The Current Status of Hydraulic Hybrid Powertrain Technology

1. The Current Status of Hydraulic Hybrid Powertrain Technology Kenneth J. Waldron Professor (Research), Stanford University Professor, University of Technology, Sydney

2. We’ve Been There Before • There was an upsurge in research on high efficiency powertrains in the wake of the OPEC oil crisis in ‘70’s and ‘80’s • Included lots of projects on hydraulic CVT’s with energy storage and regeneration: hydraulic hybrid powertrains • Hydraulics were the primary interest, relatively little on electric powertrains • In contrast to the present, the preferred energy storage device was a flywheel • Current hydraulic systems seem all to be focused on accumulators • WHY?

3. Adaptive Suspension Vehicle • Displacement controlled 18 degree of freedom system • Included regeneration using an energy storage flywheel

4. ASV Power Train • Displacement controlled hydraulic actuation • Power actuators: equal area linear actuators • Pressure regulated, valve controlled primary system with fixed displacement rotary actuators to activate swash plates • Reservoirs and small accumulators local to actuators • ≅ series hybrid with 18 “motors” • Incorporated energy storage flywheel • Powered by motorcycle engine

5. Flywheel • Primary inertia: martensitic steel rim • Containment: unidirectional kevlar-epoxy composite is part of rotating mass • Rim is press fit on hub plate: designed to drop off at 25% overspeed • Sealed aluminium casing is evacuated to low vacuum by bearing oil pump

6. Tested to Destruction • Test intended to demonstrate effectiveness of containment concept • Wheel was crippled by drilling hole through rim to ensure failure within the limits of the available drive • Failure speed above rated upper speed limit • Containment was successful in preventing ejection of rim fragments

7. Hydraulic Hybrids • Conventional wisdom is best suited for heavy vehicles used in frequent start-stop conditions • Favored by superior regeneration performance • High power density also relevant • Hydraulics best at low speed • Will run fast, but losses increase nonlinearly with speed • Relatively high energy losses over time are not important in this application • New control capabilities and new materials may extend these benefits to lighter vehicles

8. Parallel Hybrid Configuration

9. Series Hybrid Configuration

10. Power Split Hybrid Center for Compact and Efficient Fluid Power

11. Hydraulic Configuration

12. Technology Differences from Eighties • Lithium-ion battery technology was not commercially available • Lead-acid was standard technology for automotive electrics • Graphite composite material technology was not well developed • Steel accumulator weighs an order of magnitude more than graphite composite • Digital control hardware was relatively primitive • PWM was brand new technology • Modern integrated digital controllers are much more powerful

13. Hydraulics Compared to Electrics • Hydraulic pump/motor is lighter, more compact and less expensive than electric motor/generator of same power • Hydraulic motors are not subject to overheat at stall • Hydraulic pump/motors can absorb very high power densities • Regeneration is significantly more efficient in a hydraulic system • Hydraulic pump/motors require a primary actuator to drive the swash plate shaft • Electric motor/generators need solid state switches • Hydraulic systems require a parasitic generator to drive electric accessories • New technology batteries have the best energy storage characteristics • Batteries are more expensive than competing technologies

14. Flywheel or Accumulator versus Battery Storage

15. Flywheel Performance Limits Theoretical maximum energy density K is kinetic energy in system M is system mass σ is strength of rim (hoop stress) ρ is density of rim No limit on power density

16. Flywheel Issues • Simple analysis assumes thin rotor • Uniform disk reduces energy density by half • Gyroscopic moments affect handling • Use counter rotating rotors • Need speed reducer to pump/motor • Minimize windage and bearing losses • Run in low vacuum • Magnetic bearings? Hydrostatic? Hydrodynamic? • Need containment • Composite ring? Include in rotating mass?

17. Accumulator Performance Limit • Theoretical maximum energy density • Difficult to approach in practice due to upper limit imposed by system operating pressure • Relatively rapid leakage due to heat transfer

18. Accumulator Issues • Theoretical energy density not practical • Composite thin walls are fragile, back with aluminium or titanium • Accumulators are bulky, difficult to package in vehicle

19. Hydraulic System Issues • Noise • Principal source is valve porting • Reduced by running slower: means larger pump/motors • Use gear trains at engine and flywheel (if used) • Acoustically isolate pump/motors • Throttling losses • Avoid control valves • Leakage • Eliminated with proper design and maintenance • Peaky efficiency/speed characteristics • Actually no worse than electric machines • Swash plates need significant muscle

20. Pump/Motor Circuit

21. EPAM Actuators • Configured as capacitor with very extensible dielectric, compliant electrodes • Pretension to maximum actuation force • Excitation causes relaxation in stretch direction • Largest force produced when passive • Good force to weight ratio • Fast response • Moderate efficiency

22. Cross-Pull EPAM Actuator • Electrostrictive polymer actuator • Thin polymer layer with compliant electrodes deposited on both sides • Sheet is pre-tensioned • Relaxes when excited • Needs high voltage, small current • Nonlinear characteristics • Two sheets tensioned across diagonals of parallelogram frame • Durability issues

23. Binary Configuration • Cross-pull EPAM’s work well as bistable actuators • Extensive practical experience with this mode • Proven durability • Suggests use to actuate switching valves • Needed to switch from motor to pump operation • Effort needed is moderate • Normally use solenoid valves • Efficiency is moderate, but also true for solenoids

24. EPAM on Swashplate? • Swashplate shaft actuator should be fast and accurate. Does not need large motion range. Should be low loss. • Alternatives are fixed displacement hyd. motor, electric motor, EPAM • Hydraulic motor entails severe valve losses • Electric motor is heavy, bulky • EPAM is light with good bandwidth, adequate motion range

25. Summary • New technology options justify a new look at hydraulic hybrids • Propose optimal configuration study • Mechanical complexity versus electrical complexity • Serial or split configurations are most attractive • EPAM’s may provide viable option for swash plate and switching valve actuation