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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 (ω𝑤ℎ𝑒𝑒𝑙𝑠) and acceleration

(ω̇𝑤ℎ𝑒𝑒𝑙𝑠) 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 × 𝜏𝑔𝑒𝑎𝑟 × 𝜂𝑔𝑒𝑎𝑟𝑏𝑜𝑥 × 𝜏𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙 × 𝜂𝑑𝑖𝑓𝑓

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 = ω𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏𝑔𝑒𝑎𝑟 × 𝜏𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙

Tice_output = (Tmax 𝑖𝑐𝑒 × 𝑡ℎ𝑟𝑜𝑡𝑡𝑙𝑒 %) − 𝑇𝑒𝑛𝑔𝑖𝑛𝑒 𝑖𝑛𝑒𝑟𝑡𝑖𝑎 − 𝑇𝑒𝑛𝑔𝑖𝑛𝑒 𝑏𝑟𝑎𝑘𝑖𝑛𝑔

Tengine inertia = ω̇wheels × 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 = ω𝑝𝑟𝑖𝑚𝑎𝑟𝑦 × 𝑐𝑙𝑢𝑡𝑐ℎ 𝑡𝑎𝑢

ω̇ICE = ω̇𝑝𝑟𝑖𝑚𝑎𝑟𝑦 × 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 = ω𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏𝑔𝑒𝑎𝑟 × 𝜏𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙

ω̇primary = ω̇𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏𝑔𝑒𝑎𝑟 × 𝜏𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙

Tgearbox out = (T𝑝𝑟𝑖𝑚𝑎𝑟𝑦 × 𝜏𝑔𝑒𝑎𝑟 × 𝜂𝑔𝑒𝑎𝑟𝑏𝑜𝑥

) + 𝑇𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛

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 × 𝜏𝑑𝑖𝑓𝑓 × 𝜂𝑑𝑖𝑓𝑓

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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

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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 × 𝜏𝑒𝑚 × 𝜂𝑒𝑚 × 𝜏𝑑𝑖𝑓𝑓 × 𝜂𝑑𝑖𝑓𝑓

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 (𝜏𝑑𝑖𝑓𝑓 = 4.5). The power

needed from the electric motor is then –

𝑃𝑚𝑜𝑡𝑜𝑟 = 𝑉(𝑡) × 𝐹𝑥

𝐹𝑥 = 𝑀𝑎𝑥(𝑡) + 0.5𝜌𝐴𝐶𝑥𝑉(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 = ω𝑤ℎ𝑒𝑒𝑙𝑠 × 𝜏𝑒𝑚 × 𝜏𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙

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 = 𝑃𝑑𝑅𝑡ℎ𝑚 (1 − 𝑒−

𝑡

𝜏𝑡𝑚) + 𝑇𝑒𝑚𝑝𝑒𝑛𝑣

Where 𝑅𝑡ℎ𝑚 is the thermal resistance of the

heat exchange between the motor components

and the environment. The thermal time constant is

𝜏𝑡𝑚 and 𝑇𝑒𝑚𝑝𝑒𝑛𝑣 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 = 3𝑅𝑝ℎ𝑎𝑠𝑒𝑖𝑝ℎ𝑎𝑠𝑒2

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 → 𝑖𝑑𝑐 > 0 → 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑖𝑛𝑔

𝑇𝑒𝑚 < 0 → 𝐵𝑟𝑎𝑘𝑖𝑛𝑔 → 𝑖𝑑𝑐 < 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 = 𝑅𝑏𝑎𝑡𝑡𝑒𝑟𝑦𝑖𝑑𝑐2

Tempbattery = 𝑃𝑑𝑅𝑡ℎ𝑏 (1 − 𝑒−

𝑡𝜏𝑡𝑏) + 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 × 𝐾𝑃 × 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 − 𝑇𝑤ℎ𝑒𝑒𝑙 𝑖𝑛𝑒𝑟𝑡𝑖𝑎

Trear wheels = 𝑇𝑟𝑒𝑎𝑟 𝑎𝑥𝑙𝑒 − 𝑇𝑟𝑒𝑎𝑟 𝑏𝑟𝑎𝑘𝑒𝑠 − 𝑇𝑤ℎ𝑒𝑒𝑙𝑠 𝑖𝑛𝑒𝑟𝑡𝑖𝑎

Twheels inertia = 𝐽𝑤ℎ𝑒𝑒𝑙𝑠 × ω̇𝑤ℎ𝑒𝑒𝑙𝑠

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

𝑅𝑤ℎ𝑒𝑒𝑙𝑠

Fx rear wheels =𝑇𝑟𝑒𝑎𝑟 𝑤ℎ𝑒𝑒𝑙𝑠

𝑅𝑤ℎ𝑒𝑒𝑙𝑠

Where 𝑅𝑤ℎ𝑒𝑒𝑙𝑠 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 = 0.5𝜌𝐴𝐶𝑥𝑉(𝑡)2

Frolling resistace = 𝑀𝑔𝐶0

Where 𝜌 is the air density, 𝐶𝑥 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 + 𝑓1𝑉𝑡 + 𝑓2𝑉𝑡2

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 –

𝑎𝑥 𝑎𝑐𝑡𝑢𝑎𝑙 =𝐹𝑥 𝑡𝑜𝑡𝑎𝑙

𝑀

35

𝑉𝑥 𝑎𝑐𝑡𝑢𝑎𝑙 = ∫ 𝑎𝑥 𝑎𝑐𝑡𝑢𝑎𝑙

𝑡

0

ω𝑤ℎ𝑒𝑒𝑙𝑠 =𝑉𝑥 𝑎𝑐𝑡𝑢𝑎𝑙

𝑅𝑤ℎ𝑒𝑒𝑙𝑠

ω̇𝑤ℎ𝑒𝑒𝑙𝑠 =𝑎𝑥 𝑎𝑐𝑡𝑢𝑎𝑙

𝑅𝑤ℎ𝑒𝑒𝑙𝑠

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 (𝑽𝟎) – 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 = 𝑉0𝑖

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 (𝐐𝐧𝐨𝐦 [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.

𝐸𝑚𝑎𝑥 = 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.

𝑖𝑚𝑎𝑥 = Q𝑛𝑜𝑚 × 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 − 𝐾𝑄𝑛𝑜𝑚

𝑄𝑛𝑜𝑚−𝑖𝑡𝑖𝑡 + 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, 𝑉𝑒𝑥𝑝, 𝑉𝑛𝑜𝑚, 𝑄𝑒𝑥𝑝, and 𝑄𝑛𝑜𝑚.

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 − 𝐾𝑄

𝑄 − 𝑄𝑒𝑥𝑝𝑄𝑒𝑥𝑝 + 𝐴𝑒−𝐵×𝑖𝑡

And finally the SOC of the battery can be calculated as –

𝑆𝑂𝐶 = 100 (1 −𝑖𝑑𝑐

𝑄𝑛𝑜𝑚) %

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,

𝑆𝑂𝐶𝑏𝑎𝑡𝑡𝑒𝑟𝑦 = state of charge of the battery,

𝐼𝑚𝑎𝑥 = maximum current provided by the battery,

𝑇𝑒𝑚𝑝𝑏𝑎𝑡𝑡𝑒𝑟𝑦 , 𝑇𝑒𝑚𝑝𝑚𝑜𝑡𝑜𝑟 = 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) × 𝑅𝑤ℎ𝑒𝑒𝑙𝑠

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

45

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.

𝑖𝑚𝑎𝑥 = Q𝑛𝑜𝑚 × 𝐶

For example a battery with Q𝑛𝑜𝑚 = 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

𝑆𝑂𝐶𝑚𝑖𝑛 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 𝑆𝑂𝐶𝑚𝑎𝑥 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 = 𝑇𝑚𝑜𝑡𝑜𝑟, 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 =

0.5𝑇𝑚𝑜𝑡𝑜𝑟. 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)

47

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 × 𝑇𝑒𝑚𝑝𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟

𝑇𝑒𝑚 max 𝑏𝑟𝑎𝑘𝑒 = Tmotor × 𝑆𝑂𝐶max 𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 × 𝑇𝑒𝑚𝑝𝑚𝑢𝑙𝑡𝑖𝑝𝑙𝑖𝑐𝑎𝑡𝑖𝑜𝑛 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

49

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.

50

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

51

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

52

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

54

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

56

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

58

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.

60

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

62

Simulation – 1; data –

NEDC cycle for reference velocity,

Maximum charge/discharge rate 5C,

No SOC or temperature limit on battery,

Maximum regenerated energy.

Clutch always open during braking,

Simulation – 2; data –

NEDC cycle for reference velocity,

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.

65

Battery capacity [Ah]

Discharge rate 5C

𝑉𝑠𝑤𝑖𝑡𝑐ℎ [Km/h]

Clutch open

𝑉𝑠𝑤𝑖𝑡𝑐ℎ [Km/h]

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.

66

Simulation – 3; data –

NEDC cycle for reference velocity,

Maximum charge/discharge rate 10C,

No SOC or temperature limit on battery,

Maximum regenerated energy.

Clutch always open during braking,

Simulation – 4; data –

NEDC cycle for reference velocity,

Maximum charge/discharge rate 10C,

No SOC or temperature limit on battery,

Maximum regenerated energy.

Clutch always open during braking,

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

67

Battery capacity [Ah]

Discharge rate 10C

𝑉𝑠𝑤𝑖𝑡𝑐ℎ [Km/h]

Clutch open

𝑉𝑠𝑤𝑖𝑡𝑐ℎ [Km/h]

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 –

NEDC cycle for reference velocity,

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.

Simulation – 2, data –

NEDC cycle for reference velocity,

Q and corresponding 𝑉𝑠𝑤𝑖𝑡𝑐ℎ are the ones calculated in the previous simulation

to have ΔSOC = 0,

Maximum battery discharge rate = 5C,

Stop&start turned off.

Simulation – 3, data –

NEDC cycle for reference velocity,

Q and corresponding 𝑉𝑠𝑤𝑖𝑡𝑐ℎ are the ones calculated in the previous simulation

to have ΔSOC = 0,

Maximum battery discharge rate = 10C,

Stop&start turned on.

69

Simulation – 4, data –

NEDC cycle for reference velocity,

Q and corresponding 𝑉𝑠𝑤𝑖𝑡𝑐ℎ are the ones calculated in the previous simulation

to have ΔSOC = 0,

Maximum battery discharge rate = 10C,

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.

Battery capacity [Ah]

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

Battery capacity [Ah]

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.

Results analysis –

Battery capacity [Ah]

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.

Battery capacity [Ah]

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 -

NEDC cycle for reference velocity,

No traction power provided by the EM in traction,

Maximum regenerative energy and clutch always open during braking,

Battery capacity 35Ah,

Maximum battery discharge rate = 5C,

Stop&start turned on.

Maximum alternator torque limiting from 1 to maximum.

Results analysis –

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.

80

Adding an alternator to the ICE which generates electric energy when the ICE is

turned on, increases the fuel consumption but also increases the energy stored in

the battery. The additional energy stored in the battery can be used to provide

current to the motor in traction, in addition to the accessories, reducing the fuel

consumption.

A comparative list of the fuel consumption in different vehicle configuration analyzed is

given below –

Driving

mode

Vehicle configuration Fuel consumption

[l/100km]

Pure ICE No accessory connected to it 4.17

A/C + Alternator connected to it 7.18

HEV mode

with battery

capacity

35Ah @5C

and clutch

open during

braking

HEV traction + EM velocity limit 35.05km/h

+ No accessory + ΔSOC = 0

3.2

Pure ICE traction +Max Regen. Braking + no

accessory

4.17

Pure ICE traction + Max regen. Braking +

Alternator (max torque 1 Nm)

4.44

Pure ICE traction + Max regen. Braking +

Alternator maximum uses

5.45

Table 6.8 - Fuel consumption of the vehicle in different configuration

Without taking into account any accessories and powering the vehicle only by

the ICE, the vehicle consumes 4.17 liter of fuel per 100km.

Driving the vehicle in HEV mode and without taking into account the

accessories, the fuel consumption reduces by 9.7%. The battery used has a

capacity of 35Ah and discharge rate of 5C, and the EM is used in way to achieve

the battery energy balance.

The fuel consumption in pure ICE traction increases by 41.9% if only the A/C

compressor and the alternator, which charges the 12V battery in traditional

vehicles, are taken into account. This is without taking into consideration the

liquid cooling pumps.

81

Adding an alternator to the ICE, which provides a maximum resistive torque of

2 Nm, and harvesting maximum regenerative brake energy, the energy stored in

the battery increases, and the battery is capable supplying 1.8 kW of power.

Supposing that the A/C compressor is electric and consumes 1.5kW of power

from the battery, the battery is capable of powering the A/C throughout the

whole cycle. Powering the vehicle only with the ICE and not using any battery

energy for traction, the fuel consumption is 4.44 l/100 km. In comparison to the

pure ICE traction in which the A/C and alternator are both connected to the ICE,

the fuel consumption is 38.1% lower.

In similar configuration as the case above, but using the alternator without any

limit the fuel consumption is 5.45 l/100 km, which is still 24% lower than the

case in which the A/C and alternator are both connected to the ICE. Using the

alternator without any limit, the power that the battery can provide is around 3

kW which is more than enough to power the accessories and then provide some

power to EM in traction.

From the analysis performed, it is clear that the fuel consumption and subsequently the

emission in the urban driving cycles can be reduced highly by adopting the TTR HEV

technology. The analysis also provides a good approximation about the dimensioning of

the electric drive-train components. The efficiency of the regenerative braking is higher

than the traditional parallel HEVs, due to fewer components used in the electric drive-

train.

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7 Conclusion

The TTR architecture enables the ICE powered vehicles to be retrofitted into a hybrid

vehicle with minimal physical modification. This thesis has explained the modelling

and simulation of a TTR HEV architecture, with specific emphasis on the fuel

consumption reduction, battery sizing and power split strategy development. The

proposed power split control strategy is optimized to maximize the use of the electric

motor on the basis of the constraints imposed on the battery usage. The simulations are

performed in different configuration which helps to understand the battery properties

required to maximize the use of the EM and minimize the fuel consumption.

As a first approximation all the accessories (A/C compressor, pumps, lights, info-

tainment system etc.) that demand electric and mechanical power for performing

additional operational required in a vehicle are not taken into consideration. The energy

provided by the ICE and the EM is used solely to run the vehicle. In this scenario the

energy stored in the battery by the regenerative braking in the rear axle is sufficient to

propel the vehicle in hybrid or pure electric mode until a certain velocity while

maintaining the battery SOC balance. The simulation results also show that the

recoverable brake energy is higher if the clutch is left open during braking.

The battery charge/discharge rate which determines the amount of current provided by

the battery during charging and discharging process, is more important than the capacity

of the battery. If a battery pack with smaller capacity has a higher C-rate, it can provide

the necessary current to the electric motor during traction and braking, because the

torque provided by the electric motor is equivalent to the current available. A battery

capable of producing high current can power the vehicle in pure electric mode until the

velocity limit, while maintaining the battery SOC balance.

In urban driving conditions the velocity of a vehicle is often very low, and is in the

range of 0-50 km/h. So it is better to have a battery that is capable of powering the

vehicle in pure electric mode until a certain velocity even if it is very low. When the

vehicle is functioning in hybrid mode it uses the ICE which increases the fuel

consumption.

83

The energy balance changes radically if the vehicle accessories, that are not directly

responsible to drive the vehicle but are important for the proper functioning of the

vehicle, are taken into consideration. The accessories in traditional vehicles demand a

constant power supply, some from the battery others from the ICE. The accessories that

are dependent on the ICE, consume energy from the ICE and increase fuel consumption

when it is turned on. In addition also forces the ICE to be turned on even when the

vehicle is in stand-still condition. The HEV technology allows total electrification of all

the accessories removing their dependence from the ICE, thus reduces the fuel

consumption. These accessories are always present in a vehicle and must be considered

during the vehicle modelling.

The energy recovered by the regenerative braking is hardly enough to supply the power

required by the accessories, if all of them are electric. To increase the energy stored in

the battery an additional alternator can be added to the ICE. The alternator consumes

power from the ICE and charges the battery. The additional energy then can be used to

power the accessories. The fuel consumption and the energy available from the battery

are equivalent to the energy consumed by the alternator. Even with the alternator

connected the ICE, the fuel consumption is lower than pure ICE- powered vehicle due

to the electrification of the accessories.

The model proposed is highly modular and allows the addition of more components and

degrees of freedom to simulate a more realistic vehicle. The performed simulations and

corresponding results provide a preliminary idea about the dimensioning of the main

electric powertrain components. The reduction of fuel consumption due the additional

electric powertrain is highly advantageous from the environmental and economical

point of view.

84

8 Model limitations and future work

The model although allows the preliminary testing to understand the dimensioning of

the electric powertrain components, but does not take into consideration some very

important factors. The possible improvements are –

Use of more reliable thermal data of the battery and motor,

Use of more accurate battery data,

Performing the simulations using other standard driving cycles,

Use of more accurate data about the power consumption of the vehicle

accessories.

One of the biggest constraints in using the battery and the electric motor is the

temperature. The efficiency and cycle-life of both the electric motor and the battery

depends highly on the temperature, which is not taken into account during the

simulations due to the lack of reliable thermal data. The thermal model is present in the

vehicle model and the control strategy is capable of controlling the use of the electric

motor and the battery on the basis of the temperature increase. It is highly recommended

to redo the analysis done before, using the correct and reliable thermal data.

Most of the analysis has been done using only the NEDC cycle as reference velocity. It

can be interesting to perform the simulations using other driving cycles such as WLTP,

ECE, JPN15, US06 etc. This additional analysis will provide a better understanding

about the use of the electric powertrain and fuel consumption of the vehicle.

More reliable and accurate battery data will help to simulate a more realistic battery

behavior. And the use of accurate power consumption data of the vehicle accessories

will allow better management of the electric energy usage.

85

Appendix -1

Vehicle data

M = 975 [kg] = Vehicle mass

Mp = 80 [kg] = Passenger mass

R = 0.283 [m] = Rolling radius

l = 2.3 [m]= Wheelbase

a = 0.9 [m] = Distance of the front axle from the vehicle COG

b = 1.4 [m] = Distance of the Rear axle from the vehicle COG

hg = 0.55 [m] = Height from ground of CoG

j_w = 0.31 = Wheels inertia

j_eng = 0.1 = Engine + Flywheel inertia

Cx = 0.35 = Drag coefficient

FA = 2 [m2] = Frontal area

f_0 = 89.5 [N]

f_1 = 0 [N/kph]

f_2 = 0.035 [N/kph2]

𝜏𝑔𝑏 = (4.1; 2.158; 1.345; 0.974; 0.766) = Transmission ratio of the gearbox (1-5)

Gear shift time = 0.5 [s]

𝜏𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑡𝑖𝑎𝑙 = 4.5 = Transmission ratio of both the front and rear differential.

𝜏𝑒𝑚 = 2.5 = Transmission ratio of the electric motor.

η𝑒𝑚 = 0.95 = Efficiency of the differential.

86

Battery data

V = 48 [V] = Nominal Voltage

𝑅𝑐𝑒𝑙𝑙 = 0.003 [Ω] = Internal resistance of a single cell

𝜏𝑡𝑏 = 629 = Battery thermal time constant.

𝑅𝑡ℎ = 0.07 = Battery thermal resistance.

A = 0.35 = per cell

B = 2 = per cell

K = 0.041 = per cell

87

Appendix 2

Acronyms

HEV = Hybrid Electric Vehicle

EV = Electric Vehicle

TTR = Through The Road

NEDC = New European Driving Cycle

WLTP = Worldwide harmonized Light vehicles Test Procedures

ICE = Internal Combustion engine

EM = Electric Motor

4WD = 4 Wheel Drive

SOC = State Of Charge

A/C = Air-conditioning

88

List of Figures

Figure 1.1 - Series hybrid architechture .......................................................................... 12

Figure 1.2 - Parallel hybrid architecture ......................................................................... 13

Figure 1.3 - Series - parallel hybrid architechture .......................................................... 13

Figure 1.4 - Classification of vehicles based on the size of ICE and electric motor ...... 15

Figure 1.5- Through the Road (TTR) hybrid architecture .............................................. 16

Figure 2.1 - Components of the vehicle model ............................................................... 17

Figure 2.2 - Acceleration and brake signal generation ................................................... 18

Figure 2.3- Engaged gear selection ................................................................................. 19

Figure 2.4 - Clutch control .............................................................................................. 19

Figure 2.5- Torque flow in the front axle ....................................................................... 21

Figure 2.6- Components of the ICE block ...................................................................... 22

Figure 2.7 - ICE speed-torque map ................................................................................. 23

Figure 2.8 - Fuel consumption map of the ICE .............................................................. 24

Figure 2.9- Friction torque vs rotational speed for different gear engaged .................... 26

Figure 2.10- Gearbox efficiency at 1st gear vs speed at different temperature .............. 26

Figure 2.11 - Torque flow in the rear axle ...................................................................... 26

Figure 2.12 - Power required (WLTP left, NEDC right) to follow the driving cycle .... 28

Figure 2.13- Torque and velocity of the electric motor for different transmission ratio in

NEDC cycle. ................................................................................................................... 28

Figure 2.14 - Torque required from the electric motor for WLTP (left) and NEDC

(right) cycle ..................................................................................................................... 29

Figure 2.15 - Torque - Velocity curve of the electric motor ........................................... 30

Figure 2.16 - Transient temperature response ................................................................ 30

Figure 2.17 - Phase current vs torque and velocity map of the electric motor ............... 31

Figure 2.18 - Battery model ............................................................................................ 31

Figure 2.19 - Direct current vs torque and velocity map of the electric motor............... 32

Figure 3.1 - Typical discharge curve of Li-ion battery [15] ........................................... 39

Figure 4.1- Control strategy ............................................................................................ 42

Figure 4.2 - Control flow chart ....................................................................................... 43

Figure 4.3 - Maximum motor torque calculation ............................................................ 44

Figure 4.4 - Motor map that provides the maximum torque for each value of current and

velocity ............................................................................................................................ 45

89

Figure 4.5 - Limitations on SOCmin (left), limitation on SOCmax(right) .................... 46

Figure 4.6 - Maximum temperature limit ....................................................................... 47

Figure 4.7 - Clutch position control flow chart .............................................................. 48

Figure 5.1 - Reference and actual velocity of the vehicle, using only the ICE for traction

........................................................................................................................................ 50

Figure 5.2 - Velocity vs the ICE and hydraulic brake torque ......................................... 50

Figure 5.3 - Gear variation in a NEDC cycle using only ICE traction ........................... 51

Figure 5.4 - Clutch position change in a NEDC cycle using only ICE for traction ....... 51

Figure 5.5 - Reference and actual velocity of the vehicle in EV mode .......................... 52

Figure 5.6 - Velocity vs the torque provided by EM in pure EM traction. ..................... 52

Figure 5.7 - Current required by the EM in pure EV mode ............................................ 53

Figure 5.8 - Reference velocity vs actual velocity of the vehicle in hybrid mode ......... 53

Figure 5.9 - Traction torque split between ICE and EM in hybrid traction .................... 54

Figure 5.10 – Brake torque provided by the EM and hydraulic brakes .......................... 54

Figure 5.11 - Electric motor torque limiting factors ....................................................... 55

Figure 5.12 – Vehicle energy and velocity during constant acceleration traction .......... 56

Figure 5.13 - Vehicle velocity and energy during constant deceleration braking .......... 57

Figure 6.1 - Vswitch vs SOC for different Q, with no constraint on the battery ............ 60

Figure 6.2 - Current required by the EM with Vswitch = 25km/h in NEDC cycle ........ 61

Figure 6.3 - Clutch always open during braking ............................................................ 62

Figure 6.4- Clutch always closed during braking ........................................................... 63

Figure 6.5 - ICE and EM torque with Vswitch = 30km/h and Q = 20 Ah. .................... 63

Figure 6.6 - ICE and EM torque with Vswitch = 30km/h and Q = 40 Ah. .................... 64

Figure 6.7 - Clutch always open during braking ............................................................ 66

Figure 6.8 - Clutch always closed during braking .......................................................... 67

Figure 6.9- Fuel consumption [l/100km] for different battery capacity ......................... 69

Figure 6.10 - Fuel consumption [l/100 km] for different Q ............................................ 70

Figure 6.11 - Current available from the battery in different configuration @ 5C

discharge rate .................................................................................................................. 72

Figure 6.12 - Current available from the battery in different configurations @10C

discharge rate .................................................................................................................. 73

Figure 6.13 - Alternator connected to the ICE and charges the same 48V battery ........ 75

Figure 6.14 - Alternator connected to the ICE and charges a separate 12 V battery ...... 76

Figure 6.15 - Alternator torque velocity map ................................................................. 76

90

Figure 6.16 - Fuel consumption vs alternator torque ...................................................... 78

Figure 6.17 - Battery current vs alternator torque .......................................................... 78

91

List of Tables

Table 1.1- Advantages and disadvantages of different HEV architectures. ................... 14

Table 3.1 - Comparision of different battery types ........................................................ 37

Table 6.1- Vswitch for different battery capacities ........................................................ 65

Table 6.2 - Vswitch for different battery capacities ....................................................... 67

Table 6.3 - Fuel consumption with Stop&start and without ........................................... 69

Table 6.4 - Fuel consumption with stop&start and without ........................................... 70

Table 6.5 - Current available from the battery in different configurations @5C discharge

rate .................................................................................................................................. 72

Table 6.6 - Current available from the battery in different configurations @10C

discharge rate .................................................................................................................. 73

Table 6.7- Battery current and fuel consumption with increasing alternator torque ...... 77

Table 6.8 - Fuel consumption of the vehicle in different configuration ......................... 80

92

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