Abstract:
A method for controlling a hybrid electric powertrain includes, in response to a request to increase a powertrain braking force on at least one of a plurality of traction wheels, (i) commanding at least one clutch to increase a gear ratio of a transmission, and (ii) during clutch stroke, commanding an electric machine to act as a generator such that the electric machine applies a braking force to at least one of the traction wheels.

Description:
BACKGROUND 
       [0001]    U.S. Pat. No. 5,993,350 to Lawrie et al. provides a powertrain system for a hybrid vehicle. The hybrid vehicle includes a heat engine, such as a diesel engine, and an electric machine, which operates as both an electric motor and an alternator, to power the vehicle. The hybrid vehicle also includes a manual-style transmission configured to operate as an automatic transmission from the perspective of the driver. The engine and the electric machine drive an input shaft which in turn drives an output shaft of the transmission. In addition to driving the transmission, the electric machine regulates the speed of the input shaft in order to synchronize the input shaft during either an upshift or downshift of the transmission by either decreasing or increasing the speed of the input shaft. Operation of the transmission is controlled by a transmission controller which receives input signals and generates output signals to control shift and clutch motors to effect smooth launch, upshifts, and downshifts of the transmission. 
         [0002]    U.S. Pat. No. 6,019,699 to Hoshiya et al. provides a drive control system for a hybrid vehicle that prevents a delay in the application of a one-way clutch in a transmission. In this drive control system, an electric motor and an internal combustion engine are coupled to the input side of a transmission having at least one gear stage to be set by applying a one-way clutch. The drive control system comprises: a detector for detecting a coasting state in which the one-way clutch is released in a deceleration state set with the gear stage; and, an input speed raising device for driving the electric motor when the coasting state is detected, so that the input speed of the transmission may approach the synchronous speed which is the product of the gear ratio of the gear stage to be set by applying the one-way clutch and the output speed of the transmission. 
       SUMMARY 
       [0003]    A method for controlling a hybrid electric powertrain includes, in response to a request to increase a powertrain braking force on at least one of a plurality of traction wheels, (i) commanding at least one clutch to increase a gear ratio of a transmission, and (ii) during clutch stroke, commanding an electric machine to act as a generator such that the electric machine applies a braking force to at least one of the traction wheels. 
         [0004]    A hybrid electric vehicle includes a plurality of traction wheels, an engine, and a transmission mechanically connected with the engine and including at least one clutch to alter a gear ratio of the transmission. The vehicle also includes an electric machine mechanically connected with at least one of the traction wheels, and a controller. The controller is configured to, in response to a request to increase a powertrain braking force on at least one of the traction wheels, (i) command the at least one clutch to increase the gear ratio of the transmission and (ii) during clutch stroke, command the electric machine to act as a generator such that the electric machine applies a braking force to at least one of the traction wheels. 
         [0005]    A hybrid electric vehicle includes a plurality of traction wheels, an engine, and a transmission mechanically connected with the engine and including at least one clutch to alter a gear ratio of the transmission. The vehicle also includes an electric machine mechanically connected with at least one of the traction wheels, a power storage unit, and a controller. The controller is configured to, in response to a request to increase a braking force on at least one of the traction wheels, (i) command the electric machine to act as a generator such that the electric machine applies a braking force to at least one of the traction wheels until a state of charge of the power storage unit achieves a desired threshold, and (ii) command the at least one clutch to increase the gear ratio of the transmission after the state of charge of the power storage unit achieves the desired threshold. 
         [0006]    While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a block diagram of an example configuration of a hybrid electric vehicle. 
           [0008]      FIG. 2  is a block diagram of another example configuration of a hybrid electric vehicle. 
           [0009]      FIG. 3  is an example plot of transmission output torque generated in response to a request for a manual pull-in down shift. 
           [0010]      FIG. 4  is an example plot of electric machine torque generated in response to a request for a manual pull-in down shift. 
           [0011]      FIG. 5  is an example plot of net electric machine and transmission output torque generated in response to a request for a manual pull-in down shift. 
           [0012]      FIG. 6  is an example plot of on-coming transmission clutch pressure command generated in response to a request for a manual pull-in down shift. 
           [0013]      FIG. 7  is an example plot of off-going transmission clutch pressure command generated in response to a request for a manual pull-in down shift. 
           [0014]      FIG. 8  is an example plot of engine speed during a manual pull-in down shift. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    A driver of a hybrid electric vehicle may execute a manual pull-in downshift when, for example, travelling down a steep grade to achieve additional deceleration and minimize brake wear. The transmission may be downshifted into a lower gear via synchronous clutches or a coast clutch such that negative torque (braking torque) is transmitted to the driveline. 
         [0016]    A delay in achieving the desired negative driveline torque during a manual pull-in may occur in hybrid electric drivetrains (and other drivetrain configurations). This delay can be up to one second as measured from the driver command or PRNDL position movement until torque increases in the halfshafts. Delay may result from the need to stroke the oncoming transmission clutch. Delay may also result from the need to ensure that the engine does not exceed its speed limit if the transmission is downshifted. The drivetrain may wait until the vehicle speed is reduced so that when the transmission is downshifted, the engine speed will not exceed its limit. 
         [0017]    Certain embodiments disclosed herein may reduce/eliminate delays in achieving a desired negative driveline torque after the initiation of a request for a manual pull-in. As an example, an electric machine may be requested to provide negative driveline torque while a mechanical driveline is requested to perform a manual pull-in (e.g., stroke the oncoming clutch, bring the engine up to synchronous speed and transfer torque to the new ratio), provided the engine speed is less than a desired threshold for the desired gear. If the engine speed is greater than the desired threshold, the request to shift may be delayed until the engine speed is less than the desired threshold. Once the new gear is available, electric torque may be reduced as the mechanical torque is increased to provide generally consistent vehicle deceleration. 
         [0018]    As another example, the electric machine may be requested to provide negative driveline torque (possibly while the mechanical driveline remains off) until an associated battery reaches a desired state of charge. (As apparent to those of ordinary skill, the electric machine acts as a generator while providing negative driveline torque. Electrical energy generated by the electric machine may be stored in the battery.) The mechanical driveline may then be requested to perform a manual pull-in, and electric torque reduced and mechanical torque increased as described herein. 
         [0019]    Referring now to  FIG. 1 , an automotive vehicle  10  may include a drivetrain  12 . The drivetrain  12  may include tire/wheel assemblies  14   n  ( 14   a ,  14   b ,  14   c ,  14   d ), an engine  16 , electric machine  18  (e.g., electric rear axle drive), and power storage unit  19  (e.g., battery). The drivetrain  12  may also include a crank integrated starter/generator (CISG or other electric machine)  20 , transmission  22 , front differential  24 , and front half shafts  26 . As apparent to those of ordinary skill, components immediately adjacent to each other are mechanically connected. The drivetrain  12  may further include a rear differential  28 , rear half shafts  30 , and a rear prop shaft  32 . 
         [0020]    The transmission  22  may include an input  34  mechanically connected with the engine  16 , an output  36  mechanically connected with the tire/wheel assemblies  14   a ,  14   b  via the front differential  24 , one or more gears  38 , and one or more clutches  40  arranged in a known fashion. 
         [0021]    As known in the art, the CISG  20  may be used to start or stop the engine  16 ; the engine may generate motive power to drive the tire/wheel assemblies  14   a ,  14   b  via the transmission  22 , front differential  24 , and front half shafts  26 . As also known in the art, the electric machine  18  may act as a motor to generate motive power to drive the tire wheel assemblies  14   c ,  14   d  via the rear prop shaft  32 , rear differential  28 , and rear half shafts  30 ; the electric machine  18  may also act as a generator to generate electrical power for storage by the power storage unit  19 . Either or both of the engine  16  and electric machine  18  may be used to generate motive power to drive the tire/wheel assemblies  14   n.    
         [0022]    One or more controllers  42  may be in communication with the electric machine and/or transmission  22 . The controllers  42  may submit torque commands/requests to the electric machine  18  such that, for example, the electric machine consumes electrical power to generate a propulsion force for the tire/wheel assemblies  14   c ,  14   d , or consumes mechanical power to generate a braking force (negative torque) for the tire/wheel assemblies  14   c ,  14   d . The controllers  42  may submit commands/requests to the transmission  22  such that, for example, a speed ratio of the transmission  22  (e.g., the ratio of the speed of the input  34  to the speed of the output  36 ) changes via application of the clutches  40  to the gears  38  in a known fashion. As discussed below, these commands may be coordinated to provide negative driveline torque in response to a request for a manual pull-in downshift with little or no delay. 
         [0023]    Referring now to  FIG. 2 , numbered elements that differ by 100 relative to  FIG. 1  have similar descriptions to the numbered elements of  FIG. 1 . The drivetrain  112  of  FIG. 2  includes a power transfer unit  136 , front prop shaft  138 , and a coupling  140 . As known in the art, these additional components may (i) permit the engine  116  to drive any of the tire/wheel assemblies  114   n  and (ii) permit the electric machine  118  to drive any of the tire/wheel assemblies  114   n . Of course, other drivetrain configurations are also possible. 
         [0024]    Referring now to  FIGS. 3 through 8 , the operation of an engine, electric machine, transmission clutches, and controllers (such as the engines  16 ,  116 , electric machines  18 ,  118 , clutches  40 ,  140 , and controllers  42 ,  142  illustrated in  FIGS. 1 and 2 ) are described with reference to several operating modes that occur in response to a request for a manual pull-in downshift. While there are five such operating modes in the embodiments of  FIGS. 3 through 8 , any suitable number of operating modes may be used. 
         [0025]      FIG. 3  depicts conventional transmission output torque to a driveline during a manual pull-in downshift. That is, transmission output torque first overshoots (after some delay) and then undershoots its final target value. With the addition of offsetting electric machine torque to the driveline as depicted in  FIG. 4 , the net torque output of the electric machine and transmission to the driveline as depicted in  FIG. 5  has reduced overshoot and undershoot, as well as reduced delay. 
         [0026]    Mode 1: The strategy enters Mode 1 at the initiation of a manual pull-in downshift request. A controller may command an electric machine to provide negative torque (i.e., act as a generator). This torque may continue to ramp to a calibrateable value of maximum torque, which may be a function of vehicle speed. 
         [0027]    The strategy may exit Mode 1 after the controller receives notification that a transmission is ready to downshift (increase its gear ratio) via, for example, a shift ready flag or any other known technique. If the engine speed is such that it will not exceed its limit when downshifted, this may occur immediately. If the engine speed is such that it will exceed its limit when downshifted, the strategy may wait until the engine speed decreases to a suitable value before the shift ready flag is set. In other embodiments, the shift ready flag may be set when a state of charge of a power storage unit achieves a threshold value (assuming engine speed, if the engine is on, is such that it will not exceed its limit when downshifted). 
         [0028]    Mode 2: The electric machine torque command initiated in Mode 1 may continue (e.g., ramp until a calibrateable value is achieved, and then hold), if it has not already achieved the calibrateable value during Mode 1. The controller may command an on-coming transmission clutch pressure to a high value to fill the clutch then cut back to a calibrateable value needed to start the shift as known in the art. The controller may command an off-going transmission clutch pressure to a reduced calibrateable value as also known in the art. 
         [0029]    The strategy may exit Mode 2 at the expiration of a timer, detection of the torque phase, and/or detection of the shift start in a known fashion. 
         [0030]    Mode 3: The controller commands the on-coming transmission clutch pressure to increase and the off-going transmission clutch pressure to decrease in a coordinated manner as known in the art. The controller holds the electric machine torque at its current commanded value until a drop in engine speed (which corresponds to a peak in transmission output torque) is detected. (As known in the art, the described coordinated activity of the on-coming and off-going clutches causes a dip in engine speed if this coordination is biased towards a flare condition. If this coordination is biased towards a tie-up condition, the engine speed will, of course, rise and the transmission output torque will become more negative.) The controller may then command the electric machine to provide positive torque (i.e., act as a motor). (Alternatively, this command may be initiated after the strategy exits Mode 3.) This torque may continue to ramp to a calibrateable value of maximum torque, which may be a function of vehicle speed 
         [0031]    The strategy may exit Mode 3 when a speed ratio of the transmission has achieved a desired value, e.g., 5% of the final value. 
         [0032]    Mode 4: The controller may control the on-coming clutch through, for example, an open or closed loop profile, and command the off-going clutch to a pressure below its stroke pressure. The controller may command the electric machine back to, for example, zero torque (or other target value) as a function of percent shift complete. Commanding the electric machine torque to offset the inertia torque of the input to the transmission may provide a smoother shift. Keeping the electric machine torque at zero (or negative torque) may provide an elevated negative torque feel that may be desired when a manual pull-in shift is requested. Thus, this feel may be calibrated based on the particular vehicle application, and may also be calibrated for each shift type. For example, if the shift occurs immediately after the request, the driver may desire the extra inertia torque feel. If the shift occurs after several seconds to obtain better brake regeneration, the driver may not desire any torque feel as it would be delayed from the shift request. 
         [0033]    The strategy may exit Mode 4 when the speed ratio of the transmission has achieved a desired value, e.g., 90% of the final value. 
         [0034]    Mode 5: This is the end mode and provides the completion of the shift event. As known in the art, the controller may command the on-coming transmission clutch pressure to a maximum and the off-going transmission clutch pressure to a minimum, etc. 
         [0035]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.