EP1167729B2 - Piezoelectric actuator for injector - Google Patents

Description

The invention relates to a method for operating an injection valve for fuel, in which a piezoelectric actuator drives a closing member via a hydraulic coupler. It further relates to a fuel injection system having a number of such injectors.

As an essential component can be used in an internal combustion engine for introducing the fuel into the combustion chambers of the cylinder, a fuel injection system. Such a fuel injection system usually comprises a number of injection nozzles, which can be supplied individually or in the manner of a so-called common-rail system (CR system) via a central supply system with fuel. In both embodiments, each injector is usually integrated into a respective associated injection valve, via which the fuel injection is adjustable in a predeterminable manner.

The injection valves can be provided for an electrical control with a piezoelectric actuator. Such injection valve for fuel injection into the combustion chamber of an internal combustion engine with a high-pressure system or CR system is known from DE 197 328 02 known. This injection valve is designed in a double-switching manner and has a closure member, which is in one of two alternative valve seats in each case in a closed position and thereby causes a closure of the injection nozzle. In a middle position between the two valve seats, however, the closing member assumes an opening position.

For the transfer of the closure member from a closed position to the open position or from the open position to one of the closed positions, the closing member can be driven via a piezoelectric actuator. For this purpose, for example, the piezoelectric actuator is charged to a drive voltage, which is dependent on the pressure in the common rail system. Due to the drive voltage, the actuator expands in the longitudinal direction. This longitudinal extension is transmitted via a hydraulic coupler on the closing member, so that on the one hand the producible by the actuator stroke is amplified and on the other hand, the closure member is decoupled from a possible static temperature expansion of the actuator. A charging of the piezoelectric actuator thus causes via the hydraulic coupler, a transfer of the Verschließgliedes first from the first closed position to the open position and then from the open position to the second closed position. On the other hand causes a discharge of the piezoelectric actuator due to the associated contraction in the longitudinal direction via the hydraulic coupler, a transfer of the Verschließgliedes first from the second closed position to the open position and then from the open position to the first closed position.

By the movement of the closure member from one to the other closing position a short-term relief of a high-pressure valve control chamber is effected, via the pressure level, the control of a valve needle is in an open or closed position. If the closing member is thus in the opening position between the two closing positions, then a fuel injection takes place in a combustion chamber connected downstream of the injection valve.

Upon actuation of the actuator and thus upon actuation of the injection valve, as a result of the pressure conditions prevailing in the hydraulic region, part of the liquid present in the coupler, ie the fuel located there, is forced out of the coupler via a leakage gap. This effect is particularly great when the closing member is held in the closed position facing the high-pressure region, since in this case the counterforce due to the pressure in the CR system is particularly high.

For a proper functioning of the injection valve, in particular for an exact dosage of the fuel quantity to be injected as required, a precise positioning of the closure member via the drive voltage is required. The implementation of the drive voltage in a positioning of the closure member, ie in a corresponding valve lift, depends essentially on the filling state of the hydraulic coupler. In particular, the most complete possible (re-) filling of the hydraulic coupler before each injection and thus a compensation of said leakage losses is necessary. The refilling of the hydraulic coupler after an injection takes place at the injection valve according to the DE 197 328 02 as a result of a suitable system pressure of about 15 bar also via the leakage gaps, but only in the period in which the piezoelectric actuator is not driven. Accordingly, the result of refilling after injection may vary depending on the amount of fuel to be replenished and the available time interval.

When out of the DE 197 328 02 However, known injector is in any case a proper refilling of the coupler between two injections presumed without there detection of a possible false or non-filling would be provided. In other words: without further examination, a valve lift which can be accurately predicted in dependence on the drive voltage is assumed. A not or insufficiently refilled coupler can thus be determined only indirectly via errors due to faulty injections, such as speed fluctuations, without thereby a clear assignment to the cause of the error would be possible. A targeted corrective intervention is thus not possible in this injection valve, so that not in all operating conditions, a reliable Kraftstoffeinsprizung occurs.

From the WO 99/67527 For example, a method and an arrangement for controlling a capacitive actuator are known. wherein energy is applied to the actuator and a conclusion is drawn on the functioning of the actuator or an injection valve on the basis of the energy applied to the actuator and a comparison between a voltage of the actuator and reference values.

The invention is therefore based on the object, a method for operating a fuel injection system with a number of injectors, in each of which a piezoelectric actuator via a hydraulic coupler drives a closing member to specify, with a particularly reliable fuel injection is ensured. In addition, a particularly suitable for carrying out the method fuel injection system should be specified.

With regard to the method, this object is achieved according to the invention by monitoring the terminal voltage after a charging process of an actuator and used to form a diagnostic statement for the respective injector, wherein the diagnosis statement is formed on the basis of the time decrease of the terminal voltage after completion of the charging process.

To monitor the terminal voltage is disconnected after completion of the charging of the piezoelectric actuator whose power supply and instead a voltage measuring device connected to its terminals.

The invention is based on the consideration that the fuel injection system can be operated particularly reliably with regard to fuel injection by performing a direct and timely monitoring of the valve lift of the injection valves. For this purpose, a diagnostic statement regarding its valve lift is formed promptly, that is, during or immediately after the current injection cycle, for each injection valve. As has surprisingly been found, can be provided as a suitable parameter on the basis of the diagnosis statement, the temporal evolution of the terminal voltage of the respective actuator. Namely, during the charging phase, a pressure builds up in the hydraulic coupler, which also acts upon completion of the charging process on the piezoelectric actuator and in this with disconnected power supply for the pressure conditions in the coupler and consequently also for the implementation of the control voltage in the valve lift characteristic piezoelectric voltage generated. The terminal voltage at the piezoelectric actuator can thus be used without the need for a further sensor as a measurement parameter for the valve behavior with regard to the implementation of the control voltage in the valve.

Advantageous embodiments of the invention are the subject of the dependent claims.

According to the invention, the monitored terminal voltage is used to form a diagnostic statement about the fill level of the coupler of the respective injection valve. For its part, the coupler pressure, in turn, is dependent on the medium in the coupler, with the other external conditions remaining unchanged; so results in only partial filling of the coupler with fuel (effective medium: fuel-air mixture) in other pressure conditions than a complete filling of the coupler with fuel as a medium. Thus, the characteristic of the pressure conditions in the coupler terminal voltage is particularly favorable for the fill level monitoring approached.

A particularly reliable diagnostic statement can be achieved by this invention is formed based on the time decrease of the terminal voltage after completion of the charging process. After completion of the charging phase for the piezoelectric actuator, namely, the pressure in the hydraulic coupler degrades due to the outflowing medium again; This pressure drop can be detected by monitoring the terminal voltage at the actuator in the form of a voltage reduction occurring as a function of time. This voltage reduction depends to a particularly great extent on the degree of filling in the coupler: when the coupler is fully filled, a comparatively pronounced voltage dip can be detected. In contrast, this effect is significantly lower with only partially filled coupler, since in this case only air or an air-fuel mixture was compressed during charging.

Expediently, the diagnostic statement is formed on the basis of a comparison of the difference between a first measured value for the terminal voltage immediately after the end of the charging process and a second measured value for the terminal voltage after expiry of a waiting time with a limit value. For the waiting time can be given a fixed value of advantageously about 0.25 ms. Alternatively, however, the waiting time can also be selected as a function of parameters characteristic for the injection cycle, such as, for example, the activation duration or the duration of a break in injection between two injections. In particular, the waiting time can be selected such that the second measured value is determined immediately before a following control intervention, for example an increase of the drive voltage.

If a terminal voltage difference above the limit value is detected, it is concluded that a fully filled and thus functional coupler. If, however, the terminal voltage difference is smaller than the limit value, an error of the respective coupler is detected. In a particularly favorable manner, the limit value is suitably selected on the basis of characteristic operating characteristics of the respective coupler, with a voltage value of approximately 25 to 30 V in particular being able to be specified as limit value. Alternatively, the limit value can also be predetermined operating point-dependent by a central control unit, wherein advantageously a map obtained from a previous calibration measurement is used.

If a fault is detected, it is possible to distinguish between the effects of the fault by comparison with another limit value: if the terminal voltage difference also falls below a second, even lower limit value or minimum value, a "fatal error" is diagnosed which, for example, triggers an immediate shutdown of the internal combustion engine can. On the other hand, if the terminal voltage difference lies below the first limit, but above the second limit value, then a simple fault is diagnosed which, although it permits further operation of the internal combustion engine, is stored in a on-board memory for later diagnostic purposes.

Advantageously, the diagnostic statement is used directly for a corrective intervention in the fuel injection system. In this case, a target value for the drive voltage of the respective actuator for a subsequent injection cycle is specified in particular when determining a simple error based on the diagnostic statement, wherein the desired value is selected such that despite the detected insufficient filling of the coupler after activation results in the proposed valve.

With regard to the fuel injection system, the stated object is achieved with the features according to claim 8.

In order to maintain the information determined in the timely diagnosis also for later applications, for example inspections or overhaul measures, the diagnostic device is expediently connected to a data storage module.

The advantages achieved by the invention are in particular that a timely and reliable diagnosis and functional monitoring of each injector is made possible by the evaluation of the terminal voltage in a simple manner and without additional design effort. The fuel injection system can thus be operated particularly reliably, with minor deviations immediately corrective measures, for example, with regard to a tracking of the driving voltage for each actuator, can be made.

Figur 1

ein Einspritzventil eines Kraftstoffeinspritzsystems,

Figur 2

schematisch ein Kraftstoffeinspritzsystem mit einer Anzahl von Einspritzventilen nach Figur 1, und

Figuren 3a und 3b

jeweils ein Zeitdiagramm für eine Klemmenspannung.

An embodiment of the invention will be explained in more detail with reference to a drawing. Show:

FIG. 1

an injection valve of a fuel injection system,

FIG. 2

schematically a fuel injection system with a number of injectors after FIG. 1 , and

FIGS. 3a and 3b

each a time chart for a terminal voltage.

The injection valve 1 after FIG. 1 has a piezoelectric actuator 2, which comprises a number of series-connected piezo elements 4. The actuator 2 is connected on the one hand with a housing wall 6, are passed through the terminals 7 of the actuator 2, and on the other hand with a servo piston 8 positively connected. The actuating piston 8 terminates with its side facing away from the actuator 2 end face 9 from a hydraulic coupler 10. The hydraulic coupler 10 in turn acts on a guided in a connecting channel 12 actuating piston 14, at the end remote from the coupler 10, a closing member 16 is arranged. This is designed as a double-closing control valve. In a first closed position, which corresponds to a rest position of the actuator 2, it closes a first valve seat 18 of a valve chamber 20. In a second closed position, which corresponds to a maximum actuation of the actuator 2, the closing member 16 closes a second valve seat 22 of the valve chamber 20.

Via a passage in the second valve seat 22 of the valve chamber 20 is connected to a guide channel 24 which is connected on the input side via a connecting piece 26 to a pressure channel, not shown, of a common rail fuel supply system of a motor vehicle. In the guide channel 24, a nozzle needle 28 is arranged, which releases or closes a fuel outlet 30 of a branched off from the connecting piece 26 fuel passage 32 depending on an applied via the terminals 7 to the actuator 2 drive voltage Ua. To set functional pressure conditions during operation of the injection valve 1, the connecting piece 26 with an inlet throttle 34 and the guide channel 24 are provided with an outlet throttle 36.

The injection valve 1 is, together with further injectors 1, part of a fuel injection system 40, as shown schematically in FIG FIG. 2 is shown. In this case, the injection valves 1, of which in FIG. 2 the sake of clarity, only four are shown, the fuel side connected to a common supply line 42. The number of injectors provided for the fuel injection system 40 is dependent on the further requirements of the engine to be supplied, in particular, an injection valve 1 may be provided for each cylinder to be fed.

For the electrical control of the injectors 1, the fuel supply system 40 comprises a central control unit 44. This in turn has a control module 46 which is connected via lines 48 to the terminals 7 of the injectors 1 switched off. Furthermore, the control unit 44 includes a diagnostic unit 50, the to a number of the injectors 1 respectively associated and connected to the terminals 7 voltage measuring 52 is connected. In addition, the control unit 44 has a data storage module 54.

During operation of the fuel supply system 40, the control unit 44 applies a drive voltage Ua to the terminals 7 of each injection valve 1 in an injection cycle via the drive module 46. In response to this drive voltage Ua, the actuator 2 of the controlled injection valve 1 expands in its longitudinal direction, so that the actuating piston 8 moves in the direction of the hydraulic coupler 10. As a result of the pressure increase in the coupler 10 caused thereby, the actuating piston 14 with the closing member 16 arranged thereon also moves in the direction of the second valve seat 22.

Via the supply line 42 prevails in the connecting piece 26 of each injector 26, a high pressure, which may be in a common rail system, for example, between 200 and 1800 bar. This pressure acts against the nozzle needle 28 and keeps it closed, so that no fuel can escape through the fuel outlet 30. However, if now as a result of the control voltage applied to the actuator Ua the closing member 16 is moved from the first valve seat 18 to the second valve seat 22 or vice versa, then the pressure in the high pressure region of the guide channel 24 breaks down, so that the nozzle needle 28 in the direction of the valve chamber 20 back and releases the fuel outlet 30. In this case, fuel injection occurs in the associated cylinder.

Via leakage gaps, part of the medium or fuel present in the hydraulic coupler 10 is pushed out during the movement of the actuating piston 14. However, proper connection of the hydraulic coupler 10 is required for a proper relationship between the drive voltage Ua and the injected fuel quantity. Therefore, a refilling of the respective coupler 10 via a channel, not shown, is provided between two injections.

To check whether the coupler 10 of the injectors 1 have actually been properly refilled, the fuel injection system 40 is designed such that a direct and timely monitoring of all couplers 10 of the injectors 1 is made. For this purpose, a diagnostic statement regarding the respective hydraulic coupler 10 is formed promptly, that is, during or immediately after the current injection cycle, for each injection valve 1.

In order to form this diagnostic statement, after the charging process of the actuator 2 of the respective injection valve 1 has taken place, the control module 46 is disconnected from the terminals 7 of the actuator 2. Instead of the control module 46, the associated voltage measuring device 52 is connected to the terminals 7 of the actuator 2, the measured data are output to the connected diagnostic unit 50. Thus, in this phase, a monitoring of the terminal voltage Uk of the actuator 2 as a function of elapsing time. From the temporal behavior of the terminal voltage Uk, a diagnostic statement about the degree of filling of the coupler 10 of the respective injection valve 1 is formed in the diagnostic unit 50.

As has been found out, after completion of the charging phase for the piezoelectric actuator 2, the pressure in the hydraulic coupler 10 builds up again as a result of the outflowing medium; this pressure drop can be detected by monitoring the terminal voltage Uk on the actuator 2 in the form of a reduction of the terminal voltage Uk occurring as a function of time. The extent of this temporal drop in the terminal voltage Uk is also dependent on the so-called gear ratio in the coupler 10, ie the ratio of the producible stroke of Verschließgliedes 16 to the force acting on the coupler 10 change in length of the actuator 2. As from the FIGS. 3a (Time decrease of the terminal voltage Uk with fully filled coupler 10) and 3b (time drop of the terminal voltage Uk with insufficiently filled coupler 10) can be seen, the reduction of the terminal voltage Uk also depends to a particularly pronounced extent on the degree of filling in the coupler 10: at full filled coupler 10 is a comparatively pronounced voltage dip in accordance with FIG. 3a about 50 V detectable. In contrast, this effect is significantly lower with only partially filled coupler 10 and is according to FIG. 3b only about 15 V.

From the monitoring of the terminal voltage Uk on the actuator 2, a diagnostic statement is thus formed in the diagnostic unit 50 for the respective injection valve 1 in the following manner: after the charging phase of the actuator 2, the terminal voltage Uk is measured. After a waiting time of, for example, about 0.25 ms, the terminal voltage Uk is measured again. Then, the difference between the two measured values is formed and compared with a limit value. In the embodiment, a fixed limit value of about 30 V is given for this purpose; Alternatively, however, an operating point-dependent limit value from a map obtained by prior calibration and stored in the data storage module 54 can also be used. The waiting time is chosen such that the measurement of the terminal voltage Uk immediately before a subsequent control intervention, namely before a further increase in the clamping voltage Uk occurs.

If the determined difference of the terminal voltages Uk is greater than the limit value, a proper refilling of the coupler 10 is concluded as a diagnosis; no further action is taken.

However, if the determined difference of the terminal voltages Uk is smaller than the limit value, the diagnosis is based on a faulty refilling of the coupler 10. In this case, a further comparison of the difference of the terminal voltages Uk is made with a second threshold or minimum value. Through this A comparison is made according to the effects of the error: if the difference of the terminal voltages Uk also falls below the second, even lower limit value or minimum value, then a "fatal error" is diagnosed which triggers, for example, an immediate shutdown of the internal combustion engine. On the other hand, if the difference between the terminal voltages Uk is below the first limit, but above the second limit value, then a simple fault is diagnosed which, although it permits further operation of the internal combustion engine, is stored in the data storage module 54 for later diagnostic purposes.

Upon detection of a simple error in the diagnostic unit 50, a desired value for the drive voltage Ua of the respective actuator 2 is also specified for a subsequent injection cycle, wherein the desired value is selected such that despite the detected insufficient filling of the coupler 10 after a control of the intended Valve lift results.

Claims (8)

1. Method for operating a fuel injection system (40) having a number of injection valves (1), in which in each case one piezoelectric actuator (2) drives a closure element (16) by means of a hydraulic coupler (10), wherein after a process for charging the actuator (2), the actuator's (2) terminal voltage (Uk) is monitored, and is used to form diagnostic information about the filling level of the coupler (10) of the respective injection valve (1), and wherein the diagnostic information is formed by reference to the decay of the terminal voltage (Uk) over time after the charging process ends.
2. Method according to Claim 1, in which the difference between the terminal voltage (Uk) which is measured directly after the charging process ends and the terminal voltage (Uk) which is measured after a predefinable waiting time has expired is compared with a limiting value.
3. Method according to Claim 2, in which a setpoint value for the actuation voltage (Ua) of the respective actuator (2) is predefined for a subsequent injection cycle as a function of the difference between the terminal voltage (Uk) which is measured directly after the charging process ends and the terminal voltage (Uk) which is measured after a predefinable waiting time expires.
4. Method according to Claim 2 or 3, in which a time of approximately 0.25 ms, calculated from the start of the charging process, is selected as the waiting time.
5. Method according to one of Claims 2 to 4, in which a fixed voltage value of approximately 25 to 30 V is predefined as a limiting value.
6. Method according to one of Claims 2 to 4, in which a voltage value which is dependent on the operating time and is derived from a characteristic diagram which is determined by reference to a previous calibration is predefined as a limiting value.
7. Fuel injection system (40) having a number of injection valves (1) in which in each case one piezoelectric actuator (2) drives a closure element (16) by means of a hydraulic coupler (10), wherein the actuator (2) of each injection valve (1) is connected to an assigned voltage measuring device (52) which is itself connected at the output end to a diagnostic unit (50), and wherein the diagnostic unit form is diagnostic information about the filling level of the coupler (10) for the respective injection valve (1) by reference to a decay in a terminal voltage (Uk) of the actuator (2) over time after a process of charging the actuator (2) ends.
8. Fuel injection system (40) according to Claim 7, whose diagnostic unit (50) is connected to a data memory module (54).

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