Abstract:
An automotive powertrain includes a hydraulic drive circuit including at least one of: (1) a hydraulic pump/motor having a shaft fixed to for rotation with the crankshaft of an internal combustion engine and (2) a pair of pump/motors coaxially arranged to share a common shaft on which is mounted a gear of a gear set for transmitting output to the vehicle drive wheels. Hydraulic control logics are provided for control of the various pump/motors of the hybrid powertrain.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is a unique automotive powertrain design that allows highly efficient use of energy generated by an integrated internal combustion engine. Field of application is in automotive powertrains. 
     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. Current powertrains typically average only about 15% thermal efficiency. Accordingly, new approaches to improve the efficiency of fuel utilization for automotive powertrains are needed. 
     Conventional automotive powertrains result in significant energy loss, make it difficult to effectively control emissions, and offer limited potential for 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% to 90% of the fuel energy consumed by such a system is wasted as heat. Only 10%-15% of the energy is available to overcome road load, 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 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 more powerful than average road load would require. 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 the vast majority of road load experienced in normal driving is near the low end of the spectrum, the engine must operate at poor efficiency (e.g., less than 20%) much of the time, even though conventional engines have peak efficiencies 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 produce and not reclaim energy, and a simple gear transmission can only transmit it, 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 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 effectively control emissions because it requires the engine to operate at many different conditions of combustion. Operating the engine at more constant speed and/or load would allow much better optimization of any emission control devices, and the overall more efficient 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 efficiency, and would apply equally well to improved powertrains. 
     Hybrid vehicle systems have been investigated as a means to mitigate the above-described inefficiencies. A hybrid vehicle system provides a “buffer” between road load demand and 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. The effectiveness of a hybrid vehicle system depends on its ability to operate the engine at peak efficiencies, on the capacity and efficiency of the buffer medium, and on the efficiency of the transmission system that transfers power to the drive wheels. 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, engine power rotates a shaft that pumps 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 unit, spinning the shaft and producing power. See, for example U.S. Pat. No. 4,223,532 issued Sep. 23, 1980 to Samual Shiber. 
     Other U.S. Patents disclosing such hybrid powertrains include: 
     Hybrid Powertrain Vehicle—U.S. Pat. No. 5,495,912 issued Mar. 5, 1996; 
     Anti-Lock Regenerative Braking System—U.S. Pat. No. 5,505,527 issued Apr. 9, 1996; 
     Accumulator Engine—U.S. Pat. No. 5,579,640 issued Dec. 3, 1996; 
     Lightweight, Safe Hydraulic Power System &amp; Method of Operation Thereof—U.S. Pat. No. 5,507,144 issued Apr. 16, 1996; and 
     Continuously Smooth Transmission—U.S. Pat. No. 5,887,674 issued Mar. 30, 1999. 
     SUMMARY OF THE INVENTION 
     The present invention provides an automotive powertrain including a pair of drive wheels and a hydraulic circuit including at least one accumulator for receiving hydraulic fluid, storing pressure and discharging the stored pressure. The hydraulic circuit further includes first and second pump/motors or a first hydraulic pump/motor in combination with the second hydraulic pump. The first hydraulic pump/motor, operating in its motor mode, drives the drive wheels responsive to receipt of hydraulic fluid and, in a pump mode, pumps hydraulic fluid to the accumulator responsive to braking. The second hydraulic pump or hydraulic pump/motor has a shaft fixed to the crankshaft of an internal combustion engine by which it is driven, as a pump, for pumping hydraulic fluid to at least one of the accumulator and the first hydraulic pump/motor, when the latter is operating in a motor mode. Preferably, the first and second hydraulic pumps or pumps/motors are inline piston machines or, more preferably, bent-axis piston machines. 
     The present invention also provides an automotive powertrain including a pair of drive wheels, an internal combustion engine with a crankshaft for power output and a hydraulic power circuit. Hydraulic power circuit includes at least one accumulator for receiving hydraulic fluid, storing pressure and discharging the stored pressure. A gear set serves to transfer power from at least one hydraulic pump/motor to the drive wheels. In a preferred embodiment, two drive hydraulic pump/motors incorporated into the hydraulic power circuit are located on opposing sides of one gear of the gear set and share a common input/output shaft having that one gear mounted thereon. These first and second hydraulic pump/motors may operate either in a motor mode to drive the pair of drive wheels through the gearshaft or in a pump mode for pumping hydraulic fluid into the accumulator responsive to braking of the drivewheels. A third hydraulic pump or pump/motor driven by the internal combustion engine serves to pump hydraulic fluid to the accumulator and/or the first and second hydraulic pump/motors to drive those pump/motors in a motor mode, thereby powering the vehicle. Again, the pumps and/or pump/motors are preferably inline piston machines and more preferably bent-axis piston machines. The third hydraulic pump or pump/motor may have a driveshaft fixed to the crankshaft of the internal combustion engine as in the above-described aspect of the invention. 
     The present invention also provides hydraulic control logic for control of hydraulic fluid in powertrains of the types described above. More specifically, the present invention provides an automotive powertrain including a pair of drive wheels, an internal combustion engine with power output through a crankshaft and a hydraulic drive circuit. A first pump/motor, when operating in a motor mode, serves to drive the drive wheels responsive to receipt of high pressure fluid from a high pressure line and operates in a pump mode to deliver high pressure fluid to the high pressure line responsive to braking of the drive wheels. The hydraulic circuit further includes a high pressure accumulator for receiving and discharging high pressure fluid through the high pressure line and a low pressure line and a low pressure accumulator for receiving and discharging low pressure fluid through the low pressure line. The hydraulic control logic includes first and second lines connecting, in parallel, to one side of the first pump/motor to the high pressure and low pressure lines, respectively, with the first parallel line having a first valve which opens to admit high pressure fluid from the high pressure line into the one side of the first pump/motor in forward drive. The second parallel line has a second valve which opens to admit low pressure fluid from the low pressure line to the one side of the first pump/motor in reverse drive. Third and fourth parallel lines serve to connect, in parallel, a second side of the first pump/motor to the high pressure and low pressure lines, respectively. The third parallel line has a third valve which opens to admit low pressure fluid from the low pressure line to the second side of the first pump/motor in forward drive. The fourth parallel line has a fourth valve which opens to admit high pressure fluid from the high pressure line to the second side of the first pump/motor in reverse drive. Similar control logics may be provided to control operation of the second pump/motor and, optionally, a third pump/motor. First and third pump/motors may share a common shaft with a gear of a reduction gear unit as in a feature of the present invention described above. 
     The present invention also provides an automotive powertrain which, as in the other aspects of the present invention includes a pair of drive wheels, an internal combustion engine with power output through a crankshaft and a hydraulic drive circuit. The hydraulic drive circuit includes high pressure and low pressure lines and a first pump/motor operable over center, in a motor mode, for driving the drive wheels responsive to receipt of high pressure fluid from the high pressure line and for operating in a pump mode to deliver high pressure fluid to the high pressure line responsive to braking of the drive wheels. The hydraulic drive circuit further includes a second pump/motor operable over center and driven by the internal combustion engine for operation in a pump mode to deliver high pressure fluid to the high pressure line. The hydraulic drive circuit also includes high pressure and low pressure accumulators and a hydraulic control logic. Here, the hydraulic control logic includes first and second parallel lines for connecting, in parallel, one side of the first pump/motor to the high pressure line and low pressure line, respectively. The first parallel line has a first valve which opens to admit high pressure fluid from the high pressure line to the one side of the first pump/motor. The second parallel line has a valve for preventing fluid flow from the high pressure line directly into the low pressure line. In this hydraulic drive circuit, a second side of the first pump/motor is connected directly to the low pressure line. The second and optionally third pump/motor are provided with similar hydraulic control logics. 
     The hydraulic hybrid vehicle powertrain of the present invention is a unique powertrain that performs all the functions of a conventional powertrain, but at a much higher level of energy efficiency. This novel powertrain efficiently converts the kinetic energy of the moving vehicle into potential energy when decelerating (i.e., braking) the vehicle, and this energy is stored on the vehicle for subsequent re-use. The powertrain employs a unique, integrated design of various conventional and novel components necessary for energy and cost efficient operation. Also, a unique hydraulic fluid flow circuit and unique operational control logic are utilized to achieve the full energy efficiency improvements which can be realized through this new powertrain. Many of the unique features of this new powertrain apply to electric hybrid powertrains as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram of a preferred embodiment of a powertrain of the present invention; 
     FIG. 2 is a circuit diagram of a first preferred embodiment of a hydraulic control circuit; 
     FIG. 3 is a schematic diagram of a preferred arrangement for the pump/motors of the powertrain of the present invention; 
     FIG. 4 represents a table of pump/motor displacement settings for a specific speed; 
     FIG. 5 is a circuit diagram of a second preferred embodiment of a hydraulic control logic; 
     FIG. 6 illustrates a control scheme utilizing an accelerator pedal; 
     FIG. 7 is a flow chart of a preferred embodiment of a control program; 
     FIG. 8 is a schematic diagram of a modified portion of the embodiment of FIG. 1; and 
     FIG. 9 is a schematic diagram of another modification of a portion of the drivetrain of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an engine  11  is first started by and then drives pump/motor  12 . High pressure hydraulic fluid is supplied by high pressure accumulator  13 , through line  14 , through hydraulic control circuit  15 , through line  16 , to pump/motor  12 , acting as a motor to start engine  11 . Low pressure fluid is discharged from pump/motor  12 , acting as a motor through line  19 , through hydraulic control circuit  15 , through line  18 , to low pressure accumulator  17 . When started, engine  11  drives pump/motor  12  in its pump mode. 
     An electronic control circuit  10  receives various signals, including a signal from sensor  9  indicative of the position of accelerator pedal  90 , and outputs control signals to the hydraulic control circuit  15 . 
     Low pressure hydraulic fluid is supplied by low pressure accumulator  17 , through line  18 , through hydraulic control circuit  15 , and through line  16 , to pump/motor  12  operating as a pump. Pump/motor  12  operates as a pump discharges high pressure fluid through line  19 , through hydraulic control circuit  15 , and through line  14 , to high pressure accumulator  13 . If power is being demanded to drive the wheels  20 , high pressure fluid will also flow through either one or both lines  23  and  24 , to either one or both of drive pump/motors  21  and  22  operating as motor(s). Low pressure fluid is discharged from one or both of pump/motors  21  and  22  acting as motor(s) through one or both lines  25  and  26 , through control circuit  15 , through line  18 , to low pressure accumulator  17 . When the pressure in high pressure accumulator  13  reaches a predetermined maximum value, engine  11  will stop delivering power to pump/motor  12  acting as a pump by either going into idle or being turned off. When the pressure in high pressure accumulator  13  reaches a predetermined minimum value, engine  11  will resume delivering power. 
     When it becomes necessary for the vehicle to brake, power is obtained from wheels  20  by operating one or both the drive pump/motors  21  and  22  as pumps by supplying low pressure fluid through one or both lines  23  and  24 , and discharging high pressure fluid through one or both lines  25  and  26 , through control circuit  15 , through line  14 , to high pressure accumulator  13 . The design and functioning of control circuit  15  as well as the control are more fully described later. 
     FIG. 2 is a schematic of the details of the hydraulic control circuit  15 , for this first embodiment wherein the pump/motors do not go overcenter, i.e., the pump/motors do not provide for the reverse flow of hydraulic fluid while the pump/motor output shaft continues to rotate in the same direction. In this embodiment, valves  42 - 45 ,  32 ,  35 ,  52  and  55  each operate as check valves in a “closed” (checked) position. Valves  42 ,  44 ,  32  and  52  always allow high pressure flow from the pump/motors  12 ,  21  and  22  into high pressure line  14  and valves  43 ,  45 ,  35  and  55  always allow low pressure from pump/motors  12 ,  21  and  22  into low pressure line  18 . 
     The middle subcircuit  31  controls the flow of fluid to and from pump/motor  12  through lines  16  and  19 . Middle subcircuit connects one side of pump/motor  12  through line  16  to high pressure line  14  and to low pressure line  18  through, respectively (first and second), parallel lines  36  and  37 . Likewise, a second side of pump/motor  12  is connected through line  19 , in parallel, to high pressure line  14  and low pressure line  18  through parallel lines  38  and  39 , respectively. For high pressure fluid to flow from high pressure line  14  through line  16  to pump/motor  12  acting as a motor to start engine  11 , valve  32  is opened. Check valves  33  and  34  prevent high pressure fluid from flowing directly into the low pressure lines. Valve  35  must also be open (as shown in FIG. 2) when valve  32  is opened for the fluid to flow through pump/motor  12  acting as a motor with discharge through low pressure line  18  to low pressure accumulator  17 . 
     When engine  11  has started, the displacement of pump/motor  12  is rapidly reduced to zero and valve  32  is returned to the closed or checked position as shown on FIG.  2 . If the displacement of pump/motor  12  is not absolute zero or if there is some pump/motor  12  leakage, check valve  33  allows low pressure fluid to flow from low pressure line  18 , to line  37 , to line  16 , and fluid would come from pump/motor  12  through line  19 , through open valve  35  and parallel line  39  to low pressure line  18 , thus preventing the possibility of cavitation of pump/motor  12  and establishing a neutral loop for low friction, neutral spinning of pump/motor  12 . Valve  32  in its closed position operates as a check valve to prevent hydraulic lock or over pressure if the pump/motor  12  goes slightly overcenter. Neutral spinning of pump/motor  12  would be necessary after a cold engine start to allow the engine to warm up sufficiently before torque is required. When ready and needed, engine  11  drives pump/motor  12  acting as a pump. Valve  35  is first turned to the closed (or checked) position, and the displacement of pump/motor  12  is increased. Low pressure fluid flows from line  18  through check valve  33  and parallel line  37  through line  16  to pump/motor  12  acting as a pump. High pressure fluid leaves pump/motor  12  through line  19  through parallel line  38  and check valve  34  to high pressure line  14 . Fluid will flow to high pressure accumulator  13  only, to both high pressure accumulator  13  and one or both of pump/motors  21  and  22  acting as motors, or to only one or both of pump/motors  21  and  22  acting as motors. 
     If power is being demanded to drive the wheels  20 , high pressure fluid will flow from either one or both subcircuits  41  and  51 , through one or both lines  23  and  24 , to one or both drive pump/motors  21  and  22  acting as motor(s). The decision whether to use subcircuit  41 , subcircuit  51 , or both subcircuit  41  and  51  is described in U.S. Pat. No. 5,887,674 entitled “Continuously Smooth Transmission,” the teachings of which are incorporated by reference herein. Subcircuit  41  has lines  46  and  47  connecting, through line  23 , one side of pump/motor  21  in parallel with, respectively, high pressure line  14  and low pressure line  18 . Subcircuit  41  connects a second side of pump/motor  21 , through line  25 , in parallel with high pressure line  14  and low pressure line  18  through parallel lines  48  and  49 , respectively. If subcircuit  41  is commanded to supply high pressure fluid to line  23 , valve  42  is opened (shown closed or checked in FIG.  2 ), and high pressure fluid flows from line  14 , through valve  42  and parallel line  46  to line  23 . If pump/motor  21  acting as a motor is commanded to provide torque to wheels  20 , the displacement of pump/motor  21  is increased from zero to the desired level and high pressure fluid flows through line  23 , through pump/motor  21  and returns at low pressure through line  25 , through valve  45  in line  49  (shown already open in FIG.  2 ), and through line  18  to low pressure accumulator  17 . Valves  43  and  44  are in the closed or checked position (as shown in FIG. 2) to prevent high pressure fluid from flowing to the low pressure side. The checked position of valve  43  prevents cavitation of pump/motor  21  if valve  42  is in the closed position. 
     Likewise, subcircuit  51  has lines  56  and  57  connecting, through line  24 , one side of pump/motor  22  in parallel with, respectively, high pressure line  14  and low pressure line  18 . Subcircuit  51  connects a second side of pump/motor  22 , through line  26 , in parallel with high pressure line  14  and low pressure line  18  through parallel lines  58  and  59 , respectively. If subcircuit  51  is commanded to supply high pressure fluid to line  24 , valve  52  is opened (shown closed or checked in FIG.  2 ), and high pressure fluid flows from line  14 , through valve  52  to line  24 . If pump/motor  22  acting as a motor is commanded to provide torque to wheels  20 , the displacement of pump/motor  22  is increased from zero to the desired level and high pressure fluid flows through line  24 , through pump/motor  22  and returns at low pressure through line  26 , through valve  55  (shown already open in FIG.  2 ), and through line  18  to low pressure accumulator  17 . Valves  53  and  54  are in the checked position (as shown in FIG. 2) to prevent high pressure fluid from flowing to the low pressure side. The checked position of valve  53  prevents cavitation of pump/motor  22  if valve  52  is in the closed position. 
     When it becomes necessary for the vehicle to brake, power is obtained from wheels  20  by operating one or both the drive pump/motor(s)  21  and  22  as pumps by supplying low pressure fluid through one or both lines  23  and  24 , utilizing one or both subcircuits  41  and  51 . If subcircuit  41  is commanded to supply low pressure fluid to line  23 , valve  45  is turned to the closed or checked position (shown in the open position in FIG.  2 ). All other valves ( 42 ,  43  and  44 ) remain in the closed or checked position as also shown in FIG.  2 . Proportional to the brake pedal (not shown) depression, pump/motor  21  is commanded to increase its displacement and low pressure fluid will flow from line  18 , through valve  43  and parallel line  47 , through line  23 , and through pump/motor  21 , and high pressure fluid will flow through line  25 , through valve  44  and parallel line  48 , and through line  14 , to high pressure accumulator  13 . If subcircuit  51  is commanded to supply low pressure fluid to line  24 , valve  55  is turned to the closed or checked position (shown in the open position in FIG. 2) All other valves ( 52 ,  53  and  54 ) remain in the closed or checked position as shown in FIG.  2 . As with subcircuit  41  and pump/motor  21 , proportional to the brake pedal depression, pump/motor  22  is commanded to increase its displacement and low pressure fluid is caused to flow from line  18 , through valve  53  and parallel line  57 , through line  24 , and through pump/motor  22 , and high pressure fluid will flow through line  26 , through valve  54  and parallel line  58 , and through line  14 , to high pressure accumulator  13 . 
     While one, two or more drive pump/motors could be served by a single subcircuit (preferably of the design of subcircuit  41  which also provides for a reverse drive to wheels  20  as will be described later), pairing individual subcircuits and individual drive pump/motors allows the turning off of high pressure fluid to any pump/motor not being commanded to provide positive or negative (braking) torque. Turning high pressure off to a pump/motor improves efficiency by significantly reducing fluid leakage through the pump/motor to the low pressure side. Pump/motor spinning torque/friction and fluid compressibility losses are also significantly reduced when the vehicle is moving, when a clutch is not used to disengage the pump/motor from the drive train. 
     When reverse vehicle direction is commanded, subcircuit  41  is utilized. Valves  44  and  43  are opened (shown in the closed or checked position in FIG.  2 ), valve  45  is closed (shown open in FIG.  2 ), and valve  42  remains in the closed/checked position as shown in FIG.  2 . Proportional to “accelerator pedal” (not shown) depression, pump/motor  21  is commanded to increase its displacement and high pressure fluid will flow from line  14  through valve  44  and parallel line  48 , through line  25 , and through pump/motor  21 , and low pressure fluid will flow through line  23 , through valve  43  and parallel line  47 , and through line  18 , to low pressure accumulator  17 . By reversing the high and low pressure sides of pump/motor  21 , the pump/motor will rotate in the opposite direction. If higher reverse torque is desired, subcircuit  51  could be configured as subcircuit  41  and pump/motor  22  could be used to provide reverse torque as well. 
     The foregoing preferred embodiment includes the engine pump/motor  12  integrated with the engine crankshaft. The pump/motor pistons act directly on the end of the crankshaft. Since the pump/motor output shaft has become the engine crankshaft, only a common set of bearings is used, e.g., tapered roller bearing  127 , and the pump/motor plane for barrel movement in pump/motor  12  is chosen to provide forces mitigating those imposed by the engine pistons on the crankshaft, to reduce bearing forces and thus bearing friction. 
     In the foregoing preferred embodiment the engine  11  bolts directly to the transmission housing. The transmission will thus contain the engine pump/motor and actuator, as well as all flow circuits and valves necessary for power transmission control as previously described, the two drive pump/motors and actuators of the preferred embodiment integrated with the drive shaft of the gear reduction section (with the planes of barrel movement located to mitigate the gear forces), and the gears required for speed reduction integrated with the differential assembly. The transmission has two primary hydraulic line connections, one to the high pressure accumulator (high pressure line  14 ) and one to the low pressure accumulator (low pressure line  18 ). The transmission has two secondary hydraulic oil line connections, one to supply low pressure lube oil to the pump/motors and gear reduction/differential assembly and one to return lubrication oil and any pump/motor case leakage oil to a holding vessel for reuse. 
     FIG. 3 shows the key components of the preferred embodiment of the hydraulic hybrid vehicle powertrain transmission in more detail. The engine pump/motor  12  is integrated with the engine crankshaft output flange  123 . The pistons  124  of the engine bent axis pump/motor  12  act on rotating plate  125  which is attached directly to (or can be itself) the crankshaft flange  123 . Since the pump/motor output shaft and the engine crankshaft are integrated into a single shaft  126 , a common set of bearings  127  is used for both the crankshaft back main bearings and the pump/motor drive bearings. Arrows  129  indicate the flow path of hydraulic fluid through pump/motor  12 . 
     The two drive pump/motors  21  and  22  of the preferred embodiment are integrated with the drive shaft  213  of the gear reduction assembly  214  that drives the smaller of the two drive gears  225 . Gear  225  drives the larger of the two drive gears  325 . Gear  325  is mounted on a drive shaft  326  which, in turn, is attached to a conventional differential assembly (not shown) which is connected through conventional drive axles (not shown) to drive wheels in the conventional manner. The drive pump/motor pistons  216  and  226  act on the rotating plates  217  and  227  which are attached directly to opposing ends of the small gear drive shaft  213 . Since the pump/motor output shafts have been integrated with the small gear drive shaft into a single shaft  213 , common sets of bearings  218  and  228  are used for both the small gear drive shaft bearings and the pump/motor drive bearings. Arrows  219  and  215  indicate the flow path of hydraulic fluid through the pump/motors  21  and  22 , respectively, which are inline piston machines or, more specifically, bent-axis piston machines. 
     The hydraulic hybrid vehicle powertrain utilizes an operational control logic to maximize efficiency and performance characteristics. By managing the vehicle engine, the pump/motors through their associated displacement actuators, flow control valves, shut-off valves and other components of the vehicle. The electronic control system receives various inputs including drive torque demand (accelerator pedal position), vehicle speed, and pressure of the hydraulic fluid to determine output signals such as pump/motor displacements, valve positions, etc. The electronic control system controls the engine in response primarily to hydraulic system pressure, drive torque demand and vehicle speed. 
     The operational control logic is consistent with the teachings of U.S. Pat. No. 5,495,912, the teachings of which are incorporated herein by reference, with additional unique control logic for the present invention as described later with reference to FIG.  6 . Referring again to FIG. 1, one or both pump/motors  21  and  22  deliver positive (or zero) torque to wheels  20  in response to accelerator pedal  90  position by adjusting motor displacement, considering hydraulic system pressure and vehicle speed. Negative torque (i.e., braking) is delivered in a like manner in response to a signal from sensor  7  which detects brake pedal  8  position for the first portion of brake pedal depression, while the second portion of brake pedal depression phases in the conventional friction brakes (not shown) consistent with the teachings of U.S. Pat. No. 5,505,527, the teachings of which are incorporated herein by reference. Positive, zero or negative torque commands are satisfied with the highest efficiency displacement settings of the drive pump/motors available for that torque command, consistent with the teachings of U.S. Pat. No. 5,887,674, the teachings of which are incorporated herein by reference. High pressure hydraulic fluid may be turned off to any pump/motor that is set to zero displacement to reduce efficiency losses, as previously explained. Values for the best displacement settings (i.e., highest overall efficiency) for all drive pump/motors, for each possible driver torque command, are a single solution based on the hydraulic system pressure and vehicle speed. The electronic control unit  10  obtains displacement settings from correlation equations or from look-up tables, for example as illustrated in FIG. 4, stored in memory, in accordance with signals from sensor  9 , and outputs command signals to hydraulic control circuit  15 . 
     A second embodiment of the invention utilizes pump/motors that go overcenter, and substitutes a hydraulic control circuit  15  (FIG. 5) for hydraulic control circuit  15  (FIG.  2 ), but is otherwise as shown in FIG.  1 . Valves  62 ,  72  and  82  provide a check valve function in their “closed” (checked) position whereby high pressure fluid from the pump/motors is always allowed to flow into said high pressure line. In this second preferred embodiment, valves arc not needed to switch the high pressure and low pressure. Thus, when engine  11  has been started by pump/motor  12  acting as a motor and begins driving pump/motor  12  acting as a pump, the pump/motor displacement control mechanism (not shown) moves the piston stroking means overcenter, and the hydraulic fluid flow through the pump/motor is reversed. Therefore, when pump/motor  12  is operating as a pump, low pressure hydraulic fluid is supplied by low pressure accumulator  17 , through line  18 , through control circuit  15  (see FIG.  5 ), through line  19  to pump/motor  12  acting as a pump. Pump/motor  12  acting as a pump discharges high pressure fluid through line  16 , through control circuit  15  (see FIG.  5 ), through line  14 , to high pressure accumulator  13 . If power is demanded to drive the wheels  20 , high pressure fluid will also flow through either one or both lines  23  and  24 , to either one or both drive pump/motors  21  and  22  acting as motor(s). Low pressure fluid is discharged from one or both of pump/motors  21  and  22  acting as motor(s) through one or both lines  25  and  26 , through control circuit  15 , through line  18 , to low pressure accumulator  17 . 
     When it becomes necessary for the vehicle to brake, power will be obtained from wheels  20  by operating one or both the drive pump/motors  21  and  22  as pumps by driving one or both pump/motors  21  and  22  overcenter, and the flow direction of the fluid will reverse. Low pressure fluid will flow through one or both lines  25  and  26 , and discharge high pressure fluid through one or both lines  23  and  24 , through control circuit  15 , through line  14  to high pressure accumulator  13 . 
     FIG. 5 shows the details of the control circuit  15  for the second embodiment utilizing overcenter pump/motors. The lines into and out of control circuit  15  are the same as in FIG.  1  and are so labeled. In the second embodiment of FIG. 5, the middle subcircuit  60  has lines  64  and  65  connecting, through line  16 , one side of pump/motor  12  in parallel with, respectively, high pressure line  14  and low pressure line  18 . Subcircuit  60  connects a second side of pump/motor  12 , directly (without intervening valving) to low pressure line  18 . The middle subcircuit  60  controls the flow of fluid to and from pump/motor  12 . For high pressure fluid to flow from high pressure line  14  through line  16  to pump/motor  12  acting as a motor to start engine  11 , valve  62  is opened. Low pressure fluid from pump/motor  12  acting as motor is discharged through low pressure line  19 , through low pressure line  18  to low pressure accumulator  17 . 
     When engine  11  has started, the displacement of pump/motor  12  is rapidly reduced to zero and valve  62  in line  64  is returned to the closed or checked position as shown in FIG.  5 . If the displacement of pump/motor  12  is not absolute zero or if there is some pump/motor  12  leakage, optional check valve  63  in parallel line  65  allows low pressure fluid to flow to pump/motor  12  to prevent the possibility of cavitation of pump/motor  12  and establishes a neutral loop for low friction, neutral spinning of pump/motor  12 . When ready and needed, engine  11  drives pump/motor  12  acting as a pump. Pump/motor  12  is stroked overcenter and the displacement in the overcenter direction is increased. Low pressure fluid flows from line  19 , through pump/motor  12  acting as a pump. High pressure fluid is discharged into line  16 , parallel line  64  and through valve  62  to line  14 . 
     If power is being demanded to drive the wheels  20 , high pressure fluid will flow from either one or both subcircuits  70  and  80 , through one or both lines  23  and  24 , to one or both drive pump/motors  21  and  22  acting as motor(s). 
     Subcircuit  70 , which controls drive pump/motor  21 , has lines  74  and  75  connecting, through line  23 , one side of pump/motor  21  in parallel with, respectively, high pressure line  14  and low pressure line  18 . Subcircuit  70  also connects a second side of pump/motor  21 , directly (without intervening valving) to low pressure line  18 . If subcircuit  70  is commanded to supply high pressure fluid to line  23 , valve  72  is opened, and high pressure fluid flows from line  14 , through parallel line  74  and valve  72  to line  23 . If pump/motor  21  acting as a motor is commanded to provide torque to wheels  20 , the displacement of pump/motor  21  is increased from zero to the desired level and high pressure fluid flows through line  23 , through pump/motor  21  and returns at low pressure through line  25 , through line  18  to low pressure accumulator  17 . Optional check valve  73  in parallel line  75  prevents cavitation of pump/motor  21  if valve  72  is in the closed position. If subcircuit  80  is commanded to supply high pressure to line  24 , its functions and those of valves  82  and optional check valve  83  are the same as those described for subcircuit  70 , including return flow of low pressure fluid through line  26  to line  18 . 
     When it becomes necessary for the vehicle to brake, power will be obtained from wheels  20  by operating one or both pump/motors  21  and  22  as pumps. If subcircuit  70  is commanded to operate pump/motor  21  as a pump, the actuator strokes the pump/motor overcenter and to the desired displacement. Low pressure fluid flows from low pressure accumulator  17 , through line  18 , through line  25  and into pump/motor  21  acting as a pump. High pressure fluid is discharged into line  23  and flows through parallel line  74 , through valve  72 , through line  14  and into high pressure accumulator  13 . If subcircuit  80  is commanded to operate pump/motor  22  as a pump, its functions and those of valve  82  and optional check valve  83  are the same as those described for subcircuit  70 . 
     When reverse vehicle drive is commanded, one or both pump/motors  21  and  22  and subcircuits  70  and  80  are utilized. Since subcircuits  70  and  80  operate identically, subcircuit  70  alone will be used to describe operation for reverse vehicle drive. Valve  72  is opened and pump/motor  21  is stroked overcenter to the command displacement, in the same manner as when pump/motor  21  acting as a pump is stroked for regenerative braking. High pressure fluid flows from line  14  through line  74 , through valve  72 , through line  23  and into pump/motor  21  acting as a motor in the reverse direction. Low pressure fluid is discharged into line  25 , through line  18  and into low pressure accumulator  17 . 
     FIGS. 6 and 7 illustrate an operational control for the drive pump/motors which serves to encourage the vehicle driver to drive more efficiently. In the control illustrated in FIG. 6 operation of the drive pump/motors  21 ,  22  is controlled in accordance with position of the accelerator pedal  90  as detected by sensor  9 . Zone  1  of the accelerator pedal movement corresponds to that depression where only the smallest drive pump/motor  22  is operating. Within Zone  1  there are four subzones of depression. The first subzone is a “dead band,” with the control logic retaining the pump/motor at zero displacement to avoid the very inefficient, very low displacement (step  310 ). Accordingly, if pedal  90  is depressed (“YES” in step  300 ), the control routine of FIG. 7 proceeds to step  310  wherein presence of the pedal in sub-zone  1  does not result in a displacement command signal. Detection of a position within the second subzone (step  320 ) results in output of a command signal to operate the pump/motor between zero displacement and some minimally acceptable efficient displacement, considered here as ⅓ displacement. A determination is next made as to whether the pedal was depressed or released into sub-zone  2  (step  321 ). When the driver is depressing the pedal to a position within this second subzone, the control logic provides a slow response to the displacement command (e.g., taking 2 seconds to stroke from zero to a command for just less than ⅓ displacement) (step  323 ). Since the torque outputs from these commands are quite small, the driver will depress the pedal more if a more rapid torque increase is desired, thus minimizing time actually spent at less than ⅓ displacement. When the driver is releasing the pedal  90  to a position within subzone  2 , the control logic sends the displacement to zero as quickly as possible (step  322 ). If the pedal  90  is again depressed within subzone  2 , the control logic increases the displacement at a slow rate as before. The third subzone is ⅓ to full displacement, and the electronic control unit  10  (FIG. 1) sends the displacement signal which is determined as previously described to the command valve as quickly as possible (step  331 ). The fourth sub-zone is a second “dead band” (i.e., pump/motor stays at full displacement even though pedal depression suggests a “little more” torque) to keep the pump/motor  22  at the efficient, full displacement as long as possible. If the driver depresses the pedal into Zone  2  (step  350 ), the second drive pump/motor  21  begins providing torque, sending the second, larger pump/motor  21  to a displacement setting above ⅓ displacement (for this example) and simultaneously sending the smaller pump/motor  22  to zero displacement. Within Zone  2 , the electronic control unit sends a signal for the displacement of the larger pump/motor  21  to the command value as quickly as possible as it operates between ⅓ and full displacement (step  351 ) and a command signal to pump/motor  22  to go to zero displacement (step  352 ). If the driver depresses the pedal  90  into Zone  3  (step  360 ), both pump/motors  21 ,  22  are issued commands (steps  361 ,  362 ) to provide torque at the most efficient combined displacement settings to satisfy the commanded torque up to full displacement of both pump/motors. Obviously, if more than two drive pump/motors are utilized, the same logic would continue for the additional pump/motors until all are utilized at full displacement. Similar control logic would apply for regenerative braking, keyed to brake pedal depression. 
     FIG. 8 illustrates another embodiment of the invention which provides efficient torque over the large range of torque commands typical of motor vehicle operation without requiring multiple drive pump/motors or a speed change transmission between the drive pump/motor(s) and the drive wheels, and which will also provide the highest torque possible with the available drive pump/motor(s) when commanded. In the embodiment of FIG. 8 high pressure line  94  replaces high pressure line  14  in the embodiment of FIG.  1  and connects to control circuit  15  in the same manner as high pressure line  14  which it replaces. The other components shown in FIG. 8 replace high pressure accumulator  13 , but the remainder of the structure shown in FIG. 1 is included, without change, though unshown in FIG.  8 . The embodiment of FIG. 8 will “valve out” the high pressure accumulator and operate in a direct hydrostatic mode when high torque is commanded. At low and medium torque commands the drive pump/motor(s) operate in the manner previously described. For higher torque commands than can be supplied by full displacement of the drive pump/motor(s) at the instant system pressure, the high pressure accumulator is “valved out” of the hydraulic circuit, and the engine supplies sufficient hydraulic power to raise the system pressure to the commanded torque. In this manner, the drive pump/motor(s) operate at the highest efficiency (i.e., the optimum displacement and pressure) possible in satisfying the commanded torque, up to that torque corresponding to maximum displacement at maximum rated system pressure. 
     Operation of the drivetrain of FIG. 8 will now be described with reference to operation when the vehicle is at a low speed and a command for maximum acceleration (i.e., maximum torque) is received. At low vehicle speeds and at hydraulic system pressures below the maximum system pressure rating, the engine is able to deliver more power than the drive pump/motors can utilize. With the high pressure accumulator  93  open the engine  11  would pump hydraulic fluid into the high pressure accumulator, even though the drive pump/motors are at full displacement. If the system rated pressure is 5000 psi and the instantaneous system pressure is 2500 psi, the drive pump/motors  21 ,  22  can provide only one-half the torque (and power) available at 5000 psi. Therefore, removing the high pressure accumulator  93  from the hydraulic circuit immediately increases the system pressure to (for this example) 5000 psi, and maximum possible torque (and power) is provided to the wheels. Referring to FIG. 8, flow into the high pressure accumulator  93  through high pressure line  94  is terminated by closing valve  91 . The engine power must, at very low vehicle speeds, be managed to correspond to the power associated with the hydraulic fluid flow rate through the pump/motors  21 ,  22  to avoid exceeding the maximum system rated pressure. An optional, small auxiliary high pressure accumulator  95  (smaller than accumulator  93 ) may also be added to the circuit as shown in FIG. 8 to moderate the system pressure when the larger high pressure accumulator  93  is not allowed to receive fluid. As the vehicle speed and pump/motor  12  speed increase (FIG.  1 ), additional flow occurs and more power from the engine is supplied. This continues up to the maximum power output of the engine  11 . Beyond the vehicle speed associated with maximum engine power output, the displacement of the drive pump/motors  21 ,  22  is reduced while continuing to utilize the maximum power available from the engine  11  until the maximum acceleration rate command from the vehicle driver ceases (i.e., until the accelerator pedal depression is reduced). The high pressure accumulator  93  is again returned to the hydraulic circuit by opening valve  91  when the system pressure and the pressure in high pressure accumulator  93  are equal, and “normal” operation (i.e., operation as previously described) resumes. Optional check valve  92  may be added to the circuit to smooth the transition of adding the high pressure accumulator  93  back to the system. When system pressure falls below the pressure in high pressure accumulator  93 , fluid begins flowing again from high pressure accumulator  93 , through check valve  92  to the hydraulic circuit. Valve  91  can be opened without causing any pressure spike in the system since the pressure in the system and high pressure accumulator  93  are assured to be equal. 
     FIG. 9 shows still another alternative embodiment of the invention for operation in a direct hydrostatic mode of operation by “valving out” the high pressure accumulator as was just described. In FIG. 9 components identical to those of FIG. 1 are depicted with the same reference numerals and the components to which lines  16 ,  18 ,  19 ,  23 ,  24 ,  25  and  26  connect are the same as in FIG.  1 . This embodiment provides for operation in the direct hydrostatic mode at certain low and medium torque commands, as well as when high torque is commanded. The functioning of this embodiment will be described with reference to operation when the vehicle is at any speed and a command for low or medium torque is received, but the instantaneous system pressure is sufficiently high that the resultant drive pump/motor displacement is so low that it would operate at a lower than desired efficiency. Flow from high pressure accumulator  13  through high pressure line  104  is terminated by closing valve  105  (without optional valve  106  and optional check valves  107  and  108  included in the circuit). Engine power output is reduced to the power required by the vehicle, and the system pressure drops to a specified minimum value thus allowing/requiring the drive pump/motor(s)  21 ,  22  to increase displacement to maintain the commanded torque and thereby operate at an increased efficiency. Higher torque commands are easily satisfied as was previously described for the direct hydrostatic mode of operation. However, when a torque command is received that would result in an engine power output at that vehicle speed that is below the lowest efficient power output level defined for the engine  11 , then the high pressure accumulator  13  must be added back to the hydraulic circuit and thus valve  105  must be opened. To avoid a hydraulic pressure spike to the system, the system pressure must be raised to the high pressure accumulator pressure before valve  105  is opened. Then the engine  11  can continue operating at a minimum efficient power level by pumping hydraulic fluid into high pressure accumulator  13  or, if the system pressure is above the target level for the vehicle speed, the engine  11  can be turned off while vehicle power is supplied by the accumulator  13 . To minimize the difficulty of exactly matching the system pressure with the high pressure accumulator pressure before opening valve  105 , optional valve  106  and optional check valves  107  and  108  may be added to the circuit. Valve  105  and valve  106  are both open when the direct hydrostatic mode of operation is not being used. When system pressure higher than is instantly available from high pressure accumulator  13  is desired, valve  105  is closed and the high torque hydrostatic mode of operation as previously described is active. When system pressure lower than is instantly available from high pressure accumulator  13  is desired, the direct hydrostatic mode of operation for certain low and medium torque commands is provided by closing valve  106 . When system pressure rises above the pressure in high pressure accumulator  13 , fluid begins flowing again from the system into high pressure accumulator  13  through check valve  108 . Valve  106  can then be opened without causing any pressure spike in the system since the pressure in the system and high pressure accumulator  13  are assured to be equal. 
     The embodiment shown in FIG. 9 also includes a small auxiliary accumulator  109  which is utilized to start the engine  11 . When utilizing control logic which includes operation at low system pressure, the engine pump/motor  12  operating as a motor must be large enough to start the engine with lowest allowable system pressure. This results in a pump/motor larger than is needed or desired (for best efficiency) for operation as a pump. The embodiment shown on FIG. 9 allows utilization of a significantly smaller engine pump/motor  12 . Since the system pressure cycles between low values and high values, check valve  110 , interposed between said auxiliary accumulator  109  and said high pressure line  14 , allows fluid to enter auxiliary accumulator  109  and be charged to the highest pressure reached since the last engine start. A valve  111 , in a by-pass line  112  has been in the closed position since the last engine operation. When it becomes necessary to start the engine, valve  111  is opened, high pressure fluid flows to pump/motor  12  through by-pass line  112 , and pump/motor  12  operates as a motor to start the engine  11 . When the engine  11  starts, valve  111  is again closed and pump/motor  12  operates as a pump. Valve  32  and line  36  shown in FIG. 2 would not be used in this embodiment. 
     Another possible modification would mechanically isolate the engine and engine pump/motor from the remaining transmission of the preferred embodiment. Other modifications may be made by rearranging pump/motors and other components. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.