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
Weve Been There Before

• There was an upsurge in research on high
  efficiency powertrains in the wake of the OPEC
  oil crisis in 70s and 80s
• Included lots of projects on hydraulic CVTs 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 s
is strength of rim (hoop stress) ? 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 EPAMs 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
• EPAMs may provide viable option for swash plate
  and switching valve actuation