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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 greenhouse gases such as carbon dioxide. For this reason, there has been a quest for approaches to improve the efficiency of 15 fuel utilization for automotive powertrains. Current powertrains typically average only about 10 to 15% thermal efficiency.
Conventional automotive powertrains result in significant energy loss, make it difficult to effectively control emissions, 20 and offer limited potential to bring about major improvements in automotive fuel economy. Conventional powertrains 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% 25 to 90% of the fuel energy consumed by such a system is wasted as heat. Only 10%—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 30 engine power capacity and average power demand. The load placed on the engine at any given instant is directly determined 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 35 more powerful than the average power required to propel the vehicle. The efficiency 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 40 end of the spectrum, the engine must operate at poor efficiency much of the time, even though some conventional engines have peak efficiencies in the 35% to 40% range.
Another major source of energy loss is in braking. In 45 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 produce and not reclaim energy, a conventional powertrain is a one-way energy path. Braking is achieved by a friction 5Q 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 difficult to 55 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 efficient settings of the engine 60 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 65 offer incremental improvements in efficiency, 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 "buffer" 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 buffer also allows regenerative braking because it can receive and store energy from sources other than the engine. The effectiveness of a hybrid vehicle system depends on its ability to operate the engine at peak efficiencies and on the capacity and efficiency of the buffer medium. Typical buffer media include electric batteries, mechanical flywheels and hydraulic 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, producing 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 efficiency because the efficiency of the fluid power path is not as sensitive to the power demand variations, and because the engine is thus decoupled from road load, allowing it to operate at peak efficiency or be turned off. Series systems are relatively simple in concept and control, but have less efficiency 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/off operation of the engine for optimum efficiency. "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 conversion 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 difficult 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 significant reduction of size of the vehicle's internal combustion engine.
It is a further object of the present invention to provide a powertrain system which allows the vehicle's internal combustion engine to be constantly operated at near peak efficiency.
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 "buffer" for use to produce driving force (1) at
such times when the internal combustion engine alone is insufficient to provide the output torque demanded of the vehicle and (2) at times of very low power demand when engine operation would be inefficient, e.g. in a traffic jam.
Still another object of the present invention is to provide a powertrain design that allows a more highly efficient 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 efficiency.
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 transmission (CVT) having the usual movable pulley of variable effective diameter (or other multiple gear ratio transmission).
In the preferred embodiment, a reversible fluidic displacement 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 combination of a storage battery, generator/alternator 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 microprocessor 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 detenriining processor for determining an engine speed of optimum efficiency in accordance with the determined engine output torque and the sensed vehicle speed and for outputting a transmission signal, indicative of the determined engine speed. An engine speed control means controls 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 effective diameter of
the movable pulley of the CVT, responsive to the transmission signal output by the engine speed determining processor. 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 embodiment 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 controlled 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 effective 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 efficiency 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 responsive 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 powertrimming 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 efficiency, 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
on the engine, the engine is run at a power level corresponding to peak efficiency 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 5 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 10 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 15 creates a drag on the power train that reduces efficiency somewhat. The friction losses associated with this arrangement are minimal due to the small displacement of the internal combustion engine and the small amount of time in this mode of operation. 20
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 25 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 30 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 sufficient size to allow this additional power to be provided two or more times in succession. Accumulator capacity for at least one acceleration 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 efficiency 45 corresponds to loads characteristic of average road load. If more power is required of the engine in order to maintain peak operating efficiency, 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 5Q 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 efficiency 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 sufficient unused storage capacity reserved in the accumulator, regenerative braking occurs where the pump/motor acts as a pump to go charge the accumulator. If there is no capacity left in the accumulator, friction brakes are used. The system is managed so that there will normally be sufficient capacity available for regenerative braking.
If sudden acceleration is required during a cruising period, 65 this may be provided by boosting the output of the engine along the best efficiency line. After the maximum efficient
engine power output point is reached, the hydraulic subsystem 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 traffic 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) depleting the accumulator by several rapid accelerations without chance to recharge the accumulator.
The use of a small engine supplemented by an accumulator of finite energy storage capacity presents a difficulty 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 insufficient 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 system of the present invention.
FIGS. 2a, 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 propulsion 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
EMBODIMENTS
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 displacement device, e.g. a swash plate pump in which flow
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