Abstract:
A transmission shift is timed for a hybrid electric powertrain as a function of a torque capacity of an electric machine relative to a shifting torque required to change gearings of a transmission. A vehicle is being propelled by the machine, with an engine stopped, when the shift is requested. If the machine has insufficient torque capacity to change transmission gearings, then the shift request is delayed until the engine has started.

Description:
BACKGROUND OF INVENTION 
       [0001]    The present invention relates to a method of controlling a hybrid electric automotive powertrain and in particular to a method of timing transmission gearing shifts. 
         [0002]    A hybrid electric powertrain of an automotive vehicle may include both an internal combustion engine and an electric machine to provide propulsion. Commonly, while the vehicle is maintaining a constant cruising speed, the powertrain will stop the engine and use only the machine for propulsion. While at the machine only cruising speed, an acceleration request may be made. Meeting the acceleration request may require both restarting the engine and downshifting a transmission. 
         [0003]    However, the combination of restarting the engine and downshifting may result in a torque shortage that delays meeting the acceleration request. The torque shortage may reduce drivability by causing a driver of the vehicle to experience a sense of deceleration despite making the acceleration request. 
       SUMMARY OF INVENTION 
       [0004]    An embodiment contemplates a method of controlling a hybrid powertrain. A transmission shift request is received while an electric machine is propelling a vehicle and an engine is stopped. A torque capacity of the machine is determined. Completion of the shift request is timed as a function of the torque capacity relative to a shifting torque required to complete the shift request. 
         [0005]    Another embodiment contemplates a method of controlling a hybrid powertrain. A transmission shift request is received while an electric machine is propelling a vehicle and an engine is stopped. A shifting torque to complete the shift request is determined. A torque capacity of the machine is evaluated. The shift request is completed using the machine to change gearings in a transmission when the torque capacity exceeds the shifting torque. Completing the shift request is delayed while starting the engine when the shifting torque exceeds the torque capacity. The shift request is then completed using the machine and started engine to change gearings in the transmission. 
         [0006]    Another embodiment contemplates a method of controlling a hybrid powertrain. An acceleration request is received while an electric machine is propelling a vehicle and an engine is stopped. A determination is made that downshifting a transmission and starting the engine are needed to meet the acceleration request. A torque capacity of the machine and a shifting torque required to downshift the transmission by changing transmission gearing are determined. Downshifting the transmission is timed as a function of the torque capacity relative to the shifting torque. 
         [0007]    An advantage of an embodiment is that both the starting of the engine and completing of the shift request can be completed without a torque shortage. This improves driveability of the vehicle. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  is a schematic view of a hybrid electric powertrain. 
           [0009]      FIG. 2  is a flow chart of a control routine for a powertrain. 
           [0010]      FIGS. 3   a  and  3   b  are a flow chart of a control routine for a powertrain. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  schematically illustrates a hybrid electric powertrain  10  for an automotive vehicle  12 . This powertrain  10  is merely exemplary, and may take other forms, which may be front wheel drive, rear wheel drive, or all wheel drive types of powertrains. As described, the powertrain  10  is a parallel type hybrid electric powertrain but may also be another suitable powertrain known to one skilled in the art. 
         [0012]    The powertrain  10  includes an internal combustion engine  14  powering a crankshaft  16 . Interposed between the engine  14  and an electric machine  22 , which may be an electric motor or motor/generator, is an engine disconnect clutch  18 . When engaged, the clutch  18  connects the crankshaft  16  with an electric machine input  20  and transmits torque between the engine  14  and the machine  22 . In turn, the machine  22  transmits torque to a torque converter  26  through a torque converter input  24  and the torque converter  26  transmits torque to a transmission  30  through a transmission input  28 . The transmission  30  includes a plurality of gearings that are changeable to alter the input to output gear ratio, and hence rotational speed and torque output of the powertrain  10  by any suitable technique known to those skilled in the art. The transmission  30  turns a driveshaft  32  which in turn drives a differential  34 . The differential  34  transmits torque to first and second axles  36  and  38 , respectively, which drive first and second wheels  40  and  42 , respectively. A controller  44  controls operation of the powertrain  10 . 
         [0013]      FIG. 2  will now be discussed with reference to  FIG. 1 .  FIG. 2  illustrates a control routine  100  for the powertrain  10 . 
         [0014]    In a step  102 , the controller  44  receives a shift request for the transmission  30  while the vehicle  12  is being propelled by the machine  22  with the engine  14  stopped. The shift request is made to shift the transmission  30  from a present gear ratio to a new gear ratio. The controller  44 , in a step  104 , determines a shifting torque to complete the transmission shift request and, in a step  106 , a torque capacity of the machine  22 . The shifting torque is a torque required for the transmission  30  to shift from the present gear ratio to the new gear ratio. The torque capacity of the machine  22  may include a torque reserve for starting the engine  14 . 
         [0015]    In a step  108 , the controller  44  determines if the torque capacity is greater than the shifting torque. If the torque capacity is not greater than the shifting torque, then, in a step  110 , the controller  44  delays completing the shift request. In a step  112 , the controller  44  prepares the transmission  30  to complete the shift request. The shift request is prepared by reducing an off going clutch pressure for the present gearing to almost slipping and increasing an oncoming clutch pressure for the new gearing to just below a torque force in the transmission input  28 . While the shift request preparation is completed, the controller  44  starts the engine  14  in a step  114 . Once the engine is running in a step  116 , the controller  44  engages the engine disconnect clutch  18  in a step  118 . The shift request is then completed in a step  122 . 
         [0016]    If, in the step  108 , the torque capacity is greater than the shifting torque, then, in a step  120 , the controller  44  prepares the transmission  30  to complete the shift request. The shift request is then completed in the step  122 . 
         [0017]      FIGS. 3   a  and  3   b  will now be discussed with reference to  FIG. 1 .  FIGS. 3   a  and  3   b  illustrate a control routine  200  for the powertrain  10 . 
         [0018]    In a step  202 , the controller  44  receives an acceleration request for the transmission  30  while the vehicle  12  is being propelled by the machine  22  with the engine  14  stopped. In a step  204 , the controller  44  determines that starting the engine  14  is required to meet the acceleration request and in a step  206 , the controller  44  determines that downshifting the transmission from a higher gearing to a lower gearing is also required to meet the acceleration request. In a step  208 , the controller  44  determines a shifting torque to complete downshifting the transmission and, in a step  210 , a torque capacity of the machine  22 . The shifting torque is a torque required for the transmission  30  to change from the higher gear ratio to the lower gear ratio. The torque capacity of the machine  22  may include a torque reserve for starting the engine  14 . 
         [0019]    In a step  212 , the controller  44  determines if the torque capacity is greater than the shifting torque. If the torque capacity is not greater than the shifting torque, then, in a step  214 , the controller  44  delays completing the downshift. In a step  216 , the controller  44  prepares the transmission  30  to downshift while also, in a step  218 , starting the engine  14 . The transmission  30  is prepared to downshift similarly to how the transmission  30  is prepared to shift for the control routine  100 . Preparing to downshift the transmission  30  in step  216  may be simultaneous with starting the engine  14  in the step  218 . After the engine  14  is running in a step  220 , the clutch  18  is engaged to transmit torque in a step  222 , and in a step  224 , downshifting the transmission  30  is completed. Following downshifting the transmission  30 , the acceleration request is completed in a step  240 . 
         [0020]    If, in the step  212 , the torque capacity is greater than the shifting torque, then in a step  236  the controller  44  prepares the transmission  30  to downshift while, in a step  230 , the engine  14  is started. The transmission  30  is downshifted in a step  228 . Once the engine  14  is running in a step  232 , the clutch  18  is engaged to transmit torque in a step  234 . As illustrated, the steps  226  and  228  comprise a downshift subroutine  236  and the steps  230 ,  232 , and  234  comprise an engine start subroutine  238 . The downshift and engine start subroutines  236  and  238 , respectively, may occur simultaneously. Following downshifting the transmission  30  in the step  228  and engaging the clutch  18  in the step  234 , the acceleration request is completed in the step  240 . 
         [0021]    While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.