Abstract:
A method for controlling the gear release in an automated multi-speed drive in a vehicle drive with a drive motor and a separating clutch positioned between the drive motor and the multi-speed drive that the events “application of a shifting power” and “opening of the separating clutch” are determined dependent on reaching certain vehicle accelerations (a gear     —     release , a clutch     —     engage ). The vehicle acceleration (a veh ) decreases at the beginning of the shifting process due to the reduction of the fuel injection quantity (EM) supplied to the drive motor.

Description:
This application is a national stage completion of PCT/EP02/07200 filed Jun. 29. 2002 which claims priority from German Application Serial No.101 32 738.2 dated Jul. 5, 2001. 

   FIELD OF THE INVENTION 
   The invention concerns a method for controlling the gear release in an automated multi-speed drive for motor vehicles. 
   BACKGROUND OF THE INVENTION 
   A system-dependent tractive force interruption takes place when shifting gears in automatic multi-speed drives. Normally the following processes take place when shifting a gear:
         reduction of the torque transferred from the drive motor   engaging of the separating clutch   release of the old gear   engagement of the new gear   disengaging of the separating clutch   increase of the transferred torque by an amount that corresponds to the appropriate pedal position.       

   Shifting should take place quickly especially in the case of high tractive power requirements, such as in inclinations in order to avoid a slow-down of the vehicle during shifting. In addition, comfort criteria must be maintained and the load of the shift elements must remain within certain limits. Depending on the driving situation, a different time sequence is necessary for the process steps, so that the shift period, comfort, and load criteria can be maintained. 
   DE 198 02 736 A1 discloses a method where the speed of the torque decrease at the start of the shifting process and of the torque increase at the end of the shifting process is determined depending on the vehicle acceleration and the gas pedal angle. According to this known method, the clutch is engaged and the gear is released at the end of the ramp with which the torque is reduced. In certain driving situations, the timing for engaging the clutch is too early or too late compared with an optimal shifting operation. 
   The present invention is based on the task of developing a method for controlling the gear release where the time sequence of the process steps while releasing the gear can be optimized in a simple fashion. 
   This problem is solved according to the inventive method. Accordingly, the steps—application of a shift load for decoupling the shift elements and opening the separating clutch—are triggered at that point when the vehicle acceleration, which is continuously determined at each program step, reaches an assigned vehicle acceleration value. During the start of the shifting process the current initial acceleration of the vehicle is determined and stored in the memory of the electronic control system. Then the torque of the drive motor is continuously reduced by lowering the injected fuel quantity supplied by the fuel injection pump. At the beginning of the shift, a first and second vehicle acceleration value is determined dependent on the starting acceleration of the vehicle, the injected fuel quantity that is supplied to the drive motor and the engaged gear. The current vehicle acceleration is constantly compared with the first and the second vehicle acceleration values. As soon as the current vehicle acceleration reaches or falls below the first vehicle acceleration value, the actuator is applied with a shift power for the shift elements to be decoupled, so that consequently the gear is released. 
   SUMMARY OF THE INVENTION 
   The current vehicle acceleration is also constantly compared with the second vehicle acceleration value. As soon as it reaches or falls below the second vehicle acceleration value, the clutch actuator will engage the clutch. This can take place after the application of the shift power to the shift elements that are to be decoupled, especially with high starting acceleration values. As the first and the second vehicle acceleration values are determined independently from each other, it is also conceivable that the value reaches or falls below the second vehicle acceleration value first. In this case, however, the shift power is applied also to the actuator for the shift elements that are to be decoupled at the latest now because the transferred torque in the drive is so small that the shift power applied by the actuator is sufficient for decoupling the shift power elements. 
   The determined vehicle acceleration which, for example, can be measured or calculated by deducing a speed value, is a very good indicator for the actual torque that is transferred by the drive at any moment. As the state of the vehicle is extremely transient during shifting other methods that depend on the motor load often deliver results that are offset in time. The first and second vehicle acceleration values are dependent on the fuel injection quantity supplied to the drive motor and on the engaged gear or on the gear range in maps that had been stored in the memory of the electronic control system, which is accessed with each program run. 
   In an advantageous design of the invention, the torque transferred from the drive motor is reduced with a speed that depends on the starting acceleration of the vehicle and on the applied gas pedal position at the start of the shifting process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  shows a flow chart of the method according to the invention; and 
       FIG. 2  shows the time sequence of the gear release. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A method step  2  presents the start of the shift at the time t 0 , showing a status just before the shifting process. At method step  4  at the time t 0 , the current initial vehicle acceleration a start  is determined at the start of the shifting process. It is the same as the current vehicle acceleration a veh  (t 0 ). In method steps  6  and  8 , the two vehicle acceleration values a clutch     —     engage  and a clutch     —     release  are determined. The values for these vehicle accelerations depend on the initial vehicle acceleration a start , the fuel injection quantity EM that is supplied to the drive motor and the engaged actual gear in the maps of the electronic control system. The two vehicle acceleration values a clutch     —     engage  and a clutch     —     release  are stored in the electronic control system and are subsequently constantly—that is at every program run—compared with the current vehicle acceleration a veh . The current vehicle acceleration then decreases continuously due to the reduction of torque by the drive motor. The speed dM/dt with which the torque that is supplied by the drive motor is reduced is a function of the initial vehicle acceleration a start , and the gas pedal position FP (see program step  10 ). During the following program step  12 , the current vehicle acceleration a veh  is compared with the vehicle acceleration value a gear     —     release . If the current vehicle acceleration a veh  is smaller than or equal to the vehicle acceleration value a gear     —     release  a command is triggered in program step  14  causing the shift elements that are to be decoupled to be supplied with the shift power by the actuator. As soon as the torque on the shift elements is small enough, the shift elements to be decoupled are released due to the applied shift power and the gear is released. The program step  16  will run a cycle until the current vehicle acceleration value a veh  drops also below the second vehicle acceleration value a clutch     —     release  until the separating clutch is engaged in program step  18 , ending the gear release in program step  20 . 
   As long as none of the two vehicle acceleration values a gear     —     release  and a clutch     —     engage  is reached or falls below it, the scanning is repeated in a loop in program steps  12  and  22 . If the current vehicle acceleration value a veh  drops below the vehicle acceleration value a clutch     —     engage  the separating clutch is opened on program step  24  and immediately thereafter the actuator is applied with a shift power for the shift elements to be decoupled, which ends the function. 
     FIG. 2  shows the time sequence of the method steps according to the invention. The chronological course of the fuel injected quantity EM is shown in the upper part of the diagram. The reduction of the fuel injected quantity starts after the shift command at the time t 0  causing the torque reduction of the drive motor. The increase at which the fuel injected quantity is reduced during the time period t 0  to t 1  is a measure for the speed dM/dt of the torque reduction by the drive motor. This speed depends on the initial vehicle acceleration a start  and the gas pedal position FP at the time of t 0 . The speed of the reduction in torque by the drive motor is greater the greater the initial vehicle acceleration and the greater the initial gas pedal position is. 
   The course of the vehicle acceleration a veh  is shown in the second partial diagram. The current vehicle acceleration at the time t 0  is the initial vehicle acceleration a start , which is used for determining the vehicle acceleration values a gear     —     release  and a clutch     —     engage . The current vehicle acceleration a veh  is reduced as a consequence to the reduction in the fuel injected quantity at a time delay. The time delay results partially from the time-delayed reaction of the drive motor. In addition, there are several components in the drive train, especially the long universal joint shafts that deform elastically under load. The stored energy herein also decreases only slowly with the torque reduction by the drive motor. 
   The current vehicle acceleration a veh  reaches the vehicle acceleration value a gear     —     release  at the time t 2 . It can be seen in the bottom partial diagram of  FIG. 2  that the shift elements to be decoupled are applied with a shift power at this time. 
   The current vehicle acceleration a veh  reaches the vehicle acceleration value a clutch     —     engage  at the time t 3 . It can be seen in the third partial diagram of  FIG. 2  that the clutch plate starts to engage the clutch at that time. The clutch is finally engaged at the time t 4 . At the latest at that point in time, the shift elements do not transfer any torque to the shift elements to be decoupled causing them to be decoupled with a low shift power. The decoupling of the shift elements can, however, take place already before engaging the clutch if the torque has an appropriate small value due to the reduction of the fuel injected quantity. 
   According to the invention, the events “application of the shift elements to be decoupled with a shift power” and “opening of the clutch” do not always take place in the same given sequence. The vehicle acceleration values that trigger those events are determined independently from each other at the beginning of the shifting process. The coordination is significantly simplified as the values are determined at first independently from each other for different driving situations.