of the 1/5 variety which has superior mechanical properties to the 2/17. Generally speaking, alloys of 20 MGO are in common use and the trick is to design rotors around standard size blocks, 1 × 1/2 × 6 inches thick. Modern high coercivity magnets need very large currents to demagnetize the magnets and typically 3 tesla are needed to achieve full initial magnetization for about 1 millisecond.

Electric motor and drive-controller design 71

ial INDUCTION MOTOR icl CLAW TYPE PM MAGNET MOTOR

AIR GAP 0.4 MM CLAW MODULES SHORTING

POLYPHASE RINGS

(4 OFF) PERMANENT

STATOR WINDING POLYPHASE MAGNET

ROTOR SHAFT

SHAFT

ROTOR BARS

SHORTING RING STATOR

POLE STACK

CLAW SOUTHROTOR POLE

LAMINATIONS CLOSED LOOP

CLAW

i.l SURFACE MOUNTED PM MAGNET MOTOR idl UNIO TYPE MOTOR iContentional constructionl

FLUX RETURN MAGNETS FLUX

RETURN PERMANENT MAGNET

STATOR

ROTOR

STATOR

SLEEVE PERMANENT MAGNET

SHAFT

FLUX

RETURNMAGNETS

POLYPHASE STATION (PRINTED CIRCUITS)ROTOR POLYPHASE SLEEVE WINDING

CLOSED LOOP

iel BURIED MAGNET MOTOR i4 POLEl igl HOMOPOLAR MACHINE

HIGH RELUCTANCE

STATOR SOUTHPATH FIELD COIL WINDINGS ROTOR POLYPHASE

POLYPHASE

ASSEMBLY

WINDING

STATOR

CAGE FOR STARTING ROTOR

STATOR

N

S

LAMINATION

ROTOR

ROTOR SHAFT

BURIED SHAFT (non-magnetic) PERMANENT

SHAFT MAGNET NORTH

STATOR

ROTOR ASSEMBLY STARTING/DAMPING

FIELD COIL CAGE

CAGE OPEN LOOP OR CLOSED LOOP

OPEN LOOP OR CLOSED LOOP

ifl SWITCHED RELUCTANCE MOTOR ihl RELUCTANCE MOTOR

i8 STATOR POLES. 6 ROTOR POLESl

STATOR POLES WINDING

POLYPHASE WINDING

WINDING

SOLID STEEL ROTOR

STATOR

ROTOR

SHAFT

FLUX SHAFT

RETURN

DAMPING CAGE

ROTOR POLES

ROTOR

C12 345678

OPEN LOOP OR CLOSED LOOP

CLOSED LOOP

Fig. 3.12 Motor types.

Typical machine specification for 60 kW, 10 000 rpm (surface mounted) would be: stator OD 10 in, rotor diameter 7 in, active length 3 in; operating point 0.7 tesla at 666 Hz, 8 poles, 380 V, 103 A, efficiency 0.97, power factor 1; winding resistance 0.015 Ω L/L, winding inductance 300 mH L/L; iron loss 1.5 kW at 666 Hz, core Transil 270 0.35 mm non-orientated; load torque 57 Nm, peak torque 150 Nm, vector control current 100 amps for 0.7 tesla.

3.6.6 IRONLESS PM SYNCHRONOUS MOTOR

This machine has been developed by UNIQ (USA) for hub mounted motors for use in electric vehicles. It consists of a machine with both an internal and external rotor which are mechanically linked and a thin stator winding which is usually fabricated using printed circuit techniques. The result is a lightweight machine with a very high power density and low winding inductance since there is no stator iron. Performance is largely determined by the quality of permanent magnet used. The d-axis reluctance is high due to the double air gap so that the currents needed for vector control can be large compared with a conventional PM machine. Such machines have been built up to 40 kW rating at 7500 rpm with epicyclic speed reducers that are wheel-mounted.

At present such machines are costly to manufacture because of the large amount of PM material involved, which has to be of the cobalt/neodynium variety to achieve good performance. Losses are all due to stator copper which is generally operated at extremely high current density to give a very thin stator.

3.6.7 WOUND ROTOR SYNCHRONOUS MACHINE WITH BRUSHLESS EXCITATION

This machine is sometimes used for inverter drives in addition to the well-known use as an electricity generator. The presence of the exciter/rectifier means that this solution is applied at higher powers. The rotor can be salient pole or of surface slot construction at high speed. Whichever solution is chosen, the full field thermal loss in the motor is significant and a particular problem if the machine is to be run slowly at high load torques. This type of machine is used in traction drives using thyristor-based converters.

ROTATING

SHAFT

3.� Inno�ati�e dri�e scheme for DC series motors

Many DC brushed motor drive schemes for EVs use a DC shunt motor and it has been suggested that such a solution is the most appropriate5. This section investigates an alternative solution. There are many railway locomotives which successfully use series wound motors and we hope to establish that indeed this is the best solution for electric vehicles.

3.7.1 MOTOR DRIVES: WHY CHANGE THE SYSTEM?

Because the system is already subject to change brought about by new requirements and developments. First, we have the introduction of sealed battery systems. These will permit much higher peak powers than hitherto possible and consequently will run at high voltages. 216 V DC is a common standard working with 600 V power semiconductors. Second, we have the introduction of hybrid vehicles. This will result in the need for drives and motors to operate for long sustained periods – previously batteries did not store enough energy. Third, the DC series motor has the right shape of torque–speed curve for traction, constant power over a wide speed range. Fourth, DC series field windings make much better use of the field window than high voltage shunt windings where much of the window is occupied by insulation. The series field winding is a splendid inductor for use in battery charging mode. Losses in series mode are significantly reduced.

Tor�ue S�eed Cur�e

1250 rpm 5000 rpm

An example specification is typified by the Nelco N200, Fig. 3.15(a), which compares with a 240 mm stack, Fig. 3.15(b):

Shunt field Series field

N = 227 N = 12

Hot resistance 7 Ω Hot resistance 0.014 Ω

Watts 700 at 10 A Watts 500 at 189 A

So why hasn’t somebody attempted to use series motors in EVs before? They have for single quadrant low voltage systems but not on multi-quadrant, high voltage schemes. This account proposes a new control concept akin to vector control for AC machines. We will show how it is possible to achieve independent control of field current If and armature I a, with very fast response, using a transistor bridge.

3.7.2 VEHICLE DYNAMICS AND MOTOR DESIGN

A vehicle represents a large inertia load with certain elements of resistance some of which increase with speed; see Chapter 8. For a small family car, mass = 1250 kg at 60 mph (26.8 m/sec) typical cruising speed. Windage accounts for 6 kW, rolling resistance 2 kW and brake drag 2 kW, a total of 10 kW in steady state conditions. Windage varies as the 3rd power of vehicle relative velocity with respect to the wind.

Kinetic Energy = 1/2 MV2, where M = mass = 1250 kg and V = velocity in metres/sec. So we have:

SPEED (MPH) 10 20 30 40 50 60 70 80
(m/sec) 4.5 8.9 13.4 17.8 22.3 26.7 31.2 35.6
KE (kilojoules) 12.5 49.5 111 198 309 446 607 792

What this illustrates is that recovered energy below 20 mph is small, consequently regeneration only matters at high speed. It also illustrates that the inertia load, not the static resistance, is the main absorber of power during acceleration.

3.7.3 MOTOR CHARACTERISTICS

These are shown in the following table:

Voltage 216 V Rated power 45 kW, 1250–5000 rpm

Frame D 200 M-4 pole with interpoles Weight 170 kg

3 1 2

(a) (b)

Fig. 3.15 Field windings: (a) shunt field machine; (b) 3 state strategy for series field machine.

Cooling air forced, separate fan Winding, series field 245 A/216 V full load Efficiency at full load 85% Field Resistance 10 milliohm, inductance 1.2 mH Armature Resistance 30 milliohm, inductance 260 mH inc. brushgear interpoles Dimensions A = 490 mm, B = A + shaft, C = 335 mm, D = 350 mm; see Fig. 7.14

This illustrates that when the field current is strengthened in the constant power region, the armature voltage can be made to exceed the battery voltage and regenerative braking will take place. Below 1250 rpm plug braking must be used; however, the energy stored at this speed is small.

3.7.4 SWITCHING STRATEGY (SINGLE QUADRANT), FIG. 3.15

Figure 3.15(a) shows the arrangement for a 216 V, 45 kW shunt field machine with separate choppers for field and armature. There are some disadvantages with this scheme: (a) field is energized when not needed; (b) forcing factor of field is small – for a 45 kW shunt field, R = 7 ohm, I = 10 A nominal, L = 1.2 henries, t = 0.17 seconds; (c) when extended to multi-quadrant design two bridge chopper systems are needed if contactor switching is to be avoided; (d) extensive modifications are needed to provide for high power sine wave battery charging; (e) field power losses are significant (3 kW at max field).

Figure 3.15(b) illustrates the proposed new circuit which has a single 3 state switch: state (1) open-circuit; state (2) armature + series field; state (3) armature. So as an example, consider the

following situation:
Full load torque at standstill Field voltage for 245 A Armature voltage for 245 A = 2 V = 16 V D9

Ef

Fig. 3.16 Three state circuit expanded to 4 quadrant operation.

FORWARD BRAKING FORWARD MOTORING
Ia Ea Ia Ea
If If
Ef Ef
REVERSE BRAKING REVERSE MOTORING
Ia Ea Ia Ea
Ef If Ef If

so with 216 V battery: D = 2/216 in state 2 D = 16/216 in state 3

The balance of the time will be off (D = duty cycle ratio for chopper). It can be seen that by manipulating the relative times spent in each of the states, separate control of field and armature currents may be exercised. When the speed of the motor exceeds the base speed (1250 rpm) the back-EMF is equal to the battery voltage and the switch henceforth operates only in states (2) and (3). Let D = duty cycle for single quadrant chopper, then V /V = D, hence

out in

D(V5KωI IR L dI/dt) = I R + LdI/dt and

2BAfaa aaffff

V 5= (KωI + IR + L dI/dt) × (D+ D)

BAfaaaa23

where ω = motor speed, rads/sec VB = battery voltage KA = armature back-EMF constant V/amp/rad/sec (D2 + D3) D2 = duty cycle state 2 D3 = duty cycle state 3

Other symbols are self-explanatory.

3.7.5 MULTI-QUADRANT STRATEGY

Figure 3.16 illustrates the 3 state circuit when expanded to 4 quadrant operation: state 1 is all switches off; state 2 either S/S or S/S on and state 3 is either S/S or S/S on. As is clear, the

l423l234

third state is produced by having a controlled shoot-through of the transistor bridge. It may be considered that with two transistors and two diodes in series, voltage drops in the power switching path make the circuit inefficient. In fact with the latest devices: V for switches = 1.5 V at 300

ce sat

A; Vf for diodes = 0.85 V at 300 A, giving a total drop = 4.7 V. So (4.7/216) × 100 = 2.3% power loss.

When the motor loses 15% this is a small deficiency. It represents 1.2 kW at full power. As the table illustrates in Fig. 3.16, all states of motoring and braking can be accommodated. The outstanding feature of this scheme is that the full power of the armature controller can be used to force the field, giving very fast response. From Fig. 3.16, it will be seen that the 4 quadrant circuit

220/240 V AC MAINS

BATTERY

SHUNT CHOPPER

Fig 3.17 4 quadrant circuit.

consists of a diode bridge D–D and a transistor bridge S–S (D–D). D acts as a freewheel diode

l4l4589

when the transistor bridge is operated in shoot-through mode. Bridge Dl/D4 is required because the direction of armature current changes between motoring and braking. Control in braking mode is a two-stage process. At high speed the armature voltage exceeds the battery voltage and the battery absorbs the kinetic energy of the vehicle. At low speed the field current is reversed and plug braking of the armature to standstill is achieved via D9.

3.7.6 DEVICE PROTECTION IN A MOTOR CONTROLLER

Switches S1–S4 form a bridge converter and the devices require protection against overvoltage spikes from circuit inductances. The main factors are: (1) minimize circuit inductances by careful layout. The key element is the position of D9 and associated decoupling capacitor relative to Dl–D4; (2) fit 1 mF of ceramic capacitors across the DC bridge S1 /S4 plus varistor overvoltage protection.

Dl–D4 can be normal rectification grade components but D9 must be a fast diode with soft recovery. D5–D8 are built into the transistor blocks.

3.7.7 SINE WAVE BATTERY CHARGER OPERATION

With little modification the new circuit, Fig. 3.17, can be used as a high power (fast charge) battery charger with sine wave supply currents. The circuit exploits the series field as an energy storage inductor. Sl and D6 are used as a series chopper with a modulation index fixed to give 90% of battery volts. This creates a circulating current in the storage inductor. Switch S4 and diode D7 function as a boost chopper operating in constant current mode and transfer the energy of the storage inductor into the battery. Charging in this manner is theoretically possible up to 250 amps but will be limited by: (a) main supply available and (b) thermal management of the battery.

C1 100 µF

DV/DT FILTER

ARMATURE OF MOTOR

CONTACTOR K1 K2 K3 K4 K5
MOTORING 0 C 0 C 0
BATTERY CHARGER C 0 C 0 C

Experience shows that charging at 30 amps is possible on a 220 V, 30 A, USA-style house air conditioning supply. Charging at greater currents will require special arrangements for power supply and cooling. One advantage of the scheme presented is that it may be used on any supply from 90 V to 270 V.

It is also possible to adopt the circuit for 3 phase supplies in one of two ways: (1) add an additional diode arm – this would produce a square wave current shape on the supply; (2) fit a 3 phase transistor bridge on the supply – this would permit a sine wave current in each line at a much increased cost.

3.7.8 POWER DIAGRAM FOR MOTORING AND CHARGING

Figure 3.18 presents the combined circuit diagram for motoring and battery charging. Reservoir capacitors and mode contactors have been added. The capacitors function as snubbers when running in motoring mode. As drawn, to adapt to battery charging, the battery plug is moved to outlet D and the mains inserted into plug B, alternatively contactors could be used to do the job. Battery safety precautions comprise: (1) the battery is connected via a circuit breaker capable of interrupting the full short-circuit current of a charged battery; (2) this circuit breaker is to contain a trip to disconnect battery by mechanical means only; (3) battery/motor/controller are each to contain ‘firewire’ to disconnect the circuit breaker; (4) circuit breaker is to be tripped by ‘G’ switch when 6G is exceeded in any axis.

3.7.9 CONTROL CIRCUIT IN MOTORING MODE

Figure 3.19 shows the block diagram of the controller for motoring mode. The heart of the system is a memory map which stores the field and armature currents for the machine under all conditions

PWM CLOCK BATTERY VOLTAGE OSCILLATOR FEEDBACK If FEEDBACK

ENGINE BRAKING

If SHUNT CHOPPER

I OIL REF V BATT V TEMP

Fig. 3.20 Block diagram of battery charging controller.

References

  1. Hodkinson, R., Operating characteristics of a 45 kW brushless DC machine,EVS 12, Aneheim, 1995
  2. Hodkinson, R., Towards 4 dollars per kilowatt, EVS 13, Osaka, 1996
  3. Al’Akayshee et al., Design and finite element analysis of a 150 kW brushless PM machine, Electric Power Transactions, IEEE, 1998
  4. Hodkinson, R., The characteristics of high frequency machines, Drives and Controls Conference, 1993
  5. Hodkinson, R., A new drive scheme for DC series machines, ISATA 24, Aachen, 1994
  6. Jardin and Hajdu, Voltage Source Inverter with Direct Torque Control, IEE PEPSA, 1987
Further reading

Alternative transportation problems, SAE, 1996 The future of the electric vehicle, Financial Times Management Report, 1995 Battery electric and hybrid vehicles, IMechE, 1992 Electric vehicle technology seminar report, MIRA, 1992 Electric vehicles for Europe conference report, EVA, 1991

4

Process engineering and control of fuel cells, prospects for EV packages

4.1 Introduction

The first three sections of this chapter will give a history of fuel cells; describe the main types of fuel cells, their characteristics and development status; discuss the thermodynamics; and look at the process engineering aspects of fuel-cell systems. It is based on a series of lectures given by Roger Booth to undergraduates at the Department of Engineering Science at the University of Oxford, under the Royal Academy of Engineering Visiting Professor Scheme in 1999. The assistance of Dr Gary Acres of Johnson Matthey in preparing this chapter is greatly appreciated.

The remaining sections deal with the control systems for fuel cells that turn them into ‘fuelcell engines’ and considers the problems of package layout for all EVs as an introduction to the package design case studies reviewed in the following two chapters.

4.1.1 WHAT IS A FUEL CELL?

The easiest way to describe a fuel cell is that it is the opposite of electrolysis. In its simplest form it is the electrochemical conversion of hydrogen and oxygen to water, as shown in Fig.

4.1. Hydrogen dissociates at the anode to form hydrogen ions and electrons. The electrons flow through the external circuit to the cathode and the hydrogen ions pass through the electrolyte to the cathode and react with the oxygen and electrons to form water. The theoretical electromotive force or potential of a hydrogen–oxygen cell operating at standard conditions of 1 atm and 25oC is 1.23 V, but at practical current densities and operating conditions the typical voltage of a single cell is between 0.7 and 0.8 V. Commercial fuel cells therefore consist of a number of cells in series.

4.1.2 TYPES OF FUEL CELL

Fuel cells are described by their electrolyte:

Alkaline – AFC

Phosphoric acid – PAFC

Solid Polymer – SPFC (also referred to as proton exchange membrane – PEMFC)

Molten carbonate – MCFC

Solid oxide – SOFC.

Single cell 1V

The reaction shown in Fig. 4.1, with hydrogen ion transfer through the electrolyte, is only applicable to fuel cells with acid electrolytes and solid polymer fuel cells. The reactions in each of the fuel cell types currently under development1 are:

Cell Anode Cathode
AFC H2 + 2OH——> 2H2O + 2e O2 + 2H2O + 4e-——> 4OH
PAFC H2 ——> 2H+ + 2e 4e+ 4H+ + O2 ——> 2H2O
SPFC H2 ——> 2H+ + 2e 4e+ 4H+ + O2——>2H2O
MCFC H2 + CO3 = ——> H2O + CO2 + 2e O2 + 2CO2 + 4e-——> 2CO3 =
CO + CO3 =——> 2CO2 + 2e
SOFC H2+ O= ——> H2O + 2e O2+ 4e——>2O=
CO + O= ——> CO2 + 2e
CH4 + 4O= ——> CO2 + 2H2O + 8e
4.1.3 HISTORY

The concept of the fuel cell was first published in 1839 by Sir William Grove when he was working on electrolysis in a sulphuric acid cell. He noted a passage of current when one platinum electrode was in contact with hydrogen and the other in contact with oxygen. In 1842 he described experiments with a stack of 50 cells, each with one quarter of an inch wide platinized platinum electrodes and he noted the need for a ‘notable surface of action’ between the gases, electrolyte and electrodes. Over the next 90 years a number of workers published papers on both acid and alkali fuel cells, including the development of three dimensional electrodes by Mond and Langer in 1889. But it was not until 1933, when F. T. (Tom) Bacon (an engineer with the turbine manufacturers C. A. Parsons & Co. Ltd.) started work with potassium hydroxide as the electrolyte and operating at 200°C and 45 atm, that significant progress was made. The main thrust for development of fuel cells was the space programme of the early 1960s, when NASA placed over 200 contracts to study and develop fuel cells. The first major application was the use of solid polymer fuel cells developed by General Electric for on-board power in the Gemini programme. By 1960 Bacon had transferred to the Pratt and Whitney Division of United Aircraft Corporation (now United Technologies Corporation) in the USA, and led the development of the on-board power system for the Apollo lunar missions. Ninety-two systems were delivered and 54 had been used to power nine moon shots by 1965. This was followed by UTC’s development of a 7 kW stack which is used in the Space Shuttle. During the 1990s fuel-cell development accelerated, with particular interest in automotive and small distributed power systems as the main target applications2,.3,7.

4.2 Reforming and other hydrogen feedstocks

The prime source of hydrogen for fuel cells is by reforming a hydrocarbon, particularly natural gas. The hydrogen production can be carried out either at a central large-scale facility, or immediately upstream of the fuel cell, including on board for transport applications.

Steam methane reforming is the main commercially applied process for hydrogen production, with an annual global production capacity in excess of 40 million t/a and single units with capacities greater than 400 t/d. Overall thermal efficiencies (LHV) are over 65% and there is the potential to exceed 70%. The process is carried out over a nickel catalyst at temperatures of about 800°C on a sulphur-free feedstock. The main reaction is:

CH4 + H2O = CO + 3H2 + 205.1 kJ

with the endothermic heat of reaction provided by combustion of natural gas (note this chapter follows the convention of enthalpy changes as +ve when energy is absorbed by the system).

Partial oxidation is also applied on a significant commercial scale with both natural gas and liquid hydrocarbons as feedstock:

CH4 + (1/2)O2 = CO + 2H2 37.7 kJ

Both catalytic (nickel based) and non-catalytic processes are applied.

Autothermal reforming processes are being developed which combine steam methane reforming with partial oxidation, the latter providing the energy for the former. This application is of particular interest to developers of on board reforming systems for road vehicles.

The reactions are significantly more complex than given above and one of the main problems is caused by the Bouduoard reaction:

2CO = CO2 + C

which results in soot production and lower hydrogen yield. Water gas shift reaction is used to increase the yield of hydrogen from all the above processes9

CO + H2O = CO2 + H2 41.9 kJ

Hydrogen from electrolysis is also a potential fuel, but unless the electricity is produced from solar energy, the overall process is not attractive (second law of thermodynamics).

Feedstocks other than natural gas can also be used for hydrogen production.

Coal was of particular interest at the turn of the century via partial oxidation (gasification) and water gas shift:

(CH) + (1/2)O = CO + 0.4H+ HO = CO + 1.4H

0.822 222

Biomass, particularly from sustainable plantations, can also be used by gasification plus shift:

(CHO) ——> CO + H+ HO = CO + 2H

22 222

Methanol has been of interest for a number of years, both via reforming and in the direct Methanol fuel cell (DMFC), where most current developments focus on a variant of the SPFC. Basic reactions are:

anode: CH3OH + H2O ——> CO2 + 6H+ + 6e

cathode: (3/2)O2 + 6H+ + 6e-——> 3H2O

Gasoline (and diesel) on-board autothermal reforming is being developed for transport applications, as it would mean fuel-cell powered vehicles could make use of the existing fuel infrastructure. Fuel cells operating on a host of exotic fuels, including ammonia and hydrazine, have been studied, but the above are the most likely fuels to be used on a large commercial scale.

4.� �haracteristics� ad�antages and status of fuel cells

The main characteristics of fuel cells, arranged in ascending order of operating temperature, are given in the following table:

Type Electrolyte Typical °C Electrode Status Efficiency
AFC KOH 80 Pt on C 100 kW > 50
SPFC Polymer e.g. ‘Nafion’ 85 Pt on C 250 kW 45 – 55
PAFC Phosphoric Acid 200 Pt on PTFE/C 0.2–11 MW ~ 40
MCFC Li or K Carbonate 650 Ni or Ni alloy 2 MW 50 – 60
SOFC Yttria stab’d zirconia 1000 Ni/zirconia cermet 100 kW 50 – 55

The quoted status and efficiencies are typical of systems in 1999 and are based on electricity as a percentage of the lower heating value of hydrogen consumed. Efficiency is discussed in more detail in the next section.

The alkaline fuel cell has the advantages of using cheap materials; quick to start and high power density. The main disadvantages are the corrosive electrolyte and the intolerance to carbon dioxide

current limits are about 50 ppm, which means CO2 removal would be required for transport applications. The technology has been proven for space applications by IFCC (International Fuel Cells Corporation), which is a joint venture between UTC and Toshiba, and one developer (ZEVCO, who took over the work of the Belgian company Elenco) has developed a London taxi.

The phosphoric acid fuel cell has the main advantages of CO2 tolerance (up to about 20%) and moderate CO tolerance. The main disadvantages are the cost of the noble metal catalyst, lower efficiency and a current system cost of about $3000/kW. PAFC is the most commercially advanced fuel-cell technology, with over 140 ONSI (a division of IFC) 200 kW systems in operation for distributed power systems. In addition, Tokyo Electric Power have an 11 MW system, supplied by IFC and constructed by Toshiba, which started operation in 1991. Other developers include Fuji, Hitachi and Mitsubishi Electric.

The solid polymer fuel cell has many advantages, including solid non-corrosive electrolyte; quick start; long life; produces potable water; ease of volume manufacture. The main disadvantages are the use of noble metal catalyst and intolerance to sulphur and carbon monoxide. In recent years development has focused successfully on reducing the platinum loading (now less than 10% of the loading in the late 1980s) and improving tolerance to CO, which is now up to about 50 ppmv. A further disadvantage is the limited scope to use waste heat which results from the low operating temperature. The SPFC is the leading contender in the automotive market and has potential in the cogeneration and battery replacement markets. Consequently the list of active companies is large and includes Ballard, Alstom, IFC, Toyota, Plug Power, Dais Corporation, Warsitz Enterprises, Advanced Power Sources, Siemens, DeNora, Sanyo.

The Molten Carbonate fuel cell is being targeted at the large scale decentralized power generation and industrial Combined Heat and Power (CHP) markets and has the advantages of being able to support internal reforming of natural gas and produces high grade heat for CHP applications. The main disadvantages are the stability of the electrolyte; poisoning by sulphur and halogens; and there are questions about the achievable operating life. The largest unit is the 2 MW Energy Research Corporation facility commissioned in 1997. M-C Power have demonstrated at the capacity of 250 kW and have announced plans for commercial production at the rate of 2–3 MWpa, targeting customers needing between 500 kW and 3 MW CHP systems. Other developers include IFC, IHI, MTU, Ansaldo (100 kW demonstration) and ECN.

The solid oxide fuel cell has similar advantages to the MCFC, but has easier electrolyte management and is less corrosive. The main disadvantage is the high cost, the 100 kW Westinghouse unit in the Netherlands which was delivered in 1998 is reported to have cost $10 million. Development is not as advanced as with the PAFC and MCFC, but active programmes are being carried out by Westinghouse, Sulzer, Siemens, Sofco, Mitsubishi, Eltron, Zetek, Global Thermoelectric amd Ceramic Fuel Cells.

Automotive company activities were triggered by the Californian legislation for, so-called zero emission vehicles (ZEVs) which were seen as being required to improve urban air quality. With the exception of Zevco who are using AFCs, all other manufacturers’ activities are based on SPFCs and are summarized in the following table.

Company Comments
Daimler-Chrysler Probable leader (Necar I-IV); Joint venture with Ballard
(Daimler Benz/Ballard), Ford, Mazda, Shell
Mazda In-house cell development, metal hydride storage, but now with
Daimler-Benz/Ballard
Toyota In-house, hybrid vehicles both H2 (hydride storage) and MeOH
Honda In-house programme, also used Ballard
Nissan In-house programme, also used Ballard
Ford PNGV; cooperating with Plug Power and Daimler-Benz/Ballard
GM PNGV; cooperating with Ballard and Arco/Exxon for reformer
Chrysler AD Little gasoline reformer; Plug Power cell, Merged with
Daimler-Benz
Renault/PSA/Volvo/VW Joule programmes with DeNora cell

There is also development work on marine applications using SPFC, SOFC and MCFC technology3,6,9,10.

4.4 �hermodynamics of fuel cells

The efficiency of the heat engine is limited by the Carnot cycle and equals: (Th Tl)/Th A Carnot efficiency of about 70% could be achieved theoretically with upper and lower temperatures of 1000K and 300K, which would require a compression ratio of about 20:1. However,

fuel and material restrictions limit the practical efficiency to about 50%, which is achieved by modern, large, low speed diesel engines, but automotive gasoline and diesel engines achieve much lower efficiencies, particularly when averaged over a standard driving cycle.

A true direct energy conversion device is one which can convert the Gibbs free energy of a chemical reaction directly into work. A fuel cell converts the Gibbs free energy of a chemical reaction into a stream of electrons under isothermal conditions. The change in Gibbs free energy of a reaction is given by:

ΔG = H TS

rr r

Fuel-cell reactions which have negative entropy change (e.g. H2 + (1/2)O2 = H2O) generate heat and those with positive entropy change (e.g. CH + 3.5 O = 2CO + 3HO and CHOH + 1.5O =

2622232

CO2 + 2H2O) extract heat from the surroundings. For a fuel cell operating at constant temperature and pressure, the maximum electrical energy is given by the change in Gibbs free energy:

Wel = −ΔG = nFE (1)

Where n = the number of electrons in the reaction, F = Faraday’s constant (96 500° C/equivalent) and E = the reversible potential. If all reactants are at standard conditions of 1 atm and 25oC:

ΔGo = nFEo (2)

For the reaction

H2(g) + (1/2)O2(g) = H2O(l)

the Gibbs free energy change12 is 237 kJ, n = 2, and therefore the maximum reversible potential, Eo = 1.23 V. The maximum reversible potential under actual fuel-cell operating conditions can be calculated from the Nernst equation. For the general reaction:

aA + bB = cC + dD

The free energy change can be expressed:

ΔG = ΔGo + RT ln([C]c[D]d/[A]a[B]b)

Substituting equations (1) and (2) gives:

E = Eo + (RT/nF) ln([A]a[B]b/[C]c[D]d

For the hydrogen/oxygen fuel cell this can be simplified1 to:

1/2]

E = Eo + (RT/2F) ln[PH/PHO] + (RT/2F) ln[PO

222

Normal practice for conventional power generation is to use the thermal efficiency, expressed as the electrical output as a percentage of the heat of combustion of the fuel. It is common practice in Europe to use the lower heating value (LHV) or lower calorific value (LCV) (water as gas), whereas in the United States it is common practice to use the higher heating value (HHV) or Gross Calorific Value (GCV) (water as liquid). The heat of combustion is equal to −ΔH, the change in enthalpy. The thermal efficiency of a fuel cell is given by:

Gibbs free energy converted to electricity Thermal efficiency = Enthalpy change ( heat of combustion)

For a hydrogen/oxygen fuel cell with liquid water as product, the Gibbs free energy change is 237 kJ per mole, equivalent to 118.5 MJ/kg hydrogen and the higher and lower heats of combustion are2 142.5 and 121.0 MJ/kg, resulting in maximum theoretical efficiency of 83% basis HHV which most accurately represents the reaction, but for comparison to conventional power systems, would be equivalent to 98% basis LHV.

A number of definitions of fuel-cell efficiency are used in the literature, without always stating which the author means, and care is therefore required in applying the information. The voltage efficiency being one of the most frequently used:

ηE = EI/EO

Where EI is the cell potential at current I and EO is the open circuit at the cell operating conditions. If a fuel cell operated reversibly, then the efficiency would be:

η rev = ΔG/ΔH

Where ΔG and ΔH are the changes in Gibbs free energy and enthalpy (both ve). As a fuel cell does not operate reversibly, the efficiency is given by:

η = −nFEI/ΔH

Rearranging and multiplying numerator and denominator by ΔG/ΔH gives:

η = (ΔG/ΔH)EI/{(ΔG/ΔH)(ΔH/nF)}

1.5

1.0

Reversible cell potential (e)

Ideal current – potential

Cell potential, V

Losses due to activation overpotential (lack of electrocatalysis)

1.0

Linear decrease due to ohmic losses

efficiency

0.5

Rapid decrease due to

0.5

mass transport losses

Fig. 4.2 Voltage/current relationship for hydrogen/oxygen cell.

0 0.5 1.0
Current density, A cm-2
Fig. 4.3 SPFC characteristics.

As η rev = ΔG/ΔH and ΔG = nFEO, then:

η = η (E/E) = ηη

revIOrevE

As the current efficiency (ηI) is not unity and there are further system inefficiencies due to feed pretreatment, cooling, gas recycle, inversion from DC toAC, etc. (η s), the overall thermal efficiency is:

η = η ηηη

revEIs

Figure 4.2 shows11 the potential versus current relationship for a typical hydrogen–oxygen cell. A further reduction in achievable efficiency results from less than total hydrogen utilization and use of air rather than oxygen.The contribution of cathode activation overpotential, membrane, ohmic electrode and anode activation losses for a specific SPFC are shown11 in Fig. 4.3.

The above figures show why considerable R&D is being focused on improving the cell voltage efficiency ηE via improved electrocatalysis. Further extensive development is being made to improve the system efficiency η s by improved design of the stack and the ancillary systems. Overall thermal efficiencies of about 50% can be achieved at current densities of 0.7 A cm2 with state of the art SPFC stacks.

One major advantage of fuel cells is the lower sensitivity of efficiency to scale than is achievable with thermal power systems, which means that fuel cells in the kW range have very similar overall system efficiencies to those in the MW range. A further benefit is the relatively flat efficiency versus load curve compared to internal combustion engines.

4.5 Process engineering of fuel cells

The discussion in the last section refers to the system efficiency η s and an indication of the engineering role in the development of a practical fuel cell system can be seen in Fig. 4.4, which

Stack

Cell

Transmission electron micrograph
Field Flow Plate of a Johnson Matthey catalyst
showing platinum particles
distributed across the carbon
support material
Anode Membrane Cathode

Fig. 4.4 PEM fuel cell.

shows how a single SPFC cell is designed to ensure even distribution of the reactants over the electrodes and the importance of catalyst distribution. The figure also shows a group of cells built into a small stack, but before addition of the manifolding for the supply of reactants and collection of products from the stack. An indication of the pipework complexity is shown in Fig. 4.5, which is the Ballard 240 kW system used in a bus.

During the 1990s Ballard and Daimler-Benz improved the system design to the extent that in the Necar I, the whole of the normal load carrying space of the van was filled with the fuel cell system, but in the small car, called Necar III, the system takes little more space than in the internal combustion powered version. Necar IV is based on the A-class Mercedes, with space for five passengers and luggage.

Standard test conditions used by Ballard for SPFC stack systems are a pressure of 3 atm, stoichiometry of 2.0 for air and 1.5 for hydrogen. Their MK 513 stack is reported 9 to give 13 kW at 0.58 V and a power density of 300 W/kg and 300 W/l. The same reference quotes the performance of the stack developed for Daimler-Benz as achieving 32.3 kW at 0.68 V, and power densities of 1000 W/l and 700 W/kg, which meet the target set in 1990 by the US DOE for automotive fuel cells of 560–1100 W/kg.

The major challenges for fuel cells are (a) to develop systems which require similar mass and volumes to the engine plus fuel tank of current gasoline or diesel cars, at competitive prices when mass produced and (b) development of the required fuel infrastructure.

4.� �teps to�ards the fuel�cell engine

Earlier sections of this chapter, contributed by Roger Booth, have dealt with process engineering of the fuel-cell stack. Hereafter the steps leading to the development of viable fuel-cell engines are considered. While hybrid drive vehicles, using conventional battery-electric and thermal-engine power sources, provide improved fuel economy and a viable solution for urban operation, the fuel-cell powered vehicle is now seen as the long-term option. Already it is realized that thermal-engine driven vehicles can never provide the necessary fuel economy and emissions control required by world governments, primarily because the thermal engine is running at 10% of its total power potential for most of its time and there is no known way of eliminating CO2 emissions from it. Over the next 20 years, people using HC vehicles are going to face increasing fuel scarcity, increasing fuel cost and ever increasing restrictions on use and size of vehicles, because of the emissions they produce. The high operating efficiency and zero emission characteristic of the fuel-cell vehicle are strong arguments for its adoption. But the passage into the period of the hydrogen-fuel economy has to be a gradual and peaceful one, requiring considerable changes in attitude by the motoring publics worldwide.

4.6.1 FUEL CELLS: TOWARDS A PAINLESS TRANSITION

The only way to break out of the cycle of increasing fuel costs and heavier restrictions is for motorists to accept the necessity to move over to a hydrogen-fuel economy in the shorter term and to lessen its impact by replacing their current vehicles, when the time comes, with much more fuel-efficient types. There are two advantages to the hydrogen economy: if early hydrogen-fuelled vehicles are not very efficient it does not matter because pollution-wise they have zero emissions, and more importantly, the existing cost of expensive exhaust after-treatment is removed.

The publication by the OECD International Energy Agency in 1998 of ‘World Energy Outlook’, a year after the Kyoto Earth Summit, was a pivotal point in understanding world energy and pollution problems. The basic message was that, as people now live longer, energy usage and pollution rise exponentially and a ‘brick-wall situation threatening in ten years’ time means that we cannot stay as we are’. It was also from this point that the major G7 economies took global warming seriously. China, too, takes it seriously, realizing that nine-tenths of its population live in its southeastern corner delta region, which could be subject to flooding if global warming is not seriously addressed. World oil supply is expected to peak in 2010 and then tail off; this is unless the cost can be met of tapping into the vast oil deposits beneath the polar icecaps. But North American motorists remain blissfully unaware of these threatening situations. While the UK and Europe pay what gasoline and diesel actually costs, in total, US users have an enormous effective subsidy which hides the governmental costs, and keeps them oblivious to the problems involved. Fuel/vehicle taxes in the UK pay for road building, health care related to accidents, also defence costs relating to naval protection of oil rigs, whereas in the USA the petrol price paid at the pumps is the direct cost of the fuel at world market prices. Even the cost of the US fleet in the Middle East is reckoned by some to be equivalent to a subsidy of $1/gallon and of course there is no contribution to the health-care costs associated with exhaust pollution. In the US such costs are covered by road tolls and other forms of local/central taxation. It is considered that Americans pay pump prices which cover only one-third of the real costs, the other two-thirds being borne by the state and, in the USA, local oil extraction has been declining since 1975 to the point where nowadays some 70% is imported.

4.6.2 FUEL-EFFICIENT VEHICLES

The US PNGV programme described later in this chapter is an effort to provide the technology for very low fuel consumption vehicles which industry could adopt if motorists accept the need. Researchers such as Lovins at the Rocky Mountain Institute have shown that 500 mpg hypercars are possible, paving the way for some intermediate value to the 30 mpg average consumption vehicle typical in America. The 10 kW absorbed at 60 mph alone could be halved by the adoption of underfloor aerodynamics. In combination with an aerodynamic superstructure, double-acting brake cylinders and low rolling-resistance tyres, it could be cut to one-third.

In the deal that was struck after the Kyoto summit in 1997 hydrogen is to become available one day at filling stations; both Shell and Esso are already committed in Europe and Japan, and will start operating only when they consider the fuel markets are operated on an orderly basis. On the other hand, in America they are operated on a subsidized basis. In Europe, too, at least one manufacturer, VW, has shown with the Lupo that a 1.3 litre 80 mpg car can be built to USA PNGV requirements. This uses a high technology diesel engine with common rail fuel injection and is an instant starter without the use of glow-plugs or comparable devices. It has top speed of over 100 mph and nippy accelerative performance. But this performance could not be achieved on larger cars typical in the American market, that is not without hybrid drive technology as now made available on the Toyota Prius, which has interior accommodation comparable with some American cars. The US version has a 21 kW battery to improve acceleration and features full air-conditioning. In 2000, this car sells at $18 000 compared with $10 000 for an American ‘base model’ car. So far some 40 000 are in use worldwide but the UK version is two-thirds dearer at £18 000, for a much higher performance vehicle required in this market.

Such fuel-efficient vehicles should permit an increase in gasoline prices that would allow oil extraction from the polar regions, permit other fuels to compete with gasoline and permit other transport systems to compete with cars and aeroplanes. The GM Precept (Fig. 4.6) is the corporation’s PNGV car and similar ones have been developed by Ford and Chrysler. GM also intends to make the vehicle in hybrid and fuel-cell versions. It is important to note that it is an ultra-low weight vehicle made primarily in aluminium alloy with underbody streamlining, a TV camera in lieu of wing mirrors, low rolling-drag tyres and double-acting brake pistons. The latter overcome the problem of drag due to brake hang-on with conventional systems when the single-acting piston fails to withdraw, and some 2 kW can typically be lost at motorway speeds. Of course, hydro-mechanical brakes only operate rarely on EVs, which can rely on regenerative braking for light-duty work. The Precept is an order-of-magnitude change in technology compared with ordinary American-produced vehicles, but of course would cost considerably more than the $10 000 dollars now charged.

4.6.3 FUEL-CELL VEHICLE PROSPECTS

The real future is with fuel-cell cars because the Precept version so fitted will have a fuel-cell stack volume of just 1.3 ft3 and produce 70 kW continuously, 95 V at 750 A, Fig. 4.7. Plate current density is 2 A/cm2; the cell is currently world leader in PEM-stack design and if used intelligently has higher power density than a thermal engine. This is, then, the turning point, and it is underlined by Mercedes whose work on the Necar IV is showing that 38% efficiency is obtained from hydrogen in the fuel tank to power at the road wheels. This compares 13–15% for a standard thermal-engine car. With this three times improvement in fuel economy GM believes that they are going to develop fuel-cell vehicles. The major challenge over the next five years is making it at a low enough cost to be attractive.

4.6.4 BATTERY ELECTRICS/HYBRIDS IN THE INTERIM

EV technology will be needed for the future fuel-cell car but currently it cannot be proven with current traction batteries which are too heavy, too expensive and made from materials that are not in plentiful enough supply. Most good quality batteries are based on nickel technology and the reason for being in this difficulty is not because the battery research programme was a failure, but rather the reverse. The outcome is that people are using batteries for communications, camcorders and computers; they pay far more for their batteries than car manufacturers could sensibly afford. The ‘3Cs’ are prepared to pay three times what the EV builder can pay.

A better alternative to nickel technology is aluminium. This is because the nickel–metal hydride battery, to give 80 kWh needed for a 3–400 mile range (on a PNVG car), would weigh 850 kg and cost $25 000, at year 2000 prices from Ovonic. The same 80 kWh can be obtained at a weight of 250 kg with aluminium, and at a cost of just $5000; and of course the material from which it is made is the most abundant on earth, next to hydrogen. The electric car is not a failure from a performance point of view but merely waiting for its time to come when high performance batteries can be produced economically. Energy density is not the problem with aluminium, rather it is the corrosion problem which causes aluminium batteries to degrade rapidly because of the formation of aluminium hydroxide jelly. However, two years ago scientists in Finland put forward a raft of patents which overcame some of the problems; these were revealed at the 2000 ISATA conference.

For the PNVG programme, Dr Alan Rudd built an aluminium battery for the US government; this was a pump-storage one, that was totally successful apart from the above-mentioned corrosion problem – the factor which persuaded the Americans to discontinue development. The technical committees took the decision to ignore the lower voltage couples on the grounds that the higher voltage couples were sold on the basis of having fewer cells in series. This gave the lightest batteries for portable power applications but the materials involved could never be cheap enough for an electric car.

A key advantage of the aluminium battery is its ability to operate at temperatures down to –80oC, overcoming the disadvantage of many existing types. The corrosion problem is now thought to be soluble and effective EV batteries are foreseen in 5 years’ time. It is considered best therefore to hold back on battery electrics, while hybrids hold the fort, until such time as the most cost-effective high performance solution is found. If the current European development programme is successful EV makers will have D-cells (like torch batteries), each handling 150 Ah at 1.5 V DC and able to be discharged at about 500 A maximum. Batteries will be formed from matrices of such cells, as discussed in Chapter 2. This is the most effective solution to the battery-electric vehicle problem. Existing technology batteries are going to be used for energy stores in hybrid-drive vehicles where the capacity needed will be less than 5 kWh.

4.6.5 POLLUTION CONTROL MEASURES

Hybrid drive cars present an early stepping point to fuel-efficient cars but very few car makers have produced the full gamut of drag-reducing measures that could transform fuel economy, and hence CO2 emissions, with existing thermal-engine technology cars. To do so could give huge improvements at modest cost, with immediate fuel saving and emission control benefits. The other big potential benefit would be catalytic converters for diesel engines. Such engines produce as much pollution now as petrol engines did in the 1960s and are the worse by far for PM10 particulates. Converters would dramatically cut diesel engine emissions and it is only their poisoning by sulphur in conventional diesel fuel that has prevented their widespread use. Now that clean ‘City Diesel’ is beginning to be seen at filling stations there is real hope for a positive step forward. This new fuel has around 10 ppm sulphur instead of 250 ppm with conventional fuel. The final stage in pollution control is, of course, the zero emission vehicle, the main contender being fuel-celled vehicles which GM intends to introduce in 2004.

4.6.6 OVERALL ENERGY POLICY

The global perspective is that over the next 20 years, road vehicles and aircraft will switch to hydrogen fuel. The exact time will depend on how consumers react to the problems outlined above. Should consumers adopt a helpful attitude and accept the certain introduction of fuel-efficient cars soon? This would allow fuel prices to rise sufficiently for the extraction of polar oil deposits, then gasoline could still be used in 100 years’ time. Conversely by doing nothing, and continuing to drive 30 mpg cars, we shall be subject to a crash introduction of the hydrogen economy in ten years’ time. From an industry and cost perspective it would obviously be better to have a gradual transition; a sudden transition could have an economic effect similar to that of a world war. It is really vital that the G7 economies, at least, introduce fuel-efficient cars within the next five years. By staying with conventional HC fuels, and avoiding the hydrogen transition, will only lead to more regulation, slow strangulation and severe restrictions – which would be very hard to impose on the US market, for example.

When the transition does occur hydrogen will be produced first from the reformation of natural gas and then by the electrolysis of water using electricity from fusion reactors. In 2010 it is estimated there will be dual-fuel aircraft with paraffin in the wing tanks and liquefied hydrogen in tanks over the passenger compartment. Over the next 20 years, many airlines will prefer to transfer directly to hydrogen solely, as they gain major operational benefits. They will, at a stroke, double range or payload and thus be prepared to pay a higher price for hydrogen. The fuel will be supplied as a liquid at –180oC and 20 bar pressure for the aerospace market and thus high quality hydrogen will also be available in quantity for road vehicles. The reformation of natural gas will be carried out in central facilities (much more efficiently than in on-board installations) with the important proviso that energy must be extracted from the carbon in the methane, as well as from the hydrogen molecules, since there is three times the energy in carbon over hydrogen. Something like an Engelhard ion–thermal catalytic process is thus required. It is also important that after the carbon has been burnt out it should be in the form of a carbonate or a carbide (and not in the form of CO2 which would revert to the atmosphere). Although energy is released in this way during the conversion, overall, energy is consumed during the process. Some 90% of the initial energy is retained in the form of pure clean hydrogen fuel. From the point of production hydrogen can be distributed as a gas through existing natural gas pipelines. This is the likely scenario until 2050 when the exhaustion of natural gas will dictate the need for fusion reactors.

4.7 Prospects for EV package design

Electric traction was viable even before 1910 when Harrods introduced their still familiar delivery truck, with nickel–iron battery, which is still in daily use and speaks volumes for the longevity and reliability of the electric vehicle. But very important structural changes have taken place since. At the turn of the Twentieth century EVs did not have solid-state controls and sophisticated control was achieved using primitive contactor technology. Amazingly successful results were obtained with contactor-changing and field-weakening resistor solutions, as is seen in the Mercedes vehicle described at the end of the Introduction.

But the ‘writing on the wall’ for the first generation of electric cars appeared in the World War I period with the development of electric starters for thermal engines. This was followed by unprecedented improvement, by development, of the piston engine and the success of the Ford Model T generation vehicles in the 1920s which substantially outperformed early electric cars. From then on until 1960 when high-power solid-state switching devices were developed and EVs were basically used for delivery and other secondary applications. Between 1960–1980, a new generation of EVs was developed, of which mechanical-handling trucks and golf-carts were the most notable. These were based on Brushed DC motors with lead–acid batteries and some millions of golf-carts are in use today. However, with growing pollution, and fuel-availability problems in the 1970s, there was an impetus to try to build a successful passenger car, with the realization that it would take an order-of-magnitude improvement in technology to make this happen. There was, however, a significant advance with the coming of power transistors in place of thyristors, and big improvements in drive controllers resulted, epitomized by the successful Curtis controller. This was a field-effect transistor chopper that was to become almost universally used in low power DC vehicles. High performance AC drives also came into being, and four machines thus came to do battle for the EV market.

4.7.1 MOTOR CONSTRAINTS

There is now general acceptance that the brushless DC motor will be the one used now and in the future. At a conference in Toronto in June 2000, GM gave details of its latest version, with inverter, in the Precept car. Compared with their earlier induction motor drives, they have halved the size by going to the permanent-magnet motor as well as reducing current consumption, for equivalent performance, by a factor of 1.8. They thus also have an inverter of half the size of that required for an induction motor, and this has resulted, too, in substantial manufacturing cost reduction.

The disadvantage of brushed DC motors is the high unit weight for the performance obtained (and the commutator is moisture sensitive, perhaps beyond the capability of tolerating a high-pressure car wash). This is quite acceptable in industrial trucks where extra weight is often required to counterbalance handling of the payload at high moment arms, but not of course in cars. Because the frequency of the commutator in a brushed DC machine is 50 Hz, compared with 1 kHz for a brushless PM machine, the latter is smaller and lighter; also the electronics can switch at 20 times the equivalent rate of mechanical brushes, which is the basis for the weight advantage. A 45 kW brushed machine weighs 140 kg, and typically runs at 1200–5000 rpm, while an equivalent powered brushless machine operates at 12 000 rpm and weighs less than 20 kg. There can of course be a 5 kg weight penalty for a reduction gearbox but even so there is a 75% weight reduction overall. The inverter is also the cheapest of those used with any ‘AC-type’ motor.

The other contenders were the switched reluctance motor (SRM) and AC induction motor, plus the permanent magnet Pancake motor (by Lynch) which serves the specialized light vehicle market but is non-scaleable technology. Key result of the investigation into comparative methods was that one had to look at the development of the whole drive package and not just the traction motor. While once people balked at the $200 required for the permanent magnets required in brushless motors, in 2000 they appreciate that some $1000 of power electronics is saved in the inverter. The induction motor was put ‘out of court’ because traction operation requires constant power over a 4:1 speed range. Since voltage, current, speed (V, I, N) characteristics show that to do this an induction motor goes from 0.5 V at double current at the bottom end of the speed range, to V and I at the top end, so an inverter that can supply double current is required. With a brushless DC machine it can be designed to require V and I at both minimum and maximum speeds, so only half the size of converter is required. It also has a further advantage that scratches SRM from the equation. Because the SRM is force commutated (current interruption is very dissipative , a ‘hardswitching’ turn-off), power losses in the inverter are very significant compared with that associated with a brushless machine which at high speed operates with a leading power factor and so has hardly any switching losses in the inverter (which may be of very compact construction). The SRM also exhibits significant acoustic noise due to the magnetostriction of its operating dynamics. With the 4 or 6 coils in the machine which have to be moved relative to one another forces of attraction between them are up to ten times greater than the developed torque of the motor; the framework of the motor can physically distort and considerable noise thus generated.

4.7.2 WHEEL MOTORS AND PACKAGE DESIGN

While wheel motors are ideal for low speed vehicles the problem of high suspended mass rules them out for cars. Road damage can be caused at wheel hop frequencies and the perceived threat of losing traction on one wheel, by a single motor failure, would prevent any safety authority from issuing a certificate of roadworthiness. Use of such devices as active suspension makes them possible on medium speed urban buses where road wheel tyres can be as much as one metre in diameter and large brake assemblies reduce the relative weight of wheel motors. Motors driving individual back wheels are a possibility in commercial vehicles, where traction and steer forces are not shared by individual tyres, and 4 × 4 drives with a single motor power source are ideal for more expensive cars, which could tolerate the cost of multiple control systems. Wheel motors could see wider application if steels with adequate magnetic properties could be developed for lighter-weight PM motors at reasonable cost. Expensive military vehicles use such a steel, called Rotalloy, but it costs some £15 per kg in 2000. Such vehicles sometimes have individually steered and driven wheels which enable them to move sideways so perhaps cheaper future alloys of this type will improve parking manoeuvres.

At the present time safety authorities are unlikely to certificate cars with electrical rather than mechanical differential gears but a number of drive-by-wire solutions may become more feasible on EVs. Introduction of 5 kW, 42 V electrical systems is a strong possibility, that could see the replacement of many hydraulic, pneumatic and mechanical controls by electrical ones monitored electronically. These drive-by-wire systems will be a prelude to convoy control of vehicles on motorways. Many development pains have yet to be cured, however, though EV technology will be helpful in the implementation. Other advances such as starter-alternators are likely to be found on thermal-engined hybrid vehicles; these use a new kind of power electronics with silicon-carbide switching devices cooled by hot water from the engine cooling system, allowing semiconductors to operate safely at 250oC. First is due on the Mercedes 500 to be introduced in 2001 and fitment to all European and American cars is expected in two years’ time.

4.7.3 ALTERNATIVE AUXILIARY POWER

Consideration of photovoltaic power is often a pastime of EV promoters but 10–15% light to electricity conversion efficiency has precluded serious traction usage so far, though use as an auxiliary power source is important. Even at high noon in the tropics solar radiation can only generate 1 kW/m2 which means that the solar cell will produce only 150 W for each square metre. In the Honda Solar Challenger, 8 m2 of solar cells generates 1–1.5 kW, which would be nowhere near enough to provide propulsion and hotel loads (‘parasitic’ loads such as lighting and air conditioning) for a conventional car. The most hopeful traction application is for electric scooters operating in the tropics where a reasonable size photocell array, carried in the panniers then unfolded and left out in the sun, could charge the battery of a Honda 50 electric scooter in 6 hours and provide traction for 50 miles without energy being drawn from the grid important in isolated areas.

Photocells are also useful on battery-electric vehicles to ensure that the battery never gets fully discharged (particularly important with lead–acid types). They are valuable sources of auxiliary power for cooling purposes, either lowering interior temperatures on cars parked in the sun or providing refrigeration power to keep gaseous fuels in liquid form. The transformation in usage that could follow an increase in conversion efficiency may be realizable before long if the reported intensity of research bears fruit. BP and Sanyo are both world leaders in this and already enjoy market success with static arrays of low efficiency cells in tropical countries.

4.� �uel�cell �ehicles and infrastructure

Fuel cells are preferred as a primary traction power source because theoretical stack efficiency on the Carnot cycle is 83%, which is more than double that of the thermal engine, and unlike the thermal engine they become more efficient (90%) at light load operation. Stack EMF drops from 1 V at no load to 0.6 V at full load; stack efficicency thus increases at light load since auxiliary losses do not go down in the same proportion. Whether hydrogen is reformed from fossil fuels on board the vehicle (Fig. 4.8), as an interim approach to carrying compressed liquid hydrogen, is still under debate. This approach is being championed by Chrysler and is attractive in America where gasoline is sold at a subsidized price. Even with this technique overall efficiency is much higher than with a thermal engine. However, the heavy on-cost to the vehicle makes it no more than a transitory solution. The military have used hydrogen propulsion for nuclear submarines, space-craft and specialized assault vehicles for many years now and they have a complete infrastructure in place from which the civil transport market can learn, so that ‘critical mass’ has been reached in terms of knowledge base and experience. Now the development is directed at moving from cost plus to cost effective. The challenge is to make parts out of plastic that were previously made from stainless steel and achieve one-tenth of existing costs.

It was once said that on-board liquid hydrogen storage was a big problem but the latest C16 carbon fibre has resulted in a 60 litre storage tank with a weight of only 7 kg that will store 16 in3 of hydrogen. What is not widely understood is how the gas is compressed in the liquefaction process. The method of approach can be the difference between success and failure since a two-stage process is involved, a Stirling cycle stage down to –200oC and a Linde cycle from –200 to –269 oC, the latter using nearly all the energy. So it is necessary not to liquefy the gas at ambient pressure as the Linde cycle will be involved, burning up 30% of energy within the fuel getting it down to –273oC. The approach is to have pressure tanks operating at –160 to –180 oC, with a metal inner wall then glass fibre in a vacuum for an inch radial thickness followed by a carbon fibre outer wall. By putting hydrogen into a tank with no additional cooling it takes about two weeks before the liquid becomes a gas, and it blows off. Under normal conditions motorists would use a

(a)

Fig. 4.8(a) Complete GM fuel-cell chassis with POX converter capable of up to 70 kW output at 300 V DC (including AC drive train),

(b) Gasoline to hydrogen (POX) converter close up.

tank-full every two weeks so that no additional refrigeration would be required. However, if it is necessary to use refrigerated gas the basic Stirling cycle refrigerator of 10 W would keep it in liquid form indefinitely, the 10 W coming readily from a photocell array charging a small auxiliary battery. Such units have been made in Israel for 30 years and are extensively used by the military. For cars a 15 litre fuel storage tank will probably be used, having capacity for 50 cubic metres of hydrogen to provide a 400–500 mile range. Larger vehicles, trucks and buses will probably store the gas as a liquid, in view of the larger gaseous volumes otherwise involved. One should also not forget the bonus that natural gas comes out of the ground at 300–400 bar therefore not so much energy is needed to compress the gas, with the recovery of energy in the reformation process.

Generation zero fuel cells cost $8000/kW for the entire system including pumps, power conversion, controls and fuel storage. Autumn 2000 saw the construction of second generation fuel cells and are the first serious attempt at cost reduction. Many separate components will be integrated by such techniques as manifolding, for example. Plastic pumps will be employed instead of metal ones and for the first time custom-engineered chips will be used instead of standard PLCs. This will yield $2000/kW which it is hoped will be reduced to $1000/kW for the third generation by 2002. For example, an air pump used by the author’s company once weighed 18 kg and by careful production development this was reduced to 10 kg; in the latest stage of conversion from metal to plastic, it is hoped to achieve 3.5 kg, as well as the reduced cost benefits. The company build all electrical and control systems for Zetec Power’s fuel-cell engines. Zetec are opening a new factory in Cologne which will make 2000 stacks per year and thus cost effectiveness is paramount.

4.8.1 HYDROGEN DISTRIBUTION

Currently natural gas is distributed through 600 mm diameter pipes at a pressure of 500 psi. Many of these pipes can be used for hydrogen distribution and energy transport factor will increase significantly as a result with the higher energy density of hydrogen. Many other end-products and processes can be fuelled, as well as road vehicles, by this means. It has to be remembered, however, that the gas is explosive at extremely low levels of hydrogen/air mixture and it must be stored near the roof of vehicles, since the gas is lighter than air. Vehicles themselves must also be stored in well-ventilated areas. Explosive energy is considerably lower than natural gas, however, and the main requirement is to install low level concentration hydrogen sensors in the storage vicinities.

Although considerable change and expense is necessary to move to a hydrogen-fuel economy, it will be a much easier experience if it can be implemented over a substantial time period, as suggested earlier, but with minimum delay in starting the process. A hydrogen economy has the advantage that one grade and type of fuel replaces the five or six currently on offer at filling stations and domestic heating too is likely to turn to fuel cells so hydrogen will also be their source of supply.

4.� �he P��V programme� impetus for change

On 29 September 1993, the Clinton Administration and the US Council for Automotive Research (USCAR), a consortium of the three largest US automobile manufacturers, formed a cooperative research and development partnership aimed at technological breakthroughs to produce a prototype ‘super-efficient’ car. The ‘Big Three’ (Chrysler, Ford, and General Motors), eight federal agencies, and several government national defence, energy, and weapons laboratories have joined in this Partnership for a New Generation of Vehicles (PNGV). It is intended to strengthen US auto industry competitiveness and develop technologies that provide cleaner and more efficient cars. The 1994 PNGV Program Plan called for a ‘concept vehicle’ to be ready in about six years, and a ‘production prototype’ to be ready in about 10 years. Research and development goals included production prototypes of vehicles capable of up to 80 miles per gallon – three times greater fuel efficiency than the average car of 1994.

Background drivers of the initiative include a combination of high gasoline prices, and government fuel economy regulation caused new car fuel efficiency to double since 1972. However, fuel economy standards for new cars peaked at 27.5 miles per gallon (mpg) in 1989 and the average fuel efficiency of all on-road (new and old) cars peaked at 21.69 mpg in 1991, then dropped slightly in 1992 and again in 1993. Further, the large drop in real gasoline prices since 1981 and the increasing number of cars on the road are eroding the energy and environmental benefits of past gains in auto fuel efficiency. The public benefits that could derive from further improvements in auto fuel efficiency include health benefits from reduced urban ozone, ‘insurance’ against sudden oil price shocks, reduced military costs of maintaining energy security, and potential savings from reduced oil prices.

The Declaration of Intent for PNGV emphasizes that the programme represents a fundamental change in the way government and industry interact. The agreement is seen as marking a shift to a new era of progress through partnership and cooperation to address the nation’s goals, rather than through the confrontational and adversarial relationship of the past. Its intent is to combine public and private resources in programmes designed to achieve major technological breakthroughs that can make regulatory interventions unnecessary. The partnership agreement is a declaration by USCAR and the government of their separate, but coordinated, plans to achieve goals for clean and efficient cars. A further objective is to curb gasoline use by 7 billion gallons per year in 2010 and 96 billion gallons per year in 2020, while creating 200 000 to 600 000 new jobs by 2010.

At the time the agreement was struck, the president and executives from the Big Three said they hoped that PNGV research breakthroughs would ultimately make auto emissions and mileage regulations unnecessary. Chrysler’s former PNGV director, Tim Adams, noted that the partnership represents the opportunity to address more efficiently fundamental national objectives than the regulatory mandate approach. Further, car-makers say the Supercar’s advanced technologies are outside their short-term research focus, and unjustified by fuel costs or market demand for fuel efficiency. They argue that the North American market forces alone would not drive them to create an 80 mile per gallon mid-sized sedan.

Examples of applied technology would be the development of lightweight, recyclable materials, and catalysts for reducing exhaust pollution; research that could lead to production prototypes of vehicles capable of up to three times greater fuel efficiency. Examples would be lightweight materials for body parts and the use of fuel cells and advanced energy storage systems such as ultracapacitors. Using these new power sources would produce more fuel-efficient cars. Further initiatives included lightweight, high-strength structural composite plastics that are recyclable, that can be produced economically in high volume, and that can be repaired. Hybrid drive control electronics and hardware were also cited alongside regenerative braking systems to store braking energy instead of losing it through heat dissipation; also fuel cells to convert liquid fuel energy directly into electricity with little pollution.

Such advances are aimed at more efficient energy conversion power sources, viable hybrid concepts as well as lighter weight and more efficient vehicle designs. The contributions of US government agencies include the following: at its ten National Laboratories, the Department of Energy has technical expertise, facilities, and resources that can help achieve the goals of the partnership. Examples include research programmes in advanced engine technologies such as gas turbines, hybrid vehicles, alternative fuels, fuel cells, advanced energy storage, and lightweight materials. The DOE’s efforts are implemented through cost-shared contracts and cooperative agreements with the auto industry, suppliers, and others. Technologies covered include fuel cells, hybrid vehicles, gas turbines, energy storage materials and others. The Department of Defense’s Advanced Research Projects Agency (ARPA) is focused on medium-duty and heavy-duty drivetrains for military vehicles which could, in the future, be scaled down to light-duty vehicles. ARPA funds research on electric and hybrid vehicles through the Electric/Hybrid Vehicle and Infrastructure (EHV) Program and the Technology Reinvestment Project (TRP). EHV is a major source of funding for small companies interested in conducting advanced vehicle research that is not channelled through the Big Three auto-makers. NASA will apply its expertise to PNGV in three ways: by applying existing space technologies such as advanced lightweight, high strength materials; by developing dual-use technologies such as advanced batteries and fuel cells to support both the automotive industry and aerospace programmes; and by developing technologies specifically for the PNGV such as advanced power management and distribution technology. The Department of Interior involvement in PNGV-related research includes research to improve manufacturing processes for lightweight composite materials and recycling strategies for nickel– metal hydride batteries. The DOI’s Bureau of Mines has developed a system for tracking materials and energy flows through product life cycles. Life-cycle assessment of advanced vehicles and components can help to anticipate problems with raw materials availability, environmental impacts, and recyclability. This includes the worldwide availability of raw materials, environmental impacts of industrial processes, and strategies for recycling of materials.

The US OTA considers that the most likely configuration of a PNGV prototype would be a hybrid vehicle, powered in the near term by a piston engine, and in the longer term perhaps by a fuel cell. It notes that there is no battery technology that can presently achieve the equivalent of 80 mpg. Thus, the proton exchange membrane (PEM) fuel cell is seen as the more likely candidate. The DOE further stresses that meeting the fuel economy goal will require new technologies for energy conversion, energy storage, hybrid propulsion, and lightweight materials.

4.9.1 PARALLEL EUROPEAN UNION AND JAPANESE INITIATIVES CITED BY THE US GOVERNMENT

According to TASC, the European Union (EU) has formed the European Council for Automotive Research and Development (EUCAR) in response to both the US PNGV programme and accelerated vehicle development in Japan. EUCAR’s objectives are technology leadership, increased competitiveness of the European automotive industry and environmental improvements. With a leader appointed from industry, EUCAR has requested a budget of over $2.3 billion from the EU over 5 years, representing a 50% EU government cost share. This includes $866 million for vehicle technology, $400 million for materials R&D, $400 million for advanced internal combustion engine (ICE), $333 million for electric/hybrid propulsion, and $333 million for manufacturing technology and processes. An additional $638 million is targeted for control and traffic management, and $267 million is targeted for management and organization structures. The annual EU budget is expected to include $173 million for vehicle technology, $80 million for advanced ICE, $80 million for materials, $67 million for manufacturing, and $67 million for electric and hybrid vehicles. Member companies of the EUCAR cooperative R&D partnership include BMW, Daimler-Benz AG and Mercedes-Benz AG, Fiat SpA, Ford Europe, Adam Opel AG, PSA Peugeot-Citroen, Renault SA, Rover, Volkswagen AG, and Volvo AB. National initiatives include fleet purchases and demonstrations, subsidies and cooperative R&D.

OTA notes that about $700 million of the EUCAR programme is focused specifically on automotive projects. The EUCAR programme is similar in some ways to PNGV, but the research proposed in its Master Plan is broader in scope, encompassing sustainability concerns in the longer term, though with no mention of a timetable for a prototype vehicle. The Master Plan proposes work focused on product-related research on advanced powertrains and materials, manufacturing technologies to match new vehicle concepts, and the total transport system, including vehicle integration into a multimodal transport system. The primary source of funding will be the EU’s 5-year Framework IV programme. Also, in 1995, to stimulate R&D on advanced vehicles using traction batteries, the EU initiated a task force named ‘Car of Tomorrow’ that will collaborate with industry, ensure R&D coordination with other EU and national initiatives, and encourage the use of other funding such as venture capital. OTA also notes that some European nations, such as France, may be a more promising market for advanced vehicles, especially EVs, since it has more compact urban areas with shorter commute distances. France, Germany and Sweden have significant EV and other advanced vehicle programmes under way.

TASC reports that Japan has utilized the Ministry of International Trade and Industry (MITI) as the focus of industry–government cooperation to execute a similar activity with funding expected to reach $250 million per year. Its strategy is focused on market share and electric/ hybrid vehicles for the California market. Reduction of nitrous oxide emissions is also an environmental goal of the programme. The annual government share of budget is expected to include $29 million or more for vehicle technology, $40 million for advanced ICE, $20 million for materials, $5 million or more for manufacturing, and $57 million for electric and hybrid vehicles. An infrastructure project is under way at nine major sites located close to industry and covering a wide range of climates. Industry manufacturers gearing up for the 1998 California zero emission vehicle (ZEV) programme include Honda, Mazda, Nissan, and Toyota. Other Japanese manufacturers participating in the cooperative activity include Daihatsu, Mitsubishi, Isuzu, and Suzuki.

OTA notes that the Japanese programme to develop PEM fuel cells began slowly under the MITI’s New Energy and Industrial Technology Development Organization, but it is rapidly catching up with US programmes. PEM fuel cells are being actively developed and tested by some of the most powerful companies in Japan. Japanese auto manufacturers have performed research on EVs for more than 20 years, but the effort was given low priority due to problems with traction battery performance and doubts about EV consumer appeal. However, California’s adoption of the ZEV regulations raised this priority.

References

1. Appleby and Foulkes, Fuel cell handbook, Van Nostrand Reinhold, 1989

2. Blomen and Mugwera, Fuel cell systems, Plenum Press, 1993

3. Hart and Bauen, Fuel cells: clean power, clean transport, clean future, Financial Times Energy, 1998.

4. Prentice, Electrochemical engineering principles, Prentice-Hall Inc., 1991

5. Fuel cells, a handbook, US Dept of Energy 1988, DOE/METC-88/6096 (DE88010252)

6. Platinum 1991, Johnson Matthey

7. Appleby, Journal of Power Sources, 29, pp. 3–11, 1990

8. Dicks, J. L., Journal of Power Sources, 61, pp. 113–124, 1996

9. Prater, Journal of Power Sources, 61, pp. 105–109, 1996

10. Ledjeff and Heinzel, Journal of Power Sources, 61, pp. 125–127, 1996

11. Acres and Hards, Phil Trans R. Soc. Lond. A, pp. 1671–1680, 1996

12. Blomen or Perry’s Chemical Engineers’ Handbook, Sixth Edition, pp. 3–150

13. Shibata, Journal of Power Sources, 37, pp. 81–99, 1992

Further reading

Maggetto et al. (eds), Advanced electric drive systems for buses, vans and passenger cars to reduce pollution, EVS Publication, 1990

PART TWO

EV DESIGN PACKAGES/DESIGN FOR LIGHT WEIGHT

5

Battery/fuel-cell EV design packages

5.1 Introduction

The rapidly developing technology of EV design precludes the description of a definitive universal package because the substantial forces which shape the EV market tend to cause quite sudden major changes in direction by the key players, and there are a number of different EV categories with different packages. For passenger cars, it seems that the converted standard IC-engine driven car may be giving way to a more specifically designed package either for fuel-cell electric or hybrid drive. While the volume builders may lean towards the retention of standard platform and body shell, it seems likely that the more specialist builder will try and fill the niches for particular market segments such as the compact city car. It is thus very important to view the EV in the wider perspective of its market and the wider transportation system of which it might become a part.

Because electric drive has a long history, quite a large number of different configurations have already been tried, albeit mostly only for particular concept designs. As many established automotive engineers, brought up in the IC-engine era, now face the real possibility of fuel-cell driven production vehicles, the fundamentals of electric traction and the experience gained by past EV builders are now of real interest to those contemplating a move to that sector. A review of the current ‘state of play’ in sole electric drive and associated energy storage systems is thus provided, while hybrid drive and fuel-cell applications will be considered in the following chapter.

5.2 Electric batteries

According to battery maker, Exide, the state of development of different battery systems by different suppliers puts the foreseeable time availability for the principal battery contenders, relative to the company’s particular sphere of interest, lead–acid – as in Fig. 5.1a.

5.2.1 ADVANCED LEAD–ACID

The lead–acid battery is attractive for its comparatively low cost and an existing infrastructure for charging, servicing and recyclable disposal. A number of special high energy versions have been devised such as that shown at (b), due to researchers at the University of Idaho. This battery module has three cells, each having a stack of double-lugged plates separated by microporous glass mats. High specific power is obtained by using narrow plates with dual current collecting lugs and a 1:4 height to width aspect ratio. Grid resistance is thus reduced by shortening conductor lengths and specific energy is improved by plates that are thinner than conventional ones. They have higher active mass utilization at discharge rates appropriate to EV use. At an operating temperature of 110oF specific energy was 35.4 Wh/kg and specific power 200 W/kg. Over 600 discharge cycles were performed in tests without any serious deterioration in performance. The table at (c) lists the main parameters of the battery. The US company Unique Mobility Inc. have compared advanced lead–acid batteries with other proposed systems. In carrying out trials on an advanced EV-conversion of a Chrysler Minivan the company obtained the comparisons shown at (d). The graphs also show the extent to which the specific energy content of batteries is reduced as specific power output is increased. Trojan and Chloride 3ET205 are commercial wet acid batteries

Short term Medium term Long term
Improved lead–acid
Nickel–Metal Hydride
Sodium–Nickel chloride
Lithium–Ion Lithium–ion (polymeric electrolyte)

Fuel cells

(a)

PRESSURE

(f)

Cell weight 15 kg 0

(d)

Cell dimensions 10x7x6 inches

1020 30 40 5060

120

100

80

60 LUGS

SPECIFIC ENERGY (Wh/kg)

35 Wh/kg 2hr rate 200 W/kg 20% state of charge

600 cycles at 80% DOD 40 Charge/discharge efficiency 85% Battery volts 240 nominal Weight 600 kg 20 Number of cells 40 Cell voltage 6 V

(c) SPECIFIC POWER (W/kg)
Battery Type Module weight (lb) Average module voltage (volts) C/3 AMP hour Nominal Vehicle Energy capacity density kWh Wh/kg Calculated EV energy density C-cycle D-cycle 35 mph 55 mph Wh/kg Wh/kg Wh/kg Wh/kg

GC 12 V 100 68.0 11.1 72 16.0 25.9 22.1 18.0 24.9 17.0 DF8D 141.1 11.4 150 16.5 26.7 23.7 19.7 26.5 18.6 MET205 70.5 6.0 187 21.7 35.2 31.2 26.9 34.1 25.8 NI-Fe 75.0 6.3 225 33.8 54.8 54.5 50.8 57.0 49.8 NaS 1102.0 220.0 250 67.9 110.0 115.6 111.6 118.4 110.6 Adv Ph acid 141.1 11.4 225 24.7 40.1 42.9 38.7 45.9 37.5

(e)

Fig. 5.1 The lead–acid battery: (a) development time spans compared; (b) high energy lead–acid battery; (c) parameters of H-E battery; (d) battery characteristics; (e) energy-storage comparisons.

while the Sonnenshein DF80 and JCI 12V100 are gelled electrolyte maintenance-free units which involve an energy density penalty. The Eagle pitcher battery is a nickel–iron one taking energy density up to 50 Wh/kg at the 3 hour rate. The Beta and Delta units are sodium–sulphur batteries offering nominal energy density of 110 Wh/kg. Unique Mobility listed the characteristics of the batteries as at (e).

Exide’s semi-bipolar technology has both high electrical performance and shape flexibility. The very low internal resistance allows high specific peak power rates and the electrode design permits ready changes in current capacity. The flat shape of the battery aids vehicle installation. The battery is assembled in a way which allows reduced need for internal connections between cells and a lightweight grid. Coated plates are stacked horizontally into the battery box. Performance is

3.9 Ah/kg and 7.4 Ah/dm3 and shape profile is at (f).

5.2.2 SODIUM–SULPHUR

For the sodium–sulphur battery, Fig. 5.2, as used in the Ford Ecostar, the cathode of the cell is liquid sodium immersed in which is a current collector of beta-alumina. This is surrounded by a sulphur anode in contact with the outer case. The cells are inside a battery box containing a heater to maintain them at their operating temperature of 300–350°C. This is electrically powered and contained within the charge circuit. When discharging, internal resistance produces sufficient heat for the electrode but some 24 hours are required to reach running temperature from cold. In a

Battery energy IkWh) 40 40 60 85

typical EV application 100 cells would be connected in series to obtain 100 V and give a battery of 300 Ah, 60 kWh. In use, batteries would typically be charged nightly to bring them up to voltage after daily discharge and to keep the electrode molten. A typical battery installation of chloride cells is seen at (a) with expected vehicle performance, compared with lead–acid, shown at (b). The chloride cells are based on an electropheritic process while those from the Asea Brown Boveri company, used in Ecostar, are made by isostatic pressing.

The ABB cell is seen at (c); the electronic current flowing through the external load resistor during discharge corresponds to a flow of sodium ions through the electrolyte from the sodium side to the sulphur side. Voltage is from 1.78 to 2.08 V according to the degree of discharge involved. A cell with a capacity of 45 Ah has a diameter of 35 mm and length of 230 mm. Its internal resistance is 7 milli ohms and 384 cells of this type can be installed in a battery of 0.25 litre volume. An example produced by ABB has external dimensions 1.42 × 0.485 × 0.36 metres. The cells account for 55% of the total weight of 265 kg. By connecting the cells in four parallel strings of 96, the battery has an open circuit voltage of 170–200 and a capacity of 180 Ah.

The electrical energy which can be drawn from the battery is shown at (d) as a function of the (constant) discharge power. With a complete discharge in 2 hours, energy content is 32 kWh, corresponding to a density of 120 Wh/kg. Associated discharge efficiency is 92%. Complete discharge at constant power is possible in a minimum of 1 hour, and an 80% discharge in less than three-quarters of an hour. The graph at (e) shows that the battery can cope with a load of up to two-thirds of the no-load voltage for a few minutes. This corresponds to a rating of about 50 kW or 188 W/kg. The portion of the heat loss not removed by the cooling system which is incorporated into the battery is stored in the heated-up cells – and covers losses up to 30 hours. Additional heat must be supplied for longer standstill periods either from the electric mains or from the battery itself. Effective, vacuum-type, thermal insulation maintains the power loss at just 80 W so that when fully charged it can maintain its temperature for 16 days. In order to maintain the battery in a state of readiness, the battery must be held above a minimum temperature and it takes about 4–10 hours to heat up the battery from cold – but a limit of 30 freeze–thaw cycles is prescribed. Life expectancy of the battery otherwise is 10 years and 1000 full discharge cycles, corresponding to an EV road distance of 200 000 km.

5.2.3 NICKEL–METAL HYDRIDE

As recently specified as an option on GM’s EV1, the nickel–metal hydride alkaline battery, Fig. 5.3, was seen as a mid-term solution by the US Advanced Battery Consortium of companies set up to progress battery development. According to the German Varta company, they share with nickel– cadmium cells the robustness necessary for EV operation; they can charge up quickly and have high cycle stability. The nickel–metal hydride however, is superior, in its specifications relative to vehicle use, with specific energy and power some 20% higher and in volumetric terms 40% higher. Unpressurized hydrogen is taken up by a metallic alloy and its energy then discharged by electrochemical oxidation. The raw material costs are still signalling a relatively high cost but its superiority to lead–acid is likely to ensure its place as its associated control system costs are lower than those of sodium sulphur. Specific energy is 50–60 Wh/kg, energy density 150–210 Wh/litre, maximum power more than 300 W/kg; 80% charge time is 15 minutes and more than 2000 charge/ discharge cycles can be sustained.

The negative electrode is a hydrogen energy-storage alloy while nickel hydroxide is the positive electrode. An optimum design would have weight around 300 kg, and capacity of 15 kWh, with life of 2000 discharge cycles. For buses Varta have devised a mobile charging station, in cooperation with Neoplan, which will allow round-the-clock operation of fleets. This removes the need for fixed sites and allows battery charging and changing to be carried out by the bus driver in a few minutes. The mobile station is based on a demountable container which can be unloaded by a conventional truck. Trials have shown that a bus covering a daily total distance of 75 miles on a three-mile-long route needs to stop at the station after eight journeys. Discharged batteries are changed semiautomatically on roller-belt arms, by a hand-held console.

5.2.4 SODIUM CHLORIDE/NICKEL

Sodium chloride (common salt) and nickel in combination with a ceramic electrolyte are used in the ZEBRA battery, Fig. 5.4, under development by Beta Research (AEG and AAC) and Siemens. During charging the salt is decomposed to sodium and nickel chloride while during discharge salt is reformed. Its energy density of 90 Wh/kg exceeded the target set by the USA Advanced Battery Consortium (80 Wh/kg energy density, to achieve 100 miles range under any conditions and 150 W/kg peak power density to achieve adequate acceleration) and can achieve 1200 cycles in EV operation, equivalent to an 8 year life, and has a recharge time of less than 6 hours. The USABC power to energy ratio target of 1.5 was chosen to avoid disappointing short-range high power discharge of a ZEV battery and for a hybrid vehicle a different ratio would be chosen.

Each cell is enclosed in a robust steel case with electrodes separated by a β-ceramic partition which conducts sodium ions but acts as a barrier to electrons, (a). The melt of sodium/aluminium chloride conducts sodium ions between the inner ceramic wall and into the porous solid Ni/NiCl2 electrode. As a result, the total material content is involved in the cell reaction. Apart from the main reversible cell reaction there are no side reactions so that the coulometric efficiency of the cell is 100%. The completely maintenance-free cells are hermetically sealed using a thermal compression bond (TCB) ceramic/metal seal.

The cell type SL09B presently produced in the pilot production line has an open-circuit voltage of 2.58 V at 300°C with a very low temperature coefficient of 3 × 104 V/K, a capacity of 30 Ah and an internal resistance that varies between 12 and 25 mW, dependent on temperature, current and rate of discharge. This variation is because, during the charging and discharging process, the electrochemical reaction zone moves from the inner surface of the β-ceramic electrolyte into the solid electrode. During this process the length of the sodium ion path and the current-density in the reaction zone increases and so the internal resistance increases. In principle this effect is used

110 Lightweight Electric/Hybrid Vehicle Design

�5 �11 (preliminary)

Dimensions 1 x w x hI1)

Weight battery

I1)

Weight accessoriesI2)

Cell type

Thermal losses at 270oC Rapid charging

Rated energy Energy density Peak Power

I80% DOD�

2/3 OCV� 30s) Peak Power Density I80% DOD� 2/3 OCV)

OCV Capacity

Ni Cl1 + 2 Na' 2 Na Cl + Ni

Na

on the right Load
Current collector ( Pol) Nickelchloride + Sodiumaluminiumchloride Ceramic electrolyte Sodium Cell can ( Pol) 2 Na Cl + Ni Ni Cl1 + 2 Na' Na
Load
Cooling

730 x 541 x 933 x 665 x 315 mm 315 mm

194 kg 310

6 kg about 10

SL09B ML1 max. 125 W 170 W

75% in 45 min 75% in 45 min 17 kWh 29 kWh 88 Wh/kg 94 Wh/kg 15 kW 42 kW

75 W/kg 135 W/kg

284/188/142 V 302 V 60/90/120 Ah 96 Ah

(c)

N3 AlCl4 Liquid electrolyte

8' - Al1O1 Ceramic electrolyte Capillary gap Wick

Charge

(a)

N3 AlCl4 Liquid electrolyte 8' - Al1O1 Ceramic electrolyte Capillary gap Wick

Discharge

Vaccum insulation

Current terminals

Electric heater

Fig. 5.4 ZEBRA battery: (a) cell; (b) cell-box; (c) performance comparison.

to enable a stable operation of parallel connected strings of cells. But from the vehicle point of view the available power which is directly related to the internal cell resistance should not depend on the battery charge status. The redesigned cell type ML1 is a good compromise between these two requirements. The battery is operated at an internal temperature range of 270–350° C.

The cells are contained in a completely sealed, double walled and vacuum-insulated battery box as shown at (b). The gap between the inner and outer box is filled with a special thermal insulation material which supports atmospheric pressure and thus enables a rectangular box design to be utilized. In a vacuum better than 1.101 mbar this material has a heat conductivity as low as 0.006 W/mK. By this means the battery box outside temperature is only 5–10° C above the ambient temperature, dependent on air convection conditions. Cooling systems have been designed, built and tested using air cooling as well as a liquid cooling. The latter is a system in which high temperature oil is circulated through heat exchangers in the battery with an oil/water heat exchanger outside the battery. By this means heat from the battery can be used for heating the passenger room of the vehicle.

In the ML1 cell, internal resistance is reduced to increase power. The resistance contribution of the cathode is due to a combination of the ion conduction between the inner surface of the β-aluminium ceramic with the reaction zone (80%) and electric conduction between the reaction zone and the cathode current collector (20%). The ML1 has a cloverleaf section shape ceramic to enlarge its surface area over the normal circular section, with resultant twofold reduction in cathode thickness and 20% reduction in resistance. Based on this form of cell construction a new, Z11, battery has been produced with properties compared with the standard design as shown by the table at (c) and the battery is under development for series production.

5.2.5 SOLAR CELLS

According to Siemens, solar technology is a probable solution for Third World tropical countries. Solar modules are available from the company to supply 12 V, 100 Ah batteries from a 50 W solar module. The company recently installed a system on the Cape Verde Islands with a collective power output of 550 kW at each of five island sites. Even in Bavaria, the village of Flanitzhutte, which has an average 1700 hours annual sunshine period, has severed its links with the national grid with the installation of 840 solar modules, with a total area of 360 square metres, to provide peak power of 40 kW. Maintenance-free batteries provide a cushion.

Watts/sq metre

800

700

700

600

1.50

600

500

1.25

500

400

1.00

Current-

Power

mA

watts

300

.75

400

200

.50

Summer

300

.25

100

200

0

0

Winter

0 1.0 2.0 3.0 0 1.0 2.0

3.0

Volts

Volts

(a) 100

Hours from noon (GMT)

(b) 0

Fig. 5.5 Solar cell technology: (a) cell characteristics; 0 1 2 34 56789

(b) solar energy variation.

The technology of solar cells, Fig. 5.5, has been given a recent boost by the Swiss Federal Institute of Technology who claim to have outperformed nature in the efficiency of conversion of sunlight to electricity even under diffuse light conditions. The cell has a rough surface of titanium dioxide semiconductor material and is 8% efficient in full sunlight rising to 12% in diffuse daylight. For more conventional cells, such as those making a Lucas solar panel, these are available in modules of five connected in series to give maximum output of 1.3 watts (0.6 A at 2.2 V). Some ten modules might be used in a solar panel giving 13 watts output in summer conditions. Power vs voltage and current vs voltage are shown at (a) for so-called ‘standard’ and ‘typical’ operating conditions. 100 mW/cm2 solar intensity, 0° C cell temperature at sea level defines the standard conditions against 80 mW/cm2 and 25°C which represent ‘typical’ conditions at which power output per cell drops to 1 W. Temperature coefficients for modules are 0.45% change in power output per 1° C rise in temperature, relative to 0° C; cell temperatures will be 20° C above ambient at 100 mW/cm2 incident light intensity. Variation of solar energy at 52° north latitude, assuming a clear atmosphere, is shown at (b). On this basis the smallest one person car with a speed of 15 mph and a weight of 300 lb with driver would require 250 W or 50 ft2 (4.65 m2) of 5% efficient solar panel – falling to 12.5 ft2 (1.18 m2) with the latest technology cells. A 100 Wh sealed nickel– cadmium battery would be fitted to the vehicle for charging by the solar panel while parked.

The future, of course, lies with the further development of advanced cell systems such as those by United Solar Systems in the USA. Their approach is to deposit six layers of amorphous silicon (two identical n-i-p cells) onto rolls of stainless steel sheet. The 4 ft2 (0.37 m2) panels are currently 6.2% efficient and made up of layers over an aluminium/zinc oxide back reflector. The push to yet higher efficiencies comes from the layer cake construction of different band-gap energy cells, each cell absorbing a different part of the solar spectrum. Researchers recently obtained 10% efficiency in a 12 in2 (0.09 m2) module.

Rapid thermal processing (RTP) techniques are said to be halving the time normally taken to produce silicon solar cells, while retaining an 18% energy conversion efficiency from sunlight. Researchers at Georgia Institute of Technology have demonstrated RTP processing involving a 3 minute thermal diffusion, as against the current commercial process taking 3 hours. An EC study has also shown that mass production of solar cells could bring substantial benefits and that a £350 million plant investment could produce enough panels to produce 500 MW annually and cut the generating cost from 64 p/kWh to 13p.

V: voltage sensor

A: current sensor

5.2.6 LITHIUM–ION

A high energy battery receiving considerable attention is the lithium–ion cell unit, the development of which has been described by Nissan and Sony engineers1 who point out that because of the high cell voltage, relatively few cells are required and better battery management is thus obtained. Accurate detection of battery state-of-charge is possible based on voltage measurement. In the battery system developed, Fig. 5.6, cell controllers and a battery controller work together to calculate battery power, and remaining capacity, and convey the results to the vehicle control unit. Charging current bypass circuits are also controlled on a cell-to-cell basis. Maximizing lifetime performance of an EV battery is seen by the authors to be as important as energy density level. Each module of the battery system has a thermistor to detect temperature and signal the controllers to activate cooling fans as necessary.

Nissan are reported to be launching the Ultra EV in 1999 with lithium–ion batteries; the car is said to return a 120 mile range per charge. Even further into the future lithium–polymer batteries are reported to be capable of giving 300 mile ranges.

5.2.7 SUPERCAPACITORS

According to researchers at NEC Corp.2, the supercapacitor, Fig. 5.7, will be an important contributor to the energy efficient hybrid vehicle, the absence of chemical reaction allowing a durable means of obtaining high energy charge/discharge cycles. Tests have shown for multi-stop vehicle operations a 25–30% fuel saving was obtained in a compact hybrid vehicle fitted with regenerative braking. While energy density of existing, non-automotive, supercapacitors is only about 10% of that of lead–acid batteries, the authors explain, it is still possible to compensate for some of the weak points of conventional batteries. For effective power assist in hybrids, supercapacitors need a working voltage of over 100 V, alongside low equivalent series resistance and high energy density. The authors have produced 120 V units operating at 24 kW fabricated from newly developed activated carbon/carbon composites. Electric double layer capacitors (EDLCs) depend on the layering between electrode surface and electrolyte, (a) showing an EDLC model. Because energy is stored in physical adsorption/desorption of ions, without chemical reaction, good life is obtained. The active carbon electrodes usually have a specific surface area over 1000 m2/g and double-layer capacitance is some 20–30 µF/cm2 (activated carbon has capacitance over 200–300 F/g). The EDLC has two double layers in series, so it is possible to obtain 50–70° F using a gram of activated carbon. Working voltage is about 1.2 V and storable energy is thus 50 J/g or 14 Wh/kg.

The view at (b) shows a cell cross-section, the conductive rubber having 0.2 S/cm conductivity and thickness of 20 microns. The sulphuric acid electrolyte has conductivity of 0.7 S/cm. The view at (c) shows the high power EDLC suitable for a hybrid vehicle, and the table at (d) its specification. Plate size is 68 × 48 × 1 mm3 and the weight 2.5 g, a pair having 300 F capacity. The view at (e) shows constant power discharge characteristics and (f) compares the EDLC’s energy density with that of other batteries. Fuji Industries’ ELCAPA hybrid vehicle, (g), uses two EDLCs (of 40 F total capacity) in parallel with lead–acid batteries. The stored energy can accelerate the vehicle to 50 kph in a few seconds and energy is recharged during regenerative braking. When high energy batteries are used alongside the supercapacitors, the authors predict that full competitive road performance will be obtainable.

5.2.8 FLYWHEEL ENERGY STORAGE

Flywheel energy storage systems for use in vehicle propulsion has reached application in the light tram vehicle discussed in the Introduction (pages xiii, xiv). They have also featured in pilotproduction vehicles such as the Chrysler Patriot hybrid-drive racing car concept. Here, flywheel energy storage is used in conjunction with a gas turbine prime-mover engine, Fig. 5.8. The drive was developed by Satcon Technologies in the USA to deliver 370 kW via an electric motor drive to the road wheels. A turbine alternator unit is also incorporated which provides high frequency current generation from an electrical machine on a common shaft with the gas turbine. The flywheel is integral with a motor/generator and contained in a protective housing affording an internal

Anion Cation

Differential Throttle gear

Controller Charger

Polarizable electrode

(a)

Polarizable electrodeCollector electrode

(g)

Gasket (thermoplastic) 100

50 Polarizable electrode 0

(e)

Pressurize plate Time/s and terminal

10000

Stacked cells: 144 p (0.83 V/cell) Electrolyte: Sulphuric acid (40 wt%) 1000

Items Characteristics

Working voltage IV) 120 Capacitance IF) 20 100 ESR IMΩ) 78 Maximum current IA) 200

Weight Ikg) 24 Volume IL) 17 Size IW x D x 11 mm) 390 x 270 x 160

Power density IkW/kg) 1.0 Power density IkW/L) 1.4 1.0

Energy density IWh/kg) 1.7

(f)

Energy density IWh/L) 2.4 (d)

Separator (glass fibre)

(g) ELCAPA configuration.

vacuum environment. The 57 kg unit rotates at 60 000 rpm and provides 4.3 kW of electrical energy. The flywheel is a gimbal-mounted carbon-fibre composite unit sitting in a carbo-fibre protective housing. In conjunction with its motor/generator it acts as a load leveller, taking in power in periods of low demand on the vehicle and contributing power for hill climbing or high acceleration performance demands.

European research work into flywheel storage systems includes that reported by Van der Graaf at the Technical University of Eindhoven3. Rather than using continuously variable transmission ratio between flywheel and driveline, a two-mode system is involved in this work. A slip coupling is used up to vehicle speeds of 13 km/h, when CVT comes in and upshifts when engine and flywheel speed fall simultaneously. At 55 km/h the drive is transferred from the first to the second sheave of the CVT variator, the engine simultaneously being linked to the first sheave. Thus a series hybrid drive exists at lower speeds and a parallel hybrid one at higher speeds. The 19 kg 390 mm diameter composite-fibre flywheel has energy content of 180 kW and rotates up to 19 000 rpm.

5.3 Battery car conversion technology

For OEM conversions of production petrol-engined vehicles the decades up to the 1970s, and up to the present day for aftermarket conversions, is typified by that used by many members of the UK Battery Vehicle Society and documented by Prigmore et al4. Such conversions rely on basic lead–acid batteries available at motor factors for replacement starter batteries. A ton of such batteries, at traction power loading of 10–15 kW/ton, stores little more than 20 kWh. Affordable motors and transmissions for this market sector have some 70% efficiency, to give only 14 kWh available at the wheels.

5.3.1 CONVERSION CASE STUDY

The level-ground range of the vehicle can be expressed in terms of an equivalent gradient 1: h, representing rolling resistance, such that a resistance of 100 kgf/tonne is equivalent to a gradient of 1:10. If the fraction of the total vehicle weight contributed by the battery is fb then range is given by {(14 × 3600)/(9.81 × 1000)}fbh. Pessimistically h is about 30 at 50 km/h and if fb is

0.4 then cruising range would be about 60–65 km. This of course is reduced by frequent acceleration and braking.

Series-wound DC motors, Fig. 5.9, have been chosen for low cost conversions because of their advantageous torque/speed characteristics, seen at (a), given relatively low expected road speeds.

Series winding of field and armature the same current is carried by both and as it increases in magnitude so does the magnetic flux and the torque increases more than proportionally with current. Rotation of the armature creates a back-EMF in opposition to the applied voltage because the wires at the edge of the armature are moving across the field flux. Motors are designed to equalize applied and back-EMFs at operational speed. This will be low when field current is high and vice versa. The speed/current curve can be made to move up the x-axis by reducing the field current to a fixed fraction of the armature current (0.5–0.7), with the help of the field diverter resistance

nwf

1000 2000 3000 n rev/min 0 time

(c)
(e)

( b )

(b)
field diverter resistance; (c) speed-base motor characteristics;
(d)
rheostatic control; (e) parallel/series battery control.

shown but the torque for a given armature current is, of course, reduced, see (b). The efficiency of the motor is low at low speeds, in overcoming armature inertia, and again at high speeds as heating of the windings absorbs input power. Motors can thus be more highly rated by the provision of cooling fans. Average power in service should in general be arranged at 0.8 of the rated power and the transmission gear ratio be such that the motor is loaded to no more than its rated power for level-ground cruising. The motor characteristics shown at (c) are obtained by replotting the conventional characteristics on a speed base. The wide range of speeds available (up to 2:1) are around rated power and show how full field can be used for uphill running while weak field is used on the level enabling speed reduction to compensate for torque increase in limiting battery power requirements for negotiating gradients.

With little or no back-EMF to limit current at starting, resistance is added to keep the current down to a safe level, as at (d). The current is maintained at the required accelerating value, perhaps 2–4 times rated current. The starting resistance is reduced as the motor gains speed so as to keep the accelerating current constant to the point where the starting resistance is zero, at the ‘full voltage point’. Thereafter a small increase in speed causes gradual reduction in current to the steady running value. As the current is supplied from the battery at constant voltage, the current curve can be rescaled as a power curve to a common time base, as at (e). The shaded area then gives energy taken during controlled acceleration with the heavily shaded portion showing the energy wasted in resistance. So rheostatic acceleration has an ideal efficiency of about 50% up to full voltage. This form of control is thus in order for vehicle operation involving, say, twice daily regular runs under cruise conditions but unwise for normal car applications.

5.3.2 MOTOR CONTROL ALTERNATIVES

Alternatives such as parallel/series (two-voltage) rheostatic control, or weak field control, can be better for certain applications, but the more elaborate thyristor, chopper, control of motor with respect to battery (Fig. 5.10) is preferred for maintaining efficiencies with drivers less used to electric drive, particularly in city-centre conditions. It involves repetitive on-off switching of the battery to the motor circuit and if the switch is on for a third of the time, the mean motor voltage is a third of the supply voltage (16 V for a 48 V battery), and so on, such that no starting resistance is needed. Effective chopper operation requires an inductive load and it may be necessary to add such load to the inherent field inductance. Because an inductive circuit opposes change in current then motor current rises relatively slowly during ‘on’ periods and similarly falls slowly during ‘off’ periods, provided it has a path through which to flow. The latter is provided by the ‘flywheel’ diode FD, a rectifier placed across the motor to oppose normal voltage. During chopper operation, current ib flows in pulses from battery to motor while current i m flows continuously through the motor. Electronic timing circuits control the switching of the thyristors, (a).

Single ratio drives from motor to driveline are not suitable for hilly terrain, despite the torque/ speed characteristic, as the motor would have to be geared too low to avoid gradient overloading and thus be inefficient at cruise. A 5:1 CVT drive is preferred so that the motor can be kept at its rated power under different operating conditions. There is also a case for dispensing with the weight of a conventional final drive axle and differential gear by using two, say 3 kW, motors one for each driven wheel.

The behaviour of lead–acid batteries, (b), is such that in the discharged condition lead sulphate is the active material for both cell-plates which stand in dilute sulphuric acid at 1.1 specific gravity. During charging the positive plate material is converted to lead peroxide while that of the negative plate is converted into lead, as seen at (c). The sulphuric acid becomes more concentrated in the process and rises to SG = 1.5 when fully charged, the cells then developing over 2 volts. In discharge the acid is diluted by the reverse process. While thin plates with large surface area are

118 Lightweight Electric/Hybrid Vehicle Design

Chopper unit

ib

O A

A

A

O

(a)

id

FD

Applied voltage pulse on

off on

off

im

Motor

Mean

Converts water into

voltage

more acid

O

(b)

Converts acid into water

30 C kWh/t

25

t

20

15

10

5

P 2 3 4 5 7 10 20 30 4050 70 100kW/tonne

t .2 .3 .4 .5 .7 1.0 2.0 4.0 7.0 10.0 hr

(d)

chopper circuit; (b) battery charge– discharge cycle; (c) cell arrangement; (d) battery time-of-discharge curves.

intended for batteries with high discharge rates, such as starter batteries, the expansion process of the active material increases in volume by three times during discharge and the active material of very thin plates becomes friable in numerous charge/discharge cycles, and a short life results. Normal cells, (b), comprise interleaved plates with porous plastic separators; there is one more negative than positive plates, reducing the tendency to buckle on rapid discharge. Expensive traction batteries have tubular plates in some cases with strong plastic tubes as separators to keep the active material in place. Discharge rates of less than half the nominal battery capacity in amp-hours are necessary to preserve the active material over a reasonable life-span, but short bursts at up to twice the nominal rate are allowable. The graphs at (d) permit more precise assessments of range than the simple formula at the beginning of the section which assumes heavy discharge causes battery capacity to be reduced by 70–80% of normal, 25 kWh becoming 20.

When charging the gassing of plates must be considered, caused by the rise in cell voltage which causes part of the current to electrolyse the water in the electrolyte to hydrogen. Gassing commences at about 75% full charge. At this point, after 3–4 hours’ charging at 1/15th battery capacity, the rate should be decreased to 1/20th and carried on until 2.6 volts are shown at the cells. To ensure near-complete removal of the sulphate a periodic ‘soak charge’ should be provided for several hours until peak voltage remains steady at 2.6–2.8 V, with all cells gassing freely and with constant specific gravity. Such a charge should be followed by topping up with distilled water.

5.� EV development history

According to pioneer UK EV developer and producer Geoffrey Harding5, the Lucas programme was a major event in the renaissance of the electric vehicle. He set up a new Lucas Industries facility to develop battery EVs in 1974 because, as a major transport operator, he had asked Lucas to join him in an approach to a UK government department for some financial assistance to build a battery electric bus which would operate on a route between railway stations in Manchester. The reasons for his interest in this project were twofold. First, there was a major problem with the reliability of many of the diesel buses at that time and he wanted to find out whether electric buses would live up to the attributes of good reliability and minimal maintenance that had been afforded to EVs for many years. Second, a world shortage of oil at that time was causing an apparent continuous and alarming increase in the price.

Having subsequently joined Lucas and set up the new company, he was responsible for building the electric bus in question and providing technical support when it entered service. The bus – the performance of which was comparable with diesel buses, except for range – operated successfully for some years and was popular with both passengers and drivers. On the other hand, it was not popular with schedulers because its restricted range (about 70 km in city service) added yet more limitation to its uses, particularly at weekends. Nevertheless, much was learnt from the in-service operation of this vehicle which proved to be remarkably reliable. He then obtained agreement within Lucas that the battery EV most likely to succeed at that time was a 1-tonne payload van because it would be possible, with relatively minor changes to production vans, to modify the drive to battery electric without reducing either the payload volume or the weight, Fig. 5.11.

The converted Bedford vehicles underwent a significant testing programme on that company’s test track, and were in fact built on the company’s ICE van production line, interspersed between petrol- and diesel-powered versions of CF vans. This method of production was the first of its kind. Some hundreds of Bedford vans and a smaller number of Freight Rover vans were built and sold, all with a working range in excess of 80 km in city traffic, a payload of just under 1 tonne, an acceleration of 0–50 km in 13 s, a maximum speed of 85 km/h, and a battery design life of 4 years.

The vehicles had, for that time, sophisticated electronic controllers and DC/DC converters, as well as oilfired heating and demisting systems. Lucas designed and constructed the chargers and battery-watering systems. Some were sold in the USA as the GM Griffon, (a), and it was estimated that collectively their total service had exceeded 32 million km, and even today a few are still operating.

The Lucas Chloride converted Bedford CF van had two-pedal control and a simple selector for forward/reverse. Most of the vehicle’s braking was regenerative and batteries were of the tubular cell lead–acid type. Thirty-six monobloc units of 6 volts were used – connected in series to give a

FANS FOR HOOD

50 A
100 A
200 A
300

6-0

200

5-5

100

5-0

0

(b) 4-5

Capacity (Ah)

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Ampere hours discharged

(a)

CONTROLLER

VENTILATION

DIFFERENTIAL & 2ND STAGE REDUCTION

MOTOR &

1st STAGE

REDUCTION

AIR EXIT DUCTS FROM HOOD

(a) GM Griffon; (b) terminal volts per 6 V module and discharge current in amps; (c) Lucas Chloride SERIES HYBRID MODE hybrid car; (d) bi-mode drive system.

216 V, 188 Ah pack. The rear-mounted traction motor drove the wheels through a primary reduction unit coupled to a conventional rear axle, via a prop-shaft. Measured performance of the monobloc is shown at (b); an energy density of 34 Wh/kg was involved, at the 5 hour rate, and a 4 year service life was claimed. The motor used was a separately excited type in order to allow the electronics maximum flexibility in determining the power curve. It weighed 15 kg and had a controlled output of 40 kW; working speed was 6100 rpm corresponding to a vehicle speed of about 60 mph. The motor control system used an electronic bypass to leave the main thyristor uncommutated during field control. The latter uses power transistors which handle up to 25 A.

Within the Lucas development programme, which at one time employed close to 100 personnel, some work on HEVs was undertaken and one five-seat passenger car was designed and built. This utilized an electric Bedford drive system and could be operated either as a series hybrid or a parallel hybrid. The car had a maximum speed of 130 kph, and a pure-electric range of about 70 km. The Lucas Chloride hybrid, (c), has engine (3) driving through the motor (1) but midships positioning of the batteries (4) with on-board charger (5) at the rear. Clutches are shown at (6) while (7) and (8) are alternator and control unit. This used Reliant’s 848 cc engine developing 30 kW alongside a 50 kW Lucas CAV traction motor. The 216 V battery set had capacity of 100 amp-hour on a 5 hour rate. Maximum speed in electric drive of 120 km/h rises to 137 km/h in combined mode, (d).

5.4.1 ELECTRIC VEHICLE DEVELOPMENT 1974–1998

In considering the changes which have taken place in the quarter century since the start of the Lucas project, Harding argues that the developments which have taken place in electric cars are not as great as had been hoped and expected. Some hybrids, he considers, are effectively ICEVs with an electric drive which assists when required. A major problem with HEVs has been their cost, which is exacerbated by having two drive systems in one vehicle. Fortunately, the automotive industry is so good at meeting challenges of this nature that who can say what can be achieved? However, it is claimed that micro-turbines together with their associated generators and accessories can be produced cheaply, mainly because they have a very low component count. These turbines are capable of operating on a wide variety of fuels and are considered to produce a very low level of pollutants, but with one or two exceptions such as Volvo and Chrysler, these claims have not been subjected to any extensive field testing. If what is claimed proves to be true, then such vehicles would be expected to play a large part in the transport scene in the new millennium.

At present, the great hope for the future, he believes, is the fuel cell. Hydrogen is the preferred fuel for fuel cells but its storage presents a problem. One of the ways of overcoming this problem is to convert a liquid fuel, such as methanol, into hydrogen. This was done in the 5 kW unit made by the Shell Oil Company as long ago as 1964. The unit was installed in the world’s first fuel-cell powered car. Shell also produced a 300 W nett cell in 1965 which converted methanol directly into electricity, so it is not the case that this technology is new. The principal problem at the time this work was carried out was the cost of the unit. Although a number of fuel-cell powered cars

Fig. 5.12 Sinclair C10 proposal.

have been built recently by automobile manufacturers, the only vehicle so far offered for sale is the Zevco London taxi which was launched in London in July 1998. The propulsion system is a hybrid arrangement: a battery drives the vehicle and is recharged by a 5 kW fuel cell. The vehicle uses bottled hydrogen as fuel and has a service range of 145 km, and a performance similar to its diesel counterpart. This design works well because the stop-start nature of the traffic provides time for the low output of the fuel cell to replenish the energy drawn from the battery during previous spells of vehicle motion. At a later date, this type of taxi may be fitted with a cryogenic hydrogen-storage system, perhaps placed between the two layers of a sandwich-floor construction of the vehicle. With such an arrangement, it is expected that the fuel cell would be refuelled with very cold liquid hydrogen in minutes and, thereby, would extend the vehicle’s range dramatically, but only in stop-start traffic.

Harding opines that what the world really needs are vehicles fitted with fast-response, high-output fuel cells together with on-board clean reformers which would enable a liquid fuel to be turned into hydrogen on vehicles. Initially, the most likely liquid fuel would seem to be methanol, but arranging for methanol to be widely available would necessitate some large changes in infrastructure. If all this is possible, then refuelling vehicles with liquid fuel would be, in principle, little or no different from today. The eventual aim is said, by those developing high-output fuel cells, to be the development of reformers which can produce hydrogen from gasoline. In this case, only the current gasoline infrastructure would be required. Interest and investment in fuel cells is increasing, and the joint arrangements between the Canadian fuel cell company Ballard and motor industry giants Mercedes and Ford would appear to be an almost irresistible force on a course aimed at solving some daunting problems. The Ballard unit is a proton exchange membrane (PEM) fuel cell and amongst early examples of road vehicles fitted with this are buses in the USA. Quite apart from the technical problems still to be resolved, the problem of cost is very great.

5.5 Contemporary electric car technology

According to Sir Clive Sinclair, whose abortive efforts to market an electric tricycle have led him to concentrate on economical bicycle conversions, peak efficiencies of 90% are available with EVs for converting electricity into tractive energy – and that attainable electrical generating efficiencies of over 50% meant a 45% fuel conversion efficiency could be obtained compared with 30% for the petrol engine. His C10 proposal shown in Fig. 5.12 must mean his faith in the future of the electric car is still maintained.

Fig. 5.13 Road-induced electricity.

There are other initiatives, too, such as the desire to make motorway driving under very high density peak traffic conditions less dangerous and less tiring. This is generating fresh interest in reserved lanes for vehicle guidance systems. Where these additionally provide roadway-induced powering, Fig. 5.13, as described by researchers from the Lawrence Livermore National Laboratory6, a case for a car to suit relatively long-distant commuters can be made. The success of trials on GM’s Impact electric car have so far pointed to the very considerable importance of light weight, good aerodynamics and low rolling resistance but the electrical breakthrough has come in the electronics technology of the DC/AC converter. Ford, too, have had very promising prototype results from their Ecostar l car-derived van, using a transistorized DC to AC inverter.

5.5.1 HONDA ‘EV’

The state of the art in pilot-production electric cars is typified by Honda’s nickel–metal hydride battery driven electric car, Fig. 5.14; it has been given the name ‘EV’ and claims twice the range obtainable with comparable lead–acid batteried cars. The car is not a conversion of an ICE model and has 95% new componentry. It is a 3-door, 4-seater with battery pack in a separated compartment between the floor. The pack comprises 24 × 12 V batteries and rests between virtually straight underframe longitudinal members running front to rear for maximum crash protection. The motor is a brushless DC type with rare earth high strength magnets and is said to give 96% efficiency. There is a fixed ratio transmission with parking lock. Maximum torque is 275 Nm, available from 0 to 1700 rpm, the speed at which a maximum power of 49 kW is developed and remains constant up to 8750 rpm. The under-bonnet power control unit comprises management and motor ECUs, power driver, junction board, 12 V DC/DC converter, air-conditioning inverter and 110/220 V onboard charger. Its aluminium container is liquid cooled in a system shared with motor and batteries. The controller uses IGBT switching devices in a PWM system. A phase control system involves both advanced angle control and field weakening to optimize operation in both urban and motorway conditions. A heat-pump climate control system has an inverter-controlled compressor with a remote-control facility to permit pre-cooling or pre-heating of the cabin prior to driving. Energy recovery is carried out in both braking and ‘throttle-off’ coasting modes. Low rolling-drag tyres are inflated to 300 kPa and are said to have just 57% the resistance of conventional tyres. A ‘power-save’ feature automatically reduces peak power when battery state-of-charge drops below 15%. An instrument display shows range and battery-charge state as a biaxial graph with clearly marked segments which even respond to throttle pedal depression. The car has a 125 mile urban range to the FUDS standard while top speed is 80 mph. Recharge time is 8 hours from 20% to fully charged.

5.5.2 GENERAL MOTORS ‘EV1’

The latest generation GM EV1 (Fig. 5.15) is a purpose-built electric vehicle which offers two battery technologies: an advanced, high capacity lead–acid, and an optional nickel–metal hydride. The EV1 is currently available at selected GM Saturn retailers and is powered by a 137 (102 kW), 3 phase AC induction motor and uses a single speed dual reduction planetary gear set with a ratio of 10.946:1. The second generation propulsion system has an improved drive unit, battery pack, power electronics, 6.6 kW charger, and heating and thermal control module. Now, 26 valve-regulated, high capacity, lead–acid (PbA) batteries, 12 V each, are the standard for the EV1 battery pack and offer greater range and longer life. An optional nickel–metal hydride (NiMH) battery pack is also available for the Gen II model. This technology nearly doubles the range over the first generation battery and offers improved battery life as well. The EV1 with the high capacity lead– acid pack has an estimated real world driving range of 55 to 95 miles, depending on terrain, driving habits and temperature; range with the NiMH pack is even greater. Again, depending on terrain, driving habits, temperature and humidity, estimated real world driving range will vary from 75 to 130 miles, while only 10% of power is needed to maintain 100 km/h cruising speed, because of the low drag, now aided by Michelin 175/65R14 Proxima tyres mounted on squeeze-cast aluminium alloy wheels.

The 1990 Impact prototype from which the EV1 was developed had one or two more exotic features which could not be carried through to the production derivative but claimed an urban range of 125 miles on lead–acid batteries. In the Impact the 32 10 volt lead–acid batteries weighed 395 kg, some 30% of the car’s kerb weight, housed into a central tunnel fared into the smooth underpanel and claimed to have a life of 18 500 miles. The Impact weighed 1 tonne and accelerated from 0 to 100 kph in 8 seconds, maximum power of the motor being 85 kW. The vehicle had 165/65R14 Goodyear low-drag tyres running at 4.5 bar. Two 3 phase induction motors were used, each of 42.5 kW at 6600 rpm; each can develop 64 Nm of constant torque from 0 to 6000 rpm, important in achieving 50–100 km/h acceleration in 4.6 seconds. Maximum current supply to each motor was 159 A, maximum voltage 400 V and frequency range 0–500 Hz. The battery charger was integrated into the regulator and charging current is 50 A for the 42.5 Ah lead–acid batteries, which could at 1990 prices be replaced for about £1000.

Lite-Cast* suspension link

Aluminum track link Battery pack module Aluminum axle Propulsion inverter

Micro-alloy coil spring

Power steering inverter Drive unit: Battery pack Motor/gearbox/ differential

Cast aluminum cradle

Sealant Battery disconnect

Propulsion service: tires check tire

pressureValve-regulated systemlead-acid

Squeeze-cast aluminum wheel Battery tray

battery module

Electric rear brake High voltage relays

Inductive Low rolling-resistance tires charge port

Double-wishbone aluminum Power steering

suspension motor/pump Aluminum

Heat pump

Stabilizer bar brake caliper

motor/compressor

Propulsion system Electro-hydraulic coolant pump brake modulator

Fig. 5.15 GM EV1 and Impact

prototype, inset.

The EV1 can be charged safely in all weather conditions with inductive charging. Using a 220 volt charger, charging from 0 to 100% for the new lead–acid pack takes up to 5.5–6 hours. Charging for the nickel–metal hydride pack, which stores more energy, is 6–8 hours. Braking is accomplished by using a blended combination of front hydraulic disk, and rear electrically applied drum brakes and the electric propulsion motor. During braking, the electric motor generates electricity (regenerative) which is then used to partially recharge the battery pack. The aluminium alloy structure weighs 290 pounds and is less than 10% of the total vehicle weight. The exterior composite body panels are dent and corrosion resistant and are made from SMC and RIM polymers. The EV1 is claimed to be the most aerodynamic production vehicle on the road today, with a 0.19 drag coefficient and ‘tear drop’ shape in plan view, the rear wheels being 9 inches closer together than the front wheels. The EV1 has an electronically regulated top speed of 80 mph. It comes with traction control, cruise control, anti-lock brakes, airbags, power windows, power door locks and power outside mirrors, AM/FM CD/cassette and also a tyre inflation monitor system.

5.5.3 AC DRIVES

An interesting variant on the AC-motored theme, Fig. 5.16, is the use of a two-speed transaxle gearbox which reduces the otherwise required weight of the high speed motor and its associated inverter. A system developed by Eaton Corporation is shown at (a) and has a 4 kW battery charger incorporated into the inverter. A 3 phase induction motor operates at 12 500 rpm – the speed being unconstrained by slip-ring commutator systems. A block diagram of the arrangement is at (b) and is based on an induction motor with 18.6 kW 1 hour rating – and base speed of 5640 rpm on a 192 V battery pack. The pulse width modulated inverter employs 100 A transistors. The view at (c) shows the controller drive system functions in association with the inverter. In an AC induction motor, current is applied to the stator windings and then induced into the windings of the rotor. Motor torque is developed by the interaction of rotor currents with the magnetic field in the air gap between rotor and stator. When the rotor is overdriven by coasting of the vehicle, say, it acts as a generator. Three phase winding of the stator armature suits motors of EV size; the rotor windings comprise conducting ‘bars’ short-circuited at either end to form a ‘cage’. Rotation speed of the magnetic field in the air gap is known as the synchronous speed which is a function of the supply-current frequency and the number of stator poles. The running speed is related to synchronous speed by the ‘slip’.

If two alternators were connected in parallel, and one was driven externally, the second would take current from the first and run as a ‘synchronous motor’ at a speed depending on the ratio of each machine’s number of poles. While it is a high efficiency machine which runs at constant speed for all normal loads, it requires constant current for the rotor poles; it is not self-starting and will stop if overloaded enough for the rotor to slip too far behind the rotating stator-field. Normally, the synchronous motor is similar in construction to an induction motor but has no short-circuited rotor – which may be of the DC-excited, permanent-magnet or reluctance type.

The view at (d) shows a stator winding for a 2 pole 3 phase induction motor in diagrammatic form. If supply current frequency is f s, then stator field speed is f s/p for number of poles p. Rotor current frequency f r = sf p where s is the slip. Power supplied to a 3 phase motor can be expressed as EI(3)1/2f s n where n is efficiency. When a synchronous motor has no exciting voltage on the rotor it is termed a ‘reluctance’ motor which has very simple construction and, when used with power transistors, can be applied as a variable speed drive. Axial air-gap versions are possible as at (e); such an electronically commutated motor can operate with a DC source by periodic reversal of the rotor polarities.

The general form of control, with DC link inverter, for induction motors is shown diagrammatically at (f). The thyristors of the inverter are generally switched so as to route current

126 Lightweight Electric/Hybrid Vehicle Design

192 V Battery (f)

(d)

AC input charge power

(c)

through the stator winding as though it were connected to a 3 phase AC. Frequency can be varied by timing of pulses to the thyristors and is typically 5–100 Hz, to give a speed range of almost twice synchronous speed. According to engineers from Chloride EV Systems Division7, devices for building such an inverter are not yet available for current levels associated with EV traction. This situation may have already changed in the interim, however.

5.5.4 FORD E-KA: LITHIUM–ION BATTERY POWER

European Ford claim the first production-car use of lithium–ion batteries in a road vehicle by a major player in the industry. The high energy density and power-to-weight ratio of these storage units puts a prototype electric version of the small Ka hatchback, Fig. 5.17, on a par with the petrol-engined model in driving performance. Top speed is said to be 82 mph with acceleration to 62 mph in 12.7 seconds, although range between charges is still only 95 miles, but is extendible to 125 miles with a constant speed of 48 mph.

Until now Li–ion batteries have been used mainly in small consumer electronic products like notebook computers, cellular phones, baby monitors and smoke detectors. Output per cell of 3.6 volts is some three times that of nickel–cadmium and nickel–metal–hydride that they widely replace. They also retain full charge regardless of usage, can be recharged from zero to full capacity in 6 house with over 3000 repeated charge/discharge cycles, and are immune from the so-called ‘memory effect’ suffered by Ni–Cads. The basic Li–ion chemistry was initially redeveloped for automotive use by the French company SAFT SA, a leading battery manufacturer. Their advanced technology was then adapted to the e-Ka by Ford’s Research Centre in Aachen, Germany, with financial assistance for the project from the German Ministry for Education and Research. The battery pack consists of 180 cells with 28 kWh rating and weighs only 280 kg (615 lb). This is 30% the weight of the power equivalent in lead–acid batteries, and substantially less than comparable Ni–Cads and Ni–MHs. In terms of volume the Ni-ion has approximately half the bulk of all the other three.

Batteries are divided into three individual sealed ‘troughs’, each with 30 modules containing six cells. One trough is located in the engine compartment, with the other two on either side of the back axle. Nominal output of 315 V DC is transformed by a solid-state inverter to 3 phase AC for the traction motor. Heat generated by the internal resistance of these second-generation Li–ion batteries is dissipated by one of two fluid cooling systems. A second independent system cools the drivetrain, with a 65 kW (88 bhp) asynchronous motor followed by a fixed-ratio transmission driving the front wheels. Torque rating is 190 Nm (140 lb ft).

Fig. 5.17 Ford e-Ka.

Performance of the e-Ka is enhanced by a 45 kg (100 lb) weight reduction to counter the battery load. The roof and hood are of aluminium sandwich construction with a thermoplastic filling, while the front brake callipers with ceramics discs, rear drums, wheel rims and back axle are all in light alloy. Electric power steering supplied by Delphi provides further weight saving, where an electronic control module regulates the assistance needed to minimize battery demand. Brake servo and ABS system are also electric.

5.� Electric van and truck design

5.6.1 GOODS VAN TO FLEET CAR CONVERSION

Europe’s largest maker of EVs is Peugeot-Citroen whose Berlingo Dynavolt, Fig. 5.18, sets out to maximize the benefits of electric vehicles in a fleet car. It has a range extender in the form of an auxiliary generating system which does not quite make the vehicle a hybrid in the conventional sense. The generator feeds current into the traction motor rather than into the battery pack. The generator engine is a 16 ps, 500 cc Lombardini running on LPG which drives a Dynalto-style starter generator unit developing 8 kW at 3300 rpm, to supplement the supply from the 4 kW Saft Ni–Cad batteries. Company designed software controls the cut-in of the generator according to range requirements. The range is 80 km, which can be extended to 260 km with generator assistance. Series production was imminent as we went to press.

5.6.2 FORD EXT11

An early key US initiative in AC drives was the Ford EV project EXT11, Fig. 5.19, which has been exploring the use of an alternating current drive motor in the Aerostar Minivan, seen at (a). A sodium–sulphur battery was employed and a single-shaft propulsion system. A battery with the following specification was involved: nominal voltage 200 V; minimum voltage at 60 kW, 135 V; 50 kW capacity on FUDS cycling; 60 kW maximum power (20 seconds) and 35 kW continuous power (40 minutes). Much of the technology has since been carried over to the Ford Ecostar, described later. Dimensions of the battery were 1520 × 1065 × 460 mm and it was based on the use of small (10 Ah) cells connected in 4-cell series strings with parallel 8 V banks arranged to provide the required capacity. A voltage of 200 required 96 cells in series (each had a voltage of 2.076) and a parallel arrangement of 30 cells gave the 300 Ah capacity; overall 2880 cells are used and their discharge performance is shown at (b). Battery internal resistance was 30 milliohms with an appropriate thermal management system under development. The cells rested on a heater plate which incorporates a 710 W element used particularly in the initial warm-up. An associated air cooling system can dissipate 6 kW. Temperature must be maintained above 300° C to achieve an internal resistance value that will allow sensible current flow.

The US General Electric Company was also a partner in the programme, specifically on the AC drive system, at (c). This comprised a 50 bhp induction motor within a two-speed transaxle, a liquid-cooled transistor inverter being used to convert the 200 battery volts into variable-voltage 3 phase AC, (d). The induction motor was a 2 pole design with just under 13 cm stack length – within a stator diameter just under 23 cm. Calculated stall torque was 104 Nm and the transition between constant torque and constant power operation occurred at 3800 rpm. Design speed was 9000 rpm and absolute maximum 12 000 rpm. Full power was developed at a line current of 244.5

A. Control strategy relating inverter to motor is shown at (e).

Ford produced the transaxle on an in-house basis having coaxial motor and drive, (f). The motor rotor is hollow to allow one of the axle shafts to pass through it from the bevel-gear differential. The installation in a test saloon car achieved 30% gradient ability, 0–50 mph acceleration in less than 20 seconds, 60 mph top speed and 0.25 kWh/mile energy consumption. Gear ratios were 15.52 and 10.15:1, and rearward speed was obtained by electrically reversing the motor; in ‘neutral’ the motor was electrically disconnected.

5.6.3 UK EVA PRACTICE FOR CVS

In its manual of good practice for battery electric vehicles the Electric Vehicle Association lays down some useful ground rules for conceptual design of road-going electric trucks. Exploiting the obvious benefits of EV technology is the first consideration. Thus an ultra-low floor walk-through cab is a real possibility when batteries and motors can be mounted remotely. Lack of fuelling requirement, ease of start-up and getaway – also driving simplicity of two-pedal control without gearshifting – all these factors lend themselves to operations, such as busy city-centre deliveries where a substantial part of the driver’s time is spent in off-loading and order-taking. Any aspect of vehicle design which minimizes the driving task thus maximizes his or her other workload duties. Successful builders of electric trucks are thus, say the EVA, specialists in assembling bought-in systems and components. Required expertise is in tailoring a motor/battery/speed-controller package to a given application. Principles to be followed in this process are keeping top speeds and motor power as low as possible consistent with fulfilling the task; using controllers which prevent any unnecessary acceleration once the vehicle has reached running speed in stop-start work; using generously rated motors rather than overspecifying battery capacity – the latter because large batteries cost more, displace payload and waste energy providing tractive force required for their extra weight.

Whereas over a decade ago the rule of thumb applied that 1 tonne of batteries plus one of vehicle and one of payload could be transported at 20 mph over a 20 mile range including 200 stop-starts per charge, now 40/40 to 50/50 mph/miles range is feasible by careful design. Belted radial tyres can now be run at 50% above normal inflation pressures. High voltage series motors of 72 V and above have working efficiencies of 85–90% over a 3:1 speed range and electronic controllers cut peak acceleration currents and avoid resistive losses – to extend ranges by 15–20%

Heater

Transistorized DC-to-AC inverter

Integrated interior permanent-magnet AC motor and automatic transaxle