Abstract:
In a vehicle powertrain that includes an engine and an electric machine, a method for preventing rollback of a wheeled vehicle located on an incline includes determining a magnitude of wheel torque required to prevent the vehicle from rolling back, using the electric machine to produce the required magnitude of wheel torque at the wheels, transmitting engine torque to the wheels, and reducing torque produced by the electric machine while increasing engine torque such that the sum of wheel torque produced by the engine and electric machine is substantially equal to said required magnitude of wheel torque.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) having an engine, an electric machine and a multiple-speed, powershift transmission. In particular, the invention pertains to using the powertrain to prevent rollback of the vehicle on an incline. 
     2. Description of the Prior Art 
     A powershift transmission is an example of a power transmission for a motor vehicle in which there is no torque-converter. A powershift transmission is a geared mechanism producing multiple gear ratios in forward drive and reverse drive and having two input clutches, which connect a power source, such as an engine or electric motor, to two transmission shafts. A powershift transmission transmits power alternately to the two shafts using synchronized clutch-to-clutch shifts. 
     A powershift transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear. 
     Conventional vehicles with automatic transmissions have torque converters that allow the transmission to stay in gear even when the vehicle is stopped or launching on an incline. In a vehicle having a converterless transmission, such as a powershift or manual transmission, the clutches must be slipping or disengaged when the vehicle is stopped or launching on an incline to avoid vehicle stall; therefore the transmission cannot transmit torque to the wheels immediately. 
     When a vehicle without a torque converter transitions from being stopped on a positive incline to ascending the incline, delays in torque delivered to the wheels necessary to accelerate the vehicle can result in undesirable rollback of the vehicle. Rollback control is required when the driver depresses the accelerator pedal, called a “tip in,” and expects to ascend the hill. If the engine is to provide torque to the wheels to launch the vehicle, a delay occurs before torque is transmitted to the wheels due to delay in charging the intake manifold and engine cylinders with a combustible fuel/air mixture and delay in producing transmission input clutch torque capacity. 
     When a HEV stops on a positive grade, the engine is typically shut off to save fuel, but certain conditions may require the engine to continue running including charging the battery, a driver&#39;s request for front accessory drive for air conditioning, or a request for other belt driven electro-mechanical devices. Vehicle rollback of an HEV with the engine running can occur in the transition from holding the vehicle on the positive grade to accelerating the vehicle up the grade. The HEV vehicle can be held stationary on a positive incline when the driver applies the wheel brakes, or, if the brakes are released, when an electric machine provides holding torque while the engine is idling. In a HEV, the transmission can be placed in neutral and an electric machine operating as a motor may be used to hold the vehicle stationary on an incline. 
     There is a need in the industry for a technique that eliminates unintended rollback due to delay in torque delivery to the wheels of a vehicle when the vehicle operator tips-in to accelerate the vehicle on an uphill grade from a stationary vehicle condition while the engine is running. 
     SUMMARY OF THE INVENTION 
     In a vehicle powertrain that includes an engine and an electric machine, a method for preventing rollback of a wheeled vehicle located on an incline includes determining a magnitude of wheel torque required to prevent the vehicle from rolling back, using the electric machine to produce the required magnitude of wheel torque at the wheels, transmitting engine torque to the wheels, and reducing torque produced by the electric machine while increasing engine torque such that the sum of wheel torque produced by the engine and electric machine is substantially equal to said required magnitude of wheel torque. 
     An electric machine (ERAD) prevents rollback prevention of a vehicle as the driver tips into the accelerator pedal. The ERAD quickly provides torque that is transmitted to the wheels to avoid rollback. The ERAD torque is necessary to provide torque to the wheels during delays caused by engine manifold filling and the transmission engagement. If the state of charge of the battery is below a reference state of charge or ERAD temperature is greater than a thermal limit, the ERAD torque capability will be reduced which requires the engine output torque to be blended with that of the ERAD. 
     Finally, ERAD torque is blended off synchronously while engine torque increases, thereby maintaining a constant wheel torque. This provides an undetected transition as the engine is used for vehicle propulsion while preventing vehicle rollback 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a vehicle powertrain system to which rollback control can be applied; 
         FIG. 2  is a schematic diagram showing additional details of the vehicle powertrain system of  FIG. 1 ; 
         FIG. 3  illustrates the steps of a control method for preventing vehicle rollback; 
         FIG. 4  is a schematic diagram showing a function for determining required wheel torque; 
         FIGS. 5A-5C  show the variation over time of powertrain variables while vehicle rollback is controlled in response to a tip-in; and 
         FIGS. 6A-6C  show the variation over time of powertrain parameters while vehicle rollback is being controlled in response to releasing the wheel brake pedal. 
         FIG. 7  is a schematic diagram of a kinematic arrangement for a powershift transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in  FIGS. 1 and 2 , a vehicle powertrain  12  includes an engine  14 , such as a diesel or gasoline engine; a transmission  16 , such as dual wet clutch powershift transmission or another multiple ratio transmission having no torque converter; an electric machine  18 , such as an CISG driveably connected to the transmission input  20 ; and an additional electric machine  22 , such as an electric motor. Electric machine  18  provides rotating power to crank engine  14  when starting the engine and generates electric power, which is supplied directly to machine  22 , or to an electric storage battery  23  or to both of these. 
     Electric machine  22 , sometimes referred to as an electric rear axle drive unit (ERAD), is connected to the final drive of a rear axle  24  and provides propulsion capability in either an electric drive or hybrid (series/parallel) drive mode. Power output by the electric machine  22  drives vehicle wheels  26 ,  27  through ERAD gearing  28  and a final drive unit  30 , which is in the form of an inter-wheel differential mechanism. Similarly, the transmission output  32  is driveably (mechanically) connected to vehicle wheels  34 ,  35  through a final drive unit  36 , which includes an inter-wheel differential mechanism. In front wheel drive (FWD) applications, electric machine  22  could be driveably connected to the final drive  36  of the front axle at the output  32  of the transmission  16 , in which case it is referred to as an electric front axle drive (EFAD) unit. 
       FIG. 2  illustrates the input clutches  40 ,  41 , which selective connect the input shaft  20  of transmission  16  alternately to the even-numbered gears  42  and odd-numbered gears  43 ; an electronic transmission control module (TCM)  44 , which controls the input clutches and gearbox state through command signals to servos or solenoids that actuate the input clutches and gearbox shift forks/synchronizers; an electronic engine control module (ECM)  46 , which controls operation of engine  14 ; and an ISC  48 , which controls the CISG and ERAD operations. A vehicle control system (VCS), which is not shown, issues control commands to the TCM and ECM. Each of the VCS, TCM and ECM includes a microprocessor accessible to electronic memory and containing control algorithms expressed in computer code, which are executed repeatedly at frequent intervals. Data communication among the control modules, ECM  46 , VSC, TCM  44  and ISC  48  is carried on a communications bus  47 . 
     Powertrain  12  includes two power paths to the load, a mechanical path and an electrical path. Power produced by engine  14  is transmitted through transmission  16  and final drive  36  in the mechanical power path to wheels  34 ,  35 . Power produced by ERAD  22  is transmitted through ERAD gearing  28  and final drive  30  in the electrical propulsion path to wheels  26 ,  27 . 
       FIG. 3  illustrates the steps of control algorithm for preventing vehicle rollback. As shown in  FIGS. 5A-5C , the hill hold control strategy uses ERAD  22  to provide torque to accelerate the vehicle on a hill in order to prevent rollback during a period required before the engine produces output torque and the transmission transmits engine output torque to the wheels. When a tip-in occurs, the ERAD can quickly provide torque to the wheels and accelerate the vehicle to avoid rollback because the ERAD produces output torque quickly. 
     The control algorithm is called for execution by the controller at step  48  when signals produced by sensors indicate that the vehicle is stopped on an incline. As  FIG. 4  shows, the vehicle operator&#39;s demand for wheel torque is represented by the degree to which the engine accelerator pedal  50  is depressed, which depression is usually referred to as accelerator pedal position, pps. An electronic signal representing the accelerator pedal position produced by a pps sensor and an electronic signal representing the current vehicle speed (VS)  52  produced by a shaft speed sensor, are received as input by a driver demand determination function  54 , accessible in electronic memory, the function being indexed by the two input variables VS and pps and producing as its output the current desired wheel torque T W     —     DES . 
     At step  56 , a test is made to determine whether the accelerator pedal position is greater than zero or a reference pedal position. If the result of test  56  is logically true, control passes to step  58 . If the result of test  56  is false, control returns to  48 . 
     At step  58 , a test is made to determine whether the battery&#39;s state of charge (SOC) is greater than a reference SOC. If the result of test  58  is true, control passes to  60 , where a test is made to determine whether the temperature of ERAD  22  is greater than a reference temperature. If the result of test  60  is false, control passes to step  62 , where ERAD  22  produces torque, which is transmitted to the wheels to control rollback of the vehicle on the incline. 
     Provided the accelerator pedal is depressed, the battery&#39;s SOC is above the reference SOC, and the ERAD temperature is below the reference temperature, ERAD  22  and the electric power path are used at step  62  to drive the wheel load and control vehicle rollback. If the result of test  58   is  false and test  60  is true, ERAD is not currently available to provide torque to the wheels and prevent rollback. Next, control advances to step  64 , where the appropriate input clutch of transmission  16  associated with the current gear command(the subject clutch  40 ,  41 ) is commanded to stroked pressure in preparation for engagement. Steps  64  through  72  apply to conventional vehicles as well as hybrid vehicles. 
     At step  66  a test is made to determine whether engine  14  is currently producing the demanded wheel torque. If the result of test  66  is true, a check is made at step  68  to determine whether the selected gear is engaged and the subject clutch  40 ,  41  of transmission  16  is prepared for engagement. If the result of either of tests  66 ,  68  is false, control returns to step  58 . 
     If the result of tests  66  and  68  is true, the subject input clutch  40 ,  41  is engaged at step  70 , engine torque is increased such the wheel torque reaches the demanded wheel torque at step  72 , and ERAD torque is decreased at step  74  along a descending ramp concurrently with the increase in engine torque, as shown in  FIG. 5B . 
       FIGS. 5A-5C , represent the variation of various vehicle and powertrain variables during a period when vehicle rollback is controlled with the HEV stopped on an incline, having a positive slope, in response to tip-in and while the wheel brakes are released. 
     As  FIGS. 5A and 5C  show, the accelerator pedal position  80  increases following a period  76 , during which torque produced by ERAD  22  and transmitted to wheels  26 ,  27  is used to hold the HEV on the incline, the accelerator pedal  50  is off (not pressed), and vehicle speed  78  is zero. The accelerator pedal position  80  increases as the operator demands wheel torque to move the vehicle forward on the incline. Accelerator pedal position  80  later decreases slightly and remains steady while the vehicle speed  78  increases steadily. 
     In  FIG. 5B , following period  76 , ERAD torque  82  increases rapidly in response to the increase in accelerator pedal position  80 , reaches a peak, and is ramped off synchronously with the increases in engine torque and transmission output torque, thereby maintaining wheel torque without transient impulses. This provides an undetected transition as the engine is used for vehicle propulsion while preventing vehicle rollback. Engine torque  84  is at engine idle setpoint during period  76  before the rollback prevention control begins at  88 , remains low during a delay period  90  while the intake manifold and engine cylinders are filled with a combustible air-fuel mixture as a result of the tip-in, and increases rapidly after the engine is charged. Transmission output torque  92  is low during period  76 , remains constant during a delay period  94  while the torque capacity of the subject input clutch  40 ,  41  increases, and increases rapidly with engine torque. Wheel torque increases from holding level during period  76  to the new driver demanded level when accelerator pedal  50  is depressed at the beginning of the rollback prevention control  88 . 
       FIGS. 6A-6C  shows the variation of powertrain parameters with the HEV under vehicle rollback control in response to the vehicle operator releasing a wheel brake pedal. As  FIGS. 6A and 6C  show, the wheel brake pedal position or brake pressure  96  decreases rapidly to zero after being released, the operator tips-in to the accelerator pedal at  98 , and vehicle speed  100  increases steadily thereafter. 
     In  FIG. 6B , following the beginning of rollback control at  98 , ERAD torque  102  increases rapidly in response to the increase driver demanded torque as indicated by the increase in accelerator pedal position, reaches a peak, and decreases to zero while engine torque  104  increases. Engine torque  104  is at engine idle level before the rollback prevention control begins at  98 , remains low during a delay period  106  while the intake manifold and engine cylinders are filled with a combustible air-fuel mixture, and increases rapidly after the engine is charged. Transmission output torque  108  is low initially, remains constant during a delay period  110  while the torque capacity of the subject input clutch  40 ,  41  increases, and increases rapidly with engine torque. Wheel torque  112  increases rapidly when accelerator pedal  50  is depressed at the beginning of the hill-hold control  98 . 
       FIG. 7  illustrates details of a powershift transmission  16  including a first input clutch  40 , which selective connects the input  20  of transmission  16  alternately to the even-numbered gears  42  associated with a first layshaft  244 , and a second input clutch  41 , which selective connects the input  20  alternately to the odd-numbered gears  43  associated with a second layshaft  249 . 
     Layshaft  244  supports pinions  260 ,  262 ,  264 , which are each journalled on shaft  244 , and couplers  266 ,  268 , which are secured to shaft  244 . Pinions  260 ,  262 ,  264  are associated respectively with the second, fourth and sixth gears. Coupler  266  includes a sleeve  270 , which can be moved leftward to engage pinion  260  and driveably connect pinion  260  to shaft  244 . Coupler  268  includes a sleeve  272 , which can be moved leftward to engage pinion  262  and driveably connect pinion  262  to shaft  244  and can be moved rightward to engage pinion  264  and driveably connect pinion  264  to shaft  244 . 
     Layshaft  249  supports pinions  274 ,  276 ,  278 , which are each journalled on shaft  249 , and couplers  280 ,  282 , which are secured to shaft  249 . Pinions  274 ,  276 ,  278  are associated respectively with the first, third and fifth gears. Coupler  280  includes a sleeve  284 , which can be moved leftward to engage pinion  274  and driveably connect pinion  274  to shaft  249 . Coupler  282  includes a sleeve  286 , which can be moved leftward to engage pinion  276  and driveably connect pinion  276  to shaft  249  and can be moved rightward to engage pinion  278  and driveably connect pinion  278  to shaft  249 . 
     Transmission output  32  supports gears  288 ,  290 ,  292 , which are each secured to shaft  32 . Gear  288  meshes with pinions  260  and  274 . Gear  290  meshes with pinions  262  and  276 . Gear  292  meshes with pinions  264  and  278 . 
     Couplers  266 ,  268 ,  280  and  282  may be synchronizers, or dog clutches or a combination of these. 
     Although the invention has been described with reference to a powershift transmission, the invention is applicable to any automatic shift manual transmission, or automatic transmission that has no torque converter located in a power path between the engine and transmission input. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.