Method for operating a piezoelectric actuator and means for its implementation

Abstract

Method (10) for operating a fuel injector (1) at which a fuel with a variable fuel pressure (pRail) is present, and which has a piezoelectric actuator which is charged from a start of activation (tiSOE) over a charging time (tiChrg) and, after charging, is discharged again after a control period (tiET) has elapsed over a discharging time (tiDisCh), wherein the charging time (tiChrg) and the discharging time (tiDisCh) are determined as a function of the fuel pressure (pRail), the start of activation (tiSOE) is determined as a function of the fuel pressure (pRail) and the predetermined charging time (tiChrg), and the control period (tiET) is determined as a function of the fuel pressure (pRail), the determined charging time (tiChrg) and the determined discharging time (tiDisCh).

Description

[0001] The present invention relates to a method for operating a piezoelectric actuator and means for its implementation. State of the art

[0002] As described in DE 10 2007 059 540 A1, the piezoelectric actuator in a common rail piezo injector is charged or discharged by pulsed charge using charge and discharge switches. A buffer capacitor serves as the voltage source, which is charged to a target voltage, the buffer voltage, via a DC / DC converter. The buffer voltage is higher than the maximum voltage of the actuator.

[0003] Charging and discharging are controlled by a discontinuous or pulsating current waveform, respectively, and are regulated via a current shunt. Two parameters serve as control variables. The first parameter is the charging current limit during charging and the discharging current limit during discharging. The second parameter is the pulse-pause time during charging or discharging. The pulse-pause time is defined as the time between the opening and subsequent closing of the charging or discharging switch during a charging or discharging pulse.

[0004] Due to the short charging and discharging times, the mechanical design, and the specific properties of the actuator, a characteristic noise (known as a piezo ticking) is generated each time the piezo injector is activated. Depending on the installation position in the vehicle and the noise insulation measures used, this noise can be particularly noticeable when the vehicle is idling and can be perceived as annoying. During operation, however, these noises are masked by wind and road noise.

[0005] One measure to reduce noise levels is to extend the charging and discharging times when controlling the piezo injector or its actuator. This so-called anti-ticker function is only active at low rail pressures, i.e., during overrun and especially when the vehicle is idling. To increase the charging and discharging times while maintaining a constant actuator voltage, the charge quantity must be applied to or removed from the actuator over a longer period of time. With a fixed pulse pause time, the charging current limit is lowered during charging and the discharging current limit is lowered during discharging (see Fig. 2).

[0006] Corresponding methods are known, for example, from EP 1 381 764 B1 and have been used for several years to improve the noise of injection systems with piezo injectors.

[0007] Further procedures are known from the following documents: DE 10 2006 046 470 A1 discloses a method, a device, and a computer program for correcting the activation duration and activation start for a desired injection quantity with a varying charge / discharge gradient. This avoids time-consuming map measurements and only requires the measurement of one map with a constant gradient and another injection curve with a different gradient. The correction is based on the shift in the charge / discharge start and takes into account a characteristic point of the charge / discharge curve, which is determined at the factory and stored in the injector if necessary.

[0008] DE 101 13 560 A1 describes how a valve control unit sets a voltage gradient to control the piezoelectric actuator. Importantly, this gradient is dependent on the fuel pressure in the supply line. It also explains that at high fuel pressure, rapid control with a high gradient occurs. At low pressure, the gradient is reduced accordingly.

[0009] DE 101 48 217 C1 discloses a method for precisely controlling a piezo injector by dynamically adapting the control gradient to various influencing variables. The adaptation is achieved by correcting a standard gradient using correction factors based on the current values ​​of the influencing variables.

[0010] The method known from DE 10 2004 004 007 A1 optimizes the fuel injection of piezo injectors in internal combustion engines to minimize emissions. By adjusting the charging voltage gradient at the injector, the start of injection is synchronized to a specified reference point. This enables more precise metering of the fuel quantity and reduces emissions more effectively than conventional adjustment of the injection duration. In addition, the fuel quantity can be further optimized by adjusting the injection duration. Reference points are preferably derived from statistical averages or from a reference injector.

[0011] A disadvantage of corresponding anti-ticker functions is that the change in the charging and discharging time has an influence on the hydraulic behavior of the fuel injector and can lead to problems when calibrating corresponding injection systems. Disclosure of the invention

[0012] Against this background, the present invention proposes a method for operating a piezoelectric actuator and means for implementing it with the features of the independent patent claims. Preferred embodiments of the invention are the subject of the dependent patent claims and the following description. Advantages of the invention

[0013] An extension of the charging time causes the opening point of the fuel injector's switching valve, and thus the nozzle needle, to shift backward. In relation to the electrical start of energizing (SOE), the actual start of injection (SOL) is thus shifted backward. Since the nozzle needle exhibits ballistic behavior, the nozzle needle's closing point is also affected accordingly.

[0014] An extension of the discharge time causes the closing point of the switching valve and thus of the nozzle needle to be shifted (even further) backwards. In relation to the electrical control end (the beginning of the discharge), the injection end is therefore shifted, as also shown below. Fig. 3 explained.

[0015] These effects also lead to a change in the amount of fuel injected by the fuel injector by changing the injection duration.

[0016] The invention offers the advantage that these effects no longer need to be taken into account from the outset during the basic parameterization of an injection system (in particular, the activation start times and the activation duration map). Conventionally, this is the case, meaning that the parameterization of the charging and discharging times must already be completed before the activation start times and the activation duration map can be parameterized.

[0017] The basic calibration of an injection system is often the responsibility of the vehicle manufacturer, not the fuel injector supplier. This calibration usually takes place at a very early stage, as soon as appropriate fuel injector samples are available, since the calibration of all correction functions is based on the basic calibration.

[0018] However, noise emission studies are usually conducted at a later date. If this results in the need to adjust charging and discharging times, the parameters for the activation start times and the activation duration map must be recalculated, which represents a significant initial and subsequent effort.

[0019] However, the measures proposed by the invention allow for the continued use of a previously created basic parameter set by correcting the influence of the loading and unloading times on the start of injection and the activation duration via separate corrective interventions. Therefore, the loading and unloading times can also be configured after the basic parameter set has been created.

[0020] The core of the invention is a method that considers the influence of the charging and discharging time on the start of injection and the control duration, for example, in separate correction maps. Using these correction maps, the charging and discharging time can be parameterized at any time after the basic parameters have been created. Only the pressure wave correction parameters may need to be re-created, as this takes the hydraulic interval between injections into account. Pressure wave correction is known to correct the effects of a fuel pressure wave, which results from the fact that each injection causes a brief drop in fuel pressure.

[0021] A key advantage of the measures according to the invention is that the calibration of the function for optimizing noise emissions (anti-ticker function by increasing the charging and discharging time) can now be performed after the basic calibration of an injection system (start of actuation, actuation duration map) has been established. Recursion of the basic calibration after the charging and discharging time has been calibrated is no longer necessary. Only the pressure wave correction needs to be recalibrated in the relevant rail pressure range.

[0022] This results in significant cost savings through a significant reduction in calibration effort. The calibration of the correction maps can be performed once per fuel injector design by the injection system manufacturer and then adapted for all application scenarios. The calibration sequence can be adapted to the usual prioritization of the individual development steps (first basic calibration, then noise optimization).

[0023] To this end, the present invention proposes a method for operating a corresponding fuel injector, which is supplied with a fuel at a variable fuel pressure and has a piezoelectric actuator. The actuator is charged over a charging time starting at the start of the activation and then discharged again over a discharging time after the activation period has elapsed.

[0024] It is intended to specify the charging and discharging times depending on the fuel pressure. As explained above, such adjustment of the charging and discharging times to the fuel pressure occurs particularly when noise reduction is to be achieved as part of the aforementioned anti-tick function. In such cases, the charging and discharging times are extended when the fuel pressure decreases, for example, during the aforementioned overrun phases and / or when the vehicle is stationary.

[0025] The charging time and the discharging time are each determined depending on the fuel pressure. The start of the activation is determined depending on the fuel pressure and the specified charging time. The activation duration is determined depending on the fuel pressure value, the specified charging time, and the specified discharging time.

[0026] In particular, relationships, particularly in the form of characteristic maps, are used here. For example, it can be provided in particular that the start of control is determined on the basis of a first relationship which, for the prevailing fuel pressure, indicates a change in the start of injection of the fuel injector compared to a predetermined charging time, corresponding to the determined charging time. The control duration can be determined in particular on the basis of a second relationship which, for the fuel pressure, indicates a change in an injection quantity of the fuel injector compared to a predetermined charging time, corresponding to the determined charging time, and on the basis of a third relationship which, for the fuel pressure, indicates a change in an injection quantity of the fuel injector compared to a predetermined discharging time, corresponding to the determined discharging time.

[0027] The start of injection of the fuel injector, like the injection quantity, can be measured or derived from other variables. At the start of injection, the valve needle of the fuel injector opens and fuel is injected into an associated combustion chamber. Injection continues until the valve needle closes again. Depending on the opening and closing speed or the actual opening and closing times and the fuel pressure of the available fuel, changing the control of the piezoelectric actuator, in particular changing the charging and discharging times, can result in different injection starts and / or injection quantities. In other words, if the charging and discharging time of the piezoelectric actuator is extended, a fuel injector opens and closes more slowly, so that the opening time is shifted. The opening duration changes when the charging and discharging time is extended.

[0028] A computing unit according to the invention, e.g. a control unit of a motor vehicle, is configured as a means for implementing the method according to the invention, in particular in terms of programming, to carry out this method according to the invention.

[0029] Implementing the method in software form 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 available. Suitable storage media for the computer program include floppy disks, hard disks, flash memory, EEPROMs, CD-ROMs, DVDs, and others. Downloading a program via computer networks (internet, intranet, etc.) is also possible.

[0030] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0031] The invention is illustrated schematically in the drawing using an embodiment and is described in detail below with reference to the drawing. Short description of the drawings Fig. 1 shows a fuel injection system in a schematic representation. Fig. 2 illustrates the principles of a method according to the invention. Fig. 3 illustrates the principles of a method according to the invention. Fig. 4 shows an embodiment of a method according to the invention. Embodiment(s) of the invention

[0032] One in Fig. 1 schematically shown common rail fuel injection system of an internal combustion engine 100 of a motor vehicle, not shown, comprises a tank 101, from which a high-pressure pump 102 delivers fuel via a line 103 into a high-pressure accumulator 104, the so-called rail.

[0033] Fuel injectors 1, which in this case comprise piezoelectric actuators, are connected to the rail 104 via lines 105. The fuel is present at the fuel injectors 1 at a corresponding fuel pressure and can be injected into the combustion chambers 106 of the four-cylinder internal combustion engine 100.

[0034] The rail 104 is connected to the tank 101 via a pressure relief valve 107 and a return line 108. The fuel injectors 1 are also connected to the return line 108. The fuel injectors 1 can be controlled by a control unit 110 via electrical control lines 109. Similarly, the high-pressure pump 102 can also be controlled by the control unit 110 via an electrical control line 111. A rail pressure sensor 112, which is arranged on the rail 104 and detects the fuel pressure in the rail 104, is connected to the control unit 110 via a signal line 113.

[0035] In Fig. 2, the principles of a method according to the invention are illustrated using current and voltage curves in a diagram 200. The current and voltage curves are plotted as currents I and voltages U on the ordinate, respectively, against a time t on the abscissa.

[0036] Reference numeral 11 denotes a current waveform used to charge and discharge a piezoelectric actuator of a fuel injector of a fuel injection system. The start of activation is denoted by tiSOE. As mentioned, the piezoelectric actuators of corresponding fuel injectors are activated with an intermittent current waveform, which is controlled, for example, via a current shunt. Details of the activation method are illustrated, for example, in the aforementioned DE 10 2007 059 540 A1.

[0037] In the illustrated example of current profile 11, the piezoelectric actuator is charged over a charging time tiChrg, which is illustrated in diagram 200. At the end of the charging time tiChrg, the piezoelectric actuator is in a charged state. This state is maintained until the end of a control period tiET. Starting at the end of the control period tiET, the piezoelectric actuator is discharged. The discharge occurs, also with a corresponding current profile, over a discharge period tiDisCh.

[0038] In the example, the current curve 11 results in the Fig. 2 a voltage curve 13 of a charging voltage of a corresponding piezoelectric actuator.

[0039] In Fig. Figure 2 further illustrates the effects of an extension of the charging time tiChrg and the discharging time tiDisCh in the form of current and voltage curves 13 and 14, respectively. The extended periods are not specifically labeled.

[0040] As from the Fig. As can be seen in Figure 2, extending the charging time tiChrg while maintaining the same control duration tiET shortens the time during which the actuator is fully charged. The extension of the charging time tiChrg and the discharging time tiDisCh is achieved by lowering the charging and discharging current limits: As can be seen, the maximum and minimum values ​​of current waveform 11 (without extended charging and discharging times) are significantly below and above those of current waveform 12 (with extended charging and discharging times).

[0041] In Fig. 3, real voltage curves and injection rates of a correspondingly controlled actuator are illustrated in the form of a diagram 300. The diagram 300 shows voltage and injection rate curves U and Q on the ordinate against a time t on the abscissa. The voltage curve 21 corresponds to non-extended charging and discharging times (cf. voltage curve 13 of Fig. 2), the voltage curve 22 extended charging and discharging times (see voltage curve 14 of the Fig. 2). As can be seen from the associated injection rate curves 23 (without extended charging and discharging times, see voltage curve 21) and 24 (with extended charging and discharging times, see voltage curve 22), the injection quantity 9 (area under the respective injection rate curves 23 and 24) of a corresponding fuel injector changes due to the extension of the charging and discharging times. The present invention aims to compensate for this without the need to re-data the fuel injector. The start of injection is designated SOI here. Although no change with regard to this start of injection SOI can be seen in the injection rate curves 23 and 24, this can also change due to the extension of the charging and discharging times.

[0042] In Fig.Figure 4 illustrates a method according to one embodiment of the invention in the form of a schematic control diagram. The method is designated overall by 10.

[0043] Using a characteristic curve 41, a charging time tiChrg is first calculated from, for example, a measured fuel pressure pRail. Using a characteristic curve 42, a discharging time tiDisCh is calculated from the fuel pressure pRail.

[0044] A first characteristic map 31 with the input parameters fuel pressure pRail and charging time tiChrg is used to determine the amount tiSOEOffset by which the electrical control start tiSOE must be corrected in order to obtain the same injection start SOI as with the nominal charging time.

[0045] A second map 32 with the same input parameters fuel pressure pRail and charging time tiChrg is used to determine the amount tiETOffsChrg by which the control duration tiET must be corrected at the operating point defined by the rail pressure or fuel pressure pRail in order to obtain the same injection quantity as with the nominal charging time.

[0046] A third map 33 evaluates the discharge time tiDisCh, using the input parameters fuel pressure pRail and discharge time tiDisCh to determine the amount tiETOffsDisCh by which the control duration tiET must be corrected at the operating point defined by the rail pressure in order to obtain the same injection quantity as with the nominal discharge time.

[0047] The values ​​tiETOffsChrg and tiETOffsDisCh are calculated with a nominal value or default value DtiET to determine a value to be used for the activation duration tiET. The value DtiET denotes the activation duration at a nominal discharge time.

[0048] Due to the input parameter fuel pressure pRail in the three maps 31 to 33, an intentional redundancy results, although the fuel pressure pRail is already implicitly included in the charging time tiChrg and the discharging time tiDisCh in the relevant operating range (see curves 41 and 42 for determining the charging time tiChrg and the discharging time tiDisCh). However, since the charging and discharging times are applied at a later time, full flexibility is ensured at all times by taking the fuel pressure pRail into account.

[0049] The calibration of the aforementioned characteristic maps takes place, for example, on a hydraulic test bench for the respective hydraulic design of the fuel injector (nozzle flow, etc.). Devices for measuring the injection rate and injection quantity are also required.

[0050] For calibration, for example, a single injection is set with a nominal actuation duration and a nominal actuation start time. At a specific fuel pressure pRail, the injection quantity q or injection rate Q is determined for a specified charge time tiChrg, and the injection start time SOI is determined from the curve of the injection rate Q. The charge time tiChrg is then increased step by step, and for each tiChrg value, the value tiETOffsChrg by which the nominal actuation duration must be corrected to maintain the same injection quantity q or injection rate Q is determined.

[0051] At the same time, the time value tiSOEOffset that the nominal electrical control start must be corrected by to maintain the same start of injection (SOI) is determined. The step size for increasing the charging time can be adjusted based on a compromise between accuracy and the number of support points in the map.

[0052] The same applies to the discharge time: For parameterization, for example, a single injection is set with a nominal actuation duration and a nominal actuation start time. At a specific fuel pressure pRail, the injection quantity q or injection rate Q is determined for a specified discharge time tiDisCh, and the injection start time SOI is determined from the curve of the injection rate Q. The charge time tiDisCh is then increased step by step, and for each value of tiDisCh, the value tiETOffsDisCh by which the nominal actuation duration must be corrected to maintain the same injection quantity q or injection rate Q, etc.

Claims

[1] Method (10) for operating a fuel injector (1) at which a fuel with a variable fuel pressure (pRail) is present and which has a piezoelectric actuator which is charged from a start of activation (tiSOE) over a charging time (tiChrg) and, after charging, is discharged again after the elapse of an activation period (tiET) over a discharging time (tiDisCh), wherein the charging time (tiChrg) and the discharging time (tiDisCh) are determined as a function of the fuel pressure (pRail), the start of activation (tiSOE) is determined as a function of the fuel pressure (pRail) and the predetermined charging time (tiChrg), and the activation period (tiET) is determined as a function of the fuel pressure (pRail), the determined charging time (tiChrg) and the determined discharging time (tiDisCh). [2] Method (10) according to claim 1, wherein the start of control (tiSOE) is determined on the basis of a first relationship (31) which indicates for the fuel pressure (pRail) a change in a start of injection (SOI) of the fuel injector (1) compared to a predetermined charging time (tiChrg) corresponding to the determined charging time (tiChrg). [3] Method (10) according to claim 2, wherein the activation duration (tiET) is determined on the basis of a second relationship (33) which indicates, for the fuel pressure (pRail), a change in an injection quantity (9) of the fuel injector (1) compared to a predetermined charging time (tiChrg) corresponding to the determined charging time (tiChrg), and on the basis of a third relationship (33) which indicates, for the fuel pressure (pRail), a change in an injection quantity (q) of the fuel injector (1) compared to a predetermined discharging time (tiDisCh) corresponding to the determined discharging time (tiDisCh). [4] Method (10) according to claim 3, wherein the first, second and third relationships are provided in the form of characteristic maps (31-33). [5] Method (10) according to claim 3 or 4, in which default values ​​(DtiSOE, DtiET) are provided for the start of activation (tiSOE) and the activation duration (tiET), wherein correction values ​​(tiSOEOffset, tiETOffsChrg, tiETOffsDisCh) for adapting the default values ​​(DtiSOE, DtiET) are determined by means of the first, the second and the third relationship. [6] Method (10) according to one of the preceding claims, in which the charging time (tiChrg) and discharging time (tiDisCh) are extended when the fuel pressure (pRail) decreases and vice versa. [7] Method according to one of the preceding claims, which is used for operating a fuel injector (1) of a common rail internal combustion engine (100) with a piezoelectric actuator. [8] Method according to one of the preceding claims, which is used to prevent a piezo ticker. [9] Fuel injection system comprising a fuel injector (1) with a piezoelectric actuator and means adapted to carry out all steps of the method according to any one of the preceding claims. [10] Computing unit adapted to carry out the method according to one of claims 1 to 8. [11] A computer program which causes a computing unit to carry out the method according to any one of claims 1 to 8 when executed on the computing unit. [12] A machine-readable storage medium having stored thereon a computer program according to claim 11.

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