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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Q and A - HEV Energy Management June 2008