CONTROL FOR A PIEZO-ELECTRIC INJECTOR WHEN A FOOT IS RAISED FROM THE ACCELERATOR

Abstract

Disclosed is a method for discharging the pressure in a fuel supply rail of an injection system of an engine, the fuel injection rail connected to a fuel tank by piezo-electric injectors, each including a needle and a piezo-electric actuator pressing on a servo valve of the injector. The injection system includes a fuel pressure sensor and an electrical generator transmitting electric current pulses to each actuator. When the accelerator is released, a first electrical command allows determination of a moment of opening of the respective servo valve without triggering an injection. A second electrical command triggers a discharge of fuel from the fuel supply rail to the tank and therefore to discharge the pressure of the supply rail without triggering an injection. The second electrical command charges the piezo-electric actuator between a first voltage level that opens the servo valve, and a second voltage level triggering an injection.

Claims

1. A method for discharging the pressure in a fuel supply rail of a injection system of an engine of a vehicle, said fuel injection rail being connected to a fuel tank by means of a plurality of piezo-electric injectors, each piezo-electric injector comprising a needle and a piezo-electric actuator able to press on a servo valve of the injector, the injection system further comprising a fuel pressure sensor for the supply rail and an electrical generator able to transmit electric current pulses to the piezo-electric actuator of each injector, said process being implemented during a phase of release of the accelerator for which no request for fuel injection has been made, the method comprising: comparing, by a computer, on each engine cycle, a defined reference fuel pressure for the fuel supply rail with a measured fuel pressure in the fuel supply rail measured by the pressure sensor, and when the measured pressure is greater than the reference pressure: sending, by the electrical generator, of a first electrical command to the piezo-electric actuator of at least one injector of the plurality, the first electrical command comprising an electric charge pulse of defined duration and an electric discharge pulse of the piezo-electric actuator, the duration being determined so as to cause a complete charge of the piezo-electric actuator of the at least one injector, the first electrical command also comprising a defined duration corresponding to the time elapsing between the start of the electric charge pulse and the start of the electric discharge pulse of the piezo-electric actuator of the at least one injector, the duration being determined such that the needle of the at least one injector remains immobile, determining, from a value of a force exerted by the piezo-electric actuator on the servo valve during the first electrical current command, a charge time duration allowing opening of the servo valve of the at least one injector, sending, by the electrical generator, of a second electrical command to the piezo-electric actuator of said at least one injector, the second electrical command comprising an electric charge pulse of a defined duration and an electric discharge pulse of the piezo-electric actuator, and also comprising a defined duration corresponding to the time elapsing between the start of the electric charge pulse and the start of the electric discharge pulse of the piezo-electric actuator, the duration being determined from the charge time duration of the at least one injector so as to allow an opening of the servo valve of said at least one injector while keeping its needle immobile, so that the voltage at the terminals of the piezo-electric actuator is higher than a first voltage threshold triggering the opening of the servo valve and lower than a second voltage threshold triggering the opening of the needle, the duration being determined as a function of the engine speed, the pressure in the fuel supply rail and the desired pressure discharge.

2. The method as claimed in claim 1, wherein the second electrical current command also comprises a defined duration corresponding to the time elapsing between the start of the electric charge pulse and the start of the electric discharge pulse of the piezo-electric actuator of the at least one injector, the defined duration being greater than a duration allowing breakage of the inertia of the needle of the at least one injector.

3. The method as claimed in claim 1, wherein the opening duration of the servo valve is determined by measuring a voltage applied to the piezo-electric actuator and a quantity value of electric charges transferred from the electrical generator to the piezo-electric actuator of the at least one injector.

4. The method as claimed in claim 1, wherein an opening of the servo valve is detected when the force exerted by the piezo-electric actuator on the servo valve is at a maximum, the force exerted by the piezo-electric actuator on the servo valve being determined from the voltage applied to the piezo-electric actuator, from a capacitance value of the piezo-electric actuator and from a quantity value of electric charges.

5. The method as claimed in claim 1, wherein the charge duration of the second electrical command is equal to the opening duration of the servo valve, to which a defined duration is added.

6. The method as claimed in claim 1, wherein at least one time lapse not equal to zero separates the two electric commands.

7. A computer that is configured to be capable of controlling an injection system of an engine of a vehicle, said system comprising a fuel supply rail connected to a fuel tank by means of a plurality of piezo-electric injectors, each piezo-electric injector comprising a needle and a piezo-electric actuator able to press on a servo valve of the injector, the injection system further comprising a fuel pressure sensor for the supply rail and an electrical generator able to transmit electric current pulses to the piezo-electric actuator of each injector, and wherein the computer is also able to command the implementation of the steps of the method as claimed in claim 1.

8. A non-transitory computer-readable medium on which is stored a computer program, comprising code instructions for implementing the steps of the method as claimed in claim 1 when said program is executed on a computer that is configured to be capable of controlling an injection system of an engine of a vehicle, said system comprising a fuel supply rail connected to a fuel tank by means of a plurality of piezo-electric injectors, each piezo-electric injector comprising a needle and a piezo-electric actuator able to press on a servo valve of the injector, the injection system further comprising a fuel pressure sensor for the supply rail and an electrical generator able to transmit electric current pulses to the piezo-electric actuator of each injector.

9. The method as claimed in claim 2, wherein the opening duration of the servo valve is determined by measuring a voltage applied to the piezo-electric actuator and a quantity value of electric charges transferred from the electrical generator to the piezo-electric actuator of the at least one injector.

10. The method as claimed in claim 2, wherein an opening of the servo valve is detected when the force exerted by the piezo-electric actuator on the servo valve is at a maximum, the force exerted by the piezo-electric actuator on the servo valve being determined from the voltage applied to the piezo-electric actuator, from a capacitance value of the piezo-electric actuator and from a quantity value of electric charges.

11. The method as claimed in claim 3, wherein an opening of the servo valve is detected when the force exerted by the piezo-electric actuator on the servo valve is at a maximum, the force exerted by the piezo-electric actuator on the servo valve being determined from the voltage applied to the piezo-electric actuator, from a capacitance value of the piezo-electric actuator and from a quantity value of electric charges.

12. The method as claimed in claim 2, wherein the charge duration of the second electrical command is equal to the opening duration of the servo valve, to which a defined duration is added.

13. The method as claimed in claim 3, wherein the charge duration of the second electrical command is equal to the opening duration of the servo valve, to which a defined duration is added.

14. The method as claimed in claim 4, wherein the charge duration of the second electrical command is equal to the opening duration of the servo valve, to which a defined duration is added.

15. The method as claimed in claim 2, wherein at least one time lapse not equal to zero separates the two electric commands.

16. The method as claimed in claim 3, wherein at least one time lapse not equal to zero separates the two electric commands.

17. The method as claimed in claim 4, wherein at least one time lapse not equal to zero separates the two electric commands.

18. The method as claimed in claim 5, wherein at least one time lapse not equal to zero separates the two electric commands.

19. A non-transitory computer-readable medium on which is stored a computer program, comprising code instructions for implementing the steps of the method as claimed in claim 2 when said program is executed on a computer that is configured to be capable of controlling an injection system of an engine of a vehicle, said system comprising a fuel supply rail connected to a fuel tank by means of a plurality of piezo-electric injectors, each piezo-electric injector comprising a needle and a piezo-electric actuator able to press on a servo valve of the injector, the injection system further comprising a fuel pressure sensor for the supply rail and an electrical generator able to transmit electric current pulses to the piezo-electric actuator of each injector.

20. A non-transitory computer-readable medium on which is stored a computer program, comprising code instructions for implementing the steps of the method as claimed in claim 3 when said program is executed on a computer that is configured to be capable of controlling an injection system of an engine of a vehicle, said system comprising a fuel supply rail connected to a fuel tank by means of a plurality of piezo-electric injectors, each piezo-electric injector comprising a needle and a piezo-electric actuator able to press on a servo valve of the injector, the injection system further comprising a fuel pressure sensor for the supply rail and an electrical generator able to transmit electric current pulses to the piezo-electric actuator of each injector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Other features, details and advantages will become apparent from reading the following detailed description and from examining the appended drawings, in which:

[0037] FIG. 1 shows an embodiment of a method of discharging pressure in a fuel supply rail in an injection system of an engine of a vehicle.

[0038] FIG. 2 shows an embodiment of an injection system of an engine of a vehicle in which the method is implemented.

[0039] FIG. 3a shows a piezo-electric injector in the closed position.

[0040] FIG. 3b shows an enlarged view of a servo valve and control chamber of the piezo-electric injector from FIG. 3a.

[0041] FIG. 4a shows a piezo-electric injector in the injection position.

[0042] FIG. 4b shows an enlarged view of a servo valve and control chamber of the piezo-electric injector from FIG. 4a.

[0043] FIG. 5 shows, on the top graph, an example of an electrical pulse train allowing a voltage charge of a piezo-electric actuator of an injector and causing a pressure discharge of the fuel supply rail. The middle graph shows the development of the force exerted by the piezo-electric actuator on the servo valve of the injector. The bottom graph shows the opening of the servo valve caused by the electrical pulse train.

DESCRIPTION OF THE EMBODIMENTS

[0044] Reference is now made to FIG. 2 showing an embodiment of an injection system of an engine of a vehicle. The injection system 1 allows implementation of a method of discharging pressure in a fuel supply rail in an injection system of an engine of a vehicle shown in FIG. 1.

[0045] The injection system 1 comprises a fuel supply rail 4 connected to a fuel tank 3 via return lines from a plurality of piezo-electric injectors 5. The fuel present in the supply rail 4 is subjected to a defined pressure in order to promote the good combustion of the fuel during the various injection phases. Therefore it follows a reference pressure P.sub.ref determined by an engine computer (not shown). The engine computer may, for example, be a processor, a microprocessor or a microcontroller. It also has a memory which comprises code instructions for controlling the implementation of the steps of the method for discharging pressure shown in FIG. 1. The injection system 1 also comprises a pressure sensor 6 for the fuel of the supply rail 4 and an electrical generator 8. The pressure sensor 6 is used to measure whether the pressure P.sub.rail in the rail actually follows the reference pressure P.sub.ref determined by the engine computer.

[0046] A piezo-electric injector 5 of the injection system 1 is shown in more detail in FIGS. 3a, 3b, and 4a, 4b. It comprises a high-pressure fuel inlet 501, a low-pressure fuel outlet 502 to the return line from the injector 5 and hence to the tank, and an orifice 503 for injection of fuel into a combustion chamber of the engine. The injector also comprises a movable needle 53 in a first chamber 530, said first chamber being in fluidic communication with the high-pressure fuel inlet 501, said needle being movable between a first position (shown in FIGS. 3a and 3b) in which it closes the fuel injection orifice 503, and a second position in which it opens this orifice (position shown in FIGS. 4a and 4b), thus allowing the injection of fuel into a combustion chamber (not shown).

[0047] The injector also comprises a control chamber 54 (see FIGS. 3b and 4b) arranged at the end of the needle opposite the fuel injection orifice. The control chamber 54 is in fluidic communication with the high-pressure fuel inlet 501 via a restriction 540, and in fluidic communication with the low-pressure fuel outlet 502 to the tank via a second restriction 541 and a servo valve 52 arranged between the outlet 502 and the second restriction 541.

[0048] The injector also comprises a piezo-electric actuator 51 which, when it receives an electrical pulse from the electrical generator 8, is able to press on the servo valve 52. Pressing on the servo valve 52, as shown in FIGS. 4a and 4b, authorizes a circulation of fluid from the high-pressure fuel circuit of the injector towards the low-pressure outlet 502, which causes a fall in the pressure in the control chamber 54 and a movement of the needle 53 under the effect of the high pressure remaining in the first chamber 530, in order to open the orifice 503. In this way, the fuel may pass from the supply rail 4 to the combustion chamber via the orifice 503 and thus trigger an injection into said combustion chamber. This is the traditional function method of a piezo-electric injector.

[0049] The opening of the needle is not however immediate, and by controlling the opening of the servo valve via the current applied to the piezo-electric actuator, it is possible to generate a leakage current from the high-pressure inlet 501 to the low-pressure outlet 502 and the fuel tank without moving the needle, and hence without generating an injection.

[0050] In this case, the method described herein and with reference to FIG. 1 allows the fuel to pass from the supply rail 4 via the plurality of injectors 5, directing it towards the fuel tank 3, so as to discharge the pressure in the fuel rail 4. For example, when a driver releases the accelerator again, it is necessary to discharge the pressure P.sub.rail so that it can follow the reference pressure P.sub.ref determined by the engine computer. The method therefore proposes using a fuel leak created by the plurality of injectors 5 on opening of their respective servo valves 52, in order to cause the fuel to pass from the supply rail 4 to the fuel tank 3 and thus discharge the pressure P.sub.rail.

[0051] The method of discharging the pressure in a fuel rail 4 of an injection system 1 of an engine of a vehicle, described above with reference to FIG. 1, is implemented in the vehicle during a phase of release of the accelerator, i.e. when there is no request for a quantity of fuel to be injected. Reference will also be made to FIG. 5 during the description of the method. The graph at the top of the figure shows an electrical pulse train sent by the electrical generator 8 to a piezo-electric actuator 51 of an injector 5. The same graph also shows a voltage at the terminals of the piezo-electric actuator 51 in response to the electrical pulses. The graph in the middle of the figure shows the force exerted by the piezo-electric actuator 52 which receives the pulses on the servo valve of the injector 5. Finally, the graph at the bottom shows the opening O of the servo valve of the injector in response to the force applied by the piezo-electric actuator.

[0052] A first step 110 of the method comprises a comparison, on each engine cycle, between the reference pressure P.sub.ref determined by the engine computer and the pressure P.sub.rail measured by the pressure sensor 6. This is to identify whether the pressure P.sub.rail is greater than the pressure P.sub.ref so as to be able to implement the remaining steps of the method. In this way, if it is not necessary to discharge the pressure P.sub.rail in the supply rail 4, the method waits for the next engine cycle The method is therefore implemented on each engine cycle.

[0053] Advantageously, the remaining steps of the method are implemented when the difference between the pressure P.sub.rail in the rail and the reference pressure P.sub.ref is greater than a defined threshold. For example, if the difference by which the pressure P.sub.ref is exceeded reaches a defined percentage or an absolute value, the remaining steps are implemented.

[0054] A second step 120 of the method comprises the sending of a first electrical command C.sub.1 by the electrical generator 8 to the piezo-electric actuator 51 of at least one injector 5 of the plurality.

[0055] With reference to FIG. 5, the first electrical command C.sub.1 comprises an electric charge pulse I.sub.cha1 of a defined duration T.sub.cha1 and a electric discharge pulse I.sub.dcha1 of the piezo-electric actuator 51. In an electric charge pulse I.sub.cha1, the current applied is positive, causing an increase in voltage at the terminals of the piezo-electric actuator 51. In an electric discharge pulse I.sub.dcha1, the current applied is negative, causing a reduction in voltage at the terminals of the piezo-electric actuator 51.

[0056] In fact, an increase in voltage of the piezo-electric actuator 51 mechanically corresponds to an elongation of the piezo-electric unit and hence to an application of force on the servo valve 52. Conversely, a reduction in voltage of the piezo-electric actuator 51 corresponds mechanically to a shrinkage of the piezo-electric actuator 51.

[0057] The charge time duration T.sub.cha1 of the piezo-electric actuator 51 is determined so as to cause a complete charge of the piezo-electric actuator 51 of the at least one injector 5. A complete charge of the piezo-electric actuator 51 means that the piezo-electric actuator 51 is charged to allow both an opening of the servo valve 52 and an opening of the needle 53 of the injector 5. Advantageously, the piezo-electric actuator 51 of the at least one injector 5 reaches its voltage saturation level following the electric charge pulse I.sub.cha1.

[0058] According to one embodiment, the electric discharge pulse I.sub.dcha1 is symmetrical to the electric charge pulse I.sub.cha1. This means that the electric discharge pulse I.sub.dcha1 has an intensity substantially opposite to the intensity of the electric charge pulse I.sub.cha1 and that the durations of the two pulses are substantially the same (I.sub.dcha1≈−I.sub.cha1).

[0059] The first electrical command C.sub.1 also comprises a defined duration T.sub.i1 corresponding to the time elapsing between the start of the electric charge pulse I.sub.cha1 and the start of the electric discharge pulse I.sub.dcha1 of the piezo-electric actuator 51. The duration T.sub.i1 is advantageously determined such that the needle 53 of the at least one injector 5 remains immobile.

[0060] In this case, the duration T.sub.i1 is therefore advantageously determined such that, even if the charge time duration T.sub.cha1 is sufficiently long for the voltage level at the terminals of the piezo-electric actuator 51 to be greater than a threshold voltage U.sub.inj allowing opening of the needle 53, the inertia of the needle 53 keeps the latter immobile. The duration T.sub.i1 may be determined during a calibration phase by determining, via an injected quantity measuring device, a maximum duration above which the injection takes place.

[0061] An example of a first electrical current command C.sub.1 is shown in FIG. 5. This figure also shows in dotted lines the voltage threshold U.sub.inj for which the piezo-electric actuator 52 is sufficiently charged to allow opening of the needle 53.

[0062] A third step 130 of the method comprises the determination, from a value of a force exerted by the piezo-electric actuator 51 on the servo valve 52, of a charge time duration T.sub.open allowing opening of the servo valve 52 of the at least one injector 5. More precisely, the charge time duration T.sub.open is determined from a development of the force exerted by the piezo-electric actuator 51 on the servo valve 52 during the first electrical command C.sub.1 of the at least one injector 5.

[0063] In fact, the first electrical command C.sub.1, which causes a complete charge of the piezo-electric actuator 51 without producing an injection, is used in this step to estimate the charge time duration T.sub.open causing the opening of the servo valve 52. The duration T.sub.open in reality corresponds to a first voltage level U.sub.open of the piezo-electric actuator 51 for which the servo valve 52 opens without triggering the opening of the needle 53, which itself corresponds to a second voltage level U.sub.inj of the piezo-electric actuator 51.

[0064] As shown in FIG. 5, the moment of starting of the opening of the servo valve 52 corresponds to a maximum force F.sub.max. The force applied by the piezo-electric actuator to the servo valve is determined from a voltage U applied to the terminals of the piezo-electric actuator 51, from a capacitance value C of the piezo-electric actuator 51, and from a quantity value Q of electric charges transferred from the electrical generator 8 to the piezo-electric actuator 51. Thus the determination of the force exerted by the piezo-electric actuator 51 on the servo valve 52 is mathematically approximated by the following formula:

F≈U×C−Q  [Math. 1]

wherein F corresponds to the force exerted by the piezo-electric actuator 51 on the servo valve 52,
U corresponds to a voltage applied to the terminals of the piezo-electric actuator 51,
C corresponds to a capacitance value of the piezo-electric actuator 51, and
Q corresponds to a quantity value of electric charges transferred from the electrical generator 8 to the piezo-electric actuator 51.

[0065] Consequently, step 130 comprises measurement, during application of the first electrical command C.sub.1, of the voltage at the terminals of the piezo-electric actuator 51 and of the quantity of electrical charges transferred to the piezo-electric actuator 51 by the electrical generator 8, in order to deduce from these the force exerted by said actuator, and detection of the maximum force during application of the first electrical command C.sub.1.

[0066] Once the duration T.sub.open allowing opening of the servo valve 52 for the at least one injector 5 has been determined, a fourth step 140 comprises the sending of a second electrical command C.sub.2 by the electrical generator 8 to said piezo-electric actuator 51 of the at least one injector 5.

[0067] The second electrical command C.sub.2 comprises an electric charge pulse 6a2 of a defined duration T.sub.cha2 and an electric discharge pulse I.sub.dcha2 of the piezo-electric actuator 51. The charge time duration T.sub.cha2 of the piezo-electric actuator 51 is determined so as to obtain an opening of the servo valve 52 of the at least one injector 5 without triggering an injection. It is therefore determined from the duration T.sub.open of the opening of the servo valve 52, since it must be greater than this opening duration.

[0068] In addition, the charge time duration T.sub.cha2 is advantageously determined such that the voltage at the terminals of the piezo-electric actuator 51 is greater than a first voltage threshold triggering the opening of the servo valve 52 (not shown), and less than a second voltage threshold U.sub.inj triggering the opening of the needle 53.

[0069] Advantageously, the intensity associated with the electric charge pulse is I.sub.cha2 of the second electrical command C.sub.2 is substantially the same as that of the electric charge pulse I.sub.cha1 of the first electrical command C.sub.1. This means that the at least one injector 5 is under the same conditions as during first electrical command C.sub.1, and therefore the moment of opening of its servo valve 52 is substantially the same. This therefore facilitates determination of the value T.sub.cha2.

[0070] According to one embodiment, the second electrical command C.sub.2 is performed by allowing at least one time lapse T.sub.rem, not equal to zero, to pass after the end of the first electrical command C.sub.1 so as to limit the impact of the electrical remanence of the piezo-electric actuator 51. The effects of electrical remanence would disrupt the similarity between the response of the piezo-electric actuator to the electrical pulse I.sub.cha1 and the response of the piezo-electric actuator to the electrical pulse I.sub.cha2, which could modify the moment of opening of the servo valve 52.

[0071] According to one embodiment, the charge time duration T.sub.cha2 is equal to the sum of the duration T.sub.open causing the opening of the servo valve 52 and another duration T.sub.offset allowing opening of said servo valve 52 to a varying extent.

[0072] The duration T.sub.offset is thus used as a regulator as a function of the desired pressure fall. It lies strictly between a value of zero, for which T.sub.cha2 is equal to T.sub.open, and a second value allowing opening of the needle 53. It will be understood here that the closer this value comes to zero, the smaller the leakage of fuel from the at least one injector 5 to the tank 3, and hence the smaller the discharge of pressure in the fuel supply rail 4. Conversely, the greater the duration T.sub.offset, the greater the fuel leakage. In fact, if the duration T.sub.offset is too great, the voltage at the terminals of the piezo-electric actuator 51 will exceed the injection threshold U.sub.inj and therefore an injection will be triggered if this voltage is applied for a sufficiently long time.

[0073] The second value, allowing opening of the needle, is predefined on test benches using the characteristic of the observed fall in fuel pressure in the supply rail as a function of the charge time of the piezo-electric actuator. This characteristic clearly shows a charge time value above which the pressure fall is substantially intensified because of the injection of fuel into the cylinder. Thus the second value associated with the duration T.sub.offset may be determined. Naturally, the second value may be deliberately set below the critical value causing injection, as a safety measure.

[0074] It will also be understood that when the piezo-electric actuator 51 is in the desired voltage range between the opening of the servo valve and the opening of the needle 53, no injection can take place. The servo valve 52 may therefore remain open to discharge the pressure in the fuel supply rail 4 for a maximum duration, depending on the capability of the electrical generator and the development of pressure in the fuel supply rail 4, which must not influence the level of opening of the servo valve, at the risk of opening the servo valve 52 too much and triggering an injection.

[0075] To this extent, the second electrical command C.sub.2 also comprises a defined duration T.sub.i2 corresponding to the time elapsing between the start of the electrical charge pulse I.sub.cha2 and the start of the electrical discharge pulse I.sub.dcha2 of the piezo-electric actuator 51. The duration T.sub.i2 is advantageously determined such that the development of pressure in the supply rail 4 during the current combustion cycle of the engine does not influence the opening level of the servo valve 52 enough to trigger an injection.

[0076] The defined duration T.sub.i2 is thus advantageously determined to be greater than a duration allowing breakage of the inertia of the needle 53 of the at least one injector 5, since the voltage level of the piezo-electric actuator 51 is not sufficient to cause the opening of the needle 53.

[0077] The duration T.sub.i2 less the duration T.sub.open allowing the opening of the servo valve 52 of the at least one injector 5 (T.sub.i2−T.sub.open) in fact corresponds to the fuel leakage time of the at least one injector 5. Insofar as the method is implemented for each combustion cycle of the engine in the accelerator release phase, the duration T.sub.i2 is determined as a function of the engine speed, the pressure in the fuel supply rail 4, and the amount of the desired pressure fall.

[0078] An example of a second electrical current command C.sub.2 is shown in FIG. 5. The voltage at the terminals of the piezo-electric actuator 51 is thus less than the threshold voltage U.sub.inj but sufficient to open the servo valve 52 and hence cause a leakage of fuel from the supply rail 4 to the tank 3.

[0079] The method described above is therefore optimized with respect to the current operating conditions of the engine, because it allows adaptation of the fuel leak to the tank on each engine combustion cycle without risk of injection. The optimization extends beyond the operating conditions of the engine since it extends to the operating conditions of each injector, insofar as determination of the moment of opening of the servo valve is specific to each injector. The method therefore proposes an alternative to the installation of a pressure decay valve in the supply rail, while being less complex and more economic, without the addition of any further component.

[0080] Also, the sequence of only two electrical commands to the piezo-electric actuator of the injector on each engine combustion cycle during an accelerator release phase does not overly stress the piezo-electric actuator and therefore does not cause premature wear of the injector.