Abstract:
A system for a hydraulically driven vehicle includes a pump producing fluid flow at an outlet, pump-motors having variable flow rates for driving the wheels, a hydraulic rail having a pressure and connecting the pump and the pump-motors, sensors producing signals representing rail pressure, pump-motor speed, pump-motor displacement, and a controller for determining a target hydraulic system parameter, determining, based at least in part on the flow rate of the pump-motor, rail pressure, and a flow rate produced by the engine-pump, a flow rate produced by the engine-pump that is required to produce the target system parameter, and adjusting an engine operating parameter of a cylinder-pump bank such that the demanded magnitude of the system parameter is produced.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a hydraulic hybrid powertrain for vehicles, particularly to a powertrain having an engine-pump for pressurizing a hydraulic system, an accumulator for energy storage, and pump-motors for driving the wheels.  
         [0002]     In a hydraulic hybrid powertrain having a prime mover, such as an engine-pump that produces hydraulic flow at system pressure, and one or more pump-motors driving the wheels, it is desirable to operate at low system pressure to maximize the pump-motor efficiency during most operating conditions. There is, however, a mismatch in operating efficiency of the engine-pump and the efficiency of the pump-motors. The engine-pump has its highest efficiency at high system pressures. The pump-motors have their highest efficiency at lower system pressures.  
         [0003]     It is desirable to operate at a high system pressure at times of peak demand, to achieve the required power with a smaller pump-motor. Also, system pressure is directly coupled to the stored energy state from regenerative braking. In a hydraulic hybrid, therefore, it is desired that rapid transitions occur between low system pressure and high system pressure, without incurring significant energy loss. This would allow normal operation at a low system pressure, and quick access to a higher peak torque level on demand. A powertrain operating this way would realize a significant improvement in system cycle fuel economy.  
         [0004]     In a hydraulic hybrid powertrain, hydraulic flow at system pressure produced by the prime mover is used to drive one or more hydraulic pump-motors. Energy exceeding the current requirements of the powertrain is stored in a hydro-pneumatic accumulator. The pump-motors can provide regenerative braking. Kinetic energy of the vehicle produced by the pump-motors is recovered by a regenerative braking strategy and is stored in the accumulator. That energy can be supplied as required to the drive system from the accumulator. However, the system pressure necessarily decreases while the accumulator supplies this energy to the system. This drop in accumulator pressure reduces the available drive torque from the system.  
         [0005]     It is desirable that the engine and storage accumulator are decoupled so that one pump-motor can use the stored energy to drive a first set of wheels, and another pump-motor can be powered by flow from the engine at pressure up to maximum system pressure to drive another set of wheels. This technique makes more total power available and better uses stored energy.  
         [0006]     The magnitude of energy stored in an accumulator is approximately proportional to system pressure, and peak tractive output available from the pump-motors is also directly proportional to system pressure. Changing system pressure in this case requires a significant change in stored energy, and also takes time. This requires a compromise between drivability and use of energy storage to improve fuel economy.  
       SUMMARY OF THE INVENTION  
       [0007]     The invention relates to a hydraulic hybrid powertrain consisting of an engine-pump assembly, accumulator energy storage, and pump-motors driving the wheels. All components are connected to operate at a common pressure, and drive torque is modulated by changing pump-motor displacement. In such a system, peak system drive torque, and often peak engine power, require a high system pressure. Hydraulic pump-motor torque is proportional to pump displacement and pressure drop.  
         [0008]     For light load operation, such a cruising and light vehicle acceleration, a lower system pressure is desirable so that the pump-motors operate closer to maximum displacement, and at higher efficiency.  
         [0009]     Multiple accumulators, at different pressure states, can be connected to the system through valves, so that a quick transition from a low to a high pressure state can be made. Vehicle torque capability is then decoupled from stored energy, allowing more ideal pressure scheduling and use of regenerative braking, without adverse performance effects. Engine power at peak pressure can be combined with power available from stored energy, through separate pump motors, to achieve instantaneous power greater than the total engine power.  
         [0010]     A method according to this invention controls pressure in a hydraulic system, which includes an engine, a pump driven by the engine for supplying fluid to a hydraulic rail, first and second pump-motors supplied with fluid through the rail for driving a load, a main accumulator connected to the rail and containing fluid at a first pressure, and a power mode accumulator connected to the rail and containing fluid at a second pressure greater than the first pressure. The method includes the steps of monitoring a demand for an increase in a target parameter of the system. Communication is opened between the power mode accumulator and the rail, and communication is closed between the main accumulator and the rail after the demand occurs and before the target parameter is produced. A rate of fluid flow supplied by the pump to the rail is adjusted such that a combination of pressure in the rail and a rate of fluid flow to the pump-motors produces the target parameter.  
         [0011]     In another aspect of this invention a system for transmitting power to the wheels of a vehicle includes an engine-pump for producing a fluid flow, and a hydraulic rail connecting the fluid flow from pump to the pump-motor. A first pump-motor is supplied with fluid through the rail for driving a first set of wheels. A first accumulator contains fluid at a first pressure, and a second accumulator contains fluid at a second pressure greater than the first pressure. A first control valve opens and closes a hydraulic connection between the first accumulator and the rail. A second control valve opens and closes a hydraulic connection between the second accumulator and the rail. A splitting valve, located on the rail between the first accumulator and the second accumulator, opens and closes a hydraulic connection between the first accumulator and the second accumulator.  
         [0012]     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  
       [0013]      FIG. 1  is a schematic diagram of a hybrid hydraulic drive system for a vehicle to which the control of the present invention can be applied; and  
         [0014]      FIG. 2  is a schematic diagram of a control system applicable to the hybrid hydraulic system of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring now to the system illustrated in  FIG. 1 , an engine-pump  10  assembly is supplied with hydraulic fluid from a low pressure line  11 , which is hydraulically connected to a low pressure accumulator  12 . A main hydraulic rail  13 , which contains fluid pressurized at line pressure, is connected to the outlet of the pump  14 . Preferably pump  14  is a variable displacement pump. Engine  16  is preferably an internal combustion engine, such as a gasoline or diesel engine having a crankshaft, or a free piston engine having either spark ignition or compression ignition. Engine  16  drives the pump  14  and produces output torque or output hydraulic flow in response to control of one or more engine operating parameters including engine airflow, the engine throttle position, engine ignition timing, and engine air-fuel ratio.  
         [0016]     A check valve may be used to close a connection between line  11  and the pump inlet when inlet pressure exceeds pressure in line  11 . A check valve may be used to close a connection between rail  13  and the pump outlet when pressure in rail  13  exceeds pressure at the pump outlet. Otherwise, these connections are open. The pump outlet is connected by rail  13  to a front pump-motor  22  and a rear pump-motor  26 . The fluid flow rate produced by pump  14  is directly proportional to the pump displacement and its speed. Because of constraints on displacement and speed, power output by the engine  16  can be tightly constrained by line pressure, the pressure in rail  13 . Therefore, power output by the engine  16  is closely related to line pressure, the pressure in rail  13 .  
         [0017]     The front hydraulic pump-motor  22  is supplied with fluid through a valve body  24  connected to rail  13 . Pump-motor  22  is driveably connected to the front wheels of a motor vehicle. Similarly, the rear hydraulic pump-motor  26  is supplied with fluid through a valve body  28 , connected to rail  13 . The rear wheels of the vehicle are driven by pump-motor  26 . The front and rear pump-motors  22 ,  26  are variable displacement hydraulic pumps, each pump having a maximum displacement or volumetric flow rate per revolution.  
         [0018]     When an increase of torque or power must be delivered to the front wheels and rear wheels through the pump-motors  22 ,  26  while those pump motors are operating at maximum displacement, the pressure of fluid supplied to the pump motors must be increased in order to increase the output power from the pump-motors. When an increase of power must be delivered from the engine pump  14 , while pump  14  is operating at its maximum flow, the pressure of fluid at the pump outlet must be increased in order to increase the output [hydraulic] power from the pump. During normal operation, when the wheels are being driven, the pump-motors  22 ,  26  generate torque due to fluid flow from rail  13  through the pump-motors to low pressure line  11 . When the wheel brakes are braking the vehicle, the direction of torque and direction of fluid flow are reversed. Disregarding losses, torque is proportional to the product of displacement and pressure difference. Flow rate is proportional to the product of speed and displacement.  
         [0019]     The fluid outlet of the engine  16 , from which rail  13  is supplied, is connected to an engine accumulator  30 , which buffers or attenuates hydraulic pressure pulses produced by variations in engine speed and its inertia. A high pressure or power mode accumulator  32  communicates with rail  13  through a valve  34 . A spring  36  biases valve  34  to the position shown in  FIG. 1 , where check valve  38  closes a hydraulic connection between accumulator  32  and rail  13  when pressure in the accumulator is greater than rail pressure, and opens that connection when rail pressure is greater than the accumulator pressure. When electric current actuates solenoid  40 , it overcomes the effect of spring  36  and moves the valve to a second state, where a hydraulic connection between accumulator  32  and rail  13  is open through the valve  34 .  
         [0020]     A brake regeneration accumulator  42  stores energy recovered during the process of braking the drive wheels of the motor vehicle and stores that energy in the form of relatively high pressure hydraulic fluid. In accordance with the state of two control solenoids  46 ,  48 , accumulator  42  is connected to and disconnected from rail  13  through a regen shutoff/powermode valve  44 , or multiple valves arranged in series. Valve  44  has a first state in which accumulator  42  is open to rail  13  regardless of the differential pressure across the valve, a second state that closes a hydraulic connection between accumulator  42  and rail  13  when pressure in the accumulator is greater than rail pressure, and a third state that opens that connection when rail pressure is greater than the accumulator pressure.  
         [0021]     The case outlet of the front pump-motor  22  is connected through line  50  and check valve  52  to a heat exchanger  54 , filter  56 , and case drain reservoir  58 . Similarly, the case outlet of the rear hydraulic pump-motor  26  is connected through line  60  to the case drain reservoir  58 . A recovery pump  62  draws hydraulic fluid from the reservoir  58  and supplies fluid to the system through a check valve  64  and line  66 . Line  66 , which mutually connects the valve blocks  24 ,  28  and a low pressure accumulator  12 , communicates hydraulically also with the inlet side of the pump  14 . The solenoid  72  that operates valve  70  is energized as required to ensure that the positive pressure in accumulator  12  is present also at the inlets of the pump-motors  22 ,  26  and pump  14 .  
         [0022]     A splitting valve  74  has a first state that allows open communication between main accumulator  42 , the front pump-motor  22 , the pump  14 , and the rear pump-motor  26 . A second state of valve  74  divides the system in half when line pressure in rail  13 , at the left-hand side of the valve  74 , is greater than line pressure at the right-hand side of the valve and opens that connection when line pressure in rail  13  at the right-hand side of the valve is greater than line pressure at the left-hand side of the valve. A solenoid  76  controls the state of the valve  74 . The main accumulator  42  and the front pump motor  22  are to the left hand side of the valve  74 , and the pump  14  and the rear pump motor  26  are to the right-hand side of the valve  74 . Pressure relief valves  77 ,  78  allow fluid flow from rail  13  to main accumulator  42 , if valves  44  and  74  do not react quickly enough to limit a rapid increase in rail pressure. Valves  77 ,  78  minimize loss of energy in the system by providing a path between rail  13  to accumulator  42 , where the energy is stored in the form of a pressurized volume of fluid.  
         [0023]     The hydraulic fluid volume capacity of accumulators  32 ,  42  is about 10-11 gallons each. The pressure maintained in power mode accumulator  32  is about 5,000 psi. The pressure maintained in main accumulator  42  varies over a range that is principally determined by the frequency and degree of recovery of vehicle kinetic energy resulting from brake regeneration.  
         [0024]     A controller  80 , preferably a microprocessor-based controller, provides integrated control of the engine  16  and the hydraulic system. The engine and system may be controlled instead by a separate engine controller and system controller, depending upon the particular application. Controller  80  includes a microprocessor  82  in communication with input ports  84 , output ports  86 , and computer readable media  88  via a data/control bus  89 . Computer readable media  88  may include various types of volatile and nonvolatile memory such as random access memory (RAM)  90 , read-only memory (ROM)  92 , and keep-alive memory (KAM)  94 . These functional descriptions of the various types of volatile and nonvolatile storage may be implemented by any of a number of known physical devices including, but not limited to PROMs, EPROMs, EEPROMs, flash memory, and the like. Computer readable media  88  include stored data and instructions executable by microprocessor  82  to implement the method for controlling operation of the engine  16 , pump  14 , pump-motors  22 ,  28 , and solenoids  40 ,  46 ,  48 ,  72 ,  74 . The system and its components are controlled in accordance with commands produced by the controller as a result of repetitive execution of control algorithms stored in electronic memory on computer readable media  88 .  
         [0025]     Various sensors, in communication with the corresponding input ports  84  of controller  80 , monitor and produce signals representing the current operating conditions of the engine, hydraulic system, and vehicle. Information is also provided by driver inputs. The engine parameter sensors preferably include an engine throttle position sensor (TPS)  96 , which monitors the position of engine throttle valve, disposed within the engine intake. An accelerator pedal position (APP) sensor  97  may be substituted for the TPS. An accelerator pedal is operated manually by the driver to produce a demand for an output, such as torque output by the powertrain or vehicle speed. A pedal position sensor generally provides as an output, either a voltage or possibly a digital signal, which is interpreted by the controller as a software value often referred to as counts.  
         [0026]     A mass airflow sensor (MAF)  98  provides an indication of the air mass flowing through the engine intake. A temperature sensor (TMP)  100  provides an indication of the engine coolant temperature, or engine oil temperature. An engine speed sensor (NE)  102  monitors the speed of engine  16 . A rotational speed sensor, vehicle speed sensor (VSS)  104 , provides an indication of the speed of the vehicle derived from the speed of the axles, driveshaft, or individual wheels. Other sensors may be required depending on the type of engine used. The hydraulic system input sensors preferably include a pressure sensor  106 , which monitors and produces a signal representing the magnitude of line pressure in rail  13  (LP), as well as other pressure sensors, for example, for accumulators  32 ,  42  and  12 . Swashplate angle sensors (FPD) (RPD)  107  produce a signal representing the current angular position of the swashplates of the front axle and rear axle pump-motors  22 ,  26 , respectively. Pump-motor speed sensors (FPS) (RPS)  108  produce a signal representing the current speed of the front axle and rear axle pump-motors  22 ,  26 , respectively. The corresponding swashplate angular position is proportional to displacement of the front motor-pump  22  (FPD) and displacement of the rear pump-motor  28  (RPD). Temperature sensors monitor the system temperature so that action can be taken in the case of system temperatures being outside of desired limits.  
         [0027]     A brake pedal  112 , controlled by the driver, includes a pedal position sensor  112 , which provides an indication of the position of brake pedal (BPP), or the applied and released states of the brake pedal. The braking system may include additional features to enable more effective regenerative braking. Pressure sensors (P) produce signals representing the pressure in accumulators  12 ,  32 ,  42 .  
         [0028]     Depending upon the particular application requirements, various sensors may be omitted, or alternative sensors may be provided that generate signals indicative of related monitor parameters. Values corresponding to ambient or operating conditions may be inferred or calculated using one or more of the sensed parameters without departing from the spirit or scope of the present invention. For example, vehicle speed can be inferred or calculated from speed signals produced by wheel speed sensors (WS 1 ) (WS 2 ).  
         [0029]     In addition to the sensors described above, actuators, indicated generally by reference numeral  116 , communicate with controller  80  via output ports  86  to control the engine  16 , hydraulic system and vehicle in response to commands generated by the controller  80 . Actuators  116  may include actuators for timing and metering fuel (FUEL)  120 , controlling ignition angle or timing (SPK)  122 , setting the amount of exhaust gas recirculation (EGR)  124 , and adjusting the intake air using the engine throttle valve with an appropriate servomotor or actuator (TVA)  126 . Signal (S 1 , S 2 , S 3 , S 4 ) produced by controller  80  control the state of solenoids  40 ,  46 ,  48 ,  72 ,  74 .  
         [0030]     The control can be implemented in the hydraulic hybrid powertrain of  FIG. 1 . The power mode accumulator  32  is hydraulically isolated from the regen or main accumulator  42  and system by splitting valve  74  and valve  34 , or the valve  79  arranged in parallel with hydraulic valve  34 . The power mode accumulator  32  generally is maintained at a higher pressure than the pressure in the rail  13  and the system.  
         [0031]     When a demand for increased wheel torque is produced by the vehicle operator&#39;s control of the accelerator pedal (APP), a power mode can be activated, at the discretion of the control strategy, in which splitting valve  74  is open and in the state shown in  FIG. 1 , the main accumulator  42  is closed off from the rail  13  by regen shutoff/powermode valve  44 , and the engine  16 -pump  14  produces fluid flow in excess of that currently used by the pump-motors  22 ,  26 . The increase in fluid flow from pump  14  is accomplished by increasing engine-pump speed, or by increasing displacement of pump  14 , or a combination of engine-pump speed and pump displacement increases. Line pressure in rail  13  rises as a result of the excess flow produced by pump  14 , and as it exceeds the pressure of the power mode accumulator  32 , the valve  34  will allow flow will allow flow into the accumulator  32 , and the valve  34  can then be switched to a state that allows flow in both directions without any disturbance such as from a sudden rush of fluid flow. The system then operates at a relatively high rail pressure while higher drive torque is needed at the wheels.  
         [0032]     In an alternative arrangement, a flow control bypass valve  110  having an orifice of predetermined diameter is arranged in parallel with valve  34 . Valve determines the rate of fluid flow between accumulator  32  and rail  13 , and operates to raise line pressure in rail  13  more quickly than control valve  34 .  
         [0033]     When a demand for increased torque has been met or is otherwise absent, the flow from engine pump  14  is reduced, and the state of valve  34  is adjusted so that the power mode accumulator  32  is closed, thereby trapping in accumulator  32  the relatively high pressure present in rail  13  during the demand for increased wheel torque. Rail pressure then falls to the magnitude of pressure in the main accumulator  42 , which is reconnected to the system by changing the state of valve  44  to the fully opened state, after it begins to allow flow out of the accumulator  42  due to its behavior as a check valve.  
         [0034]     The magnitude of pressure in main accumulator  42  is a measure of the magnitude of energy stored in the main accumulator  42 . If the main accumulator  42  contains sufficient stored energy, i.e., a magnitude of energy sufficient to meet a demand for torque at a set of wheels, such as to accelerate the vehicle from a stopped condition to 20 mph, the splitting valve  74  may be closed, so that energy stored in main accumulator  32  only supplies fluid to one of the pump-motors  22 ,  26 , preferably the front pump-motor  22 . The other pump-motor  26  is then supplied with fluid from the engine  16 -pump  14 , and engine  16 -pump  14  and pump-motor  26  both operate at a pressure above the pressure in the main accumulator to drive the rear wheels. Pump-motor  26  produces more power or torque at the rear wheels due to the higher pressure in rail  13  than if pump-motor  26  were in communication with the main accumulator  42 .  
         [0035]     The attached schematic covers one possible system implementation. The system includes a single power mode accumulator  32  and two possible pressure modes, power mode and split power mode. However, additional power accumulators may be incorporated, allowing additional independent pressure levels, and transitions to the additional pressure levels. Piloted check valves are used for the state transitions, and are arranged to allow opening and closing events to occur by the check valve when there is no differential pressure across the check valve.  
         [0036]     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.