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小型轻量电动汽车的碰撞安全性

调研分析

Analysis and Survey of Small Lightweight

Electric Vehicle Crash Safety

(申请清华大学工学硕士学位论文)

培养单位：汽车工程系

学科：机械工程

研究生：史悦

指导教师 : 周青教授

二○一一年六月



Contents

小型轻量电动汽

车的碰撞安全性调研

分析

肖

文

小型

轻量电

动汽

车的碰撞安

全性调研

分析

史

悦



Contents

关于学位论文使用授权的说明

本人完全了解清华大学有关保留、使用学位论文的规定，即：

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作者签名：导师签名：

日期：日期：



I

摘摘摘摘要要要要

论文研究了汽车纯电动化趋势对于车辆被动安全性能产生的影响，并在事故

避免和碰撞防护等方面进行了调研，重点调研分析了以保证电动车具有与传统汽

车相同的被动安全性能为目标所采用的设计方案。

论文首先总结了一般性的汽车被动安全设计理念，以作为研究车辆小型化和

电动化对车辆被动安全带来的特殊要求的基础。

通过文献阅读和对现有电动汽车的调研，从总体上把握车用技术的最新发

展，介绍了电驱动动力系统中与车辆安全有关的因素，评估了针对新的动力系统

而做的总布置改进对车辆安全带来的可能影响，并介绍了一系列有突出特点的电

动车型上为应对新的安全要求而采用的改进措施。其中，对 Benz Smart ForTwo

车型安全性能的深入研究发现，其特有的结构设计值得在未来小型电动汽车的碰

撞安全概念设计中借鉴。

在对碰撞兼容性的分析中，发现车辆的小型化对乘员舱的刚度提出了更高的

要求，并在对 Benz Smart ForTwo 和 Reva G-Wiz 车型的具体分析中进一步证明了

这一点的重要性。

无论是在被调研的车型上，还是在一般分析中，电池组件都主要布置于位于

车辆地板或地板中央通道内。

在调研中也发现，在小型车领域，目前能够达到碰撞要求的车型往往在车长

上超过 3100 毫米且质量上大于 1000 千克。到现在为止，Euro NCAP 只对两款全

电动汽车进行了测试和评分，并给予了其中的一款车型：长度 3475 毫米而质量

为 1110 千克的三菱 i-MIEV 4 星的碰撞性能评分。同时，Euro NCAP 的测试结果还

表明先进的乘员约束系统，尤其是安全带预紧装置和可调节的气囊是该类车达到

优秀的碰撞保护性能的重要措施。对车辆碰撞吸能区的研究也进一步证明了在车

辆质量增加而吸能区减小时乘员约束系统的重要性。在调研中发现，电动车总布置的灵活性可以使在汽车碰撞前部的吸能区设计

更加均匀的变形响应成为可能。论文最后提出了一种活动的可单独变形吸能的电

池总成支撑结构概念设计，为吸收车辆碰撞时由电池总成的质量带来的附加动能

提供新的措施。

关键词：小型车，城市轿车，电动车，碰撞安全，耐碰撞性



Contents

II

Abstract

This thesis presents a study on the impact of vehicle electrification on vehicle safety and

offers solutions to ensure consistent safety performance, in both accident prevention and

protection during a crash. A general approach on vehicle safety fundamentals provides the

basis for imposing specific requirements caused by the miniaturisation and electrification of

vehicles. The safety relevant characteristics of the electric drive train components are

introduced and the influence on the packaging possibilities evaluated based on literature

research. The survey of current electric vehicles provides an overview of the state of the art

technology. A range of prominent models with noticeable specifics is introduced to presents

measures taken to ensure vehicle safety under the new requirements. A detailed inquiry of the

Smart ForTwo’s safety performance provides further conceptions in crashworthy vehicle

design and evaluates their feasibility in small lightweight electric vehicle design.

The safety fundamentals of crash compatibility as well as the examination of the Smart

ForTwo and Reva G-Wiz emphasise the importance of increased rigidity demands for the

small vehicles’ safety cell. Battery compartments in the vehicle floor and tunnel prevailed in

both the vehicles surveyed and the general analysis. The survey shows that so far safety

performance on the level of standard motorcar certification has only achieved by vehicles

longer than 3100 mm and kerb weight above 1000 kg. The Euro NCAP rated only two all-

electric models until now, awarding the 3475 mm long and 1110 kg heavy Mitsubishi i-MiEV

with 4 stars.

The results also show that good performance was only achieved by featuring advanced

restraint systems, in particular seatbelt pretensioners and adaptive airbags. Their significance

is further confirmed by the crush space study addressing the reduced crush zone space andincreased kerb weight. The study finds higher stiffness and more even deformation response

requirements for the crumple zone, which are feasible due to design freedom enabled by

electric drives. Moreover, a concept of flexible respectively crushable battery support is

established as measurement to cope with the vehicle’s increased kinetic energy due to the

battery’s weight.

Keywords: microcar, city car, electric vehicle, crash safety, crashworthiness



Contents

III

Contents

1 Introduction .................................................................................................................... 1

2 Motivation and research objective.................................................................................. 2

2.1 Need for alternative propulsion ................................................................................ 2

2.2 Electric Vehicle advantages ..................................................................................... 3

2.3 Microcars ................................................................................................................. 4

3 Background and fundamentals of vehicle crash safety .................................................. 5

3.1 Crashworthy vehicle structure .................................................................................. 5

3.2 Managing impact kinetic energy: Crash pulse .......................................................... 7

3.3 Occupant restraint system ....................................................................................... 8

3.4 Crash Compatibility ................................................................................................ 10

3.4.1 Mass effect ......................................................................................................... 10

3.4.2 Stiffness effect.................................................................................................... 11

3.4.3 Effects on mini lightweight electric vehicle design .............................................. 11

3.5 Pedestrian safety and silent electric vehicles ......................................................... 12

3.6 General crash test set-up and evaluation ............................................................... 13

3.7 Legal Vehicle Safety Regulations .......................................................................... 15

3.8 New Car Assessment Programmes ....................................................................... 16

4 Structural design and automotive layout ...................................................................... 18

4.1 Body-on-frame ....................................................................................................... 18

4.2 Unit-body ............................................................................................................... 19

4.3 Aluminium for structural elements .......................................................................... 21

4.4 Lightweight design effects ...................................................................................... 22

4.5 Vehicle classes and segmentation of electric vehicles ........................................... 22

5 Electric vehicle technology........................................................................................... 25

5.1 Electric motors and efficiency ................................................................................ 25

5.2 Battery technology ................................................................................................. 28

5.2.1 Energy Specific Density and weight ................................................................... 29

5.2.2 Range ................................................................................................................ 30



Contents

IV

5.2.3 Charging and lifetime ......................................................................................... 31

5.2.4 Safety issues ...................................................................................................... 31

5.3 Electric vehicle layout and packaging scope .......................................................... 32

5.4 Lightweight design of electric vehicles ................................................................... 34

6 Current electric vehicle layout and design .................................................................... 35

6.1 Conversion design ................................................................................................. 35

6.1.1 BMW Mini E ....................................................................................................... 37

6.1.2 Tesla .................................................................................................................. 38

6.2 Purpose design ...................................................................................................... 38

6.2.1 REVA i/ G-Wiz i .................................................................................................. 39

6.2.2 Reva NXR and NXG .......................................................................................... 42

6.2.3 Think City ........................................................................................................... 44

6.2.4 Tazzari ZERO .................................................................................................... 46

6.2.5 BMW i ................................................................................................................ 48

7 Miniaturisation of vehicles ............................................................................................ 49

7.1 Daimler Smart ForTwo Coupé ............................................................................... 49

7.2 Smart ED second generation ................................................................................. 52

7.3 Consequences in the design of mini lightweight vehicles ....................................... 52

7.4 Concept of flexible and deformable battery support ............................................... 53

8 Conclusion ................................................................................................................... 55

9 Formula symbols and indices ...................................................................................... 57

10 Literature ..................................................................................................................... 58

11 Apprendix .................................................................................................................... 66

11.1 Table of selected current mini electric vehicles ...................................................... 66



Contents

V

Table of figures

Figure 3-1: Resistance levels of consecutive deformation zones ........................................ 6

Figure 3-2: Axial folding pattern of rectangular tube ............................................................ 7

Figure 3-3: Exemplary crash pulse: vehicle and occupant deceleration history ................... 7

Figure 3-4: Equivalent square wave (ESW) vs. peak deceleration of actual curve .............. 8

Figure 3-5: Exemplary occupant and vehicle displacement in a crash test ......................... 9

Figure 3-6: Euro NCAP front impact test ........................................................................... 17

Figure 4-1: Chrysler Imperial 1966, last year of full frame construction in Imperials .......... 18

Figure 4-2: Ultra Light Steel Auto Body (ULSAB) 4 door, five passenger sedan ............... 19

Figure 4-3: Lightweight ASF (Audi Space Frame) of second generation Audi TT Coupé .. 20Figure 4-4: Rough EV classification based on survey ....................................................... 23

Figure 5-1: AC propulsion three-phase asynchronous motor in a BMW Mini E ................. 26

Figure 5-2: Electric drive configurations ............................................................................ 27

Figure 5-3: Siemens VDO eCorner motor-in-hub concept ................................................. 27

Figure 6-1: Mini E cutaway ............................................................................................... 37

Figure 6-2: REVA i / G-Wiz i ............................................................................................. 39

Figure 6-3: REVA/G- Wiz in a EURO NCAP frontal offset collision test ............................ 40Figure 6-4: REVA/G- Wiz condition after a EURO NCAP frontal offset collision test ......... 40

Figure 6-5: REVA/ G- Wiz after a collision with a Skoda Octavia ...................................... 41

Figure 6-6: Reva NXR ...................................................................................................... 42

Figure 6-7: Reva NXR’s steel spaceframe ...................................................................... 43

Figure 6-8: Think City (4 seat version) .............................................................................. 44

Figure 6-9: Think City body structure ................................................................................ 45

Figure 6-10: Battery position and several safety related devices in the Think City .............. 45

Figure 6-11: Tazzari ZERO ................................................................................................. 46

Figure 6-12: BMW ActiveE drive train ................................................................................. 48

Figure 6-13: BMW i3 carbon fibre composite body ............................................................. 48

Table of figures



Contents

VI

Figure 7-1: Smart ForTwo Coupe ..................................................................................... 49

Figure 7-2: Smart ForTwo Coupe in EURO NCAP frontal offset crash test ....................... 50

Figure 7-3: Cutaway of Smart ForTwo .............................................................................. 51

Figure 7-4: Electric drive layout in a Smart ED ................................................................. 52

Figure 7-5: Concept of flexible/ deformable battery support .............................................. 54

Table of figures



1 Introduction

1

1 Introduction

To cope with future transportation challenges new propulsion systems and

new vehicle concepts have to be established. Electric mobility is a promising hope since

serial production is announced by a range of manufacturers. In the transition from concept

cars and test fleets to serial production of all electric vehicles, however, the safety

performance has to be approached to achieve sustainable future developments in electric

mobility. Particularly when downsizing vehicles in dimension and weight is the aim to achieve

clean urban mobility.

Focusing on the crash safety this mini-thesis indentifies the challenges for small lightweight

electric vehicles and the measures that can be taken to cope with. The familiarization with

the crash safety fundamentals detects the basic challenges for miniaturisation and

electrification of vehicles on a wide range of safety related issues. A general introduction of

body, frame and chassis layouts feasible for electric vehicle design approaches the vital

vehicle structure. To distinguish the relevant differences to standard motor vehicles with

internal combustion engines the electric drive train technology is surveyed. Thereby the

focus lies on weight, size and packaging possibilities of the components. The elaboration of

battery technology offers insight in related dangers. Electric propulsion integration affects the

mass distribution and packaging, thus influences the crash safety. The investigation of the

correlation between them provides guidelines for EV packaging. Furthermore the range of

new possibilities in the drive-train set-up enabled by contemporary technology is included

and evaluated. Overall this thesis introduces the new opportunities to adapt the vehicle

design and packaging to safety demands, instead of simply replacing the traditional

components with new ones.

Based on the provided background the body design and layout, as well as safety features

and crash behaviour of current mini electric vehicles are surveyed and analysed. A selection

of vehicles with remarkable characteristics in performance, market success, safety

performance or design is introduced to evaluate the feasibility of a safe small electric vehicle

and assess measures to be taken. In addition the safety performance of the Smart ForTwo,

representing safe microcars, is examined and reviewed.

The key influences on passive crash safety are determined, the impacts of miniaturisation

and electric drive train implementation investigated and guidelines provided.



2 Motivation and research objective

2


2.1 Need for alternative propulsion

The internal combustion engine has dominated the power generation for personal vehicles

from the early beginning on and their convenience is still unachieved by any competitor. But

the world’s oil resources are diminishing and the serious and urgent issue of climate change

has to be addressed [GRE06]. There is no question that the emissions of green house gases

caused by humans have to be reduced as soon as possible and that new mobility concepts

will play a vital role in that matter [STE07]. Nowadays road transportation emits nearly a sixth

of the total annual CO2 emissions and is almost exclusively dependant on fossil fuels

[DAV10].

Nevertheless the end of the automobile era, being the product of choice for personal

transportation is not in sight as new markets are still opening and private vehicle ownership

is still rising. In China alone the quantity of civil motor vehicles is growing by about 20%

annually since 2007 [HU10]. The motorcar not only provides flexible, safe and on-demand

transportation it is moreover a symbol of freedom, independence and wealth.

There have been many studies and projects on alternative propulsion systems and fuels, but

each technology has their own big issues, yet to be solved. The main handicap of electricvehicles (EVs) has always been the energy storage. Until recently the low specific energy,

slow charging rates and cost of the batteries have hindered the EV to seriously content with

the fossil fuel run internal combustion engine. Weight and size of the often toxic batteries

prevented a competitive performance and driving range. In the past two decades, however,

battery technology advanced rapidly, accompanied by the massive appearance of mobile

devices in today’s information age. [BEC09] [LAR03]

In 2006 the Tesla Roadster, the world’s first Lithium-ion battery powered EV, set the new

benchmark with its outstanding performance. This EV demonstrates an acceleration of

0-60mph (0-97km/h) in 4 seconds, a top speed of 200km/h, and last but not least a range of

more than 322km [BER06]. It reveals how far battery technology progressed in reducing size

and weight, while maintaining the power and capacity needed to live up to the demands of a

consumer vehicle. The breakthrough of this sports car aroused the attention of the

automotive industry, the governments and the consumer. From then on a series of promising

EV models from automobile manufacturers around the world were introduced and started in

small batch production, while many governments including China, Germany and the USstarted concentrating their funding for alternative propulsion on EV technology. [JAM09]




3

2.2 Electric Vehicle advantages

Particularly with regard to urban areas the use of lightweight electric mini vehicles seems a

promising approach to several issues. It provides convenient, economical and fossil fuel

independent mobility while being energy and parking space efficient. The lack of tailpipe

emissions helps improves the local air quality and even challenges global warming, when

regenerative energy sources are taken in mind. Those hopes are justified by many

investigations addressing urban EV-fleet feasibility, consumer convenience and the

foresights for environmental friendly mobility.

EVs do not have tailpipe emissions and could be a key influence on improving the polluted

air in urban areas, which is threatening the inhabitants’ health in many cities around theworld. Even if the energy is still supplied by pollution emitting power plants, the emissions will

at least be shifted to less populated areas where those plants are conventionally located.

[KIN07]

It is often predicated that the pollution caused by electric vehicles were just outsourced to the

power plants and therefore switching from gasoline engines to electrical driven transportation

would not reduce greenhouse gas emissions. Among others investigations the Pacific

Northwest National Laboratory has rebutted these concerns. Kintner-Meyer et. al. obtained

that the personal vehicle fleet replacement could save up to 27% of greenhouse gas

pollution, which is mainly due to the more efficient drive train of EVs compared to an internal

combustion engine (ICE) driven car. Moreover the possibility of regenerative braking, which

enables EVs to store energy in the batteries by using the electric motor as generator to

convert kinetic energy while decelerating, has to be taken in mind. Even if the electric energy

comes from coal plants, using EVs is a more effective way to convert fossil energy into

vehicle movement, than the standard gasoline driven motorcar. With the increasing use of

renewable energy sources the CO2 emissions caused by the transportation sector can bedecreased even further. [KIN07]

Within half an hour extremely fast charging can already recharge batteries to sustain a range

of 100miles. Even on a convenient household electric outlet overnight charging is capable to

set up the vehicle within hours for a multiple of the average daily range of private vehicles

[KAP10].

Kintner-Meyer et. al. concludes that the majority of the personal car fleet in the US could be

charged without extension of the existing electricity infrastructure, if it was replaced by




4

plug-in vehicles. This is mainly due to night-charging and the possibility of using smart

charging to achieve near maximum utilisation of the power generation capability. The

capacity of EVs batteries is even assumed to be capable of stabilising the electricity grid and

supporting the expansion of renewable energy generation.

Scott et.al demonstrate how the plug-in vehicle owner, the general client and the provider of

the electricity network can economically profit from the plug-in fleet introduction, by using

off-peak capacities to charge the vehicles. The power generation installations’ fix costs can

be shared broader and power surplus can be preserved. [SCO07]

From an economical point of view the EV ownership will further pay back, as the maintaining

cost is fewer due to the high reliability of electric motors, which have very few moving parts

compared to the gasoline engine.

2.3 Microcars

For many years microcars have had a bad reputation for their lack of power and safety.

Despite their long history they didn’t succeed in gaining any mayor importance, with very few

exceptions, as they have always been considered the second choice for those who cannot

afford a proper car. Only in times of energy crises new models were considered due to being

very economical in purchase and sparing fuel. Nevertheless, the BMC’s Mini from 1959,being the most sold British car, proved with its immense success that a microcar could

achieve cult status against all prevailing sentiments. Yet another car, considered by many to

be a microcar, entered the market successfully in 1998, the Daimler Smart city-Coupé, which

impressively revealed that safe microcars are possible. In the 21 th century a new wave of

microcars primarily designed for emerging markets in India and China were introduced,

which were focused on low acquisition cost. The Tata Nano, though presented as the

cheapest microcar in the world and very basic in comfort and safety equipment, came close

to passing the European crash tests. [LWN04]

Microcars can be attractive and safe and in times of urbanisation and growing private vehicle

ownership, they can challenge a lot of problems. They fit well into urban working live,

transporting one or two persons environmentally friendly and being very convenient due to

their parking space efficiency.



3 Background and fundamentals of vehicle crash safety

5


Automobile safety is fundamentally categorised into active safety and passive safety. Active

safety embraces all measures taken to prevent accidents from happening. This includes

basic features such as rear and side mirrors, lights, signals, brakes and steering. Moreover

advanced driver assistance systems, which are implemented to avoid collisions, such as

anti-lock braking system (ABS), electronic stability control (ESC) and traction control as well

as reverse backup sensors, lane departure warning and pre-crash systems (collision

warning, emergency braking assist). Passive safety on the other hand covers all

technologies that help to protect the occupants’ health and lives in the event of a crash, for

instance the restraint system (including airbags and seatbelts), the seats and the vehicles

structure itself. Passive safety is commonly referred to as crash safety, while a vehicle’scrashworthiness is a measure of its ability to protect its occupants in the event of an

accident.

Some of the fundamentals will the illustrated by graphs derived from filtered accelerometer

data from a 2011 Audi A4 4-door sedan full frontal crash test at 56 km/h.

3.1 Crashworthy vehicle structure

In general a crashworthy designed car will stop as gradually and fluently as possible in acrash, while its initial kinetic energy is converted into deformation of the vehicle’s structure

and all other participating elements, for example the tires. The objective is to provide

reasonable deceleration loads, while maintaining sufficient survival space by managing the

impact kinetic energy in a beneficially. [KRA06]

A passenger car usually features crumple zones, areas specifically designed to absorb

impact kinetic energy. They spread the load introduced by the impact and provide a

reasonable resistance to it by plastically deforming, which is folding and bending. Crumple

zones are included in the front- and rear end, less commonly at the side of a car. The

passenger compartment, however, is built as rigid as possible to maintain sufficient survival

space for the occupants. It has to stay intact and prevent intrusion of any sort, for example

the penetration of parts of the own engine compartment in a standard motorcar’s frontal

crash. [KRA06]




6

Fig. 3-1: Resistance levels of consecutive deformation zones [cf. KRA04]

Figure 3-1 shows the front-end crumple zone’s three sequential crush zones and theirfunctions. The first zone provides little resistance and reduces the aggressivity in low-speed

accidents. A vehicle’s aggressivity is defined as its potential to harm crash partners, for

instance pedestrians or other vehicles. The bumper design, rigidity and replaceability have

major influences on this zone. Easily interchangeable crash-boxes can be attached

subsequent to the bumper to lower repair costs in low-speed accidents, which might happen

for example while parking manoeuvres. The central zone includes the most important impact

energy absorbers and is crucial for the protection of everyone involved in a crash. The final

stage has to be strong enough to ensure survival space inside of the rigid safety cell and

protect the passenger compartment from intrusion of the front end’s components. [BOI04],

[ZHO10]

Force level

Survival space

Self protection andcompatibility

Low-s eed rotection

Deformation path




7

Fig. 3-2: axial folding pattern of rectangular tube [BOI04]

Especially in the central, energy absorbing zone the structures have to be designed in a waythat they deform by axial folding, rather than bending. Axial folding, as illustrated in

figure 3-2, absorbs much more energy and therefore enables lighter structures to handle the

impact energy. To prevent the less efficient bending, the stability of the crumple zone

components has to be optimised. [BOI04], [ZHO10]

3.2 Managing impact kinetic energy: Crash pulse

Figure 3-3: Exemplary crash pulse: vehicle and occupant deceleration history

To characterise the impact energy absorption performance of a vehicle structure after acrash test the data from the accelerometers is processed. The sensors are mounted for

-5

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120 140 160

time [ms]

Deceleration

vehicle [g]

occupant [g]

d e c e l e a r a t i o n

[ g ]

max. occupant acceleration=37,57gt1=68,75ms

duration=119-120ms

free-flying= 14±1ms




8

example on the passenger compartment and collect the acceleration loads experienced

while the collision. First this data has to be filtered from noise then it can be plotted against

time to obtain the crash pulse - the deceleration history illustrated in figure 3-3. The ups and

downs reflect the structural response and engagement with components within the crumplezone. The deceleration usually expressed in multiples of “g”, the acceleration produced by

gravity at the earth's surface. The area beneath the plot is proportional to the resisting force

and therefore to the energy absorbed. Figure 3-4 shows the equivalent square wave (ESW)

in comparison to the actual deceleration. The ESW is the idealised pulse, without ups and

downs, and therefore the most efficient way to use the available space for energy absorption.

The peak deceleration, now a constant value over most of the time, is of less than half the

magnitude than the actual pulse, thus much lower peak forces occur. The aim is to achieve a

crash pulse as close as possible to the ESW. The major injury parameters, such as chestacceleration and the Head Injury Criterion, directly correlate with the average value of

deceleration [DUB10].

Figure 3-4: Equivalent square wave (ESW) vs. peak deceleration of actual curve

3.3 Occupant restraint system

Within the passenger compartment seatbelts, seats and airbags keep the passenger retained

in a seating position and prevent contact with the interior. In the event of an accident they

distribute the load over the occupant and give a specific resistance to slow him down. The

objective is to reduce the relative velocity between the occupant and the interior as graduallyas possible. Thereby the occupant’s kinetic energy has to be dissipated. To minimise the

-5

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120 140 160time [ms]

Deceleration

vehicle [g]

ESW

d e c e l e a r a t i o n [

g ]


g ]

ESW=18,12g


g ] tm=88,56ms

∆t=30,49ms

Peak deceleration=40,37g




9

restraint force’s mandatory magnitude, the time and area over which it is applied has to be

maximised as well as the occupant’s total displacement relative to the ground. Seatbelt

pretensioners which are triggered by sensors right before or when the vehicle gets involved

in a crash reduce the free flying time until the seatbelt applies a force by diminishing therestraint slack. Ideally as much as possible of the interior space is used to decelerate the

occupant. However, it is vital that contact of the occupant with interior surfaces are

minimised, as they are the main cause for severe injuries. Thus the maximum relative

displacement between the occupant and the vehicle, illustrated by the distance of the

displacement curves in figure 3-5, is limited. On the other hand the forces that can safely be

applied by the occupant restraint system are limited as well. Load-limiters, included in the

seatbelt system to restrict the belt force, and the airbag power are adjusted to the statistically

average human. Restraint systems that adapt to the individual occupants weight as well asthe collision type and severity are not yet a standard feature at the time of writing, but have

already been included in large executive cars [NN09a]. All elements of the vehicles interior

that could come into contact with the occupant have to be designed to reduce the risk of

injuries. This includes knee bolsters, foot rests, the steering wheel and column, the

instrument panel, doors, arm rests, pillars, roof rails, the windshield and side windows.

[KRA06] [BOI04]

Figure 3-5: Exemplary occupant and vehicle displacement in a crash test

0

100

200

300

400

500

600

700

800

900

1000

0 20 40 60 80 100 120 140 160time [ms]

Displacement

vehicle [mm]

occupant [mm]

d i s p l a c e m e n t [ m m ]

max. dynamic crush= 695,8mmtmax.crush=74,2ms

max.rel.displacement=261,8mmt3=90,9ms




10

3.4 Crash Compatibility

Crash tests against solid or padded walls represent a car’s collision with a car of a similar

mass and speed with both travelling at half the impact speed. When vehicles of different

mass are involved they will encounter different changes of velocity and therefore encounter

different decelerations. Different rigidities will affect the individual degrees of deformation,

while geometric mismatch can cause significant intrusion into the passengers’ compartments

of both vehicles, when the energy absorbing elements of both collision partners fail to meet

due to different heights. [ZHO11]

3.4.1 Mass effect

The mass-effect can be illustrated by a simplified one dimensional collision model. Twocollision partners are represented by their mass and their crumple zones by plastic springs

that will not store energy and therefore not provoke a rebound. Due to the assumption of

inelasticity, both vehicles will move together subsequent to the crash. Therefore the

individual velocity changes iii vvv −=∆ ' can be concluded from the conservation

momentum p and kinetic energy kin E :

2

21

2

22

2

11

2

22

2

11 ')(2

1'2

1'2

1

2

1

2

1combcomb vmmvmvmvmvm E ⋅+=⋅+⋅=⋅+⋅= Eq. 3.1

combcomb vmmvmvmvmvm p ')('' 2122112211 ⋅+=⋅+⋅=⋅+⋅= Eq. 3.2

Due to the equality of the speeds after the crash ( 21 ''' vvv comb== ) the individual velocity

changes and the ratio of the velocity changes are:

)( 2121

2

1 vvmm

m

v+

+

=∆ Eq. 3.3

)(21

21

1

2 vvmm

mv +

+=∆ Eq. 3.4

1

2

2

1

m

m

v

v=

∆

∆Eq. 3.5

The magnitude of the hereby calculated individual velocity changes lack of significance,because in this model no kinetic energy is absorbed by the plastic deformation, which would




11

decrease the values. Their quotient, however, can reveal the difference in the fatality risks

between the vehicles.

The fatality risk R can be estimated with an empirical formula, which compares the actual

velocity change to one of 70 mph, which is considered to be 100 % fatal [JOK08].

4

70

∆=

mph

v R Eq. 3.6

The relation between the individual fatality risk and the vehicle masses results to be:

4

1

2

2

1

=

m

m

R

R

Eq. 3.7

The result shows, that the fatality risk for occupants in a vehicle in a collision with a vehicle

with, for example, twice the mass is estimated to be 16 times greater than for the occupants

in the heavy car. The increase of risk in the lighter car, however, cannot be cancelled out by

the reduction of risk in the heavier car. The net risk of fatalities is therefore increased by

vehicles with a heavier weight and altogether lighter vehicle fleet is favourable [JOK08].

3.4.2 Stiffness effect

A car’s structural stiffness affects the vehicles total deformation in a crash and the energy

absorbing capability. A one dimensional model provides the basic correlations. Two masses,

kept apart by two springs with differing rigidities, represent the collision partners. Newton’s

third law of motion provides the interrelations between the individually absorbed crush

energy E ∆ , total deformation d and stiffness k :

1

2

2

1

2

1

k

k

d

d

E

E

==∆

∆

Eq. 3.8

The stiffer vehicle experiences less deformation and absorbs less energy. On the contrary,

the softer vehicle suffers greater deformation and the risk of intrusion is increased, because

the total available energy absorbing space is more likely to be used up.

3.4.3 Effects on mini lightweight electric vehicle design

The increased velocity change experienced by a lighter vehicle has to be taken in mind when

designing lightweight vehicles, increasing the demands for all passive safety aspects and




12

features. Due to the heavy battery system however the mass effect is less significant than it

would be in a traditional vehicle and the velocity change smaller. The increased weight,

however, demands a structure capable of absorbing more energy by increasing the crush

length and the stiffness.

3.5 Pedestrian safety and silent electric vehicles

Pedestrian safety is of significant importance, because lightweight mini EVs primarily provide

mobility in urban areas where high proportion of the traffic participants are pedestrians. Both

active and passive safeties have to be addressed to prevent accidents from happening and

to minimise the severity of resulting injuries.

The lack of engine noise in EVs is a major concern. Boxwell, however, points out that above16 km/h the wind and road noise caused by the EVs was loud enough to be easily perceived

by pedestrians as those noise sources became dominant in ICE cars above this velocity as

well. In areas where vehicles drive below this speed, there were other silent road users

besides the EVs, like bicycles, and pedestrians were watching out for them. Moreover,

similar to bicyclists who were aware of their quietness, EV drivers paid special attention to

pedestrians as well. Nevertheless the Pedestrian Safety Act of the United States’

government, which became law in 2011, includes a rule prescribing EVs to omit an audible

signal to warn pedestrians at low speeds to be on the safe side. The Nissan Leaf features

such a system that provides a signal that cannot be perceived from inside the car but is

heard by pedestrians. As vehicles are rarely produced solely for domestic markets, this

safety feature is likely to become a worldwide standard. [BOX11]

In Germany most of pedestrian fatalities and serious injuries occur in frontal collision in areas

with speed limits between 30 and 50 km/h. Therefore the measures taken to enhance the

survival chances of pedestrians in a collision with vehicles are mainly focused on the contact

areas between pedestrian and vehicle in frontal collision. In a crash a characteristic series ofcontacts occur, starting from leg vs. bumper, followed by pelvic vs. leading edge of bonnet

and head vs. bonnet respectively windshield. The car’s type and size affect the timing

between those contacts, the relative movement between the pedestrian’s body parts and the

area where the head impact occurs. The aim is increasing the deformation path by altering

the bodywork and structures beneath the contact areas to decrease the occurring forces. For

vehicles with distinct bonnets the preferred contact of head vs. bonnet, rather than a collision

into the windshield, is more likely to occur. Arrangements can be made to decrease the

forces applied by the hood, such as featuring yielding hoods and increasing of space




13

between the bonnet and the components below it, either by initially including clearance or by

actively pushing the bonnet up in a collision, before the contact occurs. For other vehicle

types, especially small vehicles, the focus must lie on yielding windshields and deformable

windshield mounting. [KRA06]

Overall the EVs lack of omitting loud motor noises has to be taken in mind in the design

process and the installation of loudspeakers to raise awareness of the vehicle can be

considered. Besides the general basics of pedestrian safe front and back design the lack of a

lengthy bonnet in a small car creates problems, as the probability of a pedestrians head

colliding with the windshield increases. Therefore special attention has to be laid on a

yielding support of the windshield and a windshield that is yielding enough itself to prevent

endangering the health of pedestrians. External airbags are another option.

3.6 General crash test set-up and evaluation

The crashworthiness of a vehicle’s structure and its restraint system can be estimated by

analytical tools and crash tests. In recent years computer-aided engineering (CAE) made the

evaluation of the structural performance with the finite element method possible. This can

help engineers in the designing process. Yet many assumptions have to be made to cope

with the complexity of the interaction of all components involved, their different materials and

their bonding, especially if a full-scale vehicle is simulated. Therefore laboratory tests are

used for the final vehicle crashworthiness assessment, particularly in vehicle certification.

[HUA02]

There are three categories of tests. Their complexity and the number of parameters analysed

is rising from component tests to sled tests and full-scale impact tests. The first mentioned

analyse the response of isolated components subjected to a load. Thereby crucial

information about energy absorption capacities and general deformation mechanisms can be

assessed. They can be used to optimise single components, but also as basis formathematical models simulating for example plastic deformation. [BOI04]

Sled tests mainly examine restraint system performance. A mechanical surrogate of a

human, commonly known as crash test dummy, is placed in a vehicles passenger

compartment mounted on a sled. The compartment includes all relevant features for the

individual test. The set-up may include the complete interior, it can, however, also be solely

composed of a single seat. The sled is then subjected to acceleration similarly to a

passenger compartment in a crash. This can be achieved by two methods: Either the sled isbrought to a certain speed and then decelerated abruptly by a brake device in a controlled




14

way, for example by deformation of specially designed plastic tubes, or it is, in reverse, very

quickly propelled from zero to a certain velocity by a well controlled force. Due to the great

magnitude of the force needed to accelerate the trolley to speeds above 50 km/h within

100-200 ms those systems are expensive. Sled tests have a good repeatability and arerelatively cheap, because they don’t demand a full vehicle to be crashed. [BOI04]

In full-scale vehicle crash tests vehicles impact with rigid or deformable barriers, poles,

pendulums, other vehicles, movable deformable barriers or pedestrian surrogates. There are

front-, side and rear impacts as well as roll-over tests and impacts with different collision

partners, such as poles. Before the crash the vehicle gets equipped with instrumentations,

such as accelerometers and cameras, to monitor the test and a data recorder. Crash test

dummies are placed into the car to mimic human beings and to collect data by a vast amountof sensors, for instance accelerometers, load sensors (measure force at specific body parts)

and motion sensors. To simulate the diversity of human beings, different dummies replicate

statistically chosen human representatives in size, age and gender. The dummies and

instrumentation are positioned and restrained in the car and ballast installed to simulate a

fully loaded vehicle. From the outside the collision is recorded by high- speed cameras from

several perspectives. About 15 high-speed cameras, several of them pointing upward from

under the place of the crash, film the event with 1,000 frames per second. The vehicle or

objects, which are collided into the vehicle, can be accelerated to their impact velocity by awinch or another vehicle. To ensure that the equipment remains in position a low level of

acceleration is required, resulting in an acceleration tracks length of at least 100m. It takes

about 100 milliseconds from the time the car hits a barrier until it stops, see figure 3-3. After

the test the data from all sensors, including accelerometers mounted on the vehicle structure

and dummies, the high- speed cameras’ film material and the deformed vehicle structure are

observed and processed. [ZHO10]

Many of the criteria used to evaluate a vehicles performance are parameters judging theinjuries a human passenger would have suffered by comparing them to estimated human

injury-load limits. Injury parameters are for example chest decal, femur load and dummy

head acceleration. Different rating systems classify the severity of the injuries, for example

the Head Impact Criterion (HIC), which is based on the accelerations that occurred. The

investigation of biomechanics to establish threshold values for mechanical loads that cause

injuries of certain degrees is very complex. To evaluate human body injury tolerances a

combination of different approaches is used, such as cadaver testing (human cadavers only

in the past, nowadays animal cadavers), volunteer testing and animal testing. [ZHO10]




15

Crash tests are run for instance by governmental institutions, vehicle manufacturers,

organisations funded by insurances, e.g. Insurance Institute of Highway Safety IIHS, and

laboratories of motor vehicle associations.

3.7 Legal Vehicle Safety Regulations

In a type approval procedure governmental agencies test new vehicles whether they cope

with the national regulations. Because of a rapid rise of car accident casualties, the U.S.

vehicle safety act was passed in the United States of America in 1966. 20 safety issues were

issued by the National Highway Traffic Safety Administration (NHTSA) after its establishment

between 1967 and 1970. Nowadays the Federal Motor Vehicle Safety Standards (FMVSS), a

series of regulations, are in effect. In the European Union as well as another 47 countries

vehicle safety is premised on the ECE regulations. In 1958 an agreement was passed under

the umbrella of the Economic Commission for Europe (ECE) to harmonise technical

specifications of motor vehicles. Since then the ECE passed over 125 regulations. Moreover

the European parliament attempts to unify the type approval in the European Economic

Community (EEC) by publishing the so called EEC- directives. The regulations in many other

countries, such as Australia (ADR), Japan, Canada (CMVSS) and Saudi Arabia (SAS), are

mostly based on the US or European regulations and conform more or less to them. They

include features of both passive and active safety. [KRA06]

Besides regulations about basic features a vehicle has to have, for instance rear mirrors,

their performance in different fields is tested, including for example the stopping distance in

brake tests and the passive safety performance with different crash tests. The national

regulations of crash tests vary in many aspects, inter alia, the number and nature of tests,

the impact speeds and directions of collisions, the crash partner’s nature, as well as the

sensors and equipment used for measurement of different values to meet different criteria.

There are frontal, side, rear, pole and roll-over crash tests. While for example the frontal

crash in Europe is at 56 km/h with a 40 % overlap against a deformable barrier, in the USA

the impact is at speed is 30 mph (48 km/h) against a rigid barrier in different angles (0°, +30°,

-30°). However they all try to simulate likely real world accidents and to assess the survival

chances for the occupants.




16

3.8 New Car Assessment Programmes

In several nations and unions, such as the United States, Europe, China and Australia, New

Car Assessment Programmes (NCAP) are run. The EURO NCAP is a safety assessment

programme, independent from governmental interests and car companies, which provides

simplified results of crash-test to guide the consumer with his purchase decision. Moreover

the aim is to encourage car manufacturers to exceed the legal regulations by implementing

the latest safety technologies and developing new innovations. Each year the ratings are

adapted to current developments. Achieving the maximum score is supposed to be a

challenge, yet possible for committing manufacturers. Their results are widely published in

automotive journals and additionally in common media, especially if a car performs unusually

poorly or extraordinarily well. Thus, these programmes pressure the automobilemanufacturers to put emphasis on passive and active crash safety. [NCP11a] [NN11b]

A five-star-rating-system is presents the safety performance of a vehicle. The top rating is

5 stars. Since 2009 it is composed of scores in four categories: adult protection, child

protection, pedestrian protection and safety assist. Every year since then the levels to

acquire a certain grade are raised (for the moment planned until 2012) and new

requirements and test methods can be added as well. Since 2009 the EURO NCAP includes

a whiplash rating into the adult protection score for instance, mainly examining the seat’s

ability to prevent neck injuries, which most likely occur in low speed rear accidents. [NCP11a]

[NN11b]

However, only the most popular vehicles are tested each year (e.g. 2009: 45 tests; 1010:

29 tests). The models are mainly chosen by the number of estimated sales. Sometimes,

however, models with extraordinary new design or functions are chosen as well, such as the

Smart ForTwo for its small size. Moreover a car manufacturer can request a specific model

to be tested. He has to fund the tests and does not have an influence on the publication of

the results. [NCP11a]




17

Fig. 3-6: Euro NCAP front impact test [cf. WIK11a]

The crash tests of both, the US and the EURO NCAP, are based on the legal regulations in

the United States and Europe respectively, but have been tightened by some degree. The

EURO NCAP’s frontal crash test for example is similar to the directive ECE-R 94. The impact

speed, however, is increased from 56 km/h to 64 km/h. The arrangement is illustrated in

figure 3-6.

The testing includes a frontal, side and pole impact as well as a whiplash and various

pedestrian protection tests. It is conducted in six laboratories around Europe. [NCP11a]



4 Structural design and automotive layout

18


An automobile can essentially be subdivided into five main assemblies: drive train, chassis,

body structure, interior and electric subsystems. The drive train embraces all components

that take part in the conversion of stored energy into the vehicle’s propulsion from the engine

via gearbox and differentials to the wheels. They all take part in the vehicle‘s propulsion by

transmitting the forces and torques from the engine to the street. The chassis transmits

forces between the vehicle and the street. It determines the properties of the driving

dynamics, including the comfort. The function of the body structures depends on its type and

whether or not it comprises the frame. It consists of the passenger compartment and the

surrounding structures that hold all other assemblies together and support them. It is

designed to perform well in withstanding all static and dynamic loads it could encounter in thevehicles life time, including the possibility of a crash. There are two basic groups of body

structures: so called body-on-frame and unit-body structures. [BOIS04]

4.1 Body-on-frame

In a body-on-frame structure the vehicle body, frame and front sheet metal (or plastic) can be

clearly differentiated. Figure 4-1 shows a Chrysler Imperial of the year 1966 with a full frame

construction on which the body is mounted. The chassis is part of the frame, which supports

all other assemblies, such as the engine, transmission, power train, suspension and body.

The vehicle body is attached onto the frame by shock absorbing body mounts to isolate high

frequency vibrations. While the vehicles body provides most of the vehicle’s rigidity in

bending and torsion, the frame and front sheet metal will absorb most of the energy in a

frontal collision. Body-on-frame is still the preferred structural design for heavy-duty vehicles

from pick-up truck to lorries. In the 70’s, however, the unit-body, or also monocoque called,

structure became predominant for personal vehicles. [BOI04]

Fig. 4-1: Chrysler Imperial 1966, last year of full frame construction in Imperials [IMP11]




19

4.2 Unit-body

In a unit-body structure vehicle body, frame and front sheet metal are combined to a single

unit to allow further weight saving and to enhance the whole vehicle rigidity. The body and

external panels have supporting and structural function. There are many different design

methods for unit-body structures depending on the materials and bonding methods used.

The two most common are portrait in Figures 4-2 and 4-3: the steel unit-body with metal

sheets and panels, represented by the concept from the Ultra Light Steel Auto Body

Programme, and the space frame, represented by the current Audi TT Coupé’s Audi Space

Frame. [BOI04]

Fig. 4-2: Ultra Light Steel Auto Body (ULSAB) 4 door, five passenger sedan [MAR11]

The unit-body steel sheet shell is now the most common construction for mass produced

cars. The sometimes very complex work pieces are manufactured as a half shells and then

welded together at included flanges. The structural stiffness is reached by hollow elements

with large height-to-width ratios and therefore large moment of inertias, as well as sheetmetal body shell components that are covered with beads and the supporting body panels

and base plate, which are attached by welding. The standard steel type is mainly spot

welded. There are several techniques to reduce the weight, including advanced

manufacturing techniques to reduce the number of parts, for instance with tailored blanks,

and the implementation of lightweight materials. Then the biggest challenge is the bonding of

the different materials, if the application exceeds the simple replacement of attached doors,

flaps and hoods. Generally unit-body constructions offer a very high degree of automation.

With sufficient high quantity and a low range of variants, the production is cheap and simple.




20

For low volume production and vehicles with many variants this design is only limited use,

since high investment for advanced press and forming tools have to be made. [GIE08b]

Fig. 4-3: Lightweight ASF (Audi Space Frame) of second generation Audi TT Coupé

[cf. CAR11a]

Space frames are truss like lightweight structures that span large volumes with few interiorsupports and are commonly known from architecture or structural engineering. Electricity

pylons and cranes are a good example for the repetitive geometric patterns joined by nodes.

In vehicle technology the main difference to unit-body or monocoque constructions is that

body panels have no or little supporting function and are simply attached to the skeletal

structure. In comparison to Figure 4-2 the ASF of figure 4-3 is lacking a roof panel and shows

the simple flanges to attach a roof panel of choice. The floor includes supporting structures in

longitudinal and transverse direction to increase the construction’s stiffness. Compared to a

standard steel unit-body up to 40 % weight reduction is possible. The aluminium spaceframe’s basic structure consists of large castings and extruded profiles interconnected by

complex cast elements, so called nodes. The result is a body which can be divided into three

modules: the passenger safety cell with high rigidity as well as front and rear sections with

high energy absorption capabilities. Aluminium is particularly suitable for the crumple

sections, as it offers a higher potential for energy absorption than steel. Therefore protection

for the occupants of both crash partners can be provided, since the crash energy can be

reduced significantly. The degree of automation that can be achieved in the production of

space frames is low and the manufacturing additionally complex due to required extravagant




21

bonding and welding techniques. The equipment investments, on the other hand, are low,

which makes small batch production is feasible. Other than the Audi A2, space frame

construction is mainly used for small batches of for example sports vehicles. Overall the

space frame is a very good choice for the contraction of a EV, because it enables lightweightdesign, good safety performance, flexible enough to fit the new demands of electric

propulsion technology and is suitable for the upcoming small batch production. [GIE08b]

Another vehicle body concept is the grid frame structure, consisting of metal tubes. Usually

thin, high-steel tubes are welded together manually to create a stiff und rigid structure. Very

low automation possibilities and complex crash failure mechanisms make it only feasible for

very small batches of mostly sports vehicles. [GIE08b]

4.3 Aluminium for structural elements

Steel has been the material of choice for vehicle structures for a long time and a lot of

experience has been gained. When using alternative materials, however, new design

approaches are necessary to achieve satisfying results, which increases not only the

development costs for the structure but also affects the attached components. Aluminium is

the only alternative material to steel and high-strength-steel, which is used for structural

elements in a larger scale today. Fibre composite materials for example evoke challenges in

efficient production and bonding, which increases cost and require a complex structure

design especially tuned for the use of fibre-composites. Therefore, with the exception of

sports cars, they are scarcely used in current production vehicles.

Mallik discusses the potential of aluminium to safe weight, while maintaining the structural

performance. He applies the classical beam theory on a hollow cantilever beam subjected to

a transversal force at the free end. Aluminium has one third of the density of steel but a lower

tensile strength and is three times as compliant. Due to those mechanical parameters an

aluminium beam has to be modified from the steel design to achieve the equivalent bendingstiffness. By simply increasing the beams thickness until it can cope with the load, however,

about 33% of weight are added, despite the lower density. If, however, the beam’s section is

increased while the thickness is kept constant about 43% of weight can be saved. A larger

beam section, on the other hand, requires more space. This illustrates that the use of

aluminium has a promising potential as lightweight material, but demands new designs, such

as the space frame. [DEB10]




22

4.4 Lightweight design effects

Reducing a vehicle’s weight has a wide range of advantages. By saving weight the car’s

efficiency is increased, because less energy needs to be converted into kinetic energy to

accelerate it to a certain speed. Moreover the acceleration and deceleration as well as the

general driving performance are increased. As a secondary effect a lighter vehicle needs a

less powerful engine, which is usually lighter. The engine’s supporting structure can then be

reduced as well. Just the same as lightweight design can increase the fuel efficiency in an

ICE vehicle, it increases the efficiency of using electric charge to propel an EV as well. This

leads to a longer range, which has always been a main aim for EV design. Moreover the

reduced mass improves the handling for example when cornering, because the inertia is

reduced which reduces the centrifugal force.

4.5 Vehicle classes and segmentation of electric vehicles

Uniform regulations to formally define vehicle segments neither exist globally nor in Europe

or the United States. Vehicle segments tend to be based on comparison to well known brand

models. Comparing for example with vehicles within the Volkswagen Company’s line of

models, vehicles can be described to be Volkswagen Polo, Golf or Passat size class or in

between the listed. There is no uniform definition of vehicle classes in the English language

and in both British and American English usage there are several different classification

structures and conditions. Focusing on vehicles in size and weight below the compact car

(Ford Focus, Toyota Corolla, Volkswagen Golf) British English differentiates most refined into

microcar (bubble car), city car and supermini. The Euro- NCAP included all of those cars in

the single category supermini until 2009, which are not longer than approximately 3900mm in

case of a hatchback. In American English usage city cars and superminis are joined in the

category subcompact. City cars in British English usage can also be compared to Japanese

kei cars, which require being below a total length of 3400mm.

Some of the common parameters used for segmentation don’t apply on EVs and are not

relevant for vehicle safety. Therefore this thesis will adapt to the most refined segmentation

in British English usage and adapt the conditions to the electric drive train and current EV

characteristics, rather than basing the categories on cylinder capacity and size. The

characteristics used in this thesis for segregation are summarised in figure 4-4. They are

based on the survey of current EVs and are only very rough guides. In case of fulfilling

contrary conditions a vehicle will be grouped into the segment with the greater compliance.




23

Microcar City car Subcompact car

EEC- class L6e, L7e M1, L7e M1

Max. speed < 45 km/h*, < 60 km/h > 60 km/h > ~80 km/h

Max. range < ~100 km > ~100 km

Power < 4 kW*, < 15 kW** < 15 kW > 15 kW

Total length < ~2500mm < ~3000mm > ~3000mm

Kerb weight < 300 kg*, < 500 kg** < ~800 kg > ~800 kg

Seats 2 2, 4

* to qualify for class L6e

** to qualify for class L7e

*** not including the battery’s weight

Fig. 4-4: Rough EV classification based on survey

National vehicle legislations often include a vehicle class of very light, small engine, three- tofour- wheeled vehicles that rather compare to scooters and bicycles than to cars. In

European legislation, for instance, class L includes such vehicles differentiating from

class L1e to L7e based on number of wheels, motorisation and weight. Those vehicles do

not have to adapt to the safety standards of the standard passenger class M1. Tricycles are

classified into class L2e and class L5e and distinguished by a maximum continuous rated

power below 4 kW and a maximum speed below 45 km/h for class L2e, respectively above

for class L5e. Class L6e vehicles have four wheels with a kerb weight up to 350 kg, without

batteries in case of an EV, a maximum speed below 45 km/h and a continuous rated powerthat does not exceed 4 kW in the case of an electric motor. A class L7e vehicle is four-

wheeled and limited to a maximum kerb weight 400 kg or 550 kg, without batteries in case of

an EV and a continuous rated power below 15 kW in the case of an electric motor. In the

U.S. there are similar regulations for so called “neighbourhood electric vehicles” (NEVs) or

low speed vehicles, which only have to pass a frontal 25 mph (40 km/h) crash test. Their

maximum speed is limited to 25 mph. Even though vehicles in this class might not give much

insight in vehicle safety, some breakthrough EVs and currently very successful models are

part of this class and will be surveyed later on. [GER11]




24

In this paper and the appended chart microcars will refer to vehicles that have to comply with

decreased safety requirements in European and American legislation and are smaller than

2500 mm in total length.

City cars will refer to EVs that are mainly designed for urban areas and fall into the category

of EEC class M1 vehicles or L7e. They have to pass class M1 crash test certification and

their maximum speed has to be high enough for highway certification, which is 60 km/h

under EEC- legislation. However, since city cars are not designed for excessive highway use

their maximum range is adapted for daily urban commuting and their maximum speed will

mostly below 100 km/h. They carry no more than 2 passengers, feature a motor with a

continuous rated power below 15 kW and are roughly below 3000 mm in total length.

Subcompact cars allow comfortable cruising at highway speeds above 80 km/h and in the

choice of battery configuration the emphasis is put on range, to allow driving for more than

100 km with a single charge. Additionally the survey of current vehicles has shown that

current EVs can be roughly grouped into two groups. The first group includes vehicles

focused on reasonable cost and light weight, roughly below 800 kg. They will be grouped to

city cars. Vehicles with emphasis on high safety, comfort and performance with a noticeable

increase of kerb weight up to 1500 kg, make up the second group, which will be referred to

as subcompact cars.



5 Electric vehicle technology

25


An electric vehicle is a vehicle that is propelled by one or multiple electric motors by

converting electric charge to kinetic energy. The electric charge is usually stored in batteries.

It is charged by connecting it to an electric socket or charging stations specially built for the

purpose. Those charging stations often provide higher voltages as the common mains

voltage and higher currency, which enables fast charging. Most EVs do not feature a

conventional gearbox but a simple differential or a direct connection to the wheels. [BOX11]

There are two other forms of electric vehicles which will not be discussed in detail: hydrogen

fuel cell cars and hybrid cars. Hydrogen fuel- cell cars charge their batteries by generating

electricity through a chemical reaction in the fuel- cell that consumes hydrogen. An

electric/combustion hybrid car features one or more electric engines besides an ICE. The

ICE is either installed parallel or in series with the electric engine. In the first case it directly

powers the wheels parallel to the electric engine/-s. In the second case it powers a generator

which will charge the batteries to extend the range of a so called range extended EV.

[BOX11]

An electric drive train offers the possibility of regenerative breaking. It uses the electric motor

as generator and thereby decelerates the vehicle. In this process the kinetic energy is

converted into electricity and can be used to charge the batteries, whereas traditional brakes

convert the kinetic energy mainly into heat. Regenerative braking can improve the rage of

EVs to up to 30 %. [BOX11]

5.1 Electric motors and efficiency

The various electric motors used for transportation greatly differ from traditional ICEs in many

aspects. This not only affects the vehicle’s driving performance but also evokes great

differences in the whole drive train set-up.

Electric motors are usually of smaller size than a comparable ICE. Figure 5-1 shows that

even the BMW Mini E’s particularly powerful 150 kW engine can easily be fit into the former

compartment of the ICE. Because the electric motors do not heat up, the complex and bulky

cooling systems of ICEs can be omitted. Overall more space gets available by the exchange

of the motors. Electric motors are also much lighter. Tesla’s 215 kW asynchrony motor

weights less than 32 kg.




26

Figure 5-1: AC propulsion three-phase asynchronous motor in a BMW Mini E [AUT11a]

The electric power train has several advantages to the ICE power train, which are mainly due

to the motor technology. The lighter and more compact electric motors have only very few

moving parts compared to over a hundred in an ICE. One of the big advantages as a result is

the motor’s very low-maintenance. Unlike in an ICE no oil is needed to reduce the wear of

parts running against each other. Electric motors barely omit audible noise and cause very

few vibrations. The electric motor and drive system’s overall efficiency is about 90 %, in

comparison to the overall efficiency of 20 % of an ICE and a gearbox. This is mainly due to

the electric motor’s physical principles, which, unlike the combustion of fuel, barely cause

any heat loss. Moreover, the lack of a complex gearbox and the low number of moving parts

add up on the efficiency. [LAR03]

Unlike the ICE and electric motor can already provide the maximum torque from standstill.

This makes clutches and gearboxes unnecessary and opens new freedoms in the drive train

layout, for instance arranging the electric motors closer to the axles or wheels. Figure 5-2

shows the currently usual set-up a) with a single motor driving a complete axle via a

differential and an alternative set-up b) with several smaller motors arranged close by the

wheels. The torque distribution and speed compensation between the left and right wheel in

the second set-up is no longer handled by a differential and therefore the motors have to be

controlled individually. This complicates the control but also enables new possibilities to

enhance active safety and drive dynamics by extending the functionality of advanced driver

assistance systems, such as ESC, anti-spin regulation (ASR) and ABS, without addition of

complex mechanical devices. [GIE08A]




27

Fig. 5-2: Electric drive configurations [cf. GIE08A]

There are several possibilities to arrange those motors, including so called wheel-hub

motors, which are placed within the rim. Siemens VDO introduced their motor-in-hub

concept eCorner in 2007 at the Frankfurt Motor Show (IAA). As can be seen in figure 5-3 the

motor is directly attached to the rim. The system weights 15 kg in total and includes an

electronic wedge brake to support the motor’s regenerative braking. A special feature is the

electronic steering, which is combined with the active damping into one module. It enableseasy implementation of four-wheel steering.

Fig. 5-3: Siemens VDO eCorner motor-in-hub concept [BRY11]

Electric

motor

Differenial

a) b)

Wheel-hub motor

Rim

Electronic wedge

brake

Electronic steering

Active damping

Electricmotor

Battery

Differential

a) b)




28

Wheel- hub motors save space within the vehicle and thereby allow new freedom to use this

space for example to accommodate batteries or to down-size the vehicle. In terms of safety

the system has several advantages and disadvantages. The drive-by-wire steering raises

concerns, because a malfunction of the automotive electronics would cause the steering tobe disabled as well. Redundancy is especially demanded for the brakes, for that reason

mechanic brakes are integrated additionally. In general wheel- hub motor systems raise the

unsprung mass, which has negative effects on the vehicle NVH (Noise, Vibration,

Harshness) and driving dynamics. Therefore sports cars use lightweight rims and brakes.

The advantages are the new design freedom, new possibilities for advanced driver

assistance systems and a lower centre of gravity. So far no EV has been produced in series

that features wheel-hub motors.

5.2 Battery technology

This thesis includes a quick overview of current battery technology with special focus on

safety related issues, such as weight, size and packaging possibilities. The battery

technology is the crucial point in the success of an EV. The battery is the component with the

highest weight, volume and cost. It provides electric energy by converting stored chemical

energy in a chemical reaction. A battery consists of several electric cells joined together.

Each electric cell includes a positive and negative electrode linked by an electrolyte. Achemical reaction between those components generates direct current (DC) electricity. By

reversing the current chemical energy can be stored in a rechargeable battery. Even though

there is a large number of materials and electrolytes that can be used to built electric cells

only few are feasible for the use in an EV. The development of batteries goes back for about

150 years to the invention of the lead- acid battery. Since then many new material

combinations have been tested and further developed, but haven’t been successful enough

to enable the widespread use for personal mobility in EVs until now. The most important

criteria in the battery technology choice are among electrical performance parameters,energy density, typical voltages, self-discharge and charge rates, as well as commercial

availability and cost. Moreover operating temperatures, cycle stability and the temporally

limited lifetime have to be considered. [LAR03]

Battery cells are produced in different shapes. The usual 12 V car battery in an ICE vehicle is

composed of several flat rectangular cells, while the widespread 18650 Li-Ion cells used in

many laptop batteries as well as in Tesla’s EV Roadster is cylindrical. Other shapes are

prismatic or hexagonal. The cells are joined and normally packed in rectangular blocks. Theelectric cells are connected in long series strings to provide a sufficient voltage and those




29

stings are connected in parallel to provide sufficient currency. In a vehicle the battery blocks

can be spread, for example over the vehicle floor. However, depending on the battery

technology several different additional components are required to ensure a safe and

efficient performance of a battery, for instance cooling equipment, pressure valves and solidcasings. Those can have a negative impact on the freedom of packaging, since they may

make the merging of battery blocks favourable. [WES01]

The most common battery technologies used today in EVs are lead-acid, NiMH (Nickel-Metal

Hydride), Nickel- Cadmium, Sodium-nickel chloride (high operating temperature, also known

as ZEBRA battery) and Li-Ion (Lithium-ion).

The lead- acid battery has been invented in 1859 and already been used for propelling a

tricycle in 1881. In comparison to other battery types it is very cheap and has until the 21 st

century been the most used battery technology for EVs. Its performance, however, is limited.

A standard lead- acid battery has a relatively very low energy density of 25- 35 Wh/kg, its

performance is severely affected by low ambient temperatures (starting below 10 °C) and

survives only about 1000 charging cycles – 3 years when charged daily. There have been

further developments, such as the valve- regulated lead- acid battery.

Nickel based batteries, broadly spread in household electronics such as shavers and electric

toothbrushes have been used in EVs since a longer period. The famous GM EV1 featured a

nickel metal hydride (NiMH) battery in its later models, which more than doubled vehicle’s

range in comparison to the older lead battery version. The EV1 could be leased from 1996 to

1999 in California and was the first serial-produced, purpose-built EV of a major automobile

manufacturer.

Lithium based batteries, most prominent the lithium ion battery, have been utilised in many

mobile devices such as cell phones and laptops. They can store more energy with less

weight than NiMH and far more than lead-acid batteries.

5.2.1 Energy Specific Density and weight

Up to this date even the most advanced battery technologies used in EVs energy storage

systems are outperformed by standard gasoline or diesel in terms of energy density and

energy specific volume. Energy density is a measure of how much energy can be stored in a

battery per mass of the battery. It negatively correlates with the batteries specific power.

Traditional fuels have an energy density of about 12000 - 13000 Wh/kg compared to

35 - 200 Wh/kg of batteries [WES01]. Berdichevsky et al. state that the Tesla Roadsters




30

Battery system stores the equivalent energy of 8 litres of gasoline [BER06]. However, it has

to be considered, that the electrical power train is much more efficient. The full efficiency

calculation comparing the fuel to propulsion with battery to propulsion, including all losses in

the power train, is complex. Nonetheless Westbrook et al. approximated in 2001 the usefulenergy density of gasoline to be 2000 Wh/kg compared to 35 Wh/kg of a lead- acid battery.

Current models, for instance the Think City or the BMW E Mini, feature battery systems with

energy densities of approximately 110 Wh/kg1. Including the electric motor efficiency of

approximately 90 %, the effective energy density in batteries in current EVs can be estimated

to be roughly 100 Wh/kg compared to the 20 time higher value of gasoline2.

Moreover the heavy concentrated weight of the batteries causes a compounding effect on

the vehicle’s kerb weight, because stronger and therefore heavier structural componentshave to be used to support them and secure them in case of a crash.

5.2.2 Range

The main issue limiting the success of electric vehicles has always been the limited range,

which is due to the batteries’ bulkiness and weight. To ensure a nominal range above

150 km the battery weight in current models is above 250 kg [cf. Tab. 1]. Westbrook et al.

summarized in 2001 that for ensuring a range of 100 km for a small EV over 400 kg of

lead- acid batteries, about 200kg of nickel- metal hydride (NiMH) or about 120 kg of

lithium- ion (Li-Ion) batteries would be required [WES01]. Moreover an ICE car can be

refuelled in minutes to cover at least another 500 kilometres or even up to 1000 kilometres in

modern cars3. Most EVs currently on the market reach only between 80 and 200 kilometres

with a single charge and full recharging can take up to 9 hours. Therefore long distance

travelling in current EVs is not possible in a comfortable manner yet, also due to the lack of a

fast charging network.

Nevertheless, Boxwell can refer to several studies, stating that for most current vehicleowners the range of EVs was sufficient to ensure their regular driving habits, as for example

the average distance travelled by an American driver was 46.5 kilometres according to the

US department of Transportation. Overall the range offered by the current battery technology

is sufficient for the mobility for the average professional life or urban commuting, making it a

feasible choice particularly for a second car. [BOX11]

1 Mini E: 28 kWh (usable) energy to a weight of 260 kg [MIN11]2

2000 Wh/kg [WES01] or 20 % of overall power train efficiency [LAR03] combined with 12000 Wh/kgenergy density of gasoline3 Golf V: 80 hp, 757 km [RPO11]; Porsche Panamera: 250 hp,1200 km [PRS11]




31

5.2.3 Charging and lifetime

EV charging duration differ greatly depending on the battery technology and its capacity as

well as the charging station technology and the electricity grid’s performance. The Tazzari

ZERO, for instance, fully charges in 9 hours at a standard European 220 Volt socket. With

the offered fast charging equipment charging is possible in 5 hours and super fast charging

to 80 % of the total capacity is possible in 1 hour [SMI11]. The E Mini’s charging time differs

from 23.6 hours on an American 110 Volt (1.3 kW) socket to 2.9 hours at 240 Volt (10.6 kW)

[MIN11].

Batteries are limited in their cycles of charging and discharging. The lifetime differs between

the different battery types and is affected by several influences, for instance by

environmental conditions, such as ambient temperatures, and the course of usage. The

battery lifespan greatly influences the EV’s maintenance cost, because a replacement is

expensive. It is defined as the time frame until a battery can only be charged to 80 % of its

total capacity. The lifespan of the Reva i/ G-Wiz i’s battery system is estimated to be only 2

years for the lead-acid and at least three years for the Li-Ion version [GOI11].

5.2.4 Safety issues

There are many safety issues that have to be addressed for any type of battery technologyon the general mechanical and electrical matters. In addition each type has its own

jeopardies.

First of all the heavy battery has to be supported adequately to prevent detaching - even in

an accident situation – because there could be consequences beyond dangers of a large

mass in uncontrolled movement but also threats evoking from the electrical nature of the

battery system. External short circuiting has to be avoided under any circumstances, to

prevent potential harm to the occupants. Voltages above 50 V can cause fatal electric shock,

if enough current is provided by the energy source and the external circumstances cause the

current to be lead though vital organs. Low voltage systems, however, are less feasible,

because they require heavy and bulky motors and higher voltages up to 350 V are usually

used in EVs. Thus, additional protection and insulation of cables and electrical components

are required to prevent contact with persons or conducting structures. These measures must

function in any situation, whether in normal usage, maintenance or accidents. Special orange

colouring is used to signal high voltage components. As protection from mechanical intrusion

and deformation the battery system is usually sheltered by a rigid surrounding casing. Afurther option is the individual protection of smaller electric cell blocks. To prevent leakage of




32

toxic, corrosive or high temperature liquids and gases self-sealing mechanisms are used.

Additionally protective devices shut down the electrical system in case of internal or external

short circuiting. They react to electric sensors detecting irregularities as well as to the

controller that triggers airbags when detecting a collision or roll-over. To connect the vastamount of battery cells or blocks many cables and connectors are required. By monitoring

the resistance, voltage and temperatures of many electric components on a detailed level as

close as possible to monitoring the individual cells, malfunctions can be detected quickly and

precise to ensure safe operation. It also helps preserving the maximum power and lifespan of

the battery. Electronic controllers, fuses and circuit breakers provide protection against

overvoltage or short-circuiting of electrical components. Ventilation disposes of escaping

gases. In addition to preventing a human to get in contact with electric current an unguided

release of large amounts of energy is naturally unwanted, as it can cause fires andexplosions. [BER06], [WEs01]

One of the main safety concerns is the thermal runaway within Li-Ion batteries, a chain

reaction that can lead to the explosion. Internal short circuiting within a cell, caused by the

melting of the separator between the electrodes, can start an uncontrolled temperature rise

accompanied by a chain of exothermal chemical reactions between the components that can

lead to a rapid pressure rise and explosion of the battery. Pressure valves can prevent the

explosion by limiting the pressure in the battery. Moreover, devices can be installed that willinterrupt the electric current, for example by braking, in case certain pressure or

temperature limits are exceeded. Moreover flammable electrolytes can be replaced by

polymers.

5.3 Electric vehicle layout and packaging scope

Similar to the standard motor vehicle different compositions of the drive are possible. Rear

wheel drive EVs obviously favour a positioning of the electric motor/-s near the rear axle, to

avoid weight gaining and space loss due to mechanical connections between the motor and

the axle or wheels, especially because no additional components like gearboxes are needed

in between. The same is true for front wheel drive EVs.

The weight distribution in a vehicle has a significant impact on its handling. An uneven weight

distribution between the sides has to be avoided. However, an uneven distribution of

passengers has to be taken in mind, especially the eventuality of driver only situation. The

guidelines for weight distribution between front and back are more complex as different

driving situations evoke contrary requirements. Overall, however, the optimum is close to a




33

balanced distribution. A low centre of gravity has positive impact on the handling, especially

in cornering. Moreover it prevents roll-overs, a common issue for sport utility vehicles

(SUVs), vans and pick-up trucks with a high centre of gravity.

The placement of the energy storage system (ESS) is very flexible, since it is connected to

the motor by relatively light and flexible electric cables. The batteries can be placed

anywhere in the vehicle where enough space and strong enough support can be provided.

There is also the possibility to split the ESS into several blocks and connect them. Potential

locations are in the back of the vehicle, the vehicle floor and in the so called “tunnel” located

on the floor between the passengers, which is traditionally used to house for example a rear

wheel driven car’s drive shaft as connection to a front motor. The front of the car is less

favourable due to safety reasons. Mechanical impact on the battery is to be avoided andfront collisions tend to have the highest impact energies. If a battery was to be designed for

absorbing kinetic energy by deformation, the threat of internal and external short-circuiting as

well as spilling toxic and hot substances would have to be contained very efficiently. In

addition a lack of components in the front-end of a car enables a more precise adaption of

the crash pulse, discussed in chapter 3.2. The roof is another less favourable position, due to

the raising of the centre of gravity. It can however be considered for busses, and other

municipal and heavy-duty vehicles. Positioning in the rear often decreases the cargo space

and exacerbates the load on the safety cell in frontal collisions, since the structure then hasto support the deceleration of the vehicle‘s heaviest component. Moreover the ESS is less

protected to exposition to mechanical loads and penetration that could be caused by

rear-end collisions.

Featuring a tunnel in the body structure increases the structural rigidity, but can somewhat

reduce the passengers comfort, especially in the rear. As tunnels are common in standard

vehicles, however, they are an overall favourable housing for batteries.

The battery’s accommodation in a sandwich floor or below the passenger seats meets the

requirements due to weight distribution best and can be protected best from deformation due

to impact from all sides of the vehicle. The only major drawback is that it could cause the

extension of the vehicle’s height, which results in an increased air resistance. For now, most

of the EVs are designed for urban commuting and because of low speeds the air resistance

will be less significant. Moreover the driver’s field of view will increase in an elevated

position. The predominant advantages of the placement at the vehicle floor prevailed and it

became predominant in current purpose built EVs.




34

5.4 Lightweight design of electric vehicles

In EVs lightweight materials and lightweight design is extensively used to challenge the issue

of the heavy batteries. Westbrook takes Honda’s hybrid model “Insight” as example, which’s

body weight was reduced by 40 % below that of a comparable steel body by the extensive

use of extruded, stamped and die-cast aluminium components and ABS composites. Such

techniques, however, do not only increase the vehicle cost significantly but also have a

negative impact on the production efficiency. On the other hand is the number of vehicles in

present series low enough for technology that is not yet developed for mass production.

[WES01]



6 Current electric vehicle layout and design

35


There are two basic approaches on the design of EVs: conversion- design and purpose-built

design. Either an existing ICE vehicle is converted into an electric vehicle by replacing the

propulsion system, while keeping the general structure, or a completely new vehicle is

designed to specifically fit the purpose of using electric propulsion. Both methods and current

supermini class models will be introduced. Those ICE models designed with alternative

propulsion taken in mind will be included in the purpose-built section. The overview presents

selected EV models’ performance, body design and choice of drive train equipment as well

as their packaging and safety performance. To demonstrate the difficulties and measures

taken in the field of EV crash safety the selection is focussed on purpose-built EVs that stand

out for their performance, size, weight, design or safety concepts. EEC-class M1 vehicles,which need to meet the standard safety regulations and demand a regular driving licence,

were preferred.

Additional models and details are summarized in table 1 in the appendix, which is neither

claimed to be complete in all details nor in the range of current models, due to the lack of

published details on the new models and the abundance of EVs currently introduced. It will,

however, provide an overview over current developments in mini EVs. Unfortunately there is

only very few information released on crash data of EVs.

6.1 Conversion design

In the conversion process of an existing vehicle with traditional propulsion the entire drive

train including the motor, gearbox and tank are removed and an electric motor, gearbox and

batteries are integrated. The advantage is the low development effort and production cost,

since the existing design of most of the vehicle can be adopted and the original production

lines can be utilised for most components. Additionally the certified original vehicle has

already proven to be safe in terms of crash safety and its performance in the converted statecan be estimated more easily. However, the new electric drive’s full potential and the new

design possibilities cannot be utilised. The placement of the heavy and bulky batteries is a

challenge and can lead to multiple disadvantages in a conversion vehicle. The batteries,

which are currently used to ensure a suitable range and the surrounding protective case, are

larger than the original fuel tank. Therefore the batteries often have to be placed at

unfavourable locations taking up room in the luggage compartment or passenger room,

especially in compact cars and smaller vehicles.




36

Additionally the batteries’ heavy weight causes difficulties beyond the increase in kerb

weight. The weight distribution shifts to the batteries, which can have negative impact on the

driving dynamics, in particular if they are placed of centre, as they often are in conversion

design vehicles. Moreover there is the risk of lifting the centre of gravity. Those parts of thevehicle structure that support the battery system often have to be reinforced, adding weight

to the structure. Furthermore it is very difficult to safe weight by downsizing those structures

that have to carry less weight, for instance the engine compartment in case of a design in

which the ICE is simply replaced by a lighter electric motor. Ultimately, the vehicle’s weight

increases beyond the increase that is due to the heavier total weight of the electric drive train

and the batteries, compared to the ICE drive train and fuel tank.

On the other hand, the conversion allows a fast entry into the field of electric propelledvehicles, especially for big motor companies, which can choose from a vast range of models.

In recent years most of the major car manufacturers have set-up at least one conversion

model EV additionally to concept studies. Some have been produced in small series and

candidates could apply for leasing one of them for a limited time. These cars, mostly used for

testing and demonstrations, have lead to a rapid increase of road experience of electric

propelled cars. Some manufacturers and converted models are: Audi (A2 electric); BMW

(Active E; Mini E); Citroen (Berlingo electrique, Saxo electrique); Dodge (Circuit EV); Fiat

(500 BEV, Doblo Micro-Vet); Peugeot (106e); Renault (Kangoo Elect’Road RE); Škoda(Elcar Tatra Beta); Renault (Kangoo Electri'cité); VW (Golf III CitySTROMer)




37

6.1.1 BMW Mini E

Fig. 6-1: Mini E cutaway [NUS11]

One example for a well tested conversion design vehicle is the BMW Mini E, which is based

on the unibody BMW Mini Cooper. The 260 kg heavy battery, consisting of 5088 Li-Ion cells,

replaces the fuel tank and rear seats. Under the bonnet in the front a 150 kW, 220 Nm

torque, three-phase asynchronous electric motor replaces the ICE as can be seen in the seethru figure 6-1. The body-in-white gains 70 kg due to reinforcements to support the battery

and to protect it in case of an accident. In total the vehicle roughly gains 300kg in weight

compared to the standard gasoline fuelled Mini Cooper model, looses the rear seats and

100 litres of cargo space, resulting in a total cargo space of only 60 litres The emphasis was

laid on the performance and safety to compare well against the standard gasoline model.

The Mini E accelerates in 8.5 seconds from 0 to 100 km/h, reaches maximum speeds up to

152 km/h and supports a range of 200 to 250 kilometres on a single charge. It features disk

brakes with brake boosters and BMW’s electronic stability control (dynamic stability control,DSC). The heavy kerb weight is well distributed over the axles, loading the front axle with

750 kg and the rear with 715 kg. [MIN11], [AUT11a]

From 2009 on more than 600 test vehicles have been offered for leasing over a period of 12

month respectively: for instance 500 in Los Angeles, California; 40 in Oxford, England and 40

in Berlin and Munich, Germany. The vehicles were leased to university members, celebrities

and selected test candidates who succeeded in the application process with reportedly over

10,000 candidates. The gained experience will be used in the creation of a new fleet of EVs




38

under the name of BMW i. At least one of them specifically designed for Megacities, which

will be addressed in the purpose-built section. [SPI11]

6.1.2 Tesla

With its exceptionally good driving performance the Tesla Roadster started drawing attention

to EV technology in 2007 particularly with regards to the capabilities of their lithium battery

system. It won several innovation and design awards, competed successfully in several

alternative propulsion rallies and gained much media attention. The combination of 6,831

type 18650 Li-Ion cells, usually used for example for laptop batteries, is the centrepiece of

the sports car. Despite the battery pack’s weight of 450 kg the original version can accelerate

from 0- 100 km/h in 3.9 seconds, reaches an electronically limited top speed of 201 km/h and

travels more than 320 km (200 miles) per charge. Since production start in 2008 until March

2011 1,650 Roadsters have been sold in 30 countries. Based on Lotus Elises’ body the

Roadster features a 65 kg light aluminium monocoque and is one of the cheapest sport cars

with a carbon fibre skin. Lotus engineering made great progress in the use of Lithium-Ion

batteries in vehicle technology and battery safety in vehicles. A five year warranty

guarantees the batteries lifespan and options for a beneficial priced replacement are offered.

6.2 Purpose design

A purpose- built EV’s body and frame are uniquely designed to meet the demands and

possibilities in the flexibility of electric propulsion. The vehicle layout and structure is

specifically adapted to the electric drive line and its components, which differ greatly in form,

size and weight from the components of a drive line with an ICE. Another difference is, that

the electric motor/-s can be mounted close to or on the driven wheels, because of their small

size and no complex gearbox is needed in between.

The purpose- design approach involves much greater effort in development and production.

Until recently the serial production of purpose- built EVs for this niche market was dominated

by small independent manufacturers, often in close collaboration with the suppliers of battery

technology. The production was dominated by manual work and small production lines. The

demand is still highly dependent on governmental legislation, for instance tax reductions and

subsidies on leasing contracts. The major car manufacturers kept to concept studies and

used exciting models of their fleet for conversion, with the exception of very few models,

such as the GM EV1. There has not been the demand for mass production of EVs with their

current performance and cost yet. Just recently, however, most major car manufacturersannounced the start of series production of purposed- built EVs in the years 2011 to 2013.




39

6.2.1 REVA i/ G-Wiz i

Mahindra Reva Electric Vehicle Company was created in 1994 in a joint venture of an

American and Indian partner and produces electric vehicles in India. Their original model

Reva, also called G-Wiz in Britain, was launched in 2001 in Bangalore and 2004 in London.

In 2009 it claimed to be the world’s EV manufacturer with the most vehicles on the road and

more than 70 million kilometres of user experience. The Reva i/ G-Wiz i is the upgraded

version of the model Reva/ G-Wiz. With an accumulated sale of 4,000 vehicles until 2011,

both models together make one of the most sold EV model up to date. About 1,200 of them

have been sold in the municipal area of London, where they benefit from free parking, tax

reductions, a favourable insurance class and charging stations. The upgrade in 2008

improved the motor’s performance, offered the choice of Li-Ion batteries and added severalsafety related features in response to a public controversy about the original model’s safety.

Disk brakes replace the drum brakes in the front and with the support of Lotus engineering

the passenger compartment’s structure has been strengthened to improve its performance in

front and side accidents. Moreover, the steering wheel column is now designed to be

collapsible, which has already become a standard in cars in the 1970s. [GOI11], [DNA11],

[DAY11a]

Fig. 6-2: REVA i / G-Wiz i [NN11c]

The Reva, as can be seen in figure 6-2, is designed to provide environmental commuting in

urban areas as alternative to bicycles and mopeds or fuel consuming cars. It is 2.6 meters in

length and weights 650 kg respectively 565 kg depending on the battery installed. The

lead-acid battery version has a range of 50 kilometres and a top speed of 75 km/h, while the




40

lighter Lithium-Ion version travels for 120 km. This microcar is individually adapted to meet

the national regulations for low- speed vehicles for each country. [GOI11]

Fig. 6-3: REVA/ G- Wiz in a EURO NCAP frontal offset collision test [DAY11b]

Fig. 6-4: REVA/ G- Wiz condition after a EURO NCAP frontal offset collision test [TOP11]

The initial version’s very poor safety performance has triggered a controversy aboutneighbourhood electric vehicle (NEV) safety standards in Britain and has been discussed in

British media and the parliament on several occasions. Being the most sold EV in Britain its

performance has been the root of a negative image of lightweight EVs, especially concerning

safety. In 2007 for example the BBC production “Top Gear”, a British television series about

motors vehicles, polemically compared the driving performance, battery life, comfort of the

passenger compartment and safety performance with a RC-car and a dining table that was

carried by four men. They executed a frontal off-set crash test with 64 km/h, in the Transport

Research Laboratory (TRL) in at Wokingham Berkshire, England, similar to the EURO NCAP




41

test. The TRL has been reported to have used outdated dummy-models, because of concern

about destroying their newer dummies in the test. The high-speed video footage of the test

shows the original G-Wiz’s very poor safety performance with a high chance of fatal injuries

to the driver. Figure 6-3 shows how the cars protection cell failed in the crash test. The frontpillar collapsed and the passenger compartment sustained severe intrusion in the whole front

area. At the same time door opened. A quick examination of the microcar’s condition after

the crash, as shown in figure 6-4, can already assess the injuries sustained by a real

passenger to be severe, especially in the area of the abdomen and the legs. [DAY11b]

Fig. 6-5: REVA/ G- Wiz after a collision with a Skoda Octavia [DAY11b], [DAY11c]

Supporters of the vehicle argue that it was not certified to travel at speeds up to the impact

speed of the test [TRE11A]. On the other hand, a fatal accident between a G-Wiz and a

Skoda Octavia in 2010 underlined the fact that NEVs are driving on public roads with

standard vehicles. Figure 6-5, as published in the United Kingdom's second biggest-selling

newspaper, shows how the vehicle was separated into several pieces in the accident. Thecrash-test and the accident show how important the passenger compartment’s rigidity is to

ensure safety for the passengers and to avoid losing public confidence. This is valid, even

though G-Wiz drivers have been involved in very few serious accidents, as the vehicles

importer GoingGreen could claim that, in 20 million miles driven in London and Bangalore

until 2007, there hadn’t been any accidents with serious injuries at all. [DAY11c]




42

6.2.2 Reva NXR and NXG

Fig. 6-6: Reva NXR [GEE11a]

The Mahindra Reva Electric Vehicle Company introduced its new generation of models on

the Frankfurt Motor Show (IAA) in 2009: the NXR and NXG. The start of new model NXR’s

production is planned in 2012 in the newly built assembly plant in Bangalore, which is

capable of an annual output of 30,000 vehicles. The NXR, shown in figure 6-6, is supposed

to be a proper city car competing with normal vehicles in performance and safety. The

Lithium-Ion version is capable of comfortably cruising at highway speed with a maximumvelocity of 104 km/h and covers a nominal range of 160 kilometres. In the NXR more

emphasise was put on the structure’s crashworthiness and safety features as in the Reva

microcar. The steel spaceframe, as can be seen in figure 6-7, includes an enlarged crush

zone in the front and is strengthened with high-strength steel elements. Side-protection

elements, airbags for the driver and passenger and naturally a collapsible steering column

have been added. The lithium version’s total weight is 850 kg respectively 900 kg for the

lead- acid version. Therefore the NXR falls into the standard passenger car class (EEC class

M1). Consequently full safety standards and crash tests will apply in the certification. [SMI11]




43

Fig. 6-7: Reva NXR’s steel spaceframe [SMI11]

With the new model Reva introduced a “remote emergency charge system” called Revive. In

case of running out of charge, while being on the road, the system is supposed to unlock

hidden capacities of the batteries to allow reaching an accessible electric plug. The driver

would send a message via cell phone to a Reva service centre, which remotely carries out

the unlocking. It is supposed to comfort customers, who are scared of running out of charge

on the road. Even though there are still no new official details released from Reva about the

system, it is likely that it allows deep discharging below the usual level. The usual level is set

to protect the batteries lifespan, which can be impaired by repeatedly deep discharging.

[TRE11b]




44

6.2.3 Think City

Fig. 6-8: Think City (4 seat version) [THI11]

Because of its small dimensions and nonetheless good crash performance, the Think City is

of special interest for this thesis. Designed for fleet applications and urban commuters, it was

the first EV to get a pan-European homologation certificate for standard motor vehicles. Over

50 sled and 20 full scale test have been conducted in the safety optimisation progress. In

addition the electric safety was tested by submerging the vehicle in salt water with activated

electrical systems and test driving it through 30 cm deep water. Their effort resulted in the

safety certification in 1999. Until December the vehicle has accumulated more than 35 million

road miles in customer experience [THA11].

It is built in Finland for the European market. A new manufacturing facility in Indiana is

producing the Think City for the USA with more than 100 workers. Until the end of 2013 raise

of the work force to more than 415 employees is planned. In the second half of 2011 Think

plans to roll out retail distribution in select cities.




45

Fig. 6-9: Think City body structure [cf. THI11]

Figure 6-9 presents the interesting composition of a high-strength steel frame in combination

with a space frame of extruded aluminium hollow profiles. The rigid bumper including frame

is responsible for most of the impact energy absorption. It also houses the battery

compartment, which is capable of fitting different types of batteries, and protects it from

impact from either direction.

Fig. 6-10: Battery position and several safety related devices in the Think City [cf. THI11]

Upperstructure

12 VoltBattery

ControlUnit

Seat belttensioner

High- VoltageComponents

Structuralreinforcements

High- VoltageBattery

Gas filledspring device

Airbag



6

Think provides a rescue s

related features clearly str

and other electrical related

6.2.4 Tazzari ZERO

Fig. 6-11: Tazzari ZERO [

The Tazzari Zero is a light

supposed to represent th

the urban commuter. It’s I

leather sports steering w

impression. The choice of

enables individuality. Dis

lightweight aluminium bod

underlined by aluminium p

use of recyclable materialconcept. [TAZ11]

The Tazzari group compa

Imola, Italy. They designed

with a mere curb weight o

and the relatively small ca

in the front and vehicle’s b

Current electric vehicle layout and design

46

heet, partly shown in figure 6-10, with the

uctured in figures. Besides the important lo

devices it also presents the advanced restr

MI11]

weight city car with an emphasis on styling

avant-garde of sporty, affordable electric

alian styling, as shown in figure 6-11, and

eel and aluminium rims, are aimed at m

twelve different exterior paintings and three

brakes, different drive modes, a low

and a wide wheelbase shall create a spo

edals and internal door handles. On the oth

and the electric drive train shall outline th

y’s foundries are specialised on aluminium

an aluminium body structure, which is the c

f 542kg. The very compact car’s total lengt

go space is limited to 180 litres separated i

ck-end. The Tazzari ZERO went into produ

most important safety

cation of the fuse box

int system featured.

and performance. It is

ehicles, designed for

features, such as the

king a contemporary

different rims colours

entre of gravity, the

ty character, which is

er hand the extended

e ecological vehicle’s

alloy casting based in

ornerstone of their EV

h is below 2.9 meters

to two compartments

tion in 2009. [TAZ11]




47

Four different driving modes are selectable: race, economy, standard and rain. The power of

the drive and the regenerative braking system’s efficiency can be adapted to a driving style

or the road conditions. This influences the acceleration and braking capabilities. The

three-phase asynchronous motor (150 Nm torque) is positioned between the safety cell andthe rear axle at the axle’s height. The aim was a low centre of gravity and even weight

distribution on both axles to enhance the road holding characteristics and stability. The

power unit is positioned slightly off-centre to balance the driver’s weight, when there is no

other passenger in the car. The position is switched to the left in the right-hand drive version.

[TAZ11]

There are several approaches on the safety improvement. In terms of active safety the four

electro-assisted disk brakes and LED technology in the turn signals and tail- and brake lightshave to be noted as well as the elaborate weight distribution, which improves the driving

dynamics. In terms of passive safety the aluminium body structure features a stiffened safety

cell and both front- and rear- crumple zones with straight beams to absorb a crash impulse

best. On the other hand advanced driver assistance systems, such as anti-lock braking

system (ABS) and electronic stability control (ESC) are missing. Furthermore the lightweight

vehicle (class L7e) it will not have to compete for standard passenger cars’ safety

performance. [TAZ11], [SMI11]




48

6.2.5 BMW i

In November 2010 BMW announced the production of two purpose built models the model i3

and i8. The i8 will be a purpose built sports car and the i3 a subcompact or compact

hatchback. Figure 6-12 shows some details of BMW’s electric drive train in the ActiveE

version of a BMW 3 series for instance the separation of the batteries in a rear and tunnel

module. The GM EV1 was also housing batteries in a tunnel, which increases the structural

stability. The i3 will feature an aluminium chassis combined with a lightweight carbon fibre

composite passenger compartment, pictured in figure 6-13, is supposed to entirely

compensate the battery’s weight. [FOC11], [ZEI11]

Fig. 6-12: BMW ActiveE drive train [cf. BMW11]

Fig. 6-13: BMW i3 carbon fibre composite body [BWI11]

Tunnel battery module

Rear battery module



7 Miniaturisation of vehicles

49


With the current range anxiety of EVs the most favourable application for electric drives are

second cars aimed on daily local mobility. Particularly in urban areas small vehicle size is

advantageous due to park space and energy efficiency. On basis of an exceptionally small

car with good safety performance, the Daimler Smart ForTwo coupé, and general vehicle

safety knowledge the dangers of vehicle miniaturisation are detected and countermeasures

suggested.

7.1 Daimler Smart ForTwo Coupé

Fig. 7-1: Smart ForTwo Coupe [CAR11b]

Standing out for its ultra compact size and nevertheless very reasonable safety performance

Daimler’s Smart ForTwo demonstrates a range of principles that should be considered in

small lightweight vehicle design. With a total length of only 2.5 meters the Smart ForTwo,

portrait in figure 7.1, is even shorter than the Reva G-Wiz. The 730 kg light car has almost no

crumple zones other than the low speed crash boxes. Nevertheless it achieved an overall

4 star rating in the EURO NCAP. One of the main reasons is the exceptionally rigid “Tridion”

safety cell, a steel safety cell reinforced with high-strength steel. With a minimum of crush

zones, the passenger compartment’s rigidity is even more important to ensure the

passengers safety. In a crash with a larger vehicle the Smart ForTwo uses the crash

partner’s crumple zone. The crash partner’ safety is retained, because its crumple zone is

designed for accidents with cars of twice the Smart’s weight. Another measure is the special

design of the front wheels and wheel arches optimised take part in the impact energy

absorption. Protected from penetration the issue of the severe velocity change remains, as a




50

ForTwo will almost certainly be pushed away from most of its potential crash partners. A

very sophisticated restrain system including several Airbags, seatbelt pretensioners and load

limiters is the solution. The newest versions also include weight sensors in the seats to

adjust the airbag deployment to the passenger. Its 5 star rated side protection by theNHTSA is partly due to the elevated seating position, which places the passengers above the

intrusion zone of another cars front, and the fact that most likely both car’s wheels and axles

will be hit by the other vehicle rather than just the rocker and side structure. In terms of active

safety the Smart ForTwo is well equipped with both ESC and ABS. Figure 7-2 provides

evidence of the superior performance compared to the slightly heavier G-Wiz in figure 6-3. In

crash tests against the more than twice as heavy Mercedes C-Class the dummy was

exposed to much higher loads. The Smart was airborne, pushed away for a distance of more

than 7 meters and spun about one and a half times until it came to a halt. Even though thedriver of the smart was likely to be severely insured, considering the size and weight, the

car’s performance was still good. [DAI11], [JAC07], [ITI11a], [ITI11b], [YOU11a]

Fig. 7-2: Smart ForTwo Coupe in EURO NCAP frontal offset crash test [NCP11b]

An interesting concept to reduce the negative effect of the lack of crumple zones is the Smart

ForTwo’s flexible support of the rear-wheel drive assembly. The Motor and gearbox are

supported by engine bearers that allow a longitudinal movement to act as dampers

absorbing a part of the assembly’s kinetic energy in a crash [JAC07]. In addition the flexibility

is partly elastic. In the rebound subsequent to a crash or in a collision with a much heavier

vehicle the Smart will move backwards. The power unit’s velocity course will lag behind the

rest of the vehicles structure. At one point the power unit comes to a complete halt when the

safety cell is already moving backwards. From then on the flexible support is supposed to




51

release the share of potential energy stored by the elastic part to reduce the safety cell’s

backward velocity. This will reduce the rebound’s magnitude and severity and therefore the

relative speed between the passengers and the safety cell, softening the ride-down. By

spreading the collision energy pulse over a longer time injury producing peak loading can bereduced.

To prevent intrusion into the passenger compartment in a rear accident the power unit

assembly is mounted in such a manner that it slides beneath the floor without intruding the

occupant space [JAC07]. Figure 7-3 shows the inclined sandwich laminate that shields the

passengers from engine components and supports the engine’s deflection. This idea can be

adapted to any EV with a compact power unit mounted in a low position.

Fig. 7-3: Cutaway of Smart ForTwo [cf. ITI11c]

The pedestrian protection has been enhanced in the newer models, since it was criticised in

the EURO NCAP and rated poorly [NCP11b]. The vehicle design lacks of the long front

bonnet that would enable the preferred head vs. Bonnet collision.

Sandwichlaminate




52

7.2 Smart ED second generation

Fig. 7-4: Electric drive layout in a Smart ED [SMA11]

The second generation Smart ForTwo ED model is built since 2009 in a small series and

tested in eight countries in Europe as well as the USA, Canada and Asia. Series production

and sales are announced for 2012. The high-temperature Zebra batteries in the sandwich

floor, similar to those used in the Think City, were replaced by Tesla’s Li-Ion batteries. As

can be seen when comparing figure 7-4 with figure 7-3 the original model’s layout onlychanged minimally to fit the electric drive, because the option of alternative propulsion was

already considered in the original design. The total weight, however, has been increased by

150 kg to a total of 890 kg. The safety features are equally to the normal fossil fuel model on

a very high standard. A roll over cache, the same motor support idea and the sandwich

laminate shield are included. Moreover the same advanced restraint system including the

weight adapting airbags and seatbelt pretensioners are added as well as the electronic

stability control. Overall it is a very good example demonstrating the possibility of electric city

cars to be small and yet perform well, provide comfort and include contemporary safety

features. [GEE11b]

7.3 Consequences in the design of mini lightweight vehicles

The kinetic energy to be absorbed in a crash is given by the vehicle’s mass and speed. It will

be consumed by the vehicle’s structural deformation and the restraint system. The load that

is directed to the occupant in this process is limited by the maximum load a human can take

without severe injuries. Therefore the maximum resistance the structure should have againstdeformation is limited to an amount that will not evoke serious injuries. The crush zone,




53

however, is restricted in size by the vehicle design, especially in case of a microcar. The

kinetic energy has to be absorbed before the whole crumple zone is deformed to prevent

intrusion into the passenger compartment or a peak force when the crash partner reaches

the stiff passenger compartment. Thus, the crush zone design and choice of its stiffness issubject to conflicting aims. Stiff enough to absorb enough energy in the given limited

deformation path and yielding enough not to induce severely dangerous decelerations of the

occupant.

For those reasons, if the goal is to minimise the vehicle and thereby shrinking the

deformation zone and length, the kinetic energy has to be reduced as much as possible by

reducing the vehicles weight. Lightweight materials and smart body design reduce the

body-in-white’s mass. The development of battery technology will reduce their mass as well – but nonetheless the energy storage system’s big weight evokes the biggest constraints to

the crush zone’s minimum size. The heavier the vehicle, the stiffer has the crush zone to be

to absorb the kinetic energy before the deformation zone is fully deformed. Nevertheless,

even if the car is designed with the best lightweight techniques available, there is always a

minimum of crush space needed, to limit the passengers’ deceleration.

The battery’s heavy weight increases the demands for the crush zones additionally. To

absorb the additional kinetic energy either the crush length has to be extended or the

stiffness increased.

7.4 Concept of flexible and deformable battery support

One interesting way of reducing the effect would be a modification of the battery support.

Similar to the Smart ForTwo’s flexible support of the rear-wheel drive assembly, energy

absorbing battery support could take part in the impact energy absorption and reduce the

requirements for the crumple zones. Depending on the EVs total weight the idea of a partly

elastic support can be considered to reduce the rebound’s magnitude. If the support iscapable of absorbing kinetic energy by plastic deformation or similar to a damper, the

deformation length for the battery’s portion of the total vehicle’s weight is increased. The

concept could be realised for example by bearers of specific material characteristics and

geometry, such as rubber retainers, but also through metal tube with an axial folding failure

mode, similar to a vehicle’s crash boxes or front rails. The stiffness of the front crumple

zone’s section illustrated in figure 3-1 is increased from stage to stage to achieve a

progressive crumpling. The absorbing battery support however, would have to be more

yielding than the central section. Then parts of the battery’s kinetic energy would beconverted into the deformation of the battery support before it would affect the frontal




54

crumple zone. The battery can be linked directly to the frontal collapsible zone’s central

section or the axle assembly. The passenger’s compartment, which is otherwise supporting

the weight, and the firewall, which protects the safety cell from intrusion, will be relieved from

the additional load created by the battery’s large kinetic energy.

Fig. 7-5: Concept of flexible/ deformable battery support

As pointed out before the restraint system is vital. Its optimisation enables smaller vehicles

with the same safety performance as vehicles with larger crumple zones. Advancedtechnology, such as multiple airbags, weight adapted airbag response and seatbelt

pretensioners should be considered despite their cost and weight, because of their very

efficient safety performance.

Front crumple zone Rear crumple zone

Battery

Flexible battery support

Low-speed

Central

Firewall



8 Conclusion

55

8 Conclusion

A fleet of small electric vehicles providing urban mobility are a promising solution to

challenge smog, greenhouse gases, the fossil fuel scarcity and lack of parking space. The

current models’ performance figures show the technology’s capability to cope with the

demands of local personal mobility, particularly as second car.

New requirements for small EVs can be specified by application of crash safety

fundamentals. When miniaturising a vehicle in length the crumple zones will be downsized as

well. This poses no threat as long as the stiffness remains on the same level, the weight is

proportionally downscaled and crash compatibility is not taken in mind. In current traffic,

however, the average weight and size of the vehicle fleet is far above the planned small

lightweight EV. Moreover, including the electrical drive train with its heavy battery increases a

vehicles weight even when extensive measures of lightweight design are taken. This leads to

increased demands for the remaining crumple zone, the safety cell and the restraint system.

Fortunately those systems have room for improvement. Advanced body and frame design

combined with the electric drive train packaging flexibility enable adjustments of the front

crumple zones to enhance the plastic deformation’s effectiveness. Stiffer responses with

crash pulses closer to the optimal equivalent square wave are feasible as the tight packaging

of frontal ICE compartments ceases. The Smart ForTwo demonstrates the positive impact of

the extraordinarily rigid safety cell. Advanced features in restrain system technology can

partly substitute the missing crumple zones by using the available interior space as extension

of the load path to decrease the required magnitude of the forces applied on the occupant.

Seatbelt pretensioners enlarge the load path by diminishing slack and advanced airbags

optimise their performance by adapting the passenger’s statue and weight. Moreover

advanced active safety systems can be included. Even though all of those measures add

weight and cost, they are well worth consideration as they advance further while becoming

more and more standard in motor cars.

To increase pedestrian safety additional attention is required besides the general basics of

pedestrian safe front and back design. To which extend the lack of engine noise at low

speeds poses a threat remains vague as there are divergent opinions among experts. The

lack of a lengthy bonnet, however, certainly increases the probability of a pedestrians head

colliding with the windshield in an accident. This threat has to be approached by yielding

windshields and supports or pedestrian airbags.



8 Conclusion

56

While the front-end of a EV is better used for cargo space or the smaller electric motor,

leaving enough freedom to design to focus on crush performance and both the front and rear

end pose threats to the battery safety to a certain extent, the accommodation of the battery

pack in the floor or tunnel proves to be the most promising. It satisfies the demands forbattery safety as well as the weight distribution. New challenges, such as high-temperature

and high-voltage safety have to be addressed by everyone involved: the manufacturers,

users and rescue workers as well as the certification and assessment institutes.

The overview of current models demonstrates the wide range of manufacturers taking

advantage of the new design flexibility to progress the safety and performance of electric

mobility. Moreover, the rapid progress in battery technology, material science and production

optimisation will further increase the feasibility of small lightweight electric vehicles.



9 Formula symbols and indices

57

9 Formula symbols and indices

ABS Anti-lock brake system

ASR Anti-spin regulation

ECE Economic Commission for Europe

ESC Electronic stability control

ESS Energy storage system

ESW Equivalent square wave

FMVSS Federal Motor Vehicle Safety Standards

ICE Internal Combustion Engine

NCAP New Car Assessment Programme

NHTSA National Highway Traffic Safety AdministrationNVH Noise, Vibration, Harshness

TRL Transport Research Laboratory



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11 Apprendix

66

11 Apprendix

11.1 Table of selected current mini electric vehicles

Production Manufacturer BMW Tesla Daimler SmartModel name Mini E Roadster Smart ED II

production small small 2008 small 2009

origin Germany USA Germany

design Conversion Conversion Purp/Conv

availability test mode on sale sale (~2012)

category EG-vehicle class M1 M1 M1

EV vehicle class supermini sports car city car

dimensions Wheelbase [m] 2467 2352

Total length [m] 3714 3946 2695

Height [mm] 1407 1127 1542

width [mm] 1683 1873 1559

battery B. weight [kg] 260 450

Battery type Li-Ion ZEBRA Li-Ion Li-Ion Li-Ion

Capacity [kWh] 21,5 28,3 35 (28 usable) 53 16,5

Voltage [V] 400 378 375

B. position rear seats rear floor

motor Motor Type async async

Power [kW] 150 215 20 / 30(peak)

Torque [NM] 220 370 120

position front comp. central rear

Drive front comp. rear rear

structure Kerb weight [kg] 1465 1240 890

Body shape hatch-back Roadster hatch-back

Doors 3 2 3

Body type unit-body monocoque unit-body

Material alu & cfk steel

Panels matl cfk abs

performance Max. v [km/h] 152 201 100

Range [km] 240 350 135

Seats 2 2 2

0-40 km/h [s]

0-50 km/h [s]

0-60 km/h [s] 6,5

0-80 km/h [s]

0-100 km/h [s] 8,5 4

cargo space [l] 60 220

turning 5,35

safety BS, power steering,

EURONCAP

160

2 or 4

6,5

16

1038

110

1658

245 - 260

floor - below seats

async

front

4,5

e impact beam, ESP,

3143

Think Global ASCity

2007

Norway

on sale

Purpose

city car

M1

1970

3

hatch-back

25 / 34 (peak)

1596

space frame

HSS & Alu

ABS



11 Apprendix

67

Production Manufacturer Mahindra Reva Tazzari GroupModel name Reva NXG ZERO

production ~2013 2009

origin Bang.- India Italy

design Purpose Purpose

availability on sale

category EG-vehicle class L7e

EV vehicle class city car city car

dimensions Wheelbase [m]

Total length [m] 2884

Height [mm] 1400

width [mm] 1550

battery B. weight [kg] 265 165 142

Battery type Ld-Acid Li-Ion Ld-Acid Li-Ion Li-Ion

Capacity [kWh]

Voltage [V] 48 48 48 72 72

B. position

motor Motor Type AC ind. AC ind. async

Power [kW] 13 13 13 25 15

Torque [NM]

position central

Drive rear rear rear

structure Kerb weight [kg] 665 565 900 850 542

Body shape targa top hatch-back

Doors 3 3

Body type space frame space frame

Material HSS alu(cast)

Panels matl

performance Max. v [km/h] 80 104 130 100

Range [km] 50 120 80 160 200 140

Seats 4 4 2

0-40 km/h [s]

0-50 km/h [s] 5

0-60 km/h [s]

0-80 km/h [s]

0-100 km/h [s]

cargo space [l] 180

turning

safety air bags o-assisted disk

EURONCAP

Purpose

3

Mahindra RevaReva NXR

~2012

Bangalore- India

M1

3280

1514

hatch-back

1560

city car

Purpose

75

2+2

7

300

1320

microcar

2640

1510

space frame

hatch-back

3

Mahindra RevaReva i/ G-Wiz i

2008

Bangalore- India

on sale

L7e

1810

3,5

HSS

larged front cru

ABS PMMA

space frame

HSS



11 Apprendix

68

Production Manufacturer StartLab/EVE Mitsubishi NissanModel name Easy Street i- MiEV Leaf

production ~2012

origin Italy/Spain

design Purpose

availability on sale

category EG-vehicle class L7e

EV vehicle class microcar supermini supermini

dimensions Wheelbase [m] 1725 2550 2700

Total length [m] 2354 3475 4445

Height [mm] 1540 1610 1550

width [mm] 1260 1475 1770

battery B. weight [kg] ca. 50 1110 272

Battery type

Capacity [kWh] 16 24

Voltage [V] 48 360

B. position

motor Motor Type

Power [kW] 4 / 7(peak) 47 80

Torque [NM] 258

position

Drive rear

structure Kerb weight [kg] 400 1520

Body shape hatch-back

Doors 2

Body type space frame

Material aluminum

Panels matl

performance Max. v [km/h] 75 130

Range [km] 80 150

Seats 2

0-40 km/h [s]

0-50 km/h [s]

0-60 km/h [s]

0-80 km/h [s]

0-100 km/h [s]

cargo space [l] 263 330

turning

safety

EURONCAP 4 5



11 Apprendix

致谢

本论文是周青教授的悉心指导下完成的，周老师深厚的学术功底、踏实的

工作态度和创新性的思维使我深感敬佩、受益匪浅。在研究工作中给予了作者

诸多指导，在此对他表示最真挚的谢意！

特此致谢。

声明

本人郑重声明：所呈交的学位论文，是本人在导师指导下，独立进行研究

工作所取得的成果。尽我所知，除文中已经注明引用的内容外，本学位论文的

研究成果不包含任何他人享有著作权的内容。对本论文所涉及的研究工作做出

贡献的其他个人和集体，均已在文中以明确方式标明。

签名：日期：

致谢和声明

thesis storz v4.1

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