Abstract:
A method for controlling a vehicle powertrain includes holding the vehicle stopped on a grade by automatically producing wheel brake torque while driver demand torque is less than wheel brake torque, automatically releasing wheel brake torque when driver demand torque equals or exceeds wheel brake torque, and launching the vehicle using engine torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to controlling the powertrain of a motor vehicle while launching the vehicle on a grade. 
         [0003]    2. Description of the Prior Art 
         [0004]    A vehicle that is stopped on a grade can be held stationary using wheel brake torque until propulsion torque, transmitted from the engine through a transmission and final drive mechanism to the wheels, exceeds brake torque. It is important to avoid unnecessary transmission clutch slip when brake torque is holding the hill, particularly when the transmission lacks a torque converter. 
         [0005]    An electronic signal representing estimated propulsion torque can be used as a measure of propulsion torque at the wheels to release brake torque. In this case, either a brief timeout occurs after neither the brake pedal nor accelerator pedal is depressed by the driver, or the brakes are applied indefinitely if the driver depresses the accelerator pedal greater than a small amount. In the latter case, the resulting propulsion torque is smaller than brake torque. 
         [0006]    The period during which propulsion torque is less than brake torque can be significant especially in heavy traffic on a grade, and would cause excessive, unnecessary clutch wear. 
       SUMMARY OF THE INVENTION 
       [0007]    A method for controlling a vehicle powertrain includes establishing first and second functions relating desired engine torque and driver demand torque corresponding to hill start assist being active and inactive, respectively; while hill start assist is active, holding the vehicle stopped on a grade by automatically producing wheel brake torque and producing engine torque derived from the first function; automatically releasing wheel brake torque when driver demand torque equals or exceeds wheel brake torque; launching the vehicle using engine torque derived from the first function and corresponding to said pedal displacement; and while hill start assist is inactive, launching the vehicle using engine torque derived from the second function. 
         [0008]    The method reduces clutch wear by lower engine torque levels when the driver is pressing the accelerator pedal to a level insufficient to launch the vehicle on the current grade. 
         [0009]    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 
         [0010]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a schematic diagram of a vehicle powertrain; 
           [0012]      FIG. 2  schematic diagram of a multiple speed, hydraulically actuated automatic transmission; 
           [0013]      FIG. 3  is a logic flow diagram of a control algorithm; and 
           [0014]      FIG. 4  is a graph showing functions used to determine a desired engine torque when hill start assist is active and inactive. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    Referring now to the drawings, there is illustrated in  FIG. 1  a motor vehicle powertrain  10 , which includes a power source  12 , such as an internal combustion engine; an engine starter motor  14 ; a dual clutch automatic transmission  16 , connected to the engine by an input shaft  17  and clutches  18 ,  20 ; an electro-mechanical actuator  25 , which varies the torque transmitting capacity of the clutches; a first layshaft  36  containing odd gears first, third, fifth and reverse gears; a second layshaft  37  containing even gears second, fourth, and sixth gears; a transmission output  22 ; final drive mechanism  24 , connected to the output  22 ; an electric storage battery  26 , which supplies electric power to the starter motor 14  and clutch actuator  25 ; and axle shafts  28 ,  29 , driveably connect to the driven wheels  30 ,  31 . 
         [0016]    A vehicle controller comprising a transmission module (TCM)  42  and an engine control module ECM  50  communicates through electronic signals mutually and with battery  26 , transmission  16 , the clutch actuator  25 , and a gear selector  44 , which moves among (P)ARK, (R)REVERSE, (N)EUTRAL, and (D)RIVE positions in an automatic mode channel  46  and between upshift (+) and downshift (−) positions in a manual mode channel  48 . The engine control module (ECM)  50  is powered by battery  26 , receives and sends signals to the starter  14  and engine  12  and receives input signals from an accelerator pedal  52  and brake pedal  54 . 
         [0017]      FIG. 1  shows the transmission  16  in the form of a powershift automatic transmission, in which the dual clutches  18 ,  20  produce a drive connection between the transmission&#39;s input  17  and layshafts  36 ,  37 . 
         [0018]      FIG. 2  illustrates an alternative in which the transmission is a multiple-speed, hydraulically actuated automatic transmission  60  having a torque converter  62 , which includes an impeller  64 , connected to the engine  12 ; a turbine  66 , hydrokinetically driven by the impeller; and a bypass clutch  68 , which alternately driveably connects the turbine to the impeller and releases that connection. Located within transmission  60  are friction control elements  70 ,  72 , i.e., clutches and brakes, whose state of coordinated engagement and disengagement produce forward drive and reverse drive. 
         [0019]    The accelerator pedal  52  and brake pedal  54  are controlled manually by depressing the respective pedal through a distance from a reference state, in which the pedal is not depressed. The accelerator pedal  52  provides input demand, i.e., drive demand torque, to the vehicle controller for changes in engine torque. Engine torque, transmitted through the transmission  16 ,  60  and final drive mechanism  24  to the wheels, powers the driven wheels  30 ,  31  with wheel torque. The accelerator pedal  54  provides demands to the vehicle controller for changes in wheel brake torque. Under certain conditions, the controller can actuate the brake system automatically to produce wheel brake torque that holds the vehicle stationary on a grade without actuating the brake pedal  54 . 
         [0020]    The vehicle controller, a microprocessor-based controller accessible to a control algorithm  76 , communicates through electronic signals transmitted on a communication bus with the engine  12 , starter  14 , transmission  16 ,  60 , gear selector  40 , accelerator and brake pedals  52 ,  54 , and a wheel brake system, which supplies brake pressure to the wheel brakes to produce the wheel brake torque that holds the vehicle stationary on a grade. The controller is accessible to data stored in electronic memory relating engine torque and accelerator pedal displacement, which indicated the magnitude of driver demand torque. 
         [0021]    As illustrated in  FIG. 3 , at step  82  of control algorithm  76  a test is made to determine whether vehicle speed (VS) is less than a reference speed, the gear selector  40  is in a forward drive position, and accelerator pedal  52  is displaced greater than a reference displacement, indicating that vehicle launch control is active. If the result of test  82  is logically false, control returns to  82 . 
         [0022]    If the result of test  82  is true, control advances to  84  where a test is made to determine whether the road grade is greater than a reference road grade, and whether brake torque is greater than a reference brake torque that will hold the vehicle stationary on the grade, indicating that hill start assist (HSA) control is active. When hill start assist (HSA) control is active, the vehicle controller actuates the brake system to produce wheel brake torque automatically at a wheel torque magnitude that holds the vehicle stationary on the road grade. 
         [0023]    If the result of test  84  is logically true, control advances to step  86  where, as illustrated in  FIG. 4 , current accelerator pedal displacement  88  is used to index a function  90  relating engine torque and accelerator pedal displacement, i.e., driver demand torque, to determine the desired engine output torque  92  while HSA is active. Function  90  may be a family of curves corresponding to the magnitude of the road grade, such that desired engine output torque  92  increases as road grade increases. 
         [0024]    If current accelerator pedal displacement  88  is greater than 94, where hill-start-assist brake release torque  96  is less than the engine torque that will produce wheel torque equal to the brake release torque, then brake torque is released and the vehicle is launched using engine torque alone. 
         [0025]    If the result of test  84  is false, indicating that hill start assist (HSA) control is inactive, control advances to step  98  where current accelerator pedal displacement  88  is used to index a function  100  to determine the desired engine output torque  102  while HSA is inactive. 
         [0026]    When the transmission includes at dual input clutches  18 ,  20  such as the powershift transmission  16  of  FIG. 1 , the torque transmitted by the active input clutch follows engine torque indirectly, thereby avoiding need for the controller to directly control clutch torque capacity. 
         [0027]    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.