1. Understanding Opportunities for Energy Management in HEVs through Degree-of-Freedom (DOF) Analysis
Tony Phillips, Senior Technical Leader
Ming Kuang, Technical Leader
Vehicle & Battery Controls
Research and Advanced Engineering
June 2008

2. Fuel economy improvement opportunities in an HEV
Overview
Fuel economy improvement opportunities in an HEV
Vehicle system control
Propulsion system degree of freedom analysis
Conventional vehicle
Powersplit HEV
Dual drive HEV
Optimization problem formulation and constraints
Summary & conclusions
Ford
Escape Hybrid
HEV Energy Management
June 2008

3. Fuel Economy (FE) Benefits of Hybridization
Engine downsizing
Conventional vehicles
Engines are normally sized for maximum acceleration
Part load operation is not optimal
Hybrid vehicles
Engine sized for sustained operation
Transient power requests are fulfilled by electric motor/battery
10-15% FE improvement
Regenerative braking
Vehicle kinetic energy normally lost to heat is recaptured as potential energy in the battery
Efficiency – 80-90% depending on configuration and component efficiency
5-20% FE improvement
Required for
maximum performance
Normal driving
operating range
HEV Energy Management
June 2008

4. Fuel Economy (FE) Benefits of Hybridization
Engine stop/start
Engine is turned off when not needed for propulsion or battery charging
~5% FE improvement
Energy Management
Effective use of propulsion system components to provide torque to the wheels while still maintaining onboard energy storage (battery charging)
5-10% FE improvement x
Engine optimal x
System optimal
HEV Energy Management
June 2008

5. Context: Vehicle System Control
Subsystem Control
Vehicle Status
Ignition Key
Position
Friction
Torque
Command
Vehicle
System
Control
(VSC)
Brake
Subsystem
Hydraulic System
PRNDL
Accelerator
Pedal Position
Engine
Torque
Command
Brake Pedal
Position
Engine
Subsystem
Steering Wheel
Angle
Transmission
Subsystem
Speed Control
Setpoint
Climate Control
Settings
Gear Command
VSC
Energy Management
Torque Control
Battery Power Control
Vehicle Stability Control …
HEV Energy Management
June 2008

6. Context: Vehicle System Control
Subsystem Control
Vehicle Status
Ignition Key
Position
Friction
Torque
Command
Vehicle
System
Control
(VSC)
Brake
Subsystem
Hydraulic System
PRNDL
Trans-Axle
Subsystem
Planetary Gear
Accelerator
Pedal Position
ring
Engine
Torque
Command
Brake Pedal
Position
Engine
Subsystem
Generator
Subsystem
sun
Steering Wheel
Angle
ring
Generator
Torque Command
Speed Control
Setpoint
Motor
Torque Command
Climate Control
Settings
Motor
Subsystem
Contactor
Command
Battery
Subsystem
High Voltage Bus
VSC
Energy Management
Torque Control
Battery Power Control
Regenerative Braking
Engine Start/Stop …
HEV Energy Management
June 2008

7. DOF Analysis – Conventional Vehicle
Governing Equations
Dynamic equation
Speed relationship
Conventional Vehicle System Configuration
teng , weng R
Tdrive_sh
tdrive_sh ,wdrive_sh
Jeng
Free Body Diagram
HEV Energy Management
June 2008

8. DOF Analysis – Conventional Vehicle
Steady State Operation
and are constrained by the driver’s demanded torque and the vehicle speed, respectively
2 equations with 3 unknowns
1 DOF
Since gear ratio, R, is discrete, it is used as the independent variable x
 Constant Pedal
Acceleration x
Gear Shift
HEV Energy Management
June 2008

9. DOF Analysis – Powersplit HEV
Governing Equations
Dynamic equations
Speed & power relationships
Loss terms
Powersplit HEV System Configuration
Free Body Diagram
HEV Energy Management
June 2008

10. DOF Analysis – Powersplit HEV
Steady State Operation
and are constrained by the vehicle speed and the driver’s demanded torque, respectively
8 equations with 10 unknowns
2 DOF
Select DOF variables
to move total system
toward max. efficiency
HEV Energy Management
June 2008

11. DOF Analysis – Dual Drive HEV
Governing Equations
Dynamic equation
Speed & power relationships
Loss terms
Dual Drive HEV System Configuration
teng , weng ,
tgen , wgen R
Twhl_f
twhl ,wwhl
Jeng
tmot ,wmot
Jmot
Twhl_r
Free Body Diagram
HEV Energy Management
June 2008

12. DOF Analysis – Dual Drive HEV
Steady State Operation
and are constrained by the vehicle’s wheel speed and the driver’s demanded torque, respectively
7 equations with 10 unknowns
3 DOF
Select DOF variables
to move total system
toward max. efficiency
HEV Energy Management
June 2008

13. DOF Variable Selection and Optimization
Consider Powersplit case:
Choice of independent variables
Pbat: Useful for controlling battery power violations and for controlling the battery state of charge
eng: Provides better control over engine power than using engine torque. Also useful for directly controlling Noise, Vibration & Harshness (NVH) attribute
Objective
Minimize fuel usage ( ) over the vehicle’s drive cycle, subject to constraints
HEV Energy Management
June 2008

14. Optimization – Constraints
Battery power limits
Required for battery life
Torque limits
Capability limits for torque producing devices
Speed limits
Capability limits for rotating devices
Battery state of charge (SOC) limits
Required to balance onboard energy at beginning & end of optimization cycle
Relationship to battery power
HEV Energy Management
June 2008

15. Summary & Conclusions
Vehicle “energy management” is the effective use of the available degrees of freedom to maximize the propulsion system efficiency
Different powertrain configurations provide different opportunities for energy management
Conventional vehicles have only a limited opportunity for exploiting a single degree of freedom
Hybrid vehicles provide an opportunity to replace low efficiency engine operation with higher efficiency electric operation
Optimization techniques can be used to determine optimal system commands
HEV Energy Management
June 2008

16. Q and A -
HEV Energy Management
June 2008