Abstract:
A method for launching an automatic vehicle transmission utilizing dual clutches in place of a torque converter avoids clutch overheating, sudden launch or roll-back by placing a clutch in a slip mode only after the vehicle&#39;s brakes have been actuated and fully disengaging the clutch upon over-heating detection. When launch is initiated, a first gear engaged and the brakes are released, the associated clutch is fully engaged to avoid overheating.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electromechanical automatic transmissions and more particularly to an acceleration launch strategy for an electromechanical automatic transmission. 
     BACKGROUND OF THE INVENTION 
     Electromechanical dual clutch transmissions that include automated electromechanical shifting mechanisms and methods are known in the art. For example, U.S. Pat. Nos. 6,463,821, 6,044,719 and 6,012,561, disclose a dual clutch electromechanical automatic transmission. 
     The dual-clutch transmission does not include a torque converter. Therefore, launch and acceleration is created by closing one of the clutches. Under normal driving conditions, the clutch is not completely engaged. Rather, it is only closed far enough to transfer engine torque without transferring torque peaks. This so called “slip control” approach enables smooth shifting as well as fast disengagement of the clutch. However, issues arise when the vehicle is held stationary on an incline. While a motor vehicle can be held on an incline with a slipping clutch, after some period of time, typically based on incline slope, vehicle weight, and temperature, the slipping clutch will overheat. To prevent overheating, the clutch may be fully engaged or disengaged. If the clutch is disengaged, the vehicle would roll back unexpectedly. If the clutch is fully engaged, the vehicle would suddenly launch. There is a need for a launch acceleration strategy which avoids these issues. 
     SUMMARY OF THE INVENTION 
     Accordingly, a method for launching an automatic vehicle transmission utilizing dual clutches in place of a torque converter recognizes a vehicle transmission launch request, engages a first gear, monitors an activation condition of a vehicle brake, fully engaging a clutch associated with the first gear when the vehicle brake is released to enable the vehicle to accelerate to an idling speed, partially disengaging the clutch to a slip condition whenever the brake is activated and fully disengaging the clutch from the slip condition whenever clutch temperature exceeds a predetermined overheating threshold. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a control system for a dual clutch electromechanical automatic transmission; 
         FIG. 2  is a flow chart illustrating the acceleration launch strategy for a dual clutch transmission according to the principles of the present invention; and 
         FIG. 3  is a graph illustrating the vehicle speed over time during the acceleration launch strategy of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     The present invention pertains to a method for controlling a dual clutch automatic transmission. Although the present invention is applicable to virtually any dual clutch transmission, the method of the preferred embodiment is illustrated with the electromechanical automatic transmission disclosed in commonly assigned U.S. Pat. No. 6,012,561, which is hereby incorporated by reference in its entirety. 
     With reference to  FIG. 1 , a powertrain controller  100  is provided for operating first and second clutch actuators  102 ,  104  and first and second shift actuators  106 ,  108 . The powertrain controller  100  provides signals to the respective driver motors  110 ,  112  of the clutch actuators  102 ,  104  as well as to the respective driver motors  114 ,  116  of the shift actuators  106 ,  108 . The powertrain controller  100  also monitors the position of the clutch actuators  102 ,  104  as well as the shift actuators  106 ,  108  via potentiometers  120 ,  122 ,  124 ,  126 , respectively. Normal and uninterrupted power shifting between gears is accomplished by engaging the desired gear prior to a shift event. The transmission can be in two different gear ratios at once, preferably with only one clutch  102 ,  104  being engaged for transmitting power during normal operation. In order to shift to a new gear ratio, the current driving clutch will be released during normal operation via the corresponding clutch actuator and the released clutch will be engaged via the corresponding clutch actuator. The two clutch actuators perform a quick and smooth shift as directed by the powertrain controller  100  which monitors the speed of the transmission input shafts  128  and  130  via speed sensors  132  and  134 , respectively, as well as the speed of the transmission output shaft  136  via a speed sensor  138 . Alternatively, the controller  100  determines the speed of the input shafts  128  and  130  based upon the known gear ratio and the speed of the driven shaft  136  as detected by sensor  138 . An engine speed sensor  140  is also provided and detects the speed of the flywheel  142 . Based upon the accelerator pedal position as detected by sensor  144 , the vehicle speed, and the current gear ratio, the powertrain controller  100  anticipates the next gear ratio of the next shift and drives the shift actuators  106 ,  108 , accordingly, in order to engage the next gear ratio while the corresponding clutch actuator is in the disengaged position. As a gear is engaged, the corresponding input shaft which is disengaged from the engine output shaft becomes synchronized with the rotational speed of the transmission output shaft  136 . At this time, the clutch which is associated with the current driving input shaft is disengaged and the other clutch is engaged in order to drive the input shaft associated with the selected gear. 
     Referring to  FIGS. 2 and 3 , the method of launching a dual clutch automatic transmission using the acceleration launch strategy  200  will now be described. With regard to the description in  FIG. 2 ,  FIG. 3  graphically illustrates speed of the motor vehicle over time using the acceleration launch strategy  200 . 
     The method  200  begins at step  202  with the motor vehicle in a “launch state”. This “launch state” occurs when the motor vehicle is at rest (i.e., motor vehicle speed equals zero), corresponding to point “A” in  FIG. 3 . 
     At the launch state, the controller  100  selects the first gear at step  204 . The controller  100  then determines if motor vehicle brakes are engaged at step  206 . If the brakes are engaged, there is no danger of the clutches overheating. If, however, the brakes are released, the controller  100  closes the first clutch at step  208 . As the clutch is closed, the motor vehicle will accelerate, indicated by the slope of the line indicated by reference “B” in  FIG. 3 . If the first clutch is already hot, slip time is shorter, therefore there is less energy generated and the motor vehicle may accelerate faster, indicated by lines “D” in  FIG. 3 . 
     The vehicle will accelerate to “idle” speed at step  210 , indicated by reference “C” in  FIG. 19 . “Idle” speed is defined as the engine speed with no throttle when in first gear. By keeping the clutch fully engaged at step  208 , the clutch is prevented from overheating. 
     Idle speed continues until such time as the controller  100  determines that the brakes have been engaged, indicated at decision block  212 . The controller  100  then allows the clutch to slip at step  214 . As a result of the braking, the vehicle speed will decrease, shown by a line indicated by reference “E” in  FIG. 19 . 
     If the clutch overheats during the slip event at decision block  216 , the controller  100  opens the clutch at step  218 . Since the brake is already engaged, there will be no unexpected rollback. The method  200  then repeats if another launch state is detected. 
     Area  302  of the graph of  FIG. 3  indicates a high speed area that would occur if the transmission were equipped with a torque converter. Area  304  indicates roll-back speed that would occur with such a transmission system. 
     The invention has been described with reference to a preferred embodiment for the sake of example only. The scope of the invention is to be determined from an appropriate interpretation of the appended claims.