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SERIES HYBRID VEHICLES:DESIGN AND CONTROL

Pierre DuysinxUniversité de Liège

Academic Year 2010-2011

ReferencesR. Bosch. « Automotive Handbook ». 5th edition. 2002. Societyof Automotive Engineers (SAE)C.C. Chan and K.T. Chau. « Modern Electric VehicleTechnology » Oxford Science Technology. 2001.C.M. Jefferson & R.H. Barnard. Hybrid Vehicle Propulsion. WIT Press, 2002.J. Miller. Propulsion Systems for Hybrid Vehicles. IEE Power & Energy series. IEE 2004.M. Ehsani, Y. Gao, S. Gay & A. Emadi. Modern Electric, HybridElectric, and Fuel Cell Vehicles. Fundamentals, Theory, andDesign. CRC Press, 2005.

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OutlineIntroduction

Operation pattern

Control strategies

Sizing of major components

Design examples

IntroductionAdvantages of series hybrid vehicles vs Internal Combustion Engines

Zero emission mode in city drivingMultiple sources of energyHigher efficiency

Disadvantages of present technologiesLimited drive range due to shortage of energy storage of in-board batteriesLimited payload and volume capacity due to heavy and bulky batteriesLonger battery charging time

Initial objective of series hybrid vehicles: extending the driverange by adding an engine / generator to charge the batteries on board

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Introduction

Typical series hybrid electric drive train

IntroductionSeries hybrid vehicle are propelled by their electric traction motorTraction motor is powered by

Battery packEngine / generator unit

Engine / generatorHelps to power the traction motor when load power is largeCharges batteries when load power is small

Motor controllerControl the traction motor to produce power required by vehicle motion

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IntroductionVehicle performances (acceleration, gradebility, max speed) are completely determined by the size and characteristics of traction motorSizing of motor and gears of transmission are the same as pure electric vehiclesDrive train control is different form pure electric vehicles because of additional engine / generatorBatteries

Act essentially as a peak power source (PPS)Can be replaced by ultra capacitors of flywheels

IntroductionObjective of this lesson:

Design of engine / generator designOperation control and strategyBattery (PPS) sizing

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Operation patternsEngine is mechanically decoupled from driven wheels

Speed and torque of engine is independent from vehicle speed and torque traction demandCan be controlled independently at any operating point on speed torque mappingIn particular control to operate at optimal generation performance(minimum of fuel consumption and emissions)Optimal regime is realizable but depends on operating modes and control strategies of drivetrain

Operation patternsSelection of several operating modes accordingly to driving conditions and desires of driver

1/ Hybrid traction modeWhen a large of amount of power is demanded (e.g. acceleration pedal is deeply depressed)Both engine / generator and batteries supply the power to the electric motor driveEngine operates in operational region for efficiency and emissionsPPS supplies additional power to meet the demand

/demand e g PPSP P P= +

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Operation patterns2/ Peak power source alone traction mode

Operating mode in which PPS alone supplies its power to meet power demand

3/ Engine / generator alone traction modeEngine / generator alone supplies power to meet power demand

/ 0demand PPS

e g

P PP

=

=

/

0demand e g

PPS

P P

P

=

=

Operation patterns4/ PPS charging from engine / generator

When energy of PPS is too low, PPS has to be charged using regenerative braking or by engine / generatorRegenerative braking is generally too small or insufficient and engine / generator is divided in two parts: car propulsion and charge of PPS

This mode is only possible if

/e g demandP P≥

/demand e g PPSP P P= −

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Operation patterns5/ Regenerative braking mode

When vehicle is braking traction motor can be used as generator, regenerative braking converts kinetic energy into energy charge of batteries

PPS brakingP P= −

Operation patternsVehicle controller commands the operation of each component according to

The traction power and torque demand using driver demandThe feedback information of components and drivetrainPreset control strategies

Objectives of controller:Meet power demand from driverOperate components in optimal efficiencyRecapture the maximum braking energyMaintain the state of charge (SOC) of batteries in a preset window

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Control strategies

Control rules programmed in vehicle controller and command operation of each component

Control rules make use of command of the driver and feedback information from drivetrain and components status

Control rules make decision on proper operation modes

Performance of drivetrain depends on controller quality and control strategy

Typical control strategiesMaximum state of charge of batteries

Engine on/off control strategies

Max state-of-charge control strategyTarget of max state-of-charge control strategy

Meet power demand given by driver

AND in the same time Maintain SOC of batteries at its highest possible level

Application: Vehicles whose performances rely heavily on Peak Power Source (frequent stop and go driving patterns)Vehicles such as military vehicles for which carrying on the mission is of the highest importance

High SOC levelGuarantees the highest performance of the vehicle at any time

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Max state-of-charge control strategy

Point A: hybrid traction mode

Point B:

If

If

/traction e gP P≥

/traction e g PPSP P P= +

/traction e gP P≤

maxSOC SOC≤

maxSOC SOC=

/traction e g PPSP P P= −

/traction e gP P=

Max state-of-charge control strategy

Point B:

If engine/ generator alone traction mode

IfPPS charging mode

/traction e gP P≤

maxSOC SOC≤

maxSOC SOC=

/traction e g PPSP P P= −

/traction e gP P=

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Max state-of-charge control strategy

Point C:

Hybrid braking mode

Point D:

Regenerative braking mode

/braking e gP P≥

/braking e g brakeP P P= +

/braking e gP P=

/braking e gP P≤

Max state-of-charge control strategy

Control flowchart of max SOC of batteries control strategy

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On-off control strategy

In some situations such as long time driving with low / moderate load on highway at constant speed, batteries can be easily charged at their max SOC level.Engine / generator is forced to work at a power that is smaller than its optimum efficiency pointEfficiency is reducedIn this case engine on-off (bang-bang controller) is appropriate

Operation is completely controlled by the SOC of the batteries or PPSEngine can be operated in its optimal efficiency map during ENGINE ON periods

On-off control strategyWhen SOC > preset top level:

Engine / generator is OFFVehicle propelled by batteries

When SOC < present bottom levelEngine / generator is ONVehicle is propelled using engine/ generator

/traction e g PPSP P P= −

traction PPSP P=

Illustration of thermostat control

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Sizing of major componentsSizing of major components of series hybrid drive train

Traction motorEngine / generatorBatteries or peak power sources

Power rating of these components is the first important step of the system design

Design constraintsAcceleration performanceHighway driving / urban drivingEnergy balance in batteries

Power rating of traction motorSimilar procedure to power rating of pure electric motor driveMotor characteristics and transmission are completely determined by vehicle acceleration performance requirementFirst iteration, acceleration performance (time to accelerate from zero to Vf)

M, mass of vehicle; γ correction factor for effective massTa: time to accelerateVb speed corresponding to switch from constant torque mode to constant power mode

Exact performance evaluation requires a verification using a simulation code

2 2 32 1( )2 3 5t f b f x f

a

mP V V m g f V S C Vt

γ ρ= + + +

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Power rating of traction motorTraction efforts and traction power vs vehicle speed with a low and a high gear ration

Low gear ratio: a-b-d-e trace during acceleration

High gear ratio: c-d-e trace

max

bVxV

=

Power rating of traction motorPower rating versus speed ratio

max

bVxV

=

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Power rating of engine / generatorEngine / generator is used to supply steady state power in orderto prevent batteries from being discharged completely

Design of engine / generator considers two driving situationsDriving for a long time with constant speed (highway driving, intercity driving)Driving with frequent start and stops (city driving)

Driving for a long time with constant speedEngine / generator and drivetrain do not rely on batteries to support operation at for instance 130 km/hEngine / generator is able to produce sufficient power to support this speed

Power rating of engine / generatorDriving in frequent stop and go

Engine / generator produce sufficient power to maintain energy storage at a certain level while enough power can be drawn to support vehicle accelerationEnergy consumption is closely related to control strategy

In the design of engine / generator system, power capability should be greater than the power selected during constant speed (highway driving) or average power when driving in urban areas (evaluated using typical standard drive cycles).

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Power rating of engine / generatorEstimation of power output needed to cruise at constant speed on a flat road

Power is much less than for acceleration

Ex. Passenger car (1500 kg) at 130 km/h: P ~ 35 kW

Effect of transmission efficiency and motor efficiency: increase by 2à to 25% of power

2/

12e g x

t m

VP m g f S C Vρηη

⎛ ⎞= +⎜ ⎟⎝ ⎠

Power rating of engine / generatorDuring stop and go patterns in urban areas, power generation by engine / generator must be equal or slightly greater than average power load to maintain the balance of PPS energy storageAverage load power:

When vehicle is able to recover kinetic energy, average power consumed in acceleration / deceleration (second term) is zero otherwise it is greater than zero

2

0 0

1 1 12

T T

ave xdVP m g f SC V Vdt m dt

T T dtρ γ⎛ ⎞= + +⎜ ⎟

⎝ ⎠∫ ∫

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Power rating of engine / generator

2

0

0

1 12

1

T

ave x

T

P m g f SC V VdtT

dVm dtT dt

ρ

γ

⎛ ⎞= +⎜ ⎟⎝ ⎠

+

∫

∫

Design of batteries and PPSBatteries must be capable of delivering sufficient power to traction motor at any timeBatteries store sufficient energy to avoid failure of power delivery during too-deep discharging

POWER CAPACITYTo fully use electric motor power capacity energy source power and engine / generator must be greater than max rated power of electric motor

max max

/ /mot mot

PPS e g PPS e gm m

P PP P P Pη η

+ ≥ ⇔ = −

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Design of batteries and PPSENERGY CAPACITY

In some driving conditions, frequent acceleration / decelerationdriving patterns result in low SOCDetermine energy changes in PPS during typical driving cycles to determine energy capacity of peak power sources

Check that

0

T

PPSE P dt∆ = ∫[ ]min max,E SOC SOC∆ ∈

Design of batteries and PPS

Instantaneous power and average power with full and zero regenerative braking in typical drive cycles

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Design of batteries and PPSRange of SOC depends on the operating characteristics of PPS.

Max efficiency of batteries in range [0,4;0,7]Ultra capacitors have a limited range rate [0,8;1,0]

max

max mincap

EESOC SOC

∆=

−

Design exampleDesign specification

M=1500 kgf = 0,01 Cx= 0,3 S=2 m²Transmission ratio efficiency ηt=0,9

Performance specificationsAcceleration time (0 to 100 km/h): 10 +/- 1 sMaximum gradebility: 30% @ low speed and 5% @ 100 km/hMaximum speed 160 km/h

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Design of traction motor sizeEquation for motor power rating

Assume x=4 (induction motor), it comes

2 2 32 1( )2 3 5t f b f x f

a

mP V V m g f V S C Vt

γ ρ= + + +

82,5tP kW=

Design of traction motor size

Characteristics of traction motor vs motor rpm and vehicle speed

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Design of gear ratioGear ratio is designed such that vehicle reaches its maximum speed at maximum motor rpmUse

Data Nnom=5000 rpm, vmax = 44,4 m/s (160 km/h) and Re=0,2794 m

It comes

max

max30en Ri

Vπ

=

3,29i =

Verification of acceleration time

Simulated acceleration time and distance vs speed

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Verification of gradebility

Traction effort and resistance of the vehicle vs speed

Verification of gradebilityGradebility is calculated using tractive effort and resistanceFrom plots of resistance vs speed it can be checked that gradebility is satisfiedImplies that for a passenger car, the power needed for acceleration is usually larger than needed for gradebilityAcceleration power determines generally power rating of motor

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Design of engine / generatorPower rating of engine / generator is designed to be capable of supporting the vehicle motion during highway drive (130 km/h) on level roadData: efficiency of transmission: 90%, motor: 85%, generator: 90%Power of 32,5 kw is sufficientIt can maintain a speed of 78 km/h with a 5% road

Design of engine / generatorSecond consideration in power rating of engine / generator: average power when driving with typical stop and go driving cycles

9,327,8949,8120ECE-1

23,0018,3077,4128US06

14,1012,6079,697,7FTP 75 highway

4,973,7627,986,4FTP 75 urban

Average powerwith no regen. Brabking (kW)

Average powerwith full regen. braking

Average speed (km/h)

Max speed (km/h)

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Design of engine / generatorCompared to power needed for gradebility and acceleration, average power of city driving is smallerP=32,5 kW is sufficient

Design of engine / generator

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Design of Power Capacity of PPSSum of output power of engine/ generator and PPS should be greater than input power of traction motor

/82,5 32,5 0,9 67,80,85

motorPPS e g

motor

PP P kWη

= − = − × =

Design of energy capacity of PPSEnergy capacity heavily depends on drive cycles and control strategyHere as power capacity of engine / generator is much greater than average power, the bang / bang controller is chosenSimulation results for FTP 75 urban driving cycle using regenerative braking

Control strategy for batteries:Maximum energy variation: 0,5 kWhOperation in the SOC range of 0,2, that is in [0,4;0,6] for optimal efficiency

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Design of energy capacity of PPSControl strategy for supercaps

Operation in the SOC range of 0,2 will limit terminal voltage to10%Total energy in supercapacitors

max 0,5 2,50,2PPS

EE kWhSOC∆

= = =∆

Design of energy capacity of PPS

Simulation results for FTP75 urban drive cycle

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Design of energy capacity of PPS

Simulation results for FTP75 highway drive cycle

Fuel consumptionFuel consumption is evaluated using simulation on two FTP75 drive cycles.For FTP75 urban driving, one estimates the fuel consumption to 17,9 km/lFor FTP75 highway drive cycles, one has a fuel consumption of 18,4 km/lFuel consumption is lower than with ICE because of high operating efficiency of the engine and the significant energy recovery from regenerative braking