Patent Publication Number: US-6983731-B2

Title: Method for operating a fuel injection system for an internal combustion engine

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for operating a fuel injection system for an internal combustion engine. 
   BACKGROUND INFORMATION 
   Published German patent document DE 100 33 343 discloses a fuel injection system for an internal combustion engine, in particular a diesel engine, that includes an injection control system for monitoring and/or for resolving a conflict upon triggering of the actuator elements, in particular a conflict management system for superimposed injection curves of piezoactuators. 
   With so-called common rail piezoactuators, only one triggering edge can be executed at a time. For reasons of combustion engineering, however, it is necessary to apply the triggering of complementary banks in such a way that injections are superimposed. This is possible, e.g., with the circuit device known from published German patent document DE 100 33 343 for interconnecting piezoelectric elements, when the charging/discharging edges of the piezoelectric elements exhibit no overlap. With overlapping edges, provision is made in the context of the fuel injection system disclosed in, e.g., published German patent document DE 100 33 343, for the triggering action with low priority (hereinafter called the low-priority triggering action) to be shifted or shortened. 
   It is an object of the present invention to detect and determine edge overlaps, and to determine therefrom the necessary degree of time shifting or shortening out of the overlap region. 
   SUMMARY 
   In accordance with the present invention, the above object is achieved, in a method for operating a fuel injection system of the kind described initially, in that the edge overlaps are determined during static and dynamic interrupts of a triggering circuit during operation of the injection system. This determination is accomplished as a function of the rotation speed and crankshaft angle of the internal combustion engine. 

   
     In this context, individual edge times are examined in pairs for overlap. Based on the determined edge overlaps, the necessary degree is determined of time shifting or shortening. 
       FIG. 1  shows a diagram of an interconnection of piezoelectric elements. 
       FIG. 2A  shows one example of charging of a piezoelectric element. 
       FIG. 2B  shows another example of charging of a piezoelectric element. 
       FIG. 2C  shows one example of discharging of a piezoelectric element. 
       FIG. 2D  shows another example of discharging of a piezoelectric element. 
       FIG. 3  shows a block diagram of triggering IC. 
       FIG. 4  shows a time sequence of interrupts known in the art. 
       FIG. 5  is a chart plotting low-priority edges versus high-priority edges, which chart shows collision regions of edge pairs in terms of angular region. 
       FIG. 6  schematically depicts the shifting of a low-priority edge later in time. 
       FIG. 7  schematically depicts the shortening of a low-priority triggering action. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 ,  60  as well as circuit arrangements for triggering the piezoelectric elements. The letter A designates a region depicted in detail, and B a region not depicted in detail, the separation of which is indicated by a dashed line c. Region A depicted in detail includes a circuit for charging and discharging piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60 . In the example shown in  FIG. 1 , piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  are actuators in fuel injection valves (in particular, in so-called common rail injectors) of an internal combustion engine. In the embodiment described, six piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  are used for independent control of six cylinders within an internal combustion engine; however, any other number of piezoelectric elements could be used for any other desired purposes. 
   Region B not depicted in detail in  FIG. 1  includes an injection control system F having a control unit D and a triggering IC E that serves to control the elements inside region A depicted in detail. Various measured values of voltages and currents are conveyed to triggering IC E from the entirety of the remaining triggering circuit for the piezoelectric elements. According to the present invention, control unit D (e.g., computer) and triggering IC E regulate the triggering voltages and triggering times for the piezoelectric elements. Control computer D and/or triggering IC E also monitor various voltages and currents of the entire triggering circuit for the piezoelectric elements. 
   In the description below, the individual elements inside region A depicted in detail will be introduced first. A general description of the operations of charging and discharging piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  then follows. Lastly, a detailed description is given of how both operations are controlled and monitored by control computer D and triggering IC E. 
   Piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  are divided into a first group G 1  and a second group G 2 , each of which encompasses three piezoelectric elements (i.e. piezoelectric elements  10 ,  20 , and  30  in first group G 1 , and piezoelectric elements  40 ,  50 , and  60  in second group G 2 ). Groups G 1  and G 2  are constituents of circuit parts connected in parallel. With group selection switches  310 ,  320 , it is possible to define which of groups G 1 , G 2  of piezoelectric elements  10 ,  20 , and  30  or  40 ,  50 , and  60  are respectively discharged by way of a common charging and discharging device (group selection switches  310 ,  320  are of no importance for charging operations, however, as described in further detail below). Piezoelectric elements  10 ,  20 , and  30  of first group G 1  are disposed in one actuator bank, and piezoelectric elements  40 ,  50 , and  60  in second group G 2  are disposed in a further actuator bank. The term “actuator bank” designates a block in which two or more actuator elements, e.g., piezoelectric elements, are immovably placed, e.g., encapsulated. 
   Group selection switches  310 ,  320  are disposed between a coil  240  and the respective groups G 1  and G 2  (the coil-side terminals thereof), and the switches are embodied as transistors. Drivers  311 ,  321 , which convert the control signals received from triggering IC E into voltages that are selectable, as necessary, for closing and opening the switches, are implemented. 
   Diodes  315  and  325  (referred to as group selection diodes) are provided in parallel with group selection switches  310 ,  320 , respectively. If group selection switches  310 ,  320  are embodied as MOSFETs or IGBTs, these group selection diodes  315  and  325  can be constituted, for example, by the parasitic diodes themselves. During charging operations, group selection switches  310 ,  320  are bypassed by diodes  315 ,  325 . The functionality of group selection switches  310 ,  320  is thus reduced to the selection of a group G 1 , G 2  of piezoelectric elements  10 ,  20 , and  30  or  40 ,  50 , and  60  only for the discharging operation. 
   Within groups G 1  and G 2 , piezoelectric elements  10 ,  20 , and  30 , and piezoelectric elements  40 ,  50 , and  60  are disposed respectively as constituents of parallel-connected piezo branches  110 ,  120 , and  130  (group G 1 ), and parallel-connected piezo branches  140 ,  150 , and  160  (group G 2 ). Each piezo branch encompasses a series circuit made up of a first parallel circuit having a piezoelectric element  10 ,  20 ,  30 ,  40 ,  50 , or  60  and a resistor (called a branch resistor)  13 ,  23 ,  33 ,  43 ,  53 , or  63 ; and a second parallel circuit having a selection switch (called a branch selection switch) embodied as a transistor  11 ,  21 ,  31 ,  41 ,  51 , or  61 , and a diode (called a branch diode)  12 ,  22 ,  32 ,  42 ,  52 , or  62 . 
   Branch resistors  13 ,  23 ,  33 ,  43 ,  53 , and  63  cause the respectively corresponding piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  to discharge continuously during and after a charging operation, since they respectively interconnect two terminals of the capacitative piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60 . Branch resistors  13 ,  23 ,  33 ,  43 ,  53 ,  63  are of sufficient size, however, to make this operation slow with respect to the controlled charging and discharging operations, as described below. The charging of any piezoelectric element  10 ,  20 ,  30 ,  40 ,  50 ,  60  within a relevant time after a charging operation is therefore to be regarded as invariable. 
   The branch selection switch/branch diode pairs in the individual piezo branches  110 ,  120 ,  130 ,  140 ,  150 ,  160 —i.e. selection switch  11  and diode  12  in piezo branch  110 , selection switch  21  and diode  22  in piezo branch  120 , etc.—can be embodied as electronic switches (i.e. transistors) having parasitic diodes, for example MOSFETs or IGBTs (as indicated above for the group selection switches/diode pairs  310  and  315 , and  320  and  325 ). 
   With the aid of branch selection switches  11 ,  21 ,  31 ,  41 ,  51 ,  61 , it is possible to define which of piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 ,  60  are respectively charged by way of a common charging and discharging device; the piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and/or  60  charged in each case are all those whose branch selection switches  11 ,  21 ,  31 ,  41 ,  51 , and/or  61  are closed during the charging operation (described below). Usually only one of the branch selection switches is closed at a time. 
   Branch diodes  12 ,  22 ,  32 ,  42 ,  52 , and  62  serve to bypass branch selection switches  11 ,  21 ,  31 ,  41 ,  51 , and  61  during discharging operations. In the example considered, each individual piezoelectric element can therefore be selected for charging operations, whereas for discharging operations, either first group G 1  or second group G 2  of piezoelectric elements  10 ,  20  and  30 , or  40 ,  50 , and  60 , or both, must be selected. 
   Returning to piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  themselves, branch selection piezo terminals  15 ,  25 ,  35 ,  45 ,  55 , and  65  can be connected to ground either using branch selection switches  11 ,  21 ,  31 ,  41 ,  51 , and  61 , or via the corresponding diodes  12 ,  22 ,  32 ,  42 ,  52 , and  62 , and in both cases additionally via resistor  300 . 
   The currents flowing between branch selection piezo terminals  15 ,  25 ,  35 ,  45 ,  55 , and  65  and ground during the charging and discharging of piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  are measured by resistor  300 . A knowledge of these currents allows controlled charging and discharging of piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60 . It is possible, e.g., by closing and opening charging switch  220  and discharging switch  230  as a function of the magnitude of the currents, to adjust the charging current or discharging current to defined average values, and/or to prevent them from exceeding and/or falling below maximum and/or minimum values, respectively. 
   In the example embodiment, the measurement itself additionally requires a voltage source  621  that supplies a voltage of, for example, 5 VDC, as well as a voltage divider in the form of two resistors  622  and  623 . The purpose of this is to protect triggering IC E (which performs the measurements) from negative voltages, which otherwise might occur at measurement point  620  and cannot be handled by triggering IC E. Negative voltages of this kind are modified by addition, using a positive voltage supplied by the aforesaid voltage source  621  and the voltage divider resistors  622  and  623 . 
   The other terminal of the respective piezoelectric element  10 ,  20 ,  30 ,  40 ,  50 , or  60 , i.e. the respective group selection piezo terminal  14 ,  24 ,  34 ,  44 ,  54 , or  64 , can be connected to the positive pole of a voltage source via group selection switch  310  or  320  or via group selection diode  315  or  325 , and via a coil  240  and a parallel circuit made up of a charging switch  220  and a charging diode  221 ; or alternatively, or additionally, connected to ground via group selection switch  310  or  320 , or via diode  315  or  325 , and via coil  240  and a parallel circuit made up of a discharging switch  230  and a discharging diode  231 . Charging switch  220  and discharging switch  230  are implemented, for example, as transistors that are triggered via drivers  222  and  232 , respectively. 
   The voltage source encompasses a capacitor  210 . Capacitor  210  is charged by a battery  200  (for example, a motor vehicle battery) and a downstream DC voltage converter  201 . DC voltage converter  201  converts the battery voltage (for example, 12 V) into substantially any other desired DC voltages (for example, 250 V), and charges capacitor  210  to that voltage. Control of DC voltage converter  201  is accomplished via transistor switch  202  and resistor  203 , which serves to measure currents picked off at measurement point  630 . 
   For cross-checking purposes, a further current measurement at measurement point  650  is made possible by triggering IC E and by resistors  651 ,  652 , and  653 , and, for example, a 5 VDC voltage source  654 ; a voltage measurement at measurement point  640  is additionally possible by way of triggering IC E and the voltage-dividing resistors  641  and  642 . 
   Lastly, a resistor  330  (referred to as the total discharge resistor), a switch  331  (referred to as the stop switch), and a diode  332  (referred to as the total discharge diode) serve to discharge piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  (if outside the normal operation, they are not discharged by the “normal” discharging operation, as described below). Stop switch  331  may be closed after “normal” discharging operations (cyclical discharging via discharge switch  230 ), and thereby connects piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  through resistors  330  and  300  to ground. Any residual voltages that might remain in piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  are thus eliminated. Total discharge diode  332  prevents any occurrence of negative voltages at piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60 , which in some circumstances could be damaged by the negative voltages. 
   The charging and discharging of all piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50  and  60 , or of a specific piezoelectric element  10 ,  20 ,  30 ,  40 ,  50 , or  60 , is accomplished with the aid of a single charging and discharging device common to all the groups and their piezoelectric elements. In the example embodiment, the common charging and discharging device encompasses battery  200 , DC voltage converter  201 , capacitor  210 , charging switch  220 , discharging switch  230 , charging diode  221 , discharging diode  231 , and coil  240 . 
   The charging and discharging of each piezoelectric element is accomplished in the same manner, and is explained below with reference to only first piezoelectric element  10  for sale of simplicity. 
   The states occurring during the charging and discharging operations are explained with reference to  FIGS. 2A through 2D , of which  FIGS. 2A and 2B  illustrate the charging of piezoelectric element  10 , and  FIGS. 2C and 2D  illustrate the discharging of piezoelectric element  10 . 
   Control of the selection of one or more piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  to be charged or discharged—the charging operation and discharging operation described below—is accomplished by way of triggering IC E and control unit D by the opening and closing of one or more of the aforementioned switches  11 ,  21 ,  31 ,  41 ,  51 , and  61 ;  310 , and  320 ;  220 ,  230 , and  331 . The interactions between the elements inside region A depicted in detail on the one hand, and triggering IC E and control computer D on the other hand, are explained in further detail below. 
   With respect to the charging operation, firstly a piezoelectric element  10 ,  20 ,  30 ,  40 ,  50 , or  60  to be charged must be selected. To charge only first piezoelectric element  10 , branch selection switch  11  of first branch  110  is closed, while all the other branch selection switches  21 ,  31 ,  41 ,  51 , and  61  remain open. To charge exclusively any other piezoelectric element  20 ,  30 ,  40 ,  50 , or  60 , or to charge several elements simultaneously, the appropriate element(s) would be selected by closing the corresponding branch selection switches  21 ,  31 ,  41 ,  51 , and/or  61 . 
   The charging operation itself can then occur, as explained below: 
   For the example embodiment considered, a positive potential difference between capacitor  210  and group selection piezo terminal  14  of first piezoelectric element  10  is generally necessary for the charging operation. As long as charging switch  220  and discharging switch  230  are open, however, no charging or discharging of piezoelectric element  10  takes place. In this situation, the circuit depicted in  FIG. 1  is in a steady state, i.e., piezoelectric element  10  retains its charge state with substantially no change, and no currents flow. 
   To charge first piezoelectric element  10 , switch  220  is closed. Theoretically, first piezoelectric element  10  could be charged by that action alone. This would result in large currents, however, which might damage the elements in question. The currents occurring at measurement point  620  are therefore measured, and switch  220  is opened again as soon as the sensed currents exceed a certain limit value. To achieve a desired charge on first piezoelectric element  10 , charging switch  220  is therefore repeatedly closed and opened, while discharging switch  230  remains open. 
   Upon closer examination, the conditions occurring with charging switch  220  closed are those depicted in  FIG. 2A , i.e., a closed circuit is created encompassing a series circuit made up of piezoelectric element  10 , capacitor  210 , and coil  240 , and a current iLE(t) flows in the circuit, as indicated in  FIG. 2A  by arrows. As a result of this current flow, positive charges are conveyed to group selection piezo terminal  14  of first piezoelectric element  10 , and energy is stored in coil  240 . 
   If charging switch  220  is opened shortly (for example, a few microseconds) after closing, the conditions depicted in  FIG. 2B  result: a closed circuit is created encompassing a series circuit made up of piezoelectric element  10 , discharging diode  231 , and coil  240 , and a current iLA(t) flows in the circuit, as indicated in  FIG. 2B  by arrows. As a result of this current flow, energy stored in coil  240  flows into piezoelectric element  10 . Corresponding to the energy delivery to piezoelectric element  10 , the voltage occurring in the latter rises, and its external dimensions increase. Once energy has been transferred from coil  240  to piezoelectric element  10 , the steady state of the circuit (depicted in  FIG. 1  and already described) is once again attained. 
   At this point in time or earlier or later (depending on the desired time profile of the charging operation), charging switch  220  is once again closed and opened again, so that the processes described above occur again. Because charging switch  220  is closed and then opened again, the energy stored in piezoelectric element  10  increases (the energy already stored in piezoelectric element  10  and the newly delivered energy are added together), and the voltage occurring at piezoelectric element  10  rises, and its external dimensions become correspondingly greater. 
   If the aforementioned closing and opening of charging switch  220  are repeated many times, the increase in the voltage occurring at piezoelectric element  10 , and the expansion of piezoelectric element  10 , take place stepwise. 
   When charging switch  220  has been closed and opened a defined number of times, and/or when piezoelectric element  10  has achieved the desired charge state, charging of the piezoelectric element is terminated by leaving charging switch  220  open. 
   With regard to the discharging operation, in the example embodiment, piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  are discharged in groups (G 1  and/or G 2 ), as described below: 
   Firstly, group selection switches  310  and/or  320  of group G 1  and/or G 2 , whose piezoelectric elements are to be discharged, are closed (branch selection switches  11 ,  21 ,  31 ,  41 ,  51 , and  61  have no influence on the selection of piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and  60  for the discharging operation, since in this case they are bypassed by diodes  12 ,  22 ,  32 ,  42 ,  52 , and  62 ). In order to discharge piezoelectric element  10  as a part of first group G 1 , first group selection switch  310  is therefore closed. 
   When discharging switch  230  is closed, the conditions depicted in  FIG. 2C  occur: a closed circuit is created encompassing a series circuit made up of piezoelectric element  10  and coil  240 , and a current iEE(t) flows in the circuit, as indicated in  FIG. 2C  by arrows. As a result of this current flow, the energy (or at least a portion thereof) stored in the piezoelectric element is transferred into coil  240 . Corresponding to the energy transfer from piezoelectric element  10  to coil  240 , the voltage occurring at piezoelectric element  10  drops, and its external dimensions become smaller. 
   When discharging switch  230  is opened shortly (for example, a few microseconds) after being closed, the conditions depicted in  FIG. 2D  occur: a closed circuit is created, encompassing a series circuit made up of piezoelectric element  10 , capacitor  210 , charging diode  221 , and coil  240 , and a current iEA(t) flows in the circuit, as indicated in  FIG. 2D  by arrows. As a result of this current flow, energy stored in coil  240  is fed back into capacitor  210 . Once the energy transfer from coil  240  into capacitor  210  has occurred, the steady state of the circuit (depicted in  FIG. 1  and already described) is once again attained. 
   At this point in time, or earlier or later (depending on the desired time profile of the discharging operation), discharging switch  230  is once again closed and opened again, so that the processes described above occur again. Because discharging switch  230  is closed and then opened again, the energy stored in piezoelectric element  10  decreases again, and the voltage occurring at piezoelectric element  10 , and its external dimensions, likewise correspondingly decrease. 
   If the aforementioned closing and opening of discharging switch  230  are repeated many times, the decrease in the voltage occurring at piezoelectric element  10 , and the contraction of piezoelectric element  10 , take place stepwise. 
   When discharging switch  230  has been closed and opened a defined number of times and/or when the piezoelectric element has achieved the desired charge state, discharging of the piezoelectric element is terminated by leaving discharging switch  230  open. 
   The interaction of triggering IC E and control computer D with the elements inside region A depicted in detail is accomplished by way of control signals that are conveyed from triggering IC E, via branch selection control lines  410 ,  420 ,  430 ,  440 ,  450 , and  460 , group selection control lines  510 , and  520 , stop switch control line  530 , charging switch control line  540  and discharging switch control line  550 , and control line  560 , to elements inside region A depicted in detail. On the other hand, sensor signals are acquired at measurement points  600 ,  610 ,  620 ,  630 ,  640 , and  650  inside region A depicted in detail, and are conveyed to triggering IC E via sensor lines  700 ,  710 ,  720 ,  730 ,  740 , and  750 . 
   In order to select piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and/or  60  for the execution of charging or discharging operations of individual or multiple piezoelectric elements  10 ,  20 ,  30 ,  40 ,  50 , and/or  60  by opening and closing the corresponding switches as described above, voltages are applied or not applied to the transistor bases by the control lines. With the aid the sensor signals, a determination is made of the resulting voltage of piezoelectric elements  10 ,  20 , and  30 , or  40 ,  50 , and  60 , on the basis of measurement points  600  and  610 , respectively, and of the charging and discharging currents on the basis of measurement point  620 . 
     FIG. 3  indicates some of the components contained in triggering IC E: a logic circuit  800 , memory  810 , digital/analog converter module  820 , and comparator module  830 . Also indicated is the fact that the fast parallel bus  840  (used for control signals) is connected to logic circuit  800  of triggering IC E, whereas the slower serial bus  850  is connected to memory  810 . Logic circuit  800  is connected to memory  810 , to comparator module  830 , and to signal lines  410 ,  420 ,  430 ,  440 ,  450  and  460 ;  510  and  520 ; and  530 ,  540 ,  550 , and  560 . Memory  810  is connected to logic circuit  800  and to digital/analog converter module  820 . Digital/analog converter module  820  is furthermore connected to comparator module  830 . In addition, comparator module  830  is connected to sensor lines  700 ,  710 ,  720 ,  730 ,  740 , and  750 , and, as already mentioned, to logic circuit  800 . 
     FIG. 4  schematically shows a time sequence of interrupts for programming the beginning of a main injection HE (to be described below in more detail) and of two preinjections VE 1  and VE 2 , as a function of the top dead center point of the crankshaft. As is evident from  FIG. 4 , in a six-cylinder engine static interrupts are generated, for example, at approximately 780 crankshaft and, for example, at approximately 138° crankshaft, and these respectively program the beginning of preinjection VE 2  and of preinjection VE 1  located immediately before main injection HE. The ends of these injections are then programmed on the basis of dynamic interrupts. It is understood that the above crankshaft angles are indicated merely by way of example, and the interrupts may also be generated at different crankshaft angles. Although only the programming of preinjections has been explained above, the same procedure may be used correspondingly for postinjections as well, however, if they are to be performed. 
   Calculation for the detection of edge overlaps is accomplished in each static and dynamic interrupt. Only overlaps between edges that are known at the time of the interrupt can be calculated. 
   In each interrupt, the following steps are performed: 
   1. The current rotation speed n is ascertained; this rotation speed n is used in the entire interrupt (i.e., the rotation speed is “frozen”). 
   2. With each interrupt, new information about edges becomes known. To ensure that only current information items are compared in pairs, the information status is updated. At each interrupt, a flag for new information items is therefore set, and a check is made as to whether triggering operations for which flags are set have already been executed, in which case the relevant flags are deleted. 
   3. A determination is made of the edge processing times with respect to an arbitrary reference, e.g., with respect to reference time t=0 at a crankshaft angle phi=0°. The known information about beginning angle, time offset, beginning, and duration is utilized, in consideration of the current rotation speed, for extrapolation. The general relationship among rotation speed n, angle phi, and time t is indicated by equation (1):
 
 n =( phi/t )* c   (1),
 
time being measured in microseconds and crankshaft angle phi in °KW, and constant c being 166,667 (rpm)/(°KW/μs).
 
   4. The individual edge times are examined in pairs for overlap. For example, only pairs belonging to different banks are tested, since overlaps within the same bank result from application errors. The safe strategy, however, is nevertheless to test every conceivable edge pair. 
   5. A priority is allocated to each injection. A specific priority is assigned to each injection on the basis of system parameters and environmental parameters. As a result, for each injection pairing a distinction is made between low-priority and high-priority triggering actions. Steps are taken to ensure that a switchover of priorities during a calculation run does not have negative consequences. For example, an overlap detection and actions may be performed in the static interrupt in accordance with the current priority constellation, and then the priorities may be switched over, i.e., modified. In the subsequent dynamic interrupt of this pairing, control would need to occur on the basis of a new priority, which would result, in the worst case, in an action against the triggering of a higher-priority injection (high-priority triggering action). Consistent priority allocation must therefore be ensured even in the context of this kind of priority switchover. This may be provided by allocating a priority set to each pairing. The size of the buffer for various priority sets must be selected in such a way that the maximum possible number of changes in the priority sets during the entire execution of a pairing can be stored. After it has been completely processed, the priority set of a pairing is replaced with the current set defined by a priority manager of the electronic triggering circuit. 
   6. In the overlap examination, the spacing in time between the respective beginnings of the two edges is ascertained. Proceeding from that spacing, a decision can be made as to whether an overlap exists. Since the edge times are based on the angles for the injections, particular attention must be paid here to 720° KW overruns. Purely in principle, a large number of implementation possibilities are conceivable in terms of spacing calculation and evaluation. In the example embodiment of the method described below, three calculations are performed. 
     FIG. 5  depicts the calculations on the basis of angle, the value of a low-priority edge A being plotted on the abscissa, and the value of a high-priority edge B plotted on the ordinate. The high-priority edge is “protected” with regions on the earlier (pre) and later (post) sides. If a low-priority edge intersects that region, an overlap exists. The regions are marked in the illustration. Regions outside 720° KW=phimax are transferred, in accordance with allocation, into the permissible regions. The results of the calculations using the following equations: 
             B   -   A           (   2   )               B   -   A   -     phi   max             (   3   )               B   -   A   +     phi   max             (   4   )               
are marked in the diagram in  FIG. 5 . Overlaps that are detected by the individual calculations are characterized in each case by the same crosshatching. The angle-based correlation is explained in  FIG. 5 ; transformation into the time region is accomplished using equation (1) explained above. An example using A=50° and B=100° yields, with equation (2), the overlap for given values of earlier (pre) and later (post) shifting.
 
   7. The degree of shift or shortening is ascertained as a function of the degree of overlap. Shifting is performed in the later direction in such a way that the low-priority beginning edge is placed after the predicted end of the high-priority edge at a distance equal to a time lead. The duration is retained upon shifting. The point in time of the dynamic interrupt, which is coupled to the beginning edge at a fixed spacing, is also shifted. Shortening occurs in such a way that the low-priority ending edge is shifted in the earlier direction. The point in time of the beginning edge is retained. The decision as to whether to shift or shorten depends on whether the beginning edge has already been processed at the moment the overlap is detected. If the beginning edge (this being understood as the beginning of execution of the combustion operation) has already been processed, a shift is no longer possible and only shortening can occur. The result is that for all overlaps of low-priority ending edges, only shortening is possible, since the point in time at which the overlap is detected can lie only in the dynamic interrupt of the low-priority injection, but the latter is associated with execution of the beginning edge. 
   As an example, a shift is depicted in conjunction with  FIG. 6 . The overlap is detected using equation (2); the resulting overlap magnitude t k  is incorporated directly into the degree of overlap. The degree of shift is expressed by expression (5) below:
 
t k +time lead+“post” protection region  (5).
 
Expression (5) applies even when the overlap was ascertained from equation (3) or equation (4).
 
   An example of a shortening of the triggering duration is depicted in  FIG. 6 . The overlap is once again detected using equation (2); the resulting overlap magnitude tk is incorporated directly into the degree of overlap. The degree of shortening is expressed by expression (6) below:
 
t k −time lead−“pre” protection region  (6).
 
Expression (6) applies even when the overlap was ascertained from equation (3) or equation (4).
 
   In addition to primary overlaps or collisions, secondary overlaps or collisions are also possible. Secondary collisions result, for example, when the low-priority beginning edge is shifted later in the static interrupt, but collides with the high-priority ending edge. The point in time at which the collision is detected then lies within the dynamic interrupt of the high-priority triggering action. With this secondary collision, the low-priority beginning edge must therefore be shifted further in the later direction. The procedure is analogous in the case of tertiary overlap or collisions. An example embodiment of the method provides that after a checking of all pairings that has ended with the detection of an overlap and associated action, another pass through all pairings is performed until either an abort criterion based on number of passes occurs, or an absence of overlaps is identified. 
   In another embodiment of the method, detection is performed of undesired overlappings between the time intervals in which one piezoelectric element is to be charged or discharged and a time interval in which the other piezoelectric element is to be charged or discharged, by calculating the utilized angle ranges and comparing them to predefined permissible angle ranges, i.e., collision-free or collision-tolerant angle ranges. 
   A “collision-free angle range” is understood as the angle range that can be covered by the various injection types of a cylinder of the internal combustion engine without causing overlaps of triggering actions of the actuators. In the case of a four-cylinder internal combustion engine with a single-bank structure, for example, the collision-free angle range is determined by dividing the 720° crankshaft angle value by the number of cylinders, i.e., four. In an internal combustion engine of this kind, the collision-free angle range is therefore 180° crankshaft angle. The “utilized angle range” is the term for the crankshaft angle range covered from the beginning of the earliest preinjection to the end of the latest postinjection. If the utilized angle range exceeds the collision-free angle region then, for example, a late injection for one cylinder overlaps an early injection for another cylinder in the same bank. As already mentioned earlier, only one actuator in a bank can be charged at one time; otherwise a charge equalization would occur that might cause disruptions in triggering. 
   In addition to the single-bank structure, several cylinders can also be grouped into a bank, several banks being triggered by the same supply unit for charging and discharging. A configuration of this kind is called a “quasi-multi-bank” structure. In this case, the angle range in which collisions of triggering actions in different banks can be resolved by an edge management system is called the “collision-tolerant” region. In this case, an exceedance beyond the collision-tolerant range plus collision-free angle range results in disrupted triggering actions. 
   Taking the example of a six-cylinder internal combustion engine with a quasi-double-bank structure, the collision-free angle range is 120° crankshaft angle, and the collision-tolerant angle range is likewise 120° crankshaft angle. The entire permissible angle range is then determined by the sum of the collision-free angle range and the collision-tolerant angle range; in the case of the six-cylinder internal combustion engine with a quasi-double-bank structure, the permissible angle range is 240° crankshaft angle. In general, the permissible angle region in an internal combustion engine having a quasi-double-bank structure can be determined by dividing the value of 720° crankshaft angle by the number of cylinders multiplied by the number of banks. 
   A main feature of the above embodiment of the method for operating a fuel injection system for an internal combustion engine is calculation of the utilized angle range and comparison with the permissible angle range, i.e., the collision-free angle range or the sum of the collision-free and collision-tolerant angle ranges. 
   An example embodiment of the method is described below. 
   In each interrupt, new information items that are used to calculate the utilized angle range become known. In each interrupt, the following steps are performed: 
   1. The current rotation speed n is ascertained; this rotation speed n is used in the entire interrupt (i.e., the rotation speed is “frozen”). 
   2. With each interrupt, new information about edges becomes known. That information is converted, using the current rotation speed n, to an angular basis. 
   3. Each newly arrived angle information item is incorporated into the calculation of the utilized angle range. From the set of known angle information items a minimum/maximum selection is made, with the goal of ascertaining the earliest and latest triggering edge belonging to one working cycle. The known utilized angle range is ascertained by differentiation from the angle information for the earliest and latest triggering edges. 
   After the dynamic interrupt of the last postinjection, the entire utilized angle range from the earliest preinjection to the latest postinjection is therefore known, the general relationship among rotation speed n, angle phi, and time t having already been explained above in the form of equation (1). 
   4. The known utilized angle range is compared with the predefined collision-free and collision-tolerant angle ranges. 
   If the ranges are exceeded, an error message is issued and the range exceedance is quantified. 
   5. In all the calculations, consideration is given to rotation speed dynamics with its effect from the time of calculation to the time of execution, i.e., triggering of the actuators. 
   The possibilities for reacting to an error message include:
         a) correspondingly shifting a low-priority injection, so that the utilized angle range once again lies within the permissible region; and   b) accounting for the error message and the degree of range exceedance in the context of the next triggering action at the same or a similar operating point.