WO2008133805A2

WO2008133805A2 - Hydro-mechanical hydraulic hybrid drive train with independent wheel torque control

Info

Publication number: WO2008133805A2
Application number: PCT/US2008/004618
Authority: WO
Inventors: Perry Y. Li; Michael Olson; Kyle True; Charles Toy
Original assignee: Regents Of The University Of Minnesota; Van De Ven, James, D.
Priority date: 2007-04-10
Filing date: 2008-04-10
Publication date: 2008-11-06
Also published as: WO2008133805A3

Abstract

A hydro-mechanical hybrid drive train (100, 200, 300) is provided The drive train includes a prime mover (102) that has an output (104 A first hydraulic pump/motor unit (110) is operably coupled to the output (104) of the prime mover (102) The first hydraulic pump/motor unit (110) has a mechanical output (112) A mechanical transmission (114) has a plurality of gear ratios, an input shaft and a mechanical output (116) The input shaft is coupled to the mechanical output (112) of t4e first hydraulic pump/motor unit (110) A planetary differential (118) is mechanically coupled to the output (116) of the transmission (114) and provides first and second mechanical outputs A second hydraulic pump/motor unit (130) is coupled to the first mechanical output of the planetary differential (118) At least one drive element (126) is operably coupled to the second mechanical output of the planetary differential (118)

Description

HYDRO-MECHANICAL HYDRAULIC HYBRID DRIVE TRAIN WITH INDEPENDENT WHEEL TORQUE CONTROL

GOVERNMENT RIGHTS The United States government has certain rights in this invention pursuant to Agency Grant No. NSF EEC-0540834 awarded by National Science Foundation.

BACKGROUND The present invention relates to drive trains. More specifically, the present invention relates to a hydro-mechanical hydraulic hybrid drive train.

A major component of global energy consumption is transportation, which currently consumes 4.8 billion barrels of crude oil per year. Of the transportation industry, passenger cars consume approximately 2 billion barrels of oil per year with a value of approximately 100 billion dollars, as of 2003. This substantial fuel consumption is a motivation for the development of technologies that improve the efficiency of passenger cars. One improvement that has recently developed in the automotive industry is the hybrid vehicle. A hybrid vehicle contains two sources of power consisting of an internal combustion engine and a secondary power source that allows for energy storage. The energy storage is used during braking events and other drive train control strategies to minimize, or otherwise reduce, fuel consumption. Two types of auxiliary power sources that have been found to be quite practical are: electric motors/generators combined with batteries and hydraulic pump/motors combined with hydraulic accumulators.

Electric hybrid vehicles have been the first hybrid technology to be mass produced for the commercial passenger market. A strength of electric hybrids is the high-energy density of electric batteries, thereby allowing for large energy storage in relatively compact and light weight batteries. One limitation of such electric hybrids is the relatively low power density of both electric motors/generators and batteries at approximately 100 to 300 W/kg. This limits acceleration and deceleration capability and the ability of the system to capture braking energy. Switching the secondary hybrid power source from the electric source to a hydraulic source realizes benefits in multiple areas. First, the power density of hydraulic pumps/motors and accumulators is very high at approximately 500-1000 W/kg. Second, hydraulic components are relatively inexpensive when compared with electrical components, such as advanced battery packs. Third, certain hybrid architectures allow for independent control of the torque at each wheel, which opens numerous possibilities for vehicle dynamics control. Fourth, recent and developing technologies such as digital hydraulic valves and high-energy density accumulators are improving the future outlook of hydraulic hybrid vehicles. One significant advantage of certain hydraulic hybrid drive train architectures is the leveraging of the intrinsically high power density of the hydraulic energy storage system through optimal engine management. Internal combustion engines create power most efficiently at relatively high power levels near the RPM of the peak torque output. Operating at other conditions decreases the energy conversion efficiency. An optimal energy management scheme runs the internal combustion near its peak efficiency, with a portion of the power being transferred to the wheels, while additional power is stored in hydraulic accumulators. Once the accumulator is charged to a desired state, the engine is shut off and power for vehicle propulsion is supplied by the accumulator. Two relatively different types of hydraulic hybrid drive train architectures in use today include the "parallel" system and the "series" system. A "parallel" system uses a traditional mechanical drive train with a hydraulic pump/motor inline between the transmission and the axle. The mechanical transmission of the parallel system provides an efficient transfer of power from the engine to the wheels, yet the engine speed is determined by the vehicle speed and the available gear ratios, thereby preventing optimal engine management. The majority of hybrid drive trains for large vehicles, such as buses, delivery vehicles and refuse trucks use a parallel drive train, which consists of a standard mechanical drive train with a hydraulic pump/motor unit inline with the transmission output. This allows energy storage and release, but does not create an infinitely variable transmission, forcing the engine operating speed to be dependent upon the vehicle speed.

In a "series" hybrid system, the engine is directly coupled to a hydraulic pump and the wheels are coupled to one or more pump/motors. This purely hydraulic transmission allows optimal engine management yet creates a less efficient power transmission path than a mechanical transmission. However, by placing a hydraulic pump/motor at each of the four wheels, independent wheel torque control is possible in the series hybrid system. The series drive train was demonstrated by the EPA in a UPS delivery vehicle. The series hybrid system allows both energy storage and an infinitely variable speed ratio between the engine and the wheels. The series hybrid suffers from lower drive train efficiency as all power is transmitted through the hydraulic circuit.

While hydraulic hybrid drive train systems provide a number of advantages over electrical hybrid drive train systems, implementations thus far have achieved some of the advantages, but have also had some limitations. Providing a hydraulic hybrid drive train that allows optimal engine management and independent wheel torque control would provide the benefits of the high power density of hydraulic pumps/motors without the limitations that have accompanied those benefits in the past. Such a system would represent a significant improvement for vehicle drive trains, and potentially save significant energy costs through increased efficiency of such vehicles.

SUMMARY OF THE INVENTION -A-

A hydro-mechanical hybrid drive train is provided. The drive train includes a prime mover that has an output. A first hydraulic pump/motor unit is operably coupled to the output of the prime mover. The first hydraulic pump/motor unit has a mechanical output. A mechanical transmission has a plurality of gear ratios, an input shaft and a mechanical output. The input shaft is coupled to the mechanical output of the first hydraulic pump/motor unit. A first input of a planetary differential is mechanically coupled to the output of the transmission. A second hydraulic pump/motor unit is coupled to a second input of the planetary differential. At least one drive element is operably coupled to the mechanical output of the planetary differential. A hydraulic accumulator is operably coupled to the first and second hydraulic pump/motor units.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of a hydro-mechanical drive train in accordance with an embodiment of the present invention.

FIG. 2 is a diagrammatic view a hydro-mechanical hybrid drive train in accordance with another embodiment of the present invention.

FIG. 3 is a diagrammatic view of a hydro-mechanical hybrid drive train in accordance with another embodiment of the present invention. FIG. 4 is a flow diagram of a method of operating a hydro- mechanical hybrid drive train in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Embodiments of the present invention generally provide a high efficiency drive train that is capable of an infinitely variable speed ratio between the prime mover and the output as well as regeneration. While embodiments of the present invention will generally be described with respect to a wheeled vehicle, embodiments of the present invention are also applicable to many other power transmission applications.

Drive trains in accordance with various embodiments of the present invention generally include a prime mover, typically an internal combustion engine, which is mechanically coupled, through a clutch, to both a variable displacement hydraulic pump/motor unit and the input shaft of a finite geared mechanical transmission. The output of the mechanical transmission is provided as an input to a planetary differential. The second input to the planetary differential is a separate hydraulic pump/motor unit. The output of the planetary differential is coupled to the final drive, the wheels in the case of a wheeled vehicle. The two hydraulic pump/motor units are both hydraulically coupled to an accumulator, which stores and releases hydraulic energy, as required.

FIG. 1 is a diagrammatic view of a hydro-mechanical drive train in accordance with an embodiment of the present invention. Double lines represent a mechanical shaft coupling two elements together, while single lines represent hydraulic coupling. Drive train 100 operates by setting the angular velocity and torque of the prime mover, illustrated diagrammatically as an internal combustion engine 102, to relatively fixed values and splitting the power through a high efficiency mechanical circuit and a speed-variable hydraulic circuit. As illustrated in FIG. 1, drive train 100 includes prime mover 102, which is illustrated as an internal combustion engine. Prime mover 102 is coupled, via shaft 104, to clutch 106 which allows an operator or driver to selectively engage drive train 100. Clutch 106 is coupled, via shaft 108, to variable displacement hydraulic pump/motor unit 110. Pump/motor unit 110 is coupled, via shaft 112, to mechanical transmission 114. Mechanical transmission 114 is any drive arrangement that includes an input shaft, and provides an output shaft and allows a plurality of ratios between the input and the output. While it is preferred that transmission 114 be a finite geared transmission, embodiments of the present invention can be practiced with continuously variable transmissions (CVT) as well. The output of transmission 114 is coupled, via shaft 116, to planetary gear 118. The output of planetary gear 118 is coupled to axle differential 120 which provides a pair of output shafts 122, 124 that are coupled to wheels 126, or other suitable drive elements. Pump/motor 110 is fluidically coupled to hydraulic fluid reservoir 128. Similarly, pump/motor 130 is fluidically coupled to reservoir 132. Pump/motor 110 functions as a pump when more mechanical energy is input from shaft 108 than provided on output 112. The fluid pumped by pump 110 during such periods is stored, or otherwise maintained, in high pressure accumulator 134. However, when clutch 106 is disengaged, and/or prime mover 102 is stopped, high pressure accumulator 134 can supply fluid pressure to pump/motors 110, 130 which then function as a hydraulic motors generating mechanical energy from the stored fluid pressure in high pressure accumulator 134. During deceleration, the system is reversed with axle differential 120 providing an input to planetary gear 118, which drives pump/motor 130 and/or pump/motor 110, and pumps hydraulic fluid from reservoir 132, 128 into high pressure accumulator 134. This allows the kinetic energy of virtually any deceleration which does not lock up the wheels 126 on the road to be recaptured and stored as pressurized hydraulic fluid in accumulator 134. Once accumulator 134 reaches a specific state of charge, prime mover 102 is turned off and de-clutched from drive train 100, while accumulator 134 powers either one or both of hydraulic pump/motor units 110, 130. Accordingly, during the braking of a wheeled vehicle, the power is absorbed by one or both hydraulic pump/motor units 110, 130 and stored in high pressure accumulator 134. A benefit of the utilization of finite geared mechanical transmission 114 is the ability to change the angular velocity of the pump/motor unit 110 coupled to the input 112 of mechanical transmission 114 by changing gear ratios. This reduces the torque variation and allows the use of a smaller pump/motor unit 110, which improves system efficiency, compactness and weight. FIG. 2 is a diagrammatic view of a hydro-mechanical hybrid drive train in accordance with another embodiment of the present invention. The embodiment illustrated in FIG. 2 provides the added feature of independent torque control for each wheel. Those skilled in the art will recognize that drive train 200 bears some similarities to drive train 100, and like components are numbered similarly. Drive train 200 still includes a prime mover 102 mechanically coupled to a hydraulic pump/motor unit 110. Additionally, pump/motor unit 110 is coupled to transmission 114 via shaft 112. Drive train 200 differs from drive train 100 in that the output of the mechanical transmission is split into two, or optionally four, separate shafts, based on whether the vehicle is a two or four wheel drive, respectively. Thus, system 200 includes right-angle gear set 220 in place of axle differential 120. Each of shafts 122, 124 is provided as the input to respective planetary differentials 204, 202. The other input to each planetary differential 202, 204 is a small hydraulic pump/motor unit 206, 208, respectively. The outputs of planetary differentials 202, 204 drive wheels 126. The embodiment illustrated in FIG. 2 allows the power to be split between the highly efficient mechanical circuit (comprising the mechanical transmission, right-angle gear set and planetary differentials 202, 204) and the infinitely variable ratio hydraulic circuit (comprising pump/motor units 110, 206, 208 and high pressure accumulator 134). An added benefit of having variable displacement pump/motors for each wheel 126 is the provision of independent torque control of each wheel. This independent torque control can be utilized in a variety of vehicle stability schemes such as traction control, stability control, rollover prevention, and anti-lock braking. When taken to the extreme, this development even allows "skid-steering" of the vehicle by rotating the wheels forward on one side of the vehicle and backward on the other side of the vehicle, allowing rotation of the vehicle without longitudinal movement.

FIG. 3 is a diagrammatic view of a hydro-mechanical hybrid drive train in accordance with another embodiment of the present invention. System 300 includes many components that are similar to system 200 illustrated with respect to FIG. 2, and like components are numbered similarly. The primary difference between system 200 and 300 is that system 300 includes a mechanical torque splitter, or transfer case 302 that is mechanically interposed between the output shaft of mechanical transmission 114 and right angle gear set 220. Splitter 302 provides, as an output, shaft 304 that is mechanically coupled to auxiliary right-angle gear set 306. Gear set 306 provides a pair of output shafts 308, 310 that are provided as inputs to planetary differentials 312, 314, respectively. Outputs of planetary differentials 312, 314 are provided to wheels 126, or other suitable drive elements, as may be desired. While splitter 302 is illustrated diagrammatically as providing two output shafts at right angles to one another, those skilled in the art will recognize that commercially available splitters such as four-wheel drive transfer cases, or all-wheel drive transfer cases provide the output shafts to be lined parallel with one another. However, the illustration of FIG. 3 is provided at right angles for clarity.

Each of planetary differentials 312, 314 is coupled to a respective pump/motor unit 316, 318, respectively. Each pump/motor unit 316, 318 is coupled, along with pump/motor units 206, 208 to high pressure accumulator 134. Thus, by suitably controlling the various pump/motor units 206, 208, 316, 318, the potential energy stored within high pressure accumulator 134 can be individually delivered, in specific ratios, to the various wheels. Additionally, if pump/motor units 206 and 318 are controlled to drive their respective wheels 126 forward, while pump/motor units 208 and 316 are controlled to drive their respective wheels backward, the vehicle will rotate without generating any appreciable longitudinal displacement.

FIG. 4 is a flow diagram of a method of operating a hydro- mechanical hybrid drive train in accordance with an embodiment of the present invention. Method 400 begins at block 402 where the prime mover is started. Preferably, the hydraulic system is maintained with enough energy to start the prime mover using stored hydraulic, such as using a hydraulic starter pump. However, embodiments of the present invention can be practiced with a conventional electric started motor. Method 400 continues at block 404 where the angular velocity (RPM) is set to a selected velocity (e.g. 4000 RPMs). While it is generally preferable to set the velocity of the prime mover to the most efficient velocity of the prime mover, embodiments of the present invention also include setting the velocity to a different setting to increase the overall efficiency of the prime mover/drive system. At block 406, the clutch is released to couple the prime mover to a hydraulic pump/motor, such as pump motor 110 (shown in FIG. 1). Once the clutch is released, method 400 continues at block 408 where a fraction of the torque from the prime mover (preferably greater than 50%) is provided to a mechanical transmission, while the remainder of the torque from the prime mover powers the hydraulic pump. At block 410, the torque from the mechanical transmission is provided to one or more drive elements, such as wheels 126. At block 412, substantially all work done by the hydraulic pump/motor unit is stored in a hydraulic accumulator.

At block 414, the method checks to see if the charge level of the hydraulic accumulator is at or above a "charged" threshold. If the charge level is not above the "charged" threshold, control returns to block 412 and the method keeps charging the accumulator and checking the charge level until the "charged" threshold is reached. Once the charged threshold is reached, control passes to block 416 where the prime mover is de-clutched and shut down. Then, at block 418, the mechanical transmission is driven by the hydraulic pump/motor using energy stored in the accumulator. At block 420, the charge level of the accumulator is checked to see if it is at or below a selected "discharged" level. If the discharged level is reached, control returns to block 402 where the prime mover is started. It should be noted that the "discharged" level is a level that is set to cause the prime mover to restart, and need not actually be a state where the accumulator contains no stored energy. Various embodiments of the present invention are applicable to a wide variety of power transmission applications. Embodiments are especially well suited to applications requiring an infinitely variable speed ratio and/or energy storage for cycling the prime mover or regeneration. Possible applications include wheeled vehicles, marine propulsion, wind generation, industrial machinery and many others.

The hybrid passenger cars that are commercially available today are electric hybrids that store energy and batteries and use electric motor/generator units for auxiliary propulsion. Embodiments of the present invention generally provide a hydro-mechanical hydraulic hybrid system.

Embodiments of the present invention are believed to be superior to the parallel hydraulic drive train because of the infinite variability in the speed ratio of the prime mover and the load. This allows the engine to operate at the most efficient angular velocity and can generate torque independent of the wheel speed, hi addition, embodiments of the present invention enable vehicle stability control through independent control of the torque at each wheel.

Embodiments of the present invention are believed to be superior to the series hydraulic drive train because of the higher power conversion efficiency. Because embodiments of the present invention are capable of transmitting the majority of the power through the mechanical circuit (transmission 114) the inherent inefficiencies of hydraulics do not create as great an effect on overall efficiency. In addition, the mechanical transmission allows the hydraulic pump/motor units to operate across a smaller torque range allowing them to be smaller and improving their efficiency. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A hydro-mechanical hybrid drive train comprising: a prime mover having an output; a first hydraulic pump/motor unit operably coupled to the output of the prime mover, the first hydraulic pump/motor unit having a mechanical output; a mechanical transmission having a plurality of gear ratios, an input shaft and a mechanical output, the input shaft being coupled to the mechanical output of the first hydraulic pump/motor unit; a planetary differential having a first input mechanically coupled to the output of the transmission, the planetary differential having a second input, and a mechanical output; a second hydraulic pump/motor unit coupled to the second input of the planetary differential; at least one drive element operably coupled to the mechanical output of the planetary differential; and a hydraulic accumulator operably coupled to the first and second hydraulic pump/motor units.

2. The hydro-mechanical drive train of claim 1 , and further comprising a clutch interposed between the prime mover and the first hydraulic pump/motor unit to selectively engage the prime mover with the first hydraulic pump/motor unit..

3. The hydro-mechanical drive train of claim 1, and further comprising a drive differential gear mechanically coupled to the output of the planetary differential, the drive differential gear having a pair of output shafts, where each output shaft is coupled to a drive element.

4. The hydro-mechanical drive train of claim 3, wherein each drive element is a wheel.

5. The hydro-mechanical drive train of claim 1 , wherein the prime mover is an internal combustion engine.

6. The hydro-mechanical drive train of claim 1 , wherein the internal combustion engine is adapted to run at a relatively constant angular velocity.

7. The hydro-mechanical drive train of claim 1 , wherein the mechanical transmission is a finite geared mechanical transmission.

8. A hydro-mechanical hybrid drive train comprising: a prime mover having an output; a first hydraulic pump/motor unit operably coupled to the output of the prime mover, the first hydraulic pump/motor unit having a mechanical output; a mechanical transmission having a plurality of gear ratios, an input shaft and a mechanical output, the input shaft being coupled to the mechanical output of the first hydraulic pump/motor unit; a right-angle gear coupled to the mechanical output of the transmission, the right-angle gear having a first and second output shafts; a first planetary differential having a first input mechanically coupled to the first output shaft of the right-angle gear, the first planetary differential having a second input and a mechanical output; a second hydraulic pump/motor unit coupled to the second input of the first planetary differential; a drive element coupled to the mechanical output of the first planetary differential; and a hydraulic accumulator operably coupled to the first and second hydraulic pump/motor units.

9. The hydro-mechanical hybrid drive train of claim 8, and further comprising: a second planetary differential having a first input mechanically coupled to the second output shaft of the right- angle gear, the second planetary differential having a second input and a mechanical output; a third hydraulic pump/motor unit coupled to the second input of the second planetary differential; a drive element coupled to the mechanical output of the second planetary differential; and wherein the hydraulic accumulator is operably coupled to the third hydraulic pump/motor unit.

10. The hydro-mechanical hybrid drive train of claim 9, and further comprising: a torque splitter mechanically interposed between the transmission and the right-angle gear, the torque splitter having an auxiliary output shaft; an auxiliary right-angle gear, coupled to the auxiliary output shaft of the torque splitter and having first and second auxiliary output shafts; a third planetary differential having a first input mechanically coupled to the first auxiliary output shaft, the third planetary differential having a second input and a mechanical output; a fourth hydraulic pump/motor unit coupled to the second input of the third planetary differential; a drive element coupled to the mechanical output of the third planetary differential; and wherein the hydraulic accumulator is operably coupled to the fourth hydraulic pump/motor unit.

11. The hydro-mechanical hybrid drive train of claim 10, and further comprising: a fourth planetary differential having a first input mechanically coupled to the second auxiliary output shaft, the fourth planetary differential having a second input and a mechanical outputs; a fifth hydraulic pump/motor unit coupled to the second input of the fourth planetary differential; a drive element coupled to the mechanical output of the fourth planetary differential; and wherein the accumulator is operably coupled to the fifth hydraulic pump/motor unit.

12. A method of operating a hydro-mechanical drive train, the method comprising: starting a prime mover and controlling the prime mover to a selected angular velocity, the prime mover generating torque; providing a fraction of the torque from the prime mover through a mechanical transmission; using the remainder of the torque from the prime mover to drive a hydraulic pump/motor; storing work from the pump/motor by charging a hydraulic accumulator; de-clutching and shutting down the prime mover once the accumulator reaches a selected charge threshold; and powering the mechanical transmission with the hydraulic pump/motor using charge from the accumulator.

13. The method of claim 12, wherein the selected angular velocity is selected based on efficiency of the prime mover.

14. The method of claim 12, wherein the selected angular velocity is selected based on efficiency of the prime mover/drive system.

15. The method of claim 12, and further comprising restarting the prime mover and repeating the method once the accumulator charge level reaches a "discharged" threshold.