Abstract:
A method of releasing a lockup clutch for a fluid coupling assembly disposed between an engine and an automatically shiftable transmission during a negative engine torque mode of operation is provided. The method includes the steps of: A) determining a current engine torque value prior to commanding the engine to ramp to a target engine torque value; B) commanding the engine to a target engine torque value wherein the target engine torque value is one of a zero and near zero value; C) holding the engine at the target engine torque value; and D) releasing or disengaging the lockup clutch when the engine is operating at the target engine torque value. A powertrain is also provided having a controller sufficiently configured to operate according to the method of the present invention.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a powertrain and method of operation, and more specifically, to a powertrain and method of disengaging or releasing a lockup clutch. 
       BACKGROUND OF THE INVENTION 
       [0002]    Modern vehicular powertrains incorporating an engine, such as an internal combustion engine, and a transmission, such as an automatically shiftable transmission, typically include a fluid coupling disposed therebetween. The fluid coupling operates to substantially decouple the engine from the transmission to allow the vehicle so equipped to come to rest without causing the engine to stall. Additionally, as in the case of a torque converter, the fluid coupling may operate to provide a measure of engine torque multiplication within the powertrain. 
         [0003]    Further, the fluid coupling may serve to impede or damp the oscillations, caused by the firing pulses of the engine, from being transmitted to the transmission and other components within the powertrain. To increase the operating efficiency of the powertrain, some fluid couplings incorporate a lockup clutch, sometimes refereed to as a torque converter clutch (TCC). The lockup clutch is selectively engageable to substantially couple the engine to the transmission thereby reducing any slip that may be occurring within the fluid coupling. In order to make the engagement and disengagement, or release, of the lockup clutch imperceptible to the operator of the vehicle, careful attention is paid to provide optimal powertrain operating conditions during such operations. 
         [0004]    The engine may provide an amount of engine braking when the vehicle is coasting or decelerating. This condition will cause the engine to “motor” or experience negative torque as a result of pumping losses. The operator of the vehicle may feel a shock or impulse should the lockup clutch be disengaged or released while the engine is experiencing a large amount of negative torque. 
       SUMMARY OF THE INVENTION 
       [0005]    A method of disengaging or releasing a lockup clutch for a fluid coupling assembly, such as a torque converter assembly, disposed between an engine and an automatically shiftable transmission during a negative engine torque mode of operation is provided. The method includes the steps of: A) commanding the engine to a target engine torque value wherein the target engine torque value is one of a zero and near zero value; B) holding the engine at the target engine torque value; and C) releasing or disengaging the lockup clutch when the engine is operating at the target engine torque value and a sufficient amount of time has elapsed to enable release of the lockup clutch. The method may further include determining or capturing a current engine torque value prior to commanding the engine to ramp to the target engine torque value. 
         [0006]    A powertrain is also provided including an engine, transmission, and fluid coupling assembly disposed between the engine and the transmission and having a lockup clutch operable to substantially lock the engine with the transmission for unitary rotation. The powertrain further includes at least one controller sufficiently configured to: command the engine to ramp to a target engine torque value during engine braking conditions, wherein the target engine torque value is zero or near zero; hold the engine at the target engine torque value; and release the lockup clutch when the engine is operating at the target engine torque value. 
         [0007]    In one embodiment, the at least one controller may include a transmission controller operable to communicate command signals to the transmission and an engine controller operable to communicate command signals to the engine. The engine controller and transmission controller may communicate with each other over a data link. The fluid coupling assembly may be a torque converter, while the transmission may be an automatically shiftable transmission. Additionally, the engine may be one of a spark-ignited and a compression-ignited internal combustion engine. 
         [0008]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic illustration of a powertrain including an engine, fluid coupling, lockup clutch, engine controller, and transmission controller consistent with the present invention; 
           [0010]      FIG. 2  is a method, in flowchart form, of disengagement or release the lockup clutch of  FIG. 1  consistent with the present invention; 
           [0011]      FIG. 3   a  is a graphical illustration of a portion of the method, shown in  FIG. 2 ; illustrating the release of the lockup clutch of  FIG. 1 ; and 
           [0012]      FIG. 3   b  is a graphical illustration of a portion of the method, shown in  FIG. 2 ; illustrating an aborted release of the lockup clutch of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring to  FIG. 1 , there is shown a powertrain  10  for a vehicle  11 , a portion of which is shown in  FIG. 1 , including an engine  12 , transmission  14 , and a final drive  16 . The engine  12  is preferably an internal combustion engine such as a spark-ignited or compression-ignited engine. The transmission  14  is preferably an automatically shiftable transmission incorporating a plurality of planetary gearsets, not shown, therein to provide a plurality of speed ratios. The final drive  16  may be a differential or other device known to those skilled in the art of vehicle design. 
         [0014]    The engine  12  includes an output shaft  18  operable to provide torque to an input shaft  20  of the transmission  14 . A fluid coupling assembly  22  is disposed between the output shaft  18  and the input shaft  20 . The fluid coupling assembly  22  is shown in  FIG. 1  as a torque converter and includes an impeller member  24 , turbine member  26 , stator member  28 , and lockup clutch  30 . The turbine member  26  and stator member  28  are disposed in toroidal fluid flow relationship with the impeller member  24 . The stator member  28  is connected via a one-way clutch device  32  to a stationary component or housing, not shown, of the transmission  14 . The lockup clutch  30  is selectively engageable to selectively lock or join the output shaft  18  with the input shaft  20  for substantially unitary rotation therewith. The lockup clutch  30 , when engaged, operates to increase the operating efficiency of the powertrain under certain driving conditions such as highway driving and other operating conditions when slip between the impeller member  24  and the turbine member  26  of the fluid coupling assembly  22  is near a minimum. 
         [0015]    The transmission  14  includes an output shaft  34  mechanically interconnected with the final drive  16 . The final drive  16  operates to transfer torque from the output shaft  34  to wheels  36  of the vehicle  11 . The powertrain  10  is also operable to absorb torque from the wheels  36  in what is commonly referred to as “engine braking”. During engine braking, torque is transferred from the wheels  36  through the final drive  16  and the transmission  14  and is at least partially absorbed by the engine  12  through the pumping losses incurred by pumping air through the cylinders, not shown, of the engine  12 . The engine  12  experiences negative torque during engine braking, that is, the engine  12  is absorbing torque from the powertrain  10  instead of providing torque to the powertrain  10  as is the case when the engine  12  is firing. 
         [0016]    A transmission controller  38  is provided to communicate control signals to, and receive operational parameter signals from, the transmission  14 . The transmission controller  38  is further operable to control the selective engagement of the lockup clutch  30 . An engine controller  40  is provided to communicate control signals to, and receive operational parameter signals from, the engine  12 . The transmission controller  38  and the engine controller  40  are preferably microprocessor based and communicate with each other over a data link  42 . The transmission controller  38  is sufficiently configured to execute a method  44 , shown in  FIG. 2 , of the present invention. 
         [0017]    Referring to  FIG. 2 , and with continued reference to  FIG. 1 , there is shown the method  44  of disengaging or releasing the lockup clutch  30  during engine braking conditions of the vehicle  11 , wherein the engine  12  experiences negative torque. The method begins at step  46  and proceeds to step  48 . At step  48 , parameters are initialized such as any timers that require resetting. Thereafter the method  44  proceeds to step  50  where the transmission controller  38  captures the current engine torque value from the engine controller  40  over the data link  42 . The current engine torque value may be obtained though lookup tables containing representative values of engine torque for various engine rotational speed and engine load conditions of the engine  12  or may be obtained through direct measurement. 
         [0018]    Having captured the current engine torque value, the transmission controller  38  will request that the engine controller  40  ramp or increase the engine torque value to a target engine torque value, as shown in step  52 . The target engine torque value is preferably zero or near zero and negative. However, those skilled in the art will recognize that the target engine torque value may be near zero and positive while remaining within the scope of that which is claimed. The engine controller  40  will command the engine  12  to increase in torque either by increasing the throttle opening in the case of a spark-ignited internal combustion engine, or increasing the fueling rate in the case of a compression-ignited internal combustion engine. 
         [0019]    The method then proceeds to step  54  where a determination is made as to whether abort conditions are present. The abort conditions may include such considerations as: whether a commanded throttle value is greater than, or equal to, a threshold value signaling a driver request for increased engine torque by depressing the accelerator pedal; whether a driver command torque value is greater than, or equal to, a threshold value signaling a driver request for increased engine torque; whether a range shift is being completed, and whether engine braking is requested. If none of the abort conditions are present, the method  44  proceeds to step  56  where the target engine torque value is held for a predetermined amount of time to enable release of the lockup clutch  30 . The lockup clutch is released at step  56 . If any of the abort conditions are present, the method  44  proceeds to step  58  where the target engine torque value is ramped toward a driver commanded engine torque value, which will allow the engine  12  to produce positive torque in response to driver requests. 
         [0020]    At step  60 , a determination is once again made as to whether abort conditions are present. The abort conditions include all of those described hereinabove and may also include considerations such as: whether the fluid coupling assembly  22  has been slipping for greater than or equal to a predetermined amount of time; whether the lockup clutch  30  has been reapplied; and whether the engine torque has been held at the target engine torque value for greater than or equal to a predetermined amount of time. If none of the abort conditions are present, the method  44  loops to step  66 , described hereinbelow. Alternately, if any of the abort conditions are present, the method  44  proceeds to step  58  where the target engine torque value is ramped toward the driver command engine torque value, which will allow the engine  12  to produce positive torque in response to driver requests. 
         [0021]    The method  44  proceeds from step  58  to step  66  where the lockup clutch release torque control is disabled. That is, the transmission controller  38  will not attempt to ramp and hold the engine torque value at the target engine torque value. The lockup clutch release torque control will remain disabled until the lockup clutch  30  is reapplied or commanded to engage. The method  44  proceeds to step  68  where a determination is made as to whether the lockup clutch  30  has been reapplied or commanded to engage. If the lockup clutch  30  has been reapplied or commanded to engage, the method returns to step  46  from step  70  to begin the method  44  again. Alternately, if the lockup clutch  30  has not been reapplied or commanded to engage, the method  44  loops to step  66  where the lockup clutch release engine torque control remains disabled. 
         [0022]    Referring now to  FIG. 3   a , there is shown a graphical illustration of a portion of the method  44 , shown in  FIG. 2 ; illustrating the disengagement of the lockup clutch  30 . The engine torque value is represented by line  72 , while the commanded engine torque value is represented by line  74 . At a time equal to zero to to, the engine  12  experiences a negative torque value T b  indicating that the powertrain  10  is in an engine braking mode of operation. In order to place the powertrain  10  in a more favorable condition to release the lockup clutch  30 , the engine  12  is commanded to produce a torque increase at t 0  to a near zero and slightly negative target engine torque value T t . The engine  12  then holds the target engine torque value T t  for a time equal to t 1 −t 0 , or t h , to allow adequate time for the disengagement of the lockup clutch  30 . This hold time, t h , is preferably a predetermined value and may be determined through experimental or analytical means. At t 1 , the control of the engine torque to the target engine torque value T t  is discontinued, and the engine torque is allowed to deviate from the target engine torque value T t . 
         [0023]    Referring now to  FIG. 3   b , there is shown a graphical illustration of a portion of the method  44 , shown in  FIG. 2 ; illustrating an aborted disengagement of the lockup clutch  30 . As with  FIG. 3   a , the engine torque value is represented by line  72 , while the commanded engine torque value is represented by line  74 . Similar to  FIG. 3   a , at a time equal to 0 to t 0 , the engine  12  experiences a negative torque value T b  indicating that the powertrain  10  is in an engine braking mode of operation. In order to place the powertrain  10  in a more favorable condition to release the lockup clutch  30 , the engine  12  is commanded to produce an increased torque at t 0  to a near zero and slightly negative target engine torque value T t . The engine  12  is held at the target engine torque value T t  until at least one of the abort conditions, described hereinabove with regard to  FIG. 2 , is met at t 1 ′ at which point, control of the engine torque to the target engine torque value T t  is discontinued and the engine torque value is commanded to deviate from the target engine torque value T t  to a driver command torque value. The engine torque at this point may deviate up to the maximum engine torque value T m , as indicated by line  72  for time values greater than t 1 ′. 
         [0024]    Smooth disengagement of the lockup clutch  30  is facilitated by the present invention by placing the powertrain  10  in a more favorable condition for disengagement. By increasing the engine torque value to a zero or near zero value during engine braking, the lockup clutch  30  may be released while reducing the impulse or shock that would otherwise occur. 
         [0025]    While the best modes for carrying out the 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 within the scope of the appended claims.