Abstract:
In a powertrain for motor vehicle that includes an engine, an electric machine, a transmission having an input driveably connected to the engine and a transmission output driveably connected to the electric machine, and a powertrain output driveably connected to the electric machine and wheels of the vehicle, a method for controlling torque during a shift includes transmitting engine torque through the transmission to the powertrain output; during a shift, operating the electric machine to modify the torque transmitted to the powertrain output; and storing energy generated by the electric machine during the shift.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and in particular to controlling torque transmitted by the output of the powertrain to the vehicle wheels while executing a gear shift. 
     2. Description of the Prior Art 
     In a conventional vehicle with a fixed-ratio transmission, the driver can experience driveline disturbances during a transmission shift event, i.e., an upshift or a downshift. The driveline disturbances occur due to the acceleration and deceleration of engine and transmission components, which acceleration and deceleration produce an inertial torque during the shift event. In the case of an upshift, the transmission output torque increases during the ratio change phase, i.e., inertia phase, of the shift as a result of the engine speed changing, as shown in  FIG. 1  at point  12 . This output torque disturbance is felt by the vehicle&#39;s occupants and severely degrades shift quality. 
     The magnitude of the output shaft torque disturbance increases the faster the upshift is performed, since the magnitude of engine deceleration is greater. By reducing engine torque produced during the upshift, as shown at point  14 , the inertial torque can be offset and the output shaft torque increase can be minimized, as shown at point  16 , thereby improving the quality of the shift. This method described with reference to  FIG. 1  is referred to as “input torque modulation” control. 
     In the case of a downshift, the transmission output torque decreases during the ratio change phase as the engine and transmission components accelerate to the synchronous speed for the lower gear, as shown in  FIG. 2  at point  18 . Moreover, as shown at point  20  during the torque transfer phase, the transmission output torque can spike near the completion of the downshift as the engine accelerates. The drop in output torque during the ratio change phase is felt by vehicle&#39;s occupants and can give the sense of an acceleration discontinuity as the downshift is performed. The output torque spike at the end of the downshift can degrade shift quality and give the occupants a feeling of a harsh or rough shift. Furthermore, the magnitude of output shaft torque drop and spike near the end of the downshift increases in proportion to speed of the downshift. By using input torque modulation, the engine combustion torque is reduced near the end of the downshift, as shown at point  22 , in order to reduce the engine&#39;s acceleration as the shift ends. As a result, the transmission output torque spike can be minimized and avoided, as shown at point  24 , thereby reducing the shift disturbance. 
     In conventional vehicle applications, the problems that can occur with input torque modulation during shifts include limited engine torque reduction authority due to other constraints such as emissions, delayed or poor engine torque response to torque modulation requests, further degrading shift quality; and wasted fuel energy and efficiency since spark retardation is commonly used for achieving torque modulation requests. 
     SUMMARY OF THE INVENTION 
     In a powertrain for motor vehicle that includes an engine, an electric machine, a transmission having an input driveably connected to the engine and a transmission output driveably connected to the electric machine, and a powertrain output driveably connected to the electric machine and wheels of the vehicle, a method for controlling torque during a shift includes transmitting engine torque through the transmission to the powertrain output; during a shift, operating the electric machine to modify the torque transmitted to the powertrain output; and storing energy generated by the electric machine during the shift. 
     Excess transmission output torque is converted into electrical energy that is stored by a battery while achieving the requested torque modulation and providing optimum shift quality. 
     Delays in crankshaft torque reduction are avoided by taking advantage of the electric machine&#39;s responsiveness, which produces an accurate magnitude of torque modulation. 
     In some cases, the electric machine and engine both reduce the total driveline output torque shift disturbance to meet the requested torque modulation level. This is useful in the case where the electric machine may not be fully available or the battery state of charge is near the maximum limit. 
     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 graph that illustrates the variation of transmission output shaft torque, gear ratio and engine torque during an upshift with input torque modulation in a conventional vehicle driveline; 
         FIG. 2  is a graph that illustrates the variation of transmission output shaft torque, gear ratio and engine torque during an downshift with input torque modulation in a conventional vehicle driveline; 
         FIG. 3  is a schematic diagram of a powertrain for a RWD HEV; 
         FIG. 4  is a schematic diagram showing propulsion and power flow in the HEV powertrain of  FIG. 3 ; 
         FIG. 5  is a schematic diagram showing vectors representing torque transmission among components of the powertrain operating in mode A; 
         FIG. 6  is a schematic diagram showing vectors representing torque transmission among components of the powertrain operating in mode B; 
         FIG. 7  is a schematic diagram showing vectors representing torque transmission among components of the powertrain operating in mode D; 
         FIGS. 8A-8D  illustrate the change of powertrain variables during a transmission upshift performed with output torque modulation; 
         FIGS. 9A-9D  illustrate the change of powertrain variables during a transmission downshift performed with output torque modulation; 
         FIG. 10  is a logic flow diagram of an algorithm for selecting the operating mode of the powertrain of  FIG. 3  during output torque modulation control; and 
         FIG. 11  is a logic flow diagram of an algorithm for providing output torque modulation transmission control in the HEV powertrain of  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3  illustrates a powertrain  26  for a hybrid electric vehicle that includes an internal combustion engine (ICE)  28 , preferably an Atkinson cycle ICE; a first electric machine  30 , preferably a crank integrated starter generator (CISG) driveably connected to the engine crankshaft  32  and able to function alternately as a motor and a generator; a fixed-ratio automatic transmission  34 , a second electric machine  38  such as an electric rear axle drive (ERAD) or electric front axle drive (EFAD) driveably connected to transmission output shaft  36  and able to function alternately as a motor and a generator; adriveline output shaft  40 , driveably connected to the second electric machine  38 ; a differential mechanism  42 ; and wheels  44 ,  45 , driveably connected to the differential  42 . 
     During a transmission shift event, the electric machine  38  that is coupled to the transmission output can be controlled to achieve accurately the transmission torque modulation request and reduce the driveline output torque shift disturbance at  40 . By using the electric machines  30 ,  38  and the powertrain  26 , torque disturbances on transmission output shaft  36  can be reduced and optimum shift quality can be achieved. Other configurations including RWD, FWD, or AWD full or mild HEV with at least one electric machine at the transmission output are also applicable. Furthermore, this concept is not limited to any particular transmission technology and includes conventional automatic, dual clutch (i.e. powershift), and converterless automatic transmissions. 
       FIG. 4  illustrates the power and energy flow in the powertrain  26 . Power produced by engine  28  and power produced by CISG  30  are combined at  50  and transmitted to the transmission input  52 . Electric power produced by both electric machines  30 ,  38  is combinable at  54  for charging the battery  56 , or is transmitted from the battery to the electric machines  30 ,  38 . Mechanical power produced by ERAD  38  is transmitted through ERAD gearing  58  to the load at the wheels  44 ,  45  through the rear final drive  42 . 
     The RWD-HEV CISG/ERAD platform shown in  FIG. 3  preferably incorporates an Atkinson cycle (4.6 L, 3V) internal combustion engine, a fixed ratio, six-speed automatic transmission and two electric machines. The first electric machine  30  (CISG) is integrated at the output  32  of the engine  28  and is connected to the impeller  60  of a torque converter transmission, thus providing starter/generator capability. The second electric machine  38  (ERAD) is coupled to the output  36  of the transmission  34  through a planetary gear set  58 , which is connected to the final drive, thus providing additional propulsion capability in either an electric drive or hybrid drive mode. 
     Major operating modes for this powertrain configuration include (1) electric drive with ERAD motoring/generating); series hybrid drive with engine running, CISG generating and ERAD motoring/generating); engine drive with CISG &amp; ERAD shutdown and conventional drive; parallel hybrid drive with engine running and CISG and ERAD motoring; engine starting with CISG motoring to start engine and the engine cranking; and engine stopped with the engine cranking or shutting down. 
     As shown in  FIGS. 5-7 , operating modes of the powertrain  10  are used to provide transmission output torque modulation during transmission shift events. Depending on the type of shift event, i.e., an upshift or downshift, level of torque modulation request, ERAD operating conditions, battery conditions, and other factors, the appropriate powertrain operating mode will be used to provide the desired output torque modulation request. 
       FIG. 5  is a schematic diagram of the powertrain  26  showing vectors representing torque transmission among components during operating mode A, in which output torque modulation occurs with ERAD  38  reducing driveline output torque during a gear shift. 
       FIG. 6  is a schematic diagram of the powertrain  26  showing vectors representing torque transmission among components during operating mode B, in which output torque modulation occurs with ERAD  38  increasing driveline output torque during a gear shift. 
       FIG. 7  is a schematic diagram of the powertrain  26  showing vectors representing torque transmission among components during operating mode D, in which torque modulation occurs with only the engine  28  reducing driveline output torque during a gear shift. 
       FIGS. 8A-8D  illustrate an example of a transmission upshift, in which output torque modulation is provided by the ERAD  38  using the power path of operating mode A, shown in  FIG. 5 . In operating mode A, ERAD  38  provides output torque modulation by operating as a generator and provide negative torque as shown at  70 , reducing the transmission output torque disturbance  72  during the shift to provide a smooth total driveline output torque  74 , provided the ERAD is available for this purpose. The ERAD  38  is available if its current temperature is lower than its thermal limit, its speed is lower than its operational speed limit, and the state of charge (SOC) of battery  56  is below the maximum allowable SOC limit. 
     By using operating mode A, excess transmission output torque  76  is converted into electrical energy that is stored by battery  56  while achieving the requested torque modulation and providing optimum shift quality. Furthermore, delays in crankshaft torque reduction are avoided by taking advantage of the ERAD&#39;s responsiveness, which produces an accurate magnitude of torque modulation. In operating mode A, both the ERAD  38  and engine  28  can also be used to reduce the total driveline output torque shift disturbance  72  in order to meet the requested torque modulation level. This combination of engine  28  and ERAD  38  is useful in the case where the ERAD may not be fully available or the battery SOC is near its maximum limit. 
       FIGS. 9D-9D  illustrate an example of a transmission downshift in which output torque modulation is provided by the ERAD  38  using both operating modes A and B. During the ratio change phase of the downshift, operating mode B can be used with the ERAD  38  in a motoring mode to produce ERAD output torque  80  so that the net total driveline output torque  82  is increased in order to offset or compensate for the decrease  84  in transmission output torque that normally occurs during the ratio change phase of a downshift. Operating mode B can only be used if ERAD  38  is available for this purpose. The ERAD  38  is available if its current temperature is lower than its thermal limit, its speed is lower than its operational speed limit, and the state of charge (SOC) of battery  56  is above the minimum allowable SOC limit. 
     The powertrain  26  changes to operating mode A in the torque transfer phase near completion of the downshift so that ERAD operates as generator to produce negative torque  86 , which reduces the net total driveline output torque in order to soften or eliminate the output torque spike  88 , which would normally occur without torque modulation. Unlike that of the conventional case, with an HEV this excess torque  89  is converted into electrical energy to be stored by battery  56  while achieving the requested torque modulation and providing optimum shift quality. 
       FIG. 10  shows the steps of an algorithm for providing output torque modulation transmission control of the HEV powertrain  26  of  FIG. 3 . After execution of the algorithm is started and the operating conditions of powertrain  10  are assessed at step  90 , a test is performed at step  92  to determine whether a gear ratio change of the transmission  34  has been requested by a transmission controller acting in response to vehicle parameters that include without limitation engine throttle position, accelerator pedal position, vehicle speed, engine speed, the position of a manually operated gear selector, and a schedule of the preferred gear ratios related to the vehicle parameters. 
     If the result of test  92  is logically positive, control advances to step  94  where a test is performed to determine whether shift output torque modulation is requested by the controller. If the result of either test  92  or  94  is logically negative, control returns to step  90 . But if the result of test  94  is positive, the magnitude of desired output torque modulation is determined at step  96 . The desired magnitude of output torque modulation is determined based on the progress of the shift event. For example, at the beginning of the ratio change phase of an upshift, the desired magnitude will ramp from zero to a negative steady-state level as the ratio change phase continues, and will ramp back to zero as the ratio change phase is completed. 
     At step  98 , the operating mode of powertrain  26  is selected in accordance with the algorithm of  FIG. 11  upon reference to current operating parameters and the desired magnitude of output torque modulation. 
     At step  100 , powertrain  26  is placed in the desired operating mode selected by the algorithm of  FIG. 11  in order to provide the desired output torque modulation during the shift event. 
     Referring now to the algorithm for selecting the desired operating mode shown in  FIG. 11 , a test is performed at step  102  to determine whether the ERAD  38  temperature is less than a reference temperature representing the maximum allowable operating temperature of the ERAD. 
     If the result of test  102  is positive, a test is performed at step  104  to determine whether the speed of ERAD  38  is less than a reference speed representing the maximum allowable operating speed of the ERAD. 
     If the result of test  104  is positive, a test is performed at step  106  to determine whether the magnitude of a request for transmission output torque modulation is less than a reference torque limit representing the current maximum torque capability of ERAD  38 . 
     If the result of any of tests  102 ,  104  and  106  is negative, control advances to step  108 , where powertrain  10  is placed in operating mode D, in which torque produced by engine  28  alone is transmitted to transmission output  36  without CISG  30  torque affecting any change in torque carried on crankshaft  52  to the transmission input  52 , i.e., CISG  30  neither produces nor draws power. Operating mode D, shown in  FIG. 7 , is that of a conventional vehicle and the engine torque will be reduced to provide the desired level of output torque modulation since CISG  30  and ERAD  38  cannot be used. 
     If the result of test  106  is positive, a test is performed at step  110  to determine whether the desired magnitude of transmission output torque modulation is negative. If the result of test  110  is positive indicating that the desired output torque modulation level is negative, a test is performed at step  112  to determine whether the battery SOC is less than a maximum allowable SOC reference. 
     If the result of test  112  is positive indicating that the battery SOC can be further increased while ERAD  38  is operated as an electric generator, at step  114  operating mode A is selected as the operating mode for powertrain  26  and ERAD  38  performs output torque modulation by converting power produced by engine  12  into electrical energy to be stored by battery  56  during an upshift while achieving the desired output torque modulation level. 
     If the result of test  112  is negative indicating that the battery SOC cannot be further increased, control advances to step  116 , where powertrain  26  is placed in operating mode D, in which torque produced by engine  12  alone is transmitted to output shaft  40  without ERAD participating in the torque modulation. 
     If the result of test  110  is negative indicating that the desired output torque modulation level is positive and the output shaft  40  torque is to be increased, a test is performed at step  118  to determine whether the battery SOC is greater than a minimum SOC. 
     If the result of test  118  is positive, indicating that the battery SOC can be further decreased, control advances to step  120 , At step  120  operating mode B is selected, indicating that ERAD  38  is available to function as a motor and to participate in output torque modulation by supplementing power produced by engine  28  during a downshift. 
     If the result of test  118  is negative, indicating that the minimum battery SOC limit has been reached, control advances to step  108 , where powertrain  26  is placed in operating mode D, in which torque produced by engine  28  alone is transmitted to output  40  without ERAD  38  torque affecting any change in torque carried on output shaft  40 . 
     The output torque modulation control can be applied to RWD, FWD, AWD full or mild HEV powertrain configurations that include at least one electric machine driveably connected to the transmission output  36 . Furthermore, the control strategy is not limited to any particular transmission technology, but can be applied to a conventional automatic transmission, a dual clutch powershift transmission, and a converterless automatic transmission. 
     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.