power management control design for a hybrid electric vehicle

8/10/2019 Power Management Control Design for a Hybrid Electric Vehicle

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Power Management Control Design for a Hybrid Electric Vehicle

Ioannis Karountzos

MEng Mechanical Engineering

Department of Mechanical Engineering Sciences

Faculty of Engineering and Physical Sciences

University of Surrey

Project Report

May 2014

Project Supervisor: Dr. Saber Fallah


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Contents

List of Figures............................................................................................................................ v

List of Tables ........................................................................................................................... viii

List of Nomenclature .................................................................................................................ix

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

1.1. Disadvantages of Conventional Vehicles .................................................................... 1

1.2. Hybrid Electric Vehicles (HEVs) .................................................................................. 3

1.2.1. Comparison of Conventional, Hybrid Electric and Electric Vehicles ...................... 3

1.2.2. Economics of Hybrid Electric Vehicles (HEVs) ..................................................... 5

1.2.3. Powertrain Configurations of Hybrid Electric Vehicles (HEVs) .............................. 6

1.2.4. Plug-in Hybrid Electric Vehicles (PHEVs) ........................................................... 10

1.2.5. Power Management ........................................................................................... 10

2. Literature Review ............................................................................................................. 12

2.1. Rule-Based Control Strategies .................................................................................. 122.1.1. Deterministic Rule-Based Control Strategies ...................................................... 13

2.1.2. Fuzzy Rule-Based Control Strategies ................................................................ 16

2.2. Optimization-Based Control Strategies ...................................................................... 17

2.2.1. Global Optimization-Based Control Strategies ................................................... 18

2.2.2. Real-Time Optimization-Based Control Strategies ............................................. 19

3. Modelling of Hybrid Electric Vehicle ................................................................................. 21

3.1. Generic Vehicle Model .............................................................................................. 21

3.1.1. Longitudinal Dynamics of the Vehicle Model ...................................................... 22

3.2. Powertrain Model of a Double-Shaft Parallel HEV with Torque Coupler .................... 23

3.2.1. Overall Gear Ratios and Efficiencies .................................................................. 23

3.2.2. Internal Combustion Engine (ICE) ...................................................................... 24

3.2.3. Electric Motor ..................................................................................................... 25

3.2.4. Battery ............................................................................................................... 26

3.2.5. Thermostat Controller Strategy .......................................................................... 27

4. Implementation of Powertrain and Control Strategy in MATLAB/Simulink ........................ 30

4.1. Vehicle and Powertrain ............................................................................................. 31

4.2. Power Management Control Strategy ........................................................................ 32

4.2.1. Braking ............................................................................................................... 32

4.2.2. Traction .............................................................................................................. 34

4.2.3. Battery ............................................................................................................... 38

4.2.4. Fuel Consumption .............................................................................................. 39

5. Results and Discussion .................................................................................................... 41

5.1. New European Drive Cycle (NEDC) .......................................................................... 41


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5.1.1. Battery SOC variation and Engine On/Off Operation .......................................... 41

5.1.2. Engine and Motor Operation .............................................................................. 43

5.1.3. Fuel Consumption .............................................................................................. 46

5.2. City Cycle (FTP-75) ................................................................................................... 46

5.2.1. Battery SOC variation and Engine On/Off Operation .......................................... 46

5.2.2. Engine and Motor Operation .............................................................................. 485.2.3. Fuel Consumption .............................................................................................. 51

5.3. Summary .................................................................................................................. 51

6. Concluding Remarks ........................................................................................................ 52

6.1. Conclusions .............................................................................................................. 52

6.2. Further Work ............................................................................................................. 52

Bibliography ............................................................................................................................ 53

Appendix A .............................................................................................................................. 56

Appendix B .............................................................................................................................. 60


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List of Figures

Figure 1-1: UK Demand for Transport Fuels (UK Government Services, 2014) ......................... 2

Figure 1-2: Market share, gasoline hybrid-electric vehicles (in %) (International Council onClean Transportation, 2013) ...................................................................................................... 5

Figure 1-3: Single-Shaft Parallel Hybrid Powertrain (Fallah, et al., 2014) .................................. 7

Figure 1-4: Double-Shaft Parallel Hybrid Powertrain (Fallah, et al., 2014) ................................. 7

Figure 1-5: Series Hybrid Powertrain (Fallah, et al., 2014) ........................................................ 8

Figure 1-6: Power-Split Hybrid Powertrain (Fallah, et al., 2014) ................................................ 9

Figure 1-7: Compound Hybrid Powertrain (Fallah, et al., 2014) ............................................... 10

Figure 2-1: Classification of Power Management Strategies (Bayindir, et al., 2011) ................ 12

Figure 2-2: Classification of Rule-Based Strategies (Bayindir, et al., 2011; Jalil, et al., 1997;Kim, et al., 2014) ..................................................................................................................... 13

Figure 2-3: Classification of Optimization-Based Control Strategies (Bayindir, et al., 2011;Delprat, et al., 2004) ................................................................................................................ 18

Figure 3-1: Generic Vehicle Model of a Double-Shaft Parallel HEV in a Backward-LookingArchitecture ............................................................................................................................. 21

Figure 3-2: Powertrain Model of a Double-Shaft Parallel HEV with Torque Coupler ................ 23

Figure 3-3: Illustration of Thermostat Controller Strategy ........................................................ 27

Figure 3-4: Flowchart of Thermostat Controller Strategy ......................................................... 29

Figure 4-1: Model of Parallel HEV in Simulink ......................................................................... 30

Figure 4-2: Model of the Vehicle's Longitudinal Dynamics ....................................................... 31

Figure 4-3: Model of the Speed & Toque Calculations ............................................................. 31

Figure 4-4: Model of Power Management ................................................................................ 32

Figure 4-5: Model of Power Management/Braking ................................................................... 33

Figure 4-6: Model of Power Management/Braking/Negative Torque Check ............................. 33

Figure 4-7: Model of Power Management/Traction .................................................................. 34

Figure 4-8: Model of Power Management/Traction/SOC>=SOCmax, for motor-alone propellingmode ....................................................................................................................................... 35

Figure 4-9: Model of Power Management/Traction/SOC


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Figure 4-12: Model of the Power Management/Traction/SOC0 ...... 37

Figure 4-13: Model of the Power Management/ Traction/ SOC0/SOC


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Figure B-3: Model of Power Management/Braking/Negative Torque Check/Wheel BrakingTorque >= Max Regen EM Torque .......................................................................................... 64

Figure B-4: Model of Power Management/Traction/SOCSOCmin ................................. 65

Figure B-6: Model of Battery/Current Calculation .................................................................... 65

Figure B-7: Model of Battery/Final Current Calculation ............................................................ 66

Figure B-8: Model of Battery/SOC and Voc Calculations ......................................................... 66


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, Power of EM for Charging the Battery (W) Power of Electric Motor (W)_ Maximum Power of Electric Moto based on EM Operating Map (W)

Power of Internal Combustion Engine (W)_ Maximum Power of Internal Combustion Engine based on ICE OperatingMap (W) Power of the Battery (W) Power Demand for Vehicle Traction or Braking (W), Internal Resistance of the Battery (Ohms)

Wheel Radius (m)

, Maximum Battery State of Charge (%), Minimum Battery State of Charge (%) Battery State of Charge (%) Torque of Electric Motor (Nm)_ Maximum Torque of Electric Motor based on EM Operating Map (Nm)

_ Torque of Electric Motor defined by the Power Management Controller(Nm) Torque of Internal Combustion Engine (Nm)_ Maximum Torque of Internal Combustion Engine based on ICE OperatingMap (Nm)_ Torque of Internal Combustion Engine defined by the Power ManagementController (Nm)

Torque Demand for Vehicle Traction (Nm)

Torque for Regeneration (Charging the Battery) (Nm) Applied Torque to the Vehicle Wheels for Vehicle Traction (Nm)TC Torque Coupler

UDC Urban Drive Cycle


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V Vehicle Speed (m/s)

, Open-Circuit Voltage of the Battery (V), Battery Terminal Voltage (V)

Vehicle Acceleration (m/s

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Overall Gear Ratio from Wheel to EM_ Gear Ratio of Electric Motor Gearbox Overall Gear Ratio from Wheel to ICE_ Gear Ratio of Internal Combustion Engine Gearbox_ Gear Ratio of the Torque Coupler-Motor Input_ Gear Ratio of the Torque Coupler-Engine Input Gear Ratio of Differential or Final Drive Ratio Overall Efficiency of Gears from Wheel to ICE Efficiency of EM based on its Efficiency Map_ Efficiency of Electric Motor Gearbox

_ Efficiency of Internal Combustion Engine Gearbox Overall Efficiency of Gears from Wheel to EM_ Efficiency of the Torque Coupler-Motor Input_ Efficiency of the Torque Coupler-Engine Input Efficiency of Differential

Density of air (kg/m3)

Rotational Speed of Electric Motor (rad/s) Rotational Speed of Internal Combustion Engine (rad/s) Rotational Speed of Vehicle Wheels (rad/s)


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1. Introduction

1.1. Disadvantages of Conventional Vehicles

Conventional cars are self-propelled vehicles that use an internal combustion engine for theirpropulsion. Most internal combustion engines burn a mixture of fuel (gasoline or diesel) and air

for providing the required power for the vehicle s traction. But, as we know these fuel types(gasoline and diesel) are petroleum products, which are derived from crude oil with the processof refinement. As a result of this, conventional vehicles have some disadvantages in regard totheir impact on the environment, the public health and the economy.

The main disadvantage of conventional vehicles is their contribution to global warming (theresult of the greenhouse effect), because of their emissions. The internal combustion enginesof these vehicles emit carbon dioxide CO2and other harmful gases (such as sulfur oxide SO2&nitrogen oxide NOx) due to the combustion of fossil fuels (gasoline or diesel). According to theUnited States Environmental Protection Agency, the most important greenhouse gas is CO2and transportation emissions is the 2nd largest source of CO2, accounting for the 31% of thetotal CO2 emissions (United States Environmental Protection Agency, n.d.). The category of

transportation emissions includes all the emissions from cars, airplanes, ships and trains. Also,it is worth mentioning that the CO2emissions in the US increased by 10% from 1990 to 2011and the use of conventional vehicles contributed to this increase, because cars were travellingmore and more miles at that time period (United States Environmental Protection Agency, n.d.).Moreover, the emissions of SO2 and nitrogen dioxide NOx are the main causes of a seriousenvironmental problem, which is called acid rain (United States Environmental Protection

Agency, 2012).

Another disadvantage of the conventional vehicles is that the combustion of fossil fuels is neverideal and the combustion products (such as carbon monoxide CO and unburned fuels) areharmful to people (Ehsani, et al., 2010). So, it is obvious that conventional vehicles areresponsible for serious environmental and health problems, because they release many

pollutants (such as CO2, CO, NOx& SO2gases) into the atmosphere, which are harmful to theenvironment and toxic to human health.

Furthermore, spark-ignition engines (type of internal combustion engines) are not very efficient,since their engine efficiency (fuel conversion efficiency) is about 30%, taking into considerationthe typical best values for specific fuel consumption and the typical heating values forcommercial petrol (Ehsani, et al., 2010). This means that the work produced per cycle by thepetrol engine is about 30% of the fuel energy supplied per cycle. But if we consider some otherfactors such as mechanical losses (due to transmission and frictions inside the engine), caraccessories (like air conditioner) and driving habits, the efficiency of regular cars is much lessthan 30%. At this point, it is worth mentioning that another reason for the poor fuel economyand efficiency of the regular cars is the dissipation of cars kinetic energy during braking

(especially under urban driving conditions). So, it is clear that the engine efficiency ofconventional vehicles is not at its maximum levels.

Moreover, conventional vehicles affect the energy dependence of a country, since hugeamounts of oil and petroleum products are imported for their final consumption in the transportsector. As far as the UK is concerned, its overall net import dependence for primary oil was45% in Q3 2013 (compared to the 42% of the same quarter last year), according to theDepartment of Energy & Climate Change (UK Government Services, 2014). In other words,the net imports of the UK for primary oil were 7.6 million tonnes only in Q3 2013, which is equalto 45% of the UKs refinery demand (UK Government Services, 2014). Primary oil is not only


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used in the transport sector, but it is also used for non-energy use and by domestic andindustrial users. In the UK, the final consumption of oil in the transport sector is almost 75% ofthe overall final consumption of oil (UK Government Services, 2014). The followingFigure 1-1(UK Government Services, 2014) shows the UK demand for transport fuels (unleaded gasoline,diesel and aviation fuels) from 2010 to Q3 2013, according to the UK Department of Energy &Climate Change (UK Government Services, 2014). As we can see in the following Figure,unleaded gasoline and diesel highly affect the final oil consumption in the transport sector. So,

it is clear that conventional vehicles have a great impact on the energy dependence of UK andits net imports of oil (including the relevant expenditures), since the consumption of gasolineand diesel accounts for the biggest part of the overall final consumption of oil.

Figure 1-1: UK Demand for Transport Fuels (UK Government Services, 2014)

The internal combustion engines of conventional vehicles run on non-renewable energysources (gasoline and diesel) and as we know the oil reserves will eventually be depleted.

According to the UK Energy Research Centre, the conventional oil production is estimated toreach its peak point before 2030 or 2020, but there is an extremely high risk for theproductions fall after the peak in 2020 or 2030 (Sorrel, et al., 2009). Moreover, the world needsto extract conventional oil from new oil resources before the timing of the peak, in order todelay the fall of oil production (Sorrel, et al., 2009). Also, it is worth mentioning that the rate ofoil production decreases increasingly and more than 60% of the existing capacity may need tobe replaced by 2030, otherwise the oil production may fall after 2030 (Sorrel, et al., 2009). So,it is clear that there are many uncertainties and risks related to the future of oil production andconsumption, affecting the oil and fuels prices. As a result of this situation, anotherdisadvantage of conventional vehicles is that their use is significantly affected by theseuncertainties and risks.

To sum up, all the disadvantages associated with the use of conventional vehicles (such asglobal warming, pollution and acid rain) are highly related to huge direct and indirect costs,which may be financial, human or both. For example, the problem of pollution induces directand indirect costs, such as health expenses for sick people (indirect costs) and expenses forreplanting forests devastated by acid rain (direct costs). The solution to these problems may beanother type of transportation, which has to be more sustainable, economically viable and ithas to result to cost savings for these problems.


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1.2. Hybrid Electric Vehicles (HEVs)

The first Hybrid Electric Vehicles (HEVs) were introduced at Paris Saloon in 1899 and thesevehicles were of parallel and series types (Ehsani, et al., 2010). Of course, other hybridvehicles of different types and powertrains have been developed and built since that time.

According to the authors Fallah et al. (2014), a hybrid electric vehicle (HEV) is a vehicle thatcombines the power outputs of an internal combustion engine and an electric motor for its

propulsion and can recover its kinetic energy using recuperation power systems. In otherwords, the traction requirements of such a vehicle are met by two propulsion power sources(internal combustion engine and electric motor): the main propulsion power source (internalcombustion engine) is responsible for the vehicles propulsionand the secondary one (electricmotor) assists the main one (Fallah, et al., 2014).

Generally speaking, hybrid electric vehicles consist of a power unit (internal combustionengine), a propulsion system (it transfers the generated power to the wheels), an energystorage system (battery) and an electric motor (Fallah, et al., 2014). The electric motor not onlyprovides the additional power for the vehicles propulsion, but it also acts as a generator,because it regenerates the power during regenerative braking (recuperating the vehicleskinetic energy). So, hybrid electric vehicles combine the advantages of electric drive with an

internal combustion engine and that is why; these vehicles are more efficient than conventionalvehicles.

At this point, it is worth mentioning that there is not only one type of hybrid electric vehicles andthese vehicles can be categorized in terms of their degree of hybridization. As it can be seen inTable 1-1 (Fallah, et al., 2014), the functionalities of the three main types of hybrid electricvehicles (micro-hybrid, mild-hybrid and full-hybrid) are presented. Looking at the same Table,we see that a micro-hybrid vehicle cannot be fully electrically driven and does not supply anyadditional torque when the engine is running, due to the small vehicles electric motor. But, thesmall motor of a micro-hybrid vehicle can be used for the functions of engine start/stop andregenerative braking (Fallah, et al., 2014). On the other hand, a mild-hybrid vehicle contains alarger electric motor (it can operate as a generator, as well), which is able to provide additionaltorque to the engine (approximately 10% of the engines maximum power), but such a vehiclecannot be fully electrically driven (Fallah, et al., 2014). So, mild-hybrid vehicles can offer thesame functionalities as micro-hybrid vehicles do, with the difference that mild-hybrid vehiclescan assist the engine providing additional torque. Finally, full-hybrid vehicles can offer the samefunctionalities as mild-hybrid vehicles do, but full-hybrid vehicles can provide more additionaltorque to the engine (at least 40% of the engines maximum power) and can be fully electricallydriven (due to their larger electric motor) (Fallah, et al., 2014).

Table 1-1: Types of HEVs in terms of degree of hybridization (Fallah, et al., 2014)

EngineStart/Stop

RegenerativeBraking

MotorAssist

Electric Drive

Micro-hybrid Yes Slight Slight No

Mild-hybrid Yes Yes Yes No

Full-hybrid Yes Yes Yes Yes

1.2.1. Comparison of Conventional, Hybrid Electric and Electric Vehicles

Since, the disadvantages of conventional vehicles have been discussed; hybrid electricvehicles need to be compared with other types of transportation such as electric and


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conventional vehicles, in order to understand the merits of hybrid electric vehicles. As far as thecomparison between hybrid electric and conventional vehicles is concerned, HEVs offerenhanced fuel economy and efficiency, lower noise levels (under driving conditions), loweremission levels, improved engine life and improved brake system life (Fallah, et al., 2014).

Also, it is worth mentioning that the HEVs operating cost is lower than the operating cost ofconventional vehicles (Fallah, et al., 2014), but the maintenance and purchasing costs of HEVsare higher than those of conventional vehicles. Other advantages of HEVs (in comparison with

conventional vehicles) are the better mileage, the lower oil dependence, the independency ofgrid electricity, the regenerative braking system (the HEV batteries are recharged periodically)and tax benefits.

On the other hand, electric vehicles (EVs) have zero emissions, since they do not have aninternal combustion engine providing the required power for traction. Also, EVs areindependent of oil, since they do not need any fuel (gasoline or diesel) for their propulsion.

Although these characteristics of electric vehicles are beneficial to the environment and people,they have some limitations regarding the distances that they can travel, their production costs(due to high battery costs) (Fallah, et al., 2014) and the recharge time and lifetime for theirbatteries (it depends on the source of electricity) (Westbrook, 2001). As a result of these, EVsare very expensive and cannot compete effectively with conventional vehicles (Westbrook,

2001). If the price of EVs was lower than the existing one, then they might not be profitablybuilt.

It is obvious that HEVs and electric vehicles offer significant advantages over conventionalvehicles. But, the economic efficiency and environmental impact of EVs depend on how theirbatteries are charged or on the source of electricity for charging their batteries. In case thatrenewable energy sources are used for supplying electricity and charging the EVs batteries,then EVs offer more advantages than even HEVs do (Granovskii, et al., 2006). Unfortunately, ifmore than 50% of electricity is generated by fossil fuels combustion, then HEVs offersignificantly more advantages than EVs and conventional vehicles do (Granovskii, et al., 2006).However, in case that the electricity that EVs need to charge their batteries is generated onboard, then EVs can compete effectively with the other two types of vehicles (Granovskii, et al.,

2006). For example, if an EV contains a gas turbine, which generates electricity with anefficiency of about 50%-60%, and a high capacity battery, then the EV can offer moreadvantages than even an HEV does (Granovskii, et al., 2006). Therefore, the use of HEVs is amore feasible solution compared to the EVs use, eliminating the disadvantages of conventionalvehicles use. EVs can be more advantageous than HEVs under some special circumstances(only if there is on board electricity generation in EVs or if the electricity comes from renewablesources).

This comparison can be better understood if we go through a case study (Sharma, et al., 2013)that analyzes the life cycle emissions of conventional vehicles, HEVs and EVs for Australiandriving conditions. According to this analysis (Sharma, et al., 2013), HEVs (in case of class-Eor executive vehicles) were found to have the lowest life cycle greenhouse gas emissions

among all the three types of vehicles, due to their enhanced fuel economy and their smallbatteries (compared to the large batteries of EVs). As far as the small vehicles (class-Bvehicles) are concerned, their electric configurations have the highest life cycle emissions, dueto their large batteries (which are charged by the grid electricity) and their consumption, incontrast to their conventional configurations which have the lowest emissions (Sharma, et al.,2013). Thus, it can be concluded that hybrid class-E vehicles are the most cost effective inorder to reduce the life cycle emissions in this class, but conventional class-B vehicles are themost cost effective in order to reduce the life cycle emissions.


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Another example that supports the use of HEVs as another type of transportation foreliminating the disadvantages of conventional vehicles is the comparison between Toyota Priusand its conventional internal combustion engine Toyota Corolla. According to this analysis, thehybrid Toyota Prius (which was the first commercial HEV) has better fuel economy, lowerpollutant and carbon dioxide emissions than the Toyota Corolla, as far as the US market isconcerned (Lave & MacLean , 2002). However, this analysis also states that HEVs (like Prius)can be a cost-effective solution to reducing emissions or improving fuel economy, only if the

fuel prices are high enough so that HEVs are attractive to consumers (Lave & MacLean ,2002). In other words, the hybrid Toyota Prius offers more advantages than the conventionalToyota Corolla does in terms of fuel economy and greenhouse gas emissions, but Prius is notso attractive to consumers because the gasoline prices are not high enough.

1.2.2. Economics of Hybrid Electric Vehicles (HEVs)

Since we have discussed the advantages of HEVs, we can have a look at the economics ofHEVs. Nowadays we see that there is an upward trend in sales of hybrid electric vehicles,because people have started realizing the benefits of hybrid electric vehicles compared to thecharacteristics of conventional vehicles. According to the International Council on CleanTransportation, the hybrid cars account for about 1% of all cars registrations in European Union

(EU), while the petrol and diesel cars account for 55% and 42%, respectively (InternationalCouncil on Clean Transportation, 2013). In Figure 1-2 (International Council on CleanTransportation, 2013), we can see the upward trend in sales of hybrid electric vehicles inEuropean Union from 2001 to 2012 and it is worth mentioning that the sales of hybrid cars inNetherlands was 4.5% of total passenger cars sales in 2012 (the highest market share ofhybrid cars in EU) (International Council on Clean Transportation, 2013).

Figure 1-2: Market share, gasoline hybrid-electric vehicles (in %) (International Council on Clean Transportation,2013)

A case study (Sharma, et al., 2012) that might explain the upward trend of the sales of HEVs isthe technical and financial analysis of conventional, electric and hybrid electric vehicles for the

Australian market. According to this analysis (Sharma, et al., 2012), the parallel HEV was themost cost effective vehicle type for this market, taking into consideration the uncertaintieswhich are related to variations in future fuel, battery and electricity prices. Also, as far as theexecutive vehicles (class-E vehicles) are concerned, the total cost of ownership of their parallelhybrid configurations was found to be unchanged to variations in fuel, battery and electricityprices (Sharma, et al., 2012). This means that HEVs of class-E may have a higher resale valuethan conventional vehicles of this class have, because the demand for HEVs increases moreand more and their price remains unchanged to the above mentioned variations.


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On the other hand, according to the same analysis, it was found that the electric or hybridelectric configurations of small vehicles (class-B vehicles) were not economically feasible, dueto their low operational energy requirements and high production cost (Sharma, et al., 2012).

As a result of this, conventional vehicles of class-B were the most cost effective vehicle type forthis market, taking into consideration their low lifetime cost (Sharma, et al., 2012).

Another reason that might explain the upward trend of the HEVs sales is the increase in fuels

(gasoline and diesel) prices in the past years. For example, according to the UK Parliament,both retail prices of gasoline and diesel were increased by about 73% from 2001 to 2012(Bolton, 2014). Therefore, if we take into consideration the benefits-advantages of HEVs(compared to EVs and conventional vehicles), the high resale value of HEVs, the costeffectiveness of HEVs and the increase of fuels prices, then the market share of HEVs hasbeen reasonably increased from 2001 to 2012 in EU. It is obvious that HEVs will attract moreconsumers in the next coming years, since they will help consumers to save money on annualfuel bills and protect the environment, maintaining a high resale value.

1.2.3. Powertrain Configurations of Hybrid Electric Vehicles (HEVs)

The powertrain configuration of an HEV is responsible for transmitting the power from the

engine and the motor to the wheels, in order the vehicle to be propelled. The main powertrainconfigurations of HEVs are parallel hybrid (single shaft and double shaft), series hybrid, power-split hybrid and compound hybrid.

1.2.3.1. Parallel Hybrid Powertrain

This configuration combines the power outputs of the internal combustion engine and theelectric motor (it can be used as a generator when needed), but each power output can beused independently or together with the other power output (Fallah, et al., 2014). Thecombination of both power outputs can be done with the use of mechanical speed-couplers ortorque-couplers. Parallel HEVs are characterized by enhanced efficiency, can travel longdistances and they are suitable for highway driving and cruising (Fallah, et al., 2014). Forexample, since this configuration allows to switch between both power outputs, the HEV can be

driven only by the electric motor (fully electric traction mode), when the vehicles speed is verylow without the internal combustion engine working (due to the engines low efficiency at lowvehicles speeds). There are two different types of the parallel hybrid powertrain which dependon engine characteristics, motor characteristics, vehicles performance specifications andpowertrain component limitations.

1.2.3.1.1. Single-Shaft Parallel Hybrid Powertrain

Figure 1-3 shows the single-shaft parallel hybrid powertrain in which the engine, the electricmotor and the transmission system are attached to the same shaft and both engine and motoroperate at the same speed (Fallah, et al., 2014). This configuration is easy to control and thetransmission system can be manual, automatic, automated manual or continuously variable

transmission (CVT) (Fallah, et al., 2014). This configuration can be categorized in pre-transmission and post-transmission parallel hybrid, depending on the position of the electricmotor. The pre-transmission parallel hybrid configuration has the motor being placed betweenthe engine & transmission system and being attached to the engine output shaft (Fallah, et al.,2014). In this configuration, which is mainly selected for mild-hybrid vehicles, both powersources can provide power to the wheels through the transmission system. The mainadvantage of this powertrain is that the generated torque is multiplied through the transmissionsystem before it is delivered to wheels, making the electric motor more efficient when it assiststhe engine during high power demands (Fallah, et al., 2014). The motor can charge the battery,


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when the vehicle is idling and the vehicle can be propelled only by the electric motor under lowand medium power demands (Fallah, et al., 2014).

Figure1-3: Single-Shaft Parallel Hybrid Powertrain (Fallah, et al., 2014)

On the other hand, the post-transmission parallel configuration has the motor being placedbetween the transmission system and final drive (Fallah, et al., 2014). The advantage of thisconfiguration is that the wasted energy during braking can be recuperated more easily than inpre-transmission configuration (Fallah, et al., 2014), because the motor is placed close to thefinal drive. However, this configuration requires a larger electric motor compared to the pre-transmission arrangement (Fallah, et al., 2014) for delivering the required traction forces,because the torque multiplication is not as high as in pre-transmission arrangement.

1.2.3.1.2. Double-Shaft Parallel Hybrid Powertrain

Figure 1-4 shows the double-shaft parallel hybrid arrangement in which the engine and themotor are mounted upon separate axes, providing power to the wheels separately (Fallah, etal., 2014). As it was mentioned above, the combination of both power outputs can be done withthe use of mechanical torque-couplers or speed-couplers. Also, in the same Figure, we can seeone variation of this powertrain, in which there are two transmission systems (gearbox for the

engine and reduction gears for the motor), which are located between the mechanical couplerand the motor/engine. The main advantage of this variation is that both the motor and theengine can operate at their optimum operating conditions, leading to enhanced overallefficiency, but the structure of this powertrain is expensive and complicated (due to theintegration of two transmission systems) (Fallah, et al., 2014).

Figure 1-4: Double-Shaft Parallel Hybrid Powertrain (Fallah, et al., 2014)

Another variation of this powertrain is that there is only one transmission system which isplaced between the mechanical coupler and the drive shaft (Fallah, et al., 2014). So, the


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transmission system handles both power outputs with the same ratios and this configuration ismainly used in vehicles with small engine and motor (Fallah, et al., 2014).

The third variation of this powertrain is that each wheel shaft is driven separately, i.e. theelectric motor drives one pair of wheels on one shaft and the engine drives the other pair ofwheels on the other shaft (Fallah, et al., 2014). Both power outputs are combined through theroad surface, i.e. batteries are charged by regenerative braking system or they can be charged

by the engine due to the power transfer through the road (Fallah, et al., 2014). The mainadvantage of this configuration, whose structure is simple, is that all wheels are driven, but itrequires lots of space, which could be used for the passengers or luggage and the enginecannot charge the battery if the vehicle is stopped (Fallah, et al., 2014).

1.2.3.2. Series Hybrid Powertrain

Figure 1-5 shows the series hybrid powertrain in which the vehicle is driven by the electricmotor, whose batteries are charged by a generator which is driven by a small internalcombustion engine (Fallah, et al., 2014). So, as it can be seen in the following Figure, there isno direct connection between the engine and the wheels, and the egnine can operate at itsoptimum operating conditions for driving the generator. The substantial difference between the

series and parallel configuration is that the motor of the series powertrain must be able toprovide much more power than the motor of the parallel powertrain, because the seriespowertrains motor is responsible for the whole vehicles traction (Fallah, et al., 2014). As it wasmentioned above, the motor in parallel powertrain assists the engine for the vehiclespropulsion when needed. The other difference between these two powertrains is that thetransmission system in the parallel powertrain is an expensive multi-gear transmission (fordriving all the wheels), while the transmission system in the series powertrain is much simplersince its motor can operate at a wide speed range providing high torque (Fallah, et al., 2014).

Also it is worth mentioning that regenerative braking system can counterbalance the powerlosses of the series powertrain, because this system recuperates the wasted vehicles kineticenergy under braking (Fallah, et al., 2014). The series powertrain is characterized by powerlosses, because the engines mechanical power is converted to electrical power and then it isconverted again to mechanical power.

Figure 1-5: Series Hybrid Powertrain (Fallah, et al., 2014)

1.2.3.3. Power-Split Hybrid Powertrain

Figure 1-6 shows the power-split (or series-parallel) hybrid powertrain in which the bestcharacteristics of series and parallel powertrains are combined, leading to a very efficienthybrid powertrain (Fallah, et al., 2014). As it can be seen in the following Figure, in this series-parallel powertrain, a mechanical link connects the engine to the wheels, in order the wheels to


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be driven directly by the engine (this is the difference between this powertrain and the seriespowertrain) (Fallah, et al., 2014). Also, the power-split powertrain has an additional electricmotor, which operates primarily as a generator (this is the difference between this powertrainand the parallel powertrain) (Fallah, et al., 2014). As a result of these, there are two powerpaths in this powertrain: the power is transferred to the wheels through the mechanical gearsystem (one path), but the power is also transferred to the wheels through the motor and thegenerator (second path) (Fallah, et al., 2014). Both power outputs are effectively combined

together by the power split device (seeFigure 1-6).

Furthermore, the motor and the engine can operate independently or together (like in parallelpowertrain) for providing the required power to the wheels and improving the overall efficiencyof the vehicle. Another advantage of this powertrain is the generation of electricity as thevehicle is driven at the same time, because the electric motor can operated as a generator(Fallah, et al., 2014). But, the main advantage of the series-parallel powertrain is the flexibilityof its power control, because the engine can be decoupled from the electric motor if needed(Fallah, et al., 2014). For example, the motor can help the engine at low speed, delivering therequired power to the wheels (for the vehicles traction) until the engine reaches its optimumoperating conditions. Therefore, the engine of the series-parallel powertrain is smaller, moreefficient and less flexible than a conventional internal combustion engine, but this powertrain is

very expensive and complicated due to its complex power management system (Fallah, et al.,2014).

Figure 1-6: Power-Split Hybrid Powertrain (Fallah, et al., 2014)

1.2.3.4. Compound Hybrid Powertrain

Figure 1-7 shows the compound hybrid powertrain which consists of one engine and twoelectric motors and it is similar to the power-split hybrid powertrain (Fallah, et al., 2014). But,the (bidirectional) functionality of the second electric motor in the compound hybrid powertrainis different from the (unidirectional) functionality of the second electric motor in the series-parallel hybrid powertrain (Fallah, et al., 2014). This means that the second electric motor in thecompound hybrid powertrain can operate as a generator or traction motor, whilst the second

motor in the series-parallel hybrid powertrain operates only as a generator. As a result of thisdifference, the compound hybrid powertrain is characterized by more operating modes than thepower-split hybrid powertrain is (Fallah, et al., 2014). For example, in the compound hybridpowertrain, the vehicle can be driven by all the available power sources, i.e. one engine andtwo electric motors. But, if we compare the compound hybrid powertrain with conventional 4-wheel drive vehicles, we see that the powertrain of the conventional 4-wheel drive vehicles isheavier, more noisy and less fuel-efficient, because of its mechanical components (such aspropeller shaft and differential) (Fallah, et al., 2014).


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Figure 1-7: Compound Hybrid Powertrain (Fallah, et al., 2014)

InFigure 1-7,we can see an example of a compound hybrid powertrain, in which the vehiclesfront axle can be driven by the electric motor, whilst the vehicles rear axle can be driven by thecombination of the engine and the second motor. As it can be understood, each axle of thevehicle can be driven separately, as far as the compound hybrid powertrain is concerned.There are two different arrangements for this type of hybrid powertrains: in the firstarrangement, the electric motor drives the vehicles front axle, whilst the vehicles rear axle isdriven by the engine and the second motor, and in the second arrangement the oppositehappens (Fallah, et al., 2014).

1.2.4. Plug-in Hybrid Electric Vehicles (PHEVs)

According to Fallah et al.(2014), plug-in hybrid electric vehicle (PHEV) is an HEV that utilizesenergy storage devices (such as batteries), which can be fully recharged by connecting theirplug to the grid and not only by the internal recharging systems. Like HEVs, PHEVs can bebuilt in different powertrains, such as series, parallel or series-parallel hybrid configurations(Fallah, et al., 2014). As far as the HEVs are concerned, the internal combustion engine is theprimary power source is and the secondary power source is the electricity. But, as far as the

PHEVs are concerned, the primary power source is electricity and the internal combustionengine is the secondary power source (Fallah, et al., 2014).

The advantages of PHEVs are that they have minimum emission levels (they can have zeroemissions on electricity mode, like battery electric vehicles), they are less dependent on fuelsthan HEVs, they have better fuel economy than conventional vehicles and they can travellonger distances than battery electric vehicles (Fallah, et al., 2014). However, since PHEVsrequire larger batteries than HEVs, PHEVs are heavier and more expensive (due to higherbatteries cost) than HEVs (Fallah, et al., 2014).

1.2.5. Power Management

To begin with, in a conventional vehicle and an HEV, there are different types of power sourcesand loads, but the power must be properly distributed to the loads. A conventional vehicle hastwo kinds of loads (mechanical propulsion & electrical loads) and two kinds of power sources,which are an IC engine (energy converter) and a battery (electrical energy storage device) (Mi,et al., 2011). However, in HEVs, there are at least two power sources, but their battery is muchlarger than the battery of a conventional vehicle (Mi, et al., 2011). It is worth mentioning that theload demand (i.e. the required power) and the load characteristics (e.g. the torque & speedcharacteristics of mechanical propulsion) vary with time (Mi, et al., 2011). Since there areseveral types of power sources in a conventional vehicle or in an HEV, this means that eachpower source has different performance, power characteristics and efficiency (Mi, et al., 2011).


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Therefore, having more than one power sources available, a particular load can be met at aspecific moment by one or more power sources in terms of load performance and powersourcesefficiency (Mi, et al., 2011).

More analytically, as we know an internal combustion engine has a better efficiency in aspecific region of its efficiency-torque-speed map, whilst an electric motor is even more efficientthan an IC engine, over a large range in the torque-speed region (Mi, et al., 2011). This means

that the electric motor will be more efficient than the internal combustion engine at a specifictorque-speed point and vice versa. For example, if we assume that an HEV has to be driven atlow speeds for a short distance, the loads demand and the characteristics of the vehicle can bemet by the electric motor (power source) without the engine running, because the motor ismore efficient than the engine at these speed levels. It can be clearly understood that, in anHEV, there are more drive modes (such as electric drive mode) than in a conventional vehicle,because the hybrid powertrain consists of more components than the conventional powertraindoes.

This is the main idea for why we need power management, in terms of vehicles performanceand efficiency, because there are several drive modes, power sources and types of loads in anHEV. The aims of a power management strategy are to maximize the vehicles efficiency and

performance and to minimize the vehicles emissions(Fallah, et al., 2014). These aims can beachieved by a power management strategy, which is responsible for controlling andcoordinating the power generation, the power flow among the vehicles subsystems and formanaging the power delivery to the vehicles loads (mechanical or electrical), in terms ofvehicles efficiency and performance(Fallah, et al., 2014).


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2. Literature Review

As it was mentioned in the previous section, HEVs are becoming more and more popular in themarket, which means that lots of research has been conducted into these vehicles regardingtheir powertrain architectures (see section 1.2.3) and their power/energy managementtechniques. This section provides an overview of the surveys of the proposed powermanagement strategies and reports the benefits and drawbacks of each power management

strategy. Also, comprehensive descriptions and comparisons of these control strategies arepresented in this section.

Each power management strategy should maximise the vehicles fuel economy, minimise thevehicles emissions and the cost of the control system, and provide safe and good vehicledriving performance (Chau & Wong, 2002). Therefore, the power management controller is acrucial factor of the HEV design process and the overall effectiveness of each strategy-controller is checked against drive cycles, like the New European Driving Cycle (NEDC) andthe US Environment Protection Agency (EPA) Drive Cycles.

As it can be seen inFigure 2-1,the power management strategies can be categorized into therule-based control strategies and the optimization-based control strategies (Bayindir, et al.,

2011). Moreover, looking at the same Figure, the rule-based control strategies consist ofdeterministic strategies and fuzzy control strategies, while the optimization-control strategiesconsist of real-time optimization techniques and global optimization techniques (Bayindir, et al.,2011). The following subsections provide a more detailed analysis of these control techniques.

Figure 2-1:Classification of Power Management Strategies(Bayindir, et al., 2011)

2.1. Rule-Based Control Strategies

To begin with, rule-based control strategies are simple and suitable for providing real-timesolutions to the power split problem in the hybrid powertrain (Kim, et al., 2012; Ambuhl, et al.,2010). Moreover, the operation of the rule-based controllers is based on simple rules, whichare independent of any prior knowledge of the drive cycle and these rules are determinedaccording to human experiences, heuristics, intuition and mathematical models (Bayindir, et al.,2011). Also, according to other studies (Salmasi, 2007; Bayindir, et al., 2011), the function ofthese controllers is based on the principle of the load-levelling, which means that the operating


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point of the vehicles primary power source is moved as close as possible to its best point (apre-set value) for every time point of the vehicle operation.

The concept of the load-levelling can be better explained with an example. Assuming that theprimary power source of an HEV is the engine, i.e. the vehicle is an ICE-dominated HEV, thenthe actual engines operating point is moved as close as possible to its most efficient point (at agiven engine speed), if the rule-based controller aims to maximise the vehicles efficiency

(Bayindir, et al., 2011). Moreover, taking into consideration the HEV of this example, if the rule-based controller aims to maximise the vehicles fuel economy, then the actual operating pointof the ICE is moved as close as possible to its best point (Bayindir, et al., 2011).

At this point, it is worth mentioning that in these strategies the best vehicles fuel economy (i.e.lower fuel consumption) can be achieved at lower ICE torques and lower ICE speeds than themost optimum/efficient point of the engine (Salmasi, 2007). So, as it can be seen the fuelconsumption of the vehicle depends on the power demand, which is based on the driverscommand for acceleration (Salmasi, 2007). Although, the implementation of rule-basedcontrollers can lead to low vehicles fuel consumption, such controllers become very sensitive ifthe rules are tuned/changed (Ambuhl, et al., 2010).Figure 2-2 shows the categorization of therule-based control strategies. The following subsections provide a more detailed analysis of the

various rule-based control strategies.

Figure 2-2: Classification of Rule-Based Strategies (Bayindir, et al., 2011; Jalil, et al., 1997; Kim, et al., 2014)

2.1.1. Deterministic Rule-Based Control Strategies

An article by Bayindir et al.(2011) states that the deterministic rule-based control strategies arebased on deterministic rules, which are defined according to the ICE efficiency, fuel andemissions maps, the human experiences, the electric motor (EM) efficiency maps, and the

analysis of the power flow/distribution in the hybrid drive train. According to another author,such controllers are set offline and all of their parameters are implemented via lookup tables(Salmasi, 2007). As we can see inFigure 2-2,these techniques can be distinguished betweenThermostat Control Strategy, Hybrid Thermostat Control Strategy, Power Follower (Baseline)Control strategy, Modified Power Follower Control Strategy, Power Split Control Strategy andState Machine Based Strategy (Jalil, et al., 1997; Bayindir, et al., 2011; Kim, et al., 2014).


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2.1.1.1. Thermostat Control Strategy

The thermostat control strategy is the most common and simplest power management strategyfor HEVs and it can be used for real-time control, since it is based on instantaneousoptimisation (Gao, et al., 2009). This control strategy tries to keep the batterys state of charge(SOC) between its upper and lower pre-defined limits, by turning the engine on and off (Jalil, etal., 1997; Onea & Babici , 2013). In this strategy, the electric motor is used as the primary

power source and the internal combustion engine is used as the secondary power source,which assists the electric motor when the power demand is high (Jalil, et al., 1997; Onea &Babici , 2013). Also, it is worth mentioning that this strategy either operates the engine at itsoptimal torque (for a given rotational speed) or not, in order to maximise/optimise the enginesefficiency (Khan, et al., 2005; Jalil, et al., 1997; Onea & Babici , 2013; Gao, et al., 2009).

Moreover, in case that the generated optimal torque by the engine is greater than the torquedemand, then the remaining torque is used by the motor for charging the battery (Khan, et al.,2005; Jalil, et al., 1997). In this strategy, if the battery SOC reaches its lower pre-defined limit,the engine turns on and operates at its most optimum/efficient point, but the internalcombustion engine will turn off when the battery SOC reaches its upper pre-defined limit, andthe cycle is repeated (Jalil, et al., 1997). But, according to the work of Onea and Babici (2013),

in this strategy, if the battery SOC is lower than its lower limit and the power demand is higherthan the available power, then both the engine and the electric motor operate simultaneously(i.e. hybrid propelling drive mode) in order to deliver the required power for the vehiclestraction.

However, the main drawback of the thermostat controller is that it does not adjust the powerflow/distribution in the hybrid powertrain according to the power demand and it does notoperate according to an optimisation criterion, such as the vehicles fuel efficiency (Khan, et al.,2005). According to the same authors, another drawback of this strategy is that the battery ischaracterised by high power losses (due to the heat generated by the battery internalresistance), because the current that goes back to the battery is usually high, since theremaining torque generated by the engine is related to this current (Khan, et al., 2005). Also, ifthe power demand is high, then the current that will flow from the battery to the electric motorwill also be high leading to high power losses, because the internal resistive losses in thebattery increase dramatically (Kim, et al., 2014).

2.1.1.2. Power Split Control Strategy

In order to improve the thermostat control strategy, Jalil et al.(1997) proposed a power splitstrategy a series HEV, which implies that the internal combustion engine and the batteryoperate only at their most efficient points. Like the thermostat control strategy, the power splitcontrol strategy can be used for real-time control, as well (Gao, et al., 2009). Also, the variablesof this strategy are the drivers command for acceleration/deceleration, the battery SOC andthe power demand, which is estimated by a high gain Proportional-Integral (PI) controller thatcontrols the acceleration of the vehicle (Jalil, et al., 1997). It is worth mentioning that this

strategy uses the same rules as the Thermostat control strategy does, for charging the batteryand thus the batterys charge efficiency was found to be the same in both strategies (Jalil, etal., 1997).

Moreover, the main difference between the thermostat and the power split strategies is that inthe latter strategy the energy delivered by the battery is reduced, because the engine is onlyused whenever it will operate at its most optimum/efficient point (Jalil, et al., 1997). Therefore, itwas found that in the power split strategy the batterys discharge efficiency and the fuelefficiency were higher by 11.6% & 11.5% respectively in the urban cycle and by 11.8% & 6.4%


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respectively in the highway cycle, when compared to the thermostat strategy (Jalil, et al.,1997).

2.1.1.3. Power Follower (Baseline) Control Strategy

Like the two previously mentioned deterministic strategies (see sections 2.1.1.1 and2.1.1.2),the power follower strategy can be used for real-time control, as well (Gao, et al., 2009). In this

strategy, which is very popular, the primary power source is the engine and the electric motor isthe secondary power source, which supplies the additional power that the vehicle needs for itstraction (Salmasi, 2007). According to the same author (Salmasi, 2007), the following rules areused by the baseline strategy:

a) The electric motor is responsible for vehicles traction, when the vehicle speed isfrom zero to a certain value.

b) If the power demand is higher than the power generated by the engine, then themotor supplies the remaining power.

c) The batteries can be charged by the motor during vehicles braking (regenerativebraking mode).

d) The engine turns off, when the power demand is less than a specific value at the

engines operating speed, in order for the engine to not operate inefficiently.e) If the battery SOC is lower than its lower limit, then the electric motor charges thebattery using the additional power generated by the engine.

In this strategy, the battery charging and discharging operations are minimised/optimised,which means that the battery power losses are reduced and the battery life is extended whencompared to the thermostat control strategy (Gao, et al., 2009). However, the maindisadvantage of this strategy is that the engine is usually operated at less efficient points thanits optimum operating points, while in the thermostat strategy the engine is usually operated atits optimal region (Kim, et al., 2014).

2.1.1.4. Hybrid Thermostat Control Strategy

In order to improve the thermostat and the power follower control strategies, Kimet al.(2014)proposed a hybrid thermostat control strategy for a series hybrid bus, which combines the twopreviously mentioned strategies. According to the same authors (Kim, et al., 2014), the hybridthermostat strategy implies that the engine is operated at its most efficient points and the highcurrents in the battery are avoided. It is worth mentioning that this strategy has manysimilarities with the power follower strategy, because the engine and the battery are used in asimilar way (Kim, et al., 2014).

In addition, the hybrid thermostat strategy is based on the thermostat strategy, but in the hybridthermostat strategy, the battery is always charged by the electric motor/generator when thebattery SOC is between its upper and lower limits (Kim, et al., 2014). Also, according to thesame authors work (Kim, et al., 2014), in the hybrid thermostat strategy if the power demand is

higher than a pre-defined standard/value, then forced generation will start. In other words, if thebattery SOC reduces from its upper limit to its lower limit, the engine will turn off in thethermostat strategy, but the engine will turn on in the hybrid thermostat strategy due to theforced generation.

As a result of this, in the hybrid thermostat strategy the fuel economy was improved by 7.78%,when compared to the thermostat strategy and by 11.28%, when compared to the powerfollower strategy (Kim, et al., 2014). Therefore, it can be seen that the hybrid thermostatstrategy overcomes the drawbacks of the thermostat and the power follower strategies, whiletaking advantage of their strengths.


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2.1.1.5. State Machine Control Strategy

According to the authors Phillips et al. (2000), the state machine strategy is another type ofdeterministic rule-based strategies, which determines the vehicles operating mode, using thefollowing three steps:

1ststep:All the possible vehicles operating states have to be defined, e.g. engine

propelling mode or hybrid propelling mode 2ndstep:All the possible transitions between the vehicles operating states have to

be defined, according to the driver commands and vehicle operating condition

3rd step: All the possible transitions have to be analysed, in order for asingle/exclusive transition to be guaranteed.

More analytically, in the 1st step of this strategy, all the possible operating states of eachvehicles subsystem (i.e. engine, motor, battery etc.) are defined and combined together, e.g.engine is turned on and the clutch is engaged, whilst the impossible combinations areeliminated, e.g. engine is turned on and the clutch is not engaged (Phillips, et al., 2000). As faras the 2ndstep is concerned, the transitions between the operating states depend on system orsubsystems faults and on changes in the driver commands and the vehicles operating

condition (e.g. engine warms up) (Phillips, et al., 2000). However, it is easily understood that allthese transitions should achieve the performance and drivability targets of the vehicle (Phillips,et al., 2000). The 3rd step implies that single transitions from one operating state to anothershould be guaranteed, because transitions to more than one operating states are possibleunder the same driver commands and vehicle operating conditions (Phillips, et al., 2000).Thats why; there are priorities, which are based on driver commands, energy managementand system faults, between all the operating states, so that single transitions are guaranteed(Phillips, et al., 2000).

For example, assuming that there is an electric motor fault (i.e. subsystem fault) and that thevehicle was in electric drive mode, the strategy implies the transition from the electric drivemode to the engine drive mode, if the power demand (based on the driver command) is equal

to the power generated by the engine. But, using the same example, if the power demand wasless than the power generated by the engine (i.e. change in the driver commands), then thestrategy would imply the transition from the electric drive mode to the charge mode, in whichthe remaining power generated by the engine would be used by the generator for charging thebattery.

However, Salmasi (2007) questioned the effectiveness of the state machine strategy, becausethis strategy has no comparative advantage over the other rule-based strategies. According tothe same author (Salmasi, 2007), the implementation of the state machine strategy may lead tofaulty supervisory control of the power flow in the hybrid powertrain, since this strategy doesnot guarantee the optimization of the vehicles performance objectives, i.e. fuel economy andemissions.

2.1.2. Fuzzy Rule-Based Control Strategies

On the other hand, fuzzy logic controllers are also rule-based controllers, but they use adecision-making property for providing real-time and suboptimal power split in the hybridpowertrain (Bayindir, et al., 2011). According to the authors Pouramasad and Montazeri (2008),fuzzy logic controllers are mainly based on the concept of load-levelling (see section 2.1),according to which the internal combustion engine is operated at its optimal region and themotor is used as a load-levelling device. So, as it can be seen, these controllers do not usedeterministic rules, compared to the deterministic controllers. Also, it is worth mentioning that


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fuzzy logic controllers can be classified into (see Figure 2-2) Conventional Fuzzy LogicControllers, Adaptive Fuzzy Logic Controllers and Predictive Fuzzy Logic Controllers (Bayindir,et al., 2011).

Another benefit of such controllers is that, they do not only optimise the power flow/distributionwithin the hybrid powertrain components (i.e. motor, engine and battery), like the deterministiccontrollers do, but fuzzy logic controllers optimise the vehicles fuel economy as well (Salman,

et al., 2000). As a result of the improved fuel economy, the vehicles emissions are alsoimproved and the battery charging pattern is well balanced, when compared to deterministiccontrollers (Lee & Sul, 1998).

Moreover, according to the work of Salmasi (2007), fuzzy logic controllers are highly tolerant ofinaccurate measurements and component variations/uncertainties and they are easilyadaptable to component variations/changes, since they can be easily tuned, if necessary. Inother words, such controllers are not sensitive to various disturbances, like the load conditionsor the different drivers commands (Lee & Sul, 1998).Thats why; Bayindiret al. (2011) statethat these controllers is the most logical solution to the problem of the power split in a hybridpowertrain (for all forms of HEVs), which can be considered as a non-linear, multi-domain andtime-varying problem.

However, fuzzy logic controllers require more computations and they are more time-consumingwhen compared to deterministic rule-based controllers (Poursamad & Montazeri, 2008). Also,according to the same authors work (Poursamad & Montazeri, 2008), the effectiveness offuzzy logic controllers can be questioned, because they usually fail to achieve a satisfactoryoverall vehicles efficiency if they have been designed based on engineering intuitions. Also,the vehicles driving performance characteristics are not usually considered by this strategy(Poursamad & Montazeri, 2008).

2.2. Optimization-Based Control Strategies

As far as the optimization-based controllers are concerned, these controllers aim to minimisethe vehicle fuel consumption and/or emissions, defining the optimal distribution of the powerdemand within all the vehicle components, i.e. engine, motor/generator and brakes (Salmasi,2007; Khan, et al., 2005). More analytically, according to the work of Bayindir et al.(2011), anoptimization-based controller defines the optimal gear ratios and the optimal output torquesfor the power sources (i.e. engine and motor), minimising the cost function, which is typicallyrepresented by the vehicles fuel consumption and/or emissions. Also, such controllers takeinto consideration the future & current torque demands and all the possible engine-motortorque pairs that meet the physical & energy limitations of each component, in order to definethe optimal operating points of the vehicles engine and motor, with or without prior knowledgeof the drive cycle (Bayindir, et al., 2011; Khan, et al., 2005).

Optimisation-based controllers are more sophisticated and complicated control methods thanrule-based controllers are, because optimal controllers allow the vehicle to reach its optimum

fuel efficiency. Thats why; optimisation-based strategies require more computations and theyare more time-consuming, when compared to rule-based strategies (Ambuhl, et al., 2010). Asan example of this statement, a recent comparative study conducted by McGehee and Yoon(2013), on control strategies for a mild HEV (see section 1.2), clearly states that theoptimisation-based control strategy allows the mild HEV to have a better fuel efficiency by12.5% (with good driving performance) over the EPA drive cycle, when compared to adeterministic rule-based control strategy. Another example of the superiority of theoptimisation-based controllers over the rule-based controllers is the findings of another study(Khan, et al., 2005), which clearly state that the implementation of an optimisation-based


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controller in a parallel HEV reduces the fuel consumption by 55.7% on a highway drive cycleand by 42.4% on a city drive cycle, when compared to a thermostat rule-based controller.

Figure 2-3 shows the classification of the optimisation-based control strategies. The followingsubsections provide a more detailed analysis of the various optimisation-based controllers.

Figure 2-3: Classification of Optimization-Based Control Strategies (Bayindir, et al., 2011; Delprat, et al., 2004)

2.2.1. Global Optimization-Based Control Strategies

As it was previously mentioned, the optimisation-based controllers may operate with or withoutprior knowledge of the drive cycle. So, according to Salmasi (2007), if an optimisation-based

controller operates over a fixed/given drive cycle (in other words, with prior knowledge of thedrive cycle), then a global solution can be found by performing global optimisation. The solutionof the global optimisation aims to minimise not only the vehicles fuel consumption andemissions throughout the given drive cycle, but also the cumulative energy loss throughout thiscycle (Bayindir, et al., 2011). Also, it is worth mentioning that several solutions of globaloptimisation have been developed for achieving the performance targets and minimising thecost function. As it can be seen inFigure 2-3, the global optimisation-based strategies consistof Linear Programming, Control Theory Approach, Dynamic Programming (DP), Stochastic DP,Game Theory, Simulated Annealing and Genetic Algorithm (Bayindir, et al., 2011; Delprat, etal., 2004).

As it was previously mentioned in section2.1.1.1,the implementation of thermostat rule-based

controllers leads to high battery power losses, but this drawback can be overcome with theimplementation of global optimisation-based controllers that are based on genetic algorithm.Moreover, according to a comparative study conducted by Amiri et al. (2009) on thermostatcontrollers and global optimisation (genetic algorithm) controllers for a series HEV, clearlystates that the latter controllers reduce the vehicles fuel consumption by 8.5% and the batteryenergy losses by 33.6%, when compared to thermostat controllers.

However, according to some authors (Salmasi, 2007; Gao, et al., 2009; Perez & Pilotta, 2009;Sciarretta, et al., 2004; Paganelli, et al., 2000), the global optimisation strategy is not directly


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In addition to the previous statement, a recent comparative study on real-time and globalcontrol strategies (Pontryagins Minimum Principle (PMP) strategy and Dynamic Programming(DP) strategy, respectively) for a power-split HEV (see section 1.2.3.3), clearly states that areal-time optimisation controller based on PMP can provide a global optimum solution,assuming that only the vehicles fuel consumption needs to be minimised (Kim, et al., 2011).

According to the same authors work (Kim, et al., 2011); the difference between the optimalcontrol results from PMP and DP strategies is almost 1%.

Another difference between the global and real-time optimisation-based controllers is that thereal-time controllers can be easily adapted in real-time varying driving conditions, without priorknowledge of the drive cycle (Salmasi, 2007; Sciarretta, et al., 2004; Chasse, et al., 2009).Thats why; Bayindir et al.(2011) and Sciarretta et al.(2004) stated that the cost function of thereal-time controllers depends on variables at the current time, such as instant fuel consumptionand stored electrical energy. Another aspect of the real-time optimisation-based strategies isthat the electrical self-sustainability of the vehicle should be guaranteed, because the vehiclesbattery is only charged by the engine and means of regenerative braking and not by anexternal device (Sciarretta, et al., 2004).

However, it is worth mentioning that real-time optimisation-based controllers do not always

provide the most optimal solution, when compared to global optimisation-based controllers. Asan example of this statement is the comparative study conducted by Paganelli et al.(2000), onECMS real-time and Simulated Annealing (SA) global strategies, which states that theimplementation of ECMS controller reduces the fuel consumption of a parallel HEV by 4%when compared to a SA controller.

At this point, it is worth comparing a real-time optimisation-based strategy, which is theEquivalent Fuel Consumption Minimisation Strategy (ECMS), with two deterministic rule-basedstrategies, which are the Thermostat Strategy (see section 2.1.1.1) and the Power FollowerStrategy (see section 2.1.1.3). According to a comparative study conducted by Gao et al.(2009), on these control strategies for a series HEV, the ECMS provides the best fuel economyover various urban and highway drive cycles, optimising the power distribution/flow between

the engine-generator pack and the battery pack as well. Also, the results that can be obtainedby the implementation of an ECMS controller are very close to the results of a globaloptimisation-based controller (Gao, et al., 2009).

Another comparison between a real-time optimisation-based controller (PMP controller) andtwo deterministic rule-based controllers (power follower and heuristics controllers) has beendone by Keulen et al.(2012), using a parallel HEV for their simulations. According to the sameauthors work (Keulen, et al., 2012), a PMP controller reduces the vehicles fuel consumptionby 7%-16% when compared to the power follower controllers, and by 3% when compared tothe heuristics controller.


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3. Modelling of Hybrid Electric Vehicle

3.1. Generic Vehicle Model

All the vehicle simulations were performed based on the backward-looking modeling approach,which first determines the power that the vehicle needs, in order to be propelled for a givendrive cycle. The second step of this modeling approach is the determination of the power

demand for each component of the vehicle powertrain and then the actual power of eachcomponent is calculated, taking into consideration its efficiency. This modeling approach canonly be used for simplified quasi-static models, because it does not take into account thedynamics of each vehicle component.

The hybrid electric vehicle (HEV), which was used in these simulations, was a double-shaftparallel hybrid electric vehicle with a mechanical torque-coupler (see section1.2.3.1.2).Figure3-1 shows a generic vehicle model of a double-shaft parallel HEV in a backward-lookingarchitecture. As it can be seen, the desired vehicle speed which is based on the given drivecycle goes back from the vehicle model to the hybrid drive train. As a result of this, it can befound out how each powertrain component is used, in order for the vehicle to follow the givendrive cycle. Also, the power management controller is responsible for dealing with the power

distribution and control between all the components of the hybrid drive train, in order for thevehicle power demand to be satisfied. It is worth mentioning that the power managementcontroller of these simulations was a thermostat controller, which is actually a rule-basedcontroller.

Figure 3-1: Generic Vehicle Model of a Double-Shaft Parallel HEV in a Backward-Looking Architecture

At this point, it is worth mentioning that the drive cycles of these simulations were the NewEuropean Drive Cycle (NEDC) and the City Cycle (FTP-75). The NEDC consists of 4 repeatedECE-15 urban drive cycles (UDC) and one extra-urban drive cycle (EUDC), as it can be seen inFigure A-1 (Fallah, et al., 2014). The NEDC is modeled as a look-up table indexed by time (inseconds) and speed (m/s). The average speed of the NEDC is 33.6 km/h, whilst its total time is1,180 seconds, its total distance is 11.017km (Barlow, et al., 2009) and its maximum speed is120 km/h (Fallah, et al., 2014). The UDC represents the driving conditions of big Europeancities and it is characterized by a maximum speed of 50 km/h and low engine loads. On theother hand, the EUDC represents the highway (or extra-urban) driving conditions and it ischaracterized by higher speeds and higher engine loads than the UDC. Generally, the NEDC ischaracterized by several constant speed periods, several idling stops and lowaccelerations/decelerations.


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Also, the FTP-75 represents a city drive cycle, which consists of three phases, which are a coldstart phase, a transient phase and a hot- start phase, as it can be seen in Figure A-2 (Fallah, etal., 2014). This drive cycle is characterized by an average speed of 31.5km/h, a maximumspeed of 91.2 km/h, a total distance of 17.77km and a duration of 1874 seconds (Fallah, et al.,2014).

3.1.1. Longitudinal Dynamics of the Vehicle Model

As far as the longitudinal dynamics and planar kinematics of the vehicle are concerned, thedrive force , the aerodynamic resistance force and the rolling resistance force are only considered. It is worth mentioning that the effects of the suspension to the vehicledynamics have not been taken into consideration.

To begin with, the summation of all forces acting on the vehicle can be given by = = .3.1

where m is the vehicle mass (kg) and is the acceleration of the vehicle, which is thesecond derivative of the vehicle position with respect to time (Fallah, et al., 2014).The drive force can be given by

= = .3.2where is the vehicle acceleration (m/s2) and it is equal to the derivative of the vehicle velocitywith respect to time. The vehicle velocity can be obtained by the drive cycle in m/s.The aerodynamic force can be given by

= 12

2 .3.3where is the air density (kg/m3), A is the frontal area of the vehicle (m2) and is thedrag coefficient (Fallah, et al., 2014).

The rolling resistance force can be obtained by = .3.4

where is the rolling resistance coefficient and g is the gravitational acceleration (m/s2)(Fallah, et al., 2014).

The vehicle needs some power

, in order to overcome the rolling resistance and

aerodynamic drag forces and to follow the given drive cycle. In other words, this poweris required for vehicle traction and it can be obtained by = ( + + ) .3.5

However, it is worth mentioning that can be the braking power that the vehicle neesin order to decelerate. Also, the torque required for vehicle traction, which is equalto the applied torque to the vehicle wheels , can be obtained by


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= = .3.6where is the rotational speed of the vehicle wheels (rad/s) and it can be obtained by

= .3.7where is the wheel radius (m).Finally, it is worth mentioning that there is no gradient resistance force acting on the vehicle,because it is assumed that the whole simulation is performed with zero road slope angle.

3.2. Powertrain Model of a Double-Shaft Parallel HEV with Torque Coupler

Figure 3-2 shows the configuration of a double-shaft parallel HEV with a torque coupler thatwas used in these simulations. As it can be seen, there are two transmission systems, one islocated between the internal combustion engine (ICE) and the torque coupler (TC) and theother one is located between the electric motor (EM) and the torque coupler (TC). Moreanalytically, there is a single reduction gear between the EM and TC, which means that the

high torque characteristics of the EM can be exploited at low speeds. On the other hand, thereis a three-gear transmission system between the ICE and TC, in order for the ICE to operate atits optimal region. The purpose of each transmission system is to amplify the torque, which isdelivered from each power source to the wheels, maintaining the operation of each powersource at its optimal region.

Figure 3-2: Powertrain Model of a Double-Shaft Parallel HEV with Torque Coupler

3.2.1. Overall Gear Ratios and Efficiencies

The overall gear ratio from the wheel to the ICE can be obtained by = _ _ .3.8


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where _ is the gear ratio of ICE gearbox, which is not constant, because it changesaccording to the vehicle speed. Also, _ is the gear ratio between the TC and ICE inputand is the gear ratio of the differential or the final drive ratio.Similarly, the overall gear ratio from the wheel to the EM can be obtained by

= _ _ .3.9where _is the gear ratio of EM single reduction gear, which is constant, because there is asingle reduction gear unit. Also, _is the gear ratio between the TC and EM input.The overall efficiency of gears from the wheel to the ICE can be obtained by

= _ _ .3.10where _ is the efficiency of the ICE gearbox, _is the efficiency between the TC andICE input and

is the efficiency of the differential.

Similarly, the overall efficiency of gears from the wheel to the EM can be obtained by = _ _ .3.11

where _ is the efficiency of the EM single reduction gear and _ is the efficiencybetween the TC and ICE input.

3.2.2. Internal Combustion Engine (ICE)

The rotational speed of the ICE can be obtained by

= .3.12As it can be seen, the rotational speed of the engine depends on the gear ratio of theengine gearbox and the rotational speed of the wheel , which depend on thevehicle speed (based on the given drive cycle).

Moreover, the torque of the ICE can be obtained by = .3.13

More analytically, the engine torque is based on the whole torque demand (or wheeltorque) , which is required for the vehicle traction in no hybrid drive mode. So, the enginefuel rate [ ]in no hybrid mode can be found in with the use of the ICEfuel rate map (Figure A-3 ofAppendix) (Fallah, et al., 2014), taking into consideration and . The engine fuel rate map is modeled as a look-up table indexed by and. The combined fuel consumption of the vehicle (in no hybrid mode) for NEDC can beobtained in 100 by


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= 11.017 100 .3.14

where the total distance of NEDC is 11.017km.

However, the engine torque_can be obtained by the ICE operating map (Figure A-4ofAppendix)(Fallah, et al., 2014), which is modeled as a look-up table indexed by and_. This operating map shows the maximum torque that the engine can provide for agiven rotational speed . The _is used by the power management controller forthe vehicle traction (not for vehicle braking), because the thermostat control strategy eitheroperates the engine (as a secondary power source) at its optimal region or not (Ehsani, et al.,2010).

The engine torque _ is defined by the power management controller and it is theengine torque required for the vehicle traction in hybrid drive mode. So, the engine fuel rate

[] in hybrid mode can be found as the engine fuel rate [ ] in no hybrid mode.The combined fuel consumption of the vehicle (in hybrid mode) for NEDC can be obtained in

100 by

= 11.017 100 .3.15

3.2.3. Electric Motor

The rotational speed of the EM can be obtained by = .3.16

So, the rotational speed of the motor depends on the rotational speed of the wheel, which depends on the vehicle speed (based on the given drive cycle).Moreover, the torque of the EM, , can be obtained by

= .3.17More analytically, the motor torque is based on the whole torque demand (or wheeltorque) , which is required for the vehicle traction. The is used by the powermanagement controller, because the thermostat control strategy uses the motor as the primarypower source for the vehicle traction (Ehsani, et al., 2010).

Also, the motor torque_can be obtained by the EM operating map (seeFigure A-5)(Fallah, et al., 2014), which is modeled as a look-up table indexed by and _.This operating map shows the maximum torque that the motor can provide either for charging

the battery or for the vehicle traction, at a given rotational speed . The _ isused by the thermostat controller for charging the battery, during the vehicle braking, when the_is lower than or equal to(Ehsani, et al., 2010).

At this point, it is worth mentioning that the motor power, ,, which charges the battery,can be obtained by


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, = .3.18where is the motor efficiency and is the torque generated by the EM that chargesthe battery during the vehicle traction or braking. The can be obtained by the EM efficiencymap (see Figure A-6) (Fallah, et al., 2014), which is modeled as a look-up table indexed by and . The may either be provided by the EM, which uses theremaining power of ICE (when the power demand is less than the power that the engine canprovide operating at its optimal region) during the vehicle traction, or be provided by the EMduring the vehicle braking (Ehsani, et al., 2010).

3.2.4. Battery

For this research, a simplified model of battery is used and the instantaneous battery State ofCharge can be obtained (as a percentage of the battery capacity) by

= 1,

.3.19

where

is the initial state of charge of the battery,

,is the theoretical capacity of

the battery and is the batter current (Fallah, et al., 2014). Also, the battery current is therate of charge, which can be obtained by (Fallah, et al., 2014)

= , .3.20Also, the rate of change of the for a time interval can be obtained by

= , , 4 , 2 , , .3.21

where ,is the open circuit voltage of the battery, ,is the internal resistance of thebattery and is the power of the battery, which is equal to , (Fallah, et al., 2014

power management control design for a hybrid electric vehicle

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