Abstract:
In an internal combustion engine, fuel is injected directly into a combustion chamber by an injector that has a piezoactuator. An electrical charge conveyed to and/or removed from the piezoactuator is ascertained by a method that is calibrated at least once during an operating time span of the internal combustion engine. To allow the calibration to be carried out or performed as often as possible, the method for ascertaining the electrical charge transferred to and/or removed from the piezoactuator may be calibrated during at least one triggering off-time (dtK) of the piezoactuator while the internal combustion engine is operating.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates firstly to a method for operating an internal combustion engine in which fuel is injected directly into a combustion chamber by an injector that has a piezoactuator, and in which an electrical charge conveyed to and/or removed from the piezoactuator is ascertained by way of a method that is calibrated at least once during an operating time span of the internal combustion engine.  
       BACKGROUND INFORMATION  
       [0002]     European patent document no. 1 138 915 refers to a method in which, during charging of a piezoactuator of an injector, the transferred quantity of electrical charge can be determined. The corresponding quantity of electrical charge transferred during discharging of the piezoactuator can likewise be determined. This is accomplished by integration of a current signal. In order to reduce errors upon integration of the current signal and thereby to increase the precision with which the transferred charge quantity is ascertained, an alignment of the integration process, to be performed at specific points in time, is proposed. This alignment is to be performed, in particular, when the internal combustion engine is started. The reason for this is that ordinary control unit concepts and output stage concepts can operate only sequentially, so that an alignment cannot occur during triggering of the output stage or the piezoactuator.  
       SUMMARY OF THE INVENTION  
       [0003]     An object of an exemplary method of the present invention is to provide that the electrical charge conveyed to and removed from the piezoactuator can be determined with even higher precision.  
         [0004]     This object may be achieved, in the context of the method, in that the method for ascertaining the electrical charge conveyed to and/or removed from the piezoactuator is calibrated during at least one triggering off-time of the piezoactuator during operation of the internal combustion engine.  
         [0005]     With the exemplary method according to the present invention, the electrical charge transferred to and removed from the piezoactuator that is ascertained can be aligned not only before the internal combustion engine is started, but also during normal operation thereof. Triggering off-times of the piezoactuator, which occur even during normal operation of the internal combustion engine, are used for this purpose.  
         [0006]     This is because, in contrast e.g. to magnetic actuators, a triggering of the piezoactuator takes place or occurs only during the actual change in length of the piezoactuator. For a change in length of this kind, a specific electrical charge is transferred to, or a specific electrical charge is removed from, the piezoactuator. Between these triggering actions are triggering off-times in which the piezoactuator, and the output stage that generally triggers it, are “idle.” 
         [0007]     With the present alignment and triggering method, even though the individual actions are executed sequentially, an alignment of the method with which the charge transferred to and removed from the piezoactuator is ascertained can be performed while the internal combustion engine is operating normally.  
         [0008]     Since alignment can be performed even during operation of the internal combustion engine, drift phenomena resulting, for example, from changes in the temperature of a control unit can be compensated for even during operation of the internal combustion engine. The precision with which the method for ascertaining the electrical charge delivered to and removed from the piezoactuator is performed may thus be greatly improved.  
         [0009]     As a result of the more precise determination, according to the exemplary embodiment of the present invention, of the actuator capacitance, the actuator temperature can be more accurately ascertained. This may have a direct effect, however, on the linear stroke characteristics of the piezoactuator, and thus on the accuracy of the opening and closing behavior of an injector equipped with the piezoactuator. An accurate knowledge of actuator capacitance thus, ultimately, also allows the internal combustion engine to be operated more optimally in terms of emissions and consumption.  
         [0010]     It is understood that the exemplary method according to the present invention can be used in the same fashion in both gasoline and diesel internal combustion engines. The use of, for example, an exhaust gas turbocharger and/or an exhaust gas recirculation system also does not conflict with utilization of the exemplary method according to the present invention.  
         [0011]     In another exemplary embodiment, the calibration be accomplished with the injector open, in the triggering off-time between the end of an opening triggering action and the beginning of a closing triggering action. An open injector is present at each injection of fuel into the combustion chamber. Thus, the calibration may be performed at almost every working cycle of a cylinder of the internal combustion chamber (except during overrun of the engine, in which the injector remains closed). Such frequent calibration allows for reaction even to short-term fluctuations in the temperature of the control unit, thus considerably improving the accuracy of the method with which the charge conveyed to and removed from the piezoactuator is determined.  
         [0012]     Calibration with the injector open may also have the advantage that the calculations required for this purpose can be performed relatively easily shortly before the injection. If it were desired instead to use the unoccupied phases between two injections for calibration, this would require laborious calculation because the end of one injection is known only shortly before the actual injection, and moreover the beginning of the subsequent injection would already have to be known. This may not usually be the case.  
         [0013]     In addition, lead corrections may be necessary because of the dynamics of the internal combustion engine, since the respective beginning of an injection is referred to the crankshaft, whereas the duration of an injection has a time reference. This entire problem may be circumvented if the calibration is performed with the injector open.  
         [0014]     Also, for each working cycle of a cylinder of the internal combustion engine, at least one secondary injection and one main injection may be provided, and the calibration be performed during a main injection. This injection type occurs more often than all other injection types, since the torque of the internal combustion engine is created principally by the main injection, and the main injection is therefore normally always performed (except during overrun or the like). In addition, the duration of the main injection is relatively long as compared with the other injection type (preinjection, postinjection, etc.), so that a comparatively long time is available for calibration.  
         [0015]     Advantageously, a check may be made before a calibration, in a rotation-speed-synchronous dynamic interrupt, as to whether the time between two triggering actions is sufficient for a calibration. The reason for this is that the triggering duration may be calculated in a dynamic interrupt of this kind immediately before the injection. The triggering duration is defined here as the time span between the beginning of charging of the piezoactuator and the beginning of discharging of the piezoactuator. Subtracting the maximum possible charging time from the beginning of charging, i.e. from triggering initiation, yields the time remaining for a calibration. Performing the check in the rotation-speed, synchronous interrupt, as with the exemplary embodiment of the present invention, which may allow this check to be performed at the latest possible point in time, and therefore with greater accuracy.  
         [0016]     The dynamic interrupt is thus also the ideal time at which to program the calibration itself. This is expressed in the exemplary method according to the present invention in which an instruction necessary for the calibration is determined in a rotation-speed-synchronous dynamic interrupt.  
         [0017]     The calibration itself may be particularly accurate if it encompasses a plurality of individual calibration actions. To identify whether the triggering duration of the piezoactuator calculated in the dynamic interrupt is sufficient for one or more calibration actions, the following procedure may be used: 
 
modulo (number of calibration actions)=triggering time/maximum time for a calibration instruction plus maximum charging time.
 
         [0018]     In another exemplary method of the present invention, the number of actions possible per working cycle of a cylinder is limited to a specific value, and only as many calibration actions as will permit all the intended injection actions to be performed are allowed during one working cycle of a cylinder. In this manner, therefore, a maximum possible number of actions may be ascertained a priori as a function of the absolute length of a working cycle, the injection actions having a higher priority than the calibration actions.  
         [0019]     This can be implemented since an action coordinator firstly identifies the number of injection actions that have been ordered, and then determines the number of calibrations still possible. This may ensure that operation of the internal combustion engine is not impaired by the calibration actions. At the same time, however, there is an assurance that a calibration can be performed as soon as the “time window” required for it is open.  
         [0020]     To a certain extent, the advantages according to the exemplary method of the present invention may already be achieved if a calibration action is scheduled not regularly at frequent intervals, but instead at least when the temperature of a control unit has changed by at least a specific value since the last calibration action. This reduces the computation load on the control unit and takes into account the fact that the temperature profile of the control unit has a considerable influence on the accuracy with which the electrical charge conveyed to and removed from the piezoactuator is determined.  
         [0021]     In addition or alternatively thereto, a calibration action may be scheduled at least after expiration of a specific time interval, the duration of the time interval increasing in a defined manner after a startup of the internal combustion engine. This takes into consideration the fact that the temperature of the control unit changes relatively significantly after the internal combustion engine is started, whereas after a certain time it remains more or less steady. Calibrations are necessary only relatively seldom during this quasi-steady phase, which relieves stress on the control unit.  
         [0022]     As an alternative to the aforesaid calibration operation with the injector open, the calibration can also be performed during an overrun condition of the internal combustion engine. During this overrun the injector is closed, i.e. is not being triggered, so that a relatively long period of time is available for calibration.  
         [0023]     Given a certain driving style or corresponding traffic conditions, however, an overrun condition of the internal combustion engine may possibly occur only seldom or not at all. In addition, a number of tests, alignment or learning processes (e.g. injection quantity calibration), and a catalytic converter regeneration are performed during the internal combustion engine&#39;s overrun shutdown, making potential calibration difficult or impossible.  
         [0024]     The exemplary embodiment and/or exemplary method of the present invention also concerns a computer program that is suitable for carrying out or performing the above method when it is executed on a computer. The computer program may be stored in a memory, in particular in a flash memory.  
         [0025]     The exemplary embodiment of the present invention further relates to an open- and/or closed-loop control unit for operating an internal combustion engine, which unit encompasses a memory on which a computer program of the aforementioned kind is stored.  
         [0026]     Also the subject matter of the exemplary embodiment and/or exemplary method of the present invention is an internal combustion engine having at least one combustion chamber, at least one injector that injects fuel directly into the combustion chamber, and at least one piezoactuator. In such an internal combustion engine, it may be advantageous if it encompasses an open- and/or closed-loop control unit of the aforementioned kind. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  schematically shows an internal combustion engine with direct fuel injection, encompassing an injector having a piezoactuator.  
         [0028]      FIG. 2  is a diagram depicting the charge state of the piezoactuator of  FIG. 1  as a function of crank angle.  
         [0029]      FIG. 3  is a diagram indicating how many injection actions are to be performed for a given pressure in a fuel system and a given rotation speed of a crankshaft of the internal combustion engine.  
         [0030]      FIG. 4  is an enlarged portion of the diagram of  FIG. 2 .  
         [0031]      FIG. 5  is an execution diagram which is used to determine whether to schedule a calibration of a method with which the electrical charge conveyed to and removed from the piezoactuator of  FIG. 1  is ascertained.  
         [0032]      FIG. 6  is a block diagram of a method for coordinating various actions during operation of the internal combustion engine of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0033]     In  FIG. 1 , an internal combustion engine bears the overall reference character  10 . It has several cylinders. of which only the one having the reference character  12  is depicted in  FIG. 1 . It encompasses a combustion chamber  14  to which combustion air is conveyed through an intake valve  16  and via an intake duct  18 . A throttle valve  20  controls the quantity of intake air conveyed, which in turn is sensed by an HFM sensor  22 .  
         [0034]     An exhaust valve  24  directs the exhaust gases into an exhaust duct  26 , where they are purified by a catalytic converter  28  that has a lambda probe  30 . Fuel is conveyed to the combustion chamber  14  by an injector  32  whose valve element (not depicted) is actuated by a piezoactuator  33 . Fuel is made available to injector  32  at very high pressure from a fuel system  34 . An ignition system  36  triggers a spark plug  38 .  
         [0035]     The rotation speed of a crankshaft  40  is picked off by a rotation speed sensor  42  which supplies a corresponding signal to an open- and closed-loop control unit  44 . HFM sensor  22  and lambda probes  30  also supply signals to open- and closed-loop control unit  44 . Open- and closed-loop control unit  44  triggers piezoactuator  33 , ignition system  36 , and throttle valve  20 , inter alia.  
         [0036]     It is known that the linear stroke characteristics of piezoactuator  33  depend on its temperature. The accuracy of the opening and closing behavior of injector  32  thus also depends on the temperature of piezoactuator  33 . This in turn has an impact on the emissions and consumption behavior of internal combustion engine  10 . An accurate knowledge of the temperature of piezoactuator  33  is therefore advantageous. One possibility for determining the temperature of piezoactuator  33  is based on knowledge of the capacitance of piezoactuator  33 . That in turn can be ascertained by determining the electrical charge conveyed to and removed from piezoactuator  33 .  
         [0037]     These charge quantities are usually determined by integrating a current signal. The result of this integration also depends, however, on secondary factors. These include, for example, the temperature dependency of the properties of the electrical circuits of open- and closed-loop control unit  44 . To allow the integration to be performed with high accuracy, an alignment or calibration is therefore necessary from time to time.  
         [0038]     Since the processor used in open- and closed-loop control unit  44  can usually operate only sequentially, however, a time window in which it is certain that the processor is not occupied with other actions must be found for this alignment. As discussed in detail below, it is proposed in the present exemplified embodiment to use as the time window a triggering off-time that is present when injector  32  is open. Consideration is given, in this context, to the fact that the calibration encompasses a plurality of individual calibration actions, in the present case a total of three.  
         [0039]      FIG. 2  depicts the present voltage U of piezoactuator  33  during one working cycle of cylinder  12 . A change in voltage U causes a change in the length of piezoactuator  33  and thus an opening or closing motion of the valve element of injector  32 . As is evident from  FIG. 2 , in the instance considered here fuel is introduced from injector  32  into combustion chamber  14  by way of a total of three individual injections. In order to open injector  32  for an injection, piezoactuator  33  must modify its length. For an opening of injector  32 , the charge state of piezoactuator  33  is changed, for that purpose, from a potential U 1  to a potential U 2 . In the reverse order, the potential is modified in order to close injector  32  and terminate the injection.  
         [0040]     In  FIG. 2 a  first preinjection bears the reference character  46 , a main injection the reference character  50 , and a first postinjection the reference character  52 . The number of possible injections depends on a variety of factors, including the fuel pressure p in fuel system  34  and the rotation speed n of crankshaft  40  (see  FIG. 3 ). Because of the energy balance of control unit  44  and the volume balance of the high-pressure fuel pump (not depicted in  FIG. 1 ), fewer injections take place at high rotation speeds (field  56  in  FIG. 3 ) than at low rotation speeds and low fuel pressure (field  58  in  FIG. 3 ).  
         [0041]     The change over time in voltage U of piezoactuator  33  for main injection  50  is depicted in enlarged form in  FIG. 4 . It is evident from this that the data governing the duration of main injection  50  are determined at a crank angle W 0  in a dynamic interrupt that bears the reference character  60  in  FIGS. 2 and 4 . Those data include the beginning of the discharging operation of piezoactuator  33 , which in the present case is located at a crank angle W 1 . The beginning of the charging operation of piezoactuator  33  is ascertained in a static interrupt that is located earlier in time than the dynamic interrupt, and is not indicated in the Figure.  
         [0042]     The beginning of the discharging operation of piezoactuator  33  is determined from a triggering duration dtA that is ascertained in dynamic interrupt  60  at crank angle W 0 . This is the time between the beginning of charging operation  62  and the beginning of a discharging operation  64  of piezoactuator  33 . Subtracting the maximum possible charging time dtL of piezoactuator  33  from triggering duration dtA yields a time span dkK that is available for other actions.  
         [0043]     The basis for all this is the fact that the processor used in open- and closed-loop control unit  44  can operate only sequentially. In the present case, the remaining “free” time dtK between the two triggering actions  62  and  64  of piezoactuator  33  is sufficient for three adjustment or calibration actions  66 ,  68 , and  70 . The fact that the processor of open- and closed-loop control unit  44  can carry out these three calibration actions  66 ,  68 , and  70  was ascertained previously by an action coordinator whose operation will now be explained with reference to  FIG. 5 .  
         [0044]     Reference character  72  in  FIG. 5  refers to the enabling of the optimum number of injections for the present operating state (driver&#39;s requested torque, rotation speed, etc.). In  74 , these injections are each given an individual priority. In block  75 , the maximum number of injections permissible under the existing operating conditions is defined. This is accomplished by way of a minimum selection that depends, inter alia, on the charge state of an output stage (block  76 ) and on the delivery volume and delivery pressure of fuel system  34  (block  78 ).  
         [0045]     If the maximum permissible number of injections defined in  75  is less than the number of injections enabled in  72  itself, a selection is made in block  80  of those injections which have the highest priority and whose quantity corresponds to the number ascertained in  75 . Only those injections are carried out. In the present exemplified embodiment a total of three injections, i.e. preinjection  46 , main injection  50 , and postinjection  52 , are permitted to be carried out.  
         [0046]     In  81 , the maximum number of actions that can be processed by open- and closed-loop control unit  44  between two static interrupts of the same type is made available (a separate static interrupt being allocated on the one hand to the preinjection and on the other hand to the main injection and postinjection, so that the number of static interrupts within two crankshaft revolutions is equal to the number of cylinders of the internal combustion engine multiplied by a factor of two). In the present exemplified embodiment it is six.  
         [0047]     A subtraction in  82  then defines the number of actions still possible for calibration, which in the present case is three, corresponding to calibration actions  66 ,  68 , and  70  of  FIG. 4 . This ensures that the injection actions take priority over the adjustment or calibration actions, but that the maximum number of calibration actions in the given circumstances can nevertheless be performed.  
         [0048]      FIG. 6  depicts a method which determines those instances in which any calibration actions at all are to be performed. The basis for this is an assumed temperature of open- and closed-loop control unit  44  that is ascertained by way of a characteristic curve  84 . The time elapsed since internal combustion engine  10  was started (block  86 ) is fed into characteristic curve  84 . Characteristic curve  84  yields as its output value the temperature of open- and closed-loop control unit  44  on the assumption of a certain starting temperature.  
         [0049]     In  88 , the difference is determined between the temperature ascertained by characteristic curve  84  and a temperature ascertained and stored at the last calibration, which is made available in block  90 . In  92 , a query is made as to whether the difference ascertained in  88  is greater than a specific temperature difference, in the present case 10 K. If so, a calibration is performed and the temperature ascertained in characteristic curve  84  is stored in memory  90 .  
         [0050]     As an alternative to this, however, a calibration action may be scheduled after expiration of a certain time interval. In order to take into account the asymptotic approach of the temperature of open- and closed-loop control unit  44  to a terminal value, the length of the time interval after the internal combustion engine is started should be increased in an appropriate manner.  
         [0051]     The operation of an internal combustion engine with direct gasoline injection has been explained in the exemplified embodiment above. It is understood, however, that the method described can also be used in internal combustion engines that are operated with diesel fuel and are configured accordingly. Internal combustion engines that have an exhaust gas turbocharger and/or an exhaust gas recirculation system can also be operated using the method described above.