Abstract:
In a method for controlling the injection amount for an injector on an internal combustion engine, dependent on a given time period and the duration of the hold phase, the energy stored in the injector is calculated either with a correction value or with the voltage and charge values at the end of the hold phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of International Application No. PCT/EP2008/051056 filed Jan. 29, 2008, which designates the United States of America, and claims priority to German Application No. 10 2007 008 201.2 filed Feb. 19, 2007, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The invention relates to a method of controlling an injection quantity of an injector of an internal combustion engine according to the features of the preamble of claims  1  and  7 . 
     BACKGROUND 
     Fuel injection devices for operating an i.c. engine have been generally known for many years. In a so-called common-rail injection system the feeding of fuel into the respective combustion chamber of the i.c. engine is effected by means of injectors. During this process a high injection pressure and precise control of the injection quantity is advantageous because this makes it possible to achieve, on the one hand, a high specific power of the i.c. engine and, on the other hand, a low emission of pollutants. 
     Control of the injection quantity is effected in this case by means of a closed control loop. The energy stored in the injector is used as a controlled variable as this energy correlates with the injection quantity. By means of a supply unit the individual injectors are then charged and discharged. Under certain conditions, as will be explained in more detail in the description of the figures, it may happen that the calculated energy stored in the injector does not correlate with the injection quantity and so the closed control loop no longer operates in an optimum manner. 
     SUMMARY 
     According to various embodiments, a method can be provided that enables more precise control of the injection quantity by way of a more precise calculation of the energy quantity stored in the injector. 
     According to an embodiment, in a method of controlling the injection quantity of a chargeable and dischargeable injector, in particular a piezoelectric injector, of an internal combustion engine, an injector voltage and an injector charge are determined, by means of which an energy, which is stored in the injector and correlates with the injection quantity, as well as an injector capacity are calculated. According to this method,—the charge value and the voltage value are determined after a definable time after the end of the charging phase of the injector, and from the values a first energy value is calculated,—given a period of time that is longer than a definable period of time for a retaining phase arising between the charging phase and the discharge phase, the voltage value and the charge value of the injector are determined anew and from the values a second energy value and a correction value are calculated, the correction value being stored in a first characteristics map, and if a period of time that is shorter than a definable time period for a retaining phase arising between the charging phase and the discharge phase, the first calculated energy value is multiplied by the correction value stored in the first characteristics map. 
     According to a further embodiment, the instant of the second voltage measurement and of the second charge measurement may be at the end of the retaining phase. According to a further embodiment, for determining the second energy value the charge determined at the end of the charging phase may be squared and divided by the calculated capacity at the end of the retaining phase and multiplied by the factor 0.5. According to a further embodiment, the correction value can be stored in a first characteristics map, in dependence upon an actuator temperature of the injector and a rail pressure. According to a further embodiment, the actuator temperature can be determined by determining a mean capacity value of all actuators over a definable number of injections, and by means of a stored second characteristics map the actuator temperature for the mean capacity value determined in each case is identified. According to a further embodiment, when the correction values are stored in the first characteristics map, the values present in the characteristics map prior to storing can be overwritten. 
     According to another embodiment, in a method of controlling the injection quantity of a chargeable and dischargeable injector, in particular a piezoelectric injector, of an internal combustion engine, an injector voltage and an injector charge are determined, by means of which an energy stored at the injector is calculated, which correlates with the injection quantity, and an injector capacity. According to this embodiment,—a mean charge value and a mean voltage value are determined after a definable time after the end of the charging phase of all injectors, and from the values a first energy value is calculated,—if the period of time of a retaining phase between the charging phase and the discharge phase is longer than a definable time period, the mean voltage and the mean charge of all injectors are determined at a definable instant, and a second energy value and a correction value are calculated, the correction value being stored in a first characteristics map, and if the period of time of a retaining phase between the charging phase and the discharge phase is shorter than a definable time period, the first calculated energy value is multiplied by the correction value stored in the first characteristics map. 
     According to a further embodiment, the instant of the second voltage measurement and charge measurement may be at the end of the retaining phase. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particulars of the invention are described in detail with reference to the drawings. These show: 
         FIG. 1 : a diagrammatic representation of a piezoelectric injector, 
         FIG. 2 : the time characteristic of the stored energy of an injector during a charging phase of the injector, 
         FIG. 3 : a flowchart for calculating the energy stored in the injector. 
     
    
    
     DETAILED DESCRIPTION 
     According to the various embodiments, by means of a second measurement of the voltage value and the charge value, the energy amount stored in the injector may be calculated more precisely. This prevents injectors with a large return stroke being charged to a lesser extent and hence injecting a lesser amount. In this way return stroke influences, which with only a single measurement lead to defective control of the injection quantity, may be prevented. In particular it is thereby possible to avoid a complex burst measurement with sensor detection analysis. 
     According to a further embodiment, it is provided that a calculated correction value is stored in a characteristics map. The correction value in this case describes the extent of the return stroke change and/or the influence of the filling state in the actuator antechamber upon the energy stored in the injector. As the return stroke of the injector may vary during operation and this has an effect upon the capacity of the injector, the calculation of the stored energy is therefore distorted if the return stroke is not taken into account. A correction value that is re-calculated at regular intervals may therefore ensure that the influence of the return stroke variation on the energy calculation is taken into account. 
     According to a further embodiment, it is provided that for the energy calculation, in the situation where the length of time of the retaining phase is longer than a definable period, only the capacity at the end of the retaining phase is used. As a result, neither a possible feed of the charge because of piezoelectric capacity variations and because of a possibly provided output filter during the retaining phase nor the discharge of the injector because of a possible parallel shunt in long retaining phases has an adverse effect upon the accuracy of the calculated energy value. 
     According to a further embodiment, by way of a calculated mean capacity of all injectors over a plurality of working cycles an actuator temperature of an injector may be determined. This makes it possible to dispense with an additional temperature sensor for measuring the actuator temperature. 
       FIG. 1  shows a diagrammatic representation of a piezoelectric injector  1 , which is composed of an actuator  8 , an injector needle  3 , a control piston  9  and a control valve  2 . The control valve  2  in this case separates an intermediate control chamber  6  from a return channel  7 , the control valve  2  being held in this position by means of a preloaded spring  11 . 
     Highly pressurized fuel passes via an input throttle  4  into the injector  1  and via an output throttle  5  into the intermediate chamber  6 . Two lines  10 ′ and  10 ″ moreover separate an actuator antechamber  12  from the return channel  7 . In this case, the actuator antechamber  12  and the return channel  7  are filled with fuel at all times. 
     When the control valve  2  opens, the highly pressurized fuel of the intermediate control chamber  6  expands and flows into the low-pressure region of the return channel  7 . This leads to a momentary pressure increase within the return channel  7  and so for a short time fuel flows out of the return channel  7  through the two lines  10 ′ and  10 ″ into the actuator antechamber  12  and hence exerts a counterforce on the movement of the actuator  8 . At the same time the injector needle  3  starts to move in the direction of the actuator  8  and therefore carries on feeding fuel through the output throttle  5  into the intermediate chamber  6  and hence also into the return channel  7 . In this case, the counterforce upon the actuator  8  is maintained until the low pressure has spread from the return channel  7  to the output throttle  5 . 
     The filling state of the actuator antechamber  12  moreover has an influence on the injector operation. In the initial state, the actuator antechamber  12  is full of fuel. In this case, however, it may happen that an air bubble has formed in the actuator antechamber  12 . Because of this air bubble the counterforce opposing the actuator movement is lower than in the case of an exclusive filling of the actuator antechamber  12  with fuel. Upon a collapse of the air bubble the counterforce increases, with the result that a greater charge has to be fed in the direction of the actuator  8 . 
       FIG. 2  shows the time characteristic of the stored energy of an injector during a charging phase of the injector. The top diagram here shows the charging pulse I fed to the injector as a function of time. The bottom diagram shows the development of the energy E stored in the injector as a function of time. Here, in the case of the energy characteristic a distinction is made between whether or not an action of force upon the movement of the actuator, as described in  FIG. 1 , occurs. The calculation of the energy stored in the injector is effected by multiplying a determined voltage value by a determined charge value and a factor 0.5. 
     A charging pulse I 0  is fed to the injector. The charging pulse I 0  in this case starts at the time t 0  and ends at the time t 2 . The calculated characteristic E 1  of the energy stored in the injector in this case rises from the start of the charging pulse I 0  at the time t 0  and runs for example linearly. With this characteristic it is ensured that the fuel from the return channel does not flow into the actuator antechamber and exert a counterforce on the movement of the actuator there. 
     The situation, where the counterforce because of an air bubble in the actuator antechamber is lower than the counterforce in the case of exclusive filling of the actuator antechamber with fuel, is not represented. The calculated energy characteristic for this situation would then, from the time t 1  onwards, fall and subsequently run linearly. 
     The energy characteristic E 2 , on the other hand, represents the characteristic from when a counterforce is exerted on the movement of the actuator. The energy characteristic E 2  in this case starts, just like the energy characteristic E 1  at the time t 0 , to rise linearly. From the time t 1  the fuel flowing into the actuator antechamber presses against the movement of the actuator. Consequently, the actuator is unable to expand as much as an unloaded actuator, and the voltage across the actuator rises. Because of the voltage rise, the value of the energy stored in the injector likewise rises steeply and then runs on linearly up to the time t 2 . In this case, for the energy calculation it is immaterial whether the determined charge has risen or fallen because the raised voltage value of the actuator dominates the value of the charge. 
     For a control operation by means of the energy E 2  stored in the injector, the closed control loop from the time t 1  determines too high an energy value. It will therefore reduce the charge supplied to the injector in order to lower the energy stored in the injector. The lower energy stored in the injector will however subsequently lead to the injection of too low a quantity of fuel. Under these conditions, therefore, the energy quantity stored in the injector no longer correlates with the injection quantity. 
       FIG. 3  shows a flowchart for calculating the energy stored in the injector. Here, in step S 1  there is determined in each case for each injector a first voltage value U 1 , a first charge value Q 1  and a rail pressure p after a definable period after the end of the charging phase of the injector. In step S 10  by means of the voltage—and charge values determined in step S 1  a first energy value EN 1  stored in the injector and a first capacity value C 1  are determined. The energy amount EN 1  stored in the injector is determined by multiplying the voltage value U 1  determined in step S 1  by the determined charge value Q 1  and the factor 0.5. The energy calculation in this case is not restricted to this example, rather other types of energy calculation are also conceivable. 
     Furthermore, for each injection a first mean capacity value Cm 1  of all capacities of the respective injectors is generated and stored. As soon as a specific number of injections has been completed by the injectors, a second mean capacity value Cm 2  of all injectors may be calculated by means of the first mean capacity value Cm 1  stored in each case for each injection. Here, it has proved advantageous to calculate the mean capacity value Cm 2  after 100 injections. By means of the calculated second mean capacity value Cm 2  an actuator temperature T may be determined by way of a stored characteristics map. 
     In step S 20  it is checked whether a retaining phase time length tm is longer than a definable period t 2 . Should this be the case, then in step S 40  at the definable time t 2  a second voltage value U 2  and a second charge value Q 2  are determined. The time t 2  is however selected in such a way that the distortions of the energy calculations because of an exertion of force by the fuel on the movement of the actuator as a result of the flow of the fuel from the return channel into the actuator antechamber no longer occur. In this respect it has proved advantageous if the time tm is selected as close as possible to the end of the retaining phase. 
     Then, in step S 50 , by means of the voltage value U 2  and charge value Q 2  determined in each case in step S 40  a second capacity value C 2  and a second energy value EN 2  stored in the injector are calculated. In this case, for calculation of the second energy value EN 2  the charge value Q 1  determined in step S 1  is squared and multiplied by a factor 0.5 and divided by the second capacity value C 2  determined in step S 50 . Based on the first energy value EN 1  determined in step S 10  and the second energy value EN 2  determined in step S 50 , in step S 60  a correction value f is determined by dividing the first energy value EN 1  determined in step S 10  by the second energy value EN 2  determined in step S 50 . 
     Furthermore, in step S 60  the correction value f is stored in a characteristics map, in dependence upon the rail pressure determined in step S 1  and upon the actuator temperature T determined in step S 10 . When these correction values are stored, the values already in the characteristics map are overwritten. By updating the correction value f it is therefore possible to ensure that an adaptation of the calculation of the energy values stored in the injector that is necessary because of a return stroke variation arising during operation is effected. A capacity variation occasioned by a return stroke variation has an effect upon the voltage determined in step S 40  and hence upon the stored energy calculated in step S 50  and hence also upon the correction value f calculated in step S 60 . 
     Should the result of the interrogation in step S 20  be that the retaining phase time length tm is shorter than a definable period t 2 , then in step S 30  a third energy value EN 3  is calculated by means of the first energy value EN 1  calculated in step S 10  and by means of a correction value f that is valid for this actuator temperature T and this rail pressure p. In this case, the energy value EN 1  is multiplied by the correction value f. 
     As an alternative to the method presented in  FIG. 3 , a mean voltage value, charge value and capacity value of all injectors may be used for the energy value calculation in the steps S 10  and S 50 .