Abstract:
A method for controlling a vehicle regenerative braking event includes maintaining a converter clutch closed while braking, while an engine connected to the impeller is running, opening the converter clutch when impeller speed reaches a reference speed difference relative to engine idle speed, and while the engine is off, opening the converter clutch when impeller speed reaches a speed required for a transmission pump, connected to an impeller, to produce line pressure at a desired magnitude.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a method for controlling a regenerative braking event in a hybrid electric vehicle by changing the torque capacity of a torque converter lock-up clutch. 
         [0003]    2. Description of the Prior Art 
         [0004]    The fuel economy benefit in a hybrid electric vehicle results mainly from its ability to perform regenerative braking. In a hybrid electric powertrain an electric machine is coupled to the wheels through a torque converter and transmission gearing. The torque converter transmits torque through the combination of the hydraulic path and the mechanic path, provided the torque converter clutch is slipping. If the torque converter clutch is fully open, torque can only be transmitted through the hydraulic path. If the clutch is fully locked, the torque can only be transmitted through the mechanical path. 
         [0005]    During regenerative braking, torque is transmitted from the vehicle wheels to the electric machine. If clutch is open, the torque converter&#39;s ability to transmit torque in the reverse direction is very limited. Any excessive regenerative torque can reduce the electric machine&#39;s speed. As a result, to recoup most of the kinetic energy using regenerative braking, the torque converter clutch should be kept locked while the vehicle is slowing down. 
         [0006]    The torque converter clutch, however, must be opened for various reasons. When impeller speed is low, that clutch must be open so that the engine does not stall. When the clutch is open, the hydraulic path serves as coupling to deliver torque smoothly to the wheels. 
       SUMMARY OF THE INVENTION 
       [0007]    A method for controlling a regenerative braking event in a hybrid electric vehicle includes maintaining a converter clutch closed while braking, while an engine connected to the impeller is running, opening the converter clutch when impeller speed reaches a reference speed difference relative to engine idle speed, and while the engine is off, opening the converter clutch when impeller speed reaches a speed required for a transmission pump, connected to an impeller, to produce line pressure at a desired magnitude. 
         [0008]    Regenerative braking torque blend-out and torque converter clutch control is coordinated such that recovery of vehicle kinetic energy can be maximized during a regeneration braking event. 
         [0009]    The method coordinates torque converter clutch operation and regenerative braking by arbitrating a trade-off between fuel economy improvement and driveability attributes. 
         [0010]    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 
         [0011]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic diagram showing a modular hybrid electric powertrain for a motor vehicle; 
           [0013]      FIG. 2  shows graphs representing the variation of various powertrain parameters during a regenerative braking event; 
           [0014]      FIG. 3  is an algorithm for coordinating torque converter clutch operation and regenerative braking; and 
           [0015]      FIG. 4  is an alternative algorithm for coordinating torque converter clutch operation and regenerative braking of a powertrain having an e-pump. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    Referring first to  FIG. 1 , a parallel hybrid electric powertrain  10  includes an internal combustion engine  12 , engine disconnect clutch  14 , electric machine or motor/generator  16 , transmission hydraulic pump  18 , torque converter  20 , torque converter lock-up clutch  22 , transmission gearing  24 , final drive gearing  26 , shafts  28 ,  29 , and driven wheels  30 . A low voltage starter  32 , powered by a low voltage battery  34 , cranks the engine while starting the engine  12  and producing sustained combustion. A high voltage battery  36  powers the electric motor/generator  16 . 
         [0017]    The torque converter  20  is a hydraulic coupling that produces a hydrokinetic drive connection between an impeller, which is driveably connected to the engine  12  when clutch  14  is closed, and a turbine, which is driveably connected to the driven wheels  30 . 
         [0018]    The torque converter lock-up clutch  22  alternately opens and closes a drive connection between the torque converter&#39;s turbine and the shaft  38 . 
         [0019]    A vehicle equipped with this powertrain  10  can produce electric drive and hybrid electric drive and can charge the battery  36  either by regenerative braking, i.e., recovering and converting kinetic energy of the vehicle during a braking event to electric energy that can be stored in battery  36 , or by using the engine to charge battery  36 . 
         [0020]    The fuel economy benefit in a hybrid electric vehicle results mainly from its ability to perform regenerative braking. In powertrain  10  motor  16  is coupled to the wheels through the torque converter  20 , transmission gearing  24  and final drive  26 . The torque converter  20  transmits torque through the combination of the hydraulic path and the mechanic path, provided the torque converter clutch  22  is slipping. If the torque converter clutch  22  is fully open, torque can only be transmitted through the hydraulic path. If the clutch  22  is fully locked, the torque can only be transmitted through the mechanical path. 
         [0021]    During regenerative braking, torque is transmitted from the wheels  30  to the electric machine  16 . If clutch  22  is open, the torque converter&#39;s ability to transmit torque in the reverse direction is very limited. Any excessive regenerative torque can reduce the electric machine&#39;s speed. As a result, to recoup most of the kinetic energy using regenerative braking, the torque converter clutch  22  should be kept locked while the vehicle is slowing down. 
         [0022]    The torque converter clutch  22 , however, must be opened for various reasons. When impeller speed is low, clutch  22  must be open so that the engine  12  does not stall. When clutch  22  is open, the hydraulic path serves as coupling to deliver torque smoothly to the wheels  30 . 
         [0023]    The control strategy coordinates operation of the torque converter clutch  22  and the electric machine  16  during a vehicle braking event, whether engine  12  is running or the engine is stopped. If engine  12  is running, its crankshaft is connected to the electric machine  16 ; therefore, the torque converter&#39;s impeller speed can not drop below the engine idle speed. If engine  12  is stopped, the electric machine  16  can be running at speeds lower than the nominal engine idle speed. If the transmission&#39;s hydraulic system line pressure is provided by the mechanical oil pump  18 , the minimal impeller speed should be determined by the minimal pressure that the pump should generate in this case. 
         [0024]    As shown in  FIG. 2 , the regenerative braking torque decreases along a ramp  40  before clutch  22  is open at  42 . In  FIG. 2 , converter clutch is closed at  64  and open at  66 , and the speed  68  of electric machine  16  varies while vehicle speed  62  declines during regenerative braking. 
         [0025]    An algorithm  44  shown in  FIG. 3  controls and coordinates torque converter clutch operation and regenerative braking. At step  46  a test is performed to determine whether vehicle wheel braking is currently occurring. If the result of test  46  is logically false, control returns to step  46 . 
         [0026]    If the result of test  46  is logically true, regenerative braking is started at step  48 . 
         [0027]    At step  50  a test is performed to determine whether the impeller speed of torque converter  20  is close to engine idle speed, i.e., within a range of about 50 rpm to 100 rpm of engine idle speed. The range is calibratable, i.e., can be adjusted for particular vehicles and vehicle components and applications. The speed range avoids delay and the risk of impeller speed and engine idle speed not matching exactly due to noise factors in the system. A speed range that is too large can cause a noticeable disturbance. 
         [0028]    If the result of test  50  is false, control returns to step  48 . 
         [0029]    If the result of test  50  is true, at step  52  a test is performed to determine whether engine  12  is running. If the result of test  52  is false, at step  54  regenerative braking continues. 
         [0030]    At step  56  a test is performed to determine whether impeller speed is close to a minimum speed required for the pump  18  to produce a required magnitude of transmission hydraulic system line pressure, i.e., within about 250 rpm to 350 rpm depending on the transmission and its components. The range is calibratable, i.e., can be adjusted for particular vehicles and vehicle components and applications. Impeller speed should not be less than a minimum speed required for desired line pressure. 
         [0031]    If the result of test  56  is false, control returns to step  54 . 
         [0032]    If the result of test  56  is true, at step  58  regenerative braking torque is blended-out along ramp  40 , and the torque converter clutch  22  is opened at  42 . 
         [0033]    Execution of algorithm  44  ends at step  60 . 
         [0034]    If the engine is running, the opening of clutch  22  is scheduled at  42 , the lowest possible vehicle speed  62  while the impeller speed is above the engine idle speed, taking drivability into consideration. If the engine is stopped during the braking event, opening of clutch  22  is delayed until the impeller speed is just above the minimal speed sufficient to provide hydraulic line pressure to the transmission, usually about 400 rpm. 
         [0035]    The powertrain  10  may include an auxiliary oil pump, i.e., a hydraulic pump that is driven by an electric motor, sometimes called an e-pump, which is a substitute for the transmission pump  18  of  FIG. 1 . An e-pump is turned on to maintain the transmission line pressure and clutch  22  is kept closed as the vehicle comes to a stop. 
         [0036]      FIG. 4  shows an algorithm  70 , which controls an e-pump. At step  72  a test is performed to determine whether vehicle wheel braking is currently occurring. If the result of test  72  is false, control returns to step  72 . 
         [0037]    If the result of test  72  is true, regenerative braking is started at step  74 . 
         [0038]    At step  76  a test is performed to determine whether the impeller speed of torque converter  20  is close to engine idle speed, i.e., within a range of about 50 rpm to 100 rpm of engine idle speed. The range is calibratable, i.e., can be adjusted for particular vehicles and vehicle components and applications. The speed range avoids delay and the risk of impeller speed and engine idle speed not matching exactly due to noise factors in the system. A speed range that is too large can cause a noticeable disturbance. 
         [0039]    If the result of test  76  is false, control returns to step  74 . 
         [0040]    If the result of test  76  is true, at step  78  a test is performed to determine whether engine  12  is running. If the result of test  78  is false, at step  80  regenerative braking continues. 
         [0041]    At step  82  a test is performed to determine whether impeller speed is close to a minimum speed required for the pump  18  to produce a required magnitude of transmission hydraulic system line pressure, i.e., within about 250 rpm to 350 rpm depending on the transmission and its components. The range is calibratable, i.e., can be adjusted for particular vehicles and vehicle components and applications. Impeller speed should not be less than a minimum speed required for desired line pressure. 
         [0042]    If the result of test  82  is true, at step  84  the e-pump is turned on, regenerative braking continues until the vehicle speed stops at  86 , and execution of algorithm  70  ends at step  88 . 
         [0043]    If the result of test  78  is true, at step  90  regenerative braking torque is blended-out along ramp  40 , the torque converter clutch  22  is opened at  42 , and execution of algorithm  70  ends at step  88 . 
         [0044]    If the engine is running, the opening of clutch  22  is scheduled at  42 , the lowest possible vehicle speed  62  while the impeller speed is above the engine idle speed, taking drivability into consideration. If the engine is stopped during the braking event, opening of clutch  22  is delayed until the impeller speed is just above the minimal speed sufficient to provide hydraulic line pressure to the transmission, usually about 400 rpm. 
         [0045]    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.