Speh Hev Parallel 2012

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PARALLEL HYBRID ELECTRIC VEHICLES:DESIGN AND CONTROL

Pierre Duysinx

Research Center in Sustainable AutomotiveTechnologies of University of Liege

Academic Year 2012-2013


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References R. Bosch. Automotive Handbook . 5th edition. 2002. Society

of Automotive Engineers (SAE)

C.C. Chan and K.T. Chau. Modern Electric Vehicle

Technology 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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Outline Introduction

Control strategies

Maximum state-of-charge of peak power source strategy

Engine on-off strategy

Sizing of major components


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Introduction Parallel hybrid drivetrains allow both engine and electric traction

motor to supply their power to the driven wheels

Advantages of parallel hybrid electric vehicles vs series hybrid

Generator not necessary (save one component) Electric traction motor is smaller

Reduce the multiple conversions of energy from the engine to thedriven wheels Higher overall efficiency

Counterparts Control of parallel hybrids is more complex because of the

mechanical coupling between the ICE engine and the driven wheels Design methodology may be valid for only one particular

configuration

Design results for one configuration may be applicable for only aparticular given environment and mission requirements


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Introduction

Configuration parallel torque coupling hybrid electric drive train


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Introduction Design 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 averageload of a stop-and-go drive pattern.

Electrical traction supplies the power to meet the peakingfluctuating part of the load.

This is an alternative option to mild hybrids


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Introduction In normal urban and highway driving, base load is much lower

than peaking load

Engine power rating is thus lower than electrical power rating

downsizing Because of better torque / speed characteristics of electric

traction motors (compared to engine) a single ratio transmissionfor traction motor is generally sufficient


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Introduction Objective 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 efficiency

Maintain battery state-of-charge (SOC) at a reasonable levels

during operation without charging from outside the vehicle Recover a 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 traction

Electric alone traction

Hybrid traction (engine + electric motor) Regenerative braking

Peak power source (batteries) charging mode

During operation, proper operation modes should be used to:

Meet the traction torque requirements

Achieve high level of efficiency

Maintain a reasonable level of SOC of PPS

Recover braking energy as much as possible


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Control strategies of parallel hybrid drive trains

Control level based on a two-level control scheme

Level 2: vehicle level = high level controller

Level 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


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Control strategies of parallel hybrid drive trains Vehicle system level controller

Control commander

Assign torque commands to low level controllers (local or

component controllers) Command based on

Driver demand

Component characteristics and feed back information from components(torque, speed)

Preset control strategies


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Control strategies of parallel hybrid drive

trains Component controllers

Engine, motor, batteries, brakes, torque coupler, gear box,clutches, etc.

Control the components to make them work properly

Control operations of corresponding components to meetrequirements from drive train and prescribed values assigned bysystem controller

Vehicle system controller has a central role in operation of drive

train Full fill various operation modes with correct control commands to

each components

Achieve a high efficiency


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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 vehicleperformance

Max state-of-charge is an adequate option.


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Maximum PPS state-of-charge strategy

Various operation modes based on power demand


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Maximum PPS state-of-charge strategyElectric Motor alone propelling mode:

If the vehicle speed is below a presetvalue Veb, a vehicle speed below which

the engine cannot operate properly insteady state

Electric motor alone supplies power tothe driven wheels

Engine is shut down or idling

,

0

0

0

e

Lm

t m

m

PPS d

m

PP

P

P

P


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Maximum PPS state-of-charge strategyHybrid propelling mode:

Load demand such as A is greater thanthe engine power

Both engine and motor have to delivertheir power to the wheels simultaneously

Engine operates at its max efficiency lineby controlling the throttle to produce Pe

Remaining power is supplied by the

electric motor


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Maximum PPS state-of-charge strategyHybrid propelling mode:

Engine operates at its max efficiency lineby controlling the throttle to produce Pe

Remaining power is supplied by theelectric motor

,

,

( ) 0

0

0

opt

e e

L e t e

m

t m

mPPS d

m

P P v R

P P

P

PP


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Maximum PPS state-of-charge strategyBatteries / PPS charging mode:

When the power demand is less than thepower produced by engine in its optimum

operation line (as in point B) When batteries are below their max SOC

Engine is operating in optimum line

Motor works as a generator and covertsthe extra power of engine into electro

power stored in batteries


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Maximum PPS state-of-charge strategyBatteries / PPS charging mode:

, ,

,

( ) 0

0

0

opt

e e

Lm e t e m m

t e

PPS c m

P P v R

PP P

P P


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Maximum PPS state-of-charge strategyEngine alone propelling mode:

When load power demand (point B) isless than power engine can produce while

operating on its optimum efficiency line When PPS has reached its maximum SOC

Engine alone supplies the poweroperating at part load

Electric motor is off

,

0

0

L

et e

m

PPS

PP

P

P


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Maximum PPS state-of-charge strategyRegenerative alone braking mode:

When braking demand power is less thanmaximum 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


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Maximum PPS state-of-charge strategyHybrid braking mode:

When braking demand power is greaterthan maximum regeneration capability of

electric motor (point C) Electric motor is controlled to provide its

maximum braking regenerative power

Mechanical brakes provide the remainingpart

max 0m braking m braking m

PPS c m braking

P P

P P


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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 train

Engine 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 andvehicle 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


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On-off control strategy 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

reachedIllustration of thermostat control


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Design of parallel hybrid components Key parameters

Engine power

Electric motor power

Gear ratio of transmissions Batteries or peak power sources

Great influence on vehicle performances and operationefficiency

Design methodology

Preliminary choice based on performance requirements

Accurate selection with detailed simulations


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Illustrative design example Design specification

M=1500 kg

f = 0,01 Re=0,279 m Cx= 0,3 S=2 m

Transmission ratio efficiency: t,e=0,9 t,m=0,95

Performance specifications

Acceleration time (0 to 100 km/h): 10 +/- 1 s

Maximum gradeability: 30% @ low speed and 5% @ 100 km/h

Maximum speed 160 km/h


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Power rating of engine Engine should supply sufficient power to support vehicle

operation at normal constant speed on both flat or mild graderoad without the help of PPS

Engine should be able to produce an average power that islarger than load power when operating in a stop-and-go pattern


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Power rating of engine Operating on highway at constant speed on flat road or mild

grade road

Illustrative example

V=160 km/h requires 42 kW

With a 4 ratio gear box

Engine allows driving

road at 5 % at 92 km/h in gear 4

road at 5 % at 110 km/h in gear 3

, ,

1( sin )

2

res

e xt e t e

P VP m g f S C V mg


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Power rating of engine Engine is able to supply the average power requirement in stop-

and-go driving cycles

The average power depends on the degree of regenerationbraking.

Two extreme cases: full and zero regenerative braking:

Full regenerative braking recovers all the energy dissipated inbraking and can be calculated as above

No regenerative braking, average power is larger so that whenpower is negative, third term is set to zero.

2

0 0

1 1 1

2

T T

ave x

dV

P m g f SC V Vdt m dtT T dt


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Power rating of engine

Instantaneous and average power with full andregenerative braking in typical driving cycles


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Power rating of engine Average 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 that the engine can produce withfull throttle during the driving cycle

0

1( / , )

T

ave e eP P v R i dtT


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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 train

Design criteria: provide acceleration performance and peakpower demand in typical drive cycles

Difficult to directly design motor power for prescribedacceleration performance

Methodology

Provide good estimates first in a preliminary approach

Final design with detailed simulations

Assumption to calculate initial estimates Steady state load (rolling resistance, areo drag) handled by engine

while dynamic load (acceleration) handled by motor


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Design of electric motor power capacity Acceleration related to the torque output of the electric motor

Power rating

Illustrative example Passenger car Vmax=160 km/h, Vb=40 km/h, Vf=100 km/h

ta (0 100km/h)=10 s, =1,04

, ,m t m t m

e

T i dvm

R dt

2 2,2

m f b

t m a

mP V V

t


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Design of electric motor power capacity


Passenger car Vmax=160 km/h, Vb=40 km/h, Vf=100 km/h

ta (0 100km/h)=10 s, =1,04

Pm= 74 kW


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Design of electric motor power capacity The approach overestimates the

motor power, because the engine hassome remaining power to acceleratethe vehicle also

Average remaining power of theengine used to accelerate the vehicle

Depends on gear ratio, so that itvaries with engaged gear box andincreases with gear box

,

1( )

a

i

t

e accel e rest

a i

P P P dt

t t

Engine remaining power: 17 kW

Pm=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 calculations have to be carried out toevaluate the vehicle performances:

Max speed Gradebility

Acceleration

Gradebility and max speed can be obtained from the diagram oftractive effort and resistance forces vs speed


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Design of electric motor power capacityIllustrative example

At 100 km/h, gradeabiltiy

of 4,6% for engine aloneand 18,14% for hybridmode


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Design of electric motor power capacityIllustrative example

Acceleration performance

for 0-100 km/h: ta=10,7 s

d=167 m


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Transmission designTransmission ratio for electric motor

Because electric motor supplies peak power and because it hasa high torque at low speed, a single ratio transmission between

motor and the driven wheels is generally sufficient to producehigh torque for hill climbing and acceleration

Transmission ratio for engine

Multi 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 vehicleperformance (acceleration and gradebility)

(+) Energy storage can be charged with the large engine power (+) Improve vehicle fuel economy because the engine operates

closer to its optimal speed

(-) More complex system

(-) Heavier and larger

(-) Complicated automatic gear shifting control


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Design of batteries and PPS Batteries and PPS are sized according to power and to energy

capacity criteria

POWER CAPACITY Battery power must be greater than the input electric power of

the electric motor

max

mPPS

m

PP


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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 required from the PPS and from the engineduring acceleration period

Illustrative example:

Energy from batteries 0,3 kWh

0

at mPPS

m

PP dt

0

ate eP P dt


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Design of batteries and PPSENERGY CAPACITY

Energy capacity must meet the energy requirements duringdriving pattern in drive cycles

For a given control energy strategy charging and dischargingpower of energy storage can be obtained from simulation

Generally energy consumption (and capacity sizing) isdominated 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


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Design of batteries and PPS Not 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 increase

Ultracapacitors: low SOC results in low voltage and affects theperformances

Flywheels: low SOC is low flywheel velocity and low voltage atelectric machine to exchange port

Only part of the stored energy can be available for use

Part available is given by a certain percentage of its SOC

min max,E SOC SOC


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Design of batteries and PPS Energy capacity of the energy storage


E= 0,3 kWh

SOCmax-SOCmin=0,3

Ecap= 1 kWh

max

max min

cap

EE

SOC SOC


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Accurate simulation When major components have been designed, the drive train

has to be simulated

Simulation on typical drive train brings useful information:

Engine power Electric motor power

Energy changes in energy storage

Engine operating points

Motor operating points

Fuel consumption


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Accurate simulation


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Accurate simulation

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