Optimal Design of a High-Speed On/Off Valve for a Hydraulic Hybrid Vehicle Application Michael Rannow Perry Li Thomas Chase Haink Tu Meng Wang Center for Compact and Efficient Fluid Power, University of Minnesota, Minneapolis, MN, USA ABSTRACT Control of hydraulic systems using high-speed on/off valves has been proposed as a way to avoid the inefficiency associated with throttling valves. However, on/off control has sources of energy loss that must be considered when designing a system, such as transition throttling, full- open throttling, compressibility, and leakage. A self-spinning rotary on/off valve achieving high switching frequencies with low actuation power has been developed. However, the design of this valve contains numerous tradeoffs that must be balanced to produce the most efficient system. This paper outlines energy loss equations and design constraint equations that are needed to apply optimization techniques to the valve design. The results of this optimization are presented for a Virtually Variable Displacement Pump, and a Virtually Variable Displacement Pump/motor, which is used as the wheel motor of a hydraulic hybrid vehicle. 7th International Fluid Power Conference Aachen 2010 1
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Optimal Design of a High-Speed On/Off Valve for a Hydraulic Hybrid Vehicle Application
Michael Rannow
Perry Li
Thomas Chase
Haink Tu
Meng Wang
Center for Compact and Efficient Fluid Power, University of Minnesota, Minneapolis, MN, USA
ABSTRACT
Control of hydraulic systems using high-speed on/off valves has been proposed as a way to
avoid the inefficiency associated with throttling valves. However, on/off control has sources of
energy loss that must be considered when designing a system, such as transition throttling, full-
open throttling, compressibility, and leakage. A self-spinning rotary on/off valve achieving high
switching frequencies with low actuation power has been developed. However, the design of this
valve contains numerous tradeoffs that must be balanced to produce the most efficient system.
This paper outlines energy loss equations and design constraint equations that are needed to
apply optimization techniques to the valve design. The results of this optimization are presented
for a Virtually Variable Displacement Pump, and a Virtually Variable Displacement Pump/motor,
which is used as the wheel motor of a hydraulic hybrid vehicle.
7th International Fluid Power Conference Aachen 2010
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NOMENCLATURE
Parameter Description 3-way Value 4-way Value Units
ρ Fluid Density 876 same kg/m3
Q Flow Rate 37.9 varies l/m
P Load Pressure 69 207 bar
Μ Fluid Viscosity 0.0387 same kg/ms
α Ratio of Shear Stress 0.2 same none
cd Orifice Coefficient 0.6 same none
βeq Fluid Bulk Modulus 3.7 x 108 1.2 x 109 Pa
Pc Check Valve Cracking Pressure 3.2 N/A bar
kspool Spool Coefficient 6.0 x 10-7 1.52 x 10-7 none
mspool Spool Flow Exponent 1.735 same none
nspool Spool Diameter Exponent 2.75 same none
Pr Relief Valve Pressure N/A 228 bar
ln Inlet Nozzle Length 4.2 9.5 mm
Dd Inlet/outlet Rail Diameter 6.8 same mm
Vin Inlet/line Volume 5 7 cm3
lmid Middle Land Thickness 1.5 same mm
ri,off Inlet Offset Fraction 0.79 same none
ro,off Outlet Offset Fraction 0.9 same none
fmin/fmean,min Minimum Frequency 75 35 (mean) Hz
de Pump/motor Displacement N/A 10 cc/rev
rm Engine Gear Reduction N/A 6.0 or 20.3 none
ωsw Transmission Shift Speed N/A 550 rpm
Vspool Fluid Volume inside Spool N/A 7 cm3
7th International Fluid Power Conference Aachen 2010
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Figure 1: Diagram of 3-way rotary spool
1 INTRODUCTION
Hydraulic actuation systems are widely used due to their flexibility, durability, and high
power density. However, they also tend to be inefficient, primarily due to the use of
throttling valves for control. Valve control varies the output by restricting the fluid and
passing unwanted flow across valves at high pressure, generating heat and energy loss.
A different approach in hydraulic systems is to use a variable displacement pump for
each actuator, which has the potential to be more efficient. However, variable pumps
tend to be bulky and expensive which limits the spread of this control technique.
A different option for avoiding inefficient throttling valves is the use of high-speed on/off
valves and energy storage elements. Tomlinson and Burrows [1] and Li, Li, and Chase
[2] demonstrated the feasibility of using an on/off valve to vary the flow rate of a fixed-
displacement pump, and Rannow et al. [3] quantified the benefits of an on/off system in
comparison to a throttling system. Cao et al. [4] used a conventional on/off valve to
create hydraulic transformers, and Brown et al. [5] developed a high-speed, motor-
driven rotary on/off valve for use in a transformer.
The high-speed rotary on/off valve in Fig. 1 uses fluid forces to provide rotary actuation
[6]. This is a 3-way valve that directs flow from the center to either the top (load) or the
bottom (tank) section. Flow enters the center section through a rhombus inlet, and the
spool can move axially with respect to the inlet. The on/off valve pulse-width-modulates
(PWM) the fluid, with the duty ratio adjusted by the axial position.
This valve utilizes constant,
unidirectional motion to achieve the high-
speed on/off action. This reduces actuation
power by eliminating the acceleration and
deceleration of the spool in traditional valves.
Since fluid momentum drives the valve,
energy lost as a pressure drop across the
valve is scavenged.
Despite the reduction in acutation power
required, sources of energy loss still exist.
7th International Fluid Power Conference Aachen 2010
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Figure 2: VVDP circuit
Figure 3: Pressure Profile for the VVDP
This paper quantifies these losses and
formulates an optimization problem to minimize
them, subject to constraints. This is done for a
3-way valve, and extended to a 4-way valve that
is currently being developed to be used with a
fixed-displacement pump/motor to create a
variable pump/motor for use in a hydraulic hybrid
vehichle. The optimization problem for the 4-way
valve will minimize the energy loss when the valve is used in a hydro-mechanical
transmission (HMT) driving the Urban Dynamometer Driving Schedule (UDDS).
2 VIRTUALLY VARIABLE DISPLACEMENT PUMP
The valve in Fig. 1 can be used in the circuit in Fig. 2. This configuration allows a
variable flow rate without a variable displacement pump. The on/off valve sends the full
flow to the load when it is on and to the tank when it is off. The accumulator stores
excess flow when the valve is on, and drives the load it is off.
On/off valves are efficient because the
pressure drop across the valve is low when
the valve is in either position (Fig. 3). In this
figure, the pressure drop is usually low;
however, during transition between on and
off, the pressure drop is high. Throttling due
to the full-open pressure drop and transition
throttling are two of the four sources of loss
in the valve. Energy is also lost from the
compressibility of the hydraulic fluid and
leakage across the barrier between the load and tank ends of the spool.
The valve design can be cast as an optimization problem, minimizing energy loss while