Abstract:
In a powertrain that includes an engine driveably connected to a load and an electric machine driveably connected to the load, a method for controlling a vehicle located on an incline against rollback includes determining a magnitude of wheel torque required to prevent the vehicle from rolling back, determining whether the electric machine has a current torque capacity that is equal to or greater than the required wheel torque, using the electric machine to produce the required wheel torque provided the current torque capacity of the electric machine is able to produce the required wheel torque, and using the engine to produce the required wheel torque provided the torque capacity of the electric machine is unable to produce the required wheel torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    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. 
         [0003]    2. Description of the Prior Art 
         [0004]    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. 
         [0005]    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. In a motor vehicle whose powertrain includes a powershift transmission the vehicle has a tendency to rollback when the driver depresses the accelerator pedal (called a tip-in) following a hill-hold condition, in which the vehicle is held stationary on an incline with or without use of the wheel brakes. The rollback condition is caused by delay in starting the engine in the case where engine is shut down, delay in engine torque due to charging the intake manifold and cylinders with a combustible fuel/air mixture, and delay in producing input clutch torque capacity. These and other delays cause delay in producing wheel torque sufficient to hold the vehicle against rollback on an incline. 
         [0006]    When a HEV comes to a stop and the battery&#39;s state of charge (SOC) is sufficient and other conditions are met, the engine is shut off. The engine could also be shut off during a hill-hold condition since the vehicle is stopped. Hill holding a HEV occurs with the driver holding the vehicle by applying the wheel brakes, or, if the wheel brakes are released, an electric machine can provide hill-holding wheel torque. 
         [0007]    Rollback prevention is required when the vehicle operator then 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 wheel torque increases sufficiently due to the delays in engine starting, manifold filling and input clutch activation. Rollback can also occur when a vehicle is ascending a hill and the wheel torque does not meet the increased road load due to increasing grade. 
         [0008]    There is a need in the industry for a technique that eliminates unintended rollback of the vehicle when (1) the vehicle operator tips-in to accelerate the vehicle on an uphill grade from a stationary vehicle condition while the engine is shutdown; (2) the vehicle operator tips-in to accelerate the vehicle on an uphill grade from a stationary vehicle condition while the engine is running; and (3) when the ERAD is the only available torque source while the engine is shutdown and the vehicle is ascending a hill and the current wheel torque capacity does not meet the increased road load. 
       SUMMARY OF THE INVENTION 
       [0009]    In a powertrain that includes an engine driveably connected to a load and an electric machine driveably connected to the load, a method for controlling a vehicle located on an incline against rollback includes determining a magnitude of wheel torque required to prevent the vehicle from rolling back, determining whether the electric machine has a current torque capacity that is equal to or greater than the required wheel torque, using the electric machine to produce the required wheel torque provided the current torque capacity of the electric machine is able to produce the required wheel torque, and using the engine to produce the required wheel torque provided the torque capacity of the electric machine is unable to produce the required wheel torque. 
         [0010]    Rollback prevention is provided by the ERAD for a vehicle that is initially stationary on an uphill grade as the driver tips into the accelerator pedal. The ERAD quickly provides torque that is transmitted to the wheels to avoid rollback. If the ERAD torque capability does not meet or exceed road load or if the thermal limitation of ERAD occurs, the engine is started and its output torque is blended with that of the ERAD. 
         [0011]    As the vehicle begins to decelerate due to an increase in the road load caused by increasing grade, the vehicle operator further tips into the accelerator pedal to continue accelerating the vehicle on the incline. A control algorithm interprets the increased accelerator pedal rate as an indication of increasing road load. The combination of increasing accelerator pedal rate and decreasing vehicle speed rate are used to infer that the operator is increasing the pedal position in order to overcome the increase road slope. If the wheel torque provided by the ERAD is not adequate to maintain vehicle acceleration, the engine is started to prevent a vehicle rollback condition. 
         [0012]    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. 
         [0013]    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 
         [0014]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic diagram of a vehicle powertrain to which the control can be applied; 
           [0016]      FIG. 2  is a schematic diagram showing additional details of the vehicle powertrain of  FIG. 1 ; 
           [0017]      FIG. 3  illustrates the steps of a control method for preventing vehicle rollback; 
           [0018]      FIG. 4  is a schematic diagram showing a function for determining required wheel torque; 
           [0019]      FIGS. 5A-5D  show the variation over time of accelerator pedal position, ERAD torque, engine torque, wheel torque, road load, vehicle speed and battery SOC while vehicle rollback is controlled; and 
           [0020]      FIGS. 6A-6C  show the variation over time of powertrain parameters while the demand for wheel torque is high and vehicle rollback is being controlled. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    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. 
         [0022]    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. 
         [0023]      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 . 
         [0024]    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 . 
         [0025]      FIG. 3  illustrates the steps of control algorithm for preventing rollback when the vehicle is either stationary or driven by ERAD  22  with the engine  14  initially shutdown. The control algorithm is called for execution by the controller at step  49  when conditions indicate that the vehicle is stopped on an incline. 
         [0026]    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, 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 to the processor 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 . 
         [0027]    At  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  58 . If the result of test  56  is false, control returns to  56 . 
         [0028]    At  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 less than a reference temperature. If the result of test  60  is true, control passes to  62 , where a test is made to determine whether the current torque producing capacity of ERAD  22  is greater than the desired wheel torque determined from function  54 . 
         [0029]    Provided the pedal position is depressed, the battery&#39;s SOC is above the reference SOC, the ERAD temperature is below a reference temperature, and the ERAD torque producing capacity is greater than the desired wheel torque, ERAD  22  and the electric power path are used at  64  to drive the wheel load, prevent vehicle rollback, and ascend an uphill grade without starting the engine. But if the result of any of tests  58 ,  60  and  62  is false, control advances to step  66  where engine  14  and the mechanical power path are used to drive the wheel load and prevent vehicle rollback. At step  66 , ERAD torque is decreased synchronous to engine torque increase until engine torque provides the required wheel torque. Preferably, as shown in  FIG. 5B , ERAD torque is controlled so that the summation with engine torque continually and smoothly provides the required wheel torque. 
         [0030]    While ERAD  22  and the electric power path are being used to prevent rollback, the control algorithm repetitively performs test  68  at frequent intervals to determine whether the rate of change of accelerator pedal position is greater than zero or a reference pedal position rate. If the result of test  68  is logically true, the algorithm repetitively performs test  70  at frequent intervals to determine whether the rate of change of vehicle speed VS is greater than zero or a reference vehicle speed change rate. If the result of test  70  is true, control returns to  64 . 
         [0031]    If the result of test  68  is false, indicating that the accelerator pedal position is not changing or is changing slowly, the control assumes that ERAD torque is preventing vehicle rollback, and control returns to  58 . 
         [0032]    If the result of test  70  is false, indicating that the vehicle acceleration is not increasing or is decreasing rapidly, the control assumes that ERAD torque is not preventing vehicle rollback, and control passes to  66 , where engine  14  and the mechanical power path are used to drive the wheel load and prevent vehicle rollback. The combination of test  68  and test  70  is used by the control to infer that the driver is depressing the accelerator pedal to overcome an increase in road load due to an increase in grade and that torque at the wheels is not adequate to maintain vehicle acceleration. Tests  68  and  70  provide the earliest indication of a rollback condition. 
         [0033]    In  FIGS. 5A-5D , at the start of phase A, the vehicle is stopped on an incline having a positive slope. The accelerator pedal position  80  increases initially as the operator demands wheel torque to ascend the incline. ERAD provides increasing torque to the wheels preventing rollback and later ERAD torque is held steady while vehicle speed is stable. The rate of change of accelerator pedal position  82  follows characteristically. Vehicle speed  88  and vehicle acceleration  90  are shown in  FIG. 5C . The battery&#39;s SOC  92  is shown decreasing linearly as ERAD  22  draws electric power from battery  23 . 
         [0034]    In  FIGS. 5A-5D , phases B, C, D and E represents the periods during which the vehicle is moving forward on an incline with increasing slope and ERAD torque  84  is used initially to move the vehicle on the incline. At the beginning of phase B, the vehicle decelerates because ERAD torque  84  is less than the road load  86 , which increases due to the road slope increasing. At the beginning of phase C, the operator senses the vehicle deceleration and tips-in by depressing the accelerator pedal in order to accelerate the vehicle on the incline. CISG  18  is used to start the engine  14 , which begins to produce positive engine torque  98  after a brief period, and wheel torque  100  increases. The increase in accelerator pedal rate and the concurrent decrease in vehicle acceleration cause the engine to start, at step  66  in  FIG. 3 . Wheel torque  100  exceeds road load  86  at the beginning of phase D due to the oncoming engine torque  98 , thereby accelerating the vehicle on the incline. 
         [0035]    In phase B, the vehicle begins to decelerate due to an increase in the road load with increasing grade. During phase C, the driver further tips into the accelerator to continue accelerating the vehicle up the grade. The control strategy infers an increasing accelerator pedal rate as an indication of increasing road load. The combination of increasing accelerator pedal rate and decreasing vehicle speed are used to infer that the operator is depressing the accelerator pedal in order to overcome an increase in slope of the grade, but that wheel torque sourced from the ERAD is inadequate to maintain vehicle acceleration. This provides the earliest indication of a rollback condition. These conditions indicate that ERAD torque  84  cannot meet the driver demanded torque due to the increasing road load  86 . These conditions are checked because desired wheel torque does not compensate for increasing road load caused by increasing grade slope. Once these conditions are detected during phase C, the engine is started to prevent a vehicle rollback condition. 
         [0036]    In phase D, the vehicle accelerates on the incline as engine  14  produces torque along with ERAD  22 , as shown by the increased wheel torque above the road load. Finally, during phase E, ERAD torque  84  is blended off synchronously while engine torque  98  increases, thereby maintaining a constant wheel torque  100 . This provides an undetected transition as the engine is used for vehicle propulsion while preventing vehicle rollback. 
         [0037]      FIGS. 6A-6C  shows the variation of powertrain parameters during vehicle rollback prevention when the operator initially and continually demands a large magnitude of wheel torque, as represented by accelerator pedal position  80  and its rate of change  82 . Desired wheel torque  110  exceeds actual wheel torque  100  and ERAD torque  84 . At  112 , CISG  18  is used to start engine  14 , which begins to produce positive engine torque  98  after a brief period. As shown in  FIG. 6B , engine torque  98  preferably increases synchronously while CISG torque  114  decreases, thereby producing a smooth torque transition between those power sources. Engine torque  98  remains constant for a period  116 . A delay in increasing transmission output torque occurs due to delay in filling, stroking and slipping the input clutch  40 ,  41 , which transmits power through the gear in which transmission  14  is operating. 
         [0038]    Engine torque  98  increases and is controlled to provide required torque to wheels. Vehicle speed  88  increases uniformly while the engine  14  is started, the input clutch  40 ,  41  of transmission  16  is engaged, and power is transmitted to the wheels  34 ,  35 . ERAD torque  84  increases quickly causing the wheel torque at wheels  26 ,  27  to approach the required wheel torque when the operator initially demands a large magnitude of wheel torque, then is ramped down at a synchronous rate to the increase of engine torque to ensure smooth torque transition at the wheels. Thereafter, ERAD  22  is shutdown at  120 . 
         [0039]      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 . 
         [0040]    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 . 
         [0041]    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 . 
         [0042]    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 . 
         [0043]    Couplers  266 ,  268 ,  280  and  282  may be synchronizers, or dog clutches or a combination of these. 
         [0044]    Although the invention has been described with reference to a powershift transmission, the invention is applicable to any conventional manual transmission, automatic shift manual transmission, or automatic transmission that has no torque converter located in a power path between the engine and transmission input. 
         [0045]    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.