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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
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