EP1841963B1 - Method for operating a fuel injection device of an internal combustion engine - Google Patents

Abstract

A fuel injection device of an internal combustion engine includes a piezoelectric actuator and a valve element that are coupled to one another. The valve element has a pressure stage. An increase in the force acting on the piezoelectric actuator is interpreted as an actual opening of the valve element, and/or a decrease in the force acting on the piezoelectric actuator is interpreted as an actual closing of the valve element, and that these be taken into account at least part of the time in the controlling of the piezoelectric actuator.

Description

State of the art

The invention initially relates to a method for operating a fuel injection device of an internal combustion engine, in which a piezoelectric actuator is coupled to a valve element of the fuel injection device, wherein the valve element has a pressure stage. The invention further relates to a computer program, an electrical storage medium for a control and / or regulating device of an internal combustion engine, and a control and / or regulating device for an internal combustion engine.

A method of the type mentioned is from the EP 1172 541 A1 known. In the local fuel injection device, a valve element in the form of a valve needle is provided, which can be hydraulically opened or closed by a pressure in a control room. The pressure in the control chamber is in turn influenced by a switching valve, which is coupled via a hydraulic coupler with a piezoelectric actuator.

The WO 03/040534 A shows a method of operating a fuel injection device of an internal combustion engine, in which a piezoelectric actuator is coupled to a valve element of the fuel injection device. Based on current and voltage, the time change of the mechanical force acting on the actuator is calculated in order to be able to detect a sudden pressure surge when the valve opens at the end of the injection process. A control unit calculates depending on the temporal change in the force acting on the actuator mechanical force the target value for the electrical voltage with which the actuator is to be acted upon.

Also known from the market is a fuel injection device in which the valve element is directly, ie without the interposition of a switching valve, also coupled via a hydraulic coupler with the piezoelectric actuator. In this case, either the voltage curve of the piezoelectric actuator can be specified during charging and discharging of the piezoelectric actuator, or a current profile is predetermined, which then leads to a desired voltage at the end of the charging or discharging process. In the latter case, the predetermined current profile can additionally be scaled by a superimposed voltage regulator so that at least the voltage levels at the end of the charging or discharging processes are set by a closed control loop.

However, the voltage gradient can not be set arbitrarily high. On the one hand, it is limited by the maximum current of an output stage, with which the piezoelectric actuator is driven, and on the other by the fact that at a high voltage gradient there is a risk that the resonance of the piezoelectric actuator is excited, resulting in destruction or at least can lead to damage of the piezoelectric actuator.

In the known fuel injection device, the "voltage swing" required for actuation of the valve element, that is to say the difference between the initial and final voltage when the piezoactuator is actuated, increases with increasing fuel pressure acting on the valve element in the opening direction. In this case, the fuel injection device is designed so that at a high fuel pressure, a large part of the available voltage lift must be used to open the valve element. After opening, the valve element accelerates and moves so far until there is an equilibrium of forces at the oppositely directed pressure surfaces of the valve element. At a high fuel pressure, this equilibrium point is reached only when the valve element is almost completely open.

Due to the conditions described, it is difficult to inject very small amounts of fuel into a combustion chamber of an internal combustion engine in the known fuel injection device. Such small and smallest injection quantities are desired especially in pilot injections ("pilot injection").

Object of the present invention is to be able to inject with a fuel injector with direct coupling between the piezoelectric actuator and the valve element as small amounts of fuel, at the same time stable operation of the fuel injection device, that is, without vibrations or resonance problems.

This object is achieved in a fuel injection device of the type mentioned above in that an increase in the force acting on the piezoelectric actuator as an actual opening of the valve element (actual injection start) and / or a drop in the force acting on the piezoelectric actuator as an actual closing the valve element (actual injection end) interpreted and that a closing operation of the valve element is initiated depending on the actual injection start. In a computer program, an electrical storage medium and a control and / or regulating device of the type mentioned, the object is achieved accordingly.

Advantages of the invention

The inventive method enables stable operation of a fuel injection device in which the valve element and the piezoelectric actuator are directly coupled, with very small injection quantities of down to 1 mm ³ / injection and at the same time very high fuel pressures. In addition, the inventive method allows an increase in the metering accuracy even with larger injection quantities, since the actual injection start and / or the actual injection end are known or is and can be taken into account in the control of the piezoelectric actuator. Furthermore, a very precise realization of a desired opening duration of the valve element is possible. As a result, the metering accuracy can also be improved in the partial and full load range of an internal combustion engine.

Background of these advantages of the method according to the invention is the fact that in a valve element with a pressure stage immediately after opening the valve element or a control of the piezoelectric actuator, an additional force in the opening direction acts on the valve element. Due to the direct coupling of the valve element with the piezoelectric actuator, this additional force also acts on the piezoelectric actuator. By detecting the change in the force acting on the piezoactuator, a timing at which the valve element actually opens (actual injection start) or a point in time at which the valve element closes (actual injection end) can be detected during operation of the fuel injection device , However, if the actual injection start or the actual injection end is known, the control of the piezoelectric actuator can be adjusted accordingly and thus the accuracy when introducing fuel into a combustion chamber of the internal combustion engine can be significantly improved.

Smallest injection quantities can be realized with the method according to the invention when the sign of a signal or a signal gradient with which the piezoelectric actuator is driven is changed as soon as an actual injection start has been detected. As a signal gradient, for example, a voltage gradient in question or - even more effective - as a signal, a current with which the piezoelectric actuator is charged or discharged. Since the sign change or the switching from unloading to loading or vice versa is regulated on the basis of a detected actual injection start of the valve element, the smallest amount of fuel can also be displayed very stably.

A further advantageous embodiment of the method according to the invention is characterized in that the actual injection start and / or the actual injection end is regulated according to a desired value. Unlike in previously known methods, therefore, not more start and / or end of the control of the piezoelectric actuator, but the actual actual injection start and / or the actual injection end is regulated, which not only accurate metering a desired amount of fuel, but also a precise implementation allows a desired injection timing. This avoids that a scattering of the delay time between Ansteuerbeginn and injection start or Ansteuerende and injection end on the fuel metering effect.

It can also be a difference between the actual injection start and actual injection end (actual injection duration) after a target value be managed. In this case, the metering accuracy of the fuel is even better.

Another important advantageous embodiment of the method according to the invention provides that a change in the force acting on the piezoelectric actuator is detected by a change in an electrical variable of the piezoelectric actuator influenced by the force. This is based on the idea that the force change acting on the piezoelectric actuator leads to a change in length. In the case of operation with a predetermined expansion curve-that is, with a "stamped-in" current profile - this results in a change in the voltage profile and, when operated with an "impressed" voltage curve, a change in the actuator current profile. This change can be detected according to the invention without further notice, so that an actual injection start or an actual injection end can be detected without an additional sensor being required.

A concrete development of this variant of the method provides that the piezoelectric actuator is discharged or charged with a predetermined voltage curve for opening the valve element, and that an actual injection start is detected when a discharge current or a charging current exceeds or falls below a limit value, wherein the limit value is formed by the product of a capacitance constant of the piezoelectric actuator and the discharge or charging voltage gradient. This method is very easy to implement.

The same applies to that variant of the method in which the piezoelectric actuator is discharged or charged with a predetermined current profile for opening the valve element, and in which an actual injection start is detected when a discharge or charging voltage gradient is detected Threshold exceeds or falls below, wherein the limit is formed by the quotient of discharge or charging current and a capacitance constant of the piezoelectric actuator.

In the two last-mentioned process variants, a knowledge of the charging and discharging strategy used is required. Regardless of such a strategy is a method in which to open the valve element, the piezoelectric actuator is discharged or charged, and in which a current component is estimated by a disturbance observer, which results from the increase of the force acting on the piezoelectric actuator, and in which an actual injection start is detected when the current share exceeds a limit. For example, a disturbance observer may be a Luenberger observer method.

All three latter variants of the method can be applied not only for the detection of an actual injection start, but in a corresponding manner with correspondingly adapted other limits for the detection of an actual injection end.

drawings

Hereinafter, particularly preferred embodiments of the present invention will be explained in more detail with reference to the accompanying drawings.

Figur 1

eine schematische Darstellung einer Brennkraftmaschine mit mehreren Kraftstoffinjektoren;

Figur 2

einen Teilschnitt durch einen Kraftstoffinjektor von Figur 1;

Figur 3

ein Diagramm, in dem eine in Öffnungsrichtung wirkende Kraft über einem Hub eines Ventilelements eines Kraftstoffinjektors von Figur 2 aufgetragen ist;

Figur 4

ein Diagramm, in dem verschiedene Betriebsparameter eines Kraftstoffinjektors von Figur 2 bei einem Einspritzvorgang über der Zeit aufgetragen sind;

Figur 5

ein Funktionsschaubild eines ersten Verfahrens zum Betreiben eines Kraftstoffinjektors von Figur 2;

Figur 6

ein Funktionsschaubild eines zweiten Verfahrens zum Betreiben eines Kraftstoffinjektors von Figur 2;

Figur 7

ein Funktionsschaubild eines dritten Verfahrens zum Betreiben eines Kraftstoffinjektors von Figur 2.

In the drawing show:

FIG. 1

a schematic representation of an internal combustion engine with a plurality of fuel injectors;

FIG. 2

a partial section through a fuel injector of FIG. 1 ;

FIG. 3

a diagram in which a force acting in the opening direction over a stroke of a valve element of a fuel injector of FIG. 2 is applied;

FIG. 4

a diagram in which various operating parameters of a fuel injector of FIG. 2 are plotted over time in an injection process;

FIG. 5

a functional diagram of a first method for operating a fuel injector of FIG. 2 ;

FIG. 6

a functional diagram of a second method for operating a fuel injector of FIG. 2 ;

FIG. 7

a functional diagram of a third method for operating a fuel injector of FIG. 2 ,

Description of the embodiments

In FIG. 1 an internal combustion engine is generally designated by reference numeral 10. It includes a plurality of combustion chambers 12 into which the fuel is injected directly from a respective fuel injector 14. The fuel injectors 14 are connected to a fuel pressure accumulator ("rail") 16, in which the fuel is conveyed by a conveyor system 18. The operation of the fuel injectors 14 is of a Control and / or regulating device 20 controlled or regulated (dashed lines). Among other things, input signals (dashed lines) from various sensors are used for this purpose FIG. 1 are not shown.

How out FIG. 2 can be seen, the fuel injector 14 comprises a housing 22 in which a needle-like valve member 24 is received longitudinally displaceable. This has an acting in the opening direction of the pressure shoulder 26 which is arranged in a pressure chamber 28 which is connected via a channel 30 with the fuel pressure accumulator 16. A likewise acting in the opening direction conical pressure surface 32 is fluidly separated from the pressure chamber 28 in the closed state of the valve element.

The opposite of the pressure surface 32 end of the valve element 24 projects with a surface 34 into a hydraulic control chamber 36 into which the high pressure of the fuel pressure accumulator prevails. The control chamber 36 is also limited by a control piston 38, the diameter of which in the present embodiment is greater than the control surface 34 of the valve element 24. The control piston 38 is fixed to a piezoelectric actuator 40, optionally with the interposition of a in FIG. 2 not shown power amplifier, is controlled by the control and / or regulating device 20.

So that the fuel injector 14 injects fuel into the combustion chamber 12, the piezoelectric actuator 40 is driven so that its length is reduced. As a result, the control piston 38 moves in FIG. 2 up. About the hydraulic coupling by means of the control chamber 36 and due to the force acting on the pressure shoulder in the opening direction force also moves the valve element 24 upwards. As a result, the high fuel pressure prevailing in the pressure chamber 28 also abuts the end-side pressure surface 32 of the valve element 24, which, after the first opening movement of the valve element 24, leads to an additional force acting in the opening direction and to an accelerated opening of the valve element 24.

Since the force acting on the valve element 24 in the opening direction increases rapidly immediately after opening, it is also referred to as a valve element with a "pressure stage". The increase in the force acting in the opening direction on the valve element 24 force F is also off FIG. 3 can be seen where this is plotted over an opening stroke H. Fuel can now pass through fuel outlet channels 42 into the combustion chamber 12.

In order to be able to inject even very small amounts of fuel with the fuel injector 14, the procedure according to a method, which now with reference to FIG. 4 is explained:

In the present embodiment, when the valve element 24 is to be closed, the piezo actuator 40 is charged. To open the valve element 24, the piezoelectric actuator 40 is discharged. In this case, in the present exemplary embodiment, the piezoelectric actuator 40 is charged or discharged with a specific, in the present case substantially linear voltage curve. For this purpose, the voltage curve or the voltage gradient is measured during discharging and charging, and the charging or discharging current is adjusted accordingly. A charge Q of the piezoelectric actuator can thereby be simplified as the sum of a voltage-dependent component Q _u = C ₀ × U and a length-dependent component Q _× = C _× × (x -) x ₀ ). The factors C ₀ and C _x are capacitance constants, U a voltage applied to the piezoelectric actuator, and x a current length of the piezoelectric actuator 40.

The length-dependent component Q _x is based on the following consideration: The force acting in the opening direction on the valve element 24 is transmitted via the hydraulic coupling of the pressure chamber 28 and the control piston 38 also to the piezoelectric actuator 40. If the valve element 24 opens, this results due to the pressure level on the valve element To a force increase ("force jump") and the piezoelectric actuator 40. This force jump leads to an additional change in length of the piezoelectric actuator 40. In order to comply with the predetermined voltage curve even under the boundary condition of the increased rate of change of the piezoelectric actuator 40, the discharge current i relative to the state at rest Valve element 24 can be increased. In the method used here, this force jump and the corresponding charge change are used to detect an actual opening of the valve element 24 (injection start) or an actual closing of the valve element 24 (injection end). The following physical principles are used for this: $$\mathrm{Q}\mathrm{=}\mathrm{\int }\mathrm{idt}\mathrm{=}{\mathrm{Q}}_{\mathrm{u}}\mathrm{+}{\mathrm{Q}}_{\mathrm{x}}\mathrm{=}{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}\mathrm{*}\mathrm{u}\mathrm{+}{\mathrm{C}}_{\mathrm{x}}\mathrm{*}\left(\mathrm{x}\mathrm{-}{\mathrm{<figure-callout\; id="0"\; label="x"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">x</figure-callout>}}_{\mathrm{0}}\right)$$

This results dissolved after the discharge or charging current of the piezoelectric actuator 40: $$\mathrm{i}\mathrm{=}{\mathrm{i}}_{\mathrm{u}}\mathrm{+}{\mathrm{i}}_{\mathrm{x}}\mathrm{=}\frac{{\mathrm{dQ}}_{\mathrm{u}}}{\mathrm{dt}}\mathrm{+}\frac{{\mathrm{dQ}}_{\mathrm{x}}}{\mathrm{dt}}\mathrm{=}{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}\mathrm{*}\frac{\mathrm{you}}{\mathrm{dt}}\mathrm{+}{\mathrm{C}}_{\mathrm{x}}\mathrm{*}\frac{\mathrm{dx}}{\mathrm{dt}}$$

It follows: $$\frac{\mathrm{dx}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{x}}}\mathrm{*}\left(\mathrm{i}\mathrm{-}{\mathrm{<figure-callout\; id="0"\; label="c"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">c</figure-callout>}}_{\mathrm{0}}\mathrm{*}\frac{\mathrm{you}}{\mathrm{dt}}\right)$$

If the discharge current i unequivocally drops below the limit value C ₀ · du / dt during discharge, this means that the valve element 24 is just opening. If the charging current i exceeds this value when the piezoactuator 40 is being charged, the direction of movement of the valve element 24 is changing. If the charging current during charging drops below this value, the valve element 24 is closing. In FIG. 4 the voltage U applied to the piezoelectric actuator 40 is designated by 44, the current i by 46, the limit value C ₀ · du / dt by 48 (dot-dashed curve) and a stroke H of the valve element 24 by 50. The start of injection takes place at the time t ₁ , the direction reversal of the valve element 24 at time t ₂ , and the injection end is at the time t ₃ .

Depending on the detected injection start of the fuel injector 14 at time t ₁ , the piezoelectric actuator 40 is closed by the control and / or regulating device 20 as soon as possible. For this purpose, after a detected start of injection t _1, the discharge current i is changed such that the gradient du / dt of the voltage u has an inverse sign. The closing of the valve element 24 is thus initiated depending on the actual opening (start of injection). Since the actual injection start and the actual injection end can be detected at the times t ₁ or t ₃ on the basis of the specified method, these can each be controlled according to a desired value. It is also possible to regulate the actual injection start at time t ₁ and a difference dt _i , which is also referred to as actual injection duration, according to a desired value.

$$\frac{\mathrm{dx}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{x}}}\mathrm{*}\frac{{\mathrm{dQ}}_{\mathrm{x}}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{x}}}\mathrm{*}\mathrm{(}\mathrm{i}\mathrm{-}\frac{{\mathrm{dQ}}_{\mathrm{u}}}{\mathrm{dt}}\mathrm{)}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{x}}}\mathrm{*}\left(\mathrm{1}\mathrm{-}{\mathrm{c}}_{\mathrm{0}}\mathrm{*}\frac{\mathrm{du}}{\mathrm{dt}}\right)$$

Ultimately, however, the fuel injector 14 or the piezoelectric actuator 40 can also be charged and discharged with a given predetermined course of Ladebeziehungsweise discharge current i. From the above equation (1), $$\frac{\mathrm{you}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\frac{{\mathrm{dQ}}_{\mathrm{u}}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\mathrm{(}\mathrm{i}\mathrm{-}\frac{{\mathrm{dQ}}_{\mathrm{x}}}{\mathrm{dt}}\mathrm{)}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\left(\mathrm{1}\mathrm{-}{\mathrm{C}}_{\mathrm{x}}\mathrm{*}\frac{\mathrm{dx}}{\mathrm{dt}}\right)$$

$$\frac{\mathrm{dx}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{x}}}\mathrm{*}\frac{{\mathrm{dQ}}_{\mathrm{x}}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{x}}}\mathrm{*}\mathrm{(}\mathrm{i}\mathrm{-}\frac{{\mathrm{dQ}}_{\mathrm{u}}}{\mathrm{dt}}\mathrm{)}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{x}}}\mathrm{*}\left(\mathrm{1}\mathrm{-}{\mathrm{<figure-callout\; id="0"\; label="c"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">c</figure-callout>}}_{\mathrm{0}}\mathrm{*}\frac{\mathrm{you}}{\mathrm{dt}}\right)$$

If, in this case, the voltage gradient du / dt during discharge is clearly above the value i / C ₀ , the valve element 24 is in the process of being opened. If the voltage gradient du / dt falls below this value, the valve element 24 closes straight.

so erhält man $$\mathrm{u}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{0}}}\mathrm{*}\mathrm{\int }\left(\mathrm{i}\mathrm{-}{\mathrm{C}}_{\mathrm{x}}\mathrm{*}\frac{\mathrm{dx}}{\mathrm{dt}}\right)\mathrm{dt}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{0}}}\mathrm{*}\mathrm{\int }\left(\mathrm{i}\mathrm{-}\frac{{\mathrm{dQ}}_{\mathrm{x}}}{\mathrm{dt}}\right)\mathrm{dt}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{C}}_{\mathrm{0}}}\mathrm{*}\mathrm{\int }\left(\mathrm{i}\mathrm{-}{\mathrm{i}}_{\mathrm{x}}\right)\mathrm{dt}$$

However, the start of injection and the injection end can also be determined independently of the charging and discharging strategy, that is, regardless of whether a specific voltage curve or a specific current profile is specified. This is done with the help of a Störgrößenbeobachter 51, for example, a Luenberger observer method (see. FIG. 5 ). Integrate the equation $$\frac{\mathrm{you}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\frac{{\mathrm{dQ}}_{\mathrm{u}}}{\mathrm{dt}}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\mathrm{(}\mathrm{i}\mathrm{-}\frac{{\mathrm{dQ}}_{\mathrm{x}}}{\mathrm{dt}}\mathrm{)}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\left(\mathrm{i}\mathrm{-}{\mathrm{C}}_{\mathrm{x}}\mathrm{*}\frac{\mathrm{dx}}{\mathrm{dt}}\right)$$

this is how you get $$\mathrm{u}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\mathrm{\int }\left(\mathrm{i}\mathrm{-}{\mathrm{C}}_{\mathrm{x}}\mathrm{*}\frac{\mathrm{dx}}{\mathrm{dt}}\right)\mathrm{dt}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\mathrm{\int }\left(\mathrm{i}\mathrm{-}\frac{{\mathrm{dQ}}_{\mathrm{x}}}{\mathrm{dt}}\right)\mathrm{dt}\mathrm{=}\frac{\mathrm{1}}{{\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0003.png,imgf0004.png"\; state="\{\{state\}\}">C</figure-callout>}}_{\mathrm{0}}}\mathrm{*}\mathrm{\int }\left(\mathrm{i}\mathrm{-}{\mathrm{i}}_{\mathrm{x}}\right)\mathrm{dt}$$

The equation (7) can be understood as a transmission path, of the input variables, however, only the current i can be measured. However, the current variable i _x that is dependent on the change in length of the piezoelectric actuator 40 or the increase in force when opening the valve element 24 can not be measured. In order to determine this, the charge or discharge current i known in the control and / or regulating device 20 is first fed to a path simulation (block 52 in FIG FIG. 5 ). In the in FIG. 5 As shown, this path simulation consists of an integrator with the integration constant C ₀ or with a time constant calculated by normalization from the integration constant C ₀ . The output of this integrator 52 is an observed voltage u _b at the piezo actuator 40.

If the quantity i _x = 0, u _b = u. However, when the piezo actuator 40 changes its length when the valve element 24 is opened or closed, and consequently i _x ≠ 0, u _b and u deviate from each other. 54, the difference between u _b and the measured voltage u is formed and supplied to a feedback element 56. This may be, for example, a simple proportional amplifier or a PI element, but also an amplifier with a second or higher order transmission behavior.

The output signal of the feedback element 56 is then applied with a negative sign to the input of the path simulation 52. This output signal now follows the per se unknown quantity i _x according to the transmission behavior of the observer 51 and can either be used directly or via a further filter element 58 as an observed signal i _{x, b} for the unknown quantity i _x . If the signal i _{x, b} falls below a defined threshold during discharge, then an opening of the valve element 24 detected, it exceeds a second threshold defined when loading, the beginning of the closing operation of the valve element 24 is detected. If, at the end of the charging process, it falls below this second or a further third threshold, this is detected as the end of injection.

How out FIG. 6 it can be seen, the feedback element and the filter member 58 can be combined to form a unit 60. For this purpose, the filtered signal is formed from weighted components of the output signal of the feedback element. The example of a PI element as a feedback element 56 can be illustrated as follows: For example, as the observed signal i _{x, b} instead of the output signal of the feedback element 56, only the I component or the sum of the I component and the factor K multiplied P-portion, where K should then be between 0 and 1. This corresponds to a filtering of the output signal with a first-order delay element.

As shown below, the path simulation of the piezoelectric actuator 40 can be adapted even more precisely to its real behavior. For example, a nonlinear behavior of the piezoactuator 40 can be simulated by a similarly non-linear integrator 52 and / or hysteresis effects can be introduced by insertion a hysteresis member 60 are considered in the route simulation (see. FIG. 7 ).

It should be noted at this point that the above methods are stored in the form of a computer program on a memory of the control and / or regulating device 20.

Claims (12)

1. Method for operating a fuel injection device (14) of an internal combustion engine (10), in which a piezo-actuator (40) is coupled to a valve element (24) of the fuel injection device (14), wherein the valve element (24) has a pressure stage, characterized in that an increase in the force (F) acting on the piezo-actuator (40) is interpreted as actual opening of the valve element (24) (t₁) and/or a decrease in the force (F) acting on the piezo-actuator (40) is interpreted as actual closing of the valve element (24) (actual end of injection) and it is taken into account at least at certain times in the actuation of the piezo-actuator (40), and in that a closing process of the valve element (24) is initiated as a function of the actual opening of the valve element (t₁).
2. Method according to Claim 1, characterized in that the sign of a signal or of a signal gradient (dU/dt) with which the piezo-actuator (40) is actuated is changed as soon as an actual start of injection (t₁) has been detected.
3. Method according to one of the preceding claims, characterized in that the actual start of injection (t₁) and/or the actual end of injection (t₃) is adjusted in accordance with a setpoint value.
4. Method according to one of the preceding claims, characterized in that a difference (dt_i) between the actual start of injection (t₁) and the actual end of injection (t₃) (actual injection period) is adjusted in accordance with a setpoint value.
5. Method according to one of the preceding claims, characterized in that a change in the force acting on the piezo-actuator (40) is sensed by means of a change in an electrical variable (i, u), influenced by the force, of the piezo-actuator (40).
6. Method according to Claim 6, characterized in that, in order to open the valve element (14), the piezo-actuator (40) is discharged or charged with a predefined voltage profile (44), and in that an actual start of injection (t₁) is detected if a discharge current or a charge current (46) exceeds or undershoots a limiting value (48), wherein the limiting value (48) is formed by the product of a capacitance constant (C₀) of the piezo-actuator (40) and the discharge voltage gradient or charge voltage gradient (dU/dt).
7. Method according to one of Claims 5 and 6, characterized in that, in order to open the valve element (24), the piezo-actuator is discharged or charged with a predefined current profile and that an actual start of injection is detected if a discharge voltage gradient or charge voltage gradient (du/dt) exceeds or undershoots a limiting value (i/C₀), wherein the limiting value (i/C₀) is formed by the quotient of the discharge current (i) or charge current (i) and a capacitance constant (C₀) of the piezo-actuator (40).
8. Method according to one of Claims 5 to 7, characterized in that, in order to open the valve element (24), the piezo-actuator (40) is discharged or charged, and in that, by means of an interference variable observer (51), a current component (I_x,b) which results from the increase in the force acting on the piezo-actuator (40) is estimated, and in that an actual start of injection is detected if the current component (i_x,b) exceeds a limiting value.
9. Method according to one of Claims 5 to 8, characterized in that it is applied in a corresponding way to the detection of an actual end of injection (t₃).
10. Computer program, characterized in that it is programmed for use in a method according to one of the preceding claims.
11. Electrical storage medium for an open-loop and/or closed-loop control device (20) of an internal combustion engine (10), characterized in that a computer program for use in a method of Claims 1 to 10 is stored in said storage medium.
12. Open-loop and/or closed-loop control device (20) for an internal combustion engine (10), characterized in that it is programmed for use in a method according to one of Claims 1 to 10.

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