Thesis Chowdhury

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POLITECNICO DI TORINO
Master of Science in Automotive Engineering
Master degree Thesis
Fuel consumption and performance analysis
of TTR hybrid 4WD vehicles
Academic Tutors: Author:
Nicola Amati Chowdhury Foyz Ahamed Polas
Andrea Tonoli
Luca Castellazzi
December 2016

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Acknowledgements
I would like to thank family for their continuous support, and love throughout my whole
life. Special thank goes to my brother for always being there for me. Without them it
wouldn’t be possible for me reach where I am today.
I would also like to express my deepest appreciation to Luca Castellazzi for his
continuous support and assistance throughout the development of this thesis. Also
thanks to Di Donato Stefano for helping me out with various problems in the thesis.
Big shout to my friends for always sticking with me in good and bad times.

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Abstract
One of the biggest concerns in the automotive industry today is the fuel consumption of
the vehicle due the destructive environmental effect of the carbon emission and the
increase of fossil fuel price. One of best solution today to reduce the fuel consumption
of vehicles is the hybrid-electric vehicle technology, which uses electric power in
conjunction with the combustion engine to drive the vehicle. The use of the electric
motor reduces the use of the internal combustion engine, thus reducing the fuel
consumption.
Most of the companies today are trying to develop HEVs instead of traditional ICE
based vehicles. The problem in the development of the HEVs is the high amount of
modification required in the vehicle chassis to support the components of the electric
powertrain.
The “Through The Road (TTR) ” HEV is a variation of 4WD parallel HEV architecture
that allows total mechanical decoupling of the ICE, which propels one axle, and the
EM, which propels the others axle of the vehicle. The TTR architecture does not require
total redesign of the vehicle chassis as the ICE and the EM are totally separate and work
independently.
The main objective of this thesis is to develop a flexible and modular model of a TTR
HEV as the first step of vehicle development. The battery and motor dimensioning is
the principle goal as the front axle components (ICE, gearbox and differential) are
standard. The modelling is done using the Matlab and Simulink framework. The vehicle
model is created based on the longitudinal dynamics (supposing that the vehicle moves
in a straight line on a planar surface), using only the main powertrain components (ICE,
EM, gearbox, clutch, battery, differential, wheels etc.) that are required to drive the
vehicle.
A controller interprets the throttle and brake power required by the driver. The control
algorithm/strategy splits the power and torque between the ICE and EM based on the
constraints on battery current, SOC, temperature, and EM characteristics. The first step
is to find out the optimum power split strategy. The second step is to find out a suitable
battery capacity that maximizes the electric motor use and provides the power to all the
electric accessories, while minimizing the fuel consumption of the vehicle.

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Throughout this thesis the following goals are tried to be achieved –
 To satisfy the drivers power demand,
 Dimension a suitable electric drive-train (battery and electric motor),
 Minimize the fuel consumption and emission,
 Maintain a reasonable level of battery SOC for self – sustaining operation (no
external charging),
 Recover maximum amount of brake energy.

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Contents
1 Introduction............................................................................................................. 11
1.1 Hybrid Electric Vehicles (HEV) ...................................................................... 12
1.1.1 Based on the architecture.......................................................................... 12
1.1.2 Based on hybridization ratio..................................................................... 14
1.2 Through The Road (TTR) HEV....................................................................... 15
2 Vehicle Model......................................................................................................... 17
2.1 Driver ............................................................................................................... 18
2.1.1 Throttle and brake pedal control............................................................... 18
2.1.2 Clutch control and gear selection.............................................................. 19
2.2 Drive – train ..................................................................................................... 20
2.2.1 Front axle.................................................................................................. 21
2.2.2 Rear axle ................................................................................................... 26
2.2.3 Brakes ....................................................................................................... 32
2.2.4 Wheels ...................................................................................................... 33
2.3 Vehicle dynamics............................................................................................. 34
3 Energy storage system (ESS).................................................................................. 36
3.1 Battery technology ........................................................................................... 36
3.2 Battery modelling............................................................................................. 38
3.2.1 Charge and discharge model..................................................................... 39
4 Control strategy....................................................................................................... 42
4.1 Traction and brake torque calculation.............................................................. 43
4.2 Maximum electric motor torque calculation.................................................... 44
4.2.1 Current limitation...................................................................................... 45
4.2.2 SOC limitations......................................................................................... 46
4.2.3 Temperature limitation ............................................................................. 47

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4.3 Clutch position control..................................................................................... 48
5 Model validation..................................................................................................... 49
5.1 Pure ICE mode................................................................................................. 50
5.2 EV mode........................................................................................................... 52
5.3 HEV mode........................................................................................................ 53
5.4 Energy validation.................................................................................................. 56
6 Simulation and results analysis............................................................................... 59
6.1 Velocity limit with no constraint on the battery usage .................................... 60
6.2 Velocity limit with current constraint .............................................................. 61
6.3 Fuel consumption............................................................................................. 67
6.4 Electric accessories .......................................................................................... 71
6.5 Fuel consumption with alternator connected to the ICE.................................. 75
6.6 Summary of the results..................................................................................... 79
7 Conclusion .............................................................................................................. 82
8 Model limitations and future work ......................................................................... 84
9 Bibliography ........................................................................................................... 92

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1 Introduction
August 8, 2016, Earth Overshoot Day, the day we began to use more from nature than
our planet can renew in the whole year [1]. Fossil fuel account for 85% of world’s
energy needs [2], simply because they are the world’s least expensive source of energy
and today almost half of it is being used in the automotive industry. In 2010 the number
of motor vehicles in the world crossed the 1 billion mark [3] and the emission from
these vehicles is one of the biggest reasons of some very concerning issues, such as
global warming, deforestation, and air pollution. In 2016 the danger is more evident
than ever as this year has been the hottest year in the recent history [4]. If the oil
discovery and consumption follows the current trend, in less than 50 years we are going
to be out of fossil fuel.
In the 21st
century the earth doesn’t have the luxury of having exponentially increasing
pollution anymore, and the fastest growing concerns of this era are - energy
consumption, environmental protection and sustainable development. To address these
issues national and international organizations have been enforcing tighter legislations
on fuel economy of on road vehicles, more specifically limitations on harmful pollutants
produce by the vehicles such as carbon dioxide, unburned hydrocarbons, carbon
monoxide, nitrogen oxides and particulate matters.
There is no doubt that the use of ICE as the primary source of energy in the motor
vehicles is neither sustainable for the environment nor the economy and eventually, in
the near future, we are not going to be able to afford them anymore. In fact the
automotive industry has been searching for an alternative source of energy, and one of
the best solution till now are full electric vehicles. The biggest caveat of the electric
vehicles today is the battery technology. The li-ion batteries of today have an average
specific energy of 0.5 MJ/kg, while gasoline and diesel have 44-48 MJ/kg [5], as a
result the electric vehicles have a much lower driving range than the ICE vehicles, and
the lack of battery charging infrastructure around the world makes it very difficult for
most of the people to use an electric vehicle in their day to day life, even if they can
afford one.
One of the best temporary solutions for these above mentioned problems today are the
Hybrid Electric Vehicles (HEV), which uses both electricity and fuel as source of
energy and reduce the fuel consumption in urban driving cycles.

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1.1 Hybrid Electric Vehicles (HEV)
According to the International Electrotechnical Commission - HEV is a vehicle in
which propulsion energy is available from two or more kinds or types of energy stores,
sources or converters, and at least one of them can deliver electrical energy. Based on
this general definition, there are many types of HEVs, such as the engine and battery,
battery and fuel cell, battery and capacitor, battery and fly-wheel and battery and battery
hybrids. However, ordinary people have already borne in mind that a HEV is simply a
vehicle having both an engine and an electric motor, and we are going to use this simple
definition of HEV throughout this thesis. Different types of HEVs are described below-
1.1.1 Based on the architecture
Traditionally, HEV were classified into two basic categories – parallel and series.
Recently with the introduction of some HEVs offering the feature of both parallel and
series hybrid the classification has been extended to 3 types-
 Series Hybrid
 Parallel Hybrid, and
 Series – Parallel Hybrid.
The series hybrid is the simplest kind of
HEV. Its engine mechanical output is first
converted into electricity using a generator.
The converted electricity either charges the
battery or can bypass the battery to propel
the wheels via the same electric motor and
mechanical transmission.
Figure 1.1 - Series hybrid architechture

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Diﬀering from the series hybrid, the
parallel HEV allows both the engine and
electric motor to deliver power in parallel
to drive the wheels. Since both the engine
and electric motor are generally coupled to
the drive shaft of the wheels via two
clutches, the propulsion power may be
supplied by the engine alone, by the
electric motor alone or by both.
In the series-parallel hybrid, the
conﬁguration incorporates the features of
both the series and parallel HEVs, but
involves an additional mechanical link
compared with the series hybrid and also
an additional generator compared with
the parallel hybrid. This type of
architecture is although more efficient but
much more complex than both the series
and parallel architectures.
The series HEV is the simplest architectures but requires a full sized electric machine to
power the vehicle as there is no direct connection between the ICE and the wheels,
which also allows the ICE to operate in the full range of its torque-speed curve. The
parallel HEV allows to have a smaller ICE and electric motor as they can provide power
to the wheels independently but this requires more complex control strategy. The series
– parallel HEV on the other hand combines the advantages of both the series and
parallel architectures but the implementation is much more complex and requires an
addition alternator.
Each one of these HEV structures has their respective advantages and disadvantages
and they are shortly explained in the table below-
Figure 1.2 - Parallel hybrid architecture
Figure 1.3 - Series - parallel hybrid architechture

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Advantages Disadvantages
Series HEV
- Least complex architecture,
- Optimum operation of the ICE
due to the total mechanical
decoupling of the ICE from the
wheels,
- Battery charging through both
ICE traction and regenerative
braking.
- Larger generator and electric
motor required,
- Higher energy loss because
energy is converted twice –
mechanical to electric then
electric to mechanical,
- Full size ICE is required if
battery is small.
Parallel HEV
- ICE and electric motor can
provide traction power
separately,
- Regenerative braking,
- Smaller dimension of both ICE
and electric motor.
- More complex architecture
with respect to series HEV,
- More complex control strategy
- Higher cost.
Series – Parallel
HEV
- Has all the advantages of both
series and parallel architectures.
- Very complex architecture and
control strategy,
- Higher cost.
Table 1.1- Advantages and disadvantages of different HEV architectures.
1.1.2 Based on hybridization ratio
Another important parameter to measure the effectiveness of HEVs is the hybridization
ratio, which is the percentage of power provided by the electric motor. Although most
of the HEVs are based on their architectures, significant difference in performance and
operation can be experienced depending on the hybridization ratio. Pure ICE and series
hybrid vehicles have a hybridization ratio of 100%, while for parallel and series-parallel
hybrid vehicles it can vary. For example if a parallel hybrid vehicle has an electric
motor rated at 50 kW and an ICE rated at 75kW then the hybridization ratio of the
vehicle is 50/(50+75) = 40%. A possible classification of today’s vehicles can be given
based on the ICE size and the electric motor size [6] –

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 Conventional ICE vehicles,
 Micro hybrids,
 Mild hybrids,
 Full hybrids,
 Plug – in hybrids,
 Electric vehicles.
1.2 Through The Road (TTR) HEV
Through The Road (TTR) is a special type of parallel hybrid for 4WD vehicle, in which
there is no mechanical connection between the ICE and the Electric motor. The ICE
provides power to one axle and the electric motor to the other axle. Due to the coupling
of power through the vehicle itself, its wheels and the road on which it moves - rather
than through some mechanical device - the vehicle is referred to as a 'through-the-road'
(TTR) hybrid.
Figure 1.4 - Classification of vehicles based on the size of ICE and electric motor

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The advantage of the TTR architecture with respect to the traditional parallel hybrid
architecture is that power is separately provided to the front and rear axle as there are no
mechanical coupling between the two prime movers, and the vehicle remains functional
in case of failure of one driveline. Moreover, very little modification to chassis is
required to implement the hybrid technology with the advantage of having better
acceleration and fuel economy with very little initial cost increase.
The main objective of this thesis is to retrofit a TTR HEV architecture in a conventional
ICE powered Fiat Panda. The motivation for having a hybrid drive-train is to reduce
fuel consumption and emissions of the vehicle. Apart from design and analysis of the
electric powertrains component sizing, the vehicle's control strategy is a critical factor in
ensuring that operational and control objectives of the hybrid vehicle are achieved.
In the following chapters we will talk about the mathematical model of the vehicle, the
power control strategy, dimensioning of the components of the electric drive-train, and
at the end the energy consumption in different conditions and possible improvements.
Figure 1.5- Through the Road (TTR) hybrid architecture

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2 Vehicle Model
The main components of the vehicle are - an ICE which provides torque to the front
axle through a clutch, gearbox and differential. When the vehicle is in traction the
electric motor provides torque to the rear axle through the differential, during braking
on the other hand, the electric motor through regenerative braking provides electric
energy to the battery and charges it. The single battery module provides electric power
to both the electric motor and to all the accessories (water pump, air conditioning,
lighting, info-tainment system etc.), which are all electric.
As there are no accessories connected to the ICE, there is less power loss and less fuel
consumption. The electric motor being able to provide torque during traction reduces
the fuel consumption and the regenerative braking generates power which otherwise
would be lost in the hydraulic brakes.
The vehicle function and modelling has been completely done on the Matlab and
Simulink framework. The Simulink model consists of 4 blocks – the driver, the control
strategy, the driveline and vehicle dynamics. All the characteristics, functionalities and
components of each block are described in detail in the following chapters. The control
strategy will be explained in detail in a separate chapter.
Figure 2.1 - Components of the vehicle model

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2.1 Driver
The driver is the block contains the “throttle” and the “brake” pedal, and the “clutch”. It
takes as input the actual and the reference velocity of the vehicle and the rotational
speed of the wheels. It tries to artificially simulate the behavior of the driver and as
output provides the throttle or brake requested by the driver, the gear number and the
clutch condition.
2.1.1 Throttle and brake pedal control
The pedal control block takes the reference and the actual velocity of the vehicle as
input and a PI controller, based on the difference between the velocities calculates the
throttle or brake signal. The pedal control has 3 different scenarios –
 Vehicle accelerating or moving at constant velocity = error > 0 = throttle signal
> 0,
 Vehicle braking = error < 0 = brake signal > 0 ,
 Vehicle totally static = error = 0 = both throttle and brake signal = 0.
The aim of the PI controller is to reduce the error between the actual and reference
velocity of the vehicle by reducing the integral of the error to near zero, which
physically represents the difference between the actual distance covered by the vehicle
and the distance that it should have covered. As a result the vehicle always covers
exactly the distance imposed by the reference cycle. When the integral of the error is
positive it means that the vehicle is not following the reference velocity and increases
the throttle to accelerate the vehicle, in the same way also it increase the brake signal
when the error is negative.
Figure 2.2 - Acceleration and brake signal generation

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2.1.2 Clutch control and gear selection
The gear choice and clutch selection block takes as input the rotational speed of the
wheels and based on that selects the engaged gear and the when to open and close the
clutch.
The gear is selected on the basis of maximum rotational speed on each gear. When the
wheels reach the maximum rotational speed the clutch opens and the vehicle shifts to
the next gear, the maximum rotational speed and gear ratio of each gear is pre-selected
[appendix - 1]. The model, independent of the driving mode (full electric or full ICE or
hybrid), continuously calculates the gear that the vehicle should work on. This way
when the vehicle shifts from full electric to hybrid mode, it already knows which gear to
engage.
The clutch command block receives the gear information and controls the opening and
closing of the clutch. The clutch command, in addition also depends on the driving
mode, which will be explained in detail in the Control Strategy chapter. The clutch tau
and clutch tau dot are the parameters without any physical meaning but helps the model
to synchronize the speed of the ICE while shifting gear. When the clutch opens to allow
gear shift, the ICE rotational acceleration rises to very high value as it doesn’t feel any
constraint. To avoid this from happening the ωice is calculated by multiplying the
ωprimary to the clutch tau, that synchronizes the ωice between consecutive gears. The
clutch tau dot does the same for the rotational acceleration of the ICE.
Figure 2.3- Engaged gear selection
Figure 2.4 - Clutch control

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2.2 Drive – train
A very simplified model of the driveline is used which takes into account all the
components needed for the power transmission from the prime movers through the
differential and gearbox (for front axle only) to the wheels. The main components of the
driveline are -
 Front axle
 ICE
 Clutch
 Gearbox
 Rear axle
 Battery
 Electric motor
 Wheels
 Front wheels
 Rear wheels
 Brakes
The driver provides the acceleration (throttle) or brake signal and based on that
information, the control strategy decides the quantity of traction torque/ power to be
provided to each axle by the prime movers (ICE and electric motor), and the brake
torque by the hydraulic brakes or regenerative braking. The driveline in addition also
takes as input the clutch command and gear number from the driver, and linear
acceleration (ax) of the vehicle, wheel rotational speed (ω 𝑤ℎ𝑒𝑒𝑙𝑠) and acceleration
(ω̇ 𝑤ℎ𝑒𝑒𝑙𝑠) from vehicle dynamics.
In output the driveline block provides the total force to wheels, which is used in vehicle
dynamics to calculate the actual velocity and acceleration of the vehicle.
In the front axle the torque is provided by the ICE based on the control strategy and
throttle valve opening, to the gearbox and is increased by the engaged gears
transmission ratio, which is transmitted to the wheels through the front differential.
Similarly in the rear axle the electric power is provided by the battery to the electric
motor based on the control strategy, which is transmitted to the rear wheels through the
rear differential. The vehicle is supposed to be driving in a straight line and both the

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front and rear differential have equal and constant transmission ratio. The total torque
transmitted to the wheels is the sum of the traction and brake torque. In traction the
brake torque is 0, and while braking, the traction torque is 0.
2.2.1 Front axle
The front axle contains 3 blocks - one for the ICE, one for gearbox and one for the
clutch. The front axle block calculates the front wheel torque based on the – clutch
command, accelerator throttle percentage, gear number, and rotational velocity and
acceleration of the wheels.
The torque provided by the ICE depends on the ICE throttle percentage, which is
provided by the control strategy. The torque available at the front wheels is the torque
of the ICE multiplied by the transmission ratio of the engaged gear (provided by the
driver block), and the differential.
𝑇𝑓𝑟𝑜𝑛𝑡 𝑤ℎ𝑒𝑒𝑙𝑠 = 𝑇𝑖𝑐𝑒 × 𝜏 𝑔𝑒𝑎𝑟 × 𝜂 𝑔𝑒𝑎𝑟𝑏𝑜𝑥 × 𝜏 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙 × 𝜂 𝑑𝑖𝑓𝑓
Below the functionalities of each component of the front axle is explained in detail.
Figure 2.5- Torque flow in the front axle

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2.2.1.1 ICE (Internal combustion engine)
The ICE block calculates the total torque provided by the combustion engine and the
total amount of fuel consumed. The inputs of the ICE are the ICE throttle percentage
and the rotational speed of the wheels and the outputs are the total torque provided by it
to the front axle and the fuel consumption.
2.2.1.1.1 ICE torque calculation
The ICE is the prime mover of the front axle and only provides power and torque to the
front wheels when it is requested by the control strategy. The torque available from the
ICE is based on the speed - torque map of the engine used and throttle percentage. The
ICE map provides the maximum toque available at a given rotational speed which
multiplied by the throttle percentage provides the toque of the ICE. The total output
toque of the engine is then the summation of the engine torque, inertial torque and the
engine braking torque. The engine braking torque only applies during braking. When
the throttle is near zero the air intake valve is almost close so the air available is the
combustion chamber is very low which creates a vacuum inside the chamber that the
cylinders have to work against. This provides some additional brake torque in addition
to the hydraulic brakes and regenerative braking. The inertial torque on the other hand is
due to the inertia of the engine components that the ICE has to overcome while it is
providing driving power.
ωice = ω 𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏 𝑔𝑒𝑎𝑟 × 𝜏 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙
Tice_output = (Tmax 𝑖𝑐𝑒 × 𝑡ℎ𝑟𝑜𝑡𝑡𝑙𝑒 %) − 𝑇𝑒𝑛𝑔𝑖𝑛𝑒 𝑖𝑛𝑒𝑟𝑡𝑖𝑎 − 𝑇𝑒𝑛𝑔𝑖𝑛𝑒 𝑏𝑟𝑎𝑘𝑖𝑛𝑔
Tengine inertia = ω̇ wheels × 𝐽𝑒𝑛𝑔𝑖𝑛𝑒
Figure 2.6- Components of the ICE block

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The engine used is a fiat naturally aspirated 0.9L twin air engine with a maximum
torque of 88 Nm at 4000 rpm. The speed-torque map of the engine is given below –
2.2.1.1.2 ICE Fuel consumption
The specific fuel consumption (q [g/CVh]) of the ICE is a function of the engine mean
effective pressure (pme[bar]) and the rotational speed (ω [rpm]). For every value of
engine speed and corresponding torque the fuel consumption map provides the value of
the specific fuel consumption. The specific fuel consumption then converted to [kg/J]
and multiplied with the engine power provides the fuel consumption of the ICE.
pme =
𝑇𝑖𝑐𝑒×𝑖 × 𝜋×2
𝑉×10
[bar]
Fuel consumption = ∫ 𝑞 × 735.5 × 3.6 × 106
× 𝑃𝑖𝑐𝑒 [𝑘𝑔]
In the calculation of the fuel consumption the Stop&Start technology is also
taken into account. The Stop&Start activates only when the ICE rotational speed is
below the minimum value that is when the ICE is idle. Moreover it is activated by the
vehicle controller only after 200s from the beginning of the driving cycle, a time
considered sufficient to warm up the engine and the after treatment devices. The
condition for the activation of the Stop&Start is then-
Figure 2.7 - ICE speed-torque map

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ωice ≤ ωice minimum &
Vehicle running for a time > 200s
A controller checks if the ICE is at idle condition or not and calculates the fuel
consumption based on that. At idle condition the ICE consumes a constant amount of
fuel otherwise the fuel consumption is calculated based on the previous equations. The
total fuel consumption is then-
Total fuel consumption = Fuel consumtionidle + Fuel consumptionmoving vehicle
2.2.1.2 Clutch
The clutch is the connection between the ICE and the gearbox. The inputs for the clutch
are the clutch command from the driver, the output torque of the ICE, the rotational
speed and acceleration in the primary. The driver provides the clutch command which
contains the information about clutch position, clutch tau and clutch tau dot. Based on
the clutch position (open or closed), the clutch transmits the Tice output to the gearbox.
𝐶𝑙𝑢𝑡𝑐ℎ 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 = 1 → 𝐶𝑙𝑢𝑡𝑐ℎ 𝑐𝑙𝑜𝑠𝑒 → 𝑇𝑝𝑟𝑖𝑚𝑎𝑟𝑦 = 𝑇𝑖𝑐𝑒 𝑜𝑢𝑡𝑝𝑢𝑡
𝐶𝑙𝑢𝑡𝑐ℎ 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 = 0 → 𝐶𝑙𝑢𝑡𝑐ℎ 𝑜𝑝𝑒𝑛 → 𝑇𝑝𝑟𝑖𝑚𝑎𝑟𝑦 = 0
Figure 2.8 - Fuel consumption map of the ICE

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The clutch also provides the rotational speed and acceleration of the ICE. It uses the
clutch tau and clutch tau dot signals to synchronize the rotational speed and acceleration
of the ICE with that of the primary.
ωICE = ω 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 × 𝑐𝑙𝑢𝑡𝑐ℎ 𝑡𝑎𝑢
ω̇ ICE = ω̇ 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 × 𝑐𝑙𝑢𝑡𝑐ℎ 𝑡𝑎𝑢 𝑑𝑜𝑡
2.2.1.3 Gearbox
The gearbox used is a 5 speed automatic transmission gearbox and the gear engaged is
provided by the driver. The gearbox receives the Tprimary, multiplies it to the
transmission ratio of the engaged gear and the efficiency of the gearbox and transmits
the torque to the differential. It also takes as input the rotational speed and acceleration
of the wheels and provides the rotational speed and acceleration in the primary.
ωprimary = ω 𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏 𝑔𝑒𝑎𝑟 × 𝜏 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙
ω̇ primary = ω̇ 𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏 𝑔𝑒𝑎𝑟 × 𝜏 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙
Tgearbox out = (T𝑝𝑟𝑖𝑚𝑎𝑟𝑦 × 𝜏 𝑔𝑒𝑎𝑟 × 𝜂 𝑔𝑒𝑎𝑟𝑏𝑜𝑥
) + 𝑇 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛
The efficiency of the gearbox is calculated for each gear and the output torque of the
gearbox depends on the efficiency of the engaged gear. The friction torque map
provides the resistive torque due to the contact between the gears, which is negative
while driving and positive while braking. The efficiency map provides the gearbox
efficiency for different temperature and rotational speed.
And finally the traction torque available to the front axle is the gearbox output torque
multiplied by the constant transmission ratio and the efficiency of the differential.
𝑇𝑓𝑟𝑜𝑛𝑡 𝑎𝑥𝑙𝑒 = 𝑇𝑔𝑒𝑎𝑟𝑏𝑜𝑥 𝑜𝑢𝑡 × 𝜏 𝑑𝑖𝑓𝑓 × 𝜂 𝑑𝑖𝑓𝑓

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2.2.2 Rear axle
The rear axle is powered by an electric motor and a battery through a fix transmission
ratio differential. The motor absorbs energy from the battery and transmits torque to the
rear axle during traction and assists during braking by regenerating brake energy and
storing it in the battery. So the working conditions of the rear axle are-
 Full electric traction – Consumes electric energy from the battery,
 Hybrid traction – Consumes electric energy from the battery,
 Regenerative braking – Charges the battery through regenerative braking.
Figure 2.9- Friction torque vs rotational speed for
different gear engaged
Figure 2.10- Gearbox efficiency at 1st gear vs speed at
different temperature
Figure 2.11 - Torque flow in the rear axle

27
The control strategy provides the activation and deactivation signal to the rear axle. The
vehicle is considered to be moving on a straight line thus the transmission ratio and the
efficiency of the rear differential is considered constant and equal to that of the front
axle. The torque available in the rear axle is then -
𝑇𝑟𝑒𝑎𝑟 𝑎𝑥𝑙𝑒 = 𝑇𝑒𝑚 × 𝜏 𝑒𝑚 × 𝜂 𝑒𝑚 × 𝜏 𝑑𝑖𝑓𝑓 × 𝜂 𝑑𝑖𝑓𝑓
2.2.2.1 Electric motor
The electric motor used is a permanent magnet 3 phase brushless motor with rated
power of 10 kW and peak power of 30 kW and rated torque 30 Nm and maximum
torque of 90 Nm. The permanent magnet motor is used due to its high energy density
and to avoid the presence of exciting circuits. The rated and peak power and torque of
the electric motor is selected based on the analysis done over different driving cycles,
such as NEDC, WLTP, ECE etc. such that the vehicle can complete the cycle only
using electric traction without taking into account the battery capacity.
A very simple vehicle model, considering the vehicle as a rear wheel drive pure electric
vehicle, is used to analyze the power characteristics of the motor. The power required
for the vehicle to follow the driving cycle can be directly calculated from the velocity
profile V(t) of the cycle. The vehicle data used is identical throughout the full thesis and
is given is the appendix. The efficiency of the powertrain is considered to be constant
and 𝜂 = 0.8, during traction and
1
𝜂
during regenerative braking. The transmission ratio
of the rear differential is also considered to be constant ( 𝜏 𝑑𝑖𝑓𝑓 = 4.5). The power
needed from the electric motor is then –
𝑃 𝑚𝑜𝑡𝑜𝑟 = 𝑉(𝑡) × 𝐹𝑥
𝐹𝑥 = 𝑀𝑎 𝑥(𝑡) + 0.5𝜌𝐴𝐶 𝑥 𝑉(𝑡)2 (𝑎𝑒𝑟𝑜𝑑𝑦𝑛𝑎𝑚𝑖𝑐 𝑓𝑜𝑟𝑐𝑒) + 𝑀𝑔𝐶0 (𝑟𝑜𝑙𝑙𝑖𝑛𝑔 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒)
The power vs velocity curve for NEDC and WLTP cycle in the figure below shows that
the power required in most of the operation points around 10 kW, in particular the
regions for which the electric motor operation is chosen (urban driving condition where
V < 50 km/h). The rated power, which the electric motor can provide continuously is
then chosen to be 10 kW. The peak power chosen is 30 kW and available only in
transient conditions for a very short period.

28
The other most important parameter for the electric machine is the maximum torque
available from it. The torque from the electric motor is transmitted through a fixed
transmission ratio speed reducer and differential. The transmission ratio between the
motor and the wheels is a very important parameter, because for a small transmission
ratio the torque required from the motor becomes very high and for higher transmission
ratio the torque required is lower but the has an effect on the rotational speed of the
motor.
Figure 2.12 - Power required (WLTP left, NEDC right) to follow the driving cycle
Figure 2.13- Torque and velocity of the electric motor for different
transmission ratio in NEDC cycle.

29
We can see from the figure above that increasing the transmission ratio decreases the
torque required from the electric motor and increases the number of working points
regarding the rotational speed. The transmission ratio of the electric motor is the chosen
to be 2.5. The maximum motor torque has been chosen based on the same analysis as
that for the cycle, with transmission ratio of 4.5 for the differential and 2.5 for the
motor.
As we can see from the figure above the torque required by the vehicle to follow the
cycle both in traction and braking is maximum around 70 Nm. The maximum torque of
the electric motor is then chosen to be 90 Nm.
2.2.2.1.1 Electric motor torque
The control strategy controls the torque provided by the electric motor both in traction
and braking. The torque – velocity map of the electric motor provides the maximum
torque available from the motor for every speed. A 3-d map of the motor provides the
direct current absorbed by the motor for each value of torque and velocity. The map of
the electric motor already takes into account the efficiency of the motor. The control
strategy provides the information about the torque requested from the electric motor and
using the motor maps and EM throttle the motor provides the torque.
ωem = ω 𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏 𝑒𝑚 × 𝜏 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙
Tem = 𝑇𝑒𝑚 𝑚𝑎𝑥 × 𝑡ℎ𝑟𝑜𝑡𝑡𝑙𝑒 % 𝑒𝑚
Figure 2.14 - Torque required from the electric motor for WLTP (left) and NEDC (right) cycle

30
2.2.2.1.2 Electric motor temperature
One of the most important parameter of the electric motor is the temperature as the
efficiency of the motor decreases at temperatures out of the optimum operating zone
temperature. A very simple mono dimensional thermal model is used to calculate the
temperature of the motor. The dissipated power (𝑃𝑑), which is the difference between
the electric power consumed and the mechanical power provided by the electric motor,
is what generates the heat inside the motor. And the equation of the temperature rise is-
Tempem = 𝑃𝑑 𝑅𝑡ℎ𝑚 (1 − 𝑒
−
𝑡
𝜏 𝑡𝑚) + 𝑇𝑒𝑚𝑝 𝑒𝑛𝑣
Where 𝑅𝑡ℎ𝑚 is the thermal resistance of the
heat exchange between the motor components
and the environment. The thermal time constant is
𝜏 𝑡𝑚 and 𝑇𝑒𝑚𝑝 𝑒𝑛𝑣 is the environment temperature which is considered constant. The
power dissipated is due to the joule effect inside the motor and the phase current and
phase resistance is responsible for this.
𝑃𝑑 = 3𝑅 𝑝ℎ𝑎𝑠𝑒 𝑖 𝑝ℎ𝑎𝑠𝑒
2
The phase current absorbed is provided by a 3-D map of the electric motor which takes
provides the instantaneous phase current for each velocity and torque.
Figure 2.15 - Torque - Velocity curve of the electric motor
Figure 2.16 - Transient temperature response

31
2.2.2.2 Battery
The modelling of the selection procedure of the battery will be described in detail in the
following chapters. In this section the battery model will be explained very briefly just
to understand how it provides electricity to the motor and temperature model of the
battery.
2.2.2.2.1 Battery current
The battery provides energy to the motor through an AC/DC converter which is not
considered in the model, and we suppose that the battery directly provides the DC
current to the motor. A 3-D map calculates the instantaneous DC current requested by
the motor at each torque and velocity, and the battery is discharged during traction and
charged during regenerative braking.
Figure 2.17 - Phase current vs torque and velocity map of the electric
motor
Figure 2.18 - Battery model

32
𝑇𝑒𝑚 > 0 → 𝑇𝑟𝑎𝑐𝑡𝑖𝑜𝑛 → 𝑖 𝑑𝑐 > 0 → 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑖𝑛𝑔
𝑇𝑒𝑚 < 0 → 𝐵𝑟𝑎𝑘𝑖𝑛𝑔 → 𝑖 𝑑𝑐 < 0 → 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑐ℎ𝑎𝑟𝑔𝑖𝑛𝑔
2.2.2.2.2 Battery temperature
For the battery temperature the same mono dimensional model of the motor has been
used. The only difference is the power dissipated is calculated based on the DC current
and battery internal resistance instead of phase current and phase resistance.
𝑃𝑑 = 𝑅 𝑏𝑎𝑡𝑡𝑒𝑟𝑦 𝑖 𝑑𝑐
2
Tempbattery = 𝑃𝑑 𝑅𝑡ℎ𝑏 (1 − 𝑒
−
𝑡
𝜏 𝑡𝑏) + 𝑇𝑒𝑚𝑝𝑒𝑛𝑣
2.2.3 Brakes
The hydraulic brakes only come into play when the brake torque required is not enough
to provide sufficient braking torque. The activation and deactivation signal for the
hydraulic brakes are provided by the control strategy. The brake pressure ratio between
the front and the rear axle is considered to be constant with 70% brake pressure
provided by the front brakes and 30% by the rear brakes. The driver provides the brake
Figure 2.19 - Direct current vs torque and velocity map of the
electric motor

33
signal and the control strategy checks if the brake energy provided by the regenerative
braking is sufficient to follow the velocity profile. The regenerative braking is provided
only by the rear axle as the front axle is totally separated from the electric motor. The
brake signal is then multiplied by the pressure – torque coefficient Kp to calculate the
total hydraulic brake torque.
Thydraulic brakes = 𝑏𝑟𝑎𝑘𝑒%ℎ𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑏𝑟𝑎𝑘𝑒𝑠 × 𝐾 𝑃 × 100
Tfront brakes = 0.7 × 𝑇ℎ𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑏𝑟𝑎𝑘𝑒𝑠
Tfront brakes = 0.3 × 𝑇ℎ𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑏𝑟𝑎𝑘𝑒𝑠
2.2.4 Wheels
The torque available to the front and rear wheels is provided separately by the ICE and
the electric motor based on the information provided by the control strategy. 𝑇𝑓𝑟𝑜𝑛𝑡 𝑎𝑥𝑙𝑒
and 𝑇𝑟𝑒𝑎𝑟 𝑎𝑥𝑙𝑒 are provided by the ICE and the electric motor respectively. 𝑇𝑟𝑒𝑎𝑟 𝑎𝑥𝑙𝑒 is
positive traction and negative in braking while 𝑇𝑓𝑟𝑜𝑛𝑡 𝑎𝑥𝑙𝑒 is always positive. The brake
torque for both front and rear axle is provide by the hydraulic brakes and are zero
during traction. The wheel inertia provides a resistive torque both in traction and
braking.
Tfront wheels = 𝑇𝑓𝑟𝑜𝑛𝑡 𝑎𝑥𝑙𝑒 − 𝑇𝑓𝑟𝑜𝑛𝑡 𝑏𝑟𝑎𝑘𝑒𝑠 − 𝑇 𝑤ℎ𝑒𝑒𝑙 𝑖𝑛𝑒𝑟𝑡𝑖𝑎
Trear wheels = 𝑇𝑟𝑒𝑎𝑟 𝑎𝑥𝑙𝑒 − 𝑇𝑟𝑒𝑎𝑟 𝑏𝑟𝑎𝑘𝑒𝑠 − 𝑇 𝑤ℎ𝑒𝑒𝑙𝑠 𝑖𝑛𝑒𝑟𝑡𝑖𝑎
Twheels inertia = 𝐽 𝑤ℎ𝑒𝑒𝑙𝑠 × ω̇ 𝑤ℎ𝑒𝑒𝑙𝑠

34
2.3 Vehicle dynamics
To calculate the energy consumption of the vehicle, the handling and comfort
phenomenon of the vehicle is not very influential and is not considered in this thesis.
The vehicle is considered to be moving in a straight line on a planar surface neglecting
the pitch and heave motion of the vehicle. The equations governing the vehicle
dynamics are then –
Fx front wheels =
𝑇𝑓𝑟𝑜𝑛𝑡 𝑤ℎ𝑒𝑒𝑙𝑠
𝑅 𝑤ℎ𝑒𝑒𝑙𝑠
Fx rear wheels =
𝑇𝑟𝑒𝑎𝑟 𝑤ℎ𝑒𝑒𝑙𝑠
Where 𝑅 𝑤ℎ𝑒𝑒𝑙𝑠 is the radius of the wheel. The total longitudinal force acting on the
vehicle is then –
Fx total = 𝐹𝑥 𝑓𝑟𝑜𝑛𝑡 𝑤ℎ𝑒𝑒𝑙𝑠 + 𝐹𝑥 𝑟𝑒𝑎𝑟 𝑤ℎ𝑒𝑒𝑙𝑠 − 𝐹𝑟𝑒𝑠𝑖𝑠𝑡𝑖𝑣𝑒
The resistive force working on the vehicle is the summation of the aerodynamic force
and the rolling resistance at the tire ground contact patch.
Faerodynamic = 0.5𝜌𝐴𝐶 𝑥 𝑉(𝑡)2
Frolling resistace = 𝑀𝑔𝐶0
Where 𝜌 is the air density, 𝐶 𝑥 is the aerodynamics coefficient, A is the front area of the
vehicle, and 𝐶0 is the rolling resistance coefficient. The total resistive force can also be
well approximated by a parabolic equation of the vehicle velocity and can be written as
follows –
𝐹𝑟𝑒𝑠𝑖𝑠𝑡𝑖𝑣𝑒 = 𝑓0 + 𝑓1 𝑉𝑡 + 𝑓2 𝑉𝑡
2
Where 𝑓0, 𝑓1, 𝑓2 are referred to as cost down coefficients and are evaluated by means of
standard procedure. The instantaneous actual velocity and acceleration of the vehicle
and the wheels can now easily be calculated as –
𝑎 𝑥 𝑎𝑐𝑡𝑢𝑎𝑙 =
𝐹𝑥 𝑡𝑜𝑡𝑎𝑙
𝑀

35
𝑉𝑥 𝑎𝑐𝑡𝑢𝑎𝑙 = ∫ 𝑎 𝑥 𝑎𝑐𝑡𝑢𝑎𝑙
𝑡
0
ω 𝑤ℎ𝑒𝑒𝑙𝑠 =
𝑉 𝑥 𝑎𝑐𝑡𝑢𝑎𝑙
ω̇ 𝑤ℎ𝑒𝑒𝑙𝑠 =
𝑎 𝑥 𝑎𝑐𝑡𝑢𝑎𝑙
The actual velocity of the vehicle is then used in by the driver to calculate the error
between the reference and actual velocity of the vehicle. It is also used in the calculation
of resistive forces acting on the vehicle. The rotational velocity and acceleration of the
wheels is used by the clutch and gearbox and electric motor to determine the gear
selected and the torque provided by the ICE and the electric motor. Thus the model is
recursive and whenever a signal is used as a feedback a memory block with a unitary
step delay is used. The memory block provides as feedback the values at an instant
before the present value of the variable, such as to calculate the resistive forces on the
vehicle the velocity is used as feedback and a memory block provides 𝑉𝑥 (𝑡 − 1) instead
of 𝑉𝑥 (𝑡) for the correct calculation of the resistive forces.

36
3 Energy storage system (ESS)
The biggest caveat in the hybrid and electric vehicle technology today is the energy
storage system (ESS). The efficiency and all electric range of the HEVs depend on their
ESS, which is not only used to store large amount of energy but also should be able to
release it quickly according to the load demand [7]. The important characteristics of
vehicular ESSs include energy density, power density, lifetime, cost, and maintenance.
Currently, batteries and ultra-capacitors (UCs) are the most common options for
vehicular ESSs. Batteries usually have high energy densities and store the majority of
onboard electric energy. On the other hand, UCs have high power densities and present
a long life cycle with high efficiency and a fast response for charging/ discharging [8].
A fuel cell (FC) is another clean energy source; however, the long time constant of the
FC limits its performance on vehicles. The efficiency of the FC is dependent on the
amount of power drawn from it. Generally, the more power drawn, the lower the
efficiency. At present, no single energy-storage device could meet all requirements of
HEVs and electric vehicles (EVs). Batteries have widely been adopted in ground
vehicles due to their characteristics in terms of high energy density, compact size, and
reliability.
3.1 Battery technology
Batteries have a much lower energy density than fossil fuel and recharging a high
capacity battery requires much more time that just filling up the fuel tank. Different
battery technologies have been under development since the early 1900s. Batteries are
mainly characterized by their life cycle, energy and power density and energy
efficiency.
 The life cycle represents the number of charging and discharging cycles possible
before it loses its ability to hold a useful charge (typically when the available
capacity drops under 80% of the initial capacity). Life cycle typically depends
on the depth of discharge (DOD).
 When charging and discharging a battery not all energy, delivered to the battery,
will be available due to battery losses, which are characterized by the battery
efficiency.

37
 The speciﬁc energy and power describe, respectively, the energy content
(determining the vehicle range) and the maximum power (determining the
vehicle acceleration performance) in function of the weight of the battery.
A battery can be optimized to have a high energy content or it can be optimized to have
a high power capability. The ﬁrst optimization is important for battery electric vehicles,
while the second is mainly required for hybrid drive trains. The most common battery
technologies developed for the HEV and EV application are –
 Lead – acid batteries are available in production volumes today, yielding a
comparatively low-cost power source. In addition lead–acid battery technology
is a mature technique due to its wide use over the past 50 years [9]. They have a
limited cycle when operated at a higher DOD rate and the energy and power
density of the battery is low due to the weight of lead collectors [10].
 Nickel – Metal hydride (NiMH) have a twice the energy density of that of the
lead-acid batteries, and they are recyclable and harmless to the environment
[11]. They can operate at high voltages and wider temperature range. But the
memory effect in NiMH batteries reduces their usable power [12].
 Lithium – ion (Li – ion) batteries have high energy density, good high
temperature performance, longer cycle life, negligible memory effect and are
recyclable. The cost of the li-ion batteries is higher than NiMH batteries but the
fast development of the li-ion batteries in the past decade has brought the cost
down.
 Lithium – polymer batteries are similar to li-ion batteries in terms of energy
and power density, and have lower cost. However they have a much lower cycle
life in comparison to the li-ion batteries.
A comparative table of different battery technologies is given in the table below-
Battery type Specific energy
[Wh/kg]
Cycle life Efficiency Cost
[Wh/$US]
Lead – Acid 33-42 500-800 50-95% 0.25
NiMh 50-90 1000-1200 60-75% 1.0
Li-ion 150-200 1000-1500 80-90% 1.25
Li-polymer 120-180 300-500 80-90% 1.20
Table 3.1 - Comparision of different battery types

38
3.2 Battery modelling
Finally a li-ion battery technology selected as the energy storage element. The important
parameters in the in the modelling of a battery are briefly described below –
 Nominal voltage (𝑽 𝟎) – is the open circuit voltage of the battery in the fully
charged condition. The nominal voltage should be sufficient to fulfill the power
requirement of the motor –
P = 𝑉0 𝑖
And to keep the power dissipated by the joule effect to a sufficiently low level.
PD = 𝑅𝑖2
Where 𝑖 is the current supplied by the battery and R is the equivalent resistance.
 Nominal capacity (𝐐 𝐧𝐨𝐦 [𝑨𝒉]) – is the maximum amount of charge that can
be delivered by the battery when it is fully charged. The total energy capacity
of the battery depends on the nominal capacity and the nominal voltage of the
battery.
𝐸 𝑚𝑎𝑥 = Qnom × 𝑉0
 State of charge (SOC) – is the ratio of the charge available in the battery at
any instant and nominal capacity of the battery. The amount of energy
available from the battery depends on the SOC.
𝑆𝑂𝐶 =
Q
𝑄nom
=
E
𝐸max
 Charging and discharging rate (C-rate) – provides the amount of continuous
current that the battery can provide or consume during discharging and
charging for a certain amount of time. A 5Ah battery rated at 1C means that it
can provide 5A current for an hour. It is a very important factor for a HEV and
EVs because of the load variation required by the electric motor in different
driving condition.
𝑖 𝑚𝑎𝑥 = Q 𝑛𝑜𝑚 × 𝐶

39
3.2.1 Charge and discharge model
The battery model used is a slightly modified version of the Shepherd model but can
accurately represent the voltage dynamic when a variable current is applied and takes
into account the open circuit voltage (OCV) as a function of the SOC. The battery
voltage is the obtained by using the equation [13] [14] [15] –
𝑉𝑏𝑎𝑡𝑡 = 𝐸0 − 𝑅𝑖 − 𝐾
𝑄 𝑛𝑜𝑚
𝑄 𝑛𝑜𝑚−𝑖𝑡
𝑖𝑡 + 𝐴𝑒−𝐵×𝑖𝑡
Where –
𝐸0 = battery constant voltage [V]
𝐾 = polarization constant [V/Ah]
A = exponential zone constant [V]
B = exponential zone time constant [1/Ah]
R = battery internal resistance [Ω]
I = battery current [A] (positive while discharging, negative while charging)
Now, these parameters can be easily extracted from the battery discharge curve
provided by the manufacturer.
Figure 3.1 - Typical discharge curve of Li-ion battery [15]

40
A typical discharge of li-ion batteries can be divided into three zones –
 Exponential area – represents the exponential potential drop at the beginning of
the discharge,
 Nominal area – represents the steady state and the amount of charge that can be
extracted from the cell before it reaches the nominal voltage,
 Total discharge – start at the end of the nominal are and represents the total
discharge of the cell when the potential of the cell drops rapidly to the cut-off
voltage.
The discharge curve which provides the values of 𝑉𝑓𝑢𝑙𝑙, 𝑉𝑒 𝑥𝑝, 𝑉𝑛 𝑜𝑚, 𝑄 𝑒𝑥𝑝, and 𝑄 𝑛𝑜𝑚.
The values of the parameters can now easily be extracted by using the following
equations [15] –
At fully charged state –
𝑉𝑓𝑢𝑙𝑙 = 𝐸0 − 𝑅𝑖 + 𝐴
At the end of the exponential zone –
𝐵 = −
3
𝑄exp
𝑉𝑒𝑥𝑝 = 𝐸0 − 𝑅𝑖 − 𝐾
𝑄
𝑄 − 𝑄 𝑒𝑥𝑝
𝑄 𝑒𝑥𝑝 + 𝐴𝑒−𝐵×𝑖𝑡
And finally the SOC of the battery can be calculated as –
𝑆𝑂𝐶 = 100 (1 −
𝑖 𝑑𝑐
𝑄 𝑛𝑜𝑚
) %
The battery model used in this thesis is based on some assumption to simplify the
battery behavior –
 The parameters extracted from the discharge curve are supposed to be the same
during charging and discharging,
 The internal resistance of the battery is constant during charging and discharging
and is independent of the amplitude of the current,
 Negligible self – discharge and memory effect (true for li-ion batteries),
 The temperature does not affect the model behavior,

41
 The battery capacity does not change with the amplitude of current that is the
model behaves the same way for different C-rate.

42
4 Control strategy
The main goal of the control strategy is to split the power/torque, required by the
vehicle, between the ICE and the electric motor, in order to minimize the fuel
consumption and maximize the efficiency of the vehicle in urban driving cycles. The
main controlled parameters are –
 𝑇𝑒𝑚 = the amount of torque provided by the electric motor in traction,
 𝑇𝑒𝑚 𝑏𝑟𝑎𝑘𝑒 = amount of brake torque provided by the electric motor during
braking,
 𝑇𝑖𝑐𝑒 = amount of torque provided by the ICE,
 𝑇ℎ𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑏𝑟𝑎𝑘𝑒 = amount of brake torque provided by the hydraulic brakes.
 Clutch position.
The controlled parameters depend on –
 𝑇𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑣𝑒ℎ𝑖𝑐𝑙𝑒 = total traction torque required by the vehicle,
 𝑇𝑏𝑟𝑎𝑘𝑒 𝑣𝑒ℎ𝑖𝑐𝑙𝑒 = total brake torque required by the vehicle,
 𝑆𝑂𝐶 𝑏𝑎𝑡𝑡𝑒𝑟𝑦 = state of charge of the battery,
 𝐼 𝑚𝑎𝑥 = maximum current provided by the battery,
 𝑇𝑒𝑚𝑝 𝑏𝑎𝑡𝑡𝑒𝑟𝑦, 𝑇𝑒𝑚𝑝 𝑚𝑜𝑡𝑜𝑟 = temperature of the battery and electric motor,
The inputs of the control strategy is then, the throttle and brake pedal signal from the
driver which provides the amount of traction and brake torque required by the vehicle.
The battery model provides the instantaneous SOC and the temperature of the battery.
The electric motor model provides the current required by the motor and the
temperature.
Figure 4.1- Control strategy

43
A flowchart of the control is given below –
The maximum torque available from the electric motor both in traction and braking is
then compared to the traction and brake torque required by the vehicle.
 In traction if the maximum electric motor torque is higher than the torque
required by the vehicle, the vehicle works as a pure electric vehicle and the ICE
is turned off. And when it is lower the vehicle works as a hybrid vehicle using
energy from both the electric motor and ICE.
 In braking if the electric motor torque is enough to provide the requested brake
power than the hydraulic brakes are not used and brake energy is regenerated.
Otherwise both the hydraulic brakes and regenerative braking is used together.
4.1 Traction and brake torque calculation
The control strategy checks the throttle and brake signal provided by the driver. If the
throttle signal is positive then the traction torque required by the vehicle is –
𝑇𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑣𝑒ℎ𝑖𝑐𝑙𝑒 = 𝑡ℎ𝑟𝑜𝑡𝑡𝑙𝑒% × (𝐹𝑥 𝑓𝑟𝑜𝑛𝑡 𝑤ℎ𝑒𝑒𝑙𝑠 + 𝐹𝑥 𝑟𝑒𝑎𝑟 𝑤ℎ𝑒𝑒𝑙𝑠) × 𝑅 𝑤ℎ𝑒𝑒𝑙𝑠
If the brake signal is positive then the brake torque required is –
𝑇𝑏𝑟𝑎𝑘𝑒 = 𝑏𝑟𝑎𝑘𝑒% × 𝐾𝑝
Where kp is the brake pressure coefficient.
Figure 4.2 - Control flow chart

44
The maximum torque available from the electric motor both in traction and braking is
constrained by various factors such as the SOC, temperature and current. Below the
calculation method of the maximum electric motor torque is explained.
4.2 Maximum electric motor torque calculation
A flow chart of the maximum motor torque calculation method is given below –
Figure 4.3 - Maximum motor torque calculation

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4.2.1 Current limitation
The maximum current provided by the battery depends on the capacity and the C-rate of
the battery, as we have seen in battery model chapter.
𝑖 𝑚𝑎𝑥 = Q 𝑛𝑜𝑚 × 𝐶
For example a battery with Q 𝑛𝑜𝑚 = 20 Ah and C5 rating can provide 100A current
continuously without any battery degradation. The C-rate of the battery is provided by
the battery manufacturer.
A 3-D map of the electric motor calculates the maximum torque that it can provide at
any rotation speed and current. The maximum value of current is selected based on the
battery used and for each rotational speed of the maximum torque (Tmotor) is calculated
by the map.
For example if the maximum current provided by battery is 200A, the map for every
value of rotational speed check how much torque the motor provide without consuming
a current more than 200A.
Figure 4.4 - Motor map that provides the maximum torque for each value of
current and velocity

46
4.2.2 SOC limitations
The battery model provides the instantaneous SOC of the battery. The maximum and
minimum SOC of the battery is selected to avoid overcharging and over-discharging the
battery which can be harmful for the battery pack. The control strategy then checks the
state of charge (SOC) of the battery. During traction if the SOC is lower than the
𝑆𝑂𝐶 𝑚𝑖𝑛 then the battery can not provide any current to the motor and the traction torque
is completely provided by the ICE. If the SOC is higher than the 𝑆𝑂𝐶 𝑚𝑎𝑥 than during
braking the battery can not consume current and the brake torque is completely
provided by the hydraulic brakes.
The Tmotor is then multiplied by the SOC multiplication factor, which is provided by a
two maps one for SOCmax and one for SOCmin.
𝑇𝑒𝑚 max 𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 = Tmotor × 𝑆𝑂𝐶min 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟
𝑇𝑒𝑚 max 𝑏𝑟𝑎𝑘𝑒 = Tmotor × 𝑆𝑂𝐶max 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟
In traction if the battery SOC is above 15% then the multiplication factor is 1 and
𝑇𝑒𝑚 max 𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 = 𝑇 𝑚𝑜𝑡𝑜𝑟, but and for SOC < 10% 𝑇𝑒𝑚 max 𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 = 0. When the SOC
is between 10 to 15% the control strategy starts to limit the torque as the multiplication
factor varies from 0 to 1. For example, if the SOC is 12.5% then 𝑇𝑒𝑚 max 𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 =
0.5𝑇 𝑚𝑜𝑡𝑜𝑟. Similarly, the brake torque of the EM is not limited until the SOC is higher
than 85% and when it SOC reaches the maximum value the control strategy starts to
limit the brake torque until SOC=90% and above that the brake torque is 0. The limit
values of the SOC can be varied based on the battery data provided by the manufacturer
as different batteries have different SOC range.
Figure 4.5 - Limitations on SOCmin (left), limitation on SOCmax(right)

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4.2.3 Temperature limitation
The temperature is another important parameter for the safety and the efficiency of the
electric motor and the battery. Both the electric motor and the battery have an optimum
operating temperature range which is usually provided by the manufacturer. The control
strategy receives the instantaneous temperature information from the battery and the
motor block and uses a similar method as the SOC limitation to limit the motor torque.
When the temperature is below the maximum temperature there is no limit on the
maximum torque, and the control strategy starts to limit the torque above a temperature
value near the maximum until it reaches the maximum value above which the torque
becomes zero.
The temperature limit values, similar to the SOC limit, can also be changed based on the
working temperature range provided by the battery and motor manufacturer.
Finally the maximum torque provided by the electric motor is –
𝑇𝑒𝑚 max 𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 = Tmotor × 𝑆𝑂𝐶min 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 × 𝑇𝑒𝑚𝑝 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟
𝑇𝑒𝑚 max 𝑏𝑟𝑎𝑘𝑒 = Tmotor × 𝑆𝑂𝐶max 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 × 𝑇𝑒𝑚𝑝 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟
Figure 4.6 - Maximum temperature limit

48
Based on the maximum torque available from the electric motor in traction and braking
the control strategy calculates the 𝑡ℎ𝑟𝑜𝑡𝑡𝑙𝑒% 𝐸𝑀, 𝑡ℎ𝑟𝑜𝑡𝑡𝑙𝑒%𝑖𝑐𝑒, 𝑏𝑟𝑎𝑘𝑒% 𝑒𝑚 and
𝑏𝑟𝑎𝑘𝑒%ℎ𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑏𝑟𝑎𝑘𝑒𝑠.
4.3 Clutch position control
Having a robotized automatic transmission the driver doesn’t actually have to control
the gear selection and thus the clutch position. The clutch connects the gearbox to the
ICE, thus the clutch position during braking is important. If the clutch is closed during
braking, the engine brake and the friction loss in the front drive-train provides
additional brake torque as those components continuously resist the wheel motion. Due
the assistance of this resistive torques during braking, the brake energy is not
regenerated to the maximum possible amount.
The control strategy is capable of controlling the clutch position. During braking if the
battery SOC higher than the maximum permitted limit, the EM should not provide any
energy to the battery, and the regenerative braking torque should be limited. In this
situation the hydraulic brakes must be used to brake the vehicle. If the clutch is closed
in the front axle components assist the braking, as explained before, thus the hydraulic
brakes are less loaded, which increases the life of the hydraulic brakes. So the control
strategy on closes the clutch during braking if the SOC is higher than the maximum
limit and lefts it open when SOC is lower to maximize the regenerated brake energy.
Figure 4.7 - Clutch position control flow chart

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5 Model validation
The model validation is based on the energy flow analysis. It is important to make sure
that the model is capable of following the imposed driving cycle, and the control
strategy splits the power required by the vehicle between the powertrain components.
The data used for the model validation are –
 NEDC cycle as reference velocity,
 Battery capacity 30Ah and maximum current 5C in hybrid mode,
 No battery constraint in EV mode,
 SOC limit zone - 20-25% for SOCmin and 80-85% for SOCmax, with initial
SOC=45%.
 Temperature limit 60-70˚C for TEMPmax for both motor and battery,
And the parameters checked for validation are –
 Error between the reference and actual velocity of the vehicle,
 Power and energy provided by the ICE and the EM.
 Transition between the ICE and EM during traction, and regenerative brake and
hydraulic brake during braking,
 Limits on the maximum EM torque in traction and braking.
The cooling of the battery and the motor is not taken into account because of the lack of
data. And for the temperature limit only that of the battery is taken into account.
Different configurations of the vehicle are used to check the proper functioning of the
powertrain components. The configurations tested are –
 Vehicle powered only by the ICE (Pure ICE mode),
 Vehicle powered only by the EM (EV mode),
 Vehicle powered by both ICE and the EM (hybrid mode)
In each of these configurations the torque and power provided by the ICE, EM and
hydraulic brakes are checked. The proper functioning of the driver which includes, gear
selection, clutch control and the throttle and brake percentage requested are also
checked.

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5.1 Pure ICE mode
In pure ICE mode the vehicle is similar to the traditional ICE powered vehicles, in
which traction torque is provided only by the ICE. No additional accessories such as
A/C compressor, pumps, alternator etc. are taken into account.
The figure above shows that using the torque provided by the ICE only the vehicle is
capable of following the NEDC cycle with very little error. This means that the throttle
and brake pressure required by the vehicle is also correct. Below the torque provided by
the ICE in traction and hydraulic brakes during braking are given. The peaks in the ICE
torque are due to the gear shift, when the ICE crankshaft is left to rotate free and the
model is not capable of completely reducing this effect.
Figure 5.1 - Reference and actual velocity of the vehicle, using only the ICE for traction
Figure 5.2 - Velocity vs the ICE and hydraulic brake torque

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The figures below show that the gear selection based on the vehicle velocity also works
perfectly. In addition it can also be seen that the control of the clutch position is also
correct, as the clutch opens during gear shift and stays open when the vehicle is stopped
or braking.
The calculated fuel consumption of the vehicle is –4.218 l/100km, which is coherent
with the data provided by the producer of the engine.
Figure 5.3 - Gear variation in a NEDC cycle using only ICE traction
Figure 5.4 - Clutch position change in a NEDC cycle using only ICE for traction

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5.2 EV mode
The vehicle can be considered as a pure electric vehicle by using a very high capacity
battery, capable of providing all the current required by the electric motor. The figure
below shows that the vehicle follows the driving cycle with very little error.
The figure below shows the torque provided by the EM if the pure EV mode. The
electric motor is capable of powering the vehicle almost the whole cycle accept for the
higher velocity where the torque is not enough to provide the required acceleration and
the ICE is turned on by the control strategy for a short while. During braking on the
other hand the EM is capable of providing the brake torque throughout the whole cycle
and the hydraulic brakes are never used.
Figure 5.5 - Reference and actual velocity of the vehicle in EV mode
Figure 5.6 - Velocity vs the torque provided by EM in pure EM traction.

53
The current required by the electric motor both in traction and braking is given below –
5.3 HEV mode
In pure hybrid mode the vehicle uses power from both the ICE and EM for traction. The
vehicle follows the NEDC with very little error also in hybrid mode.
The torque split between the ICE and EM in traction and between the EM and hydraulic
brake in braking is given in the figure below. During traction the vehicle uses power
from both the ICE and the EM, as the maximum electric motor torque is not sufficient
to provide the required traction torque. The peaks in the ICE torque are due to the
simulators synchronization limitation during the gear shift. During braking the EM
torque being limited, is not enough to provide sufficient brake power and the model
uses the hydraulic brakes to provide the brake power requested.
Figure 5.7 - Current required by the EM in pure EV mode
Figure 5.8 - Reference velocity vs actual velocity of the vehicle in hybrid mode

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The motor torque without limit depends only on the rotational speed of the motor as we
can see from the figure above the maximum motor torque is around 88.5Nm and varies
with the rotational speed. By limiting the current available to the motor to 5C (150A),
the maximum torque reduces to 22.5 Nm. The battery SOC limits (both maximum and
minimum) comes into play only in some situations and limits the torque in those points.
The last and one of the most important limitation is the temperature with comes into
play in more often than not and limits the torque to a much lower value. As we can see
from the figure of the multiplication factor, the temperature limiting factor comes into
play after only 300s while the SOC never goes higher than 80% and maximum SOC
limit is not effecting at all, but the minimum SOC limit is comes into play for a short
time at the end of the cycle when it goes below 25%.
Figure 5.10 – Brake torque provided by the EM and hydraulic brakes
Figure 5.9 - Traction torque split between ICE and EM in hybrid traction

55
Figure 5.11 - Electric motor torque limiting factors

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5.4 Energy validation
It is also important to understand the model behavior from the energetic point of view
and the energy provided by the electric motor and ICE should be equal to the energy
required by the vehicle both in traction and braking. During the energy analysis the
NEDC driving cycle is not considered, instead a constant acceleration in traction and a
constant deceleration in braking is used. The rest of the parameters remain equal as
before.
In traction –
Initial velocity = 0 km/h
Final velocity = 50 km/h
Acceleration = 2 𝑚𝑠−2
The figure 5.4 shows that the vehicle follows the reference speed curve with a relatively
low error. And the total energy required by the vehicle is (yellow curve) is the same as
the sum of the energy provided by the EM and the ICE (purple dotted curve). The
energy provided by the ICE is takes into account all the losses in the gearbox, clutch,
front differential and the inertia of the wheels. The energy provided by the EM takes
into account the loss in the rear differential and rear wheel inertia.
Figure 5.12 – Vehicle energy and velocity during constant acceleration traction

57
The same analysis has been done to realize the braking energy by supposing a constant
deceleration of the vehicle. In braking –
Initial velocity = 50 km/h
Final velocity = 0 km/h
Deceleration = 2 𝑚𝑠−2
The figure 5.5 shows that the vehicle follows the reference velocity perfectly and the
total brake energy required (yellow line) is the sum of the brake energy provided by the
EM and the hydraulic brakes (purple dotted line).
Figure 5.13 - Vehicle velocity and energy during constant deceleration braking

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Finally we can say the control strategy and driveline components modelled works
perfectly for any driving cycle imposed with any vehicle configuration used, as the
control strategy is able to perfectly split the torque between the driveline components
and the driveline components are able to follow the driving cycle imposed with very
little errors. The ICE is capable of powering the vehicle throughout the whole cycle and
the front axle powertrain components functions perfectly. The EM is not capable of
powering the vehicle at higher velocities as it dimensioned only to power the vehicle in
urban driving condition at lower velocities. The control strategy is capable of managing
the use of the electric motor on the basis of the constraints imposed.

59
6 Simulation and results analysis
Various simulations are done to understand how to minimize the fuel consumption in
traction and to maximize the regenerated energy during braking. The fuel consumption
depends on the amount of torque provided by the ICE in traction, which depends on the
amount of torque available from the EM in traction and the regenerated energy depends
on the amount of torque the EM can provide during braking. The electric motor torque
available both in traction and braking depends on the battery SOC, current and
temperature and the temperature of the electric motor. During the simulations the
temperature limit is not taken into account due to the lack of dependable data. So to
maximize the EM torque the battery SOC and current are the biggest limitations. The
simulations are done to find out a suitable battery capacity that can provide enough
current to the motor in traction and braking without overcharging or over-discharging
the battery. The simulations are done to figure out the –
 Clutch control during braking,
 Battery capacity,
 Maximum vehicle velocity with pure electric or hybrid traction,
 Energy loss in different driveline components,
 Fuel consumption.
The vehicle not being a plug-in HEV, the battery can’t be charged using external power
sources. It is important to find out a suitable battery capacity that will make sure that the
battery is never over charged or over discharged. The simulations have been done to
find out a battery capacity for which the initial and the final battery SOC for the
imposed driving cycle is the same, this way the electric motor only uses the exact
amount of energy in traction, that has been regenerated by regenerative braking. It is
also important to find out the velocity limit till which the electric motor provides
traction power. In real driving condition the driving cycle is never known and so it is
not possible for the control strategy to make sure that the battery SOC at the beginning
and the end of driving cycle is the same.
The velocity limit of electric motor traction depends on the amount of energy that the
battery can provide which in terms depends on the battery capacity and the constraint on
the battery usage.

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6.1 Velocity limit with no constraint on the battery usage
Simulation data –
 NEDC cycle for reference velocity,
 No SOC, current or temperature constraint on the battery,
 Maximum regenerated energy during braking,
 Clutch always open during braking,
 𝑉𝑠𝑤𝑖𝑡𝑐ℎ = maximum vehicle velocity till which EM provides traction power.
Result analysis
It is interesting that the maximum velocity of electric motor traction for which we can
have the initial and final SOC is independent of the battery capacity if there are no
maximum and minimum current limits on the battery. As we can see from the figure 6.1
if there are no battery constraints such as SOC, temperature and current, the velocity
limit is the same for any battery capacity. It is also clear that the SOC varies highly for a
battery with small capacity than a battery with large capacity. The SOC of the battery is
the percentage of energy available in the battery thus larger variation of SOC means
higher usage of the battery energy.
Having a small battery can be convenient as the vehicle uses a higher percentage of
battery energy. Moreover a battery with smaller capacity also has a smaller mass and
Figure 6.1 - Vswitch vs SOC for different Q, with no constraint on the battery

61
takes a smaller footprint in the vehicle chassis. But this is without taking into account
the current limit of the battery. The electric motor to be able to follow the NEDC cycle
with maximum electric motor traction velocity of 25km/h requires a maximum current
of 180A which means that a battery of 20Ah has to charge and discharge at 9C rate.
Continuously charging and discharging with 9C rate will highly degrade the battery life
and the battery temperature will rise faster.
Final remarks –
 𝑉𝑠𝑤𝑖𝑡𝑐ℎ is independent of the battery capacity if there are no constraint on the
battery.
 Battery with small capacity enables higher percentage of battery energy usage
with faster battery degradation and lower cycle life.
 Battery with large capacity provides lower percentage of battery energy usage
with higher battery cycle life.
6.2 Velocity limit with current constraint
Limiting the current provided by the battery reduces the amount of toque available from
the electric motor. Multiple simulations have been done in order to figure out the
variation of 𝑉𝑠𝑤𝑖𝑡𝑐ℎ with different battery capacities and current limits. The simulations
are done with clutch always open during braking and clutch always closed during
braking to understand the effect of energy loss in the front axle during braking.
Figure 6.2 - Current required by the EM with Vswitch = 25km/h in NEDC cycle

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Simulation – 1; data –
 Maximum charge/discharge rate 5C,
 No SOC or temperature limit on battery,
 Maximum regenerated energy.
 Clutch always closed during braking,
Results analysis
The only difference between the 2 simulations is that for the first simulation the clutch
remains always open during braking thus no brake assist from the engine brake and
friction losses so the brake energy is provided by the regenerative braking and hydraulic
brakes only. For the second simulation having the clutch always closed during braking
the regenerated energy is lower than first case due to constant assist by the engine brake
and friction losses.
Figure 6.3 - Clutch always open during braking

63
As we can see for both cases having a limited current the velocity limit varies with
battery capacity. The velocity limit decreases with increasing the capacity. The reason
behind this is that increasing the for smaller battery capacity the maximum current
provided by the battery is lower which proportionally limits the maximum torque
provided by the electric motor. Having a smaller amount of torque available from the
EM the vehicle has to turn on the ICE before the vehicle reaches 𝑉𝑠𝑤𝑖𝑡𝑐ℎ and the vehicle
runs in hybrid mode instead of pure hybrid so the electric motor can be used till a higher
velocity.
Figure 6.4- Clutch always closed during braking
Figure 6.5 - ICE and EM torque with Vswitch = 30km/h and Q = 20 Ah.

64
As we can see from the figure 6.5 that for a battery with capacity 20 Ah the ICE is
turned on from the beginning of the driving cycle, much before the vehicle reaches the
limit velocity because the EM torque is not enough to run the vehicle till the limit
velocity and when the vehicle reaches limit velocity the EM stops providing power.
For a capacity of 40Ah (figure-6.6) on the other hand, the current provided by the
battery is high enough for the motor to provide enough torque to run the vehicle in pure
electric mode until it reaches the limit velocity. So it is clear that having a larger
capacity battery the velocity limit is lower but the ICE is used less so the fuel
consumption will be lower.
Now the difference between simulation 1 and 2 is the clutch position during braking.
Having the always open during braking the regenerated energy is higher as there are no
losses in the front axle by the engine brake and the friction losses. So more energy is
stored in the battery which can be used in traction and the velocity limit can be
increased. As we can see from figure 6.3 and 6.4 the velocity limit is for which ΔSOC =
0, is higher in the case of the clutch always open during braking than the clutch always
closed. Having the clutch always open the 𝑉𝑠𝑤𝑖𝑡𝑐ℎ is above 35km/h and for the same
battery capacities but with clutch closed during braking the 𝑉𝑠𝑤𝑖𝑡𝑐ℎ is below 35km/h.
Figure 6.6 - ICE and EM torque with Vswitch = 30km/h and Q = 40 Ah.

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Battery capacity [Ah]
Discharge rate 5C
𝑉𝑠𝑤𝑖𝑡𝑐ℎ [Km/h]
Clutch open
Clutch closed
20 37.7 34.55
25 39.2 33.5
30 35.7 32
35 35.05 31.35
40 35.35 31.3
Table 6.1- Vswitch for different battery capacities
Final Remarks –
 Limiting the battery current the 𝑉𝑠𝑤𝑖𝑡𝑐ℎ is no more independent of the battery
capacity,
 For higher capacity, current limit is higher, motor torque is higher, and vehicle
can run in pure electric mode till higher velocity thus discharging the battery
more so 𝑉𝑠𝑤𝑖𝑡𝑐ℎ for which ΔSOC is 0 is lower.
 Lower battery capacity, lower current limit, lower maximum motor torque, ICE
intervenes earlier as the EM is not able to provide traction torque, thus battery
discharge slow and the 𝑉𝑠𝑤𝑖𝑡𝑐ℎ is higher.
 Having the clutch always open during braking – no loss in engine brake and
friction losses in front axle – higher regenerated energy – battery charged more
and more energy available from battery to use during traction – 𝑉𝑠𝑤𝑖𝑡𝑐ℎ for
which ΔSOC is 0 is higher.
 Having the clutch always closed during braking –constant loss in engine brake
and friction losses in front axle during braking – lower regenerated energy –
battery charged less and lower energy available from battery to use during
traction – 𝑉𝑠𝑤𝑖𝑡𝑐ℎ for which ΔSOC is 0 is lower.

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Results Analysis
Increasing the maximum charging and discharging rate to 10C increases the current
provided by the battery and but decreases the battery cycle life and increases the
temperature. Increasing the discharge rate from 5C to 10C all the velocity vs ΔSOC
behavior remains the same and the difference between the clutch open during braking
and clutch closed during braking also remains the same.
Figure 6.7 - Clutch always open during braking

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Discharge rate 10C
Clutch open
Clutch closed
10 35.7 34.7
15 37.7 32
20 36.1 31.34
25 36.48 31.3
30 36.49 31.3
Table 6.2 - Vswitch for different battery capacities
Final Remarks –
 A battery with 10C discharge rate provides same results as a battery with twice
capacity and 5C discharge rate, for what regards the ΔSOC behavior.
6.3 Fuel consumption
The fuel consumption data is calculated for different battery capacities and current limit
with the corresponding 𝑉𝑠𝑤𝑖𝑡𝑐ℎ for which ΔSOC is 0, which is calculated in the previous
section. While calculating the fuel consumption it is also important to understand the
usefulness of the stop&start technology. So the fuel consumption has been calculated
once using the stop&start and once without it. The stop&start system shuts down and
Figure 6.8 - Clutch always closed during braking

68
restarts the ICE to reduce the time the engine spends idling, thus reducing the fuel
consumption and emission. This system can easily be implemented in HEVs as the
electric motor can launch the vehicle and the ICE is not needed to be turned on for very
low velocities. In this model, as described in the 2nd
chapter, the stop&start is activated
when the rotational speed of the ICE is below a preselected value. The rotational speed
of the ICE is the rotational speed of the wheels multiplied by the transmission ratio of
the differential and the gearbox. When the vehicle speed is near zero, the rotational
speed of the wheels is really low too thus the rotational speed of the ICE is low. This
limit value of the ICE rotational speed is preselected and when the ICE speed below the
limit it is turned off and the traction power is provided by the electric motor only.
Simulation – 1, data –
 Q and corresponding 𝑉𝑠𝑤𝑖𝑡𝑐ℎ are the ones calculated in the previous simulation
to have ΔSOC = 0,
 Maximum battery discharge rate = 5C,
 Stop&start turned on.
to have ΔSOC = 0,
 Stop&start turned off.
to have ΔSOC = 0,

69
to have ΔSOC = 0,
 Stop&start turned off.
Results analysis –
The fuel consumption significantly lower in the case of using the stop&start than case in
which the stop&start is not used. So it is better always better to use this system. We can
also from figure 6.9 that the fuel consumption decreases with increasing the battery
capacity. As we have seen in the previous simulations that with a lower battery capacity
the 𝑉𝑠𝑤𝑖𝑡𝑐ℎ is higher but the ICE is turned on much before the vehicle reaches the
velocity limit and more fuel is consumed.
Discharge rate 5C
Fuel consumption [l/100km]
With Stop&start Without Stop&start
20 3.79 4.34
25 3.71 4.26
30 3.47 4.09
35 3.26 3.93
40 3.24 3.91
Table 6.3 - Fuel consumption with Stop&start and without
Figure 6.9- Fuel consumption [l/100km] for different battery capacity

70
Discharge rate 10C
Fuel consumption [l/100km]
With Stop&start Without Stop&start
10 3.79 4.34
15 3.47 4.09
20 3.24 3.91
25 3.23 3.91
30 3.23 3.91
Table 6.4 - Fuel consumption with stop&start and without
Figure 6.10 shows the fuel consumption for different battery capacity but with much
higher current limitation. The behavior is similar as the fuel consumption is lower using
the stop&start system and it decreases with increasing battery capacity. But the fuel
consumption reaches a steady state value above the capacity of 20Ah and remains
constant for higher capacities. What this means is that for battery capacity above 20Ah
with a maximum discharge rate of 10C the battery can provide enough current to the
motor to run the vehicle till it reaches 𝑉𝑠𝑤𝑖𝑡𝑐ℎ. With higher capacity batteries basically
the vehicle reaches the velocity limit before it reaches the current limit. So from the
point of view of fuel consumption it enough to have a battery of 20Ah which can
provide continuous current of 10C or 200A, equivalently a battery with 40Ah capable of
providing maximum continuous current of 5C or 200A.
Figure 6.10 - Fuel consumption [l/100 km] for different Q

71
Final remarks –
 Use the Stop&Start technology to significantly reduce the fuel consumption,
 Increasing battery capacity decreases the fuel consumption,
 A battery that can provide 200A continuous current is enough to have the
minimum fuel consumption.
6.4 Electric accessories
Till now we have only thought of the battery as a source that provides energy only to
the electric motor. But to a car is a more electrical product than a mechanical one, and
contains a lot of electric components such as – lights, info-tainment system, sensors etc.
that requires continuous electric power. Usually in normal there is an alternator
connected to the ICE through a belt that generates current and stores in a 12V battery
and the accessories consume energy from that battery. Also there are accessories such
as the coolant pump, air conditioning pump, pump for power steering which are all
connected to the ICE through a belt drive system and all these components continuously
consumes energy from the ICE increasing the fuel consumption. In a HEV having a
bigger battery capable of storing higher amount of energy it is possible to use electric
energy for all the accessories and removing their dependence on the ICE to reduce fuel
consumption.
If the battery provides constant current to the accessories it has to reduce the current that
it provides to the electric motor and always having the initial and final SOC equal. The
main objective of the following simulations is to find out the maximum current the
electric motor can provide to the accessories without providing any power in traction,
similar to micro hybrid vehicles. To have the ΔSOC = 0, the constant current that the
battery can provide to the accessories is equal to the current generated in regenerative
braking and the tests have been done once with the clutch closed, and once with the
clutch open during braking, because the regenerated energy varies with the selection of
the clutch position.
Simulation 1; data –
 NEDC as reference velocity,
 No traction power provided by the electric motor,
 The regenerated energy is supplied to a constant current load,

72
 Clutch once open and once closed during braking,
 Maximum discharge rate – 5C.
Simulation 2; data –
 NEDC as reference velocity,
 No traction power provided by the electric motor,
 The regenerated energy is supplied to a constant current load,
 Clutch once open and once closed during braking,
 Maximum discharge rate – 10C.
Discharge rate 50C
Current available from battery [A]
Clutch open Clutch closed
15 13.24 12.53
20 17.18 16.09
25 20.94 18.5
30 22.8 19.65
35 23.55 20.01
40 24.11 20.01
Table 6.5 - Current available from the battery in different configurations @5C discharge rate
Figure 6.11 - Current available from the battery in different configuration @ 5C discharge rate

73
From figure 6.11 we can see that the current that the battery can constantly provide to
the accessories, which is equivalent to the energy produced by regenerative braking
increases with increasing battery capacity. The current is also higher in the case of the
clutch open during braking than the clutch closed during braking. Increasing battery
capacity increases the amount of current that the battery can absorb which equivalently
increases the brake torque provided by the electric motor so the regenerative braking is
used more. With the clutch open during braking decreases the losses in the front axle as
we have already discussed before.
Discharge rate 5C
Current available from battery [A]
Clutch open Clutch closed
15 13.24 12.53
20 17.18 16.09
25 20.94 18.5
30 22.8 19.65
35 23.55 20.01
40 24.11 20.01
Table 6.6 - Current available from the battery in different configurations @10C discharge rate
Now it is interesting to see in the figure 6.12 that the maximum current value hits steady
state above battery capacity of 20Ah with maximum charge rate of 10C.Which means
that the maximum current the motor needs to provide the necessary brake power in the
NEDC cycle is about 200A. The maximum current the battery can provide with the
energy produced by the regenerative braking without any constraint is about 24A.
Having a battery of 48V, the power it can constantly provide to the accessories is –
Figure 6.12 - Current available from the battery in different configurations @10C discharge rate

74
𝑃 = 𝑉 × 𝐼
𝑃 = 48 × 24 = 1.15 𝑘𝑊
The air-conditioning pump, when connected to the ICE can absorb up-to 3kW of power
at higher velocities. But using an electric air-conditioning system it absorbs constant
around 0.8-1 kW power. The power steering absorbs 0.5kW, the lighting and the info-
tainment system can absorb about 0.2 – 0.5 kW, the cooling system pump absorbs about
0.5 kW of power. So the vehicle accessories absorb about 1.5-2 kW of power on
average.
So, only with regenerative braking the energy generated is not enough to provide
current to all the accessories in a NEDC cycle. The previous statement is true only in
the case that the driving cycle is NEDC and all the accessories are turned on and
working at the same time. In real life condition, the driving cycle is never the same and
there are additional accessories that sip power from battery. Even if the battery is
capable of constantly providing power to the accessories, it is not able to provide any
power in traction. To increase the energy stored in the battery, it can be helpful to add
an alternator to the ICE that charges the battery when the ICE is turned on. This way the
fuel consumption of ICE increases a little but the energy stored in the battery also
increases and the additional energy provided by the alternator can be used by the
electric motor to provide power in traction which can reduce the fuel consumption as
the ICE can be used less. Now it is interesting to see the difference between the fuel
consumption increment due to energy absorbed by the alternator and the reduction due
to the electric motor used in traction.

75
6.5 Fuel consumption with alternator connected to the ICE
There is a little modification needed to be done in the vehicle driveline to attach the
alternator to the ICE. There can be 2 different possibilities –
1. The alternator charges the same 48V battery used in the rear axle. And a single
battery provides electric power to the accessories and the vehicle lighting and
info-tainment system
2. The alternator charges a separate traditional 12V battery which provides electric
energy to the vehicle lighting and ino-tainment system. The regenerated energy
charges the 48V battery and provides power in traction and to the pumps
(figure– 6.14)
Figure 6.13 - Alternator connected to the ICE and charges the same 48V battery

76
The first architecture is chosen for the analysis in which the alternator charges the same
48V battery which provides electric power to all the electric accessories. The alternator
is connected the ICE through a belt with a transmission ratio of 2.26 between the ICE
and the alternator. The torque – velocity map of the alternator is given below –
Figure 6.14 - Alternator connected to the ICE and charges a separate 12 V battery
Figure 6.15 - Alternator torque velocity map

77
Having the alternator connected to the ICE it constantly consumes energy from the ICE
and charges the battery when the ICE is turned on. The additional fuel consumption
depends on the maximum torque of the alternator. What is important to see is the
additional amount of current, equivalently the power, the battery can provide due to
alternator energy generation. The simulations have been done by limiting the maximum
alternator and registering the fuel consumption and maximum battery current provided
by the battery to have equal initial and final SOC.
Simulation – 1; data -
 No traction power provided by the EM in traction,
 Maximum regenerative energy and clutch always open during braking,
 Battery capacity 35Ah,
 Maximum alternator torque limiting from 1 to maximum.
The maximum alternator torque is varied to understand the characteristics of the
alternator to be used. Having the alternator torque = 0 means no alternator is connected
to the ICE and the ICE is providing power only to move the vehicle and no accessories
Maximum alternator
Torque [Nm]
Fuel
consumption[l/100km]
Battery discharge current and power
Current [A] Power [kW]
1 4.44 29.78 1.429
2 4.67 38.03 1.825
3 4.91 46.17 2.216
4 5.12 53.35 2.560
5 5.29 59.32 2.847
6 5.62 62.53 3.0
Maximum 5.7 66.15 3.11
Table 6.7- Battery current and fuel consumption with increasing alternator torque

78
are connected to it. So the ICE consumes 4.45 l/100km fuel if the vehicle is running
with only the ICE with no assistance from the EM in traction, without taking into
account any of the accessories. Increasing the maximum alternator torque increases the
fuel consumption and with the maximum alternator torque of around 13Nm the fuel
consumption is around 5.7 l/100km.
From the figure 6.17 we can see the amount of power the battery can provide to the
electric accessories with the alternator connected to the ICE. Increasing the maximum
alternator torque increases the energy it stores in the battery during traction. With 0
alternator which is no alternator connected to the ICE the battery can provide 1.15 kW
power which is generated by regenerative braking. And with maximum alternator
torque of about 13Nm the battery can provide about 3.2 kW power to the while having
Figure 6.17 - Battery current vs alternator torque
Figure 6.16 - Fuel consumption vs alternator torque

79
equal initial and final SOC. As we have discussed before the accessories require about
1.5-2 kW power, so by having 3.2 kW power available from the battery it can provide
enough energy to the accessories and then some in traction.
Final remarks –
 Adding an alternator to ICE increases the fuel consumption of the vehicle,
 Increases the amount of energy stored in the battery and the battery can provide
more current to the electric accessories and in traction,
6.6 Summary of the results
On the basis of the results obtained in the previous simulations the following decisions
can be made –
 With the clutch closed during braking, the friction loss in the front axle and the
engine brake provides some additional brake energy, reducing the regenerated
energy in the rear axle. So, it is better to leave the clutch open during braking to
maximize the regenerated energy.
 Without any constraint on the battery the 𝑉𝑠𝑤𝑖𝑡𝑐ℎ for which the initial and final
SOC is equal is independent of the battery capacity.
 Imposing the constraint on the maximum current provided by the battery, the
𝑉𝑠𝑤𝑖𝑡𝑐ℎ and fuel consumption decreases with increasing battery capacity.
 A battery with a smaller capacity and higher discharge rate is equivalent to a
battery with higher capacity and smaller discharge rate.
 A li-ion battery with capacity 20Ah and discharge rate of 10C is capable of
providing the necessary current to the electric motor in traction. Which allows to
use only the electric motor traction until the vehicle reaches 𝑉𝑠𝑤𝑖𝑡𝑐ℎ.
 Using the stop&start technology reduces the fuel consumption, so it is better to
always use it.
 Using electric A/C compressor and cooling pump, removes their dependence on
the ICE and reduces the fuel consumption.
 The use of the electric accessories requires a constant current from the battery.
The energy generated only by the regenerative braking is not enough to provide
constant current to the accessories and the current required for traction.

Thesis Chowdhury

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