WO2012038543A1 - Determining the closing time of a control valve of an indirectly driven fuel injector - Google Patents

Abstract

A method is described for determining the closing time of a control valve (620) of an indirectly driven fuel injector (600), which control valve (620) has a coil drive. The method has (a) switching off of a current flow through a coil (622) of the coil drive, with the result that the coil (622) is currentless, (b) detection of a time profile of a voltage which is induced in the currentless coil (622), wherein the induced voltage is generated by decaying eddy currents in a magnetic circuit of the coil drive and by a movement of a magnet armature (630) of the coil drive relative to the coil (622), (c) evaluation of the detected time profile of the voltage which is induced in the coil (622), and (d) determination of the closing time on the basis of the evaluated time profile. Furthermore, a corresponding device and a computer program for carrying out the stated method are described.

Description

description

Determining the closing time of a control valve of an indirectly driven Kraftstoffin ector

The present invention relates to the technical field of the control of indirectly driven Kraftstoffin ectors. In particular, the present invention relates to a method for determining the closing time of a control valve of an indirectly driven one having a coil drive

Fuel injector. The present invention further relates to a corresponding device and a computer program for determining the closing time of a coil drive having a control valve indirectly driven

Fuel injector.

In indirectly driven fuel injectors, a so-called solenoid actuator controls the valve piston of a control or injection valve. Servo valve, with which the pressure conditions between a control room and a valve room are influenced. The movement of the valve needle of the fuel injector is determined by the prevailing force relationships, which are determined by a spring and the pressures in the control chamber and in the valve chamber. These pressure conditions can be controlled by controlling the

Check the control valve.

FIG. 6 shows a schematic representation of such an indirectly driven fuel injector 600. The fuel injector 600 has an outer housing 602 and an inner housing 604. Within the inner housing 604 is a slidably mounted valve needle 610, which is biased by a spring 612. This spring urges the valve needle 610 down, so that in an initial state, an off ^¬ opening 614 of the Kraftstoffinj ector is closed 600th

In an upper part of the fuel injector 600 shown in FIG. 6, there is a control drive 620 Control drive 620 includes a solenoid 622 located within an iron yoke 624. The control drive 620 further comprises a piston 630 and an armature 630, which is displaceably mounted and between a through a lower stop surface of the iron yoke 624 and a seat 632 of

Control valve 620 can be moved. The piston 630 is mechanically coupled via a coupling element 628 with a spring 626. The spring 626 is located inside the solenoid 622. The fuel injector 600 further has a control chamber 642, which is connected via a high pressure line 640 to a common rail system 650. On the rail system, a pressure sensor 652 is mounted, with which the pressure in the rail system can be monitored by a control unit, not shown. The control chamber 642 is connected via a thin channel, not shown in Figure 6 with a valve chamber 644. Through this channel, fuel can flow at a relatively low flow rate. The control chamber 642 is further connected to (a) via a line not provided with a reference numeral and (b) via the control valve 620 to a low-pressure line 646. The low-pressure line is often referred to as leakage system 646.

If an injection should be triggered, the solenoid 622 is energized by applying a voltage U_Solenoid. The arrival control of the solenoid 622 may, for example, current carried ^¬ regulated. The current generates a magnetic force (in FIG. 6 denoted by F_solenoid), which acts on the piston 630 of the control valve 620. Once this magnetic force, the force exerted by the spring 626 (in Figure 6 designated F_Feder), the 620 fixed in the non-energized case in the ge ^¬ closed position, overcomes the control valve, the piston 630 in the direction of the solenoid 622 and the lower stop surface of the iron yoke 624 accelerates. As a result, the control valve 620 is opened and the high-pressure fuel can escape from the control chamber 642 in the low pressure line 646. The resulting pressure difference between the pressure in the 644 valve space and the pressure in the control chamber 642 then accelerates the valve needle 610 of the fuel injector 600 upward and the outlet opening 614 is released.

If the injection is to be terminated, the current flow through the solenoid 622 is interrupted. The magnetic force builds up, and as soon as the magnetic force exceeds the force of spring 626 under ^¬, the valve piston of the control valve 620 is accelerated in the closing position down. The high pressure in the control chamber 642 is rebuilt and the valve needle 610 of the fuel injector 600 is accelerated down to the closed position.

The amount of fuel to be injected thus depends directly on the control of the control valve 620. The dynamic behavior of the control valve 620 is then mainly influenced by the opening and closing operation ^¬. Tolerances in the opening and closing behavior of the control valve 620 lead directly to a dispersion of the injection quantity. The opening process is characterized by the temporal force build-up of the solenoid 622 and the iron yoke 624 on the piston 630 and the spring force of the spring 626 counteracting this force build-up. The force build-up is again due to the geo ^¬ metric dimensions of the actuator (solenoid 622 and iron yoke 624), the electrical and / or magnetic parameters of

Solenoids 622 and essentially determined by the excitation current or by the gradient of the excitation current through the solenoid 622.

The following equation (1) describes the magnetic force F _MA G acting on the piston 630:

It is

W _MAG stored in the solenoid 622, magnetic energy, 5 _AIR of the air gap between the lower abutment surface of the iron yoke 624 and the armature 630, N is the number of turns of the solenoid

 I is the current through the solenoid 622,

μ _Γ the relative permeability of the material of the iron yoke

624

 A is the cross-sectional area of the solenoid 622,

μο the magnetic field constant and

l _{Fe is} the length of the magnetic field lines in the iron yoke 624.

To ensure the most reproducible and robust opening of the fuel injector 600, it is known to energize the solenoid 622 in such a way that a magnetic saturation is achieved as fast as possible. In this area, changes in the current lead only to comparatively small changes in the magnetic force and thus hardly affect the opening dynamics of the control drive 620 and thus also of the fuel injector 600.

However, the closing action of the control valve 620 is still strongly affected by parasitic effects. These include u.a.

(a) a time- and / or temperature-dependent remanence in the Mag ^¬ netkreis, which leads to a blurring in the reduction of the magnetic force and thus to a blur at the closing time,

 (b) changing environmental conditions such as the pressure in the low pressure line 646, the temperature (affects the

 Compressibility and viscosity of the fuel), the exact duration of actuation of the solenoid 622 and the exact pressure in the high pressure line 640, and

 (c) an injector-individual tolerance with respect to the distance between the control drive 620 and the seat 632 and with respect to the exact spring force of the spring 626, which also lead to an injector-individual tolerance in the closing time.

All of these effects (a), (b) and (c) result in a pronounced quantity tolerance of the individual injectors, which can also change over the lifetime (wear) and the constantly changing operating conditions of the injectors. The invention has the object to improve the quantity accuracy of indirectly driven Kraftstoffin ectors.

This object is solved by the subject matters of the independent claims. Advantageous embodiments form the guide vorlie ^¬ constricting invention are described in the dependent claims.

According to a first aspect of the invention, there is provided a method of determining the closing timing of a spool drive control valve of an indirectly driven

Fuel ector described which is suitable in particular for a control valve of an indirectly driven diesel fuel ^¬ injector for an internal combustion engine of a motor vehicle. The described method comprises (a) switching off a current flow through a coil of the Spulenan ^¬ drive, so that the coil is de-energized, (b) detecting a time course of an induced voltage in the currentless coil, wherein the induced voltage by decaying eddy currents (c) evaluating the detected time history of the voltage induced in the coil, and (d) determining the closing time based on the evaluated time course ,

The described closing timing detection method is based on the recognition that a problem caused by the movement of the Mag ^¬ netankers by induction voltage signal can be used in the coil of the coil drive of the control valve to order the sequence of movement of the armature to CHARACTERI ^¬ Sieren and from the closing time of the control valve to investigate. In this case, the voltage signal in the coil caused by the movement due to the remanent magnetic field of the magnet armature is typically greatest when the magnet armature is located immediately before its stop or closed position. This is because in the de-energized state of the coil immediately before the stop of the moving magnet armature, the relative speed between the magnet ^¬ anchor and the coil is maximum. The voltage curve in the currentless coil of the control valve of the indirectly driven Kraftstoffin ektors indu ^¬ ed voltage is thus determined at least partially by the movement of the armature. By a suitable evaluation of the time profile of the induced voltage in the coil, the proportion can be determined, at least to a good approximation, based on the relative movement between the armature and the coil. In this way, information about the course of motion is automatically obtained, which allow accurate conclusions about the time of the maximum speed and thus also about the time of closing the valve. Thus, according to the invention, the voltage induced by the speed change of the control valve armature in the coil of the coil drive is detected to detect the closing operation of the control valve and the exact closing time of the control valve, which is often referred to as a servo valve, is determined from the course of this voltage.

The spool drive of the control valve is often referred to as a servo drive and / or solenoid drive. The coil of the coil drive is often referred to as a solenoid.

The period of time within which the coil of the coil drive of the control valve is de-energized is often referred to as free-running phase.

Using the method described systematic changes in the closing time can he be considered ^¬ recognized and in subsequent activations for further strokes of the control valve in a simple and effective manner. In this way, it is also possible to compensate for ector-individual variations (from injector to injector) with regard to actually injected fuel quantities. The same applies to a suitable compensation of fluctuations in the injected amount of fuel, wherein

Variations over the life and / or depending on the operating condition of a fuel injector can occur. The described method has the advantage that it can be carried out online in an engine control unit. If, for example, the closing behavior of the control valve and thus also the dynamics and the injection quantities of the entire fuel injector change, it can be described with reference to FIG

Closing time detection method detects this change automatically and compensated by a correspondingly modified control in a suitable manner.

It should be noted that in contrast to directly driven Kraftstoffin ectors in which a valve needle is directly coupled to a solenoid drive and which are typically used in the injection of gasoline or super-gasoline, in the case indirectly treated here ^¬ fuel inj There is no direct coupling between the opening of the electromagnetic solenoid drive and the fuel injector needle. The needle of the indirectly driven treated here fuel injector which typi cally ^¬ is used for the injection of diesel fuel is, rather hydraulically moved by a momentary pressure difference between a control chamber and a valve chamber of the fuel injector.

According to one exemplary embodiment of the invention, the evaluation ^comprises calculating the first time derivative and / or the second time derivative of the detected time profile of the voltage induced in the coil.

The closing time can be determined by the occurrence of a local minimum or maximum or by the occurrence of a point of inflection in the detected time profile of the voltage induced in the coil. These characteristic points in the voltage curve can be detected with a particularly high accuracy by the described evaluation of a curve, which corresponds to the first and / or the second time derivative of the actually measured voltage curve. It should be noted that, according to the application, further time derivatives can also be used and / or taken into account in the above-described evaluation of the recorded time profile of the voltage induced in the coil.

According to a further exemplary embodiment of the invention, the evaluation of the detected time profile of the voltage induced in the coil comprises a comparison of the detected time profile of the voltage induced in the coil with a reference voltage profile.

The reference voltage curve can be chosen such that it describes the proportion of the induced voltage, which is caused by decaying eddy currents in the magnetic circuit. As a result, in the context of evaluating the detected voltage curve, it is possible to ^obtain particularly accurate information about the actual movement of the magnet armature.

The reference voltage curve can be determined individually for each fuel injector and preferably in a defined operating state of the fuel injector.

According to a further exemplary embodiment of the invention, the comparison comprises a difference between (a) the detected time profile of the voltage induced in the currentless coil and (b) the reference voltage profile. This has the advantage that a Spannungssig ^¬ nalverlauf is determined in a particularly simple manner, which is particularly suitable for accurate detection of the closing time.

Depending on the sign of the difference formation described, the actual closing time of the control valve may be correlated with a local maximum or a local minimum in the curve obtained by the subtraction.

According to a further exemplary embodiment of the invention, the method further comprises determining the reference voltage profile. This determination of the reference voltage curve points in turn to (a) acting on the coil with at least one reference voltage pulse having a pulse duration and a pulse height, which are dimensioned such that the control valve is just opened so short that due to the hydraulic delay within the indirectly driven fuel ^¬ injector (b) measuring the time course of an induced in the electroless coil by decaying eddy currents in the magnetic circuit and by a movement of the armature voltage, the measured time course of the is determined ^¬ reference clamping voltage gradient. This has the reference voltage ^¬ running is determined under operating conditions where the armature is usually fully deflected and still takes place ection no fuel in the advantage. Thus, the method described during the ongoing operation of a motor vehicle can be performed online without affecting the operation of the running engine of the motor vehicle.

Due to the large displacement of the armature of such a specific reference voltage curve is the electromechanical behavior of the control valve particularly accurate again and he ^¬ enables thus a particularly high accuracy in the

Closing time determination.

According to a further exemplary embodiment of the invention, the determination of the reference voltage curve further comprises (a) impinging the coil with a sequence of different voltage pulses, the pulse duration and / or the pulse height being increased successively, and (b) measuring a fuel pressure in one the Kraftstoffinj ector coupled high-pressure fuel storage, wherein the reference voltage pulse is that one pulse of the sequence of different voltage pulses, in which for the first time a short-term pressure drop in ^¬ within the high-pressure fuel storage occurs, the pressure drop is greater than a predetermined threshold.

The fuel high-pressure accumulator may in particular be a so-called common rail system. The pressure measurement, which preferably with a time resolution in the range of a few microseconds or less, can be performed by ^¬ a pressure sensor, which includes, for example, a piezoelectric sensor.

Thus, a threshold (dP_ref) dependent on the pressure (P_rail) in the high-pressure accumulator can be defined for a pressure drop caused by the actuation of the control valve. The threshold can be calibrated such that it is ensured that the control valve has been actuated for a defined period of time and thus a defined amount

Fuel from the high-pressure accumulator can escape into the low-pressure accumulator of Kraftstoffin ector. However, this amount of fuel is so low that no movement of the valve needle of the fuel injector and thus fuel injection takes place.

If, for example, the pressure drop (dP) reaches this predefined calibratable threshold as a function of the duration of the electrical control (ti) of the control valve, the voltage curve associated with these control parameters corresponds to the reference voltage curve in the closing region of the control valve. This reference voltage profile can be stored individually for each fuel injector in a control unit and can also be updated regularly in order to compensate for changes in the injection system in a suitable manner.

The method described can thus be carried out in an advantageous manner within a closed loop, wherein a pressure drop to be detected with at least one predetermined strength represents the controlled variable. Thus, the method described is particularly robust both to manufacturing tolerances in the manufacture of the fuel injector and to so-called drifts in the course of a

(longer) operation of the fuel injector. Both possible drifts in the operation of the fuel injector and tolerances of the fuel injector are automatically taken into account in the determination of the reference signal. The described determination of the reference signal may from time to time during operation a combustion engine having the fuel injector.

According to a further exemplary embodiment of the invention, the evaluation of the detected time profile of the voltage induced in the coil comprises a comparison of a time derivative of the detected time profile of the voltage induced in the coil with a time derivative of the reference voltage profile. In this case, for example, the difference between (a) the time derivative of the detected time characteristic of the voltage induced in the coil and (b) the time derivative of the reference voltage profile can be calculated.

The closing time can then be determined by a local maximum or by a local minimum (depending on the sign of the difference). Again, the evaluation, which includes both the calculation of the two time derivatives as well as the difference, on a time interval be ^¬ restrict, in which the expected closing time is.

According to a further embodiment of the invention, the evaluation of the time profile detected is performed in the coil induced voltage only within a time interval containing the expected closing time ^¬. This has the advantage that the evaluation must be only within a limited time range Runaway ^¬ leads so that the described method can be performed reliably even with a relatively small computing power. An unnecessary evaluation in time areas in which the closing time with high security is not, can be avoided in a simple and effective way.

The beginning of the time interval can be given for example by the expected closing time minus a predetermined time period At. The end of the time interval may, for example, be given by the expected closing time plus a further predetermined period of time At '. In this case, the predetermined period of time Δt and the further predetermined period of time ^Δt 'can be the same. At and At 'should be smaller than that Expected and experimentally easy to determine time difference between the first closing time and a possibly occurring second closing time, which follows after the bouncing of the armature to the first closing time. This means that a possibly occurring second closing time is outside the observation time window given by At and At '.

The time window of the closing of the control valve can thus be defined as a calibratable time segment from the voltage curve of the coil. The time start of this time window can coincide in particular with the time end of the control of the coil. The length of the time window in which the closing of the control valve takes place is calibratable and may have a typical length of at least about 0.1 to 0.8 milliseconds.

According to a further aspect of the invention, an apparatus for determining a closing time of a control valve having a coil drive of an indirectly driven

Fuel injector described. The device is particularly suitable for a control valve of an indirectly driven DieselFuelin ector for an internal combustion engine of a motor vehicle. The described device comprises (a) a turn-off unit for switching off a current flow through a coil of the coil drive, so that the coil is de-energized, (b) a detection unit for detecting a time course of an induced voltage in the currentless coil, wherein the induced voltage by decaying eddy currents are generated in a magnetic circuit of the spool drive and by a movement of a magnetic armature of the spool drive relative to the spool, and (c) an evaluation unit. The evaluation unit is set up (cl) for evaluating the detected time profile of the voltage induced in the coil, and (c2) for determining the closing time based on the evaluated time profile.

The device described is also based on the recognition that an induction caused by the movement and the remanent magnetism of the magnet armature of the control valve Voltage signal in the coil of the control valve can be used to cha ^¬ characterize the movement of the armature and to determine the closing time. At least in the so-called. Freewheeling phase of the fuel in ector that is the movement of the armature associated induced voltage signal is typically at its greatest when the Re ^¬ lativgeschwindigkeit between the armature and the coil is maximum. This is usually the case when the magnet armature is immediately before its stop or before its closed position.

According to a further aspect of the invention, a computer program for determining a closing time of a coil drive having control valve of an indirectly ^¬ driven fuel in ector, in particular a control ^¬ valve of an indirectly driven Dieselmotoreninj ector for an internal combustion engine of a motor vehicle described. The computer program, when executed by a processor, is arranged to control the above-mentioned method of determining a closing time.

The processor can be realized for example by means of a motor control unit.

For the purposes of this document is the mention of such Com ^¬ computer program equivalent to the concept of a Pro ^¬ program element, a computer program product and / or a computer-readable medium containing instructions for controlling a computer system to the operation of a system or a method in to coordinate suitably to achieve the effects associated with the method according to the invention.

The computer program may be implemented as a computer-readable instruction code in any suitable programming language such as JAVA, C ++, etc. The computer program can be stored on a computer-readable storage medium (CD-ROM, DVD, Blu-ray Disc, removable drive, volatile or non-volatile memory, built-in memory / processor, etc.). Of the Instruction code may program a computer or other programmable device such as, in particular, an engine control unit of a motor vehicle to perform the desired functions. Further, the computer program may be provided in a network, such as the Internet, from where it may be downloaded by a user as needed.

The invention can be implemented both by means of a computer program, i. software, as well as by means of one or more special electrical circuits, i. in hardware or in any hybrid form, i. using software components and hardware components.

It should be noted that embodiments of the invention have been described with reference to different subject ^matters . In particular, some embodiments of the invention are described with method claims and other embodiments of the invention with apparatus claims. However, it will be readily apparent to one skilled in the art upon reading this application that, unless explicitly stated otherwise, in addition to a combination of features associated with a type of subject matter, any combination of features that may result in different types of features is also possible Subject matters belong.

Further advantages and features of the present invention will become apparent from the following exemplary description of presently preferred embodiments. The individual figures of the drawing of this application are merely to be regarded as schematic and not to scale.

Figure 1 shows various applied to the coil of a control valve of an indirectly driven Kraftstoffin ektors voltage curves for different lengths to ^¬ control periods.

Figure 2 shows various measured in a high-pressure accumulator of an indirectly driven Kraftstoffinj injector Pressure curves for the already used for Figure 1 different long drive times.

 Figure 3 shows an enlarged view of the voltage waveforms shown in Figure 1 in the region of the closing time of the control valve, wherein the time zero point used for the time axis is selected in each case so that it coincides with the switching off of the electric control of the control valve.

 FIG. 4 shows (a) the difference between the voltage curve 4 and the voltage curve 3 of FIG. 3 and (b) the difference between the voltage curve 5 and the voltage curve 3 from FIG. 3.

 FIG. 5 shows a schematic representation of the correction of a

Control of a control valve of an indirectly attached ^¬ excessive fuel into ector based on a previously determined accurately and injector-determination of the close timing of the control valve.

 FIG. 6 shows a schematic representation of one of FIG

 Prior art known indirectly driven fuel injector.

It should be noted that the embodiments described below constitute only a limited selection of possible embodiments of the invention. In particular, it is possible to suitably combine the features of individual embodiments with one another, so that for the person skilled in the art with the variants explicitly described here, a multiplicity of different embodiments are to be regarded as obviously disclosed.

FIG. 1 shows the corresponding voltage curves U_solenoid on the coil or on the solenoid of the control valve for five different electrical activation periods ti of a control valve of an indirectly driven fuel injector. The electrical control starts at 50 με each. The end of the electrical control can be seen on a steep drop in the corresponding voltage curve up to a value of less than -30 volts. After that, the voltage rises along one of the Inductance of the coil of certain increase asymptotically back to the voltage value 0 at. The marked with "1" voltage profile ti is approximately 15 με, ie the clamping ^¬ voltage drop takes place at about 65 με. For the voltage curve marked "2", ti is approximately 25 με, ie the voltage drop occurs at approximately 75 με. In the voltage curve marked "3" and plotted with a thick line, ti (= ti (3)) is approximately 50 με, ie the voltage drop takes place at approximately 100 με. In the case of the voltage curve labeled "4", ti is approximately 92 με, ie the voltage drop takes place at approximately 142 με. In the case of the voltage curve marked "5", ti is approximately 158 is, ie the voltage drop takes place at approximately 208 με. According to the embodiment shown here, at t = 50 is in each case an increase of the voltage to a boost voltage of about 30 V. This is followed by switching to a holding voltage of about 6 V. This switching takes place in a known manner by means of a two-point controller, so that in the voltage curve, which is particularly well seen for the curve "3" (thick line), oscillations occur.

The underlying the voltage curves "1" and "2" controls are so short that no opening of the control valve is triggered. As can be seen from Figure 1, also the holding voltage of about 6 V is not achieved. The control "3" leads to a brief opening of the control valve. The duration of the electrical control ti and thus the opening duration of the control valve is so short that due to the hydraulic delay no injection is triggered. In the controls "4" and "5", the opening of the control valve is long enough so that an injection is triggered by the indirectly driven injector.

Figure 2 shows the pressure curve in a high pressure accumulator (for example a common rail system) of at least one indirectly driven fuel injector having been ^¬ injection system. The solenoid of the control valve of the fuel injector is injected with the Actuated driving pulses, which are based on the voltage curves shown in FIG. Idar. As already stated above, in the controls "1" and "2" no opening of the control valve takes place, the pressure P_rail in the high pressure accumulator remains unaffected. In the controls "3", "4" and "5" actuation of the control valve causes at least some leakage of fuel from the control chamber 642 in the low pressure line 646 (see Figure 6). Thus escapes

Fuel from the high pressure accumulator via the control valve in the low pressure region and there is a brief pressure drop in the high pressure accumulator.

FIG. 2 also plots an arbitrarily selected reference pressure P_Ref, which is just reached during the pressure drop dP (3) from the curve "3". By a pressure measurement resp. a measurement of the pressure drop for drive times of different lengths can thus be determined, at which actuation time the excellent curve "3" is present. This means that the control valve is just opened so short that due to the hydraulic delay within the indirectly driven Kraftstoffin ector is just no fuel injection.

Figure 3 shows an enlarged representation of the m m FIG voltage curves illustrated 1 in the region of the closing time ^¬ point of the control valve. In order to compare the different clamping ^¬ voltage gradients better with each other, they were shifted along the time axis so that the end of the electrical drive in each case at t = 0 is με.

As can be seen from FIG. 3, the voltage curves corresponding to the drives "1" and "2" show a simple smooth progression. The respective voltage approximates asymptotically to the voltage value 0 in accordance with a negative exponential function with a negative exponent. The curves corresponding to the voltage curves "4" and "5" each show a turning point, which correlates with the closing of the control valve. The inflection point WP4 in the chip History "4" is about 23 με. The inflection point WP5 in the voltage curve "5" is about 37 με. As already explained above, there is no complete opening of the control valve in control "3". The speed of the valve piston of the control valve when closing is therefore very low and in the corresponding voltage curve "3" (thick line) thus no pronounced turning point can be seen.

FIG. 4 shows (a) the difference between the voltage curve "4" and the voltage curve "3" of FIG. 3, and (b) the difference between the voltage curve "5" and the voltage curve "3" from FIG Maxima at times, which indicate the closing times 4 and 5 for the drives "4" and "5". For this closing time determination, the voltage curve "3" shown in FIGS. 1 and 3 was used as a reference.

In order to determine the reference voltage signal (waveform "3" in Figures 1 and 3) as accurately as possible, the relevant indirectly driven Brennstoffin ector can be acted upon by test pulses with increasing electrical driving time ti and simultaneously to control the course of the pressure in the high-pressure accumulator (eg Common Rail System) are monitored (see Figure 2).

After exceeding a defined injector-individual activation duration (see ti (3) in FIG. 1), a sufficiently great force acting on the valve piston is generated in the control valve of the indirectly driven fuel injector. As a result, the control valve opens for a time dependent on the electrical control time and fuel can escape from the high-pressure accumulator in the low-pressure region. As already described above, this causes the pressure in the high-pressure accumulator to drop by an amount dependent on the activation duration of the control valve (see FIG. 2). Thus, according to an exemplary embodiment of the invention, a threshold (dP (3)) of the pressure drop dependent on the pressure in the high-pressure reservoir (P_rail) can be defined on the basis of the control of the control valve (see FIG. 2). The threshold can be chosen to be calibrated so as to ensure that the

Control valve has been operated for a defined period - ie a defined amount of fuel from the high-pressure accumulator can escape into the low-pressure accumulator. However, this fuel quantity is so low that no movement of the valve needle 610 (see FIG.

Reaches the rail pressure drop dP (ti) as a function of activating the control valve, this predefined threshold, the associated with these control parameters voltage waveform in the closing area of the control valve corresponds to the reference voltage ^¬ extending U_ref (see curve "3" in Figures 1 and 3). This reference voltage profile can be stored in ektorindividuell in an engine control unit and updated regularly to compensate for changes in the injection system.

FIG. 5 shows a schematic representation of the correction of a control of a control valve of an indirectly driven fuel injector based on a previously determined exact and injector-specific determination of the closing time of the control valve.

In the left diagram, the electrical voltage at the solenoid of the control valve during the active energizing phase and the subsequent freewheeling phase is dagestellt. The

Energization phase is by a pre-controlled time ti

characterized. The oscillations or interruptions within the Bestromungsphase result in a known manner from a two-step control of the respective voltage applied to the coil. During the subsequent freewheeling phase, which begins with the end of the energization phase, the closing operation of the servo valve takes place. In the illustration used here corresponds to the time of closing the

Control valve a local minimum in the voltage curve. This local minimum occurs after a time t_s after the end of the energization phase.

In the upper middle diagram of FIG. 5, two quantity characteristics q_inj_l and q_inj_2 are shown schematically as a function of

electrical drive time ti shown. The two

Quantity curves, which indicate the injection quantity per injection pulse, can either be two different ones

Be assigned fuel in ectors or one and the same Kraftstoffin ector under different operating conditions. In the lower middle diagram of FIG. 5, the slide-related closing times determined from the voltage curve are shown as a function of the pre-controlled electrical drive time ti. The corresponding curves are marked t_s_inj_l and t_s_inj_2. In the lower middle diagram, a reference line designated by t_s_ref is also plotted, which can be used for normalization, as described below. In the upper middle diagram, the uncorrected quantity curves q_inj_l and q_inj_2 show due to the injector dependent and / or operating point dependent

Closing behavior of the control valve pronounced

Nonlinearities and pronounced differences between different fuel injectors.

The right-hand diagram shows the set characteristic curves of the two injectors corrected with the aid of the measured closing time with reference to the reference closing time t_s_ref. The correction results from the following equations: ts_kor = t_s_ref - t_s (2) ti_kor = ti + ts_kor (3)

As can be seen from the right-hand diagram of FIG. 5, the quantitative characteristics corrected according to equations (4) and (5) are in good approximation one above the other. Thus, by an appropriate correction operational and / or

Injektorindividuelle quantity variations largely eliminated and thus, in particular for small injection quantities, a particularly high accuracy of the injection quantity can be achieved.

The diagrams of Figure 5 thus illustrate a basic strategy for the optimized control of a indirectly driven by a control valve Kraftstoffin ector. From the voltage curve of the solenoid of the control valve, the closing time ts of the control valve is determined each time the valve is actuated. This closing time is compared with the nominal closing time t_s_ref, which is also taken into account when determining the generic and injector-independent activation time. Based on this comparison, an injector-individual and operating point-dependent correction component is formed in accordance with equation (2) and taken into account in the on-control via equation (3). In this way, injector-specific and operating point-dependent tolerances in the closing behavior of the fuel injector indirectly driven by a control valve can be taken into account in a simple yet effective manner and compensation can be made for the resulting quantity tolerances.

Reference sign list

600 indirectly fueled fuel injector

602 outer casing

604 inner casing

 610 valve needle

 612 spring

 614 outlet opening

 620 control drive / control valve

622 coil / solenoid

 624 iron yoke

 626 spring

 628 coupling element

 630 piston / anchor control valve

632 seat control valve

 640 high pressure line

 642 control room

 644 valve space

 646 low pressure line / leakage system

650 Common Rail System

 652 Sensor for rail pressure

Claims

claims 1. A method for determining the closing time of a coil drive having a control valve (620) of an indirectly driven Kraftstoffin ector (600), in particular a control valve (620) of an indirectly driven diesel ^¬ fuel injector (600) for an internal combustion engine of a motor vehicle, the method comprising  Switching off a current flow through a coil (622) of the coil drive, so that the coil (622) is de-energized, Detecting a temporal progression of a in the current-free coil (622) induced voltage, the induced voltage by decaying eddy currents in a magnetic circuit of the Spu ^¬ lena drive and by a movement of a magnet armature (630) of the coil drive relative to the coil (622) is generated,  Evaluating the detected time profile of the voltage induced in the coil (622), and Determining the closing time based on the evaluated from ^¬ temporal course. 2. Method according to the preceding claim, wherein the evaluating comprises calculating the first time derivative and / or the second time derivative of the detected time profile of the voltage induced in the coil (622). 3. The method according to any one of the preceding claims, wherein the evaluation of the detected time course of the induced voltage in the coil (622) comprising comparing the detected time profile of the voltage induced in the coil (622) with a reference voltage profile ("3"). 4. The method according to the preceding claim, wherein the comparison comprises a difference between (a) the detected time profile of the voltage induced in the currentless coil (622) and (b) the reference voltage curve ("3"). 5. The method according to any one of the preceding claims 3 to 4, further comprising  Determining the reference voltage waveform ("3"), wherein the determination of the reference voltage waveform ("3") has - An energizing the coil (622) with at least one reference voltage pulse having a pulse duration and a pulse height, which are dimensioned such that the control valve (620) is just opened so short that due to the hyd ^¬ raulischer delay within the indirect driven Kraftstoffin ector (600) is still no fuel injection, and, immediately after the end of Refe ^¬ rence voltage pulse, - Measuring the time course of in the de-energized coil (622) by decaying eddy currents in the magnetic circuit and by a movement of the armature (630) induced voltage, wherein the measured time course of the determined reference chip ^¬ nungsverlauf ("3"). 6. The method according to the preceding claim 5, wherein the determination of the reference voltage waveform ("3") further comprises  - Applying the coil (622) with a sequence of different voltage pulses, wherein the pulse duration and / or the pulse height is successively increased, and - measuring a fuel pressure in an input coupled to the power ^¬ stoffinj ector (600) Fuel high-pressure accumulator (650) wherein said reference-voltage pulse is one pulse of the sequence of different voltage pulses, wherein the first time a momentary pressure drop within the motor ^¬ material-high-pressure accumulator (650) occurs, which drop in pressure greater than a predetermined threshold (dP (3)) is. 7. The method according to any one of the preceding claims 3 to 6, wherein the evaluation of the detected time course of the induced voltage in the coil (622) comparing a time derivative of the detected time history of the voltage induced in the coil (622) with a time derivative of the reference voltage waveform ("3"). 8. The method according to any one of the preceding claims, wherein the evaluation of the detected time profile of the induced voltage in the coil (622) is performed exclusively within a time interval which contains the expected closing time. 9. An apparatus for determining a closing time of a coil drive having a control valve (620) of an indirectly driven Kraftstoffin ector (600), in particular a control valve of an indirectly driven diesel ^¬ fuel injector (600) for an internal combustion engine of a motor vehicle, comprising the device  a turn-off unit for switching off a current flow through a coil (622) of the coil drive, so that the coil (622) is de-energized,  a detection unit for detecting a time course of a voltage induced in the de-energized coil (622), the induced voltage being generated by evanescent eddy currents in a magnetic circuit of the coil drive and by movement of a magnet armature (630) of the coil drive relative to the coil (622) , and  an evaluation, set up for evaluating the detected time profile of the voltage induced in the coil (622) and for determining the closing time based on the time course evaluated from ^¬ . 10. Computer program for determining a closing time of a coil drive having a control valve (620) of an indirectly driven Kraftstoffinj injector (600), in particular a control valve (620) of an indirectly driven diesel fuel injector (600) for an internal combustion engine of a motor vehicle, wherein the computer program, when executed by a processor configured to control the method of any one of claims 1 to 8.