Abstract:
A method for operating an injection system, in particular an injection system of an internal combustion engine having piezoelectrically triggered injectors, in which a basic voltage is applied to the injectors in the closed state, and a bottom voltage is applied to open them. The bottom voltage is individually adjusted for each injector.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and to a control device for operating an injection system, in particular an injection system of an internal combustion engine, having piezo-electrically triggered fuel injectors, in which a basic voltage is applied to the fuel injectors in the closed state, and a bottom voltage is applied to open them. 
       BACKGROUND INFORMATION 
       [0002]    Generic injection systems are referred to as common rail injection systems and used both in Otto and in diesel combustion engines. The injection system usually includes one piezoelectric fuel injector per cylinder of the internal combustion engine. 
         [0003]    In particular in stratified operation of an Otto combustion engine, what is generally known as micro-quantities in the range of approximately 1 mm 3  are injected during each injection. It has become apparent that the deviations of the injection quantity between the piezoelectric injectors of the cylinders are so considerable that they manifest themselves in a reduced exhaust-gas quality and in lower driving comfort as a result of greater irregular running. 
         [0004]    Therefore, it is an object of the present invention to provide a method and a control device for implementing the method, which reduce the deviations of the injection quantities among different piezoelectric injectors. 
       SUMMARY OF THE INVENTION 
       [0005]    This problem is solved by a method for operating an injection system, in particular an injection system of an internal combustion engine, having piezo-electrically triggered injectors, in which a basic voltage is applied to the injectors in the closed state, and a bottom voltage is applied to open them, the bottom voltage being individually adjusted for each injector. The bottom voltage is a voltage value which is lower than the basic voltage, to which value the voltage applied to a piezo element of the piezoelectric injector is intermittently lowered so as to achieve micro quantities in the injection. 
         [0006]    It is preferably provided that the bottom voltage for each injector is determined from an average value of the bottom voltage and an individual correction value for the injector. The average value of the bottom voltage (average value across a plurality of piezoelectric injectors of an internal combustion engine) may be determined from, e.g., a setpoint injection quantity and an associated average bottom voltage stored in a control device. In this context the average bottom voltage may be ascertained in a test over a large number of piezoelectric injectors, for example, and stored in the control device as type-specific value. 
         [0007]    It is preferably provided that the correction value includes a correction factor, the bottom voltage for each injector being determined from the average value of the bottom voltage multiplied by the correction factor. As an alternative or in addition, the correction value may include an offset, and the bottom voltage for each injector is determined from the average value of the bottom voltage to which the offset is added. 
         [0008]    The correction value may be specific to an operating point, i.e., different correction values (correction factor or offset) may be stored in the control device for different average values of the bottom voltage (and thus different setpoint injection quantities). 
         [0009]    The problem mentioned in the introduction is also solved by a computer program having program code to implement all of the steps according to a method of the present invention when the program is executed on a computer, and also by a control device for operating an injection system, in particular an injection system of an internal combustion engine, having piezo-electrically triggered injectors, in which a basic voltage is applied to the injectors in the closed state, and a bottom voltage is applied to open them, wherein the bottom voltage is individually adjusted for each injector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a sketch of a voltage characteristic over time at a piezoelectric injector. 
           [0011]      FIG. 2  shows a sketch of the injection quantity over the bottom voltage for a plurality of piezoelectric injectors. 
           [0012]      FIG. 3  shows a flow chart of the method. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  shows trigger voltage U An  of a piezoelectric injector over time t. Trigger voltage U An  is specified in Volt, time t has been plotted in microseconds μs. Trigger voltage U An  is the voltage applied at the piezo element. A piezoelectric injector (piezo injector) having a direct needle is assumed here, i.e., a piezoelectric injector in which the valve needle is directly set in motion by a piezoelectric actuator. In the direct needle control, the nozzle needle is actively triggered and moved by a direct actuating element. The actuating element is a piezoelectric actuator, which directly controls the nozzle needle via a hydraulic coupler. 
         [0014]      FIG. 1  shows the triggering of such a piezoelectric injector for the purpose of achieving what is referred to as micro-quantities. Micro-quantities are injected, e.g., in the pre-injection of an Otto engine in stratified-charge operation. The injection process is induced by lowering a constant basic voltage U Ba  of, for example, 190 Volt, to what is generally known as bottom voltage U Bo . The injection quantity is a function of the difference between basic voltage U Ba  and bottom voltage U Bo . Bottom voltage U Bo  is kept constant for a holding period t H , and raised again to basic voltage U Ba  once the holding period, which amounts to a constant 38 μs in the present example of  FIG. 1 , has elapsed.  FIG. 1  shows three exemplary voltage characteristics having different bottom voltages U Bo  of 30 Volt, 50 Volt and 70 Volt, respectively. Voltage gradient ΔU/Δt when lowering the voltage from basic voltage U Ba  to bottom voltage U Bo  (grad_ 1 ) and also voltage gradient ΔU/Δt when increasing the voltage from bottom voltage U Bo  to basic voltage U Ba  (grad_ 2 ) are mainly specified by the capacitance of the piezo elements and the displacement current of a driver stage for controlling the piezo elements and may thus assume different values. The injection quantity to achieve micro-quantities is thus realized solely via the control of bottom voltage U Bo , i.e., basic voltage U Ba  is kept constant, and the quantity is adjusted only by varying bottom voltage U Bo . The sum of the falling edge from basic voltage U Ba  to bottom voltage U Bo  as well as holding period t H , and the rising edge from bottom voltage U Bo  to basic voltage U Ba , is kept to a minimum. The voltage differential between basic voltage U Ba  and bottom voltage U Bo  becomes smaller due to the increase in bottom voltage U Bo , the entire control period, i.e., the time from the start of the reduction of basic voltage U Ba  to bottom voltage U Bo  and the reattainment of basic voltage U Ba  starting from bottom voltage U Bo , is reduced, which leads to a decrease in the injection quantity. Bottom voltage U Bo  may be increased until the pre-injection is omitted completely, i.e., the injection quantity becomes equal to zero. Given a constant basic voltage U Ba , constant holding period t H , and constant voltage gradients when lowering the voltage or when increasing the voltage, the injection quantity is therefore solely a function of bottom voltage U Bo . That is to say, the injection quantity for an injector is representable as being a function of a single variable—bottom voltage U Bo . 
         [0015]    In  FIG. 2 , injection quantity Q has been plotted in cubic millimeters per injection process mm 3 /H (here, the injection process is abbreviated by H as in Hub (lift)) over bottom voltage U Bo  in Volt for a plurality of injectors, which are denoted by EV 1  through EV 4 . To this end, four identical type PDN25B injectors were operated at different bottom voltages U Bo  with a constant basic voltage of U Ba  amounting to 190 Volt, and the individually achieved injection quantity Q was measured in cubic millimeters for each injection process H. The bottom voltage was increased in two-Volt increments. With each increment, fifty injection quantities Q were measured for each injection process. From this, a lift/lift deviation was determined for each piezoelectric injection valve (also referred to as injector), for one, and deviations Ex/Ex among the individual piezoelectric injectors were determined for another. The curves of the lift/lift deviations are shown in the lower region of  FIG. 2  and denoted by H/H, and the Ex/Ex deviations can be found in the curves above, which are denoted by EV 1  through EV 4 . On the one hand, it can be gathered from the illustration of  FIG. 2  that the injection quantity decreases with increasing bottom voltage U Bo ; on the other hand, a relatively high deviation is noticeable among the examined injectors. The lift/lift deviations are relatively low, so that their effects may be disregarded. However, the curves of injection quantity Q over bottom voltage U Bo  of the same-type piezoelectric injectors used in the testing, which are denoted by EV 1  through EV 4 , are considerably higher. In the example of  FIG. 2 , to inject a quantity of one cubic millimeter per injection, piezoelectric fuel injector  1  requires a bottom voltage U Bo  of 86 V, whereas piezoelectric fuel injector  4  requires a bottom voltage U Bo  of 97 V for the same injection quantity. The two other piezoelectric injectors  2 ,  3  require a voltage of approximately 91 V for this purpose. Voltage range dU for achieving identical injection quantities of the injectors thus amounts to 11 V in this case. If all piezoelectric injectors  1  through  4  in an internal combustion engine are operated at the same averaged bottom voltage U Bo  of 91 V, then this results in a deviation of the injection quantity for the four piezoelectric fuel injectors of 0.8 mm 3 /H. At the bottom voltage of 91 V, piezoelectric injector  1  produces an injection quantity of approximately 0.8 mm 3 /H; at a bottom voltage U Bo  of 91 V, piezoelectric injector  4  produces an injection quantity of approximately 1.6 mm 3 /H. Such differences in the pre-injection quantity in different cylinders increase the emissions. In order to then equalize the piezoelectric injectors for the micro-quantities on the side of the control device, bottom voltages U Bo  of the different injectors are corrected in an injector-specific manner. The correction is induced via a factor K Bo  or an offset C Bo . In the selected exemplary operating point having a setpoint injection quantity (desired injection quantity) Q of 1 mm 3 /H and a rail pressure of 800 bar, bottom voltage U Bo     —     M  averaged over the four piezoelectric injectors amounts to 91 V. As can be inferred directly from  FIG. 2 , the bottom voltage for piezoelectric injector  1  at a setpoint injection quantity Q of one cubic millimeter per injection amounts to 87 V, so that a factor of 0.9451 is applied to the average value of bottom voltage U Bo     —     M  in order to arrive at required bottom voltage U Bo  of 87 V. This corresponds to an offset C Bo  with respect to the averaged bottom voltage of −5 V. In the example of  FIG. 2 , piezoelectric injector  4  requires a higher bottom voltage U Bo  in order to realize 1 mm 3 /H and a factor &gt;1 must therefore be applied. In general, the factor is calculated in the following way: 
         [0000]        K   Bo   =U _required/ U _average 
         [0000]    U_required denotes the bottom voltage at each piezoelectric injector that is required in order to achieve the setpoint injection quantity. U_average denotes the bottom voltage, averaged across all piezoelectric injectors, required so as to achieve the setpoint injection quantity. In the example from  FIG. 2 , bottom voltage U Bo     —     4  required for the piezoelectric injector for an injection quantity of 1 mm 3 /H amounts to 97 V, U_required=97 V. The averaged bottom voltage U_average amounts to 91 V, from which a correction factor K Bo     —     4  of 1.066 results for piezoelectric injector  4 . The offset voltage for piezoelectric injector  4  is 6 V. If this injector-specific correction is then applied, the injectors are equalized on the side of the control device, which results in a reduction of the Ex/Ex deviations of the pre-injection quantity. 
         [0016]    The determination of correction factors K Bo  or voltage offsets C Bo  explained on the basis of the example of a setpoint injection quantity Q of one cubic millimeter per injection, must be implemented separately for each individual operating point. To this end, the operating points in a range of injection quantities per injection that is used for micro-quantities and thus may be utilized for an equalization, could be examined in a stepwise manner, for instance. In a range of the setpoint injection quantity between 0.4 cubic millimeter per injection and 3 cubic millimeter per injection, for example, with steps of 0.1 cubic millimeter per injection, correction values for the individual injectors could be determined. The correction values are then stored in a control device in the form of a table of a characteristic map, for example. 
         [0017]    As an alternative, it is possible to determine a constant correction value across the entire range. To this end, an average correction value may be ascertained, which is determined from correction values across a range as previously explained, or it is possible to use a correction value at a single working point, as explained with the aid of  FIG. 2 , for example, for the entire range. In this case, instead of a table having a plurality of values specific to operating points, a single value may be used for each piezoelectric injector. 
         [0018]      FIG. 3  shows a flow chart of an associated working process. In a step  101 , a setpoint injection quantity Q setpoint  is first determined for each injection process. This value may be provided by a control device of an internal combustion engine in a manner specific to an operating point, for instance. Then, in a step  102 , average value U Bo     —     M  is determined for the average bottom voltage of all piezoelectric injectors. The average value may be stored as an average setpoint value for a model line of a piezoelectric injector, which represents the average value across a large number of identical piezoelectric injectors. An individual correction factor K Bo  is then determined for each piezoelectric injector in step  103 . The index (n) here is meant to illustrate that this value is determined individually for every piezoelectric injector. The values may be taken from, e.g., a table, which is stored in a memory. In step  104 , the value of the bottom voltage for each piezoelectric injector n is then determined as the product from injector-specific factor K Bo     —     (n)  and average bottom voltage U Bo     —     M . Instead of factors K Bo     —     (n) , it is also possible here to determine offsets C Bo  in an analogous manner.