Abstract:
A system according to this invention for transmitting power hydraulically to and from the wheels of a motor vehicle includes an accumulator containing fluid at relatively high pressure, a reservoir containing fluid at lower pressure, and a pump/motor driveably connected to the wheels and having an inlet, an outlet, and a variable volumetric displacement for pumping fluid between the accumulator and the reservoir. A first circuit connects the inlet to the reservoir and the outlet to the accumulator. A second circuit, which connects the inlet to the accumulator and the outlet to the reservoir, includes a first path having a low flow rate capacity, a second path having a higher flow rate capacity. A controller switches between pumping operation and motoring operation, opens and closes the first path during motoring operation, and reduces the displacement before switching between pumping operation and motoring operation. The first path includes a first valve responsive to the controller for opening and closing a connection between the accumulator and the inlet, and a first orifice arranged in series with the first valve having a relatively low flow rate capacity. The second path, arranged in parallel with the first path between the accumulator and the inlet, includes a second valve responsive to the controller for opening and closing a connection between the accumulator and the inlet, and a second orifice arranged in series with the second valve having a higher flow rate capacity than that of the first orifice.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a hybrid vehicle powertrain having an engine and a hydraulic drive. More particularly, the invention pertains to switching a pump/motor between pumping operation and motoring operation.  
         [0002]     Hydraulic Power Assist (HPA) is a type of hydraulic hybrid vehicle, in which energy from regenerative braking or from an engine is stored in a hydro-pneumatic accumulator, and the conversion between mechanical power and hydraulic power is achieved through high pressure pump/motor having a variable volumetric displacement. In an HPA system, using stored energy from regenerative braking to help accelerate the vehicle reduces the burden on the engine and reduces fuel use.  
         [0003]     Because of the high power density available with such hydraulic systems, it is possible to recover efficiently a significant portion of braking energy with an HPA system comprised of a single pump/motor and storage accumulators. With a 7000 lb. vehicle and a pump/motor whose maximum displacement is 150 cc., an HPA system can recover 72 percent of the available braking energy in the Environmental Protection Agency (EPA) city cycle. The pump/motor operates for long periods at higher displacements and with a relatively high cycle average efficiency of 88 percent. With a return of 56 percent of the braking energy to the drive wheels (72 percent recovered in braking, and 88 percent transfer efficiency in both pumping and motoring), it is possible to recover 56 percent of the vehicle kinetic energy (or 75 percent of the velocity) while accelerating, neglecting road load friction. In the EPA city cycle it was possible to fill the hydraulic system when braking from 30 mph and then moderately accelerate again to about 22 mph using only stored energy from the HPA system.  
         [0004]     A hydraulic or pneumatic pump/motor operates in a pumping mode and a motoring mode. When changing operating modes in a hybrid hydraulic vehicle between motoring and pumping, the inlet and outlet ports of the pump/motor must be switched between connections to high pressure and low pressure sources by changing the state of several valves in a hydraulic system. This switching creates a sudden release of energy, which can place a large shock loading on the system. The control method of this invention minimizes the large shock associated with this high speed pressure switching.  
         [0005]     In the pumping mode, hydraulic fluid is moved from a low-pressure reservoir to a high-pressure accumulator. The pump outlet pressure rises as the pump rotates and very quickly opens a check valve to begin forcing fluid into the accumulator. In the motoring mode, high pressure fluid leaving the accumulator drives the pump/motor in rotation and returns to the reservoir.  
         [0006]     While pumping or motoring, displacement of the pump/motor can be independently controlled to vary the volume of fluid moved during each revolution of the pump/motor rotor between its inlet and outlet ports. When switching from pumping to motoring, it is necessary to connect the accumulator to either the inlet port or the outlet port of the pump/motor, and to connect the reservoir to the other of the two ports.  
         [0007]     High pressure solenoid valves accomplish this switching, which must be done carefully to prevent unsafe or unpleasant conditions for the vehicle occupants. For example, when switching from pumping to motoring, a valve opens to allow high pressure fluid to flow to the inlet port of the pump/motor. There is an immediate rise in pressure in the hydraulic line causing a noisy shock wave to propagate toward the pump/motor that may vibrate the components. In addition, if the pump/motor has a positive, non-zero displacement, the wheels of the vehicle will be driven by torque transmitted from the pump/motor causing the vehicle to move.  
       SUMMARY OF THE INVENTION  
       [0008]     While pumping or motoring, displacement of the pump/motor can be independently controlled to vary the volume of fluid moved during each revolution of the pump/motor rotor between its inlet and outlet ports. When switching from pumping to motoring, it is necessary to connect the accumulator to either the inlet port or the outlet port of the pump/motor and to connect the reservoir to the other of the two ports.  
         [0009]     High pressure solenoid valves accomplish this switching, which must be done carefully to prevent unsafe or unpleasant conditions for the driver or other vehicle occupants. For example, when switching from pumping to motoring, the valve opens to allow high pressure fluid to go to the inlet port of the pump/motor. There is an immediate rise in pressure in the hydraulic line causing a noisy shock wave to propagate toward the pump/motor that may vibrate the components. In addition, if the pump/motor has a positive, non-zero displacement, the wheels of the vehicle will be driven by torque transmitted from the pump/motor causing the vehicle suddenly, unexpectedly to move.  
         [0010]     To prevent these problems, a proportional valve, whose output is applied to the swashplate of the pump/motor to establish the magnitude of pump/motor displacement, is controlled so that there is no displacement when switching between pumping and motoring. There is a hydraulic circuit leading from the accumulator to the inlet port. This hydraulic circuit has its own solenoid valve and more importantly has a restriction that limits flow. When a switch to motoring occurs, this low flow rate valve is opened first so that the pressure in the lines and on the inlet port rises relatively slowly. After the pressure has risen to a sufficient level, the valve in the main flow path is opened. If, after motoring for a predetermined period, the pressure in the accumulator is not sufficient to provide useful work, the pump/motor is put into a non-motoring mode to prevent cavitation, which is very noisy and can damage hydraulic components. The switching must occur before exhausting accumulator pressure to prevent noise and vibration from being transmitted to the occupants of the vehicle. In this case, the displacement control-proportional valve is ramped down to zero at a controlled rate before the mode control valves are switched.  
         [0011]     A system according to this invention for transmitting power hydraulically to and from the wheels of a motor vehicle includes an accumulator containing fluid at relatively high pressure, a reservoir containing fluid at lower pressure, and a pump/motor driveably connected to the wheels and having an inlet, an outlet, and a variable volumetric displacement for pumping fluid between the accumulator and the reservoir. A first circuit connects the inlet to the reservoir and the outlet to the accumulator. A second circuit, which connects the inlet to the accumulator and the outlet to the reservoir, includes a first path having a low flow rate capacity and a second path having a higher flow rate capacity. A controller switches between pumping operation and motoring operation, opens and closes the first path during motoring operation, and reduces the displacement before switching between pumping operation and motoring operation. The first path includes a first valve responsive to the controller for opening and closing a connection between the accumulator and the inlet, and a first orifice arranged in series with the first valve having a relatively low flow rate capacity. The second path, arranged in parallel with the first path between the accumulator and the inlet, includes a second valve responsive to the controller for opening and closing a connection between the accumulator and the inlet, and a second orifice arranged in series with the second valve having a higher flow rate capacity than that of the first orifice.  
         [0012]     The system switches operation of a hydraulic drive system for a vehicle between pumping and motoring by determining whether the pump/motor should enter pumping operation or motoring operation, alternately entering and exiting pumping operation and motoring operation, controlling the volumetric displacement during pumping operation and motoring operation, and decreasing the volumetric displacement to substantially zero displacement before exiting pumping operation and motoring operation.  
         [0013]     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic diagram of a powertrain for a hydraulic hybrid motor vehicle that operates in a brake regenerative mode and power assist mode;  
         [0015]      FIG. 2  is a schematic diagram of a brake pedal for use in controlling the brake regeneration mode of the powertrain of  FIG. 1 ;  
         [0016]      FIG. 3  is a schematic diagram of a hydraulic system for the vehicle showing the pump/motor, accumulator, reservoir, control valves and hydraulic lines connecting them;  
         [0017]      FIG. 4  is diagram of logic for controlling brake regeneration in a deadband range of brake pedal position; and  
         [0018]      FIG. 5  is a logic diagram for controlling the hydraulic system of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     Referring now to the drawings, there is illustrated in  FIG. 1 a  hydraulic hybrid powertrain  10  for driving the rear wheels  12  and front wheels  14  of a motor vehicle. A power source  16 , such as an internal combustion engine, is driveably connected to a transmission  18 , preferably an automatic transmission that produces multiple ratios of the speed of the engine and the speed of an output shaft  20 . Suitable alternative transmissions include those that are manually operated, and those that produce continuously variable speed ratios or infinitely variable speed ratios, having chain drive, belt drive or traction drive mechanisms. The transmission output shaft  20  is continually driveably connected to the rear wheels  12  through a rear driveshaft  22 , rear axle shafts, and a rear differential mechanism. A transfer case  24  selectively transfers a portion of the torque carried by output shaft  20  to a front driveshaft  28 , which is driveably connected to the front wheels  14  through a front differential mechanism and front axle shafts. The vehicle, therefore, can operate in all-wheel drive or four-wheel drive modes.  
         [0020]     A hydraulic pump/motor  26  having a variable volumetric displacement is continually driveably connected to the transmission output shaft  20  and to the rear driveshaft  22 . When torque is transmitted in a positive torque directional sense, i.e., from the engine to the wheels, the engine  16  drives the pump/motor  26  through the transmission  18  and output shaft  20 , and the rear wheels  12  through the driveshaft  22 . When torque is transmitted in the negative torque direction, from the wheels to the engine, the rear wheels  12  drive the pump/motor  26  through rear driveshaft  22  and the transfer case  24 . A dog clutch located in the transfer case  24  produces a releasable drive connection between the pump/motor  26  and the front driveshaft  28 . A reservoir  36  containing hydraulic or pneumatic fluid at relative low pressure is connected through check valves and fluid lines  38  to the pump motor  26 , as described with reference to  FIG. 3 . Similarly, an accumulator  40  containing hydraulic or pneumatic fluid at relative high pressure is connected through check valves and fluid lines  42  to the pump motor  26 .  
         [0021]     While accelerating the vehicle with hydraulic power assist, high pressure fluid in accumulator  40  drives the pump/motor  26 , and the wheels  12 ,  14  are driven in rotation by the pump/motor, which operates then as a fluid motor. During operation in the brake regeneration mode, the vehicle is decelerated at least partially by recovering vehicle kinetic energy in the form of pressurized hydraulic fluid contained in accumulator  40 . In the brake regeneration mode, the pump/motor  26  pumps fluid from reservoir  36  to the accumulator  40 . The wheels  12  drive the pump/motor  26  through the rear axle and driveshaft  22 , and the pump/motor  26  pumps fluid from reservoir  36  across a pressure differential between the pump inlet, which communicates with reservoir  36 , and the pump outlet, which communicates with accumulator  40 . Fluid entering the accumulator  40  compresses nitrogen contained in a bladder in the accumulator  40 , and the accumulator is pressurized.  
         [0022]     Referring now to  FIG. 2 , in a conventional vehicle when the foot brake pedal  50  is applied, the vehicle decelerates due to friction braking, i.e., frictional contact of brake pads or brake shoes on wheel brake rotors or drums. The kinetic energy of the vehicle is converted by this frictional contact to heat, which is dissipated. In a deadband parallel regenerative braking system, a space  52  is located between connecting rods  54 ,  56 , which connect a brake master cylinder  58  and the foot brake pedal  50 . The space  52  causes the brake pedal to move from the rest position shown in  FIG. 2  through a first portion of its full displacement before hydraulic brake pressure is generated in the master cylinder due to movement of the piston  60  within the master cylinder  58 . This delays the application of the wheel friction brakes as the pedal is being displaced. The range of brake pedal displacement in which no friction braking occurs, called the “deadband” region, is preferably about 30 percent of the full range brake pedal displacement beginning when the brake pedal is at rest and not displaced.  
         [0023]     A tension spring  68 , fastened to a brake lever  64  between the fulcrum  66  and the pedal  50 , provides a force sensed by the vehicle operator and resisting brake pedal displacement in the deadband range. The force of spring  68 , produced when depressing the brake pedal  50 , compensates for the absence of a hydraulic pressure force opposing pedal displacement and piston movement in the master cylinder while the pedal is in the deadband range. A power brake canister  76  contains a piston  78 , which is actuated by engine vacuum to increase the force applied to connecting rod  54  by depressing the brake pedal  50 .  
         [0024]     A brake pedal position transducer  70  produces an electronic signal  72  as input to controller  74  representing the position of the brake pedal  50 . Controller  74  operates under control of a microprocessor, which executes programmed control logic for controlling the hydraulic system of  FIG. 3  and the vehicle powertrain. The controller  74  receives input signals produced by other sensors representing fluid pressure at various places in the hydraulic system, volumetric displacement of the pump/motor, the magnitude of a variable swashplate angle that alters the displacement of the pump/motor, displacement of the accelerator pedal  44  and brake pedal  64 , various inputs produced by the vehicle operator and powertrain system inputs. The controller  74  issues command signals, received by solenoid-operated hydraulic control valves of the hydraulic system causing the valves to produce various system operating states and transitions among those states.  
         [0025]     Pressure in the hydraulic brake system  80 , which actuates the friction brakes  82 , changes as pressure in the master cylinder  58  changes due to displacement of piston  60  in the cylinder as the brake pedal  50  is depressed and released. When the brake pedal  50  is depressed beyond the deadband range sufficiently to close the space  52 , brake system pressure forces the brake pads  82  into frictional contact with the brake disc  84 , to which a wheel  12  is fixed.  
         [0026]     In addition to the friction brakes, the vehicle is braked also by a regenerative brake system. While the brake pedal  50  is depressed, the operating states of hydraulic pump/motor  26  are changed between a pump state and motor state in response to command signals produced by controller  74 .  
         [0027]     The mode valve  88  is switched between the closed state shown in  FIG. 3  and an open state by a solenoid  86  in response to command signals from controller  74 . A low flow rate valve  92  is switched between the closed state shown in  FIG. 3  and an open state by a solenoid  94  in response to command signals produced by controller  74 .  
         [0028]     Preferably the pump/motor  26  is a bent-axis variable displacement unit whose maximum displacement is 150 cc per revolution, and available commercially from Ifield Technology, Inc. At peak pressure of about 5000 psi., the pump/motor produces approximately 600 ft-lb of braking torque in the pumping mode or acceleration torque in the motoring mode to the driveshaft  22 . Displacement of the pump/motor is varied by changing the angular disposition of a swashplate. System fluid in a pressure range 2500-5000 psi. controls the swashplate angle. A PID control system continually produces a command signal tending to minimize the difference between the current swashplate angle and the angle corresponding to the desired magnitude of torque produced by the pump/motor  26 .  
         [0029]     A four-way swashplate control valve  96 , also called a proportional valve, changes the variable displacement of the pump/motor  26  in response to commands issued by controller  74 . Solenoid  98  changes the state of valve  96  among three states, a center position where the inlet and outlet of valve  96  are mutually disconnected, a left-hand position where the angular disposition of the swashplate and displacement of the pump/motor  26  decrease, and a right-hand position where the swashplate angle and displacement of the pump/motor  26  increase. Proportional valve  96  is switched between its states by a solenoid  98  in response to command signals from controller  74 .  
         [0030]     Poppet valves  100 ,  102  move rightward from the position of  FIG. 3  to open a hydraulic connection between reservoir  36  and the inlet  90  of the pump/motor  26  through lines  104 ,  106 ,  108 ,  110 . Poppet valves  100 ,  102  move leftward from the position of  FIG. 3  to open a hydraulic connection between the outlet  112  of the pump/motor  26  and reservoir  36  through lines  124 ,  116 ,  106 ,  104 . Poppet valve  118  moves rightward from the position of  FIG. 3  to open a hydraulic connection between accumulator  40  and the inlet  90  of the pump/motor  26  through lines  114 ,  120  and  110 . Poppet valve  122  moves leftward from the position of  FIG. 3  to open a hydraulic connection between outlet  112  of the pump/motor  26  and accumulator  40  through lines  124 ,  126 ,  113  and  114 . Poppet valves  118  and  122  are closed in the positions shown in  FIG. 3   
         [0031]     An isolation valve  128 , controlled by solenoid  130  in response to command signals from controller  74 , alternately opens and closes a connection between accumulator  40  and an inlet of valve  96 .  
         [0032]     In operation, to place the hydraulic system in the pumping operation mode, isolation valve  128  opens a connection from accumulator  40  to the proportional valve  96 , which is moved to the right-hand state, where variable force solenoid  98  is prepared to increase displacement of the pump/motor  26  by increasing the swashplate angle. Poppet check valves  100 ,  102  are moved rightward to connect reservoir  36  to the inlet port  90  of the pump/motor  26  through hydraulic lines  104 ,  106 ,  108  and  110 . Check valve  118  closes line  120  from the accumulator  40 , but check valve  122  opens line  126  to the accumulator  40  through line  114  when pump/motor  26  is turning and pressure at the pump outlet  112  exceeds the pressure in the accumulator  40 . These steps complete a hydraulic circuit from the reservoir  36  to and through the pump/motor  26 , and from pump/motor to the accumulator  40 . Preferably the control signal applied to solenoid  98  is an electric current in the range 0-2 amps. The swashplate angle and displacement of the pump/motor  26  changes in proportion to the magnitude of the current signal at solenoid  98 .  
         [0033]     Pump displacement is directly related to the torque necessary to rotate the pump rotor at a given hydraulic pressure. When the brake pedal  50  is in the deadband range, the system operates in the pump mode, and vehicle braking is entirely accomplished by the pump  26 . If the brake pedal is displaced past the deadband range, vehicle braking is accomplished by a combination by regenerative braking and friction braking in the correct proportion to achieve the vehicle deceleration rate desired by the vehicle operator.  
         [0034]     Before switching the hydraulic system from pumping operation mode to the motoring mode, the proportional valve  96  first causes the pump/motor displacement to be zero so that cavitation of the pump/motor is prevented during the transition. Proportional control is also prevented, i.e., if the controller determines that a positive swash angle is desired in order to meet powertrain system requirements, the controller nonetheless maintains pump/motor displacement at zero until the transition of the system to the motoring mode is completed. Isolation valve  128  is closed upon a command from controller  74  to its actuating solenoid  130 . Then the low flow rate valve  92  is opened, which forces poppet check valves  100 ,  102  leftward, thereby closing line  106  from line  108 , and opening line  116  to reservoir  36  through lines  104  and  106 . This opens a hydraulic connection between reservoir  36  and the pump/motor outlet  112 . With the hydraulic system so disposed, the accumulator is connected through line  114 , restriction orifice  132 , valve  92  and lines  108 ,  110  to the inlet  90 . The low flow rate valve  92  is opened for a period of about 200 ms until the system is pressurized sufficiently by accumulator  40 . Controller  74  includes a countdown timer, which expires in about 200 ms. after the transition to pumping mode begins.  
         [0035]     Then when the timer expires, the low flow rate valve  92  closes and the mode valve  88  opens to the accumulator pressure, which moves poppet check valve  118  rightward, thereby opening a high flow rate connection between accumulator  40  and the pump/motor inlet  90  through line  114 , valve  118 , and lines  120 ,  110 . These steps complete the transition to the motoring mode. Thereafter, controller  74  permits proportional control, and the pump/motor displacement follows input from the accelerator pedal representing desired wheel torque increases and decreases.  
         [0036]     Referring now to  FIG. 4 , after the vehicle operator depresses the brake pedal, the extent to which the brake pedal is depressed  150 , called “brake pedal position,” is used to determine the current desired vehicle deceleration rate  152 . Brake system hydraulic pressure  154  at the wheel brakes is used with the brake pedal position  150  to determine the corresponding vehicle deceleration rate due to applying the friction brakes  156 . Parasitic drag on the vehicle  158  due to tire friction and air friction, and the effects of engine braking are used to determine vehicle deceleration due to these factors. The vehicle deceleration rates  152 ,  156 ,  158  are added algebraically at summing junction  160  to produce a net vehicle deceleration rate  162 .  
         [0037]     At  164 , the vehicle mass is multiplied by the net vehicle deceleration rate,  162  to produce the magnitude of force, which if applied to the vehicle, would produce the net vehicle deceleration rate  162 .  
         [0038]     That force is converted at  166  to an equivalent wheel torque  168  using the tire size and a nominal coefficient of friction between the tires and the road surface. At  170 , the wheel torque required to maintain the current vehicle speed is calculated. At summing junction  172 , the magnitude of the difference between torques  168  and  170  is calculated to determine the change in wheel torque  174  necessary to stop the vehicle from the current speed at the desired deceleration rate  152 .  
         [0039]     At  176 , that differential torque  174  is divided by the axle ratio to determine the magnitude of torque  178  that must be deducted from the torque transmitted by the driveshaft  28  to the pump/motor  26  in order to produce the desired vehicle deceleration rate  152 . Then at  180 , the pump displacement corresponding to torque  178  is calculated. The controller  74  produces a command signal that is transmitted to solenoid  98  of the a proportional valve  96  in order to change the angular position of the swashplate and to change the displacement of the pump/motor  26  to the pump displacement calculated at  180 .  
         [0040]     The brake hold control uses the hydraulic drive system for braking a stopped vehicle against creeping while automatic transmission  18  is in gear despite there being little or no vehicle kinetic energy to recover by regenerative braking. The brake hold control determines whether (1) the transmission  18  is in gear, i.e., whether a gear selector controlled by the vehicle operator is a drive range, (2) the brake pedal  50  is depressed, and (3) the vehicle is stopped or has a speed that is equal to or less than a low reference speed. The position of the gear selector is controlled by the vehicle operation by moving a selector among forward drive, park, neutral and reverse drive ranges, called PRNDL positions.  
         [0041]     If these conditions are true, and provided an accelerator pedal  44  is not depressed, the brake hold control is activated. Mode valve  88  is placed in the pump position by solenoid  86  in response to a control signal from controller  74 . Isolation valve  128  is energized by solenoid  130 , thereby connecting the accumulator  40  to the inlet of swashplate control valve  96 , so that displacement of the pump motor  26  can be increased, preferably linearly, to its maximum displacement, through operation of solenoid  98  in response to commands from controller  74 . Displacement of the pump/motor  26  is increased such that the magnitude of negative torque transmitted to the wheels  12  by the pump/motor  26  is greater than the magnitude of positive torque transmitted from the engine through the transmission  18  and its torque converter to the wheels  12 . In this way the vehicle wheels  12  are braked sufficiently so that the vehicle will not creep due to the effect of the idling engine transmitting torque to the wheels through the torque converter of the automatic transmission. This control requires minimal brake pedal effort to keep the vehicle stopped in an idling condition.  
         [0042]     Controller  74  determines the magnitude of torque produced by the engine on the basis of engine speed, engine throttle position, mass air flow and other pertinent engine parameters. The transmission gear ratio and axle ratio are then used to determine by calculation the torque transmitted to the wheels by the idling engine. That torque is comparable to the torque  170  of  FIG. 4 . The displacement of the pump/motor  26  that will produce enough negative torque at the wheels to react to the idle torque is determined as described with reference to step  178 . Then the controller produces a command signal that is transmitted to solenoid  98  for the proportional valve  96  to change the angular position of the swashplate and the displacement of the pump/motor  26  to a displacement slightly greater than the pump displacement calculated at  178 .  
         [0043]     Referring to  FIG. 5 , after being initialized at  200 , the control executed by controller  74  first checks at  202  whether the poppet, flow, mode, isolation and proportional valves are closed. Then at  204  a check is made to determine whether the pump mode entry conditions are met. The pump mode is entered if the controller determines a need for increased torque, vehicle speed is less than about 30-40 mph, pressure in accumulator is less than a predetermined magnitude, and other similar powertrain system conditions. If those conditions are logically true, at  206  isolation valve  128  is placed in its ON state by the controller  74  issuing a command signal to its actuating solenoid  130 . The proportional valve  96  is ramped to its desired displacement magnitude by changing the magnitude of current supplied to solenoid  98  at step  208  and full proportional control is initiated at  210 . When the pump mode exit conditions are present, at  212  the proportional valve  96  is ramped down to produce zero pump/motor displacement and torque at  214 . The pumping mode exit conditions are essentially the opposite of the corresponding entry conditions.  
         [0044]     If the pump entry conditions are logically false, a check is made at  216  to determine whether the motor entry conditions are logically true. If so, proportional control is prevented at  218 , the isolation valve  128  is placed in its ON state at  220  by issuing a command signal to its actuating solenoid  130 , the low flow valve  92  is placed in its ON state at  222 , and low flow timer is set. The motoring mode entry conditions include a powertrain condition for which torque produced by the pump/motor is desired to drive the vehicle wheels, the presence of a sufficient magnitude of fluid pressure and volume in the accumulator, vehicle speed in a range 0-30 mph, and additional powertrain system conditions. A check is made at  224  to determine whether the low flow timer has expired. If so, at  226 , the mode valve  88  is placed in its ON state, and low flow valve  92  is turned OFF. Next at  228 , full proportional control is enabled. A check is made at  230  to determine whether the motor exit conditions are logically true. If so, at  232  the proportional valve  96  begins to ramp motor displacement and torque output by the pump/motor  26  down to zero. When the proportional valve has completed the linear decrease of pump/motor displacement to zero, as indicated by a positive test at  234 , at  236  the mode valve  88  is closed at  236 .  
         [0045]     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.