Patent Publication Number: US-2012037122-A1

Title: Method and device for controlling a fuel metering system for an internal combustion engine

Description:
BACKGROUND INFORMATION 
     In a fuel metering system equipped with a common rail system, it is normally provided that when the internal combustion engine is switched off, the actuators which influence the fuel pressure in the high-pressure accumulator of the fuel metering system are activated in such a way that the pressure drops to atmospheric pressure. 
     Modern internal combustion engines are frequently equipped with a so-called automatic start/stop system. In such an automatic start/stop system, it is provided that the internal combustion engine is switched off as a function of the state of the internal combustion engine and/or the state of the driven vehicle. This normally occurs when the vehicle comes to a stop. As soon as the driver desires to continue driving and this is indicated by activating a control element, for example the gas pedal, the internal combustion engine starts automatically. 
     If the rail pressure in an internal combustion engine equipped with such an automatic start/stop system is reduced when the internal combustion engine is switched off, the fresh start of the internal combustion engine may possibly be delayed significantly. Furthermore, the hydraulic components are severely stressed. 
     SUMMARY OF THE INVENTION 
     The method of the present invention makes it possible to restart the internal combustion engine very rapidly in start/stop operation. Moreover, the reduced pressure reduction/buildup cycle results in significantly less stress on the hydraulic components. According to the present invention, this is achieved in that the fuel pressure is controlled at a first value when the internal combustion engine is switched off. If a further condition is present, the fuel pressure is controlled at a second value. The second value normally corresponds to the value which is usual when the internal combustion engine is switched off. Preferably, the second value assumes the value zero or it assumes a value corresponding to atmospheric pressure. In one particularly advantageous embodiment, it is provided that the first value corresponds to a fuel pressure normally used at idle. 
     As a further condition, it is preferably checked if the driver activates a control element indicating the intention of shutting down the vehicle. This means that a check is made as to whether the driver intends to switch off the vehicle driven by the internal combustion engine. 
     If the fuel pressure falls below a threshold value, the fuel pressure is controlled at a second value. This has the advantage that the pressure does not drop below a predetermined value. If the pressure threatens to drop below the predetermined value, the consequence is that when the driver switches off the vehicle, the pressure in the rail has dropped too much and it might no longer be possible to open the pressure regulating valve. In this case, a restart of the vehicle using a cold engine will be delayed. 
     It is furthermore advantageous if the fuel pressure is set to an operating point-dependent value if the rotational speed rises above a specific rotational speed value. 
     This means that as a further condition, it is checked whether the rotational speed has risen above a threshold. If this is the case, a transition may be made to the normal activation of the actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of the device according to the present invention. 
         FIG. 2  shows a flow chart for elucidating the method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows important elements of the device according to the present invention in a block diagram. A controller is denoted as  100 . The controller activates an actuator  110 . Actuator  110  is an actuator that controls the pressure in a fuel system. The fuel metering system is preferably a so-called common rail system. In such a common rail system, actuator  110  may be embodied as a so-called pressure regulating valve that connects an area having high pressure with an area having low pressure. Controller  100  activates actuator  110  in such a way that the pressure in the high-pressure area assumes a specific value. In a simplified specific embodiment, it may be designed as a controller and in an improved specific embodiment as a regulator. In this case, the pressure is detected and compared with a setpoint. Actuator  110  is activated as a function of the comparison. As an alternative or in addition, a controllable high-pressure pump may be designed as an actuator for a pressure regulating valve. It may also be activated as a controller or as a regulator. 
     Controller  100  processes the output signals of various sensors  120 . These sensors are in particular a rotational speed sensor and/or a pressure sensor, the first supplying a rotational speed signal N and the second a pressure signal P corresponding to the pressure in the high-pressure area. Furthermore, an automatic start/stop system  130  which supplies a signal S is provided. This start/stop controller is designed in such a way that it switches the internal combustion engine off under specific conditions of the vehicle and/or the internal combustion engine. For example, the internal combustion engine is switched off if the automatic start/stop system recognizes that the vehicle is not moving. If the start/stop controller recognizes that the driver intends to continue driving the vehicle, the start/stop controller starts the internal combustion engine and makes continued driving possible. 
     Furthermore, a control element  140  is provided which is activated by the driver and indicates if the vehicle is in operation or has been permanently stopped. This control element is in particular an ignition switch designed as an ignition key or as switching means activated by the driver. The state of this control element indicates if the driver intends to stop the vehicle permanently. It is normally provided that the vehicle is operated if control element  140  is activated. If the vehicle stops, the automatic start/stop system normally switches the internal combustion engine off. If the driver indicates through a corresponding activation of control element  140  that he intends to stop the vehicle permanently, the internal combustion engine is also switched off. This means that the internal combustion engine is switched off and on as a function of the position of the control element as well as a function of start/stop controller  130 . 
     According to the present invention, it is provided that if the internal combustion engine is switched off via the control element, the rail pressure in the high-pressure area of the common rail system may be reduced and if the internal combustion engine is switched off via the start/stop controller, the rail pressure is not reduced but is instead left at its value or in a preferred embodiment is set to a value used at idle. As a result, the rail pressure is more rapidly available for a fresh start of the internal combustion engine and a restart is more rapidly possible. 
     Normally, the start/stop controller recognizes that the vehicle is stopping. In this case, actuator  110  is activated in such a way that the rail pressure does not drop. If a fresh start then occurs, the rail pressure is immediately available. If the driver activates control element  140  after the stop, this is also recognized and actuator  110  is activated in such a way that the rail pressure drops to the value normally present in a vehicle that is switched off. This value to which the rail pressure is reduced normally assumes the value zero. However, embodiments are also possible in which the rail pressure is set to a different value. 
     A specific embodiment of the method of the present invention is presented below based on a flow chart in  FIG. 2 . A first query  200  checks if control element  140  is activated in such a way that the driver intends to stop the vehicle. If this is the case, actuator  110  is activated in step  210  in such a way that the fuel pressure assumes a second value that is normally present when an internal combustion engine is switched off. 
     If, however, query  200  recognizes that a corresponding signal of control element  140  is not present, query  220  checks if a start/stop controller  130  emits a signal indicating that the internal combustion engine is being shut down. If this is not the case, the actuator is activated in step  225  in such a way that the fuel pressure assumes its normal value. This value is normally specified as a function of the operating state of the internal combustion engine. Preferably this is implemented by specifying a corresponding setpoint value for the rail pressure or a corresponding control value for actuator  110 . 
     However, if query  220  recognizes that a signal of the start/stop controller is present which indicates that the internal combustion engine is being switched off, query  230  follows. This query  230  checks if another condition is present. In particular, this query  230  checks if fuel pressure P is lower than a threshold value PS. If this is the case, actuator  110  is activated in step  210  in such a way that the rail pressure is reduced to the value that is normally present when the internal combustion engine is switched off. If query  230  recognizes that value P of the rail pressure is not lower than threshold value PS, query  240  checks if rotational speed N is higher than a threshold value. If this is not the case because the rotational speed is lower than a threshold value NS, actuator  110  is activated in step  250  in such a way that the rail pressure is set to a first value. This first value is normally selected in such a way that it corresponds to the value used at idle. If the value of rotational speed N is higher than the threshold value, an activation occurs in step  210  in such a way that the fuel pressure assumes the second value. As an alternative, it could also be provided here that actuator  110  is activated in step  220  in such a way that it assumes an operating point-dependent value. 
     This means that if a signal is present in control element  140  indicating that the driver is switching the vehicle off, actuator  110  is activated in step  210  in such a way that the fuel pressure assumes a second value. This second value corresponds to the value that the rail pressure normally assumes if the internal combustion engine is shut down for an extended period of time. This value normally corresponds to the value of the ambient pressure. 
     If no signal from control element  140  indicating that the internal combustion engine is being switched off is present and start/stop controller  130  also provides no signal requesting that the internal combustion engine be switched off, actuator  110  is activated in such a way that the rail pressure assumes a third value. This third value is normally specified as a function of the operating state of the internal combustion engine, in particular the rotational speed and load of the internal combustion engine. The pressure during operation of the internal combustion engine is controlled or regulated at this third value. 
     If a signal of the start/stop controller causing the internal combustion engine to be switched off is present, actuator  110  is activated in such a way that the fuel pressure assumes a first value. This value is normally larger than the second value. The first value is selected in such a way that a rapid start of the internal combustion engine is made possible. 
     This means that when the internal combustion engine is switched off by a start/stop controller, the rail pressure is not reduced to atmospheric pressure. 
     In a common rail system having a pressure regulating valve, it is therefore provided that the start/stop controller does not open the pressure regulating valve when the internal combustion engine is switched off. This means that the pressure regulating valve is opened in step  210  in a common rail system having a pressure regulating valve. On the other hand, the pressure regulating valve is kept closed in steps  225  and  250 .