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The Current Status of Hydraulic Hybrid Powertrain Technology. Kenneth J. Waldron Professor (Research), Stanford University Professor, University of Technology, Sydney. We’ve Been There Before.
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The Current Status of Hydraulic Hybrid Powertrain Technology Kenneth J. Waldron Professor (Research), Stanford University Professor, University of Technology, Sydney
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?
Adaptive Suspension Vehicle • Displacement controlled 18 degree of freedom system • Included regeneration using an energy storage flywheel
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
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
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
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
Power Split Hybrid Center for Compact and Efficient Fluid Power
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
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
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
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?
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
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
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
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
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
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
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
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