Hybrid City Bus Design Evaluation Using System Level Simulations

1. 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

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
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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
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4. Bus specification – detailed
Subsystem Parameter Value
Vehicle
Total massa, m
10 000, 12 000, 14 000
kg
b From CMc to FAc 3.0 m
b From CMc to RAc 1.5 m
CMc height 1.0 m
Frontal area A 7.0 m2
Drag coefficient, Cd 0.4
Drive motor
Nominal torquea 660, 880, 1060 Nm
Nominal speed 1900 rpm
Generator
Nominal torque 310 Nm
Nominal speed 2200 rpm
Diesel engine Power 56 kW
Energy storage
Capacity 0.74 kWh
Power 300 kW
DC-DC converter Efficiency 95 %
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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
x   
m x d
 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
sin 
d
v
d
F F mg
t
i
d
L   
d v Ri L i
d d s q q
d
t
d
3
    e af q d q d q T  P  i  L  L i i
2
d

J 
 T  T 
B
r e L r
r
t
d
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6. Electrical powertrain and control
 Similar power electronics in generator model
 Connected to DC bus with capacitor power electronics
 Tool: Simulink/ Simscape
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7. Results

8. Energy saving estimation for varied bus weight
and propulsion motor generator torque
Nominal
case
Nominal
case
Mass: 10, 12, and 14 tonnes Max torque: 660, 860, and 1060 Nm
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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
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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
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11. Conclusions and future
work

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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13. TECHNOLOGY FOR BUSINESS