Abstract:
A device having a monitoring circuit for a knock control and a control circuit which initiates substitute measures in the event of a fault in the knock control, depending on the output signal of the monitoring circuit, is described. The control unit is designed, for example, so that the number of fuel injection operations per operating cycle of a cylinder of the engine is increased as the substitute measure.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method in which a firing angle and/or an ignition time of a gasoline or diesel engine having direct injection is regulated with the help of a knock control for preventing knocking during engine operation. In fault-free operation of a knock control, fuel is injected once per operating cycle into the combustion chamber of a cylinder of the engine. In addition, combustion is monitored by the knock control. 
     BACKGROUND INFORMATION 
     A known measure in the event of failure of knock control of gasoline engines is a safety retardation of the firing angle relative to the crankshaft position, the cylinder standing at 0° (crank angle) at top dead center, TDC. The firing angle is typically retarded by 12° to 15° (crank angle). At such a firing angle, engine knocking does not occur. However, one disadvantage of this is the loss of engine sufficient due to the safety retardation. 
     The problem on which the present invention is based is to provide a simple method of preventing knocking in the event of failure of a knock control, so that it will also permit operation of the engine at a high efficiency even in the event of failure of the knock control. In addition, a device is also to be provided for this method. 
     It is already known from Japanese Patent Application No. 56 066,426 to increase the quantity of fuel injected as a knock-reducing measure when a knock sensor fails. 
     SUMMARY OF THE INVENTION 
     The relationships in direct fuel injection are utilized for the present invention. Due to the design of the injection system, fuel is atomized in an especially fine spray. Different rotational movements of air layers are achieved due to the air flow created in the intake and compression strokes. There are different mixing ratios in the combustion chamber due to the injection of fuel in the intake stroke as well as the compression stroke. Around the spark plug there is a rich fuel-air mixture surrounded by lean layers at the edge of the combustion chamber. Lean layers, i.e., layers in which the mixing ratio of fuel to air in kg is less than 1:14.8, are not as flammable as layers in which the fuel mixture is rich. A fundamental cause of knocking is spontaneous ignition in boundary areas occurring in addition to the main ignition. 
     The present invention is based on the finding that whereas the knocking tendency of direct injection engines is already reduced, it may be further decreased by multiple injection and/or a different injection quantity during a operating cycle of the engine. For example, in the case of a gasoline engine, there is a single injection during the intake stroke and the compression stroke or the injection quantity is increased. This measure yields a very large gradient of the mixing ratio from the spark plug area to the edge of the combustion chamber. This gradient counteracts engine knocking. Since knocking is already limited in this way, it is no longer necessary to make a safety retardation or at any rate a smaller angle may be used for the safety retardation. A considerable loss of engine efficiency may be prevented by the method according to the present invention, because the firing angle need not be retarded by the usual large crank angle values of 12° to 15°. 
     In a refinement of the method according to the present invention, the firing angle is controlled as a function of the engine rotational speed and/or engine load after the change in injection quantity. The engine rotational speed and engine load are the essential characteristic variables which influence the firing angle. However other characteristic variables may also be taken into account to improve the control. 
     The present invention also relates to a device for preventing knocking in the event of failure of a knock control. This device makes it possible to implement the method steps of the method according to the present invention and it includes a monitoring circuit for a knock control and a control unit which initiate substitute measures to prevent knocking in the event of a fault in the knock control, depending on the output signal of the monitoring circuit. The control unit is designed so that the substitute measure in the case of a gasoline engine involves, for example, a transition from a single injection of fuel per operating cycle of a cylinder of the engine to two injections of fuel per operating cycle. The technical effects and considerations described above for the method according to the present invention also apply to the device according to the present invention. 
     In a next refinement, the control unit controls the firing angle as a function of the engine rotational speed and/or engine load. Thus the essential characteristic variables which are crucial for the adjustment of the firing angle are also taken into account with appropriate measure. Although regulation in a closed-loop control circuit is no longer possible, control is performed nevertheless to be able to adjust an acceptable firing angle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of part of a control circuit for activating multiple injection in the event of failure of a knock control. 
     FIG. 2 shows a block diagram of another part of the control circuit. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a block diagram of part of a control circuit  10  for activating multiple injection in the event of failure of a knock control of a direct injection gasoline engine (not shown). Control circuit  10  contains an AND logic element  12  at whose one input an activation signal kr is applied and at whose other input a fault signal f is applied. Activation signal kr and fault signal f are generated by a controller of the firing system. Activation signal kr has the switch state logic ONE when the knock control is to be active. Fault signal f has the switch state logic ONE when there is a disturbance in the knock control. AND logic element  12  gates the values of the activation signal and the fault signal according to the logic AND function and outputs a request signal hkss whose signal value of logic ONE causes switching over from single injection to double injection. The letter sequence hkss denotes homogeneous anti-knock safety setpoint. 
     The method steps and components required for switching over the type of injection are not the object of the present invention and therefore will not be explained in greater detail. For example, the position of the throttle valve may be altered in switching over. In addition, the injection nozzles are actuated according to another scheme. 
     If the switching over is performed successfully, the signal value of a control signal hks (homogeneous anti-knock safety protection) is switched to logic ONE. However, if the switching over cannot be performed, the signal value of control signal hks remains at the value of logic ZERO. For example, this is the case when the fault causing the failure of the knock control also prevents the system from switching over to double injection. 
     FIG. 2 shows the other part of control circuit  10 . Control circuit  10  contains another AND logic element  20  at whose one input is applied fault signal f and control signal hks at its other input. AND logic element  20  gates the signal values applied to its inputs according to the logic AND function. The output of AND logic element  20  is connected to the input of a NOT element  22  which outputs at its output a signal value inverted relative to the signal value applied at its input. The output of NOT element  22  is connected to the switching over input of a switching unit  24 . Depending on the signal at the switching input, switching unit  24  switches between two switch states  0  and  1 . 
     Control circuit  10  also contains an engine characteristics map unit  26  in which are stored various ignition characteristics maps for the double injection mode of operation. Engine characteristics map unit  26  has a plurality of inputs, of which FIG. 2 shows inputs for inputting an rotational speed signal nmot and a load signal rl (relative air filling). Engine characteristics map unit  26  reads a value for the firing angle out of a memory unit (not shown), depending on the signal values of the rotational speed signal nmot and load signal rl, and outputs an engine characteristics map value kfw. Engine characteristics map unit  26  may be either an analog or digital unit. Then rotational speed signal nmot, load signal rl, and engine characteristics map value kfw are either analog or digital signals accordingly. 
     Engine characteristics map value kfw is applied to the one input of switching unit  24  and is output at the output of switching unit  24  in switch state  0  of switching unit  24  (see output signal  28 ). The other input of switching unit  24  is connected to output signal sv of a knock control unit (not shown) for retardation, adjusting the firing angle in a regulating operation. Output signal sv goes to the output of switching unit  24  in switch state  1  of switching unit  24 . 
     Control circuit  10  contains another switching unit  30  at whose switching input activation signal kr is applied. Depending on the signal value of activation signal kr, switching unit  30  operates in two switch states  0  and  1 . Signal value of logic ZERO is constantly applied at the one input of switching unit  30 . If control signal kr has the value logic ZERO, then in switch state  0  of switching unit  30 , value ZERO applied at the input is output at the output of the switching unit and is used as input signal dwkrz for a firing angle setpoint unit  32 . 
     The other input of switching unit  30  is connected to the output of switching unit  24 , so that in switch state  1  of switching unit  30 , output signal  28  stipulates the course of input signal dwkrz. 
     Firing angle setpoint unit  32  also has an input for a cylinder counting signal zzyl whose signal value indicates the cylinder in whose combustion chamber an ignition is to be executed. Firing angle setpoint unit  32  outputs a firing angle signal  34  which specifies the firing angle for all cylinders of the engine in succession. 
     The part of control circuit  10  shown in FIG. 2 operates as follows with fully functional knock control. Fault signal f and control signal hks have the signal value of logic ZERO. A logic signal  36  at the output of AND logic element  20  therefore also has the signal value of logic ZERO. A switching signal  38  at the output of NOT element  22  has the signal value of logic ONE because of the inversion of logic signal  36 , so that switching unit  24  is switched to switch state  1 . Output signal sv output by anti-knock regulating unit stipulates the course of output signal  28 . If activation signal kr has a value of logic ZERO, no knocking is detected and no knock control is necessary. In this case switching unit  30  has switch state  0  so that input signal dwkrz has the value of logic ZERO. Firing angle setpoint unit  32  outputs a firing angle signal  34 , which is not corrected with regard to knock control. 
     However, if activation signal kr has the value of logic ONE in the case of fully functional knock control because knocking is detected, then switching unit  30  operates in switch state  1 . In switch state  1 , output signal  28  of switching unit  24  stipulates the course of input signal dwkrz. Firing angle setpoint unit  32  therefore outputs a firing angle signal  34  which is corrected with the help of the firing angle specified by the knock control to counteract engine knocking. 
     If there is a disturbance in the function of the knock control, then fault signal f first has a value of logic ONE and control signal hks has a value of logic ZERO. Logic signal  36  therefore continues to have a value of logic ZERO. The operation of control unit  10  corresponds to the operation described above. However, if the signal value of control signal hks is switched to the value ONE on the basis of the processes illustrated above in FIG. 1, then the value of logic signal  36  changes to the value of logic ONE. Control signal hks has the value of logic ONE as soon as the system switches to double injection. The signal change in logic signal  36  results in a change in the signal value of switching signal  38 . Switching signal  38  then has the value of logic ZERO so that switching unit  24  is switched to switch state  0 . Engine characteristics map value kfw which depends on the current engine rotational speed (see rotational speed signal nmot) and on the current engine load (see load signal rl) then stipulates the value of output signal  28 . If no knocking of the engine is detected, activation signal kr has the value of logic ZERO and the firing angle setpoint unit does not perform a correction of the firing angle with regard to knock control. However, if knocking of the engine is detected, the activation signal kr has the value of logic ONE. Switching unit  30  operates in switch state  1  and the course of output signal  28  stipulates the course of input signal dwkrz. Firing angle setpoint unit  32  corrects the firing angle so that knocking is counteracted. The engine characteristics map stored in engine characteristics map unit  26  for the double injection is used for correction. 
     If engine knocking no longer occurs, then activation signal kr again has the value of logic ZERO and switching unit  30  switches back to switch state  0 . Fault signal f and control signal hks remain at the value of logic ONE, however. 
     LIST OF REFERENCE NOTATION 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 0, 1 
                 switch state 
               
               
                   
                 10 
                 control circuit 
               
               
                   
                 12 
                 AND logic element 
               
               
                   
                 kr 
                 activation signal 
               
               
                   
                 f 
                 fault signal 
               
               
                   
                 hkss 
                 request signal 
               
               
                   
                 hks 
                 control signal 
               
               
                   
                 20 
                 AND logic element 
               
               
                   
                 22 
                 NOT element 
               
               
                   
                 24 
                 switching unit 
               
               
                   
                 26 
                 engine characteristics map unit 
               
               
                   
                 nmot 
                 rotational speed signal 
               
               
                   
                 rl 
                 load signal 
               
               
                   
                 kfw 
                 engine characteristics map value 
               
               
                   
                 28 
                 output signal 
               
               
                   
                 sv 
                 output signal 
               
               
                   
                 30 
                 switching unit 
               
               
                   
                 dwkrz 
                 input signal 
               
               
                   
                 32 
                 firing angle setpoint unit 
               
               
                   
                 zzyl 
                 cylinder counting signal 
               
               
                   
                 34 
                 firing angle signal 
               
               
                   
                 36 
                 logic signal 
               
               
                   
                 38 
                 switching signal