VWGoA - Ex. 1004
`Volkswagen Group of America, Inc. - Petitioner
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`U.S. Patent
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`Mar. 5, 1996
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`Sheet 3 of 5
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`5,495,912
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`U.S. Patent
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`Mar. 5, 1996
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`Sheet 4 of 5
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`5,495,912
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`U.S. Patent
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`' Mar. 5, 1996
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`Sheet 5 of 5
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`5,495,912
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`1
`HYBRID POWERTRAIN VEHICLE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The invention is a unique automotive hybrid powertrain
`design that allows highly efficient use of energy generated
`by an integrated internal or external combustion engine. The
`field of application is in propulsion systems for motor
`vehicles.
`2. The Prior Art
`
`The growing utilization of automobiles greatly adds to the
`atmospheric presence of various pollutants including green-
`house gases such as carbon dioxide. For this reason, there
`has been a quest for approaches to improve the efficiency of
`fuel utilization for automotive powertrains. Current power-
`trains typically average only about 10 to 15% thermal
`efficiency.
`Conventional automotive powertrains result in significant
`energy loss, make it difiicult to efl°ectively control emissions,
`and oifer limited potential to bring about major improve-
`ments in automotive fuel economy. Conventional power-
`trains consist of an internal combustion engine and a simple
`mechanical transmission having a discrete number of gear
`ratios. Due to the inefficiencies described below, about 85%
`to 90% of the fuel energy consumed by such a system is
`wasted as heat. Only l0%—15% of the energy is available to
`propel the vehicle, and much of this is dissipated as heat in
`braking.
`Much of the energy loss is due to a poor match between
`engine power capacity and average power demand. The load
`placed on the engine at any given instant is directly deter-
`mined by the total road load at that instant, which varies
`between extremely high and extremely low load. To meet
`acceleration requirements, the engine must be many times
`more powerful than the average power required to propel the
`vehicle. The efliciency of an internal combustion engine
`varies significantly with load, being best at higher loads near
`peak load and worst at low load. Since engine operation
`experienced in normal driving is nearly always at the low
`end of the spectrum,
`the engine must operate at poor
`efliciency much of the time, even though some conventional
`engines have peak efliciencies in the 35% to 40% range.
`Another major source of energy loss is in braking. In
`contrast to acceleration which requires delivery of energy to
`the wheels, braking requires removal of energy from the
`wheels. Since an internal combustion engine can only pro-
`duce and not reclaim energy, a conventional powertrain is a
`one-way energy path. Braking is achieved by a friction
`braking system, which renders useless the temporarily
`unneeded kinetic energy of the vehicle by converting it to
`heat.
`
`The broad variation in speed and load experienced by the
`engine in a conventional powertrain also makes it diflicult to
`effectively control emissions because it requires the engine
`to operate at many different conditions of combustion.
`Operating the engine at more constant speed and load would
`allow much better optimization of any emission control
`devices, and the overall more eflicient settings of the engine
`would allow less fuel to be combusted per mile traveled.
`Conventional powertrains offer limited potential to bring
`about improvements in automotive fuel economy except
`when combined with improvements in aerodynamic drag,
`weight, and rolling resistance. Such refinements can only
`offer incremental improvements in efliciency, and can apply
`equally well with improved powertrains.
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`Hybrid vehicle systems have been investigated as a means
`to mitigate the foregoing inefficiencies. A hybrid vehicle
`system provides a “bufler” between the power required to
`propel the vehicle and the power produced by_the internal
`combustion engine in order to moderate the variation of
`power demand experienced by the engine. The bufler also
`allows regenerative braking because it can receive and store
`energy from sources other than the engine. The eflectiveness
`of a hybrid vehicle system depends on its ability to operate
`the engine at peak efficiencies and on the capacity and
`efficiency of the bufler medium. Typical buffer media
`include electric batteries, mechanical flywheels and hydrau-
`lic accumulators.
`
`To use a hydraulic accumulator as the buffer, a hydraulic
`pump/motor is integrated into the system. The pump/motor
`interchangeably acts as a pump or motor. As a pump, the
`pump/motor uses engine or “braking” power to pump
`hydraulic fluid to an accumulator where it is pressurized
`against a volume of gas (e.g., nitrogen). As a motor, the
`pressurized fluid is released through the pump/motor, pro-
`ducing power.
`There are two general classes of hydraulic hybrid vehicle
`systems. A “series” system routes all of the energy produced
`by the engine through a fluid power path and so it is the fluid
`power side that experiences the variable road load. This
`improves efliciency because the efliciency of the fluid power
`path is not as sensitive to the power demand variations, and
`because the engine is thus decoupled from road load, allow-
`ing it to operate at peak efliciency or be turned off. Series
`systems are relatively simple in concept and control, but
`have less efliciency potential than other systems because all
`energy must be converted to fluid power and back to
`mechanical power to propel the vehicle. They also depend
`on frequent on/ofl operation of the engine for optimum
`efliciency. “Parallel” systems split power flow between a
`direct, almost conventional mechanical drive line and a fluid
`power path. Thus, some of the energy is spared the conver-
`sion to fluid power and back again. The most common
`context for such systems are in a “launch assist” mode where
`the hydraulic system serves mainly to store braking energy
`and to redeliver it to assist in the next vehicle acceleration.
`The parallel system, because it requires both a conventional
`and a hydraulic power path to the wheels, tends to be more
`complex than the series system and more difiicult to control
`for smoothness. Depending on the specific design, both
`series and parallel systems allow some reduction of engine
`size but both still tend to require a relatively large engine.
`For example, U.S. Pat. No. 4,223,532 (Sep. 23, 1980),
`issued to Shiber, discloses a hydraulic hybrid transmission
`system which utilizes two pump/motors and is based on a
`theory that encourages intermittent engine operation.
`
`SUMMARY OF THE INVENTION
`
`Accordingly, it is an object of the present invention to
`provide a hybrid powertrain system which allows for sig-
`nificant reduction of size of the vehicle’s internal combus-
`tion engine.
`It is a further object of the present invention to provide a
`powertrain system which allows the vehicle’s internal com-
`bustion engine to be constantly operated at near peak
`efliciency.
`It is yet a further object of the present invention to provide
`a hybrid propulsion system wherein presently unneeded
`power generated by the internal combustion engine can be
`stored in a “bufl"ef’ for use to produce driving force (1) at
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`such times when the internal combustion engine alone is
`insuflicient to provide the output torque demanded of the
`vehicle and (2) at times of very low power demand when
`engine operation would be ineflicient, e.g. in a traflic jam.
`Still another object of the present invention is to provide
`a powertrain design that allows a more highly eflicient use
`of energy generated by the internal combustion engine than
`heretofore possible.
`Still another object of the present invention is to provide
`a hybrid powertrain propulsion system which allows for
`extreme variations in road load while maintaining high
`efliciency.
`The present invention provides a unique “parallel” hybrid
`propulsion system and method of operation which meet the
`above-stated objectives. Specifically, the hybrid powertrain
`vehicle of the present invention includes a vehicle frame
`supported above a road surface by drive wheels rotatably
`mounted thereon. A primary engine, e.g. an internal or
`external combustion engine, mounted on the vehicle frame
`provides output engine power and an output shaft in a
`conventional manner. A power storage device is also
`mounted on the vehicle frame to serve as a “buffer”, i.e. for
`storing and releasing braking and “excess” engine power. A
`first drive train serves to transmit the engine power to the
`drive wheels and includes a continuously variable transrnis-
`sion (CVT) having the usual movable pulley of variable
`efl“ective diameter (or other multiple gear ratio transmis-
`sion).
`In the preferred embodiment, a reversible fluidic displace-
`ment means or “reversible pump/motor,” is interposed
`between a fluid pressure accumulator and the first drivetrain
`to output motor power to the first drivetrain, driven by the
`accumulator fluid pressure in a first mode and to operate as
`a pump, driven by the first drivetrain, to store fluid pressure
`in the accumulator in a second mode. In other embodiments
`the power storage device could be, for example, the com-
`bination of a storage battery, generator/altemator and an
`electric motor.
`
`A second drivetrain serves to connect the power storage
`device to the first drivetrain thereby defining a “parallel”
`propulsion system.
`Control of the propulsion system is provided for, in part,
`by three sensors, i.e. a vehicle speed sensor, a power storage
`sensor, e.g. a pressure sensor for sensing fluid pressure
`within the accumulator and a torque (or power) demand
`sensor for sensing torque (or power) demanded of the
`vehicle by the driver, e.g. a sensor for “throttle” pedal
`position or “accelerator” pedal depression. A microprocessor
`includes comparing means for comparing the sensed value
`of stored power with a predetermined minimum value for
`stored power and for generating a demand signal upon a
`determination that the sensed value for stored power is at or
`below the predetermined minimum value. The microproces-
`sor also includes a torque output determining means for
`determining an additional torque in accordance with the
`demand signal and for determining an engine output torque
`as the sum of the sensed torque demand and the additional
`torque. The microprocessor also includes an engine speed
`determining processor for determining an engine speed of
`optimum efliciency in accordance with the determined
`engine output torque and the sensed vehicle speed and for
`outputting a transmission signal,
`indicative of the deter-
`mined engine speed. An engine speed control means con-
`trols the rotary speed of the output shaft of the engine by
`changing the gear ratio of the transmission. In the preferred
`embodiment this involves changing the eifective diameter of
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`the movable pulley of the CVT, responsive to the transrnis-
`sion signal output by the engine speed determining proces-
`sor. An engine load controller controls engine power by
`controlling the fuel feed to the primary combustion engine
`responsive to the transmission signal. A mode controller
`serves to switch the power storage device between power
`storing and power release modes. In the preferred embodi-
`ment the mode controller serves both to convert operation of
`the fluid displacement means between the first and second
`modes of operation, responsive to the demand signal, and to
`vary the displacement of the fluid displacement means
`responsive to the sensed fluid pressure.
`Optionally, a secondary, e.g. internal combustion, engine
`is mounted on the vehicle frame to provide for additional
`engine capacity which might be needed, for example,
`to
`climb a particularly steep grade. When a secondary engine
`is mounted on the vehicle, a secondary engine clutch is
`interposed between the output of the secondary engine and
`the first drive train for matching the output speed of the
`secondary engine with the output of the primary engine.
`The propulsion system of the present invention optionally
`further includes a free wheel clutch interposed between the
`transmission (CVT) and the drive wheels for disengaging
`the drive wheels from the first drive train responsive to a
`signal indicating zero power demand.
`In the present invention the propulsion system is con-
`trolled by sensing vehicle speed, sensing fluid pressure
`within a fluid pressure accumulator and sensing power
`demanded of the vehicle by the driver. A reversible fluidic
`displacement device (pump/motor) is switched between a
`pump mode and a motor mode responsive to torque demand
`and available fluid pressure stored in the accumulator. The
`sensed fluid pressure is compared with a predetermined
`minimum fluid pressure and, if determined to be below the
`predetermined fluid pressure, a demand signal is generated.
`The additional torque necessary for adequately raising fluid
`pressure is determined in accordance with the demand signal
`and an engine output torque is determined as the sum of the
`sensed torque demand and the determined additional torque.
`An engine speed controller controls the rotary speed of the
`output shaft by changing the eifective diameter of a movable
`pulley of the CVT responsive to a transmission signal. An
`engine speed processor, in turn determines an engine speed
`of optimum efliciency in accordance with the determined
`engine output torque and the sensed vehicle speed and
`outputs a transmission signal indicative of the determined
`engine speeds. The output power of the internal combustion
`engine is controlled by controlling fuel feed thereto respon-
`sive to the transmission signal.
`In contrast to the prior art, the present system requires
`only one pump/motor in the primary drivetrain and uses the
`hydraulic subsystem in such a way as to utilize a very small
`prime engine and keeps the engine on as much as possible.
`The invention is a unique type of “parallel” system, but
`can operate in a series configuration as well. The system of
`the present invention includes a very small engine sized near
`the average power requirement rather than the peak power
`requirement. The hydraulic subsystem acts as a power-
`trimming device to “trim” the power demand experienced by
`the engine. That is, the hydraulic subsystem’s main purpose
`is to keep the engine operating as close as possible to its peak
`efliciency, by placing additional load on the engine at times
`of low propulsion power demand and delivering additional
`power at times of high or peak propulsion power demand. In
`the present invention a single hydraulic pump/motor and an
`accumulator achieve both functions. To place additional load
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`on the engine, the engine is run at a power level correspond-
`ing to peak efliciency and the excess power is routed through
`the hydraulic pump/motor (operating as a pump) into the
`accumulator where it is stored with very little energy loss. To
`deliver additional power, the stored energy is discharged to
`the powertrain through the hydraulic pump/motor (operating
`as a motor).
`In its simplest configuration, a clutching arrangement
`between the transmission and wheels allows free-wheeling
`when no power is needed from the powertrain. However, for
`simplicity, no clutching is provided between the engine,
`hydraulic pump/motor, and transmission. Therefore,
`the
`engine may occasionally be motoring while the pump/motor
`is charging the accumulator during regenerative braking or
`when delivering small amounts of power by itself. This
`creates a drag on the power train that reduces efliciency
`somewhat. The friction losses associated with this arrange-
`ment are minimal due to the small displacement of the
`internal combustion engine and the small amount of time in
`this mode of operation.
`The present invention includes at least two configurations
`for hydraulic regenerative braking. In the first embodiment,
`friction brakes are activated first, after which hydraulic
`braking is phased in. This method reduces the sophistication
`of the controls that would be needed to effect a smooth
`routing of power from the wheels, and allows safety in case
`of a hydraulic system failure. In the second embodiment,
`hydraulic braking occurs first with friction brakes added as
`a backup system. This second embodiment is somewhat
`more complex to control, but is the preferred embodiment
`because it maximizes the recovery of braking energy.
`When accelerating from a stop, the engine provides power
`to the wheels through the non-hydraulic portion of the
`driveline. If more power is needed than the engine can
`provide, additional power is supplied by the pump/motor
`acting as a motor. The accumulator is of suflicient size to
`allow this additional power to be provided two or more times
`in succession. Accumulator capacity for at least one accel-
`eration is needed for regenerative braking and capacity for
`another is needed as backup in case a stop does not allow
`regenerative braking.
`When cruising speed is reached and power demand drops
`off to a low level, the engine output matches the road load
`because the engine is small enough that its peak efliciency
`corresponds to loads characteristic of average road load. If
`more power is required of the engine in order to maintain
`peak operating efliciency, an additional load is provided by
`charging the accumulator through the pump/motor acting as
`a pump. If the accumulator can accept no more charge, the
`pump/motor is set to zero displacement and the engine
`merely runs at a reduced power output. Since the engine is
`sized close to the average power load during cruising, there
`is little or no sacrifice in efliciency at this setting. The engine
`can also be turned off and the accumulator can drive the
`pump/motor acting as a motor, if the load is very low as
`would occur in low speed, stop and go traffic.
`When braking occurs, and if there is sufiicient unused
`storage capacity reserved in the accumulator, regenerative
`braking occurs where the pump/motor acts as a pump to
`charge the accumulator. If there is no capacity left in the
`accumulator, friction brakes are used. The system is man-
`aged so that
`there will normally be suflicient capacity
`available for regenerative braking.
`If sudden acceleration is required during a cruising period,
`this may be provided by boosting the output of the engine
`along the best efliciency line. After the maximum eflicient
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`engine power output point is reached, the hydraulic sub-
`system is activated to retrieve additional power from the
`accumulator via the pump/motor.
`When the car creeps along at a very low speed, as in a
`traflic jam, the engine is turned off and the pump/motor and
`accumulator are used to drive the car. This is better than
`using the engine alone in such a mode because a pump/
`motor can operate at a good efficiency even at low speeds
`and low power demands.
`Through proper choice of component sizes and control
`system optimization, the system can be designed to optimize
`various goals. For instance, one could minimize the chance
`of either: a) encountering a fully charged accumulator when
`regenerative braking energy becomes available, or b) deplet-
`ing the accumulator by several rapid accelerations without
`chance to recharge the accumulator.
`The use of a small engine supplemented by an accumu-
`lator of finite energy storage capacity presents a difliculty in
`ascending long grades. Just as with acceleration, ascending
`a grade requires an unusually large amount of power, but
`unlike an acceleration a long grade requires this power for
`an extended period of time. Since the theory of operation of
`the invention is to provide a large portion of acceleration
`power by means of a hydraulic accumulator, a long grade
`would deplete the accumulator in short order and the vehicle
`would be left with insuflicient power.
`As an alternative to an extremely large accumulator
`capacity, a second engine, which can be inexpensive and of
`only moderate durability due to its occasional use, may be
`clutched in to supplement the power of the primary engine
`and pump/motor for an unlimited time.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a schematic diagram of a first embodiment of a
`vehicle equipped with a hybrid powertrain propulsion sys-
`tem of the present invention.
`FIGS. 211, 2b, 2c and 2d are graphs of engine load versus
`engine speed in various modes of operation of the system
`depicted in FIG. 1.
`FIG. 3 is a schematic illustration of a vehicle equipped
`with a second embodiment of a hybrid powertrain propul-
`sion system in accordance with the present invention.
`FIG. 4 is a schematic illustration of a vehicle equipped
`with a third embodiment of a hybrid powertrain propulsion
`system in accordance with the present invention.
`FIG. 5 is a schematic illustration of a vehicle equipped
`with a fourth embodiment of a hybrid powertrain propulsion
`system in accordance with the present invention.
`FIG. 6 is a logic flow diagram for control of operation of
`a vehicle by a microprocessor in accordance with the present
`invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIIVEENTS
`
`FIG. 1 illustrates an embodiment of the present invention
`suitable for driving a three to four thousand pound vehicle.
`A very small internal combustion engine 1 (e.g. 20 hp)
`provides energy to the system. The energy is transmitted
`along the driveshaft 2, which constitutes a first drivetrain,
`and can be routed either to the transmission 3,
`in this
`embodiment a continuously-variable transmission (CVT), or
`to the pump/motor 7 (acting as a pump in the second mode)
`or both. The pump/motor 7 is a reversible hydraulic dis-
`placement device, e.g. a swash plate pump in which flow
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`reversal is inherent to the pump or a bent axis pump wherein
`flow reversal is by valving external to the pump, capable of
`operating either in a first mode as a motor or in a second
`mode as a pump. The pump/motor 7 has a variable displace-
`ment. Energy routed to the pump/motor 7 (acting as a pump)
`is used to pump fluid to the accumulator 6, pressurizing the
`fluid B against a volume of gas A. Energy routed to the
`transmission flows along the lower driveshaft 9 past the
`freewheel clutch 4 to the wheels 5. The pump/motor 7 is
`switched between its first and second modes and its dis-
`
`placement is varied by a pump/motor controller 20, respon-
`sive to a signal FPs.
`When the power demanded at the wheels 5 is larger than
`the power deliverable by the engine 1 alone, additional
`power is provided by the pump/motor 7 (acting as a motor
`in the first mode). In this mode the pressurized fluid in the
`accumulator 6 flows to the pump/motor 7 (acting as a
`motor), creating mechanical power that flows along the
`drive shaft 30 to driveshaft 2, to the transmission 3 and flows
`to the wheels as already described. The hydraulic accumu-
`lator 6, pump/motor 7 and shaft 30 constitute a second
`drivetrain, “parallel” to the first drivetrain.
`Indicated at 26 is an engine control device, e.g. a fuel
`injection pump, which controls fuel feed to the engine 1,
`responsive to a signal Es which is a function of engine speed.
`Signal Es may be computed by processor 18 or may be a
`signal received directly from an rpm sensor 40.
`The control hardware for operation of the vehicle includes
`a vehicle speed sensor, e.g. rpm sensor 12, which detects the
`rotational speed of the drive shaft downstream of the free-
`wheel clutch 4, a pressure sensor 16 for detecting the
`pressure within the fluid pressure accumulator 6 and gener-
`ating a signal Ps representative of the detected pressure and
`a power demand sensor 14, e.g. a sensor for detecting
`position of the “accelerator pedal.” A first processor 42
`receives the signal Ps representative of the fluid pressure
`detected by sensor 16 and compares that detected fluid
`pressure with a predetermined minimum fluid pressure and
`generates a demand signal FPs upon determination that the
`sensed fluid pressure is below the predetermined minimum
`fluid pressure. That demand signal FPs is sent to the pump
`controller 20 for conversion of the pump/motor 7 to the
`second mode for operation as a pump, to store energy in the
`accumulator 6 in the form of fluid pressure.
`A second processor 44 determines an additional power in
`accordance with the demand signal FPs and an engine output
`power as the sum of the power demand sensed by 14 and the
`determined additional power. A third processor 46 deter-
`mines the engine speed of optimum efliciency in accordance
`with the determined total engine output power, and with the
`sensed vehicle speed outputs a transmission signal Ts,
`indicative of the determined optimum engine speed to the
`engine speed controller 24. Controller 24 regulates engine
`speed responsive to the signal Ts by changing the effective
`diameter of pulley 22 of the CVT 3. Processors 42, 44 and
`46 may optionally be combined into a single microprocessor
`18 including a memory 48. The signal Ts is determined by
`reference to a two dimensional map stored in memory 48
`wherein values for optimum eflicient power and engine
`speed are correlated. Knowing the desired engine speed and
`the vehicle speed from sensor 12, signal Ts is computed.
`This control system is likewise applicable to the other
`embodiments described hereinbelow.
`
`An optional secondary engine 10 can provide yet addi-
`tional reserve power. In this case an electronically controlled
`clutch 11 is engaged through which the power from engine
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`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`60
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`65
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`10 feeds into the system. The secondary engine 10 provides
`backup power for severe or repeated accelerations and for
`continuous operation to maintain speed up long and/or steep
`grades. The secondary engine 10 and clutch 11 can be
`installed as shown (to supply power to the drive shaft 2) or
`to supply power to drive shaft 9 directly. The engine 10 may
`be electronically started and clutch 11 engaged responsive to
`a signal SEs generated as a function, for example, of the
`sensed “accelerator pedal” position and detected accumula-
`tor fluid pressure. The clutch 11 serves to engage the
`secondary engine at the output speed of the primary engine.
`The primary engine 1 and the secondary engine 10,
`in
`combination, might be regarded as the functional equivalent
`of a variable displacement engine.
`When zero power is demanded at the wheels, the vehicle
`is changed over to a coasting mode, responsive to a signal
`Cs from the microprocessor 18, by disengagement of the
`freewheeling clutch 4. In this manner the vehicle is isolated
`from rotational friction losses in the drivetrain so that all of
`the kinetic energy of the vehicle is available for overcoming
`rolling resistance and aerodynamic drag. The clutch 4 is
`normally engaged and is disengaged only when zero power
`demand is detected by sensor 14.
`When the driver brakes, regenerative braking occurs.
`Kinetic energy is transferred from the wheels 5 past the
`clutch 4 through the transmission 3 along the drive shaft 2
`into the pump/motor 7 (acting as a pump). The pump/motor
`7 pressurizes fluid and thereby stores the energy in the
`accumulator 6 in the same manner as described above.
`
`Through fluid pressure in accumulator 6, the pump/motor
`7, operating in its first mode as a motor may be used to start
`engine 1, thereby eliminating need for a conventional starter
`motor.
`
`The operation of the invention will be more clearly
`understood in reference to FIGS. 2A—2D. In the following
`discussion the term “optimum efliciency” refers to a range of
`speed and load, i.e. (power) at which the efliciency of the
`engine 1 is deemed reasonably near its optimum efliciency,
`between points A and B.
`FIG. 2A is a graph which represents instances (Mode 1)
`when the power demanded is greater than that deliverable at
`optimum efliciency by the engine 1 (point B) in the embodi-
`ment of FIG. 1. In this case, that portion of load which
`exceeds B is provided by the pump/motor 7 (acting as a
`motor), while the engine 1 provides the rest. In embodiments
`where the engine and pump/motor shafts are not clutched or
`geared, the engine 1, pump/motor 7, and transmission 3
`input shaft would operate at the same speed. A clutching
`arrangement or a gear reduction could be incorporated
`therein without changing the basic function of this mode.
`FIG. 2B illustrates the operation of the system of FIG. 1
`in a mode 2, i.e. when power demanded of engine 1 is within
`the range of optimum efliciency (between power levels A
`and B). This power demanded of engine 1 is determined by
`microprocessor 18 considering power demanded by driver
`14 and whether power should be supplied to or extracted
`from the accumulator 6. If there is no need to replenish the
`accumulator 6, all of the power is provided by the engine 1,
`and the pump/motor 7 is stroked to zero displacement (i.e.,
`neutral position) by controller 20 where it neither pumps
`fluid into the accumulator 6 nor provides power to the
`system.
`FIG. 2C illustrates the situation where the engine 1 can
`satisfy the driver power demand, and there is need (i.e., the
`accumulator energy level has reached a predetermined mini-
`mum level, but the engine 1 can operate at an optimum
`
`10
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`10
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`

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`5,495,912
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`9
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`power level, point (b)) or desire (i.e., need to operate the
`engine at its optimum efliciency as indicated by driver power
`demand point (a)) to replenish the accumulator (mode 3).
`While road load demanded is represented by either of the
`points (a) or (b) shown in FIG. 2C, the power output of the
`engine is increased along the optimum efficiency line to a
`point at which suflicient excess power is generated, illus-
`trated here by the point (c). The excess power that does not
`go to road load is fed into the pump/motor 7 (acting as a
`pump) which stores it in the accumulator 6 for future Mode
`1 or Mode 4 events.
`
`FIG. 2D illustrates mode 4 wherein an unusually small
`road load is experienced. In this case, the engine cannot
`deliver such a small amount of power at acceptable efli-
`ciency and significant pressure exists in the accumulator 6.
`The fuel flow to the engine 1 is turned ofl°, and the pump/
`motor 7 (acting as a motor) provides power by itself.
`Regenerative braking can be thought of as an extension of
`Mode 4 (FIG. 2D), in which power demand is zero and the
`vehicle must decelerate at a rate greater than rolling resis-
`tance and aerodynamic drag would provide. The driver
`activates the brakes, which in turn activate the pump/motor
`7 (acting as a pump) which pressurizes fluid as previously
`described using the vehicle’s kinetic energy taken through
`the drive shaft 2, transmission 3 and lower drive shaft 9. This
`results in a deceleration similar to that caused by friction
`braking, but the energy is saved in the accumulator 6 rather
`than discarded.
`
`An alternate embodiment adapted for operation which is
`expected to involve more extensive stop and go driving is
`shown in FIG. 3. In continual stop and go driving, a mode
`is invoked in which the pump/motor directly drives the
`vehicle without assistance from the engine. In this case a
`clutch 8 is provided between the engine 1 and pump/motor
`7 so as to disconnect the engine 1 in this mode and prevent
`friction associated with operation of the engine 1.
`Yet another embodiment is shown in FIG. 4, wherein a
`second pump/motor 13 is provided between the transmission
`and the wheels. This configuration would allow regenerative
`braking energy to proceed through the sec