Hybrid City Bus Design

Evaluation Using System

Level Simulations

ISIE 2014, Istanbul, Turkey

Teemu Halmeaho, Pekka Rahkola, Jenni

Pippuri, and Kari Tammi

Kari.Tammi@vtt.fi, +358 50 348 7902

VTT Technical Research Centre of Finland

211/12/2014 2

Objectives

Design hybrid bus (4th evolution version)

Study good (optimal) combination for power train (generator,

motor, battery)

Study vehicle performance on actual bus line (true loading)

Parameter study regarding to bus performance (*)

Bus weight

Motor torque

Regenerative braking under different friction conditions (*)

Regeneration capability

Bus stability

(*) = focus of this presentation and ISIE paper

311/12/2014 3

Bus specification – general

Serial hybrid powertrain (or range-extended electrical vehicle)

Model includes

Vehicle longitudinal dynamics

Drive (propulsion) motor-generator

Battery

Diesel-generator (range-extender)

Necessary power electronics for generator, energy storage and

drive motor

411/12/2014 4

Bus specification – detailed

Subsystem Parameter Value

Vehicle

Total massa, m10 000, 12 000, 14 000

kgb From CMc to FAc 3.0 mb From CMc to RAc 1.5 m

CMc height 1.0 m

Frontal area A 7.0 m2

Drag coefficient, Cd 0.4

Drive motorNominal torquea 660, 880, 1060 Nm

Nominal speed 1900 rpm

GeneratorNominal torque 310 Nm

Nominal speed 2200 rpm

Diesel engine Power 56 kW

Energy storageCapacity 0.74 kWh

Power 300 kW

DC-DC converter Efficiency 95 %

511/12/2014 5

Dynamic bus model

Mechanical domain

Longitudinal dynamics, Newtonian mechanics (rigid body)

Tyres, traction forces as a function of tyre slip

4-speed gearbox (!, 4 gear ratios + final drive)

Electrical domain

Permanent magnet (PM) drive motor & PM generator

Operated on linear range

dq model, no saturation

No field weakening needed to take into account

Energy storage, ultra capacitor

Resistance, no ageing

Power electronics

Field oriented control

Ideal switching

sind

dmgFF

t

vm dx

x

qqsddd

d iLRivt

iL

d

d

qdqdqafe iiLLiPT 2

3

rLer

r BTTt

J

d

d

611/12/2014 6

Electrical powertrain and control

Similar power electronics in generator model

Connected to DC bus with capacitor power electronics

Tool: Simulink/ Simscape

Results

811/12/2014 8

Energy saving estimation for varied bus weight

and propulsion motor generator torque

Mass: 10, 12, and 14 tonnes Max torque: 660, 860, and 1060 Nm

Nominal

case

Nominal

case

911/12/2014 9

Regenerative braking and stability

Rear wheel drive

Pure regenerative braking may be

unstable (no front brakes)

High gear always ensures stable

behaviour if mechanical front

brakes applied

Black curve: ideal brake force

distribution (front rear)

Stable below black curve for all

friction levels (front wheels

block first)

Vehicle deceleration ratio

z = a / g

1011/12/2014 10

Regeneration and stability

Energy regeneration works on dry

asphalt, no stability lost

Friction demand ~ 0.55 for rear wheel

braking with 1060 Nm motor generator

Friction demand ~ 0.3 for acceleration

Limited friction limited regeneration

Largest friction demand occurs at slow

speeds ~10 km/h

< 0: braking

Conclusions and future

work

11/12/2014 12

Conclusions

• Limited friction is real problem for busses operating in cold climate (and heavy rain)

• Slip control recommended for regenerative braking

• Front axle regeneration (and possibly four wheel drive) helps but increases cost

– Control of optimal front – rear distribution

• Parameters were fixed for electrical bus

• Commercial version is to be built

Future research work

• Lateral stability when cornering

• 3D mechanical model

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