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PARALLEL HYBRID ELECTRIC 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
Control strategiesMaximum state-of-charge of peak power source strategyEngine on-off strategy
Sizing of major components
IntroductionParallel hybrid drivetrains allow both engine and electric traction motor to supply their power to the driven wheelsAdvantages of parallel hybrid electric vehicles vs series hybrid
Generator not necessaryElectric traction motor smallerReduce multiple conversions of energy from engine to driven wheels not necessary
Higher overall efficiency
CounterpartControl of parallel hybrids more complex because of mechanical coupling between and driven wheelsDesign methodology may be valid for only one particular configurationDesign results for one configuration may be not applicable for agiven environment and mission requirements
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Introduction
Configuration parallel torque coupling hybrid electric drive train
IntroductionDesign methodology of parallel hybrid drivetrains with torque coupling which operate on the electrically peaking principle
Engine supplies its power to meet the base load operating at a given constant speed on flat and mild grade roads or at an average load of a stop-and-go drive pattern.Electrical traction supplies the power to meet the peaking fluctuating part of the load.
This is an alternative option to mild hybrids
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IntroductionIn normal urban and highway driving, base load is much lower than peaking loadEngine power rating is thus lower than electrical power ratingBecause of better torque / speed characteristics of electric traction motors (compared to engine) a single ratio transmissionfor traction motor is generally sufficient
IntroductionObjective of this lesson:
Design of parallel hybrid electric drivetrain with torque coupling
Design objectives:Satisfy the performance requirements (gradeability, acceleration, max cruising speed, etc.)Achieve high overall efficiencyMaintain battery state-of-charge (SOC) at reasonable levels during operation without charging from outside the vehicleRecover maximum of braking energy
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Control strategies of parallel hybrid drive trains
Available operation mode in parallel hybrid drive train;Engine alone tractionElectric alone tractionHybrid traction (engine + electric motor)Regenerative brakingPeak power source (batteries) charging mode
During operation, proper operation modes should be used to:Meet the traction torque requirementsAchieve high level of efficiency Maintain a reasonable level of SOC of PPSRecover braking energy as much as possible
Control strategies of parallel hybrid drive trains
Control level based on a two-level control schemeLevel 2: vehicle level = high level controllerLevel 1: low level controller = subordinate controllers
Engine, motor, brake, battery, etc.
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Control strategies of parallel hybrid drive trains
Overall control scheme of the parallel hybrid drive train
Control strategies of parallel hybrid drive trains
Vehicle system level controllerControl commanderAssign torque commands to low level controllers (local or component controllers)Command based on
Driver demandComponent characteristics and feed back information from components (torque, speed)Preset control strategies
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Control strategies of parallel hybrid drive trains
Component controllersEngine, motor, batteries, brakes, torque coupler, gear box, clutches, etc.Control the components to make them work properlyControl operations of corresponding components to meet requirements from drive train and prescribed values assigned by system controller
Vehicle system controller has a central role in operation of drive train
Full fill various operation modes with correct control commands to each componentsAchieve a high efficiency
Maximum PPS state-of-charge strategy
When vehicle is operating in a stop-and-go driving pattern, batteries must deliver its power to the drive train frequently.PPS tends to be discharged quickly. So maintaining a high SOC is necessary to ensure vehicle performanceMax state-of-charge is an adequate option.
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Maximum PPS state-of-charge strategy
Various operation modes based on power demand
Maximum PPS state-of-charge strategy
Motor alone propelling mode:If vehicle speed is below a preset value Veb, vehicle speed below which engine cannot operate properly in steady state Electric motor alone supplies power to the driven wheelsEngine is shut down or idling
,
0
0
0
e
Lm
t m
mPPS d
m
PPP
PP
η
η−
=
= >
= <
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Maximum PPS state-of-charge strategy
Hybrid propelling mode:Load demand such as A is greater than engine powerBoth engine and motor have to deliver their power to the wheels simultaneouslyEngine operates at its max efficiency line by controlling the throttle to produce Pe
Remaining power is supplied by the electric motor
Maximum PPS state-of-charge strategy
Hybrid propelling mode:Engine operates at its max efficiency line by controlling the throttle to produce Pe
Remaining power is supplied by the electric motor
,
,
( ) 0
0
0
opte e
L e t em
t m
mPPS d
m
P P v RP P
P
PP
ωη
η
η−
= = >
−= >
= <
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Maximum PPS state-of-charge strategy
Batteries / PPS charging mode:When the power demand is less than the power produced by engine in its optimum operation line (as in point B)When batteries are below their max SOCEngine is operating in optimum lineMotor works as a generator and coverts the extra power of engine into electro power stored in batteries
Maximum PPS state-of-charge strategy
Batteries / PPS charging mode:
, ,,
( ) 0
0
0
opte e
Lm e t e m m
t e
PPS c m
P P v R
PP P
P P
ω
η ηη
−
= = >
⎛ ⎞= − <⎜ ⎟⎝ ⎠= >
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Maximum PPS state-of-charge strategy
Engine alone propelling mode:When load power demand (point B) is less than power engine can produce while operating on its optimum efficiency lineWhen PPS has reached its maximum SOCEngine alone supplies the power operating at part load Electric motor is off
,
00
Le
t e
m
PPS
PP
PP
η=
==
Maximum PPS state-of-charge strategy
Regenerative alone braking mode:When braking demand power is less than maximum regeneration capability of electric motor (point D)Electric motor is controlled to work as a generator to absorb the demand power
, 0m braking L t m m
PPS c m braking
P P
P P
η η−
− −
= <
=
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Maximum PPS state-of-charge strategy
Hybrid braking mode:When braking demand power is greater than maximum regeneration capability of electric motor (point C)Electric motor is controlled to provide its maximum braking regenerative powerMechanical brakes provide the remaining part
max 0m braking m braking m
PPS c m braking
P P
P P
η− −
− −
= <
=
Maximum PPS state-of-charge strategy
Flowchart of max SOC of PPS strategy
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On-off control strategy
Similar strategy to the one used in series hybrid drive trainEngine on-off strategy may be used in some operation conditions
with low speed and moderate accelerations
When engine can produce easily enough extra power to recharge quickly the batteries
Engine on-off is controlled by the SOC of PPS or batteries
When SOC reaches its max level, engine is turned off and vehicle is propelled in electric motor only mode
When SOC reaches again its low level, engine is turned on and propelled by the engine in PPS charging mode until max SOC is reached
On-off control strategyWhen SOC reaches its max level, engine is turned off and vehicle is propelled in electric motor only mode
When SOC reaches again its low level, engine is turned on and propelled by the engine in PPS charging mode until max SOC is reached
Illustration of thermostat control
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Design of parallel hybrid componentsKey parameters
Engine powerElectric motor powerGear ratio of transmissionsBatteries or peak power sources
Great influence on vehicle performance and operation efficiency
Design methodologyPreliminary choice based on performance requirementsAccurate selection with accurate simulations
Illustrative design exampleDesign specification
M=1500 kgf = 0,01 Re=0,279 m Cx= 0,3 S=2 m²Transmission ratio efficiency: ηt,e=0,9 ηt,m=0,95
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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Power rating of engineEngine should supply sufficient power to support vehicle operation at normal constant speed on both flat or mild grade road without the help of PPS
Engine should be able to produce an average power that is larger than load power when operating in a stop-and-go pattern
Power rating of engineOperating on highway at constant speed on flat road or mild grade road
Illustrative exampleV=160 km/h requires 42 kWWith a 4 ratio gear boxEngine allows driving
road at 5 % at 92 km/h in gear 4road at 5 % at 110 km/h in gear 3
, ,
1( ² sin )2
rese x
t e t e
P VP m g f S C V mgρ θη η
= = + +
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Power rating of engineEngine supplies average power requirement in stop-and-go driving cycles
The average power depends on the degree of regeneration braking. Two extreme cases: full and zero regenerative braking:
Full regenerative braking recovers all the energy dissipated in braking and can be calculated as aboveNo regenerative braking, average power is larger so that when power is negative, third term is set to zero.
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0 0
1 1 12
T T
ave xdVP m g f SC V Vdt m dt
T T dtρ γ⎛ ⎞= + +⎜ ⎟
⎝ ⎠∫ ∫
Power rating of engine
Instantaneous and average power with full and regenerative braking in typical drive cycles
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Power rating of engineAverage power of engine must be greater than average power load.Problem is more difficult than in series hybrid because engine is coupled to the driven wheels
Engine rotation speed varies with vehicle speed Engine power varies with rotation speed and vehicle speed
Calculate the average power the engine can produce with full throttle during the driving cycle
0
1 ( / , )T
ave e eP P v R i dtT
ω= =∫
Power rating of engine
Average power of a 42 kW engine
42 kW Engine is OK
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Design of electric motor power capacity
Electric motor function is to supply peak power to drive trainDesign criteria: provide acceleration performance and peak power demand in typical drive cyclesDifficult to directly design motor power for prescribed acceleration performanceMethodology
Provide good estimatesFinal design with accurate simulation
Assumption to calculate initial estimatesSteady state load (rolling resistance, areo drag) handled by engine while dynamic load (acceleration) handled by motor
Design of electric motor power capacity
Acceleration related to the torque output of the electric motor
Power rating
Illustrative examplePassenger car Vmax=160 km/h, Vb=40 km/h, Vf=100 km/hta (0 – 100km/h)=10 s, γ=1,04
, ,m t m t m
e
T i dvmR dtη
γ=
( )2 2
,2m f bt m a
mP V Vt
γη
= +
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Design of electric motor power capacity
Illustrative examplePassenger car Vmax=160 km/h, Vb=40 km/h, Vf=100 km/hta (0 – 100km/h)=10 s, γ=1,04Pm= 74 kW
Design of electric motor power capacity
The approach overestimates the motor power, because the engine has some remaining power to accelerate the vehicle alsoAverage remaining power of the engine used to accelerate the vehicle
Depends on gear ratio, so that it varies with engaged gear box and increase with gear box
,1 ( )a
i
t
e accel e resta i
P P P dtt t
= −− ∫
Engine remaining power: 17 kWPm=74 – 17 = 57 kW
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Design of electric motor power capacity
When power rating of engine and electric motor are initially designed, more accurate calculation has to be carried out to evaluate the vehicle performances:
Max speedGradebilityAcceleration
Gradebility and max speed can be obtained from the diagram of tractive effort and resistance forces vs speed
Design of electric motor power capacity
Illustrative example
At 100 km/h, gradebiltiy of 4,6% for engine alone and 18,14% for hybrid mode
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Design of electric motor power capacity
Illustrative example
Acceleration performance for 0-100 km/h:
ta=10,7 sd=167 m
Transmission designTransmission ratio for electric motor
Because electric motor supplies peak power and it has high torque at low speed, a single ratio transmission between motor and driven wheel is generally sufficient to produce high torque for hill climbing and acceleration
Transmission ratio for engineMulti gear transmission between engine and wheels can enhance the vehicle performances
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Transmission designMulti gear transmission ratio between engine and wheels
(+) Increase remaining power of the engine and vehicle performance (acceleration and gradebility)(+) Energy storage can be charged with the large engine power(+) Improve vehicle fuel economy because engine operates closer to its optimal speed
(-) More complex system(-) Heavier and larger(-) Complicated automatic gear shifting control
Design of batteries and PPSBatteries and PPS are sized according to power and to energy capacity criteria
POWER CAPACITYBattery power must be greater than the input electric power of the electric motor
maxm
PPSm
PPη
≥
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Design of batteries and PPSENERGY CAPACITY
Related to energy consumption in various driving patterns (mainly full load acceleration and typical driving cycles)Evaluate energy drawn from PPS and from engine during acceleration period
Illustrative example: Energy from batteries 0,3 kWh
0
at mPPS
m
PP dtη
= ∫
0
at
e eP P dt= ∫
Design of batteries and PPSENERGY CAPACITY
Energy capacity must meet the energy requirements during driving pattern in drive cycles
For a given control energy strategy charging and discharging power of energy storage can be obtained from simulation
Generally energy consumption (and capacity sizing) is dominated by full load acceleration
0 00( )at
PPS c PPs dE P P dt− > − <∆ = −∫
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Design of batteries and PPS
Simulation results for FTP75 urban driving cycle
Max energychange: 0,11 kWh
Design of batteries and PPSNot all energy stored can be used to deliver power to the drive train
Batteries: low SOC will limit power output and reduce efficiencybecause of internal resistance increaseUltracapacitors: low SOC results in low voltage and affects the performancesFlywheels: low SOC is low flywheel velocity and low voltage at electric machine to exchange port
Only part of the stored energy can be available for usePart available is given by a certain percentage of its SOC
[ ]min max,E SOC SOC∆ ∈
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Design of batteries and PPSEnergy capacity of the energy storage
Illustrative example∆E= 0,3 kWhSOCmax-SOCmin=0,3Ecap= 1 kWh
max
max mincap
EESOC SOC
∆=
−
Accurate simulationWhen major components have been designed, the drive train has to be simulatedSimulation on typical drive train brings useful information:
Engine powerElectric motor powerEnergy changes in energy storageEngine operating pointsMotor operating pointsFuel consumption
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Accurate simulation
Accurate simulation
