Abstract:
A method for controlling a vehicle torque converter lockup clutch during a deceleration coasting event includes producing slip across the clutch by reducing the clutch&#39;s torque capacity, decreasing said slip by increasing said torque capacity, and maintaining slip across the clutch.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to control of a motor vehicle torque converter clutch during negative torque coast downs to avoid lash and clunk. 
     2. Description of the Prior Art 
     During an aggressive deceleration fuel shutoff (ADFSO) coasting event of a vehicle equipped with an automatic transmission, when its torque converter clutch transitions from hard lock, i.e., with zero slip, to a controlled negative slip, preferably about negative 40 rpm, unlocking the converter clutch can cause fuel supply to the engine to resume. This is primarily due to a “stick-slip” phenomenon wherein the dynamic coefficient of friction of the clutch in its slipping state differs from the static coefficient of friction in its locked state. 
     Due to the stick-slip occurrence, the torque transmitting capacity of the torque converter clutch must be substantially decreased to generate slip and then increased soon thereafter to maintain a controlled amount of negative slip. If the clutch actuating hydraulic pressure doesn&#39;t increase fast enough, then destroking the clutch causes the fuel injectors to pump fuel into the engine cylinders. If clutch actuating pressure increases too fast, then the clutch torque capacity is too large, reducing slip across the clutch causing a bump that can be felt by vehicle occupants. Destroking the clutch causes a loss of fuel economy and the related bump is attributed to harsh gear shifting of the transmission. 
     Different vehicles begin generating negative converter clutch slip at different clutch actuation pressures due to variability in transmission hardware, braking rate, and engine torque differences. 
     A need exists in the industry for a technique during a deceleration fuel shutoff coasting event to increase slip across the torque converter clutch before it decreases too far and to increase its torque capacity smoothly without causing destroke or bumps. 
     SUMMARY OF THE INVENTION 
     A method for controlling a vehicle torque converter lockup clutch during a deceleration coasting event includes producing slip across the clutch by reducing the clutch&#39;s torque capacity, decreasing the magnitude of negative clutch slip by increasing said torque capacity, and maintaining slip across the clutch. 
     The method provides a consistent, acceptable control of the converter clutch during a deceleration coasting event without unlocking the torque converter, which produces a reduction in fuel economy. The method avoids a harsh bump that resulted from prior control method. 
     Clutch slip reliably indicates that torque capacity of the converter clutch has decreased sufficiently before beginning the catch mode torque capacity increase. 
     The increase in clutch torque capacity in the “catch mode” can account for the different clutch braking rates and hardware variables that occur across a range of converter clutches to which the algorithm is applied. 
     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 cross section of a torque converter to which the control strategy can be applied; and 
         FIG. 2  illustrates an algorithm for controlling the torque converter lockup clutch during a deceleration fuel shutoff coasting event. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to  FIG. 1 , a torque converter  10  includes a bladed impeller wheel  12  connected to the crankshaft  14  of an internal combustion engine, a bladed turbine wheel  16 , and a bladed stator wheel  18 . The impeller, stator and turbine wheels define a toroidal fluid flow circuit, whereby the impeller is hydrokinetically connected to the turbine. The stator  18  is supported rotatably on a stationary stator sleeve shaft  20 , and an overrunning brake  22  anchors the stator to shaft  20 , thereby preventing rotation of the stator in a direction opposite the direction of rotation of the impeller, although free-wheeling motion in the opposite direction is permitted. 
     The torque converter assembly  10  includes a lockup clutch  24  located within a torque converter housing  25 , which is secured to the impeller  12 . The lockup clutch  24  alternately engages and disengages a drive connection between the housing  25  and a damper  26 . The damper  26  is located in a torque path between clutch  24  and a turbine shaft, which is the transmission input shaft  28 . The damper  26  may incorporate dual or single-stage compression springs  30 ,  32 . 
     When clutch  24  is fully engaged or slipping, i.e., while there is a speed difference between its input and output, damper  26  attenuates transitory torque fluctuations between the engine crankshaft  14  and input shaft  28 . When the clutch is disengaged, the hydrokinetic connection between the impeller  16  and turbine  16  mitigates transient torque disturbances. 
     The clutch  24  is alternately engaged and disengaged in accordance with the magnitude of clutch apply pressure communicated to a hydraulic cylinder  34  through an axial passage  36  formed in the input shaft  28  and a radial passage  38 . A closed piston  40 , sealed on housing  25  by O-rings  42 ,  44 , moves rightward within the cylinder to force the discs of clutch  24  into mutual frictional contact, and leftward to allow the discs to disengage mutually. 
     When clutch  24  is engaged, the turbine and impeller are mechanically connected and hydrokinetically disconnected; when clutch  24  is disengaged, the turbine and impeller are hydrokinetically connected and mechanically disconnected. Fluid contained in the torque converter  10  is supplied from the output of an oil pump and is returned to an oil sump, to which an inlet of the pump is connected hydraulically. 
       FIG. 2  illustrates an algorithm for controlling the lockup clutch  24  during a deceleration, negative torque, coasting event, in which torque is transmitted from the wheels of a motor vehicle to its engine through the lockup clutch of an automatic transmission. 
     After execution of the algorithm begins at step  50 , a test is made at step  52  to determine whether the vehicle is coasting, i.e., moving with the accelerator pedal fully released, the engine running, the wheels transmitting torque to the engine through the lockup clutch. Preferably the engine throttle is closed or substantially closed. 
     If the result of test  52  is logically false, control returns to step  50 . If the result of test  52  is logically true, control advances to step  54  where a test is made to determine whether the converter lockup clutch  24  is engaged, i.e., applied. 
     If the result of test  54  is false, control returns to step  52 . If the result of test  54  is true, control advances to step  56  where a test is made to determine whether the electronic controller, which is controlling lockup clutch  24 , has commanded that the clutch be fully engaged or hard-locked. 
     If the result of test  56  is false, control advances to step  72  where the controller adjusts the torque capacity of clutch  24  such that slip across the clutch continuously slips. Execution of the algorithm ends at step  74 . 
     If the result of test  56  is true, clutch  24  is hard locked at step  58  by increasing the clutch actuating pressure in volume  34 . 
     At step  60  a test is made to determine whether the controller has commanded that the lockup clutch  24  be slipping. If the result of test  60  is false, control returns to step  58 . 
     If the result of test  60  is true, at step  62  the torque transmitting capacity of clutch  24  is reduced at step  62  decreasing the actuating pressure in volume  34 . Preferably the torque capacity is reduced first by a step function and then gradually by reducing pressure in volume  34  linearly with time or along a descending nonlinear ramp. 
     At step  64  a test is made to determine whether slip across clutch  24  has been produced, thereby indicating that the clutch is disengaging. Preferably, clutch slip produced at step  64  should be sufficient to overcome noise in electronic signals produced by sensors whose output represents the rotational speed on opposite sides of clutch  24 . If the result of test  64  is false, control control returns to step  62 . 
     If the result of test  64  is true, the torque transmitting capacity of clutch  24  is increased at step  66  by increasing the actuating pressure in volume  34 . Preferably the torque capacity is increased at step  66  first by a step function and then linearly or along an ascending ramp that increases with time. As step  66  is performed, torque capacity of clutch  24  continues to ramp up slowly, thereby increasing torque capacity enough to slow the rate of change of slip and preventing the converter from destroking. Preferably the “catch” mode step  66  does not entirely eliminate slip across clutch  24 . 
     At step  68 , a test is made to determine whether the magnitude of slip across clutch  24  is more positive than the clutch slip that occurred at a previous loop or execution of the algorithm, or another loop before the last loop. If the result of test  68  is false, control returns to step  66 . 
     If the result of test  68  is true, indicating that clutch slip is becoming more positive, at step  70  the torque capacity of clutch  24  is reduced slightly by decreasing the actuating pressure in volume  34 , preferably producing a low magnitude stepwise reduction in torque capacity, which ensures that clutch slip occurs continuously at step  72 . This final step down in clutch torque capacity keeps the converter clutch slipping smoothly in the negative domain in a controlled fashion. 
     Preferably the increase in clutch torque capacity or “catch mode,” which is initiated at step  66  in response to slip across clutch  24 , accounts for the different clutch braking rates and hardware variables that occur across a range of converter clutches to which the algorithm is applied. Clutch slip reliably indicates that torque capacity of the converter clutch  24  has decreased sufficiently before beginning the catch mode torque capacity increase. 
     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.