Method for determining a characteristic time of a fuel injection

Abstract

Method for determining a characteristic time (t EB ) a fuel injection carried out by means of a fuel injector (10) of an internal combustion engine, wherein a signal (S) of a sensor (50) for detecting fuel pressure changes in the fuel injector (10) for an injection of the fuel injector (10) is detected and corrected from the start of an actuation of a switching valve (40) of the fuel injector (10) by a difference signal (D) with a duration (Δt), which difference signal (D) of a difference between two test signals (S1, S2) of the sensor (50) for two different control periods (Δt A ) of the fuel injector (10) in a period between a start of actuation of a switching valve (40) for closing the fuel injector (10) and the closing of the fuel injector (10), in each case with respect to the test signal (S1) for the shorter activation period, and where the corrected signal (S K ) the characteristic time (t EB ) is determined.

Description

[0001] The present invention relates to a method for determining a characteristic time of a fuel injection carried out by means of a fuel injector of an internal combustion engine, as well as a computing unit and a computer program for carrying out the method. State of the art

[0002] Modern internal combustion engines are equipped with fuel injectors that allow fuel to be injected into the combustion chambers in a targeted manner. For precise control of the internal combustion engine and compliance with emissions and performance requirements, it is advantageous to record characteristic timings of the injection processes, particularly the opening and closing of the fuel injector valves, and thus the start and end of injection, or the amount of fuel delivered, as accurately as possible.

[0003] In fuel injectors where opening and closing is carried out directly by solenoid valves, piezo actuators or the like, the electrical control variables themselves can often be used to detect such characteristic times.

[0004] In so-called servo injectors, in which a servo valve or switching valve is initially controlled to actuate the injection nozzle, there is no direct relationship between the electrical control variables and the opening and closing times of the fuel injector, its nozzle needle, or injection valve. Therefore, additional sensors can be used in such fuel injectors, for example, to detect the fuel pressure in a control chamber of the fuel injector or any deformation of the fuel injector, particularly in the area of ​​its high-pressure bore.

[0005] From EP 2 944 799 A1, for example, a method for determining such characteristic times is known, in which such a sensor is mounted on the holding body of the fuel injector and whose sensor signal essentially reflects the deformation of the holding body in the region of the high-pressure bore running through the holding body below this sensor.

[0006] From the documents DE 10 2009 045 307 A1, DE 10 2011 087 418 B4, DE 10 2014 211 334 B3, general examples of procedures are known for obtaining reference profiles from test controls and comparing these with the actual signal curves of the electrical control variables.

[0007] DE 10 2009 045 307 A1 discloses a method for operating a valve, in particular a fuel injection valve, by applying a control current to an electromagnetic actuator. The method comprises the following steps: performing at least one test control with a predetermined control current, recording the temporal profile of at least one electrical operating variable of the actuator during the test control, evaluating the profile for the presence of a characteristic feature for an operating state or a change in the operating state of the valve, and, if necessary, repeating the steps if the feature is not found.

[0008] DE 10 2011 087 418 B4 discloses a method for determining the opening behavior of a fuel injector with a coil drive. A weaker electrical test excitation compared to normal operation prevents magnetic saturation of the coil drive. The time at which the injector reaches its opening position is determined based on the measured current curve. Based on this time, the time at which the injector would reach its opening position under normal excitation is determined.

[0009] DE 10 2014 211 334 B3 discloses a method for characterizing the hydraulic coupling element of a piezo injector. The coupling element converts the stroke of the piezo actuator into a pressure difference that opens the nozzle needle. The piezo actuator is initially driven with a small charging current, so that the nozzle needle remains closed. The actuator is then discharged with a large current, breaking the connection between the piston and the pin. The time until the piston hits the pin is measured and used to characterize the coupling element. Disclosure of the invention

[0010] According to the invention, a method for determining a characteristic point in time of a fuel injection performed by a fuel injector of an internal combustion engine, as well as a computing unit and a computer program for implementing the method, are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

[0011] A method according to the invention is used to determine a characteristic point in time for a fuel injection performed by a fuel injector of an internal combustion engine. Servo fuel injectors or fuel injectors with a switching valve are particularly suitable as fuel injectors, since in these cases, a direct determination of characteristic points in time from control variables is not possible. Characteristic points in time are understood to be points in time such as the start of injection or the opening of the fuel injector (or its nozzle needle) as well as the end of injection or the closing of the fuel injector (or its nozzle needle).

[0012] In this case, a signal from a sensor for detecting fuel pressure changes in the fuel injector for an injection of the fuel injector is detected and corrected by a difference signal from the start of an actuation of a switching valve of the fuel injector.

[0013] The difference signal corresponds to the difference between two sensor test signals for two different fuel injector activation durations in a period between the start of actuation of a switching valve to close the fuel injector and the closing of the fuel injector, each with reference to the test signal for the shorter activation duration. The difference signal thus has a specific duration, which results from the test signal with the shorter activation duration as the duration between the start of actuation of the switching valve to close the fuel injector and the closing of the fuel injector. The correction duration is expediently determined by the duration of the difference signal.

[0014] The corrected signal is then used to determine the characteristic timing. The fuel injector can then be conveniently controlled, particularly in subsequent injections, taking the determined characteristic timing into account.

[0015] The signals detected by such a sensor include features that can be used to determine characteristic times such as the opening and closing of the fuel injector. However, the actuation of the switching valve - for example, the charging or discharging of a piezo actuator - leads to hydraulic effects, i.e. pressure changes in the fuel in the fuel injector that are not directly correlated with the pressure changes caused by opening or closing. An interference signal is therefore superimposed on the actual or relevant signal. Especially with short control times or short injection durations, this leads to deviations in the detection of the start of injection or the so-called injection start delay duration, which indicates the delay between the start of injection and the start of control of the fuel injector.

[0016] The proposed method makes it possible to remove such interference signals from the signal. This takes advantage of the fact that the signals for different activation durations are initially the same, or at least almost the same, and only differ from one another when the switching valve is actuated to close the fuel injector (then at the signal for the shorter activation duration). From the moment the activation of the switching valve begins – for example, when the piezo actuator is discharged – until the moment the fuel injector or its nozzle needle closes, the signal for the shorter activation duration is overlaid by the interference signal from the activation of the switching valve, while the signal for the longer activation duration is not. Care should be taken to ensure that the two activation durations are sufficiently far apart, i.e.preferably at least by the relevant period of time from the start of the actuation of the switching valve to close the fuel injector.

[0017] By calculating the difference between two such signals—referred to here as test signals—the interference signal, or the disturbance superimposed on the actual signal, can be determined very precisely and then subtracted from the corresponding detected signal during a subsequent injection. The signal corrected in this way allows for a significantly more precise determination of the characteristic timing, especially for short activation periods.

[0018] Preferably, the characteristic time point includes the opening of the fuel injector, and the beginning of the actuation of the switching valve during injection includes the beginning of the actuation of the switching valve to close the fuel injector. Especially with short actuation durations, i.e., when the switching valve is closed again shortly after opening, the signal is already overlaid by the interference signal in the areas relevant to the opening of the fuel injector.

[0019] The characteristic time is advantageously determined from the corrected signal by determining a second derivative of the corrected signal and determining the characteristic time based on an extremum, in particular a minimum, in the second derivative. While the opening of the fuel injector only causes a kink in the signal itself, this kink in the second derivative can be identified as a minimum and thus very easily determined. It should be noted that, if necessary, only a difference quotient calculation may be considered as the derivation, since the signal itself is usually only quasi-continuous or present at measurement points.

[0020] It is advisable to determine the difference signal initially and / or repeatedly over the service life of the fuel injector. For example, this can be done before the first real use of the internal combustion engine, and the difference signal can be stored in an executing processing unit. Repeated determination also allows for any deviations over the service life of the fuel injector to be taken into account.

[0021] A piezo fuel injector with a switching valve actuated by a piezo actuator is preferred as the fuel injector, as this allows for particularly fast and precise switching operations, but also generates relevant interference signals.

[0022] Advantageously, a piezoelectric sensor, a piezoresistive sensor, or a capacitive sensor is used as the sensor, which is arranged in particular on a support body of the fuel injector. Additionally or alternatively, the sensor can also be designed such that the sensor can detect the opening and / or closing of the fuel injector based on pressure changes occurring in the fuel injector during the respective injection.

[0023] To detect the pressure curve, the sensor can be arranged at a suitable location in the fuel injector, for example on the circumference of a holding body of the fuel injector, in particular at the location of a high-pressure bore. Such a holding body or its high-pressure bore is exposed to the pressure changes in the fuel injector. Pressure changes lead to an elastic deformation of the holding body in the area of ​​the high-pressure bore. The deformation of the holding body or the line is used as the variable to be detected by the sensor. The sensor required for this purpose can be attached to the holding body or the supply line and does not have to be installed in either the high-pressure or low-pressure section of the injector. The sensor is preferably arranged near or above the high-pressure bore, since the expansion of the holding body due to pressure changes is greatest at such locations.

[0024] A computing unit according to the invention, e.g. a control unit of a motor vehicle, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0025] Implementing the method in the form of a computer program is also advantageous, as this results in particularly low costs, especially if an executing control unit is also used for other tasks and is therefore already present. Suitable data storage media for providing the computer program include, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0026] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0027] The invention is illustrated schematically in the drawing using an embodiment and is described below with reference to the drawing. Short description of the drawings Fig. 1 shows schematically a fuel injector with a sensor as can be used in the context of a method according to the invention. Fig. 2 to 4 show signals within the scope of a method according to the invention. Fig. 5 shows, using various signals, a correction when carrying out a method according to the invention in a preferred embodiment. Fig. 6 shows corrected signals when carrying out a method according to the invention in a preferred embodiment. Fig. 7 shows a relationship between injection start delay time and control time with and without use of a method according to the invention. Embodiment(s) of the invention

[0028] In Fig. Figure 1 schematically shows a fuel injector with a sensor, such as can be used in a method according to the invention. The fuel injector 10 can be installed in a combustion chamber (not shown) of an internal combustion engine. The fuel injector 10 serves to inject fuel into the combustion chamber of the internal combustion engine and can be part of a common rail injection system.

[0029] The fuel injector 10 has an injector housing (or a holding body) 11, to which, for example, a nozzle body 12 facing the combustion chamber of the internal combustion engine and a further stationary functional group 13 arranged between the nozzle body 12 and the holding body 11 are connected. The nozzle body 12, the functional group 13, and the holding body 11 are non-positively fixed to one another by a nozzle clamping nut 14. One or more injection openings 15 are formed in the nozzle body 12. The functional group 13 contains a servo valve or switching valve 40 with a piezo actuator 41 (shown only schematically), as well as hydraulic functional elements (not shown) for transmitting the switching state of the servo valve to the nozzle needle. The holding body 11 typically accommodates the piezo actuator and the associated switching chain for actuating the servo valve via the piezo actuator.

[0030] Furthermore, a substantially blind hole-shaped recess 16 is formed in the nozzle body 12, in which a nozzle needle 18 is arranged so as to be axially movable along a longitudinal axis. Fig. In the lowered position of the nozzle needle 18 shown in Figure 1, it closes the injection openings 15 indirectly, forming a sealing seat 19 on the inside of the nozzle body 12.

[0031] The blind hole-shaped recess 16 is connected to a high-pressure fuel accumulator (the so-called rail) 31 via an inlet channel 30 and a fuel supply line 60.

[0032] The nozzle needle 18 is subjected to an axial force acting in the direction of the sealing seat 19 by a nozzle spring 25. The pressure in the recess 16 causes a hydraulic force on the nozzle needle 18 pointing away from the sealing seat 19. On the end face of the nozzle needle 18 facing away from the sealing seat 19, a variable pressure applied there usually causes a likewise variable axial force on the nozzle needle, pointing in the direction of the sealing seat 19. The variable pressure and thus the axial force generated by it can be controlled by the switching state of the servo valve. The servo valve (not shown here) thus influences the force balance between opening and closing axial forces on the nozzle needle 18 and thus controls its opening and closing movement.

[0033] The inlet channel 30 has two bore sections 32, 33 in the holding body 11, the axes of which are pivoted at an angle relative to each other. While the bore section 32 runs essentially parallel to the longitudinal axis of the holding body 11, the second bore section 33 inclines toward the high-pressure connection 34 of the injector 10. In addition, the second bore section 33 typically accommodates a filter element 36, which has the task of keeping disruptive particles away from the fuel injector 10 or reducing them to a harmless size.

[0034] Furthermore, a sensor 50, in particular with a piezo element, is provided for detecting the holding body deformation in the region of the inlet channel 30. The sensor is arranged here on the outer peripheral wall of the holding body 11. Advantageously, the sensor is arranged directly above the inlet channel 11, particularly advantageously at the intersection of the two bore sections 32, 33. The sensor 50 is connected via a suitable electrical connection cable to a processing unit 80 designed as a control unit, which also serves, at least indirectly, to control the fuel injector 10.

[0035] The operation of the fuel injector 10 and the sensor 50 is explained as follows: In the Fig. 1, the nozzle needle 18 closes the sealing seat 19. This prevents fuel from being released via the injection openings 15 and a uniform, essentially constant, high pressure (system pressure) prevails in the high-pressure fuel reservoir 31, the fuel supply line 60, the inlet channel 30 and in the blind hole-shaped recess 16.

[0036] Actuating the fuel injector 10 or its actuator causes the nozzle needle 18 to lift upwards via the servo valve or switching valve 40, thereby opening the sealing seat 19. Fuel then flows from the blind-hole-shaped recess 16 through the injection openings 15 into the combustion chamber of the internal combustion engine. This leads to a drop in pressure in the blind-hole-shaped recess and at the end of the inlet channel 30 facing the nozzle body 12. This drop in pressure propagates in the form of a pressure wave traveling at the speed of sound through the inlet channel 30 and the fuel supply line 60 toward the high-pressure fuel accumulator 31. As a result, with a defined time delay after the opening of the sealing seat 19, there is a drop in pressure in the inlet channel at the level of the sensor 50, resulting in an elastic deformation of the retaining body 11 at this point.This elastic deformation is converted by the sensor into a usually falling voltage signal.

[0037] If the control of the piezo actuator 41 ends, the direction of movement of the nozzle needle 18 is reversed and it begins to move towards the sealing seat 19. As soon as the nozzle needle 18 reaches the sealing seat 19, the injection ends. This causes a pressure increase in the blind hole-shaped recess 16, which then propagates in the form of a pressure wave through the inlet channel 30 and the fuel supply line 60 to the high-pressure fuel accumulator. As a result, with a defined time delay after the sealing seat 19 closes, there is a pressure increase in the inlet channel 30 at the level of the sensor 50 and thus an elastic deformation of the holding body 11 at this point. This elastic deformation is converted by the sensor into a usually increasing voltage signal.

[0038] In the Fig. Figures 2 to 4 show signals that may occur within the scope of a method according to the invention. An injection rate R, a voltage U, and a current I are plotted against time t.

[0039] In Fig. 2 are first the injection rate R of a fuel injector during the injection of fuel and the corresponding signal S of a sensor, as it is related to Fig. 1. Here it can already be seen that the start of injection t EB or an opening of the fuel injector, i.e. an increase in the injection rate R, as well as an end of the injection or a closing of the fuel injector, i.e. a decrease in the injection rate R, can be found in the signal S by means of a kink that changes into a falling edge, or by means of a rising edge.

[0040] In Fig. Figure 3 shows a current I curve as used to control the fuel injector or the switching valve. First, the switching valve is controlled to open the fuel injector (control start t AB ), i.e. the piezo actuator is charged, later the switching valve is controlled to close the fuel injector, i.e. the piezo actuator is discharged.

[0041] In the corresponding signal curve, disturbances in the signal curve can be seen in the two areas B1 and B2, which are due to the actuation of the switching valve or the charging and discharging of the piezo actuator.

[0042] In Fig. 4 are now the signal S (here only in a section compared to the Fig. 2 and Fig. 3) and an associated derivative signal S'' are shown. The derivative signal S'' corresponds to the second derivative of the signal S, which can be obtained by suitable processing of the signal S. In the derivative signal S'', the kink in the signal S, which corresponds to the opening of the fuel injector, now appears as a minimum. Such a minimum can be found or identified much more easily and accurately than the kink in the signal S.

[0043] As already mentioned, the influence of the interference signal, especially in the case of short activation times of the fuel injector, can now be Fig. 3, to signal S can lead to the actual injection start or the corresponding injection start delay time being determined incorrectly. For short activation times, this area B2 shifts to the left towards the area of ​​the bend. Please also refer to the Fig. 7 and the associated description.

[0044] In Fig. Figure 5 illustrates a correction using various signals when implementing a method according to the invention in a preferred embodiment. For this purpose, a current I and a voltage U are plotted against time t.

[0045] The upper diagram shows two different current waveforms I1 and I2, namely for a short activation period (waveform I1) and a long activation period (waveform I2). The middle diagram shows the corresponding signals, here in the sense of test signals, S1 and S2. Here, it can be seen that the two test signals initially overlap, but diverge when the switching valve is activated, here at the beginning of the time range Δt.

[0046] The lower diagram shows a differential signal D, which corresponds to the difference between the two test signals S1 and S2. Within the time range Δt shown, which ends with the closing of the fuel injector, the differential signal represents the interference signal caused by the actuation of the switching valve.

[0047] This difference signal D can now be subtracted from this signal in a detected signal from the start of the actuation of the switching valve in order to obtain a corrected signal.

[0048] In Fig. Figure 6 shows corrected signals when implementing a method according to the invention in a preferred embodiment. For this purpose, a current I and a voltage U are plotted against time t.

[0049] In the upper diagram, in addition to the current I, which is similar to the one according to Fig. 3 is, however, for a shorter activation duration, the corresponding signal S itself and thus for a short activation duration, the test signal S2, i.e. a signal for a long activation duration, as well as a according to explanations to Fig. 5 corrected signal S K shown.

[0050] Since no interference signal is present in the test signal S2 due to the fact that the switching valve has not yet been activated in the section shown here, this test signal S2 can be used as a reference. Comparing the test signal S2 with the corrected signal S K It can be seen that the corrected signal S K is very well cleaned of the interference signal and therefore allows a more precise determination of the start of injection.

[0051] In Fig. 7 is a ratio of injection start delay time Δt EV , i.e. the delay between the start of injection and the start of control, to the control duration Δt Awith (course V2) and without (course V1) use of a method according to the invention.

[0052] The comparison shows that without correction, starting from control durations of approximately 250 µs down to shorter control durations, a longer injection start delay duration is determined than actually exists (this should be at least essentially constant, regardless of the control duration and except for very short control durations where no injection is possible at all). However, the correction within the scope of the invention allows a significantly more precise injection start delay duration to be determined. Consequently, more precise fuel metering is also possible.

Claims

[1] Method for determining a characteristic time (t EB ) a fuel injection carried out by means of a fuel injector (10) of an internal combustion engine, wherein a signal (S) of a sensor (50) for detecting fuel pressure changes in the fuel injector (10) for an injection of the fuel injector (10) is detected and corrected from the start of an actuation of a switching valve (40) of the fuel injector (10) by a difference signal (D) with a duration (Δt), which difference signal (D) of a difference between two test signals (S1, S2) of the sensor (50) for two different control periods (Δt A ) of the fuel injector (10) in a period between a start of actuation of a switching valve (40) for closing the fuel injector (10) and the closing of the fuel injector (10), in each case with respect to the test signal (S1) for the shorter activation period, and where the corrected signal (S K ) the characteristic time (t EB ) is determined. [2] Method according to claim 1, wherein the characteristic time (t EB ) an opening of the fuel injector and the start of the actuation of the switching valve (40) during injection comprise a start of the actuation of the switching valve (40) to close the fuel injector (10). [3] Method according to claim 1 or 2, wherein the characteristic time (t EB ) based on the corrected signal (S K ) is determined by taking a second derivative (S K '') of the corrected signal is determined and the characteristic time (t EB ) is determined using an extremum in the second derivative (S''). [4] Method according to one of the preceding claims, wherein the difference signal (D) is determined initially and / or repeatedly over a service life of the fuel injector (10). [5] Method according to one of the preceding claims, wherein a piezo fuel injector with a switching valve (40) actuatable by means of a piezo actuator (41) is used as the fuel injector (10). [6] Method according to one of the preceding claims, wherein a piezoelectric sensor, a piezoresistive sensor or a capacitive sensor is used as the sensor (50), wherein the sensor is arranged in particular on a holding body of the fuel injector (10), and / or wherein by means of the sensor (50) an opening and / or closing of the fuel injector (10) is detected on the basis of pressure changes occurring in the fuel injector (10) during the respective injection. [7] Method according to one of the preceding claims, wherein the fuel injector (10), in particular in subsequent injections, taking into account the determined characteristic time (t EB ) is controlled. [8] Computing unit (80) configured to carry out a method according to any one of the preceding claims. [9] Computer program which causes a computing unit (80) to carry out a method according to one of claims 1 to 7 when executed on the computing unit (80). [10] A machine-readable storage medium having stored thereon a computer program according to claim 9.

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