Abstract:
Within the scope of a load reduction method of automatic transmissions or clutches of a motor vehicle during emergency program shifting operations, an engine torque reduction is carried out during the emergency program shifting operation from a normal shifting program to an emergency program (NP).

Description:
FIELD OF THE INVENTION 
   This invention relates to a load reducing method for automatic transmissions during emergency program shifting operations. 
   BACKGROUND OF THE INVENTION 
   In the case of an operating error or failure of the transmission, emergency program shifting operations are carried out in order to change over the transmission to fail-safe state in which further travel with emergency running properties is possible. 
   Vehicles with high powered motorization and particularly shifting elements thereof become considerably loaded during an emergency shifting operation. The clutches concerned thus reach their load limits already after a few emergency program shifting operations under full load at high rotational speed of the engine, and this negatively affects the service life and accordingly the durability. 
   Constructional measures, such as enlargement of the clutch or increase of the number of discs for reducing the thermal load of the clutch that generates, are not adequate due to the installation space required. 
   An optimization of the lubrication or cooling or an increase in the amount of lubricant would not completely solve the problem. Besides, such a procedure would negatively affect the lubrication conditions in the rear-mounted unit of the transmission. In addition an extensive permanent testing would be needed on the test bench, which is associated with considerable expense and is time consuming. 
   For the above reasons, the use of thermally stabler clutch linings also is not the optimum solution. 
   The problem on which this invention is based is to indicate a method for load reduction of transmissions or clutches during emergency program shifting operations, which method can be economically and reliably applied without further mechanical constructional measures. 
   SUMMARY OF THE INVENTION 
   It is proposed to obtain a direct reduction of the clutch load during emergency shifting operations by reducing the engine torque. 
   The engine torque can be implemented both in an electrical and in a mechanical emergency running during emergency program shifting operations provided by the inventive method. 
   In an electrical emergency running which, according to the prior art, is triggered by diagnostic functions, there have hitherto been disconnected only the outputs of the electronic transmission control (EGS) which concern the hydraulic actuators such as magnetic valves and pressure regulators, the electronic transmission control being further activated. The emergency program shifting operation then takes place strictly hydraulically. 
   According to the invention, it is, on the contrary, provided to implement an engine torque reduction by a corresponding requirement of the electronic transmission control or a standard about the controller area network bus (CAN-BUS) on the engine control. The engine engagement or the engine torque reduction is carried out via functions already existing in the engine control, such as firing angle adjustment within the scope of an engine engagement and/or injection scattering in the engine control during the time of the emergency program shifting operation. 
   Within the scope of a variant of the invention, it is proposed to carry out the engine torque reduction according to functionality requirements and comfort considerations depending on the operating conditions at the moment of the emergency program shifting operation. Such operating conditions can be, for example, the gear introduced, load and rotational speed of the engine. 
   In the case of a mechanical transmission emergency running (EGS-not operative) and/or a CAN bus interruption, the reduction of the engine torque is carried out during the emergency program shifting operation via an additional functionality in the engine control. 
   The torque load of the transmission, during emergency program shifting operations, can be immediately and significantly reduced by the above described method. The clutches can thus be protected and a longer duration is ensured. 
   The method further proves to be economical and simple inasmuch as very expensive and lengthy mechanical optimizations are not needed. 

   
     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  is the function sequence according to the invention with reference to the following time diagrams:
         a) a state time diagram of the emergency program shifting operation,   b) a rotational speed time diagram,   c) a clutch pressure time diagram and   d) a state time diagram of the engine engagement;       
       FIG. 2  is a block shift pattern for an electrical emergency running according to the invention; and 
       FIG. 3  is a block shift pattern for a mechanical emergency running according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For the case of an electrical emergency running the inventive method functions as follows: 
   The occurrence of an error is diagnosed and detected at the moment t_ 1  by the electronic transmission control and then the emergency program NP is activated, as can be seen from the above diagram in  FIG. 1 , which shows a state time diagram of the emergency program shifting operation. According to the invention, at the moment t_ 1  an engine engagement ME is also introduced in order to reduce the engine torque T_mot. This is illustrated in the rotational speed time diagram in  FIG. 1  (second diagram from top): the engine rotational speed n_mot is reduced beginning at t_ 1  in direction to the synchronization point of the emergency program speed. 
   According to the clutch pressure time diagram from  FIG. 1 , the shifting clutches concerned are, at the same time, hydraulically activated (disconnecting clutch shown in dotted line, engaging clutch in solid line). The bottom diagram in  FIG. 1  is a state time diagram for the engine engagement ME; the state  1  corresponds to an activation of the engine engagement ME and is effected at a moment t_ 1 . 
   The intensity of the engine engagement ME is here function of the operating conditions at the moment t_ 1  of the emergency program shifting operation, preferably of the engine rotational speed n_mot and/or of the shifting interval and/or of the engine torque T_mot. 
   The electronic transmission control delivers the nominal value standard for the engine torque T_mot and a time duration t_ME for the engine engagement ME which is triggered during the emergency program shifting operation. 
   The synchronization point of the emergency program sequence is reached at the moment t_ 2 , the engine rotational speed n_mot (rotational speed time diagram) reaching here a minimum value in order subsequently to rise again. 
   At the moment t_ 3 , the main pressure on the emergency gear clutch is reached (see the clutch pressure time diagram); the engine engagement ME is then terminated at the moment t_ 4  (“ME=0”). 
     FIG. 2  is shown a block gear shift-diagram of the inventive method for the case of an emergency running during which the electronic or electrohydraulic control of the transmission still is operative and likewise the communication of the electronic transmission control unit to the electronic engine control unit is in variably intact. After activation of the emergency program NP by the electronic transmission control, the electrical emergency running is stated and the time period t_ME is established the same as the intensity of the engine engagement ME. In the next step, the engine torque T_mot is reduced, via the engine engagement ME, for the time period t_ME in order then to change over to the emergency running operation. 
   The sequence of the method for the case of a mechanical emergency running or of a failure of the CAN bus is made clear in FIG.  3 . Contrary to the above described electrical emergency running, the functions are assumed by the electronic,engine control, since the electronic transmission control can no longer perform any functions: when the electronic transmission control (EGS) or the CAN bus are inoperative, that is, when the engine control receives no signals from the EGS, the emergency program NP is activated and the time period t_ME and intensity of the engine engagement ME as function of the operating parameters are determined by the electronic engine control. The engine torque T_mot is then reduced, via the engine engagement ME, for the time period t_ME and changed over to the emergency running operation. 
   The inventive method offers an active clutch protection combined with an increased shifting comfort and ensures a mechanical protection of the transmission during every shifting transition from normal operation with a normal shifting program to an emergency running program. 
   Furthermore, the method is simple and can be implemented without added input variables or operands, it being possible to divide the implementation of the operation between the transmission control and the engine control. 
   
     
       
             
           
             
             
           
         
             
                 
             
             
               References 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               NP 
               emergency program 
             
             
               n_mot 
               engine rotational speed 
             
             
               ME 
               engine engagement 
             
             
               T_mot 
               engine torque 
             
             
               P_kuppl 
               clutch pressure 
             
             
               t_1 
               moment of activation of the emergency program 
             
             
               t_2 
               moment of reaching the synchronization point of the 
             
             
                 
               emergency shifting operation 
             
             
               t_3 
               moment of reaching the main pressure on the emergency gear 
             
             
                 
               clutch 
             
             
               t_4 
               moment of termination of the engine engagement 
             
             
               t_ME 
               duration in time of the engine engagement