Abstract:
A method and a device for controlling an internal combustion engine having a plurality of cylinders. A fuel metering system includes a fuel reservoir. A fuel level signal is provided which characterizes the fuel level of the fuel reservoir. When the fuel level signal drops below a threshold value, a subset of the cylinders is shut off.

Description:
BACKGROUND INFORMATION  
       [0001]    If, in an internal combustion engine, air enters into the fuel line and there, in particular into the injectors, considerable problems occur when the internal combustion engine is restarted. This is the case, for example, when the driver of the vehicle does not fill the tank in a timely manner and the fuel in the tank has been fully depleted, and the fuel pumping system aspires air which penetrates into the high-pressure zone. Such aspiration of air usually results in the internal combustion engine stalling immediately. The air may be easily removed from the lines, the pump, and the reservoir such as the rail, by de-aerating. If the air reaches the injectors, the injectors are much more difficult to de-aerate. 
         [0002]    In particular in the case of injectors containing a hydraulic coupler, air penetration and the subsequent restart of the internal combustion engine are problematic. 
         [0003]    It is known that such aspiration of air and the problem condition caused thereby may be avoided by shutting off the internal combustion engine in a timely manner. This means that the internal combustion engine is shut off when the tank contains a certain residual quantity of fuel. Such a shutoff is prohibited by law in certain countries. If such a measure of shutting off the internal combustion engine when the tank contains a residual quantity is not provided, a fuel tank running on empty and thus penetration of air into the fuel metering system cannot be prevented. 
       SUMMARY OF THE INVENTION  
       [0004]    The device according to the present invention and the method according to the present invention have the advantage over the related art that restart of the internal combustion engine after running on empty is considerably simplified and in the case of an almost empty tank the internal combustion engine may be driven on, if necessary, until the tank is completely empty. Furthermore, the driver&#39;s attention is called to the almost-empty tank by the measures according to the present invention. 
         [0005]    This is achieved according to the present invention by shutting off a subset of cylinders when there is a certain residual quantity of fuel in the tank. The cylinders are preferably shut off by suppressing the control of the injectors of these cylinders. This means that below a certain residual quantity of fuel in the tank a subset of the injectors is shut off. As a result, no air reaches the injectors of these cylinders. After the tank is filled, a start including the previously shut off cylinders is possible. For this purpose, a fuel level signal is provided which characterizes the fuel level in the fuel reservoir. 
         [0006]    The number of cylinders that are shut off is selected in such a way that a restart of the internal combustion engine is possible with the aid of these shut-off cylinders, while a limp-home operation of the internal combustion engine is possible using the remaining cylinders. One-half of the cylinders is preferably shut off, while the other half of the cylinders continues to operate. 
         [0007]    The cylinders are shut off as a function of a fuel level signal which shows the residual fuel quantity. A tank state signal or a signal of a fuel level indicator may be used as such a fuel level signal. Otherwise it may also be provided that the regular signal of an on-board computer regarding the residual range is used. It may thus be provided that the measure is initiated when reaching a certain residual range. 
         [0008]    Furthermore, it is possible to draw a conclusion about the tank contents from the rail pressure dynamics and the rail pressure stability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]      FIG. 1  shows a block diagram of a common rail system. 
           [0010]      FIG. 2  shows a first flow chart which describes the shut-off of the internal combustion engine. 
           [0011]      FIG. 3  shows a second flow chart, which shows the restart. 
       
    
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows the important elements of a fuel metering system which is referred to as a common rail system. The procedure according to the present invention is described below using the example of a common rail system. The procedure according to the present invention, however, is not limited to a common rail system, but may also be used in other fuel metering systems. 
         [0013]    A fuel reservoir is labeled  10 . The fuel is pumped therefrom by pumps (not depicted) to a high-pressure accumulator  20  via a line  15 . From this high-pressure accumulator  20 , the fuel reaches the combustion chambers (not depicted) of the internal combustion engine via injectors  31 ,  32 ,  33 , and  34 . Fuel metering by the injectors is controlled by actuators  41 ,  42 ,  43 , and  44 . These actuators are controlled with the aid of a control unit  50 . 
         [0014]    The fuel is pumped by a pre-supply pump and/or high-pressure pump into high-pressure accumulator  20 . Due to the control of the injectors via actuators  41 ,  42 ,  43 , and  44 , injectors  31 ,  32 ,  33 ,  34 , respectively, are controlled in such a way that, at a predefined point in time, they enable fuel metering and, at a second predefined point in time, they end fuel metering. If the fuel is depleted in fuel reservoir  10 , air reaches the high-pressure accumulator, and thus also injectors  31  through  34 , via line  15 . If air reaches high-pressure accumulator  20  or the injectors, the pressure rapidly drops in high-pressure accumulator  20  and injection is no longer possible. 
         [0015]    In particular in the case of injectors that include a so-called hydraulic coupler, the following problem arises. If only a small amount of air is in the injector, in particular in the coupler, control of fuel metering is no longer possible. De-aerating these injectors is also difficult, since a very high pressure is required therefor, which may only be provided when the engine is running. 
         [0016]    Therefore, according to the present invention the following procedure is provided. Fuel level F of the tank is analyzed in a step  210 . Different signals are available for this purpose. Thus, in a simple specific embodiment, a tank transducer showing the fuel quantity in the tank may be directly analyzed. Furthermore, systems are known in which an on-board computer calculates a residual range of the vehicle. This signal may also be used as fuel level F. Furthermore, it is recognized according to the present invention that, shortly before reaching the minimum fuel level and thus before imminent air aspiration, the rail pressure becomes unstable and the rail pressure dynamics change. 
         [0017]    Penetration of air into the high-pressure system causes an upswing of the rail pressure amplitude in the rail pressure control. This is detected by a suitable functionality, in particular in steady-state driving states. In dynamic driving states, a lack of fuel supply is recognized by analyzing the rail pressure increase gradient compared to the required rail pressure increase. In addition, by monitoring the control parameters for deviations with respect to the values in normal operation, a conclusion is drawn regarding air in the fuel or lack of fuel supply. 
         [0018]    If a fuel level F below a threshold value S is recognized in step  220 , a subset of the injectors is shut down in step  240 . At the same time, a bit is set indicating that a shutoff has taken place. 
         [0019]    If query  220  recognizes that the fuel level is still sufficient, normal operation takes place in step  230 . This is followed again by step  210 . 
         [0020]    The number of injectors to be shut off is selected in such a way that the number of shut-off injectors is sufficient for reliable engine start. On the other hand, the number of injectors that are not shut off is selected in such a way that reliable limp-home operation is possible. In an internal combustion engine having four cylinders, normally two cylinders are shut off. In an internal combustion engine having a higher number of cylinders, preferably a higher number of cylinders is shut off. Preferably only two cylinders are further operated when reaching minimum fuel level S. 
         [0021]    According to the present invention, a subset of the injectors is thus no longer controlled in the event of an indication of an existing or imminent penetration of air into the fuel metering system. This takes place, for example, when a fuel level in tank  10  drops below a calibratable level and/or in the event of a recognizable rail pressure instability. The shut-off prevents air from penetrating into the injectors; therefore restart is possible using these injectors after the tank is refilled. 
         [0022]    These injectors, which are no longer controlled, remain closed until a sufficient tank level or a sufficiently stable rail pressure is present again. At least as many injectors are always shut off as needed for the next start and at most as many as may be omitted for a safe limp-home operation. Timely shut-off prevents or at least reduces penetration of air into these shut-off injectors. At the next start, these injectors are thus still in a condition that enables a function of the hydraulic coupler. Injection is thus ensured which allows the engine to restart after the cause of the air penetration is removed and possibly to de-aerate the rest of the system. 
         [0023]    The restart is depicted in  FIG. 3  as a flow chart. If a desired start of the internal combustion engine is recognized in step  250 , a check is made in  260  of whether a shut-off of the internal combustion engine occurred during the previous operation. This takes place preferably via the bit set in step  240 . If this is the case, query  270  checks whether the fuel level is in a range in which reliable operation is possible, i.e., it is checked whether the fuel level is greater than threshold value S. If this is the case, the internal combustion engine is started in step  290 , special measures being possibly initiated to drive out the residual air from the fuel system. If query  260  detects that no shut-off is taking place, normal start occurs in step  280 . If query  270  recognizes that fuel level F is insufficient, the start operation is terminated in step  295 . 
         [0024]    Alternatively it may also be provided that a check is made in step  270  of whether the tank has been filled.