Method for operating a fuel injector

A method is for operating a fuel injector, in particular of an internal combustion engine of a motor vehicle, the fuel injector including a piezoelectric actuator for driving a valve needle, which is connected, preferably hydraulically, to the actuator, and the control voltage of the piezoelectric actuator being analyzed to infer an operating state of the fuel injector. According to this method, the second time derivative of the control voltage and/or a variable depending on the second time derivative of the control voltage is/are analyzed.

FIELD OF THE INVENTION

The present invention relates to a method for operating a fuel injector, in particular in an internal combustion engine of a motor vehicle, the fuel injector having a piezoelectric actuator for driving a valve needle connected, preferably hydraulically, to the actuator, and a trigger voltage of the piezoelectric actuator being analyzed to infer the operating state of the fuel injector.

BACKGROUND INFORMATION

Such a method is described in DE 10 2006 003 861. In this method, a voltage applied to the piezoelectric actuator after triggering of the actuator is checked for deviations from a predefinable voltage value to infer a closing instant of a nozzle needle of the fuel injector.

SUMMARY

Example embodiments of the present invention provide a method of the type defined in the introduction so that more accurate information about an operating state of the fuel injector is obtained.

According to example embodiments of the present invention, a method of the type defined at the outset includes analyzing the second time derivative of the trigger voltage and/or a variable depending on the second time derivative of the trigger voltage.

Through the analysis of the second time derivative of the trigger voltage of the piezoelectric actuator and/or a variable depending thereon according to the present invention, an accurate conclusion about characteristic operating states of the piezoelectric actuator and/or the valve needle connected to it and about the fuel injector as a whole is possible.

A zero crossing of the second time derivative, in particular from positive values to negative values, may advantageously be ascertained in particular during the opening of the fuel injector to infer a characteristic operating state. There is a feedback effect of the valve needle on the piezoelectric actuator after a certain period of time during the opening of the fuel injector, this being detectable based on the aforementioned zero crossing of the second time derivative of the trigger voltage of the piezoelectric actuator. This feedback effect corresponds to a characteristic operating state of the fuel injector and the analysis according to example embodiments of the present invention thus allows accurate monitoring of the corresponding operating state.

Furthermore, it is possible in a particularly advantageous manner for a maximum of the second time derivative to be ascertained, in particular during the closing of the fuel injector, to infer a characteristic operating state. Diring a closing operation of the fuel injector, the valve needle striking its closing seat and the associated sudden deceleration of the valve needle in turn result in a feedback effect on the piezoelectric actuator in the form of the maximum described above. The analysis according to example embodiments of the present invention for such a maximum allows accurate monitoring of the instant at which the valve needle has reached its closing seat.

A simple and at the same time particularly accurate analysis may be performed if the trigger voltage of the piezoelectric actuator is sampled, preferably at a fixed sampling frequency and if the variable depending on the second time derivative of the trigger voltage is formed from the sampling values thereby obtained.

The variable depending on the second time derivative of the trigger voltage is advantageously obtained by the following equation in particular:
ddu[k]=(u[k+j]−u[k+1])−(u[k]−u[k−j+1])
where u[k] is a sampling value for the trigger voltage at a discrete instant k, and where j is a predefinable constant for which it holds that j=5, for example. A method that is particularly easy and efficient to implement is to ascertain the variable which depends on the second time derivative of the trigger voltage using the equation given above according to example embodiments of the present invention, to obtain the information in question by using a computation unit, such as that provided in a control unit for operation of the fuel injector. This allows more accurate recognition of breaks in the signal curve in the case of sampled signals than by analysis of the mathematically accurate second derivative.

Operation of the fuel injector is regulated as a function of the second time derivative of the trigger voltage and/or of the variable depending on the second time derivative of the trigger voltage in a particularly advantageous manner. In doing so, the information about the operating state(s) of the fuel injector obtained according to example embodiments of the present invention may be advantageously used to achieve a predefinable operating performance, in particular one that is constant over time, and to compensate for manufacturing-related tolerances, aging effects and the like.

To achieve the desired operating performance of the fuel injector, a charging current and/or a discharge current, in particular the corresponding threshold values, are advantageously predefined.

Furthermore, it may also be advantageous to predefine a charging time for charging the piezoelectric actuator to be sure that the piezoelectric actuator is fully charged for subsequent triggering back to the corresponding nominal voltage and/or output voltage. It is also possible to predefine a discharge time accordingly. Alternatively or in addition to predefining a charging time and/or discharge time, a certain trigger voltage may also be predefined as a cutoff criterion for a discharge operation and/or a charging operation.

An increase in the reliability of the method according to example embodiments of the present invention is obtained because the analysis is performed in only one or more predefinable time windows.

Implementation of the method according to example embodiments of the present invention in the form of a computer program capable of running on a computer and/or a computation unit of a control unit and suitable for executing the method is of particular importance. The computer program may be stored on an electronic storage medium, for example, such that the storage medium may in turn be contained in the control unit, for example.

Additional advantages, features and details are derived from the following description in which various exemplary embodiments of the present invention are depicted with reference to the drawings. The features mentioned may be provided either individually or in any combination.

DETAILED DESCRIPTION

FIG. 1shows a fuel injector10of a motor vehicle equipped with a piezoelectric actuator12. Piezoelectric actuator12is triggered by a control unit20, as indicated by the arrow inFIG. 1. Furthermore, fuel injector10has a valve needle13, which may sit on a valve seat14in the interior of the housing of fuel injector10.

If valve needle13is lifted up from valve seat14, fuel injector10is opened and fuel is injected. This state is depicted inFIG. 1. If valve needle13is seated on valve seat14, fuel injector10is closed. The transition from the closed state to the open state is accomplished with the help of piezoelectric actuator12. To do so, an electric voltage, also referred to below as trigger voltage U, is applied to actuator12and induces a change in length of a piezostack situated in actuator12, the piezostack in turn being utilized to open and/or close fuel injector10.

Fuel injector10also has a hydraulic coupler15. For this purpose, a coupler housing16, in which two pistons17,18are guided, is provided inside fuel injector10. Piston17is connected to actuator12and piston18is connected to valve needle13. A volume19is enclosed between two pistons17,18and transfers the force exerted by actuator12to valve needle13.

Coupler15is surrounded by fuel11under pressure. Volume19is also filled with fuel. Volume19may adapt to the particular length of actuator12for a longer period of time via the guide gap between two pistons17,18and coupler housing16. However, volume19remains almost unchanged in the case of brief changes in the length of actuator12, and the change in length of actuator12is transferred to valve needle13.

To obtain information about an operating state of fuel injector10, the method according to example embodiments of the present invention is performed as described below, this method being stored in the form of a computer program on an electronic memory element (not shown) and optionally being provided in control unit20to be processable by a computation unit of control unit20.

FIG. 2aschematically shows the time characteristic of trigger voltage U with which actuator12is triggered to induce the opening and subsequent closing of fuel injector10(FIG. 1) and thus to trigger fuel injection.

At the start of the triggering operation at t=t0, actuator12is charged to an output voltage U0and thus is at its maximum length and/or maximum actuator stroke, so that valve needle13, which is linked to actuator12(FIG. 1), is in contact with its valve seat and/or closing seat14, and fuel injector10is closed accordingly.

Subsequent discharging of actuator12initially causes a drop in trigger voltage U in time interval [t0, t2]. During this time, actuator12has become accordingly shorter so that valve needle13has been moved away from its valve seat14. Investigations by the applicant have shown that approximately after instant t2, moving valve needle13has a feedback effect on coupler15and thus also on actuator12, possibly resulting in a temporary increase in trigger voltage U in interval of time [t2, t4]. The continued discharge causes a further drop in trigger voltage U by instant t5, which represents the end of the discharge operation.

Subsequent charging of actuator12takes place starting at instant t6to instant t9, valve needle13having already reached its valve seat14again at instant t7and being greatly decelerated accordingly. This results in another corresponding feedback effect of valve needle13on actuator12coupled to it, thereby lowering its electric capacitance. Consequently, trigger voltage U rises more steeply after instant t8than previously, i.e., at times t<t7, although for trigger current I there is no essential change (seeFIG. 2b). Discharging is concluded at instant t9, when trigger voltage U again has its output value U0which is required for the next fuel injection.

According to example embodiments of the present invention, the second time derivative of trigger voltage U and/or a variable depending on the second time derivative of trigger voltage U is/are advantageously analyzed to detect the feedback effect of valve needle13at instant t4.

Trigger voltage U is preferably sampled, in particular at a fixed sampling frequency, and a variable ddu, which depends on the second time derivative of trigger voltage U, is formed from sampling values u[k] thereby obtained. The sampling frequency may be 200 kHz, for example.

Variable ddu, which depends on the second time derivative of trigger voltage U, is obtained from individual sampling values u[k] using the following equation:
ddu[k]=(u[k+j]−u[k+1])−(u[k]−u[k−j+1]),
where u[k] denotes a sampling value for trigger voltage U at a discrete instant k, and where j is a predefinable constant, for which it holds that j=5, for example. Formation of variable ddu is expedient in particular because it requires only addition and/or subtraction and thus may be performed rapidly and efficiently by a computation unit of control unit20accordingly.

A schematic time characteristic of variable ddu is given inFIG. 3afor the opening operation of fuel injector10(cf. interval of time [t0, t5] fromFIG. 2a) and is given inFIG. 3bfor the closing operation of fuel injector10(cf. interval of time [t6, t9] fromFIG. 2a), where discrete instants k correspond to the corresponding values of time t plotted on the abscissa.

AsFIG. 3ashows, variable ddu to be considered according to example embodiments of the present invention has a zero crossing from positive to negative values at instant t3(see alsoFIG. 2a), this crossing being ascertained and/or analyzed as part of the analysis according to the present invention. By ascertaining chronological position t3of this zero crossing, a characteristic operating state of fuel injector10may be ascertained independently of wear phenomena or changes in operating conditions, this operating state corresponding to approximately half a maximum stroke of valve needle13.

In other words, instant t3of the zero crossing of variable ddu indicates the instant at which valve needle13has completed half its maximum stroke, regardless of altered operating conditions and wear phenomena. The chronological correspondence of half the maximum stroke and the turning point in trigger voltage U and/or the zero crossing of variable ddu are applicable for the design of fuel injector10described here. In principle, the actuator stroke may also equal a different percentage amount of the maximum stroke on occurrence of the turning point in trigger voltage U.

Analogously, as shown inFIG. 3b, there is a maximum for variable ddu during the closing of fuel injector10and this maximum corresponds to the break in trigger voltage U in the range of instant t7, t8(seeFIG. 2a). In other words, by ascertaining instant t8at which the maximum of variable ddu occurs, it is possible to determine the instant of the actual closing of fuel injector10, at which valve needle13has reached its valve seat14, regardless of altered operating conditions and wear phenomena.

Thus, the corresponding operating state of fuel injector10may always be determined by analyzing variable ddu independently of operating conditions and age phenomena, which vary over time and involve in particular an electric capacitance or a lift capacity of actuator12, valve seat14, etc.

Operation of fuel injector10is regulated in a particularly advantageous manner as a function of variable ddu and/or its analysis. Therefore, it is possible to regulate the zero crossing (seeFIG. 3a) and/or the maximum (seeFIG. 3b) to occur at a particular determined fixed instant t3, t8, for example, so that the corresponding operating state is also always established at these instants t3, t8.

In other words, the turning point in trigger voltage U, instant t3, and the break in the closing operation, instant t8, may be equated in time for successive fuel injection operations—with regard to an onset of triggering t0so that a uniform time characteristic of the needle stroke of valve needle13is achievable over almost the entire lifetime of fuel injector10and corresponds to a corresponding uniform injected quantity of fuel. This effectively prevents drift in the quantity of fuel injected due to mechanical wear and aging, i.e., fatigue, of actuator12.

To achieve chronological equality of the characteristic operating state, for example, charging current and/or discharge current I (FIG. 2b) may be adjusted accordingly. In particular, threshold values provided for charging current and/or discharge current I may be set as a function of variable ddu and/or its time characteristic.

The charging time during which actuator12is recharged after being discharged may also be regulated to be sure that actuator12is charged up again after the actual closing of fuel injector10at instant t8(FIG. 2a) until reaching output voltage U0, which is required for a subsequent fuel injection operation.

It is possible in general to use the operating states ascertained using the method according to the present invention and/or to use the correlating breaks and/or turning points in the time characteristic of trigger voltage U as regulating features and to link them with different manipulated variables such as the start of triggering, for example. The turning point in trigger voltage U during the opening of fuel injector10may be preferably linked to the start of triggering, and a break in trigger voltage U during closing of fuel injector10may be linked to the duration of triggering.

Particularly reliable detection of the characteristic operating states of fuel injector10is ensured when the analysis of variable ddu is performed only in predefinable time windows following a zero crossing and/or a maximum. The time windows are preferably to be selected as a function of actual triggering of actuator12in an advantageous manner, so that the features of the zero crossing and/or maximum that are to be ascertained occur in the time window as unambiguously as possible. For example, two such time windows T1, T2are represented by curly brackets inFIGS. 3a,3b.

In a total of four curves,FIG. 4aschematically shows a time characteristic of needle stroke h of valve needle13as it occurs under different operating conditions and/or different wear states without the use of the method according to the present invention when opening a fuel injector. As shown inFIG. 4a, there are very different time characteristics for needle stroke h and thus also accordingly different injected fuel quantities depending on the operating conditions and/or wear state.

According to example embodiments of the present invention, the time equality of the turning point of trigger voltage U at instant t3results in a much more uniform needle stroke characteristic h, as shown inFIG. 4b, under different operating conditions and/or with a different wear state.

FIG. 4cschematically shows a time characteristic of needle stroke h of valve needle13as it occurs under different operating conditions and/or different wear states without the use of the method according to example embodiments of the present invention when closing a fuel injector. AsFIG. 4cshows, there are very different time characteristics for needle stroke h and thus also accordingly different injected fuel quantities depending on the operating conditions and/or wear state

According to example embodiments of the present invention, the time equality of the break in trigger voltage U at instant t8results in a much more uniform needle stroke characteristic h, as illustrated inFIG. 4d, even under different operating conditions and/or wear states and thus results in an injected fuel quantity that is largely independent of different operating conditions and/or wear states.

The time equality of the operating states of fuel injector10described above allows the quantity of fuel injected to be maintained accurately over the entire operating time and/or lifetime of fuel injector10if the injection rate remains almost constant in the opened state.

In addition to the advantageous time equality of characteristic operating states of fuel injector10based on variable ddu and a corresponding regulation, the method according to example embodiments of the present invention also allows desired valve needle dynamics to be predefined by setting charging current and/or discharge current I.

The break in trigger voltage U, which occurs in time interval [t7, t9] in the present example according toFIG. 2a, may occur at high trigger currents I and even after the end of electric current feed, i.e., at t>t9inFIG. 2adepending on the design of the hydraulic component of fuel injector10. Again in this case, the break in trigger voltage U may be recognized by the method according to example embodiments of the present invention described above and used as a regulating feature.

The method according to example embodiments of the present invention may also be advantageously used to recognize an operating state of fuel injector10in which valve needle13reaches a stroke stop during opening. The stroke stop (not shown) limits in particular the movement of valve needle13to a maximum stroke, which corresponds to the completely open state of fuel injector10. The feedback effect of valve needle13on piezoelectric actuator12changes on reaching the stroke stop in such a way that the change in trigger voltage U over time undergoes a corresponding relatively great change. This change is advantageously recognized as a local minimum in the second time derivative of trigger voltage U, so that by using the method according to example embodiments of the present invention, it is possible to determine when the stroke stop is actually reached by valve needle13.

As already described in conjunction with the additional operating states of fuel injector10that are of interest, by stipulating corresponding manipulated variables, the instant at which valve needle13reaches the stroke stop may thus also be advantageously regulated, thereby increasing the accuracy during injection of the fuel quantity. A time equality for valve needles13of multiple fuel injectors10reaching the needle stroke stop may be implemented by way of a corresponding regulation in particular through the analysis of the local minimum of the second time derivative of trigger voltage U corresponding to the reaching of the needle stroke stop.

An electric current feed of piezoelectric actuator12, which is necessary for opening fuel injector10, is ended at instant t5in the exemplary embodiment described above (seeFIG. 2b). As of this instant t5, valve needle13first moves further toward the stroke stop in the direction of opening and in doing so exerts pressure on piezoelectric actuator12, which results in an increase in trigger voltage U immediately after instant t5(seeFIG. 2a). As soon as valve needle13has reached its stroke stop, trigger voltage U remains essentially constant until rising again at instant t6due to renewed electric current feed to piezoelectric actuator12.

Depending on the design of fuel injector10and/or its hydraulic components, other operating states that may occur and are associated with characteristic changes in the trigger voltage may be detected by the method according to example embodiments of the present invention and their actual chronological occurrence may be advantageously made the object of corresponding regulating methods, preferably with the goal of finding an equivalency over multiple fuel injectors10.