Abstract:
A method for testing the function of a high-pressure pump having a plurality of pump elements, which each define a respective work chamber which is in communication, via a suction valve with a low-pressure region, from which fuel can be aspirated, and via a pressure valve with a high-pressure region which includes a central high-pressure fuel reservoir, serving to supply fuel to an internal combustion engine, into which high-pressure reservoir the high-pressure pump pumps the fuel aspirated from the low-pressure region, and the pressure of which is detected by a rail pressure sensor. The values detected by the rail pressure sensor are used in the built-in state of the high-pressure pump in operation of the engine for testing the function of the high-pressure pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2005/052705 filed on Jun. 13, 2005. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for testing the function of a high-pressure pump having a plurality of pump elements, which each define a respective work chamber which is in communication, via a suction valve with a low-pressure region, from which fuel can be aspirated, and via a pressure valve with a high-pressure region which includes a central high-pressure fuel reservoir (rail), serving to supply fuel to an internal combustion engine, into which high-pressure reservoir the high-pressure pump pumps the fuel aspirated from the low-pressure region, and the pressure of which is detected by a rail pressure sensor. 
     2. Prior Art 
     In conventional testing methods, the high-pressure pump is removed from the internal combustion engine and tested on a special test stand. 
     The object of the invention is to furnish a method for testing the function of a high-pressure pump having a plurality of pump elements, which each define a respective work chamber which is in communication, via a suction valve with a low-pressure region, from which fuel can be aspirated, and via a pressure valve with a high-pressure region which includes a central high-pressure fuel reservoir (rail), serving to supply fuel to an internal combustion engine, into which high-pressure reservoir the high-pressure pump pumps the fuel aspirated from the low-pressure region, and the pressure of which is detected by a rail pressure sensor, which can be performed simply and economically and nevertheless makes a reliable statement to be made about the functional capability of the high-pressure pump. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     In a method for testing the function of a high-pressure pump having a plurality of pump elements, which each define a respective work chamber which is in communication, via a suction valve with a low-pressure region, from which fuel can be aspirated, and via a pressure valve with a high-pressure region which includes a central high-pressure fuel reservoir (rail), serving to supply fuel to an internal combustion engine, into which high-pressure reservoir the high-pressure pump pumps the fuel aspirated from the low-pressure region, and the pressure of which is detected by a rail pressure sensor, this object is attained in that the values detected by the rail pressure sensor are used in the built-in state of the high-pressure pump in operation of the engine for testing the function of the high-pressure pump. As a result, the removal of the high-pressure pump and the special test stand can both be dispensed with. 
     A preferred exemplary embodiment of the method is characterized in that an adapter is connected to the rail pressure sensor in order to forward the pressure values detected to an external evaluation unit. The adapter is for example an intermediate plug, with an interface for a connection cable, in particular an oscilloscope cable. The evaluation unit is preferably an oscilloscope, or a testing device with the function of an oscilloscope. It is also possible to use a control unit that is integrated with the engine. 
     A further preferred exemplary embodiment of the method is characterized in that the testing is done in the idling mode of the engine. The testing can also be done in other defined operating states of the engine. However, in the context of the present invention, unambiguous results were attained upon testing while idling. 
     A further preferred exemplary embodiment of the method is characterized in that the raw signal of the rail pressure sensor is used as the measured value for the rail pressure. In the context of the present invention, it was discovered that the signal of the rail pressure sensor, from a control unit belonging to the internal combustion engine, was only limitedly usable for testing the function of the high-pressure pump. With the raw signal of the rail pressure sensor, markedly better results were attained, especially at relatively high step-up ratios between the pump rpm and the engine rpm. 
     A further preferred exemplary embodiment of the method is characterized in that a pressure regulating valve, with which the high-pressure pump is equipped, is opened in order to increase the pumping quantity of the high-pressure pump. In the open state, the pressure regulating valve opens up a connection between the high-pressure region and the low-pressure region. By the purposeful opening of the pressure regulating valve, the pumping quantity of the high-pressure pump is artificially increased. 
     A further preferred exemplary embodiment of the method is characterized in that the pumping quantity of the high-pressure pump is increased by opening a pressure limiting valve, with which the high-pressure fuel reservoir is equipped. The pressure limiting valve is closed in normal operation of the engine, and for safety reasons it does not open until a maximum allowable pressure, for example of 1800 bar, in the high-pressure fuel reservoir is exceeded. The pressure limiting valve then regulates the pressure in the high-pressure fuel reservoir to a reduced value, for instance of 800 bar, in order to make emergency operation possible. The pumping quantity of the high-pressure pump is artificially increased by the purposeful opening of the pressure limiting valve. 
     A further preferred exemplary embodiment of the method is characterized in that a metering unit, which is connected upstream of the high-pressure pump, is opened, in order to increase the pumping quantity of the high-pressure pump, until the pressure limiting valve opens. Via the opened metering unit, more fuel reaches the preferably suction-throttled high-pressure pump than is needed for instance in the idling mode of the engine. The artificially increased pumping quantity quickly causes the pressure limiting valve to open. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, characteristics and details of the invention will become apparent from the ensuing description, taken in conjunction with the drawings, in which: 
         FIG. 1  is a schematic illustration of a fuel injection system, with a high-pressure pump which has a pressure regulating valve; 
         FIG. 2 , a schematic illustration of a fuel injection system, with a high-pressure pump, upstream of which a metering unit is connected; 
         FIG. 3 , a graph plotting the voltage of a rail pressure sensor over time for an intact high-pressure pump; 
         FIG. 4 , a graph plotting the voltage of the rail pressure sensor over time, if the suction valve is defective; 
         FIG. 5 , a graph plotting the voltage of the rail pressure sensor over time, if the pressure valve is defective; 
         FIG. 6 , a graph plotting the voltage of the rail pressure sensor over time with an open pressure limiting valve and an intact high-pressure pump; 
         FIG. 7 , a graph plotting the voltage of the rail pressure sensor over time with an open pressure limiting valve and a defective suction valve; and 
         FIG. 8 , a graph plotting the voltage of the rail pressure sensor over time with an open pressure limiting valve and a defective pressure valve. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a common rail fuel injection system is shown schematically. From a low-pressure container  1 , which is also called the fuel tank, with the aid of a fuel feed pump  2  via a connecting line  3 , fuel is pumped to a high-pressure pump  4 . An overflow valve  6  is disposed in the connecting line  3 . The low-pressure container  1 , the fuel feed pump  2 , and the connecting line  3  are subjected to low pressure and are therefore associated with the low-pressure region. 
     A pressure regulating valve  8  is mounted on the high-pressure pump  4  and is connected to the low-pressure container  1  via a line  9 . A high-pressure line  10  also begins at the high-pressure pump  4  and furnishes the fuel, subjected to high pressure, to a high-pressure fuel reservoir  12 , which is also known as a common rail. From the high-pressure reservoir  12 , with the interposition of flow limiters  13 , high-pressure lines  14  lead away, which furnish the fuel, subjected to high pressure, from the high-pressure reservoir  12  to injection valves  15 , which are also known as injectors and of which for the sake of simplicity only one is shown in  FIG. 1 . The high-pressure line  10 , the high-pressure reservoir  12 , the high-pressure line  14 , and the injection valve  15  contain fuel subjected to high pressure and are accordingly associated with the high-pressure region of the fuel injection system. 
     From the fuel injection valve  15 , a return line, which has two portions  16  and  17 , leads to the low-pressure region  1 . A pressure holding valve  18  is connected between the two portions  16  and  17  of the return line. The pressure holding valve  18  serves to maintain a minimum pressure in the portion  16  of the return line of approximately 1.0 bar. The operation of the fuel injection system is controlled by an electronic control unit  19 . 
     In  FIG. 2 , a fuel injection system similar to that in  FIG. 1  is shown. The fuel injection system includes a high-pressure pump  20 , which is driven by a drive shaft  21 , which has an external shaft portion  22 . The ends of three pistons  24 ,  25  and  26 , arranged in a star pattern, are in contact with the external shaft portion  22 . The ends of the pistons  24  through  26  remote from the drive shaft  21  define work chambers  28 ,  29  and  30 , which are also called pump chambers. The work chambers  28  through  30  are each in communication, via a respective suction valve  32 ,  33  and  34  and with the interposition of a metering unit  36 , with a low-pressure region  38 . 
     The work chambers  28  through  30  furthermore communicate, via pressure valves  40  through  42 , with a high-pressure fuel reservoir  44 , which is also known as a common rail, or rail for short. From the high-pressure fuel reservoir  44 , high-pressure lines  46  through  49  lead to fuel injection valves (not shown). The high-pressure fuel reservoir  44  communicates with the low-pressure region  38  via a pressure limiting valve  52 . A rail pressure sensor  55  is also mounted on the high-pressure fuel reservoir  44 , and by way of it the pressure in the high-pressure fuel reservoir  44  is detected. 
     The high-pressure pump  20  serves to pump fuel out of the low-pressure region  38  into the high-pressure fuel reservoir  44 . Upon fuel intake, the suction valves  32  through  34  open, while the pressure valves  40  through  42  are conversely closed. Via the metering unit  36 , the pumping quantity of the high-pressure pump  20  can be controlled. When fuel is pumped into the high-pressure fuel reservoir  44 , the suction valves  32  through  34  are closed and the pressure valves  40  through  42  are open. A dot-dashed line  58  indicates that the metering unit  36 , the suction valves  32  through  34 , and the pressure valves  40  through  42  are integrated with the high-pressure pump  20 . 
     In the testing method of the invention, the high-pressure pump, in idling mode of the vehicle, is tested without access to the control unit integrated into the vehicle, and without removing the high-pressure pump from the vehicle. The pumping quantity of the high-pressure pump is artificially increased in testing, by opening either the pressure limiting valve  52  (see  FIG. 2 ) or the pressure regulating valve  8  (see  FIG. 1 ). In the process, the pressure regulating valve must be constantly supplied with current, or a switch to pressure regulation with the pressure regulating valve must be made. The switch to the pressure regulating valve regulation can be done automatically when the metering unit is unplugged. As a result, it is possible to assess the function of the suction valves and pressure valves separately, as will be explained below. It is also possible, with the aid of suitable software functions, to automate the course of the test. 
     In the testing method of the invention, a rail pressure sensor cable adapter is used as an intermediate plug, with a pickup for an oscilloscope cable. For evaluating the signals of the rail pressure sensor, an oscilloscope is used. Alternatively, an oscilloscope function of an existing testing device can be used. 
     The method according to the invention functions as follows: The engine is in the idling mode. The rail pressure sensor cable adapter is plugged in. In a first measurement, the pressure limiting valve is closed. The engine runs at 600 rpm. The step-up ratio between the pump rpm and the engine rpm is 5:3. Accordingly, the high-pressure pump runs at 1000 rpm. 1000 rpm is equivalent to 16.66 revolutions per second. Thus if the pressure valve is defective, for instance if particles have become stuck in the valve or the seat is not tight, a characteristic rail pressure oscillation occurs at 16.66 revolutions per second. The frequency of this oscillation is independent of whether it is a suction valve or a pressure valve that is defective. The associated period is 0.06 seconds. At this point in operation, the rail pressure is measured synchronously with injection, or in other words shortly before each injection. Injection takes place upon every second revolution. As a result, for six cylinders, there are five injections per second for each cylinder, and hence thirty injections per second in all. This is equivalent to 30 Hz or 0.033 seconds. The oscillation can thus be only inadequately detected via the rail pressure sensor signal by the control unit integrated into the internal combustion engine. In addition, the signal is filtered once again in the control unit. Especially at step-up ratios higher than 5:3, detection via the control unit cannot be recommended. Therefore in an exemplary embodiment of the method of the invention, the raw signal of the rail pressure sensor is used as the measured value for the rail pressure, rather than the signal from the control unit integrated into the vehicle. 
     In  FIG. 3 , the raw signal of the rail pressure sensor is plotted in volts, over time in seconds. The injection quantity is set at 10 mg. The pressure limiting valve is closed. Over the period of time observed, the raw signal of the rail pressure sensor has a relatively constant value of approximately 1.4 volts. The rail pressure is accordingly stable. 
     In  FIG. 4 , a suction valve is defective. In comparison to  FIG. 3 , no substantial distinction can be seen, since if a suction valve is defective, the two pump elements that remain furnish enough replenishing quantity, and because of the low pumping quantities in idling, no oscillation of high amplitude occurs. The pressure valve in the element having the defective suction valve remains constantly closed. 
     In  FIG. 5 , the raw signal of the rail pressure sensor is plotted over time when a pressure valve is defective. As  FIG. 5  shows, the raw signal of the rail pressure sensor fluctuates between approximately 1.3 and 1.5 volts. The oscillation occurs because the defective pressure valve does not close. Accordingly, in the pumping stroke of the associated piston, a certain quantity is indeed pumped into the high-pressure fuel reservoir. However, in the ensuing intake stroke of the piston, this quantity is reaspirated via the defective pressure valve. Thus a certain quantity of fuel is shifted back and forth in the high-pressure region, leading to the oscillation shown in  FIG. 5 . 
     In a second part of the testing method of the invention, the engine is still running in the idling mode. Via a special function of the control unit, the metering unit is opened in order to increase the pumping quantity. The increased pumping quantity causes the pressure limiting valve to open. The same effect is attained if a pressure regulating valve at the high-pressure pump is opened into the low-pressure region. 
     In  FIG. 6 , it can be seen that in this artificial elevation of the rail pressure, the raw signal of the rail pressure sensor increases from approximately 1.4 to approximately 2.5 volts. 
     In  FIG. 7 , the raw signal of the rail pressure sensor is plotted over time when a suction valve is defective. At the higher pressure, an oscillation at the same frequency as in  FIG. 5  results, since because of the defective suction valve, one pump element is not pumping. At the elevated pressure and the increased pumping quantity, the failure of the pump element is not compensated for by the other two pump elements. 
     In  FIG. 8 , the raw signal of the rail pressure sensor is plotted over time when a pressure valve is defective. Once again, an oscillation of the same frequency occurs. 
     By way of a comparison of two pumping operations with the pressure limiting valve open and the pressure limiting valve closed, it can be ascertained whether it is a suction valve or a pressure valve in the high-pressure pump that is defective. By the testing method of the invention, if it is suspected that a high-pressure pump is defective, the function of the pumping of all the pump elements can be tested, and thus indirectly the metering unit can be excluded as the source of the defect. Moreover, uneven pumping of the pump because of suction valves with different opening pressures can be detected, since the different opening pressures lead to a similar oscillating behavior. 
     The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.