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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
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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
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Contents
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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
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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
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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
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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
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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 space- frame ...................................................................... 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
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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
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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.
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2 Motivation and research objective
2
2 Motivation and research objective
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]
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2 Motivation and research objective
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
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2 Motivation and research objective
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.
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3 Background and fundamentals of vehicle crash safety
5
3 Background and fundamentals of vehicle crash safety
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]
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3 Background and fundamentals of vehicle crash safety
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
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3 Background and fundamentals of vehicle crash safety
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
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3 Background and fundamentals of vehicle crash safety
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 ]
d e c e l e a r a t i o n [
g ]
ESW=18,12g
d e c e l e a r a t i o n [
g ] tm=88,56ms
∆t=30,49ms
Peak deceleration=40,37g
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3 Background and fundamentals of vehicle crash safety
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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
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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
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3 Background and fundamentals of vehicle crash safety
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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
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3 Background and fundamentals of vehicle crash safety
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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
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3 Background and fundamentals of vehicle crash safety
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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
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3 Background and fundamentals of vehicle crash safety
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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]
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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.
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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]
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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]
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4 Structural design and automotive layout
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]
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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.
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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
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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]
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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.
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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]
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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.
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5 Electric vehicle technology
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.
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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]
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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)
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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
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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
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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]
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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
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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
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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.
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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]
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6 Current electric vehicle layout and design
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.
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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)
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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
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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.
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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
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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
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6 Current electric vehicle layout and design
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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]
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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 space- frame, 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]
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6 Current electric vehicle layout and design
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Fig. 6-7: Reva NXR’s steel space- frame [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]
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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.
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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
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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]
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6 Current electric vehicle layout and design
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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]
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6 Current electric vehicle layout and design
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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
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7 Miniaturisation of vehicles
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7 Miniaturisation of vehicles
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
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7 Miniaturisation of vehicles
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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
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7 Miniaturisation of vehicles
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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
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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,
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7 Miniaturisation of vehicles
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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
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7 Miniaturisation of vehicles
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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
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8 Conclusion
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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.
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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.
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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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10 Literature
58
10 Literature
[AUT11a] SCHRIEBER, H.www.autobild.de
Fahrbericht Mini E: Elektrisch, exklusiv, eng
[BEC09] BECKER, T.A.; PI, I.S.; TENDERICH, B.Electric vehicles in the United States: A new model with forecasts to 2030Centre for Entrepreneurship and Technology Technical Brief, 2009
[BER06] BERDICHEVSKY, G.; KELTY, K.; STRAUBEL, JB; TOOMRE, E.The Tesla Roadster Battery SystemTesla Motors Inc, 2006
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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
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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
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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
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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。
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致谢和声明