Method for determining the activation voltage of a piezoelectric actuator of an injector

A method for determining the activation voltage of a piezoelectric actuator of at least one injector which is used to inject a liquid volume under high pressure into a cavity, in particular into a combustion chamber of an internal combustion engine, the activation voltage being varied as a function of the pressure used to pressurize the liquid volume. A drift of the activation voltage (voltage requirement) required for a predefined lift of a control valve of the injector is controlled on an injector-specific basis by controlling the difference between the cutoff-voltage threshold and the final steady-state voltage to a setpoint value predefined for one operating point.

BACKGROUND INFORMATION

German Patent Application No. DE 100 32 022 describes a method for determining the activation voltage for a piezoelectric actuator of an injector, which provides for first measuring the pressure prevailing in a hydraulic coupler indirectly, prior to the next injection event. The pressure is measured in that the piezoelectric actuator is mechanically coupled to the hydraulic coupler, so that the pressure induces a corresponding voltage in the piezoelectric actuator. This induced voltage is used prior to the next injection event to correct the activation voltage, inter alia, for the actuator. An induced voltage that is too low is indicative of a missed injection. The injector is preferably used for injecting fuel for a gasoline or diesel engine, in particular for common-rail systems. In this context, the pressure prevailing in the hydraulic coupler also depends, inter alia, on the common-rail pressure, so that the activation voltage is varied as a function of the common-rail pressure. The voltage requirement of a piezoelectric actuator depends first and foremost on the pressure prevailing in the valve chamber, as well as on the coefficient of linear expansion of the piezoelectric actuator. The voltage required for properly operating the injector at one operating point is the so-called voltage requirement, i.e., the relationship between voltage and lift at a specific force which is proportional to the common-rail pressure.

German Patent No. DE 103 15 815.4 discusses deriving the active voltage requirement of an injector from the voltage difference between the maximum actuator voltage and the final steady-state voltage.

It is problematic in this regard, however, that the voltage requirement of an injector drifts over the service life of the injector. The effect of this drift is that the actuator voltage that is predefined as a function of one operating point does not ensure a proper operation of the injector at a predefined operating point. This leads to errors in the injection quantity which, in turn, cause negative exhaust-emission levels and negative noise emissions. In the least favorable case, a failure of the injection and thus of the injector may even occur, namely when the lift no longer suffices for opening an injection-nozzle needle.

Therefore, an object of the present invention is to compensate for this voltage requirement drift.

SUMMARY OF THE INVENTION

This objective is achieved by a method for determining the activation voltage of a piezoelectric actuator of an injector. The method according to the present invention makes it possible to compensate for the voltage requirement drift by adapting the setpoint voltage value, thereby ensuring that the required, nominal actuator excursion is attained and ensuring a proper and desired operation of the injector over the entire lifetime. In addition, by adapting the voltage requirement, the advantage is derived, in principle, that a very high voltage allowance is not needed for the activation, so that a considerable benefit is gained with respect to the power input/power loss. Moreover, the adaptation of the voltage requirement may also be used for diagnostic purposes, for example in order to output an error message in response to an unacceptably high drift of the voltage requirement.

The control of the voltage requirement drift is advantageously carried out during one driving cycle of a vehicle having the internal combustion engine, correction values ascertained during the driving cycle being stored in a non-volatile memory. This makes it feasible, in particular, for the correction values stored in the memory to be used in a later driving cycle, as initialization values for a further compensation of the voltage requirement drift.

To ensure that an adaptation is only carried out in response to an actual voltage requirement drift, i.e., that no readjustment is made in response to only temporary, relatively small deviations, caused, for example, by temperature effects, an enable logic is preferably provided, which enables an adaptation of the voltage requirement drift as a function of parameters characterizing the internal combustion engine and/or the injector.

These parameters include, for example, the temperature of the internal combustion engine and/or the common-rail pressure and/or the steady state of the voltage control and/or the state of the charging time control and/or the steady state of other secondary feedback control circuits and/or the number of injections and/or the control (activation) duration and/or the injection sequence per combustion cycle, i.e., effectively, the injection pattern (preinjection(s), main injection, post injection(s)).

The voltage requirement is compensated at various operating points very advantageously with respect to the common-rail pressure, the correction values being stored in correction characteristics maps, which are then also stored in the non-volatile memory, for example in an E2-PROM.

DETAILED DESCRIPTION

FIG. 1schematically depicts an injector1, known from the related art, having a central bore. In the upper part, an actuating piston3having a piezoelectric actuator2is introduced into the central bore, actuating piston3being fixedly coupled to actuator2. A hydraulic coupler4is upwardly delimited by actuating piston3, while in the downward direction, an opening having a connecting channel to a first seat6is provided, in which a piston5having a valve-closure member12is situated. Valve-closure member12is designed as a double-closing control valve. It closes first seat6when actuator2is in the rest phase. In response to actuation of actuator2, i.e., application of an activation voltage Ua to terminals +, −, actuator2actuates actuating piston3and, via hydraulic coupler4, presses piston5having closure member12toward a second seat7. Disposed in a corresponding channel, below the second seat, is a nozzle needle11, which closes or opens the outlet in a high-pressure channel (common-rail pressure)13, depending on which activation voltage Ua is applied. The high pressure is supplied by the medium to be injected, for example fuel for a combustion engine, via a supply channel9; the inflow quantity of the medium in the direction of nozzle needle11and hydraulic coupler4is controlled via an inflow throttling orifice8and an outflow throttling orifice10. In this context, hydraulic coupler4has the task, on the one hand, of boosting the lift of piston5and, on the other hand, of uncoupling the control valve from the static temperature-related expansion of actuator2. The refilling of coupler4is not shown here.

The mode of operation of this injector is explained in greater detail in the following. In response to each activation of actuator2, actuating piston3is moved in the direction of hydraulic coupler4. Piston5having closure member12, moves toward second seat7. In the process, a portion of the medium, for example of the fuel, contained in hydraulic coupler4is forced out via leakage gaps. For that reason, hydraulic coupler4must be refilled between two injections, in order to maintain its operational reliability.

A high pressure, which in the case of the common-rail system may amount to between 200 and 2000 bar, for example, prevails across supply channel9. This pressure acts against nozzle needle11and keeps it closed, preventing any fuel from escaping. If actuator2is actuated at this point in response to activation voltage Ua and, consequently, closure member12moved toward the second seat, then the pressure prevailing in the high-pressure region diminishes, and nozzle needle11releases the injection channel. P1denotes the so-called coupler pressure, as is measured in hydraulic coupler4. A steady-state pressure P1, which, for example, is 1/10 of the pressure prevailing in the high-pressure portion, ensues in coupler4, without activation Ua. Following the discharging of actuator2, coupler pressure P1is approximately 0 and is raised again in response to refilling.

At this point, the lift and the force of actuator2correlate with the voltage used for charging actuator2. Since the force is proportional to the common-rail pressure, the voltage for a required actuator excursion must be adapted as a function of the common-rail pressure to ensure that seat7is reliably reached. The voltage required for properly operating the injector or injector1at one operating point is the so-called voltage requirement, i.e., the relationship between voltage and lift at a specific force which is proportional to the common-rail pressure. German Patent No. DE 103 15 815.4 discusses how the individual, active voltage requirement of an injector can be derived from the voltage difference between the maximum actuator voltage and the final steady-state voltage.

This voltage requirement drifts over the lifetime of injector1. The effect of this drift is that the actuator voltage that is predefined as a function of one operating point no longer ensures a proper operation of injector1at the specified operating point, which leads to errors in the injection quantity, thereby entailing consequences for exhaust-emission levels/noise emissions, culminating in a failure of the injector, namely when the lift no longer suffices for opening nozzle needle11. The method described in the following makes it possible to compensate for this voltage requirement drift on an injector-specific basis.

An idea underlying the present invention is to compensate for the voltage requirement drift by adapting the setpoint voltage value, thereby ensuring that the required, nominal actuator excursion is attained and enabling the proper and desired operation of injector1to be ensured over its entire lifetime. Thus, on the one hand, the functioning of actuator2is ensured, but on the other hand the injection quantity errors described above are also avoided.

In principle, by adapting the voltage requirement in this manner, the need is also eliminated for activation processes that require a very high voltage allowance. This is advantageous, in particular, with respect to the power input/power loss of a control system. Moreover, actuator2is subject to less wear, since there is no need for actuator2to be operated over an entire lifetime with a very large voltage allowance, which is associated with too high of a power surplus in the valve seat.

Moreover, by monitoring the correction intervention of the adaptation, a diagnostic may also be performed on the entire injector, for example when an unacceptably high drift of the voltage requirement is ascertained.

The adaptation of the voltage requirement drift is based on automatically controlling the voltage difference between cutoff-voltage threshold Ucutoffand the measured, final steady-state voltage Ucontrol(compareFIG. 2), in an injector-specific manner, to a setpoint value ΔUsetpointwhich is required for one operating point and which correlates with the required actuator excursion of an injector that has not drifted, i.e., that is performing nominally. This control intervenes correctively by adapting the setpoint actuator voltage in an injector-specific manner, as is described in greater detail below in conjunction withFIG. 3.

An actuator setpoint voltage Usetpointis calculated in an arithmetic logic unit310. During the driving cycle, difference ΔUactualbetween cutoff voltage Ucutoffand control voltage Ucontrolis continually determined. This difference ΔUactualis compared to a predefined quantity ΔUsetpoint, the difference between quantity ΔUsetpointand ΔUactualbeing determined in a node320. This difference eΔUforms the input quantity for a PI controller, for example, in which various controllers331,332,33nare provided for each of the individual cylinders. In these controllers, cylinder-specific correction signals S1, S2, Snare defined in each instance and output, n describing the number of cylinders.

The correction values are either multiplied by setpoint voltage Usetpointdetermined in arithmetic logic unit310or, alternatively, added to it, as indicated by nodes341,342. The thus ascertained corrected values Usetpointcorrare fed to an actuator-voltage control device350, which determines cutoff-voltage threshold Ucutoff. At this point, this cutoff-voltage threshold Ucutoffis utilized, together with the ensuing final steady-state voltage Ucontrol, in turn, to determine difference ΔUactual.

Correction values S1, S2, . . . Snlearned during one driving cycle are preferably stored following termination of the driving cycle in a non-volatile memory360, for example in an E2-PROM, and used before the beginning of the subsequent driving cycle as initialization values for the further adaptation, as schematically depicted inFIG. 3by an arrow362denoted by “INIT”. It is noted at this point that, to calculate voltage difference ΔUactualfor the method described above, maximum voltage Umax(compareFIG. 2) cannot be used, as described in German Patent No. DE 103 15 815.4, but rather cutoff-voltage threshold Ucutoff, since Umaxis not available as a usable quantity in a generally known engine control unit, in which this control is also executed. The voltage requirement drift is also compensated, however, when the cutoff voltage Ucutoffquantity is used.

To ensure that the adaptation is only carried out in response to an actually existing voltage requirement drift, i.e., that controllers331,332,33nonly control in this case and not, for instance, in response to temporary, relatively small deviations, caused, for example, by temperature effects, by the dynamic operation, etc., an enable logic circuit is provided in a circuit unit370, which monitors typical parameters for enabling the adaptation. These parameters of the internal combustion engine and/or of the injector include, for example, the temperature of the internal combustion engine and/or the common-rail pressure and/or the steady state of the voltage control and/or the state of the charging time control and/or the steady state of other secondary feedback control circuits and/or the number of injections and/or the control (activation) duration and/or the injection sequence per combustion cycle, i.e., effectively, the injection pattern (preinjection(s), main injection, post injection(s)). A steady state of the voltage control is verified, for example, by comparing quantities Usetpointcorrand Ucontrol. Only if Usetpointcorrand Ucontrolconform, are PI controllers331,332. . .33nenabled by circuit unit370, so that difference ΔUactualmay be adapted to ΔUsetpoint, as described above, thereby making it possible for the voltage requirement drift to be adapted.

If, on the other hand, the test reveals that the actuator voltage control is not steady-state, thus, when Usetpointcorrdeviates from Ucontrol, PI controllers331,332, . . .33nare deactivated by enable-logic circuit unit370, and correction values S1, S2, . . . Snremain unchanged, i.e., are, to a certain extent, frozen. The setpoint voltage value continues to be corrected at switching points341/342using values S1, S2, . . . Snlearned up to that point. Such a “freezing” of the correction values is possible since the injector drift occurs very slowly.

The method described above may initially be carried out only at one operating point (common-rail pressure), and the acquired correction values used for all operating points. To enhance the accuracy, the method may also be carried out at a plurality of different operating points (common-rail pressures).

Moreover, it should be pointed out that the comparison of an injector-specific correction value S1, S2, . . . S3, which represents a measure of the deviation of the voltage requirement from the standard, to a predefinable threshold value, may additionally be used for diagnostic purposes. In this manner, it is possible to diagnose the system including actuator2, coupler4, and the control valve, which is constituted of valve-closure member12.