Abstract:
An injector has a valve pin whose position is adjustable according to the pressure prevailing in a control chamber and which blocks a fluid flow through at least one injection port in a closed position. The injector has a control chamber that is hydraulically coupled to a high-pressure fluid reservoir, a piezo actuator, and a control valve which hydraulically connects and disconnects the control chamber, the piezo actuator acting upon said control valve. A combination of a triggering period for the piezo actuator and electric power that is to be fed to the piezo actuator is varied from a predefined initial combination to a target combination in which a pressure curve is detected that is characteristic of a movement of the control valve out of the closed position thereof without fluid being proportioned through the injection port.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a U.S. national stage application of Internation Application No. PCT/EP2005/056442 filed Dec. 2, 2005, which designates the United States of America, and claims priority to German application number DE 10 2005 001 498.4 filed Jan. 12, 2005, the contents of which are hereby incorporated by reference in their entirety.  
       TECHNICAL FIELD  
       [0002]     The invention relates to a method and a device for controlling an injector which is embodied in particular for metering fuel to an internal combustion engine.  
       BACKGROUND  
       [0003]     Increasingly stringent statutory provisions relating to the permissible pollutant emissions of internal combustion engines which are disposed in motorized vehicles make it necessary to adopt various measures by means of which the pollutant emissions can be reduced. One starting point in this endeavor is to reduce the pollutant emissions generated during the combustion process of the air/fuel mixture. The formation of soot in particular is greatly dependent on the preparation of the air/fuel mixture in the respective cylinder of the internal combustion engine. In order to achieve a very good fuel mixture preparation, fuel is increasingly metered under very high pressure. In the case of diesel internal combustion engines, the fuel pressures reach as high as 2000 bar. Injection valves having a piezo actuator as the final control element are becoming increasingly widely accepted for applications of this kind. Piezo actuators are characterized by very short response times. Injection valves of this type are thus possibly suited for metering fuel several times over within one working cycle of a cylinder of the internal combustion engine fuel.  
         [0004]     A particular good fuel mixture preparation can be achieved if one or more pre-injections, which are also referred to as pilot injections, are performed prior to a main injection, with possibly a very small fuel mass being required to be metered for the individual pre-injection. Precise control of the injection valve is very important in particular for cases of this type.  
         [0005]     A method for detecting injection events of an injection valve having a piezoelectric actuator is known from WO 01/63121. The injection valve comprises an injector body having a control chamber to which is assigned a control valve which controls the fuel pressure in the control chamber. The piezoelectric actuator acts on the control valve. A voltage is applied to the piezoelectric actuator in such a way that the resulting stroke of the piezoelectric actuator actuates the control valve. An axial movement of a nozzle needle away from a valve seat is detected as a function of an increase in the voltage drop at the piezoelectric actuator. A termination of the movement of the nozzle needle is detected on the basis of an abrupt decrease in the voltage at the piezoelectric actuator.  
       SUMMARY  
       [0006]     Precise control of an injectorcan be achieved by a method and device for controlling an injector comprising a nozzle needle whose position is adjustable as a function of a pressure in a control chamber and which in a closed position prevents a fluid flow through at least one injection port and otherwise releases said fluid flow, the control chamber which is hydraulically coupled to a high-pressure fluid accumulator, a piezo actuator, and a control valve which in its closed position hydraulically decouples the control chamber from a low-pressure chamber which outside of the closed position hydraulically couples the control chamber to the low-pressure chamber and upon which the piezo actuator acts, wherein the device is operable to and the method comprises the step of varying a combination of a control period of the piezo actuator and an electrical energy to be supplied to the piezo actuator starting from a predefined initial combination to a target combination in which a pressure curve is recorded which is characteristic of a movement of the control valve out of its closed position without a metering of fluid taking place through the injection port.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     Exemplary embodiments of the invention are explained below with reference to the schematic drawings, in which:  
         [0008]      FIG. 1  shows an internal combustion engine having a plurality of injectors,  
         [0009]      FIG. 2  shows one of the injectors according to  FIG. 1 ,  
         [0010]      FIG. 3  shows a first magnified view of a section of the injector according to  FIG. 2 ,  
         [0011]      FIG. 4  shows a second magnified view of a further section of the injector according to  FIG. 2 ,  
         [0012]      FIG. 5  is a flowchart of a program for controlling the injector,  
         [0013]      FIGS. 6A  to  6 D show time curves of control signals for the injectors, and  
         [0014]      FIG. 7  shows a time curve of one of the control signals during a charge period. 
     
    
     DETAILED DESCRIPTION  
       [0015]     In a method and a corresponding device for controlling an injector, which can also be referred to as an injection valve, the injector has a nozzle needle whose position is adjustable as a function of a pressure in a control chamber and which in a closed position prevents a fluid flow through at least one injection port in a nozzle body and otherwise releases said fluid flow. The injector further comprises a control chamber which is hydraulically coupled to a high-pressure fluid accumulator, a piezo actuator, and a control valve which in its closed position hydraulically decouples the control chamber from a low-pressure chamber, which outside of its closed position hydraulically couples the control chamber to the low-pressure chamber, and upon which the piezo actuator acts. A combination of a control period of the piezo actuator and electrical energy supplied to the piezo actuator is varied starting from a predefined initial combination to a target combination in which a pressure curve is recorded which is characteristic of a movement of the control valve out of its closed position, without a metering of fluid taking place through the injection port. The target combination is a simple and precise measure for an idle stroke of the piezo actuator, which idle stroke is an extremely important parameter in particular in connection with the metering of the smallest possible fluid quantities through the injector.  
         [0016]     In this way it is thus easily possible to determine at arbitrarily predefinable time intervals over the operating period of the injector valve the target combination as a measure for the current idle stroke, which can change considerably over the lifetime of the injector and possibly can also be subject to temporary fluctuations. Influencing variables for the idle stroke include, for example, the temperature, wear-and-tear, and aging of the piezo actuator. Depending on the target combination, the electrical energy to be supplied to the piezo actuator and/or the control period of the piezo actuator can then be corrected for subsequent injection operations. The target combination can, however, also be used advantageously in the context of controlling the injector for other purposes, such as, for example, a diagnosis.  
         [0017]     According to an embodiment, the combination of the control period and the electrical energy to be supplied is varied starting from the predefined initial position to the target combination in an operating state in which an actuating element for adjusting a fluid delivery to the high-pressure fluid accumulator is in its closed state. This has the advantage that the recorded pressure curve is then more independent of interference factors which are caused by a delivering of the fluid to the high-pressure fluid accumulator. Thus, it is then easier to precisely detect the pressure curve which is characteristic of the movement of the control valve out of its closed position without a metering of fluid taking place through the injection port. A further consequence of this is in turn that the target combination can be determined more precisely.  
         [0018]     According to a further embodiment, the combination of the control period and the supplied electrical energy is varied by means of a charge period during which the piezo actuator is supplied with electrical energy, which charge period is longer than that during an operation phase of the injector in which a metering of fluid is intended. In this way the generation of sound which can be perceived as unpleasant by a user can be reduced.  
         [0019]     According to a further embodiment, the combination of the control period and the supplied electrical energy is varied by means of a discharge period during which electrical energy is removed from the piezo actuator, which discharge period is longer than that during the operation phase of the injector in which a metering of fluid through the injection port is intended. In this way, too, the sound which is perceived as unpleasant by a user is reduced.  
         [0020]     According to a further embodiment, a commencement of succeeding control periods is varied in relation to the respective cylinder segment. In this way the sound spectrum can be set in a targeted manner by the controlling of the control valve in order thereby to make the subjective perception of the sound by the user as less objectionable.  
         [0021]     According to a further embodiment, a termination of succeeding control periods is varied in relation to the respective cylinder segment. In this way, too, the sound spectrum generated as a result of the controlling of the control valve can be set in a targeted manner in order thereby to make the subjective perception of the sound by the user as less objectionable.  
         [0022]     According to a further embodiment, the commencement of succeeding control periods for different injectors, all of which are assigned for example to an internal combustion engine, is varied in relation to the respective cylinder segment. In this way, too, the sound spectrum generated as a result of the controlling of the respective control valves can be suitably set.  
         [0023]     According to a further embodiment, the termination of succeeding control periods for different injectors is varied in relation to the respective cylinder segment. In this way, too, the sound spectrum generated as a result of the controlling of the respective control valves can be easily set.  
         [0024]     Elements of identical construction or function are identified by the same reference symbols throughout the figures.  
         [0025]     An internal combustion engine ( FIG. 1 ) comprises an intake tract  1 , an engine block  2 , a cylinder head  3 , an exhaust tract  4  and a feed device  5  for fuel. Embodied in the internal combustion engine are a plurality of cylinders Z 1  to Z 4  to which injectors  7 ,  9 ,  11 ,  13  are assigned, respectively. A crankshaft  15  is provided to which is assigned a crankshaft angle sensor  17  which measures the current crankshaft angle, from which a rotational speed of the crankshaft can then also be derived.  
         [0026]     The feed device  5  for fuel comprises a fuel tank  19  and a low-pressure area  21  which is either directly hydraulically coupled to the fuel tank  19  or hydraulically coupled to the fuel tank  19  via a low-pressure pump  23 . Preferably also provided is a regulator  25  by means of which a pressure can be set in the low-pressure area  21 . A high-pressure pump  29  is hydraulically coupled on its input side to the low-pressure area  21  via a volume flow control valve VCV. The high-pressure pump  29  is hydraulically coupled on its output side to a high-pressure fluid accumulator  31  and thus delivers fluid, in particular fuel, to the high-pressure fluid accumulator  31 . By means of the volume flow control valve VCV it is possible to set a volume flow which is delivered to the high-pressure fluid accumulator  31  by the high-pressure pump  29 . The volume flow control valve VCV can be embodied separately from the high-pressure pump  29  or also as a single structural unit with the high-pressure pump  29 .  
         [0027]     Disposed on the high-pressure fluid accumulator  31  is a fuel pressure sensor  33  which measures the pressure in the high-pressure fluid accumulator. The measured signal of the high-pressure sensor is thus representative of a pressure curve of the fluid contained in the high-pressure fluid accumulator  31 . The injectors  7 ,  9 ,  11 ,  13  are hydraulically coupled to the high-pressure fluid accumulator  31  via a respective high-pressure terminal ( FIG. 2 ).  
         [0028]     Also provided is a control device  35  which generates actuating signals for controlling actuating drives of the internal combustion engine as a function of measured variables which are recorded by sensors. The control device  35  has corresponding inputs via which the measured signals of the sensors can be recorded, as well as a program memory and a data memory and an arithmetic-logic unit and output stages for controlling the actuating drives of the internal combustion engine.  
         [0029]     The injectors  7 - 13  are identical in design and are explained in more detail below with reference to FIGS.  2  to  4 . The injector  7 - 13  comprises an injector housing  37  which accommodates the high-pressure terminal  39  which is hydraulically coupled to the high-pressure fluid accumulator  31 . The high-pressure terminal  39  is hydraulically coupled via a first high-pressure borehole  41  to a control valve  43  which is accommodated in the injector housing  37 . A second high-pressure borehole  45  extends from the high-pressure terminal  39  through the injector housing  37  into a nozzle body  47 .  
         [0030]     Also embodied in the injector housing  37  is a low-pressure chamber  49  which is hydraulically coupled to the low-pressure area  21 . If the low-pressure pump  23  is present, the low-pressure chamber  49  can also be directly hydraulically coupled to a return fuel line to the fuel tank  19 .  
         [0031]     Embodied in the injector housing  37  is a recess  51  of the injector housing  37  in which a control piston  53  is disposed and guided. A control chamber  55  of the control valve  43  adjoins an end face  69  of the control piston  53  and is embodied in the free space between the end face and the control valve  43 .  
         [0032]     A free space of the recess  51  of the injector housing  37  in the area of an end of the control piston  53  which faces away from the control chamber  55  is hydraulically coupled to the low-pressure chamber  49 . A nozzle needle  57  is mechanically coupled to the control piston  53  and is disposed in a recess  59  in the nozzle body  47 . In a closed position of the nozzle needle  57 , the latter prevents a fluid flow through an injection port  61  which is embodied in the nozzle body  57 . Outside of the closed position, the nozzle needle  57  releases the fluid flow through the injection port  61 . If a plurality of injection ports  61  are present, the nozzle needle  57 , in its closed position, prevents the fluid flow through all its assigned injection ports  61  and otherwise releases said fluid flow.  
         [0033]     The position of the nozzle needle  57  is dependent on a balance of forces which act on the nozzle needle  57 . A first force is caused by the pressure in the control chamber  45  which acts on the end face  69  of the control piston. A second force is coupled in due to the pressure which acts upon the surface of a high-pressure shoulder  63  and upon the needle dome  65 . A third force is caused by a spring force of a nozzle spring  67 . The position of the nozzle needle is dependent on the balance of the first to third forces.  
         [0034]     The control chamber  55  is hydraulically coupled to the first high-pressure borehole  41  via an inlet throttle  71 . The control chamber  55  can also be hydraulically coupled to the low-pressure chamber  49  via an outlet throttle  73  as a function of the switch position of the control valve  43 .  
         [0035]     Associated with the control valve  43  is a piezo actuator  75  whose axial extension is dependent on the electrical energy supplied to it. The piezo actuator  75  acts on a valve body  77  of the control valve  43  and thus determines the switch position of the control valve  43 . An idle stroke between the piezo actuator  75  and the valve body  77  is given by a play between the piezo actuator  75  and the valve body  77  in a state in which no electrical energy is supplied to the piezo actuator  75 . However, the idle stroke also includes a continuous increase in force, caused by an elastic twisting of the piezo actuator  75  in the injector  7 - 13 , when electrical energy is supplied until the control valve opens. Supplying electrical energy within the idle stroke of the piezo actuator  75  consequently does not lead to a lengthening of the piezo actuator  75  which acts on the side facing the valve body  77  when the piezo actuator  75  is in contact with the valve body  77 . In particular at very high pressures in the control chamber  55 , the valve body  77  can for example already be in contact with the piezo actuator  75  even when the latter is not supplied with electrical energy.  
         [0036]     Thus, if the control valve  43  is in its closed position, no fluid can be discharged via the outlet throttle  73  and consequently the pressure increases in the control chamber  55  until it reaches approximately the pressure in the high-pressure fluid accumulator  31 . If the control valve  43 , i.e. in particular the valve body  77 , moves out of its closed position, fluid can flow out from the control chamber  55  via the outlet throttle  73  past the valve body  77  to the low-pressure chamber  49 . As a result the pressure in the control chamber  55  drops as a function of the ratio of the throttle effects of the outlet throttle and the inlet throttle and, if the valve body  77  releases such a small free cross-section, the discharging of fluid is throttled in this area too, also as a function of the position of the valve body  77 .  
         [0037]     As the pressure in the control chamber  55  drops, the first force acting on the nozzle needle  57  via the control piston  53  also decreases.  
         [0038]     Even when the control valve  43  is located in its closed position there is a leakage from the control chamber  55  to the low-pressure chamber  49 . The leakage is caused by fluid which flows from the control chamber  55  through a gap between the wall of the recess  51  of the injector housing  37  and the control piston  53  through to the low-pressure chamber  49 . Said leakage is pressure-dependent and increases as the pressure in the control chamber rises, possibly reinforced by a slight widening of the gap which occurs at high pressures due to the high forces.  
         [0039]     Alternatively the nozzle needle  57  can be hydraulically coupled to the control chamber  55  and consequently the control piston  53  can be dispensed with. In this case the leakage of fluid in the control chamber  55  is negligible when the control valve  43  is in its closed position.  
         [0040]     A program for controlling the injector  7 - 13  is resident in a program memory in the control device  35  and is executed during the operation of the internal combustion engine, and moreover preferably for each of the injectors  7 ,  9 ,  11 ,  13 .  
         [0041]     The program for controlling the injectors according to the following steps is suitable for determining a measure for the idle stroke of the control valve.  
         [0042]     The program is started in a step S 1  ( FIG. 5 ) in which variables are initialized where applicable. The program can be started basically at any time during the operation of the internal combustion engine. Preferably the program is started when no fuel is being delivered to the high-pressure fluid accumulator  31  and preferably also when no fuel is to be metered into the combustion chambers of the cylinders Z 1  to Z 4  via the injectors  7  to  13 .  
         [0043]     In a step S 3 , a charge period T_L and preferably a discharge period T_EL are determined. The charge period T_L, and/or the discharge period T EL can correspond to those times that are provided for the operation of the injectors  7 - 13  with metering of fuel. However, they can also be different from these values, in particular greater and accordingly either permanently predefined or specified as a function of at least one operating variable of the internal combustion engine, where operating variables include the measured variables of the sensors and variables derived therefrom.  
         [0044]     In a step S 5 , a control period T_CTRL of the piezo actuator  75  is assigned a start control period T_CTRL_ST. In addition, an electrical energy E to be supplied to the piezo actuator  75  is assigned a start energy E_ST. The start energy E_ST and the start control period T_CTRL_ST for the piezo actuator  75  are preferably predefined such that a correspondingly executed control pulse definitely does not result in fuel being metered through the injection port  61  into one of the combustion chambers of the cylinders Z 1  to Z 4 . The control period T_CTRL_ST is preferably representative of a time period which commences with the start of the supplying of electrical energy to the piezo actuator  75  and which terminates with a discharge process of the piezo actuator. Preferably the discharge process is started at the end of the time period.  
         [0045]     In a step S 7 , which is preferably but not necessarily present, a check is then made to determine whether the volume flow control valve VCV is in its closed state CL, in which no or only a slight leakage flow of fluid flows through the volume flow control valve VCV to the high-pressure pump  29  and consequently also no fluid or only the leakage flow of the volume flow control valve VCV is delivered to the high-pressure fluid accumulator  31  by the high-pressure pump  29 .  
         [0046]     If the condition of step S 7  is not met, the program is terminated in a step S 15 .  
         [0047]     Otherwise the processing is continued in a step S 9  in which the piezo actuator  75  is activated for the control period T_CTRL while being supplied with the electrical energy E with the charge period T_L and the discharge period T_EL determined in step S 3 . The electrical energy E is supplied to the piezo actuator  75  preferably by direct specification of the electrical energy E that is to be supplied, though it can also be supplied by means of a corresponding specification of a current profile or also of a voltage curve, in which cases temperature-dependent changes in capacitance of the piezo actuator should preferably at least be taken into account.  
         [0048]     The time position of the respective control period T_CTRL relative to the crankshaft angle can then be freely selected if a corresponding function for converting an actuation of an output stage of the control device  35  allows this at an arbitrary moment in time and also if the output stage is so designed. Frequently, however, the control device  35  is embodied for controlling the piezo actuator  75  in each case only within the cylinder segment ZS 1 -ZS 4  assigned to the respective cylinder Z 1  to Z 4 . The cylinder segment ZS 1  to ZS 4  is understood to mean that crankshaft angle, and consequently a corresponding time period, which results from the crankshaft angle for a working cycle of the internal combustion engine divided by the number of cylinders. In the case of an internal combustion engine with a working cycle of 720° crankshaft and, for example, four cylinders, a cylinder segment is thus equal to 180° crankshaft angle. The time period assigned to the respective cylinder segment is then dependent on the current rotation of the crankshaft  15 . The cylinder segments assigned to the respective cylinders have a predefined position in relation to a reference angle position of the crankshaft which can be, for example, a top dead center of the piston upon ignition.  
         [0049]     In a step S 11 , a check is then made to determine whether the pressure curve P_V recorded by the high-pressure sensor  33  has a characteristic pressure curve P_V_M for a movement of the control valve  43  out of its closed position without fluid being metered through the injection port  61 . The characteristic pressure curve P_V_M is preferably determined in advance by appropriate trials or simulations and stored in the data memory of the control device  35 . Preferably it is determined such that the valve body  77  and hence the control valve  43  moves only the minimum possible amount from its closed position and consequently a minimal flow of fuel flows from the control chamber  55  through the outlet throttle  73  past the control valve  43  to the low-pressure chamber  49  when the pressure curve P_V corresponds to the characteristic pressure curve P_V_M.  
         [0050]     Preferably the condition of step S 11  is also met when the pressure curve P_V lies in a predefinable, preferably narrow, value range window about the characteristic pressure curve P_V_M. The characteristic pressure curve P_V_M can be represented for example by a predefinable change in the pressure or a predefinable change rate of the pressure, which is also referred to as a gradient, or also a further correspondingly representative quantity.  
         [0051]     If the condition of step S 11  is not met, in a step S 17  an incrementation period DT is added to the control period T_CTRL. In addition or also as an alternative thereto, in step S 17  an incrementation energy DE is added to the electrical energy E that is to be supplied. The incrementation period DT and the incrementation energy DE are set to such small values that it can be ensured with a high degree of probability that in a corresponding controlling of the injector  7 - 13  in a subsequent pass through step S 8  there will continue to be no metering of fuel through the injection port  61 .  
         [0052]     Depending on the embodiment of the program, it is also possible in step S 17  to vary only either the control period T_CTRL or the electrical energy E to be supplied. This can also be done differently in succeeding passes through step S 17 . Following step S 17 , processing is then resumed in step S 7 .  
         [0053]     If, on the other hand, the condition of step S 11  is met, the control period T_CTRL is assigned to a target control period T_CTRL_Z in a step S 13 . In step S 13  in addition, the electrical energy E to be supplied is assigned to a target energy E_Z. The method is then terminated in step S 15 .  
         [0054]     The target energy E_Z determined in step S 13  and the target control period T_CTRL_Z can be a measure for the idle stroke of the control valve  43 , either each taken individually or in combination.  
         [0055]     As a function of, for example, reference values for the target energy E_Z or, as the case may be, the target control period T_CTRL_Z that are also stored in the control device  35 , for the subsequent operation of the injector  7  to  13  with metering of fuel through the injection port  61 , control times or also electrical energies to be supplied that are provided therefor can then be adjusted accordingly and thereby a very precise metering of the desired fuel quantity can be achieved. Furthermore, the target control period T_CTRL_Z and/or the target energy E_Z can also be used for diagnosis of the injector  7 - 13 . Thus, for example, if one or both values of the target control period T_CTRL_Z or the target energy E_Z are exceeded, it can be deduced that there is a fault in the injector and appropriate measures can be initiated, consisting for example in no further actuating of the injector or in a warning message to a user of a vehicle in which the internal combustion engine is disposed.  
         [0056]     The program for controlling the injector according to  FIG. 5  is preferably executed for each of the injectors  7 - 13  and consequently different control periods T_CTRL_Z and target energies E_Z can then also be determined for each of the injectors  7 - 13 . By this means injector-specific differences can thus be compensated for and as a result a consistent metering of fuel across the cylinders Z 1  to Z 4  of the internal combustion engine can be guaranteed.  
         [0057]     The program according to  FIG. 5  is preferably executed in an overrun phase of operation of the vehicle, during which no fuel is delivered to the cylinders of the internal combustion engine, or also executed immediately after the internal combustion engine has been turned off.  
         [0058]     In these operating states a user of the internal combustion engine expects to perceive no noises caused by an actuating of the injectors  7  to  13 . For this reason it is important in ensuring that the user of the vehicle experiences a feeling of great comfort to minimize the sound emissions caused by the execution of the program according to  FIG. 5  in such a way that the user of the vehicle at least subjectively does not perceive them or at least does not perceive them as unpleasant. Toward that end, in addition to a suitable selection of the charge period T_L or the discharge period T_EL, as described with reference to step S 3 , further measures can also be advantageously performed which are explained in the following with reference to  FIGS. 6A  to  6 D and  7 .  
         [0059]     In  FIGS. 6A  to  6 D, control pulses assigned in each case to the respective injection valves  7 - 13  of the respective cylinders Z 1  to Z 4  are plotted, said control pulses being generated in each case during the execution of step S 9  in the respective injector  7  to  13 . Control pulses for the piezo actuator  75  of the injector  7  which is assigned to the cylinder Z 1  are in each case executed in the cylinder segment ZS 1  assigned to the cylinder Z 1 . This applies analogously to the control pulses of the remaining injectors  9 - 13 . The height of the control pulses can be representative of the energy E supplied during the control pulse. The respective control pulses are varied in relation to the respective cylinder segment with regard to their start and/or end, which can be seen by reference to their different position relative to the respective cylinder segment ZS 1 -ZS 4 . In addition, the end and/or the start of the respective control period T_CTRL of control pulses assigned to different injectors  7 - 13  can be varied relative to one another. This is also the case in relation to the different position of the control pulses relative to the respective start of the respective cylinder segment ZS 1  to ZS 4  in the signal shapes of  FIGS. 6A  to  6 D. By means of a corresponding fine-tuning of this varying in relation to the respective cylinder segment it is possible to generate in a targeted manner a desired sound spectrum which is, for example, either not perceived at all by the user or is perceived only as hissing or which is assimilated into other operating noises of the internal combustion engine.  
         [0060]     The illustration of the control pulses in  FIGS. 6A  to  6 D does not necessarily have to correspond to the actual signal shape of the corresponding physical quantities. In particular the electrical energy supplied to the piezo actuator  75  is removed again at the end of the control period.  
         [0061]     Referring to  FIG. 7 , a possible shape of a control pulse is depicted in greater detail. During the control period T_CTRL, electrical energy is supplied to the piezo actuator  75  for the predefined charge period T_L. This is effected preferably by means of corresponding energy pulses which are preferably varied in their height and hence in the supplied power PEL when the energy E to be supplied is varied. Directly following the termination of the control period T_CTRL, the piezo actuator  75  is then discharged again by means of corresponding discharge pulses of opposite polarity and moreover over the discharge period T_EL.  
         [0062]     The term “fuel” is used in the exemplary embodiments purely by way of example for a special fluid. It can, however, also be replaced by fluid.