Abstract:
A controller and a control strategy minimizes shift shock in a hybrid electric vehicle during a downshift conducted while the vehicle is in a regenerative braking mode by maintaining total powertrain torque at a desired target during the downshift. The controller has three preferable modes including modulating just engine torque, modulating just electric motor torque or simultaneously modulating both motor and engine torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention pertains to the art of hybrid vehicle powertrains and, more specifically, modulating torque in a hybrid vehicle powertrain during a ratio change of the transmission that occurs during regenerative braking. 
         [0003]    2. Discussion of the Prior Art 
         [0004]    A hybrid vehicle powertrain typically includes an electric motor, such as a high voltage induction motor, wherein driving torque of an engine is supplemented with electric motor torque produced by the electric motor. The combined engine torque and electric motor torque is transferred to vehicle traction wheels through a multiple ratio power transmission mechanism. A wet clutch assembly may be included in the power flow path between a torque input element of the multiple-ratio power transmission mechanism and a crankshaft of the engine. An example of a hybrid electric vehicle powertrain of this type is disclosed in U.S. Pat. No. 6,585,066, which is assigned to the assignee of the present invention. 
         [0005]    Attempts have been made to reduce power losses normally associated with torque converter automatic transmissions by adding an electric motor. A powertrain configuration of this type combines the performance of an internal combustion engine with the advantages of an electric motor that complements the speed and torque characteristics of the engine. The hybrid arrangement also permits the engine to be deactivated when the vehicle is at rest or disconnected from the power flow path of the powertrain as the electric motor supplies driving torque. Such a hybrid arrangement improves fuel economy while reducing undesirable exhaust gas emissions. 
         [0006]    During a process commonly referred to as regenerative braking in a hybrid powertrain of this type, charging a high voltage battery during vehicle deceleration collects the kinetic energy stored in the moving vehicle. During regenerative braking, required braking torque is allocated between a set of friction brakes and the electric motor, which acts as a generator. The amount of braking torque required as the vehicle decelerates is apportioned in real time by a control system between the hydraulic, mechanical friction braking hardware and the electric powertrain regenerative braking. The apportionment of wheel braking torque between friction and regenerative braking is balanced through the deceleration process to achieve as much regeneration as possible to improve fuel economy. 
         [0007]    In the case of a coasting downshift for a hybrid electric vehicle, the regenerative braking function coincides with the coast mode. In some hybrid electric vehicles, since the motor is located between the engine and the transmission, the coast downshift is done with a significant level of negative torque at the input to the transmission. This negative regenerative braking input torque is sometimes much higher than the negative input torque typically experienced in conventional powertrain vehicles with step ratio transmissions during coasting or braking deceleration. This operating condition differs from operating conditions found in conventional powertrains, where coasting downshifts are done with only a slight negative or positive torque at the transmission input. The negative torque in the hybrid powertrain will cause shift shock in a manner similar to that found in a power-on upshift in a conventional powertrain. For example, during a power-on upshift, the conventional transmission remains initially in the upshifted torque ratio and a torque ratio change takes place before speed ratio change. During the speed ratio change, there is no significant change in wheel torque. The length of the shift depends on the amount of torque that the engine is producing and the amount of the effective inertia mass connected to the engine which is felt by the driver as a shock. Shift quality may be improved by controlling transmission input torque such as by reducing transmission input torque during a power-on upshift by retarding the engine spark to reduce engine output torque. This improves both the durability of the on-coming friction element and the smoothness of the upshift event. Torque modulation using spark retardation will satisfy the timing and repeatability requirements to satisfy shift quality targets, but this wastes some energy during the shift, which can only reduce torque, not increase it. Torque modulation also can be accomplished by using a fuel cut-off to reduce engine torque, but restoring engine torque following a shift event often is not repeatable using fuel control. In a conventional powertrain using a hydrokinetic torque converter, a coast mode occurs whenever the accelerator pedal is off, both with and without braking. As the vehicle slows, a coasting downshift must be executed to keep the engine speed within the desired range. In the case of downshifts during regenerative braking, drivability problems result if shift shock is not addressed. 
         [0008]    Prior solutions to this problem have addressed the idea of removing regenerative torque during shift events by switching from regeneration to friction braking and back again. However, such methods have the problem that the transfer to friction braking leads or lags the duration of the shift event and such solutions tend to require overly complicated control systems. Therefore there exists a need in the art for a system that can maintain good shift quality when performing a downshift between gear ratios during regenerative braking in a hybrid vehicle. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to a controller and a control strategy for reducing shift shock in a hybrid electric vehicle powertrain during a downshift conducted while the vehicle is in a regenerative braking mode by maintaining total powertrain torque constant at a desired target during the downshift. The hybrid electric vehicle has an engine, an electric motor, a battery connected to the electric motor, a set of drive wheels, an automatic transmission including multiple gear ratios for receiving a first input torque generated by the engine and delivering a total powertrain output torque to the set of drive wheels, and a controller configured to effect a downshift between gear ratios during regenerative braking while maintaining the output torque at a constant value. 
         [0010]    Although shifting during regenerative braking tends to cause variations in the total powertrain output torque that is felt as a shift shock, the controller reduces the shift shock by sending control signals to the powertrain to control the amount of total powertrain output torque delivered from the transmission to the set of drive wheels. Specifically, the controller regulates a downshift between gear ratios during regenerative braking while maintaining the output torque constant during the downshift. The controller also provides control signals to the engine and motor for generating a first input torque with the engine and generating a second input torque with the motor. The controller preferably takes further measures to reduce shift shock, such as maintaining the total powertrain output torque at a constant level by modulating the first input torque or second input torque and varying a friction braking force applied to drive wheels of the vehicle through friction brakes to counteract variations which tend to be caused by the regenerative braking in the transmission during the downshift. The controller has three preferable modes including: modulating just engine torque; modulating just electric motor torque; or simultaneously modulating both electric motor and engine torque. Preferably, the controller removes the regenerative braking before the downshift and reinstates the regenerative braking after the downshift. In another embodiment, the controller regulates the amount of regeneration braking during both torque and inertia phases of the downshift. In particular, the controller also decreases an amount of friction braking during a torque phase of the downshift and restores the amount of friction braking during the inertia phase. The controller determines timing for applying friction braking by predicting a lag time associated with the friction braking and compensating for the lag time. 
         [0011]    Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of preferred embodiments when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic diagram of a hybrid vehicle powertrain including an internal combustion engine, an electric motor, a friction braking system and a multiple-ratio automatic transmission; 
           [0013]      FIG. 2  is a graph of a downshift event in the powertrain of  FIG. 1  from a high gear configuration to a low gear configuration with inertia effects compensated for by modulating torque from the electric motor; 
           [0014]      FIG. 3  is a graph of a downshift event in the powertrain of  FIG. 1  from a high gear configuration to a low gear configuration with inertia effects compensated for by modulating torque from the engine; 
           [0015]      FIG. 4  is a graph of a downshift event in the powertrain of  FIG. 1  from a high gear configuration to a low gear configuration with inertia effects compensated for by modulating torque from the electric motor and torque from the engine; and 
           [0016]      FIG. 5  is a graph of a downshift event in the powertrain of  FIG. 1  from a high gear configuration to a low gear configuration with inertia effects compensated for by modulating torque from the electric motor and extending the time required for the shift. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    With initial reference to  FIG. 1 , there is schematically shown a hybrid electric vehicle powertrain system  10  for a hybrid electric vehicle. As depicted, an internal combustion engine  20  has an output shaft  22  connected to an input shaft  24  of an electric motor  30  through an engine clutch  32 . Internal combustion engine  20  is also connected to a starter motor  34  used to start engine  20 . Starter motor  34  is also connected to a battery  36  through wiring  38  so as to be also used as a generator to produce electric energy that is stored in a battery  36 . Electric motor  30  has an output shaft  42  that is connected to an input shaft  44  of an automatic transmission  50  through a motor clutch  52 . Electric motor  30  is also linked to battery  36  through a connecting wiring  53 . Transmission  50  includes multiple gear ratios and is connected to a drive shaft  54  that, in-turn, is connected to a differential  56 . Left and right drive wheels  60 ,  62  are connected to differential  56  through left and right axles  64 ,  66 . With this arrangement, multiple gear ratio automatic transmission  50  transmits a powertrain output torque  68 , indicated at  68 , to drive wheels  60 ,  62 , while drive wheels  60 ,  62  are provided with friction brakes  70  for applying a braking force to slow the hybrid electric vehicle. 
         [0018]    Engine  20  is preferably an internal combustion engine, such as a gasoline or diesel powered engine, and is a primary source of power for powertrain system  10 . As noted above, when running, engine  20  can provide power to starter motor  34  so that motor  34  will generate electric energy for storage in battery  36 . Engine  20  also provides power through engine clutch  32  to electric motor  30  so that electric motor  30  can act as a generator and produce electric energy for storage in battery  36 . More specifically, engine  20  generates a first input torque  72  that is supplied to electric motor  30 . To drive the vehicle with engine  20 , at least a portion of first input torque  72  passes through motor  30  to multiple ratio transmission  50  through motor clutch  52 . Depending on the particular operating mode of the hybrid electric vehicle as will be detailed further below, electric motor  30  will either send power to battery  36  or convert electric energy stored in battery  36  into a second input torque  74  that is also sent to multiple ratio transmission  50 . When generating electrical power for storage in battery  36 , electric motor  30  obtains power either from internal combustion engine  20  in a driving mode or from the inertia in the hybrid electric vehicle as motor  30  acts as a brake in what is commonly referred to as a regenerative braking mode. Depending on whether engine clutch  32  and motor clutch  52  are engaged or disengaged determines which input torque(s)  72 ,  74  is transferred to transmission  50 . For example, if engine clutch  32  is disengaged, only second torque  74  is supplied from motor  30 . However, if both clutches  32 ,  52  are engaged, then first and second input torques  72 ,  74  are supplied by both engine  20  and motor  30 . Of course, if drive torque is only desired from engine  20 , both clutches  32  and  52  are engaged, but motor  30  is not energized, such that first input torque  72  is only supplied by engine  20 . 
         [0019]    Automatic transmission  50  preferably includes several planetary gearsets (not shown) that are selectively placed in different gear ratios by selective engagement of a plurality of friction elements  72 - 74  in order to establish the desired multiple drive ratios. For instance, friction elements  72 - 74  can be constituted by an oncoming friction element  72  and an offgoing friction element  73  and a forward clutch  74 . Basically, transmission  50  is automatically shifted from one ratio to another based on the needs of the hybrid electric vehicle. Transmission  50  then provides powertrain output torque  68  to transmission output shaft  54  connected to differential  56  that ultimately drives wheels  60 ,  62 . The kinetic details of transmission  50  are not important to the present invention and can be established by a wide range of known transmission arrangements, such as the transmission found in U.S. Pat. No. 7,223,201, which is specifically incorporated herein by reference. Other examples of transmissions that can be employed with the invention are found in U.S. Pat. No. 7,128,677, which is also incorporated herein by reference. While these transmission arrangements are presented as examples, any multiple ratio transmission that accepts torque input from an internal combustion engine and an electric motor and then provides torque to an output shaft at the different ratios is acceptable. 
         [0020]    System  10  also includes powertrain control unit  80  and a brake control unit  85  collectively constituting a vehicle controller. Based on repositioning a brake pedal  92 , a driver provides a total braking torque requirement signal  94  when the driver wishes to slow the hybrid electrical vehicle. The more the driver depresses pedal  92 , the more braking torque is requested. Brake control unit  85  functions to apportion the total braking torque between a powertrain braking torque signal  95 , representing the amount of torque to be obtained by regenerative braking, and friction braking torque signal  96 , representing the amount of torque to be obtained through friction brakes  70 . In response, powertrain control unit  80  sends a motor torque signal  98  to electric motor  30  representing the requisite amount of torque to be provided by regenerative braking. Powertrain control unit  60  also receives torque ratio signals  99  from transmission  50  regarding shifting from one speed ratio to another, such as during a gear shift as discussed in more detail below with reference to  FIGS. 2-5 , and sends an engine torque signal  100  to engine  20  indicating how much engine torque is required at a given time. A powertrain torque signal  101 , representing an amount of total powertrain torque  68 , is also sent to control unit  80  during the gear shift. 
         [0021]    Turning now to  FIGS. 2-5  there are shown four different embodiments of the invention setting forth strategies for reducing shift shock in a downshift during regenerative braking which can be employed individually or in combination in accordance with the invention. More specifically,  FIG. 2  illustrates an embodiment wherein electric motor torque is varied;  FIG. 3  presents an embodiment wherein internal combustion engine torque is varied;  FIG. 4  shows an embodiment varying both motor torque and engine torque; and finally  FIG. 5  presents an embodiment wherein the length of the shift is extended and motor torque is varied. In each figure, an exemplary downshift from a 3 rd  gear to a 2 nd  gear is presented, with the X-axis representing time and the Y-axis representing either torque, torque ratio or speed depending on the particular curve of interest. 
         [0022]    With specific reference to  FIG. 2 , a shift  103  is shown having five basic phases. A first or boost phase  104  is where on-coming friction element  72  is boosted to fill its friction element actuator quickly while off-going friction element  73  has its pressure set to a value just sufficient to hold input torque. In a second or start phase  105 , off-going element  73  continues to hold transmission  50  in its current gear ratio while oncoming element  72  is still stroking. In a third or torque phase  106 , off-going element  73  begins releasing, reducing its torque capacity, and oncoming element  72  continues increasing its torque capacity which results in the input torque transferring from the off-going element  73  to the oncoming element  72 . Once a predetermined percentage of a coasting downshift speed change is completed, torque phase  106  is complete. In a fourth phase  108 , oncoming element  72  continues to control the transmission input speed up to the new speed ratio. In the fourth phase, which is an inertia phase  108 , transmission  50 , under the effects of regenerative braking, will tend to vary powertrain torque  68  as the shift completes. Inertia phase  108  is exited when the shift is nearly complete. In an end phase  110 , the oncoming pressure command is increased to a maximum command at the completion of the shift. 
         [0023]      FIG. 2  further shows a plot of speed of the input shaft in RPM  120 , torque ratio  125 , motor torque  130 , required powertrain torque  135 , actual powertrain torque  140 , friction brake torque  145  and wheel torque  150 , which are all plotted as a function of time during the shift. As shown, speed  120  decreases during boost  104 , start  105  and part of torque phase  106 . During inertia phase  108 , an increase  170  occurs as the gear ratios are shifted from a higher gear ratio  171  to a lower gear ratio  172 , causing an inertia effect. Torque ratio  125  changes as shown by ramp  160  in torque phase  106 . Motor torque  130  has a ramp  162  corresponding to ramp  106  in torque phase  106 . In a conventional control system, no modulation of motor torque  130  would occur following ramp  162 , thereby yielding a constant torque represented by dotted line  173 . An inertia effect from ratio speed increase  170  will, if not compensated for, show up as a dip  174  in powertrain torque  140 , as well as a dip  176  in wheel torque  150  which, in turn, is felt as a shift shock. However, when motor torque  130  is modulated in accordance with the invention and represented by bump  180 , then powertrain torque  140  can be maintained constant as shown at  184 , resulting in wheel torque  150  also being constant as shown at  186 . Therefore, in accordance with one aspect of the invention, the torque signal  98  applied to electric motor  30  is regulated to modulate motor torque  130  such that powertrain torque  140  can be maintained constant, thereby avoiding shift shock during a downshift with regenerative braking. 
         [0024]    In accordance with another aspect of the invention, the shift shock can be avoided in a downshift during regenerative braking by controlling engine torque as represented in  FIG. 3 . Initially, it will be recognized that  FIG. 3  is similar to  FIG. 2  such that only the differences will be discussed. Instead of using motor torque  130  to compensate for the inertia effects, a shift  200  is performed with modulated engine torque  230  such that, after increasing torque at ramp  262  to compensate for ratio change  160 , an extra amount of torque shown at  280  is provided to hold powertrain torque  140  constant as shown at  284  resulting in wheel torque  150  being held constant as shown at  286 . Shift  300  shown in  FIG. 4  is also similar to shift  103  of  FIG. 2  and, once again, only the differences will be discussed. Shift  300  uses both motor torque  330  and engine torque  331  to compensate for shift shock. Here, motor torque  330  is modulated such that, after increasing torque at ramp  362  to compensate for ratio change  160 , an extra amount of torque shown at  380  is provided. Engine torque  331  is also modulated such that after increasing torque at ramp  363  to compensate for ratio change  160 , an extra amount of torque shown at  381  is provided. The additional amounts of torque  380  and  381  act in combination to hold powertrain torque  140  constant as shown at  384 , resulting in wheel torque  150  being held constant as shown at  386 . Extra amounts of torque  380 ,  381  not have to have the same profile as shown. Therefore, the sum of torques  380 ,  381  are used to compensate for the inertial effects caused during shift  300  so that wheel torque  150  is maintained constant. 
         [0025]    In  FIG. 5 , a shift  400  is shown that compensates for inertial effects by both modulating motor torque  430  and slowing shifting. Specifically, both the inertia phase  108  and the end phase  110  are lengthened and motor torque  430  is modulated much earlier. That is, motor torque  430  rises in start phase  104  with ramp  431  and is held relatively high through torque phase  106  as shown at  432 . Further, modulation is shown at  434  and  436  during inertia phase  108  and torque  430  drops in end phase  110  as shown at  438 . Motor torque  430  causes powertrain torque  440  and required powertrain torque  445  to significantly rise in start phase  105  and considerably drop in end phase  110 , but stay constant through shift  400  in both torque phase  106  and inertia phase  108 . Friction brake torque  145  is also modulated as shown at  450  so that wheel torque  150  stays constant through all phases of shift  400 . 
         [0026]    Based on the above, it should be readily apparent that the present invention sets forth various ways in which torque can be modulated for a downshift during regenerative braking in a hybrid vehicle such that shift shock is prevented or at least significantly minimized. In particular, input torques, friction braking and/or downshift timing is regulated to maintain output torque substantially constant. In connection with the invention, substantially constant at least requires no significant ramping or spiking of the output torque such that the output torque is, for all practical purposes, held constant through the downshift in order to substantially minimize or prevent shift shock during downshifting in a hybrid vehicle. In connection with the downshift, it should be understood that the torque and inertia phases of the downshift which are important in connection with maintaining the output torque substantially constant for controlling shift shock. In any case, although described with reference to preferred embodiments of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. In general, the invention is only intended to be limited by the scope of the following claims.