DE102018218229A1 - Method of determining a charge constant - Google Patents

Abstract

The invention relates to a method for determining a charge constant of a piezoelectric needle closing sensor of a fuel injector. The charge constant is ascertained from at least one maximum (S) of a sensor signal (S) of the needle closing sensor that occurs after a nozzle needle of the fuel injector is closed.

Description

The present invention relates to a method for determining a charge constant of a piezoelectric needle closing sensor of a fuel injector. Furthermore, the present invention relates to a computer program which is set up to carry out each step of the method, and to a machine-readable storage medium on which the computer program is stored. Finally, the invention relates to an electronic control device which is set up to determine a charge constant by means of the method.

State of the art

The operation of a fuel injector for direct fuel injection connected to a high-pressure fuel accumulator (common rail) is still a major challenge. Although the armature stroke can now be set to an accuracy of ± 1 µm, wear and runtime effects and various operating conditions mean that the armature stroke that is actually present or achieved can vary considerably. Armature strokes that are too small are particularly problematic for the injector function because they can result in seat throttling and thus a significantly reduced injection quantity. An excessively high anchor stroke can cause increased wear and tear.

The charge constant d ₃₃ a piezoelectric needle closing sensor (NCS) is an important parameter when operating a regulated fuel injector. It is also referred to as the piezoelectric longitudinal effect and describes the relationship between the mechanical force acting on the piezo element of the needle closing sensor and the electrical field generated parallel to it, which generates the sensor signal. The use of such a piezoelectric needle closing sensor is in the DE 10 2014 204 098 A1 described. On the one hand, the charge constant allows conclusions to be drawn about the functionality of the needle closing sensor, and on the other hand, with knowledge of the charge constant, other variables relevant to injector operation can be derived from the NCS signal, such as the armature stroke.

During operation and over the running time of the needle closing sensor, the charge constants are scattered by up to 30%. Previous methods for determining the charge constants, which are carried out, for example, as part of a factory test, are insufficiently accurate. Since parts that have an impermissible deviation of the charge constant from the target state cannot be identified with certainty, parts that are actually functioning may therefore be incorrectly sorted out.

Disclosure of the invention

In the method for determining a charge constant of a piezoelectric needle closing sensor of a fuel injector, the charge constant is determined from at least one maximum of a sensor signal of the needle closing sensor that occurs after a nozzle needle of the fuel injector is closed. When the needle closes, the valve chamber pressure of the fuel injector increases from a driving pressure to the rail pressure, that is to say the pressure that is present in the high-pressure fuel reservoir that supplies the fuel injector with fuel. This increase is visible in the signal curve of the needle closing sensor and can be easily evaluated in the form of the maximum of the signal. The invention is based on the finding that there is a proportionality between this maximum and the charge constant of the piezoelectric needle closing sensor, so that the charge constant can be determined in a simple manner from the maximum.

Before the needle closing is completed, that is, the nozzle needle of the fuel injector has returned to its seat, there are usually pressure fluctuations in the valve chamber of the fuel injector, which affect the signal from the needle closing sensor. In order nevertheless to enable a reliable determination of the maximum of the sensor signal, it is preferred that the sensor signal be filtered.

Superimposed pressure vibrations lead to a strong dependency of the maximum on the activation period of the fuel injector or on the exact time of the needle closing. In order to ensure the comparability of different fuel injectors or different running times of a fuel injector with regard to the maximum and thus the determination of the charge constant, the phase position of these pressure fluctuations should be put on an equal footing. In one embodiment of the method, this is achieved in that the determination is carried out at a predetermined needle closing duration of the nozzle needle, which is in particular more than one millisecond. The needle closing period is understood to mean the period from the reversal of direction of the nozzle needle movement to the complete closing of the needle valve of the fuel injector.

In another embodiment of the method, the phase position of the pressure vibrations is assimilated by averaging maxima of the sensor signal with different actuation times of the needle closing sensor. These drive times are in particular more than 500 microseconds. The charge constant is then determined from the mean value obtained.

In a particularly simple implementation of the method, the determination is carried out by multiplying a maximum of the sensor signal or an average of maxima of the sensor signal of several needle closing processes by a factor. Here the direct proportionality between the charge constant and the value of the maximum is used.

This correlation can be falsified via the final needle closing speed of the nozzle needle. This depends on the seat geometry and the blind hole geometry of the needle valve. It can be measured indirectly in the course of a factory test via the course of an injection rate of the fuel injector. It is therefore preferred that when determining a maximum of a derivative of the injection rate is taken into account in a needle closing process. This is not possible when determining the drift behavior of the charge constants during operation of the fuel injector. However, since the final needle closing speed does not drift significantly, it does not significantly falsify the relationship between the maximum of the sensor signal and the charge constant over the service life of the fuel injector. It is further preferred that values of a reference injector are taken into account in the determination, in order to allow an expected injector behavior to be incorporated into the determination. The values are in particular an expected maximum of the sensor signal, an expected charge constant and an expected maximum in terms of amount of the derivation of the injection rate.

The computer program is set up to carry out each step of the method, in particular if it runs on a computing device or on an electronic control device. It enables the method to be implemented on a conventional electronic control unit without having to make any structural changes. For this purpose, it is stored on the machine-readable storage medium.

By loading the computer program onto a conventional electronic control unit, the electronic control unit is obtained which is set up to determine a charge constant by means of the method.

Figure list

• 1 zeigt schematisch mehrere Kraftstoffinjektoren mit Nadelschließsensoren, deren Ladungskonstanten mittels Ausführungsbeispielen des erfindungsgemäßen Verfahrens ermittelt werden können.
• 2 zeigt in einem Diagramm den zeitlichen Verlauf eines Ventilraumdrucks eines Nadelhubs und eines Sensorsignals eines Nadelschließsensors in einem Ausführungsbeispiel des erfindungsgemäßen Verfahrens.
• 3 zeigt in einem Diagramm die Abhängigkeit eines Maximums eines Sensorsignals eines Nadelschließsensors von der Ansteuerdauer eines Kraftstoffinjektors für sechs verschiedene Kraftstoffinjektoren.
• 4 zeigt in einem Diagramm eine Abhängigkeit eines Maximums eines Sensorsignals eines Nadelschließsensors eines Kraftstoffinjektors in Abhängigkeit von der Nadelschließdauer für sechs unterschiedliche Kraftstoffinjektoren.
• 5 zeigt in drei Diagrammen den zeitlichen Verlauf einer Einspritzrate und der Ableitung einer Einspritzrate eines Kraftstoffinjektors sowie des Sensorsignals seines Nadelschließsensors.

Embodiments of the invention are shown in the drawings and are explained in more detail in the following description.

• 1 schematically shows several fuel injectors with needle-closing sensors, the charge constants of which can be determined by means of exemplary embodiments of the method according to the invention.
• 2nd shows in a diagram the time course of a valve chamber pressure of a needle stroke and a sensor signal of a needle closing sensor in an embodiment of the method according to the invention.
• 3rd shows in a diagram the dependence of a maximum of a sensor signal of a needle-closing sensor on the activation period of a fuel injector for six different fuel injectors.
• 4th shows in a diagram a dependency of a maximum of a sensor signal of a needle closing sensor of a fuel injector as a function of the needle closing time for six different fuel injectors.
• 5 shows in three diagrams the time course of an injection rate and the derivation of an injection rate of a fuel injector and the sensor signal of its needle-closing sensor.

Embodiments of the invention

1 shows a high-pressure fuel accumulator 1 , in which fuel is stored, by means of six fuel injectors 2nd in the internal combustion engine 3rd to inject a motor vehicle. The fuel injectors 2nd are by means of an electronic control unit 4th controlled. The high-pressure fuel accumulator 1 has a rail pressure sensor 11 on to the pressure p ₁ in the high-pressure fuel reservoir 1 to eat. With the fuel injectors 2nd in the present case, fuel injectors such as those from the DE 10 2009 029 549 A1 are known. This document is made entirely part of these exemplary embodiments by reference. Any fuel injector 2nd has, among other things, a switching valve 21 , a control room 22 , a valve room 23 , a needle valve 24th with a nozzle needle 25th and a piezoelectric needle closing sensor 26 on. The needle closing sensor 26 is provided to a sensor signal S to the electronic control unit 4th to deliver which is proportional to the pressure p ₂₃ in the valve compartment 23 is.

2nd shows the change in pressure p ₂₃ in the valve compartment 23 with time t when the nozzle needle 25th opened and closed again. This manifests itself in a change in the needle stroke H ₂₅ , which is zero when the nozzle needle is closed and reaches a maximum when the needle is reversed. The needle reversal denotes the point in time at which the opening movement of the nozzle needle changes into a closing movement. The period from when the needle is reversed until the needle is completely closed 25th is the needle closing time t _close .

The pressure p ₂₃ in the valve compartment 23 initially corresponds to the pressure p ₁ in the High pressure fuel accumulator 1 . As the needle opens, the pressure drops p ₂₃ in the valve compartment 23 decreases sharply and remains for the majority of the time the needle stroke increases H ₂₅ at this low value. At the end of the opening movement, the pressure increases p ₂₃ back on and then swings during the needle closing period t _close by a value called the driving pressure p _driving referred to as. Once the nozzle needle 25th is completely closed, the pressure rises p ₂₃ again and then swings around the pressure p ₁ in the high-pressure fuel reservoir 1 . The filtered sensor signal S of the needle closing sensor 26 maps this pressure increase. Immediately after the nozzle needle is completely closed, it reaches a maximum smax.

3rd shows how for the fuel injectors 2nd the maximum smax changes each time they have different actuation durations _not can be controlled. The activation period _not denotes the duration of the current supply to the fuel injector 2nd for actuating the switching valve 21 and corresponds to the period from the start of the increase in the needle stroke H ₂₅ until the needle reverses.

4th shows how each of the six fuel injectors 2nd the maximum smax changes when the needle closing period changes t _close changes.

In a first embodiment of the method according to the invention, the charge constant d ₃₃ each of the piezoelectric needle closing sensors 26 calculated according to Formula 1: $$d_{33}=k_1\cdot S_{max}$$

Inscribed k ₁ a constant that acts as a factor and for a defined needle closing period t _close which is more than 1 millisecond.

5 shows how the injection rate Q and their first derivative Q ' for three different fuel injectors 2nd change with time t. In addition, the course of the sensor signal S of these fuel injectors 2nd after closing the nozzle needle 25th in 5 shown. The derivative Q ' passes through a minimum, which is a maximum amount Q ' _max represents. For a determination of the charge constant d ₃₃ in a factory test with a defined needle closing time t _close in one embodiment of the method according to the invention the relationship according to formula 2 applies: $$S_{max}-S_{ref}=\alpha \cdot \left(d_{33}-d_{33\_ref}\right)+\beta \cdot \left(Q{\text{'}}_{max}-Q{\text{'}}_{ref}\right)$$

Sref denotes the maximum smax of a gold reference fuel injector, d _{33_ref} denotes the charge constant d ₃₃ the golden reference fuel injector and Q ' _ref denotes the maximum amount Q ' _max the injection rate Q of the golden reference fuel injector. The factors α and β depend on the fuel injector type. In the present case, they can be determined by means of a simulation. By solving Formula 2 according to the charge constant d ₃₃ Formula 3 results: $$d_{33}=\frac{S_{max}-S_{ref}-\beta \cdot \left(Q{\text{'}}_{max}-Q{\text{'}}_{ref}\right)}{\alpha }+d_{33\_ref}$$

$$d_{33}=k_1\cdot S_{max}-k_2\cdot Q{\text{'}}_{max}+k_3$$

Formula 3 can only be converted into Formula 4 and then into Formula 5: $$d_{33}=\frac{1}{\alpha }\cdot S_{max}-\frac{\beta }{\alpha }\cdot Q{\text{'}}_{max}+\frac{S_{ref}+\beta \cdot Q{\text{'}}_{ref}}{\alpha }+d_{33\_ref}$$

$$d_{33}=k_1\cdot S_{max}-k_{\mathrm{2nd}}\cdot Q{\text{'}}_{max}+k_{\mathrm{3rd}}$$

As in the first exemplary embodiment of the method, the maximum Smax with a first constant is also here k ₁ multiplied. This is done with a second constant k ₂ multiplied maximum amount Q ' _max the injection rate Q subtracted and a third constant k ₃ is added. While the first constant k ₁ and the second constant k ₂ from the injector-specific factors α and β result, includes the third constant k ₃ Values of the golden reference fuel injector.

The method according to the invention cannot only be used for a factory test of the fuel injectors 2nd be used, but also to drift the charge constant d ₃₃ in the operation of the fuel injectors 2nd to determine. While for the factor α the same value as used in the factory test is used for the factor β a value of zero is used to represent the final needle closing speed of a fuel injector 2nd no longer drifts significantly in operation. So that applies k ₂ = 0, which gives formula 6: $$d_{33}=k_1\cdot S_{max}+k_{\mathrm{3rd}}$$

While the constant k ₁ not operating from the constant k ₁ differs during the factory test, has the constant k ₃ a different value during operation, since the term β · Q ' _{ref takes on} a value of 0.

Instead of the maximum smax at a defined needle closing time t _close to determine, it is alternatively also possible in all of the exemplary embodiments of the method described above, for several needle closings of the fuel injector 2nd with different activation times _not the respective maxima Smax of the sensor signal S to determine and then use their mean in formulas 1 to 6.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102014204098 A1 [0003]
• DE 102009029549 A1 [0014]

Claims (10)

Method for determining a charge constant of a piezoelectric needle-closing sensor (26) of a fuel injector (2), wherein the charge constant of at least one maximum (S _max ) of a sensor signal (S) of the needle-closing sensor (2) that occurs after a nozzle needle (25) of the fuel injector (2) is needle-closed 26) is determined. Procedure according to Claim 1 , characterized in that the sensor signal (S) is filtered. Procedure according to Claim 1 or 2nd , characterized in that the determination takes place at a predetermined needle _{closing time} (Δt _closing ) of the nozzle needle (25). Procedure according to Claim 1 or 2nd , characterized in that the determination is carried out by averaging maxima (S _max ) of the sensor signal (S) at different actuation _durations (Δt _anst ) of the needle _{closing sensor} (26) and determining the charge constant from the mean value obtained. Procedure according to one of the Claims 1 to 4th , characterized in that the determination is carried out by multiplying a maximum (S _max ) of the sensor signal (S) or an average of maxima (S _max ) of the sensor signal (S) of a plurality of needle closing processes by a factor (ki). Procedure according to Claim 5 , characterized in that when determining a maximum amount of a derivative (Q ') of an injection rate (Q) is taken into account in a needle closing process. Procedure according to Claim 5 or 6 , characterized in that when determining values of a reference fuel injector are taken into account. Computer program, which is set up each step of the method according to one of the Claims 1 to 7 perform. Machine-readable storage medium on which a computer program Claim 8 is saved. Electronic control device (4) which is set up to set a charge constant by means of a method according to one of the Claims 1 to 7 to determine.

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