Abstract:
The invention relates to a control and regulation method for an internal combustion engine having a common rail system, wherein a rail pressure is regulated in normal operation in that a first actual rail pressure is determined via a first filter from the rail pressure, an offset is calculated from a target rail pressure and the first actual rail pressure, a variable is calculated via a pressure regulator from the offset and a PWM signal having a first PWM frequency (f 1 ) for activation of a control process is determined in dependence on the variable. A second actual rail pressure (pCR 2 (IST) is determined via a second filter, a load rejection is recognized if the second actual rail pressure (pCR 2 (IST)) exceeds a first threshold value (GW 1 ), upon exceeding the first threshold value (GW 1 ) the PWM signal switches over from the first PWM frequency (f 1 ) to a second PWM frequency (f 2 ) and the rail pressure is controlled upon exceeding a second threshold value in that the PWM signal is temporarily set to a PWM value that is higher than that of normal operation.

Description:
The present application is a 371 of International application PCT/EP2009/007989 filed Nov. 9, 2009, which claims priority of DE 10 2008 058 720.6, filed Nov. 24, 2008, the priority of these applications is hereby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a method for the open-loop and closed-loop control of an internal combustion engine with a common rail system, in which, during normal operation, the rail pressure is controlled by closed-loop control, and, when a load reduction is detected, a change is made from closed-loop control to open-loop control, wherein, during the open-loop control operation, the PWM signal is temporarily set to a PWM value that is higher than in normal operation in order to act on the controlled system. 
     In a common rail system, a high-pressure pump delivers the fuel from a fuel tank to a rail. The admission cross section to the high-pressure pump is determined by a variable suction throttle. Injectors are connected to the rail. They inject the fuel into the combustion chambers of the internal combustion engine. Since the quality of the combustion is decisively determined by the pressure level in the rail, this pressure is automatically controlled. The closed-loop high-pressure control system comprises a pressure controller, the suction throttle with the high-pressure pump, the rail as the controlled system, and a filter in the feedback path. In this closed-loop high-pressure control system, the controlled variable is the pressure level in the rail. The measured pressure values in the rail are converted by the filter to an actual rail pressure and compared with set rail pressure. The control deviation obtained by this comparison is then converted to a control signal for the suction throttle by the pressure controller. The control signal corresponds, e.g., to a volume flow in the unit of liters/minute. The control signal is typically electrically generated as a PWM (pulse-width-modulated) signal of constant frequency, for example, 50 Hz. The closed-loop high-pressure control system described above is disclosed by DE 103 30 466 B3. 
     Due to the high dynamic response, a load reduction is an event that is difficult to control from the standpoint of automatic control engineering. For example, after a load reduction, the rail pressure can rise with a pressure gradient of up to 4000 bars/second. If, for example, the internal combustion engine is being operated at a steady rail pressure of 1800 bars, and if the PWM frequency is 50 Hz, corresponding to a period duration of 20 ms, the rail pressure can rise by up to 80 bars before there is a response to the load reduction by the change in the PWM signal. Further complicating the situation is the fact that determination of the pressure signal, computation of the correcting variable, and output of the PWM signal occur at different times. In the most unfavorable case, the resulting lag time can be up to two PWM periods. This lag time is critical, because the maximum rail pressure is limited by a passive pressure control valve, which opens, for example, at 1950 bars. 
     To improve the reliability of the closed-loop pressure control in a load reduction, DE 10 2005 029 138 B3 proposes that the control operation be changed from closed-loop control to open-loop control. In the open-loop control operation, the PWM signal for activating the suction throttle is temporarily set to an increased PWM value by a step function, which accelerates the closing process of the suction throttle. 
     To improve the dynamic response for large jumps in set value, DE 40 20 654 A1 proposes that the end of the pulse of the PWM signal or the frequency of the PWM signal track the actual development of the set values and actual values. However, the basic prerequisite for this method is the synchronous start of the PWM signal and the determination of the set/actual values. This method is out of the question for closed-loop pressure control in the common rail system, because asynchronicity of closed-loop pressure control and PWM signal is normally the case. In addition, during a load reduction, frequency tracking in the sense of a frequency increase with a subsequent frequency reduction is not technically feasible due to the high pressure gradient. 
     To reduce the pressure oscillations in the rail, which are set in motion by the suction valve, DE 103 30 466 B3 proposes a frequency change of the PWM signal. To this end, the angular separation of two injections and the frequency of the PWM signal are used to compute a critical engine speed, at which the frequencies of the PWM signal and the injection are almost equally large, and from this a speed range is defined. If the engine speed passes through this speed range, the PWM signal is changed from a first frequency, for example, 100 Hz, to a second frequency, for example, 120 Hz. As a result of the frequency change, the high-pressure closed-loop control system is stabilized in the range around the critical speeds. 
     Proceeding from the temporary PWM assignment during a load reduction, as described in DE 10 2005 029 138 B3, the objective of the present invention is to further optimize the closed-loop pressure control during a load reduction. 
     As described in DE 10 2005 029 138 B3, a first filter is used to compute the first actual rail pressure, from which the control deviation is computed. At the same time, a second actual rail pressure is computed by a second, faster filter. A load reduction is then detected by virtue of the fact that the second actual rail pressure exceeds a first limit. When the load reduction is detected, the PWM signal is then switched from a first frequency, for example, 50 Hz, to a second, mush higher frequency, for example, 500 Hz. If the second actual rail pressure subsequently exceeds a second limit, the operation is changed to open-loop control with the temporary PWM assignment. The optimization thus consists in the fact that the lag time between the detection of the load reduction and the output of the PWM signal is shortened. This has the advantage of a significant reduction of the rail pressure overshoot after the load reduction. 
     The function is ended then the second actual rail pressure falls back below the first limit reduced by a hysteresis value. When the function ends, the PWM signal is then switched from the second frequency back to the first, lower frequency. Since the higher PWM frequency is set only during a short interval of time, the dissipation and the heat generation of the switching transistors in the electronic engine control unit remains within the specifications given by the semiconductor manufacturer. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment of the invention is illustrated in the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a system diagram. 
         FIG. 2  is a block diagram of a closed-loop high-pressure control system. 
         FIG. 3  shows a load reduction in the form of a time-dependency diagram. 
         FIG. 4  is a state diagram. 
         FIG. 5  is a program flowchart. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a block diagram of an electronically controlled internal combustion engine  1  with a common rail system. The common rail system comprises the following mechanical components: a low-pressure pump  3  for delivering fuel from a fuel tank  2 , a suction throttle  4  for controlling the volume flow, a high-pressure pump  5 , a rail  6 , and injectors  8  for injecting fuel into the combustion chambers of the internal combustion engine  1 . 
     The internal combustion engine  1  is controlled by an electronic engine control unit  9  (ECU). Input variables of the electronic engine control unit  9  shown in  FIG. 1  are the rail pressure pCR, which is detected by a pressure sensor  7 , the engine speed nMOT, and a variable EIN. The variable EIN is representative of other input signals, for example, input signals for the oil temperature or fuel temperature. The output variables of the electronic engine control unit  9  shown in  FIG. 1  are a PWM signal PWM for activating the suction throttle  4 , a power-determining signal ve for activating the injectors  8 , and a variable AUS. The power-determining signal ye characterizes an injection start and an injection duration. The variable AUS represents additional control signals for controlling the internal combustion engine  1 , for example, a control signal for activating an AGR valve. Naturally, the common rail system illustrated here can also be realized as a common rail system with individual accumulators. In this case, the individual accumulator is integrated in the injector  8 , and then the individual accumulator pressure pE is an additional input signal of the electronic engine control unit  9 . 
       FIG. 2  is a block diagram of the closed-loop high-pressure control system for automatically controlling the rail pressure. The input variable is a set rail pressure pCR(SL). The output variable corresponds to the raw value of the rail pressure pCR. A first actual rail pressure pCR 1 (IST) is determined from the raw value of the rail pressure pCR by means of a first filter  15 . This value is compared with the set rail pressure pCR(SL) at a summation point A, and a control deviation ep is obtained from this comparison. A correcting variable is computed from the control deviation ep by means of a pressure controller  10 . The correcting variable represents a volume flow qV 1 , whose physical unit is liters/minute. In an optional provision, the computed set consumption is added to the volume flow qV 1 . The volume flow qV 1  is the input variable for a limiter  11 , which can be made speed-dependent by using nMOT as an input variable. The output variable qV 2  of the limiter  11  is then converted to a PWM signal PWM 1  in a computing unit  12 . In this regard, the PWM signal PWM 1  represents the duty cycle, and the frequency fPWM corresponds to the base frequency, for example 50 Hz (period duration 20 ms). Fluctuations in the operating voltage and the ucl admission pressure are also taken into consideration in the conversion. The PWM signal PWM 1  is the first input variable of a switch  13 . A second input variable of the switch  13  is a PWM signal PWM 2 . Depending on the position of the first switch  13 , the output signal PWM of the switch  13  corresponds either to the signal PWM 1  or to the signal PWM 2 . The solenoid coil of the suction throttle is then acted upon by the PWM signal PWM. This changes the displacement of the magnetic core, and the output of the high-pressure pump is freely controlled in this way. The high-pressure pump, the suction throttle, and the rail represent a controlled system  14 . A consumption volume flow qV 3  is removed from the rail  6  through the injectors. The closed-loop control system is thus closed. 
     As DE 10 2005 029 138 B3 discloses, this closed-loop control system is supplemented by the temporary PWM assignment unit. The components of the temporary PWM assignment unit are a second filter  17  for computing a second actual rail pressure pCR 2 (IST), a functional block  18  for determining a signal SZ 1  for activating the first switch  13 , and a PWM assignment unit  16 . During operation with closed-loop control, the first switch  13  is in position a, i.e., the correcting variable qV 1  computed by the pressure controller  10  is limited and converted to a PWM signal PWM 1 , which acts on the controlled system  14 . If the second actual rail pressure pCR 2 (IST) exceeds a limit, here: the second limit GW 2 , the functional block  18  changes the signal level of the signal SZ 1 , which causes the first switch  13  to be switched to position b. In position b, a PWM value PWM 2  that is increased relative to normal operation is temporarily output by the PWM assignment unit  16 . In other words, the operation is changed from closed-loop control to open-loop control. The temporary PWM assignment can be realized, as illustrated, in step form. After the expiration of a predeterminable period of time, the switch  13  then changes back to position a, so that closed-loop control is reestablished. 
     In practical operation, the PWM signal is provided with a low PWM frequency fPWM, for example, 50 Hz, by the corresponding drive software. Therefore, the PWM value can be updated in 20-ms time intervals. The low PWM frequency achieves the result that, first, the slide of the suction throttle moves, i.e., only the sliding friction needs to be overcome, and, second, the dissipation of the switching transistors in the electronic engine control unit remains within specifications. The pressure controller  10  is computed by the engine software with constant scanning time. If the pressure controller  10  detects a quantitatively increasing control deviation ep, it may be that a PWM period started shortly before. Therefore, the new, increased PWM duty cycle cannot be set until the beginning of the next PWM period, i.e., at the earliest after the expiration of the 20-ms time interval. This in turn means that the rail pressure pCR continues to rise during the current PWM period and also at the beginning of the next PWM period. Due to the asynchronicity of PWM signal and pressure controller scanning, a corresponding lag time thus develops. 
     This is where the invention comes in, namely, the block diagram of  FIG. 2  additionally contains a functional block  19  and a second switch  20 . During operation with closed-loop control, the second switch  20  is in position a, in which the first frequency f 1  (50 Hz) determines the frequency fPWM. If the second actual rail pressure pCR 2 (IST) exceeds a first limit GW 1  (this is the case when a load reduction occurs), the functional block  19  sets the triggering signal SZ 2  for activating the second switch  20  to a second value, which causes the switch  20  to change to position b. The frequency fPWM then equals the second frequency  12  of, for example, 500 Hz. The PWM signal PWM 1  is now updated every 2 ms. 
     If the second actual rail pressure pCR 2 (IST) exceeds the second limit GW 2 , the temporary assignment is activated. If the second actual rail pressure pCR 2 (IST) falls below the difference of a first limit and a hysteresis value, the switch  20  changes back to position a, so that the PWM frequency fPWM is again identical with the first frequency f 1 . 
       FIG. 3  shows a load reduction in the form of a time-dependency diagram.  FIG. 3  comprises the four graphs  3 A to  3 D, which show the following, in each case as a function of time:  FIG. 3A  the behavior of the second actual rail pressure pCR 2 (IST),  FIG. 3B  the value of the PWM signal PWM,  FIG. 3C  the PWM signal in a pulse/pulse interval diagram according to the prior art, and  FIG. 3D  the PWM signal in a pulse/pulse interval diagram according to the invention. The pressure curve drawn as a solid line in  FIG. 3A  corresponds to the PWM signal of  FIG. 3C , and the pressure curve drawn as a broken line in  FIG. 3A  corresponds to the PWM signal of  FIG. 3D . The illustrated example is based on a first PWM frequency of 50 Hz, which corresponds to a time interval of 20 ms, and a second PWM frequency of 500 Hz, which corresponds to a time interval of 2 ms. The set rail pressure was kept constant. 
     The course of the method according to the prior art is as follows: 
     Before time t 1 , the internal combustion engine is operated in a steady state at a rail pressure of 1800 bars. In the steady state, the rail pressure is subject to closed-loop control. At time t 1  the load is reduced, which leads to an increase in the rail pressure. An increase occurs in both the first actual rail pressure pCR 1 (IST) computed by the first filter ( FIG. 2 :  15 ) and the second actual rail pressure pCR 2 (IST) computed by the second filter ( FIG. 2 :  17 ). The increasing first actual rail pressure pCR 1 (IST) causes a quantitatively increasing control deviation, which the pressure controller converts to an increasing PWM signal ( FIG. 3B ), thereby causing the suction throttle to be moved in the closing direction. If the second actual rail pressure pCR 2 (IST) then exceeds the second limit GW 2  at time t 3  (here: 1900 bars, point B in  FIG. 3A ), the temporary PWM assignment is activated, so that a change is made from closed-loop control to open-loop control. Therefore, starting at time t 3 , the PWM signal is increased to 100% for 20 ms ( FIG. 3B ). At time t 3 , a new PWM period has just begun, so that the PWM increase is not immediately effective. See the enlarged segment in  FIG. 3C . The PWM increase does not become effective until 20 ins later, i.e., one period interval later, at time t 5 . Pulse D in  FIG. 3C  corresponds to the PWM value of 100% in  FIG. 3B . Due to this lag time, the second actual rail pressure pCR 2 (IST) continues to rise and reaches its maximum value at 2030 bars. 
     The course of the method according to the invention is as follows: 
     At time t 2  the second actual rail pressure pCR 2 (IST) exceeds the first limit GW 1  (here: 1850 bars, point A in  FIG. 3A ). When the first limit GW 1  is exceeded, a change is made to the second PWM frequency of 500 Hz ( FIG. 3D ). At time t 3  the second actual rail pressure pCR 2 (IST) then exceeds the second limit GW 2  (here: 1900 bars, point B in  FIG. 3A ). When the second limit GW 2  is exceeded, the temporary PWM assignment is activated, i.e., a change is made from closed-loop control to open-loop control. The pulse E in  FIG. 3D  (starting at time t 4 ) corresponds to the PWM value of 100% in  FIG. 3B . This time, the lag time until the PWM increase takes effect is again a whole period duration, but now this is only 2 ms. All together, the PWM increase thus takes effect 18 ms earlier. As a result, the second actual rail pressure pCR 2 (IST) rises only to 1940 bars this time. Therefore, the switching of the PWM frequency reduces the high-pressure overshoot by 90 bars. In  FIG. 3A , this is represented by the reference symbol dp. 
     The increase in the PWM frequency is deactivated when the second actual rail pressure pCR 2 (IST) falls below the first limit GW 1  by a predeterminable hysteresis value pHY, for example, 30 bars, at point C. As a result, the frequency is changed back from the second frequency of 500 Hz to the first frequency of 50 Hz (see  FIG. 3D  at time t 6 ). Since, in accordance with the invention, a change to a high PWM frequency is made only during the high-pressure overshoot (time t 2 /t 6 ), the heat generation of the high power stage remains within allowable hardware specifications despite the large number of transistor switching operations. 
     The switching logic of the invention is shown in  FIG. 4 . In steady-state operation, the PWM frequency fPWM is set to the first frequency f 1 , for example, 50 Hz. If the second actual rail pressure pCR 2 (IST) becomes greater than or equal to the first limit GW 1 , then the PWM frequency tPWM is set to the second frequency f 2 , for example, 500 Hz. A switch back to the first frequency f 1  occurs when the second actual rail pressure pCR 2 (IST) falls below the limit GW 1  by an amount equal to the hysteresis value pHY. 
       FIG. 5  is a program flowchart of the method of the invention. At S 1  a flag is initialized with a value of 0, and the frequency fPWM of the PWM signal is set to the value f 1 , for example, 50 Hz. An interrogation at S 2  checks the value of the flag. If the value is 1, program control passes to the routine with the steps S 6  to S 8 . However, if the value of the flag is 0, program control passes to the routine with the steps S 3  to S 5 . During the first pass through the program (flag=0), a check is then made at S 3  to determine whether the second actual rail pressure pCR 2 (IST) has reached or exceeded the first limit GW 1 . If this is not the case (interrogation result S 3 : no), the internal combustion engine is operating in the steady state, and the program flow continues at A. However, if a load reduction is detected at S 3  (interrogation result S 3 : yes), then at S 4  the frequency is changed to the second frequency t 2 , for example, 500 Hz. The PWM value can now be changed with a 2-ms time interval. At S 5  the flag is then changed to a value of 1, and the program flow continues at A. If the interrogation at S 2  shows that the flag has a value of 1 (interrogation result S 2 : yes), then at S 6  a check is made to determine whether the second actual rail pressure pCR 2 (IST) is less than or equal to the switch-off threshold. The switch-off threshold is set to the difference between the first limit GW 1  and the hysteresis value pHY. If the second actual rail pressure pCR 2 (IST) has not yet fallen below the switch-off threshold, the program flow continues at A. If the second actual rail pressure pCR 2 (IST) has reached or fallen below the switch-off threshold (interrogation result S 6 : yes), then at S 7  the frequency fPWM of the PWM signal is switched from the second frequency f 2  back to the first frequency f 1 . At S 8  the flag is then set back to its initialization value of 0, and the program flow continues at A. 
     LIST OF REFERENCE NUMBERS 
     
         
           1  internal combustion engine 
           2  tank 
           3  low-pressure pump 
           4  suction throttle 
           5  high-pressure pump 
           6  rail 
           7  pressure sensor (rail) 
           8  injector 
           9  electronic engine control unit (ECU) 
           10  pressure controller 
           11  limiter 
           12  computing unit PWM signal 
           13  first switch 
           14  controlled system 
           15  first filter 
           16  PWM assignment unit 
           17  second filter 
           18  functional block 
           19  functional block 
           20  second switch