Abstract:
A method for controlling a vehicle powertrain during launch includes controlling slip across a first clutch that transmits engine torque through the first clutch and the current gear while a transmission operates in a current gear other than a launch gear, disengaging the first clutch, engaging the launch gear, and controlling slip across a second clutch that transmits engine torque through the second clutch and the launch gear.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for controlling a vehicle powertrain during a vehicle launch event using an engine and a stepped ratio transmission. 
     2. Description of the Prior Art 
     A powershift transmission is a geared mechanism having two input clutches, which alternately connect a power source, such as an engine to two transmission input shafts. 
     The transmission produces multiple gear ratios in forward drive and reverse drive though operation of 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 principally with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated principally 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 gearing for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the gearing for operation in the next gear. 
     Because a dual clutch transmission has no torque converter to provide damping, such transmissions begin and end each gear shift with the holding clutch, i.e., the input clutch through which engine torque is transmitted to the transmission input shaft for the target gear, slipping. To provide acceptable shift quality, it is necessary to maintain the correct slip without excess flare and without locking the holding clutch. At the same time, the input clutches must maintain sufficient torque at the output shaft to provide consistent acceleration before, during and after the shift. 
     The magnitude slip across the input clutches must be closely controlled. 
     A vehicle equipped with a dual clutch powershift transmission can be stopped but the gear in which the transmission is operating may not be its launch gear, i.e., first gear. When the driver applies the accelerator pedal to launch the vehicle, an unresponsive undesirable performance feel can be produced. 
     SUMMARY OF THE INVENTION 
     A method for controlling a vehicle powertrain during a vehicle launch event includes controlling slip across a first clutch that transmits engine torque through the first clutch and the current gear while a transmission operates in a current gear other than a launch gear, disengaging the first clutch, engaging the launch gear, and controlling slip across a second clutch that transmits engine torque through the second clutch and the launch gear. 
     The control method combines launching with gear shifting to produce a smooth launch event. 
     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 showing an automotive vehicle powertrain; 
         FIG. 2  is a logic flow diagram showing an algorithm for controlling a normal gear shift and launch gear shift from a current gear of the transmission of  FIG. 1 ; and 
         FIG. 3  shows the variation of engine speed and vehicle speed during a launch shift and normal shift. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIG. 1 , the powertrain  10  for a motor vehicle includes a first power source  12 , such as an internal combustion engine; an automatic transmission  14  producing multiple forward and reverse gear ratios; a transmission input shaft  18  connected to the engine shaft; a transmission output  24 , connected through a final drive unit and differential mechanism  26  to the front axles  28 ,  30 ; and front wheels  32 ,  33 , respectively driven by the axles shafts  28 ,  30 . In some drive configurations the rear wheels  34 ,  35  may be driven through a differential mechanism  36  and rear axles  22 ,  23 . 
     To produce forward or reverse drive, a forward or reverse gear of the transmission must be engaged between input  18  and output  24 . The input clutch  38 ,  39  that is associated with the engaged gear must be engaged to complete a drive path between engine  12 , through input shaft  18  and transmission  14  to the vehicle wheels  32 ,  33 . 
     An electronic engine control module (ECM)  24  controls operation of engine  12 . An electronic transmission control module (TCM)  27  controls operation of transmission  14  and the input clutches  38 ,  39 . 
     A sensor  40  produces a signal  42 , transmitted to ECM  24  and TCM  27 , representing the extent to which an accelerator pedal  44  is depressed from a reference position. A speed sensor  46  produces a signal  48 , transmitted to ECM  24  and TCM  27 , representing vehicle speed. 
     Generally launch gear is low gear, i.e., the forward gear having the greatest speed ratio produced by transmission  14 . 
     When vehicle speed is less than a reference speed, accelerator pedal  44  is depressed, and the launch gear of transmission  14  is not engaged, TCM  27  produces controlled slip in the input clutch  38 ,  39  through which torque is transmitted in the current gear to the driven wheels  32 ,  33 . This controlled slip allows engine speed to significantly increase beyond the launch gear speed, i.e., the engine speed that would accelerate the vehicle from a stop and produce an ideal vehicle launch that is smooth and without excessive noise, vibration or harshness with the transmission operating in the launch gear. After the launch gear is engaged, a shift from the current gear to the launch gear is produced with significant but controlled slip for the launch gear to launch the vehicle. 
       FIG. 2  shows an algorithm for controlling the shift to the launch gear. At step  50  a test is performed to determine whether the accelerator pedal is depressed. If the result of test  50  is true, at step  52  a test is performed to determine whether vehicle speed is less than a reference speed. 
     If production of the current gear and launch gear requires engagement of the same input clutch  38 ,  39 , the transmission produces a shift from the current gear to another current gear, which requires engagement of the other input clutch  38 ,  39  than the clutch required to be engaged when the transmission produces launch gear. The off-going clutch is the clutch  38 ,  39 , whose engagement is required for the transmission to operate in and produce the current gear. The oncoming clutch is the clutch  38 ,  39 , whose engagement is required for the transmission to operate in and produce the launch gear. 
     If the result of test  52  is true, at step  54  closed loop control of slip across the off-going clutch is performed on the basis of the rate of change of engine speed or as a function of accelerator pedal displacement from its reference position. At step  54  TCM  27  issues command signals, which control slip in the off-going input clutch  38 ,  39 , through which engine torque is transmitted in the current gear to the driven wheels  32 ,  33 . 
     At step  56  a test is performed to determine whether engine speed is within a reference range of launch speed. 
     If the result of test  56  is true, at step  58  the current gear is disengaged, the off going clutch is released, and a test is performed to determine whether the launch gear is engaged and whether the associated input clutch  38 ,  39  of transmission  14  through which torque will be transmitted in the launch current gear to the driven wheels  32 ,  33  is prepared for the vehicle launch. 
     If the result of either test  56  or  58  is false, control returns to step  54 . 
     If the result of test  58  is true, at step  60  closed loop control of slip across the oncoming clutch  38 ,  39  is performed as a function of engine speed. At step  60  TCM  27  issues command signals, which control slip across the oncoming input clutch  38 ,  39 , through which engine torque will be transmitted in the launch gear to the driven wheels  32 ,  33 . 
     At step  62  the ECM  24  and TCM  27  cooperate in launching the vehicle using engine  12  and transmission  14 . 
     If the result of either test  50  or  52  is false, indicating that either the accelerator pedal is not depressed or the vehicle speed is greater than the reference speed, normal shifting is executed under control of the ECM  24  and TCM  27 . 
     At step  64 , TCM  27  issues command signals, which control slip in the off-going input clutch  38 ,  39 , through which torque is transmitted in the current gear to the driven wheels  32 ,  33 . 
     At step  66 , a test is performed to determine whether the launch gear is engaged and whether the associated input clutch  38 ,  39  of transmission  14  through which torque will be transmitted in the launch current gear to the driven wheels  32 ,  33  is prepared for a gear shift. 
     If the result of test  66  is false, control returns to step  64 . 
     At step  68 , TCM  27  issues command signals, which control slip in the oncoming input clutch  38 ,  39 , through which torque will be transmitted in the oncoming gear to the driven wheels  32 ,  33 . 
     At step  70 , the oncoming input clutch  38 ,  39  is locked or fully engaged. 
       FIG. 3  shows the variation of engine speed  80  and vehicle speed  82  during a launch shift, and engine speed  84  and vehicle speed  86  during a normal shift. Each shift starts at  90 . The normal shift ends at  92  and the launch shift ends at  94 . 
     For the launch shift, closed loop control  54  of slip in the off-going input clutch  38 ,  39  occurs during period  96 , closed loop control  60  of slip in the oncoming input clutch  38 ,  39  occurs during period  98 , and launch control  62  occurs during period  102 . 
     For the normal shift, closed loop control  64  of slip in the off-going input clutch  38 ,  39  occurs during period  96 , and closed loop control  68  of slip in the oncoming input clutch  38 ,  39  occurs during period  100 . 
     For the launch shift, engine speed  80  increases rapidly during closed loop control  54  of slip in the off-going input clutch  38 ,  39 , and remains substantially constant to the end of the shift  94  during closed loop control  60  of slip in the oncoming input clutch and during launch control  62 . 
     For the normal shift, engine speed  84  increases at a lower rate during closed loop control  64  of slip in the off-going input clutch  38 ,  39 , and during closed loop control  68  of slip in the oncoming input clutch to the end of the shift  92 . Thereafter, engine speed increases more rapidly. 
     For the launch shift, vehicle speed  82  increases rapidly during closed loop control  54  of slip in the off-going input clutch, during closed loop control  60  of slip in the oncoming input clutch and during launch control  62 . 
     For the normal shift, vehicle speed  86  increases less rapidly during closed loop control  64  of slip in the off-going input clutch, and during closed loop control  68  of slip in the oncoming input clutch to the end of the shift  92 . Thereafter, engine speed increases at a higher rate. 
     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.