Abstract:
A method of using a torque converter bypass clutch to launch a vehicle, mitigate transient vibration, and mitigate vehicle natural frequency harshness. The method uses the torque converter when the bypass clutch power capacity is approaching its limit, when the vehicle load is high, or the vehicle is on a grade, where normally the bypass clutch would launch the vehicle.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to controlling the bypass clutch of a torque converter for an automotive vehicle. 
   A conventional automatic transmission includes a torque converter, located in the power path between an engine crankshaft and transmission input shaft. A torque converter includes a bladed impeller wheel driveably connected to the engine crankshaft, a bladed turbine wheel driveably connected to the transmission input shaft, a bladed stator wheel, and a toroidal chamber containing pressurized hydraulic fluid for producing a hydrokinetic connection between the impeller and turbine. The torque converter attenuates torque transients and vibrations, increases torque transmitted to the turbine from the impeller at low speed, and provides a smooth transition during gear ratio changes. Because of slippage between the input and output, the torque converter has a low operating efficiency. 
   Current automotive automatic transmissions use a converter bypass clutch to improve fuel economy primarily at highway vehicle speed. When the bypass clutch is fully engaged, it produces a mechanical drive connection between the impeller and turbine, thereby replacing the hydrokinetic drive connection. When the bypass clutch is fully disengaged, the mechanical drive connection is functionally replaced by the hydrokinetic drive connection. Usually a spring damper arranged in series with the bypass clutch is used to reduce engine torque fluctuation transmitted to the driveline. However, the bypass clutch, damper, control and strategy are usually not optimized to produce maximum fuel economy under city driving conditions. 
   There is a need for the torque converter, its bypass damper, bypass clutch, and control strategy to participate toward improving performance feel during certain transient conditions and to contribute more toward improvement in fuel economy under in city driving conditions. It is preferred that improved fuel economy and performance be realized without employing new automatic transmission architecture, such as the dual wet or dry input clutches used in powershift transmissions to replace and simulate the performance of the torque converter. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the torque converter bypass clutch is used to launch the vehicle, and the bypass clutch is locked or modulating slip during the full city driving cycle. The control strategy is appropriate for truck applications because it has a torque converter available for use in high load conditions. 
   The control is preferably, but not exclusively applied to a torque converter that includes a damper having dual stage springs, a multi-plate clutch actuated by a closed piston and a variable force solenoid. The torque converter control produces improved fuel economy; pleasing performance and feel; and excellent noise, vibration and harshness characteristics. 
   In various embodiments, the control is applicable to participate in vehicle launch, transient events, and lugging, all of which require special attention when operating a vehicle on a typical light duty drive cycle without the torque converter being open. The torque converter, operating under this control strategy can be used during aggressive driving, while pulling heavy loads, or in severe off-road driving conditions. 
   In one embodiment of this invention for controlling the bypass clutch of a torque converter during an event, a first function for determining target clutch slips during the event is defined, and a second function for determining target wheel torques during the event is defined. An updated target clutch slip is determined repetitively from the first function, and an updated target wheel torque is determined repetitively from the second function. The torque capacity of the clutch and the engine output torque are changed such that the current wheel torque becomes aligned more closely with the target wheel torque. 
   In another embodiment of the invention, a first function for determining target clutch slips during the event is defined. An updated target clutch slip is determined repetitively from the function. The torque capacity of the clutch is changed such that the current clutch slip becomes aligned more closely with the target clutch slip. 
   The method improves city drive schedule fuel economy. 
   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 be apparent to those skilled in the art. 

   
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment when considered in the light of the accompanying drawings in which: 
       FIG. 1  is a cross section of a torque converter to which the control strategy can be applied; 
       FIG. 2  is a schematic diagram that shows various sensors and actuators for use with the torque converter control strategy; 
       FIG. 3  is a schematic diagram illustrating steps for controlling the operating states of the bypass clutch during vehicle launch events; 
       FIG. 4  is a schematic diagram illustrating steps for controlling the bypass clutch during a transient events; and 
       FIG. 5  is a schematic diagram illustrating steps for controlling the bypass clutch to avoid powertrain torque transients occurring at modal or resonant frequencies of the vehicle. 
   

   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 bypass clutch  24  located within the torque converter housing  25 . The torque output side of lockup clutch  24  includes a damper  26 , located between the impeller and a turbine shaft, which is the transmission input shaft  28 . The damper  26  may incorporate dual or single-stage compression springs. 
   The damper  26  is directly connected at one end to the turbine  16  and at the other end to input shaft  28 . The bypass clutch  24  is connected between the housing  25  and damper  26 . When clutch  24  is slipping, i.e., there is a speed difference across the clutch, it attenuates transitory torque fluctuations from the crankshaft  14  to input shaft  28 . When the clutch is disengaged, the torque converter can mitigate 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  30  through an axial passage  32  formed in the input shaft  28  and a radial passage  34 . A closed piston  36 , sealed on housing  25  by O-rings  38  and  39 , moves rightward within the cylinder to force the clutch discs 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 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  shows various sensors and actuators that communicate with an engine controller  40  and transmission controller  42 , which communicate mutually via multiplex communication messages. A signal produced by a sensor  46  represents displacement of an accelerator pedal, which is controlled manually by the vehicle operator and is a component of an electronic throttle control (ETC). The time rate of change of displacement of the accelerator pedal  48 , preferably calculated between sampling intervals, is another controller input. A signal representing the selected range of a gear selector or PRNDL, also controlled manually by the vehicle operator, is produced by a sensor  50 . A signal representing the state of the brake pedal, controlled manually by the vehicle operator, is produced by a sensor  52 . 
   Other inputs to the engine controller  40  include signals produced by sensors representing intake mass air flow sensor and other engine operating parameters, from which engine load  54  and engine torque are determined; engine throttle position  56 ; engine coolant temperature  58 ; barometric pressure, accessory load, and engine speed  60 . Other inputs to the transmission controller  42  include signals produced by sensors representing turbine speed  62 ; temperature of the automatic transmission fluid (ATF)  64 ; the magnitude of pressure that actuates the bypass clutch  24  or the corresponding magnitude of electric current supplied to a variable force solenoid that controls a bypass clutch valve  66 ; and vehicle speed (VS)  68 , which is preferably determined from the speed of the transmission output shaft and the gear ratio of the final drive. 
     FIG. 3  illustrates steps for controlling the operating states of the bypass clutch  24  during vehicle launch events. The clutch states include slipping, full engagement, and full disengagement. Vehicle launch is a term indicating the process of accelerating the vehicle from rest or a nearly stopped condition, usually in the lowest forward or reverse gear. 
   A launch event is detected when the following initial conditions are satisfied: the transmission is producing the lowest gear; the PRNDL is in the drive position  70 : the brake pedal is off  71 ; VS is about zero  72 ; the accelerator pedal is displaced less than about one-half of its full travel  73 ; the time rate of change of accelerator pedal displacement is less than a reference rate  74 ; engine coolant temperature  75  is normal ambient or greater; and the temperature of ATF in transmission sump  76  is normal ambient or greater. The viscosity of ATF affects powertrain performance; therefore if ATF temperature is less than about 20° F. the torque converter is opened at step  112 . 
   When the initial conditions are met, the launch control strategy begins at step  78 , where a target wheel torque is determined. Target wheel torque, which is represented graphically by the function  80 , is defined for a vehicle launch event with reference to the position or displacement of the accelerator pedal  46 , and the current length of the period that begins at the start of the vehicle launch event. 
   At step  82 , a target clutch slip is determined. Target clutch slip, which is represented graphically by the function  84 , is defined for a vehicle launch event with reference to the displacement of the accelerator pedal  46 , and the current length of the period that begins at the start of the vehicle launch event. 
   Both clutch slip and engine output torque can be used as modulated variables to control the clutch during a vehicle launch event. An inner control loop for determining the magnitude of current wheel torque and current clutch slip is entered. At step  86 , the magnitude of current supplied to the bypass clutch solenoid is changed to align current clutch slip with the target wheel torque. At step  88 , solenoid current supplied to the clutch solenoid causes clutch apply pressure to actuate piston  36 , located in the cylinder  30  of the servo that actuates clutch  24 . The torque capacity of clutch  24  corresponding to the apply pressure is produced as shown in the graph of function  90 , which relates clutch apply pressure to clutch torque capacity. If engine output torque is to be a modulated variable, at step  91  the engine throttle opening is changed to align current wheel torque with the target wheel torque. 
   At step  92 , transmission input shaft speed is determined from the output of sensor  62 . At step  94 , engine speed (NE) is determined from the output of sensor  60 . At step  96 , engine output torque is determined from engine throttle position  91  and engine speed  94 . 
   At step  98 , the current magnitude of clutch slip is calculated by subtracting transmission input speed  92  from engine speed  94 . Current clutch slip is fed back to step  82 , where the current accelerator pedal position and the current period length of the vehicle launch event are used with function  84  to determine an updated target clutch slip and to determine any change required to the electric current supplied to the clutch solenoid for a change in clutch torque capacity. 
   At step  100 , the gear ratio in which the transmission is currently operating and the constant gear ratio of the final drive are determined. Wheel torque is calculated at step  102 , as the product of the combined gear ratio  100  and engine torque  96 . Wheel torque is fed back to step  78 , where the current accelerator pedal position and the current period length of the vehicle launch event are used with function  80  to determine an updated target wheel torque clutch and to determine any change required to the engine throttle position. Then the control loop is executed again. 
   If current wheel torque  102  is greater than the target wheel torque  78 , slip across the clutch  24  may be reduced by increasing clutch apply pressure. This reduces engine speed and torque, decreases the torque amplification produced by the hydrokinetics of the torque converter, and decreases wheel torque. If current wheel torque  102  is less than the target wheel torque  78 , slip across the clutch  24  may be increased by decreasing clutch apply pressure. This raises engine speed and torque, increases the torque amplification produced by the hydrokinetics of the torque converter, and increases wheel torque. If wheel torque is greater than the target wheel torque, the engine throttle opening may be reduced and the magnitude of engine output torque is reduced. If wheel torque is less than the target wheel torque, the engine throttle opening may be increased, thereby increasing the magnitude of engine output torque. In these ways, clutch slip and engine output torque may be modulated to produce the target wheel torque during a vehicle launch. 
   The control procedure is repeated continually until the vehicle launch event terminates or until a clutch energy condition or a vehicle load condition occurs, as described below. 
   A vehicle load monitor  104  contains a function  106  relating vehicle speed (VS)  68  and time during the vehicle launch. The function  106  includes an expected, acceptable vehicle load line  108  and a range  110  below line  108 , in which the vehicle is heavily loaded or on a grade. When vehicle speed is lower than an expected speed at the same time, the vehicle load status overrides the closed loop and causes control to pass to step  112 , where the torque converter  10  is fully open, i.e., bypass clutch  24  is fully disengaged. 
   A clutch energy monitor  114  contains a clutch energy function  116 , preferably determined empirically by measuring temperature at critical areas of bypass clutch  24  for a range of magnitudes of engine torque and clutch slip during the period while the clutch is slipping to control the vehicle launch. The current magnitude of energy being applied to the clutch while the clutch is slipping is calculated from the current engine torque  96  and the current slip speed  98 . When current clutch energy is greater than the acceptable magnitude defined by function  116 , control passes to step  112 , where the torque converter  10  is fully open and bypass clutch  24  is fully disengaged, thereby discontinuing the supply of friction energy to the clutch. 
   The clutch  24  should be fully engaged or modulating to a desired slip speed after the transmission completes an upshift to second gear. If the vehicle is equipped with deceleration fuel shut-off capability, clutch  24  is fully engaged or modulating slip during a deceleration event to avoid stalling the engine. 
   Refer now to  FIG. 4 , where a strategy for controlling bypass clutch  24  and torque converter  10  during transient events is illustrated. A transient event is detected when any of the following initial conditions is satisfied: the status of the brake pedal is changed  120  between on and off states; the time rate of change of positive or negative accelerator pedal displacement is greater than a reference rate  122  indicating a tip-in or tip-out; an upshift or downshift between transmission gears has been commanded or is underway  124 ; or a driveline torque reversal is about to occur  126 . A torque reversal is a change between a positive torque condition, wherein torque is transmitted from the engine through the driveline to the driven vehicle wheels, and a negative torque condition, wherein torque is transmitted from the vehicle wheels through the driveline to the engine. 
   When any of these or other transients is detected, control passes to step  128 , where a target clutch slip is determined from the defined function  130 ,  132 ,  134  that corresponds to the detected transient. For example, function  130  applies to a gear shift event and shows the variation of target slip over time since the transient began, the maximum slip being about 10 rpm. Function  132  applies to a torque reversal event and shows the variation of target slip over time since that transient began. Function  134  applies to a tip-in event and shows the variation of target slip over time since the transient began, the maximum slip rising rapidly to about 50-100 rpm and declining exponentially thereafter. 
   Clutch slip and engine output torque can be modulated to produce the target clutch slip during a vehicle transient event. If clutch torque is to be a modulated variable, control passes to step  136 , where the magnitude of electric current supplied to the bypass clutch solenoid is set such that clutch actuating pressure and the torque capacity of the clutch cause the current clutch slip to become aligned with the target clutch slip. At step  138 , solenoid current is converted to the magnitude of apply pressure at clutch  24 , and the magnitude of torque capacity of the clutch is determined from function  90  of  FIG. 3 , which relates clutch apply pressure to clutch torque capacity. 
   If engine output torque is to be a modulated variable, as it would be for a torque reversal transient  126 , at step  140  engine output torque is ramped down to reduce the characteristic harshness called “clunk” that is associated with driveline lash and a torque reversal. At step  142 , transmission input shaft speed is determined from the output of sensor  62 . At step  144 , engine speed is determined from the output of sensor  60 . Engine output torque is determined at step  146  from the engine throttle position  140  and engine speed  144 . At step  148 , the current magnitude of clutch slip is calculated by subtracting transmission input speed  142  from engine speed  144 . These data are fed back to step  128 , where they are used with the appropriate function  130 ,  132 ,  134  to update the target clutch slip and to determine any required change to the clutch torque capacity and engine throttle position. Then the control loop is executed again. The control procedure is repeated continually until the transient event terminates or until a clutch energy condition occurs, as described below. 
   Clutch energy monitor  150  contains a clutch energy function  152 , preferably determined empirically by measuring temperature at critical areas of bypass clutch  24  for a range of magnitudes of engine torque, clutch slip and the length of the period during which energy is supplied to the clutch  24 . The magnitude of energy being applied to the clutch is determined from function  152  using independent variables time since beginning the transient control and current clutch slip. When current clutch energy is greater than the acceptable magnitude of clutch energy defined by function  152 , control passes to step  154 , where the torque converter  10  is fully open and bypass clutch  24  is fully disengaged. 
   If current clutch energy is less than the magnitude defined by function  152 , the current slip speed  148  is fed back to step  128 , where an updated target clutch slip is determined. The transient control strategy then minimizes clutch slip error by either increasing clutch apply pressure to reduce current clutch slip to the target slip, by reducing clutch apply pressure to increase current clutch slip to the target slip, or by modulating engine output torque to reduce engine throttle position, as discussed above for a torque reversal transient. 
   Refer now to  FIG. 5 , where a strategy for controlling bypass clutch  24  and torque converter  10  to avoid engine torque fluctuation (firing frequency) forcing functions occurring at or near known resonant frequencies of the vehicle is illustrated. Such events, as perceived by the vehicle passengers are called “boom” or “moan” during lugging operation. Lugging refers to powertrain operation at low engine speed, high transmission gears, and high engine load. The lugging event control is initiated when the following initial conditions are present: engine speed is low  160  causing the engine ignition firing frequency  162  of an gasoline engine or combustion frequency of a diesel engine to be determined with reference to the engine speed produced by sensor  60  and the number of currently operating cylinders; engine load is high  164 ; and the transmission is operating in a mid to high range gear  166 , e.g. in third through sixth gear of a six-speed transmission. Each vehicle type  168  will have had a natural frequency/mode map  170  defined and available to the controller. These data are used at step  172 , where a defined target lugging slip modulation function  174  is used to determine a target clutch slip. The target clutch slip function  174  defines peak amplitudes, which occur over the range of the engine firing frequency at the frequencies corresponding to the vehicle natural frequency/mode map  170 . Function  174  illustrates that the target slip amplitude increases with an increasing magnitude of engine load. 
   When lugging control begins, control passes to step  172 , where the target clutch slip for the current firing frequency  162  and current engine load  54  are determined from frequency map or function  174 . At step  176 , the magnitude of electric current supplied to bypass clutch solenoid is set such that the clutch-apply pressure and the corresponding clutch torque capacity produce the target clutch slip, as defined by function  90 . 
   At step  178 , transmission input shaft speed is determined from the output of sensor  62 . At step  180 , engine speed is determined from the output of sensor  60 . At step  182 , the current magnitude of clutch slip is calculated by subtracting transmission input speed  178  from engine speed  180 . 
   At step  184 , the engine throttle position is determined from the output of sensor  56 . Engine output torque is determined at step  186  from engine throttle position  182  and engine speed  180 . 
   Clutch energy monitor contains a clutch energy function  190 , preferably determined empirically by measuring temperature at critical areas of bypass clutch  24  for a range of magnitudes of engine torque and clutch slip. The magnitude of energy currently being applied to clutch  24  is determined at step  188  for the current engine torque  186  and current clutch slip  182  and compared the clutch energy defined by function  190 . 
   If the magnitude of energy applied to the clutch during the lugging control becomes greater than the acceptable magnitude of energy defined by function  190 , control passes either to step  192 , where the torque converter  10  is fully open and bypass clutch  24  is fully disengaged, or preferably to step  194 , where a shift to another gear occurs. 
   If the magnitude of energy applied to the clutch during the lugging control is less than the acceptable magnitude of energy defined by function  190 , current clutch slip  182  is fed back to step  172 , where target clutch slip is updated and any change required to clutch torque capacity to align current clutch slip with the updated clutch slip is determined. The control strategy then minimizes clutch slip error by either increasing clutch apply pressure to reduce current clutch slip to the target slip, or by reducing clutch apply pressure to increase current clutch slip to the target slip. 
   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.