Abstract:
A system for braking the wheels of a hydraulic hybrid vehicle includes a brake pedal having a range of pedal displacement including a deadband displacement range, an accumulator containing fluid at relatively high pressure, a reservoir containing fluid at lower pressure, a pump/motor having variable volumetric displacement connected to the accumulator and reservoir, and driveably connected to the wheels; a system responsive to brake pedal displacement in the deadband range for placing the pump/motor in a pump state wherein the pump/motor is driven by the wheels and pumps fluid from the reservoir to the accumulator; and a control valve for changing the volumetric displacement of the pump/motor in response to displacement of the brake pedal.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates in general to a hybrid vehicle drive system having a primary power source, such as a conventional internal combustion engine, and another power source, such as a source of high pressure pneumatic or hydraulic fluid. More particularly the invention pertains to braking the wheels of a hydraulic hybrid vehicle. 
   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. 
   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. 
   SUMMARY OF THE INVENTION 
   Using regenerative braking energy for vehicle acceleration can provide a significant fuel economy benefit without the complications of engine start-stop capabilities or cruise load leveling. Since HPA can provide this fuel economy benefit without significant changes to the conventional powertrain, it is possible to achieve the fuel economy benefit without adversely affecting vehicle performance. 
   It is also possible to significantly augment vehicle performance over the engine-only powertrain, especially in a heavier vehicle. Fuel economy and performance benefits can be optimized for a given application. 
   A system for braking the wheels of a hydraulic hybrid vehicle includes a brake pedal having a range of pedal displacement including a deadband displacement range, an accumulator containing fluid at relatively high pressure, a reservoir containing fluid at lower pressure, a pump/motor having variable volumetric displacement connected to the accumulator and reservoir, and driveably connected to the wheels; a system responsive to brake pedal displacement in the deadband range for placing the pump/motor in a pump state wherein the pump/motor is driven by the wheels and pumps fluid from the reservoir to the accumulator; and a control valve for changing the volumetric displacement of the pump/motor in response to displacement of the brake pedal. 
   The invention relates to a method for braking the wheels of a vehicle that includes an accumulator containing fluid at relatively high pressure, a reservoir containing fluid at lower pressure, a pump/motor having variable volumetric displacement connected to the accumulator and reservoir are driveably connected to the wheels, and a brake pedal having a range of pedal displacement. A desired vehicle is determined on the basis of the pedal displacement, and a magnitude of braking force to decelerate the vehicle at the desired deceleration is determined. A wheel torque corresponding to the required braking force, a net wheel torque to stop the vehicle at the desired deceleration from a current vehicle speed, and a torque magnitude to be applied to the pump/motor by the wheels based on the net wheel torque are determined. Then the pump displacement corresponding to the magnitude of torque to be applied to the pump/motor by the wheels to produce the desired deceleration rate is determined. Finally, the magnitude of pump displacement is changed to the pump displacement corresponding to the torque magnitude to be applied by the wheels to the pump/motor. 
   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 
       FIG. 1  is a schematic diagram of a powertrain for a hydraulic hybrid motors vehicle that operates in a brake regenerative mode and power assist mode. 
       FIG. 2  is a schematic diagram of a brake pedal for use in controlling the brake regeneration mode of the powertrain of  FIG. 1 . 
       FIG. 3  is a hydraulic circuit diagram showing the pump/motor, accumulator, control valves and hydraulic lines connecting them. 
       FIG. 4  is diagram of logic for controlling the brake regeneration mode in a deadband range of brake pedal position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings, there is illustrated in  FIG. 1  a hydraulic hybrid powertrain  10  for driving the rear wheels  12 ,  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 producing 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 powertrain can be adapted to drive the front wheels  14  instead, and may include a transfer case for operating in all-wheel drive or four-wheel drive modes. 
   A pump/motor  26  having variable displacement is driveably connected to the transmission output shaft  20  and to a driveshaft  22 . When torque is transmitted in a positive torque direction, from the engine to the wheels, output shaft  20  drives the pump/motor  26 ; when torque is transmitted from the wheels to the engine, the negative torque direction, driveshaft  22  drives the pump/motor  26 . 
   During the power assist mode, while the vehicle is accelerating, pressure in accumulator  40  is released, high pressure fluid drives the pump/motor  26 , and the wheels  12  are driven in rotation by the pump/motor, which operates then as a fluid motor. The motor  26  drives the wheels  12  through the driveshaft  22 , differential  23  and the axles  30 ,  32 . 
   During the brake regeneration mode, while the vehicle is decelerating while being braked, vehicle kinetic energy or momentum is initially reduced by causing the wheels  12  to drive the pump/motor  26  through the axles  30 ,  32  and driveshaft  22 . The pump/motor  26  operates during the brake regeneration mode as a pump across a pressure differential between the pump inlet  112 , which communicates with reservoir  36 , and the pump outlet  90 , which communicates with accumulator  40 . The pump/motor  26  pumps fluid from reservoir  36  to the accumulator  40 . Fluid entering the accumulator  40  compresses nitrogen contained in a bladder located in the accumulator  40 , and the accumulator is pressurized. 
   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. 
   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 brake pedal position transducer  70  produces an electronic signal carried on line  72  to an electronic controller  74 , the signal representing brake pedal position. Controller  74  operates under control of a microprocessor, which executes programmed logic. 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 . 
   Pressure in the hydraulic brake system  80 , which actuates friction brakes  82 , changes when pressure in the master cylinder  58  changes due to movement of piston  60  as the brake pedal  50  is displaced. When the brake pedal  50  is depressed beyond the deadband range sufficiently to close the space  52 , brake system pressure forces the brake pads into frictional contact with the brake disc  84 , to which a wheel  12  is fixed. 
   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  and supplied to a solenoid  86 , which operates a mode valve  88 . When valve  88  is in the position shown in  FIG. 3 , the pump/motor  26  is connected hydraulically to the high pressure accumulator  40 , and the system operates in the motor mode, in which the wheels  12 ,  14  are driven by the motor  26  being actuated by high pressure fluid from accumulator  40 . When the state of valve  88  is changed by solenoid  86  in response to a command signal from controller  74 , the pump/motor  26  is connected hydraulically to the low pressure reservoir  36 , and the system operates in the pump mode, in which the wheels  12 ,  14  drive pump  26 , which pumps fluid from reservoir  36  to accumulator  40 . 
   A swashplate control valve or proportional valve  96  changes the variable displacement of the pump/motor  26  in response to commands issued by controller  74 . 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. 
   Solenoid  98  changes the state of valve  96  among three positions or states, a center position where the inlet and outlet of valve  96  are mutually disconnected, a left-hand position where displacement of the pump/motor  26  decreases, and a right-hand position where displacement of the pump/motor  26  increases. 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 . The reservoir  36  is connected to an inlet of valve  96  through a check valve  99 . When valve  96  is in the left-hand state, the state shown in  FIG. 3 , accumulator  40  is connected through valves  128  and  96  to the pump/motor  26 . Pressure from accumulator  40  changes the angular position of a swashplate in the pump/motor  26  tending to increase the swashplate angle and decrease the volume of fluid that passes through the pump/motor  26  during each revolution, its volumetric displacement. When valve  96  moves to the right-hand state illustrated in  FIG. 3 , accumulator  40  is connected through valves  96  and  128  to change the angular position of the swashplate, tending to decrease the swashplate angle and increase volumetric displacement of the pump/motor  26 . 
   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  150 ,  156 ,  158  are added algebraically at summing junction  160  to produce a net vehicle deceleration rate  162 . 
   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. 
   That force is converted at  166  to an equivalent wheel torque 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  166  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 . 
   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  22  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 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 calculated pump displacement calculated at  180 . 
   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.