Abstract:
A method for controlling a torque converter clutch in an automatic transmission of a vehicle that includes an engine, an accelerator pedal and wheels, the converter clutch alternately connecting and disconnecting the engine and transmission when the clutch is engaged and disengaged. The method includes determining that the vehicle is either ascending a grade or operating in a loaded condition, determining that the clutch is engaged, determining whether the vehicle is operating in a curve, preventing disengagement of the clutch if the vehicle is either ascending a grade or operating in a loaded condition and the vehicle is operating in a curve, and allowing disengagement of the clutch if the vehicle is neither ascending a grade nor operating in a loaded condition.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The preferred embodiment relates generally to an automatic transmission for automotive vehicles and, in particular, to controlling the lockup clutch of a torque converter for an automatic transmission. 
         [0003]    2. Description of the Prior Art 
         [0004]    An automotive vehicle includes an internal combustion engine, which converts fuel into rotational energy having torque and speed characteristics, and a powertrain, which transmits rotational energy from the engine to the vehicle&#39;s wheels. A transmission, which produces step changes in speed ratio, includes a hydrodynamic torque converter, which transmits engine torque to an input member of a gearbox. 
         [0005]    The torque converter includes a toroidal chamber containing fluid, a bladed impeller coupled for rotation to the engine crankshaft, a turbine coupled for rotation to the input shaft, a stator for redirecting fluid flow from the turbine to the impeller, and a lock-up or bypass clutch for locking the impeller and turbine such that they rotate at the same speed. 
         [0006]    The torque converter clutch can be controlled to increase the operating efficiency and performance of the powertrain and to reduce the temperature of the fluid in the torque converter. In the absence of extraordinary operating conditions, the converter clutch is engaged when a vehicle transmission is operating in its highest gear. Short term, repeated engagement and disengagement of the converter clutch increases the temperature of the clutch and fluid in the torque converter and produced undesired noise, vibration and harshness in the vehicle&#39;s powertrain issues. 
         [0007]    When a motor vehicle tows a load up a grade with the engine throttle open, i.e., with the throttle position high, the torque converter clutch is engaged to improve efficiency and assist with powertrain cooling. As the vehicle approaches a curve on the grade, the driver would normally tip-out, i.e., reduce displacement of the engine throttle and the demanded engine output torque by releasing the accelerator pedal. This would cause the torque converter clutch to disengage and the transmission control to produce an upshift, i.e., change to a higher gear than the current gear. After the vehicle has exited the curve, the operator would again tip-in, i.e., increase displacement of the engine throttle and the demanded engine output torque by depressing the accelerator pedal, which action would probably cause a down shift and eventually a re-engagement of the torque converter clutch. When the torque converter clutch repeatedly disengages under these operating conditions, heat is generated and the additional gear shifts can have detrimental effects on the vehicle&#39;s noise, vibration and harshness (NVH). 
       SUMMARY OF THE INVENTION 
       [0008]    A method for controlling a torque converter clutch in an automatic transmission of a vehicle applies to a vehicle that includes an engine, an accelerator pedal and wheels, the converter clutch alternately connecting and disconnecting the engine and transmission when the clutch is engaged and disengaged. The method includes determining that the vehicle is either ascending a grade or operating in a loaded condition, determining that the clutch is engaged, determining whether the vehicle is operating in a curve, preventing disengagement of the clutch if the vehicle is either ascending a grade or operating in a loaded condition and the vehicle is operating in a curve, and allowing disengagement of the clutch if the vehicle is neither ascending a grade nor operating in a loaded condition. 
         [0009]    A system for controlling the torque converter clutch includes a torque converter clutch alternately connecting and disconnecting the engine and transmission when the clutch is engaged and disengaged, respectively, an accelerator pedal for at least partially controlling operation of the engine, an accelerator pedal sensor producing a signal representing a position of the accelerator pedal, and a controller in communication with the transmission, the engine, the clutch, and the accelerator pedal sensor, the controller being configured to determine that the vehicle is either ascending a grade or operating in a loaded condition, determine that the clutch is engaged, determine whether the vehicle is operating in a curve, prevent disengagement of the clutch if the vehicle is either ascending a grade or operating in a loaded condition and the vehicle is operating in a curve, and allow disengagement of the clutch if the vehicle 
         [0010]    When the torque converter clutch is engaged because the system detects a high vehicle load or a large grade disengaging the converter clutch or allowing an upshift for a tip-out is delayed. The delay provides time to detect operation of the vehicle on a curve and to activate the shift inhibit control logic. 
         [0011]    Before activating the curve inhibit, this control requires the system either to activate engine braking currently or to determine that the reason for the converter clutch being engaged is because of high vehicle load. 
         [0012]    The control detects operation on a curve using the wheel speed sensors available in vehicles equipped as such. It does not require lateral acceleration speed sensors. The control prevents cycling of the converter clutch between engaged and disengaged states. 
         [0013]    The control avoids NVH issues by inhibiting converter clutch disengagement and back-out upshifts while towing up hills and around curves, condition in which NVH is most critical. This improve drivability and powertrain cooling such that auxiliary cooling packs to cool the transmission fluid while the vehicle is towing a load are not required. 
         [0014]    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 
         [0015]    These and other advantages will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
           [0016]      FIG. 1  is a schematic diagram of a motor vehicle powertrain; 
           [0017]      FIG. 2  is schematic diagram showing sensors, control algorithms executed by the PCM, and powertrain components controlled by the PCM; 
           [0018]      FIG. 3  is a logic flow diagram illustrating a shift scheduling control algorithm; and 
           [0019]      FIG. 4  is a logic flow diagram illustrating a converter clutch scheduling algorithm. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    In  FIG. 1 , the powertrain of vehicle  10  is controlled by a system  12 , which includes a controller or powertrain control module (PCM)  14 , which includes a electronic microprocessor, electronic memory, and communication ports communicating through a controller area network (CAN) with an engine  16  and an automatic transmission  18 . The PCM  14  is connected directly to the engine  16  and the transmission  18 ; however, other configurations are possible. In one such configuration, the engine  16  and the transmission  18  have separate controllers, for example, an engine control module (ECM) and a transmission control module (TCM), which communicate directly with each other. A vehicle system controller (VSC) could also be used to communicate with a TCM and an ECM, for example, on the CAN. Similarly, a controller, such as the PCM  14 , can be used in vehicles having different configurations from the one illustrated in  FIG. 1 , such as hybrid electric vehicles (HEV), and fuel cell vehicles. 
         [0021]    The vehicle  10  also includes a transmission input shaft  20 , which connects the engine crankshaft  17  to the transmission  18 , and a transmission output shaft  22 , which connects the transmission  18  to the vehicle wheels  24 . Collectively, the engine  16 , transmission  18 , transmission converter clutch  50 , and shafts  17 ,  20 ,  22  comprise the powertrain. 
         [0022]    An accelerator pedal  26  and a brake pedal  28  are operated by a vehicle operator to selectively increase and decrease the speed of the vehicle  10 . The accelerator pedal  26  includes an accelerator pedal sensor  30 , which communicates with the PCM  14 . Similarly, the brake pedal  28  includes a first brake pedal sensor, or brake position sensor  32 . 
         [0023]    Accessible to the PCM  14  and stored in the electronic memory are computer programs that control at least two different shift modes. The first shift mode is a normal mode, which may be used when the vehicle  10  is not towing or hauling heavy cargo. The second shift mode, or grade/tow mode, may be used when the vehicle  10  is towing or hauling heavy cargo. The PCM  14  is programmed with a number of shift points for each of the shift modes. The shift points include upshift points for defining when the transmission  18  is allowed to shift to a higher gear, and downshift points for defining when the transmission  18  is allowed to shift to a lower gear. Each shift point is programmed into the PCM  14  and is defined by the vehicle speed and accelerator pedal position. The PCM  14  signals the transmission  18  to shift to a higher or lower gear, when a shift point is reached. 
         [0024]      FIG. 2  schematically illustrates the organization of the PCM  14 . A signal representing the speed of the transmission output shaft  22  and signals produced by sensors  30  and  32 , representing the degree to which the accelerator pedal and brake pedal are depressed, are input at  40 ,  41  to a converter clutch scheduling control algorithm  42  and a gear shift scheduling algorithm  44  located in the PCM  14 . 
         [0025]    Signals produced by sensors, representing the speeds of the wheels  24 ,  25  at each lateral side of the vehicle  10 , are input at  46  to the gear shift scheduling algorithm  44 . Upon executing the gear shift scheduling algorithm  44  using the current wheel speed signals, the PCM  14  determines whether the vehicle is operating in a curve. If the vehicle  10  is operating in a curve, a signal  45  indicating that the curve inhibit control is activated is supplied to the converter clutch scheduling routine  42 . Upon executing the converter clutch scheduling control algorithm  42  a signal  47  representing a request to engage or disengage the converter clutch is sent to a converter clutch control algorithm  48 , which is accessible to the PCM  14 . 
         [0026]    The transmission  18  includes a hydrodynamic torque converter  50 , which transmits engine torque to transmission input shaft  20 . The torque converter  50  includes a toroidal chamber  52  containing fluid, a bladed impeller  54  coupled for rotation to the engine crankshaft  17 , a bladed turbine  56  coupled for rotation to the input shaft  20 , a stator  58  for redirecting fluid flow from the turbine to the impeller, and a lock-up or bypass clutch  60  for mechanically connecting the impeller and turbine so that they rotate at the same speed and disconnecting the impeller and turbine so that they rotate mutually independently. 
         [0027]    The converter clutch control algorithm  48  produces output commands  62 , which cause an actuator of the converter clutch  60  alternately to engage and disengage the converter clutch. When the gear shift scheduling algorithm  44  produces a gear change request  64 , a pressure control algorithm  66  responds to the request by issuing a command signal  68  to clutch and band brake solenoids  70 , which hydraulically actuate respective friction control elements of the transmission  18 , thereby causing upshifts and downshifts in response to the commands  68 . 
         [0028]    The shift scheduling algorithm  44  includes a grade/tow algorithm, which is executed by the PCM  14  when the control system detects a high vehicle load or a large grade. The PCM  14  detects the presence of a high vehicle tow load or operation of the vehicle on a steep grade when, for the current position of the accelerator pedal  26 , the current vehicle acceleration is lower than would be expected if the vehicle were unloaded or not operating on a grade. A look-up table containing magnitudes of vehicle acceleration corresponding to the current gear and accelerator pedal position can be used to determine the expected range of vehicle acceleration, which can then be compared to the current vehicle acceleration to determine whether current vehicle acceleration is abnormally low. 
         [0029]      FIG. 3  illustrates steps of the shift scheduling algorithm  44  used by the PCM  14  to select between normal shift scheduling operation and a curve inhibit operation, which is a control strategy that maintains the converter clutch  60  in the engaged state when operation of the vehicle in a curve is detected. At step  72 , a test is made to determine whether both the converter clutch  60  is in the engaged state and the grade/tow algorithm, is currently activated. If the test at  72  is logically true, a curve inhibit control algorithm is selected at  74  for execution by the PCM  14 . But if test  72  is logically false, at step  76  shift scheduling remains in, or returns to normal operation, and the current execution the shift scheduling algorithm ends at  78 . 
         [0030]    The curve inhibit algorithm preferably determines whether a vehicle is operating in a curve with reference to a difference in speed of the wheels on a given axle, such as wheels  24 ,  25 . The curve inhibit algorithm inhibits the gear shift by monitoring the current lateral acceleration rate by inferring that the vehicle is operating in a roadway curve. This is done by mathematical computations comparing the relative wheel speeds after informing the PCM  14  of known axle track dimensions. The system then decides whether to delay the shift event and converter clutch event based on comparison of current lateral acceleration to a reference vehicle lateral acceleration. 
         [0031]    Lateral acceleration of the vehicle can be calculated using the wheel speeds at a reference axle from the following: 
         [0000]    
       
         
           
             
               Lateral 
                
               
                   
               
                
               Acceleration 
             
             = 
             
               
                 
                   ( 
                   
                     vehicle 
                      
                     
                         
                     
                      
                     speed 
                   
                   ) 
                 
                 2 
               
                
               
                 
                   ( 
                   2 
                   ) 
                 
                 
                   ( 
                   T 
                   ) 
                 
               
                
               
                 
                   ( 
                   
                     
                       wheel 
                        
                       
                           
                       
                        
                       speed 
                        
                       
                           
                       
                        
                       24 
                     
                     - 
                     
                       wheel 
                        
                       
                           
                       
                        
                       speed 
                        
                       
                           
                       
                        
                       25 
                     
                   
                   ) 
                 
                 
                   ( 
                   
                     
                       wheel 
                        
                       
                           
                       
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                       speed 
                        
                       
                           
                       
                        
                       24 
                     
                     + 
                     
                       wheel 
                        
                       
                           
                       
                        
                       speed 
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                       25 
                     
                   
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         [0000]    wherein T is the track of the given axle, and each wheel speed is calculated from (rotational speed)*(rolling circumference of the wheel accounting for tire wear, tire pressure and wheel load). Examples of the magnitude of wheel track for two vehicles are front track 5.1151 and 5.2214 and rear track 5.1768 and 5.2313, respectively. If the vehicle is accelerating laterally, disengagement of the converter clutch  60  is prevented by the curve inhibit algorithm, provided vehicle speed is within a predetermined range, and the current transmission gear is in a predetermined range. 
         [0032]      FIG. 4  illustrates a portion of the converter scheduling algorithm  42  that relates to control of the converter clutch  60  while the vehicle is loaded, ascending a steep grade or in a curve. At step  80 , a test is made to determine whether both the converter clutch  60  is in the engaged state and the grade/tow algorithm, is currently activated. If the test at  80  is logically true, control advances to step  82 . But if test  80  is logically false, indicating that the vehicle is not operating on a grade or loaded, control moves to step  84  where unlocking the torque converter clutch  60  can occur, i.e., a command  62  to maintain clutch  60  engaged is removed, control returns to normal shift scheduling, and the current execution of the algorithm ends at  86 . 
         [0033]    At step  82 , a test is made to determine whether the curve inhibit algorithm is active. If test  82  is logically false, indicating that the vehicle is not operating on a curve, control moves to step  84  where unlocking the torque converter clutch  60  can occur, i.e., a command  62  to maintain clutch  60  engaged is removed, control returns to normal shift scheduling, and the current execution of the algorithm ends at  86 . If the test at  82  is logically true, indicating that the vehicle is not operating in a curve, a test is made at  88  to determine whether a clutch-unlock inhibit timer is expired. 
         [0034]    If the clutch-unlock inhibit timer is expired at  88 , indicating expiration of period of predetermined length since issuing command  62  to inhibit unlocking clutch  60 , closed pedal unlocking of the torque converter clutch  60  is permitted at  90 , and the current execution of the algorithm ends at  86 . Therefore, clutch  60  may be disengaged in response to the vehicle operator tipping out of accelerator pedal  26 . 
         [0035]    If the timer is not expired at  88 , control passes to  92  where closed pedal unlocking of the torque converter clutch  60  is prevented until the clutch-unlock inhibit timer expires and the necessary conditions, shown in  FIG. 4 , are met. The current execution of the algorithm ends at  86 . 
         [0036]    Therefore, when the torque converter clutch  60  is engaged because the system detects a high vehicle load or a large grade, disengaging the converter clutch or allowing an upshift for a tip-out is delayed for a predetermined period. The delay is used to prevent aborting the inhibit mode for short term transient changes in the entry conditions. 
         [0037]    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.