U.S. patent application number 13/232677 was filed with the patent office on 2012-01-05 for engine-off power steering system.
Invention is credited to Charles L. Gray, JR., David James Haugen, Mark Stuhldreher.
Application Number | 20120000725 13/232677 |
Document ID | / |
Family ID | 39496653 |
Filed Date | 2012-01-05 |
United States Patent
Application |
20120000725 |
Kind Code |
A1 |
Stuhldreher; Mark ; et
al. |
January 5, 2012 |
Engine-Off Power Steering System
Abstract
Motor vehicles are equipped with a center-closed power steering
system to provide power steering assistance in engine-off
conditions for reduced fuel consumption. The power steering system
includes a hydraulic pump mechanically driven by the engine, a high
pressure accumulator, a steering gearbox fluidly connected to the
accumulator, and a center closed valve mechanically connected to a
driver-operated steering wheel. The valve is configured to
selectively control flow of pressurized fluid from the accumulator
to the steering gearbox to provide power steering assistance.
Inventors: |
Stuhldreher; Mark;
(Pinckney, MI) ; Gray, JR.; Charles L.; (Pinckney,
MI) ; Haugen; David James; (Ann Arbor, MI) |
Family ID: |
39496653 |
Appl. No.: |
13/232677 |
Filed: |
September 14, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11999884 |
Dec 7, 2007 |
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13232677 |
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60873471 |
Dec 7, 2006 |
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Current U.S.
Class: |
180/421 |
Current CPC
Class: |
B62D 5/065 20130101;
B62D 5/0837 20130101 |
Class at
Publication: |
180/421 |
International
Class: |
B62D 5/06 20060101
B62D005/06 |
Claims
1. A non-hybrid motor vehicle with power steering system,
comprising: a vehicle frame; one or more drive wheels rotatably
mounted on said vehicle frame; an internal combustion engine
mounted on said vehicle frame, providing all propulsive power to
said drive wheels for propelling the non-hybrid motor vehicle; a
hydraulic pump, mechanically driven by the internal combustion
engine, for pressurizing fluid; an accumulator storing under
pressure fluid pressurized by the hydraulic pump; a steering
gearbox fluidly connected to the accumulator; a center closed valve
mechanically connected to a driver-operated steering wheel mounted
in the vehicle, the valve configured to prevent flow of pressurized
fluid from the accumulator to the steering gearbox except when said
valve is opened by rotation of the steering wheel away from a
neutral central position, the valve further configured to allow
flow of pressurized fluid from the accumulator to the steering
gearbox when the valve is open, to provide supplemental mechanical
assistance in steering the motor vehicle upon rotation of the
steering wheel away from a neutral central position, even when the
internal combustion engine is turned off.
2. The vehicle of claim 1, further comprising a spring return means
to assist return of steering to the neutral center position when
torque is no longer being applied by the driver to rotate the
steering wheel.
3. The vehicle of claim 1, wherein the valve is further configured
when open to meter the flow of pressurized fluid from the
accumulator to the steering gearbox in relative proportion to the
extent of rotational force applied to the steering wheel.
4. The vehicle of claim 3, wherein the valve is a rotary valve
comprising: a hollow cylindrical sleeve with an input port fluidly
connected to the accumulator, and an output port fluidly connected
to the steering gearbox; and a shaft rotatably positioned within
the sleeve, wherein a portion of the outer surface of the shaft is
configured to sealingly engage an inner surface of the sleeve in a
first shaft rotation position to prevent flow of the pressurized
fluid between the input port and the output port, and further
including a cavity in the outer surface of the shaft configured to
allow flow of the pressurized fluid between the input port and the
output port in a second shaft rotation position.
5. The vehicle of claim 4, wherein a V-shaped notch is provided on
the outer surface of the shaft between the cavity and the portion
of the outer surface of the shaft that is configured to sealingly
engage the inner surface of the sleeve, in order to provide a
smoother transition in driver feel from the first shaft rotation
position, in which no mechanical assistance from the power steering
system is provided in steering the motor vehicle, to a second shaft
rotation position in which full mechanical assistance from the
power steering system is provided in steering the motor
vehicle.
6. The vehicle of claim 4, wherein a notch is provided on the inner
surface of the sleeve on an edge of the output port, in order to
provide a smoother transition in driver feel from the first shaft
rotation position, in which no mechanical assistance from the power
steering system is provided in steering the motor vehicle, to a
second shaft rotation position, in which full mechanical assistance
from the power steering system is provided in steering the motor
vehicle.
7. A method of reducing fuel consumption in the non-hybrid vehicle
of claim 1, comprising: turning the internal combustion engine off
intermittently in driving conditions when there is no driver torque
demand; sending pressurized fluid from the accumulator to the
steering gearbox for power steering assistance upon turning of the
non-hybrid vehicle; cutting off flow of the pressurized fluid from
the accumulator to the steering gearbox when the non-hybrid vehicle
is not being turned or when power steering assistance is not
required.
8. A method of reducing fuel consumption in a hydraulic hybrid
motor vehicle with an internal combustion engine, comprising:
storing pressurized fluid in a high pressure hydraulic accumulator
on the hydraulic hybrid motor vehicle; turning the internal
combustion engine off intermittently in driving conditions; sending
the pressurized fluid to a steering actuation system for power
steering assistance upon turning of the hydraulic hybrid motor
vehicle; monitoring vehicle speed; reducing the pressure of the
pressurized fluid supplied to the steering actuation system when
the monitored vehicle speed is above a first threshold; cutting off
flow of pressurized fluid from the accumulator to the steering
actuation system when the hydraulic hybrid motor vehicle is not
being turned or when power steering assistance is not required.
9. The method of claim 8, further comprising adjusting the pressure
of the pressurized fluid supplied to the steering actuation system
as a function of vehicle speed.
10. The method of claim 8, further comprising using a spring return
means to assist return of steering to the neutral center position
when torque is no longer being applied by the driver to rotate the
steering wheel.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/999,884, "Engine-Off Power Steering System," filed Dec.
7, 2007, which claims priority from U.S. Provisional Application
60/873,471, "Methods for Engine-Off Operation of Motor Vehicles,"
filed Dec. 7, 2006.
FIELD OF THE INVENTION
[0002] The present invention relates to power steering systems and
methods for controlling engine operation in motor vehicles for
improved fuel efficiency.
DESCRIPTION OF THE RELATED ART
[0003] There is a pervasive and ongoing desire worldwide to reduce
fuel consumption in motor vehicles in a cost-effective manner. One
approach for the reduction of fuel consumption has been
investigation of hybrid vehicles. One of the ways in which hybrid
vehicles can reduce fuel consumption is through turning the engine
off and running on stored energy when desired. However, hybrid
vehicles are costly, requiring the addition of expensive components
(e.g., an electric or hydraulic motor and large energy storage
device) to the vehicle. Another approach for the reduction of fuel
in some vehicles has been the promotion of programs to reduce
unnecessary engine idling in parked conditions, for example, in
long-haul trucks. Generally these latter programs utilize either
auxiliary power units (APUs) which are carried on the vehicle, or
the provision of electrical outlets at truck stops and overnight
resting points, etc.
[0004] Current obstacles to implementation of engine-off operation
of passenger vehicles in moving conditions include the need for use
of power steering, power brake assistance, and air conditioning.
These accessory power demands exist even when there is no demand
for propulsive power from the engine (e.g., coasting or braking
conditions), which thereby limit possible engine-off operation in
motor vehicles.
[0005] Hydraulic power steering systems are widely used in motor
vehicles to reduce the steering effort required of a driver in
turning the motor vehicle. In a hydraulic power steering system, a
hydraulic pump (conventionally driven by the engine) is used to
pump fluid, creating a pressurized fluid flow which may
subsequently be used as needed to provide mechanical assistance in
turning the wheels (e.g., through use of the pressurized fluid to
create movement of a piston mechanically connected to a steering
gearbox and the wheels).
[0006] Current hydraulic power steering systems are uniformly each
center-open systems, meaning that hydraulic fluid is pumped
constantly through the system, even when the steering wheel remains
in the center position and no mechanical assistance for steering is
needed. As a result, hydraulic power steering systems significantly
increase motor vehicle fuel consumption because of the need to
continue running the engine to provide this constant pumping work
being performed.
[0007] Some investigations have been made into reducing the fuel
consumption caused by center-open hydraulic power steering systems.
For example, center-closed electro-hydraulic power steering systems
have occasionally been proposed for hybrid electric vehicles. The
term "center-closed" (or interchangeably "center closed" or
"closed-center") herein relates to a control valve for the power
steering system, meaning that the control valve closes off flow of
the hydraulic fluid when the valve is in a central position and no
power steering assist is needed, such as when the steering wheel is
in a central, non-rotated position. By stopping the pumping of
fluid through the power steering system except when power
assistance is needed, a closed center power steering system can
thereby reduce fuel consumption caused by the power steering system
by eliminating the need to continuously flow power steering fluid
pumped by an engine-driven pump. Two examples of center-closed
electro-hydraulic power steering system proposals are U.S. Pat. No.
5,209,317 to Schnelle, and SAE Paper 950580, "Integrated
Electro-Hydraulic Power Steering System with Low Electric Energy
Consumption," by Keiji Suzuki. However, electro-hydraulic power
steering systems nevertheless still entail significant energy loss,
such as conversion losses in the conversion of the mechanical
rotation of the engine output shaft to electricity and then back to
mechanical rotation to drive the hydraulic power steering pump.
[0008] Applicant has previously disclosed the possibility of using
a center-closed hydraulic power steering system in a hydraulic
hybrid vehicle, as a high pressure accumulator for readily
supplying pressurized fluid would already be present on the
vehicle. See "Progress Report on Clean and Efficient Automotive
Technology under Development at EPA: Interim Technical Report,"
(EPA 420-R-04-002, January 2004).
[0009] As noted in U.S. Pat. No. 5,641,033 to Langkamp, one of the
reasons center-closed power steering systems have not been
successfully commercialized is because, inter alia, "it has proved
to be difficult to achieve satisfactory control characteristics"
with previously-attempted center-closed power steering systems.
OBJECT OF THE INVENTION
[0010] It is therefore an object of the present invention to reduce
fuel consumption in motor vehicles in a cost-effective manner.
[0011] It is also an object of the present invention to provide a
hydraulic power steering system with reduced fuel consumption and
satisfactory control characteristics.
[0012] It is also an object of the present invention to enable
greater engine-off operation of motor vehicles in moving conditions
such as coasting or braking conditions.
SUMMARY OF THE INVENTION
[0013] In one embodiment of the invention, a motor vehicle with a
conventional (non-hybrid) mechanical drivetrain is provided with a
center-closed hydraulic power steering system. The power steering
system includes a hydraulic pump mechanically driven by the engine,
a small hydraulic pressure accumulator storing fluid pressurized by
the pump, a steering gearbox (or other steering actuation system)
fluidly connected to the accumulator; and a center closed valve
mechanically connected to a driver-operated steering wheel. In
order to provide for functional operation of the power steering
system, the center-closed valve is configured to prevent flow of
pressurized fluid from the accumulator to the steering actuation
system except when said valve is opened by rotation of the steering
wheel away from a torque-neutral central position, with the valve
further configured to allow flow of pressurized fluid from the
accumulator to the steering gearbox when the valve is open, to
provide supplemental mechanical assistance in steering the motor
vehicle upon rotation of the steering wheel away from a
torque-neutral central position.
[0014] In such a power steering system, power steering is made
available to the driver even when the combustion engine is turned
off, through use of the pressurized fluid stored in the
accumulator, thereby enabling engine-off operation of the vehicle
in order to reduce fuel consumption. Pressure in the accumulator is
monitored and may be maintained by drawing energy from the
vehicle's engine as may be needed.
[0015] For commercially acceptable steering control characteristics
in this fuel efficient power steering system, the center-closed
valve is preferably configured in accordance with a preferred
embodiment of the invention in order to meter the flow of
pressurized fluid from the accumulator to the steering actuation
system in the right relationship to the extent of rotational force
applied to the steering wheel, for better driveability and a
natural-feeling steering response. In this preferred embodiment,
the valve is a rotary valve comprising a hollow cylindrical sleeve
with an input port fluidly connected to the accumulator and an
output port fluidly connected to the steering gearbox; and a shaft
rotatably positioned within the sleeve, wherein a portion of the
outer surface of the shaft is configured to sealingly engage an
inner surface of the sleeve in a first shaft rotation position to
prevent flow of the pressurized fluid between the input port and
the output port. The valve further includes a cavity in the outer
surface of the shaft configured to allow flow of the pressurized
fluid between the input port and the output port in a second shaft
rotation position. Finally, a tapered or notched transition surface
is provided on the outer surface of the shaft between the cavity
and the portion of the outer surface of the shaft that is
configured to sealingly engage the inner surface of the sleeve, in
order to provide a smooth transition in terms of feel to the driver
from the first shaft rotation position, in which no mechanical
assistance from the power steering system is provided in steering
the motor vehicle, to the second shaft rotation position, in which
full mechanical assistance from the power steering system is
provided in steering the motor vehicle. In this manner, acceptable
steering characteristics for the vehicle are provided in a fuel
efficient center-closed power steering system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic view of a motor vehicle with power
steering system in accordance with principles of the present
invention.
[0017] FIG. 2A is a cross-sectional view of a preferred power
steering control valve, cutting along the longitudinal axis of the
valve.
[0018] FIG. 2B is a side outer view of an inner shaft of the
preferred power steering control valve of FIG. 2A.
[0019] FIG. 3 is a cross-sectional view of the control valve of
FIGS. 2A and 2B, perpendicular to the longitudinal axis of the
valve, with the valve in the center-closed position.
[0020] FIG. 4 is an additional cross-sectional view of the control
valve as in FIG. 3, with the valve in an open position upon turning
of the steering wheel.
[0021] FIG. 5 shows the valve in conjunction with a rack and pinion
steering gearbox.
[0022] FIG. 6 is a three-dimensional view of a section of the inner
shaft of the preferred power steering control valve of the present
invention, showing the preferred closed-center shaft port of the
present invention.
[0023] FIG. 7 shows an alternative preferred embodiment of the
power steering control valve of the present invention.
[0024] FIG. 8 is a flow chart of sample engine-off operating
control logic for a conventional powertrain vehicle with a power
steering system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Referring to FIG. 1, a schematic view of a power steering
system for a motor vehicle 10 is set forth. Motor vehicle 10 may be
either a conventional or hybrid vehicle. For purposes of this
application, the terms "conventional" motor vehicle and
"non-hybrid" motor vehicle will be used interchangeably, referring
to the great majority of passenger vehicles that have a single
drivetrain solely and directly powered by an internal combustion
engine, with no on-board propulsive power being provided to the
drive wheels through a secondary or "hybrid" power source (such as
an electric or hydraulic motor).
[0026] Referring to FIG. 1, internal combustion engine 11
mechanically drives hydraulic pump 12. Hydraulic pump 12 is shown
in FIG. 1 being mechanically driven by the engine through an
accessory drive belt 13, but could also be mechanically driven
through integration on the engine output shaft 14, or by other
means, as is understood in the art. Optional clutch device 13b
provides the option for declutching of accessory drive belt 13 from
engine 11. Pump 12 is preferably a high-efficiency pump, such as a
bent-axis pump, but could be any type of fixed or variable
displacement hydraulic pump.
[0027] Hydraulic pump 12 receives low pressure fluid from low
pressure reservoir 15 through line 16. Fluid is pressurized through
operation of pump 12, and pumped through high pressure line 17 for
storage in high pressure accumulator 18. Optional check valve 17b
in high pressure line 17 is provided to allow declutching of pump
12 from engine 11, as mentioned above. High pressure accumulator 18
preferably includes a closeable (shut-off) valve (not shown). High
pressure accumulator 18 is also fluidly connected to power steering
control valve 19 through high pressure fluid line 20. In this
regard, it should be noted that since fluid may be provided
directly from accumulator 18 to control valve 19 for power steering
assistance, power steering assistance in the invention is not
dependent on simultaneous or continuous pumping of fluid from a
pump such as pump 12.
[0028] Optionally, because the operating pressures of hydraulic
accumulator 18 may vary significantly (e.g., by thousands of pounds
per square inch in hydraulic hybrid vehicle applications), pressure
regulator 7 (e.g., a commercially available electronic pressure
regulator) is further positioned between high pressure accumulator
18 and power steering control valve 19 to maintain the fluid
supplied to control valve 19 at desired pressures. The pressure of
fluid supplied to control valve 19 may be kept relatively constant
or may alternatively be adjusted (e.g., by signal to regulator 7
from an electronic controller, not shown) to reduce fluid pressure
above a threshold vehicle speed (e.g., above 15-20 mph) and/or to
adjust the fluid pressure as a function of vehicle speed (e.g.,
gradually reducing the fluid pressure as vehicle speed increases,
with power steering assistance stopping at speeds above about 40
mph), for better control characteristics. Return line 21 fluidly
connects power steering control valve 19 with low pressure
reservoir 15.
[0029] Steering of motor vehicle 10 in FIG. 1 proceeds as follows.
The rotation of steering wheel 22 of vehicle 10 by a driver causes
rotation of steering shaft 23, which is connected to power steering
control valve 19. Operation of power steering control valve 19 is
proportionally controlled through rotation of steering wheel 22, as
will be described hereafter. Control valve 19 is further fluidly
connected with steering actuation system 24 through two fluid lines
25 and 26 and corresponding ports 40 and 41 (shown in FIG. 2A). For
example, for right hand turns, control valve 19 may operate to
allow flow of high pressure fluid from high pressure fluid line 20
through port 41 and fluid line 25 to port 27 of steering actuation
system 24, with fluid line 26 returning low pressure fluid from
steering gearbox 24 to the low pressure reservoir 15 through
control valve 19. For the purposes of this application, the term
"steering gearbox" used at times herein shall refer not only to a
conventional steering gearbox, but also to any steering actuation
system, with or without gears, that performs the same or similar
function.
[0030] Conversely, for left hand turns, control valve 19 may
operate to cause flow of high pressure fluid from high pressure
fluid line 20 through port 40 and fluid line 26 to port 28 of
steering gearbox 24, with fluid line 25 now returning low pressure
fluid from steering gearbox 24 to the low pressure reservoir 15
through control valve 19.
[0031] A cross-sectional view along the longitudinal axis of a
preferred power steering control valve 19 is shown in FIG. 2A. As
seen in FIG. 2A, one end of control valve inner shaft 29 is rigidly
connected to steering shaft 23 by means of lock pin 30. Output
shaft 31 is connected to steering shaft 23 through a torsion bar 32
in the conventional manner. Cylindrical sleeve 33 surrounds inner
shaft 29. As is known, twisting of the torsion bar 32 occurs under
a sufficient torque from turning of the steering wheel 22 and
steering shaft 23, and inner shaft 29 rotatably changes position
within cylindrical sleeve 33 around a central longitudinal axis of
the control valve 19.
[0032] A side outer view of inner shaft 29 is presented in FIG. 2B.
As can be seen in FIG. 2B, various ports 34, 35, and 36 are cut
into inner shaft 29. Small radial center port 34 is positioned in
inner shaft 29 to be in fluid connection with high pressure
accumulator 18 through high pressure fluid line 20. A more
three-dimensional view of shaft port 34 within inner shaft 29,
showing preferred structure for the shaft port 34, is shown in FIG.
6. Larger radial ports 35 and 36 are positioned in the inner shaft
29 to be in fluid communication with low pressure reservoir 15
through return line 21, as shown further in FIG. 3.
[0033] Referring now to FIG. 3, a cross-sectional view of the
control valve along plane X of FIGS. 2A and 2B is presented,
perpendicular to the axis of the valve. In FIG. 3, the control
valve is presented in a center position, where the steering wheel
is not being turned (i.e. is torque-neutral). As can be seen in
FIG. 3, in the center position, high pressure fluid from line 20
communicates with small shaft port 34, but is prevented by lands 42
and 43 and sleeve inner surface 44 from communicating with either
line 25 or line 26 to the steering gearbox 24. As such, control
valve 19 is closed in this center position, and no flow of fluid
from high pressure accumulator 18 through control valve 19
occurs.
[0034] Referring now to FIG. 4, the situation of turning of the
steering wheel 22 to cause a clockwise movement of inner shaft 29
within sleeve 33 will be discussed. As shown in FIG. 4, upon a
clockwise movement of inner shaft 29 in turning conditions, land 42
is no longer in sealing contact with sleeve inner surface 44, and
thereby high pressure fluid from line 20 is now able to flow
through port 34 to port 41 and line 25 to steering gearbox 24,
thereby actuating power steering assistance.
[0035] Steering gearbox 24 may comprise a rack and pinion
arrangement, or recirculating ball and wormgear arrangement, which
are both well-known in the art, or may use other steering actuation
means. As an example, for a rack and pinion arrangement as shown in
FIG. 5, high pressure fluid flows from line 25 to chamber 46 within
cylinder 45. Chamber 46 is bounded by sliding piston 47. The high
pressure fluid causes sliding piston 47 to move within cylinder 45,
forcing low pressure fluid on the other side of piston 47, within
chamber 48, to exit the cylinder 45 and return via line 26 to
sleeve port 40, shaft port 36, and low pressure return line 21 to
low pressure reservoir 15 as shown in FIGS. 4 and 1. This movement
of sliding piston 47 within cylinder 45 causes linear movement of
rack 50 with teeth 51 within steering gearbox 24, thereby causing
rotary movement of pinion 52 for power steering assistance, as
understood in the art.
[0036] Optionally, means may be provided, such as a balanced spring
return means, to assist the return of steering to a center position
when the driver is no longer applying torque to turn the steering
wheel. Available spring return means could include, for example, a
rotary (watch) spring positioned on steering shaft 23, or opposing
compression springs acting on rack 50 within steering gearbox
24.
[0037] For improved steering control characteristics, and as shown
best in FIG. 6, V-shaped notches 53 are cut into land edge 55 where
the side walls of shaft port 34 meet lands 42 and 43 on the outer
surface of shaft 29. As can be further understood from FIGS. 3, 4,
and 6, upon rotation of inner shaft 29 with respect to sleeve 33 in
either a clockwise or counterclockwise direction, V-shaped notches
53 result in a more gradual opening of flow between port 34 to
respective port 40 or 41. This results in a smoother initiation of
power steering assistance upon rotation of steering wheel 22 by the
driver, with less of an undesirable on/off feeling to the power
steering system. As less preferred alternatives to V-shaped notches
53, other means for providing a smooth transition in the initiation
of power steering assistance for a closed-center valve system could
include, for example, a beveled edge or chamfer at edge 55,
U-shaped notches, or notches of other various shapes, angles,
depth, or other dimensions. V-shaped notches 53 are preferred over
a simple chamfer (see, e.g., Fukumura, "Center Closed Rotary Servo
Valve for Power Steering," SAE Paper 960929 (1996)). This is
because, as the valve rotates open, the opening begins at closer to
a "zero-volume" opening at single point 56 before increasing in
both depth and width of the opening. This provides improved control
characteristics and a smoother transition in the initiation of the
power steering assistance.
[0038] It will also be appreciated in the art that the
closed-center power steering system of the present invention
provides a further advantage of comfortable highway steering when
the steering wheel is in the central position, as there is no power
steering flow within that central deadband steering wheel position
and it thus solves the problem of over-sensitive steering in such
conditions.
[0039] Turning now to FIG. 7, an alternative second embodiment of
control valve 19 is presented. As can be understood from FIG. 7,
tapered edges or notches 54 on the inner surface of sleeve 33 are
substituted in place of notches 53 of FIGS. 3, 4, and 6, with
similar function and result, as will be readily understood in the
art.
[0040] The power steering system of the present invention enables
greater engine-off operation of vehicles including hydraulic
hybrid, hybrid electric, or conventional non-hybrid motor vehicles.
For example, FIG. 8 presents sample engine-off operating logic to
reduce fuel consumption for a conventional powertrain vehicle, such
as presented in FIG. 1. In Step S1 of the control logic run by an
engine controller (not shown) in FIG. 1, it is determined whether
there is a current driver demand for propulsion power (torque) from
the engine (e.g., whether the acceleration pedal is being pressed).
In the negative condition, a second determination is made in step
S2 whether any vehicle accessories, e.g., air conditioning, other
than power steering, are currently demanded by the driver that
would require the engine to remain on. Please note that in step S2,
the reference to accessories in demand refers here only to
accessories other than power steering that would require the engine
to remain on. If the result is negative for step S2, in step S3 a
determination is made as to whether the energy storage level in
high pressure accumulator 18 is above a minimum threshold X (e.g.
by sensing of fluid pressure through a pressure sensor within or
operatively connected to accumulator 18, or by other known means in
the art to detect or determine energy storage level within an
accumulator). The value of threshold X will be understood to be
determined as a matter of design choice considering factors such as
accumulator volume and typical power steering demand levels for the
particular vehicle weight and application, etc. If the pressure in
the high pressure accumulator 18 remains above the minimally
acceptable threshold X, then the engine controller shuts off
operation of engine 11 in Step S4 (e.g., by cutting fuel supply to
engine 11). If the contrary determination is made in any of steps
S1, S2, or S3, then the vehicle will operate with the engine on
(e.g., by resuming or continuing fuel supply to engine 11), as
stated in Step S5.
[0041] Regarding FIG. 8, it will be understood that increased
engine-off operation and reduction of fuel consumption can
additionally be obtained by further minimizing the need for engine
operation to run accessories. Thus, for example, in place of a
conventional air conditioner, a cooling block could be used to
further reduce the need for engine operation, similar to air
conditioning systems being tested on certain hybrid electric
vehicle prototypes for reduction of engine operation in future
hybrid electric vehicles. Likewise, an engine-off on-demand power
brake assist system may be implemented to increase available
engine-off operation, using (for example) an accumulator or other
energy storage device to provide power for power brake assistance
when needed.
[0042] From the foregoing it will be appreciated that, although
various specific embodiments of the invention have been set forth
herein, further modifications could also be made without deviating
from the spirit and scope of the invention. For example, it will be
well-understood that either more or fewer hydraulic lines may be
utilized to perform the same functions as the hydraulic lines
mentioned herein. Therefore, the scope of the present invention is
intended to be limited solely by the claims presented herein.
* * * * *