Patent Application
20050072854
The invention relates to a method for controlling a piezo-actuated fuel-injection valve.
The fuel injection procedure in diesel engines is normally carried out in several stages, with one or more advanced injections or afterinjections being associated with each main injection, with the amount of injected fuel being small compared with the amount for the main injection, to achieve a smooth combustion characteristic.
For a precise dosing of the fuel quantities, particularly the small amounts and for optimization of the injection timepoints, fast-switching valves are necessary, with piezo-actuated fuel-injection valves being increasingly used.
Because of the small maximum longitudinal variation of the piezo-elements (stacks) used, the piezo actuator operates a hydraulic servo-valve that then moves the main valve. By means of an electronic control device, the electrical control of the piezo actuator is performed in such a way that the required fuel quantity is injected.
Because it is not possible to detect fuel quantities or mechanical movements in the injection valve, the duration of application and the amplitude of the electrical control signals during the injection of small amounts of fuel are designed so that a reliable injection takes place. Because of safety reservations with regard to pressure fluctuations in the fuel supply line, parameter tolerances of the system and the wide operating temperature range, fuel quantity overdosing is therefore entailed, particularly during advanced injection and afterinjection. Up to now, inference was drawn for this purpose from the charge fed to the piezo actuator or energy exerted on the piezo-actuation.
From DE 196 44 521 A1, a method is known for controlling a capacitive correcting element of a fuel-injection valve, whereby an energy quantity allocated to this stroke is applied to achieve a constant stroke.
The object of the invention is to provide a method, by means of which it is possible to determine whether advanced injection, main injection or afterinjection of the fuel is taking place and that also enables a more accurate determination of the amount of fuel in each advanced injection, main injection and afterinjection.
The object can be achieved by a method for control of a piezo-actuated fuel-injection valve during advanced, main or after injection, by means of a piezo actuator and a servo-valve actuated by same, to detect an opening of the servo-valve and determine the injection duration, comprising the steps of:
The object can also be achieved by a method for control of a piezo-actuated fuel-injection valve comprising the steps of:
The calculation can be performed with a non-linear actuator model. A first and second time window can be provided, the variations in longitude at the start and end of the first time window can determine a first tangent, and the variations in longitude at the start and at the end of the second time window can determine a second tangent and wherein both tangents intersect at a timepoint. The timepoint can be assessed as the opening point of the servo-valve if the tangent has a definably steeper angle compared with the abscissa than the tangent, and otherwise a faulty injection can be detected. At a timepoint assessed as the opening timepoint of the servo-valve, a tolerance band between an upper limit and a lower limit can be specified for the first time derivation of force, and the time in which the value of the first derivation moves within this tolerance band after timepoint can be assessed as the injection duration. The timepoints defining both time windows or the limits of the tolerance band can be stored in maps as timepoints allocated at least to the energy applied to the piezo actuator, the fuel pressure in the rail or the actuator temperature. The timepoints, stored in the maps, that determine the time windows can also be adapted relative to the timepoint determined in the particular proceeding earlier injection operation.
The method in accordance with the invention is based on the detection and assessment, with the aid of a non-linear actuator model, of the longitudinal variations of, and the forces exerted by, the piezo actuator from the electrical signals (of the current applied to the piezo actuator and the voltage established therefrom) during a control input, and on an adaptive method for evaluating the variations in longitude at the piezo actuator and in the forces occurring on it.
The actuator model contains the non-linear relationships between load, voltage and mechanical deflection, and also parameters relative to the working point. The actuator model also takes account of the dielectric hysteresis of the piezo actuator. This enables the actuator model to draw conclusions regarding the mechanical variables from the electrical variables and the simulation of the piezo actuator in the area of pulse-type deflection.
It is thus possible to reliably determine a faulty or correct injection function and the duration (amount) of injection of the injection valve and to adapt the control signals so that the required minimum fuel injection takes place without overdosing.
An exemplary embodiment in accordance with the invention is explained in more detail in the following with the aid of schematic drawings.
The drawings are as follows:
The dotted curve so shows, to differentiate from curve s1, the main pattern of the beginning of the longitudinal variation (expansion) of a piezo actuator during an incorrect injection operation. The curve increases as a flat curve without showing a kink, reaches a maximum and then drops again, i.e. the lost motion is not entirely measured. The maximum of the curve of the longitudinal expansion of a piezo actuator depends mainly on the energy applied to the piezo actuator, i.e. the greater the amount of energy the greater the longitudinal expansion s.
The beginning of the opening of the servo-valve therefore lies approximately at timepoint tA of the curve s1. This opening of the servo-valve is an absolute precondition for a succeeding injection. The actual injection takes place with a distinct delay because as the servo-valve opens the pressure in the valve chamber slowly reduces and only then does the actual injection valve open. The presence of the “kink” in the travel is an indication that there is sufficient energy in the piezo to open the servo-valve.
The method in accordance with the invention for determining the opening timepoint tA of the servo-valve is explained in the following. The timepoint tA varies, for example, according to the energy E applied to the piezo actuator and the fuel pressure in the rail p acting against it, and also the actuator temperature T, etc. It is thus empirically known.
By means of maps that take account of these relationships, a first time window W1 (determined by timepoints t1 and t2) just before timepoint tA [tA=f (E, p, T . . . )] and a second time window W2 (determined by timepoints t3 and t4) just after this timepoint tA are defined.
A first straight-line—tangent t1—is determined by the longitudinal variations at timepoints t1 and t2 and a second straight-line—tangent T1′—is determined by the variations in longitude at timepoints t3 and t4. Both these tangents, shown in bold in
Due to wear, the position of timepoint tA can shift over a long period. Therefore, it is provided that timepoints t1 to t4, that determine time windows W1 and W2, stored in the maps are also stored relative, i.e. adapted, to the timepoint tA determined in the preceding earlier injection operation.
A determination of the injection duration takes place only if a correct injection with a defined start of injection was determined beforehand.
The fuel injection duration D is determined by means of the force F acting on the piezo actuator. This force F is determined, as the longitudinal variation s, from the electrical signals (from the current applied to the piezo actuator and the increase in voltage resulting therefrom), with the aid of the non-linear actuator model already mentioned.
a shows the main pattern of the force F1 acting on a piezo actuator during a fuel injection operation or during a faulty injection (F0, shown dotted).
The force F rises at the start of the control operation and reaches its maximum approximately at timepoint tA, then changes to an approximately horizontal pattern (in the event of a faulty injection it reduces slowly) and on shutoff first jumps to the negative and then jumps to the positive, before it again becomes zero.
The first time derivation dF1/dt of the force F is used in accordance with the invention to determine the injection duration D. The pattern of the first derivation dF1/dt of the force F (
With a correct injection operation, this derivation dF1/dt reaches its maximum da where the force F1 rises most steeply, then becomes negative when the force drops off and reaches a plateau around the value zero da where the force F1 has a horizontal pattern, before it first becomes negative on shut-off, and then positive finally goes to zero.
In the event of a faulty injection, the derivation dF0/dt (shown dotted in
In accordance with the invention, a tolerance band for the value of the first derivation is placed in the area of the aforementioned plateau, with an upper value g1 (for positive dF/dt) and a lower value g2 (for negative dF/dt). Both these values are shown dotted in
As long as the first derivation dF1/dt, after timepoint tA, is within this tolerance band, determined between timepoints t5 and t6 in
In the manner described, it can be determined for each control input of a piezo actuator, for advanced, main or afterinjection, whether a correct or faulty injection takes place, when the injection begins and how long it persists.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10143501.0 | Sep 2001 | DE | national |
This application is a continuation of copending International Application No. PCT/DE02/03226 filed Sep. 2, 2002 which designates the United States, and claims priority to German application no. 101 43 501.0 filed Sep. 5, 2001.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/DE02/03226 | Sep 2002 | US |
| Child | 10795015 | Mar 2004 | US |
