Abstract
A method for controlling a piezoelectric actuator of a fuel injector of an internal combustion engine of a vehicle, the actuator acting on valve elements to open or close the injector, respectively enabling or stopping the injection of fuel into a combustion chamber of the engine, includes the steps of: applying to the actuator a first nominal electric charge required to open the injector in accordance with the torque requested and the engine speed, to open the valve elements for fuel injection, instructing the closure of the injector to stop the fuel injection, by applying an electric discharge to the actuator to close the valve elements, the method applied from an on-board engine control unit during operation, and including applying to the actuator between these 2 steps at least one second electric charge to polarize the actuator during an opening phase of the injector and during fuel injection.
Claims
1. A method for controlling at least one piezoelectric actuator of a fuel injector of an internal combustion engine of a vehicle, the at least one piezoelectric actuator acting on a valve in order to open or close the injector, respectively enabling or stopping the injection of fuel into a combustion chamber of the engine, comprising: applying to the piezoelectric actuator a first nominal electric charge that is a nominal control charge which is required to open the injector, in accordance with the torque requested and the engine speed, for a given electrical control time to open the valve of the injector for an injection of a quantity of fuel into the combustion chamber corresponding to the electrical control time; instructing the closure of the injector in order to stop the injection of the quantity of fuel, by an application of an electric discharge to the piezoelectric actuator in order to close the valve; and applying to the piezoelectric actuator at the nominal control charge, after the application thereof and before the instructing the closure of the injector, at least one second electric charge that is a polarization charge, in addition to the nominal control charge, to polarize the piezoelectric actuator during an opening phase of the injector and during the injection of the fuel into the combustion chamber while not substantially modifying the quantity of fuel injected for the given electrical control time, wherein the control method is applied from an engine control unit on-board the vehicle during operation.
2. The method as claimed in claim 1, wherein the polarization charge which is defined by a corresponding current profile applied to the piezoelectric actuator is separated from the nominal control charge over time.
3. The method as claimed in claim 1, wherein the first electric charge and second electric charge are obtained by the respective application of first and second electrical voltages that are a nominal charge voltage and a polarization voltage, respectively, the second electrical voltage being greater than the first electrical voltage.
4. The method as claimed in claim 3, further comprising applying the voltage to the piezoelectric actuator over time, wherein a charge level is provided in the development of the voltage applied to the piezoelectric actuator over time, the charge level being located at the level of the first electrical voltage or the nominal charge voltage.
5. The method as claimed in claim 1, wherein the injection of fuel through the injector is carried out several times involving at least one main injection, the charge or polarization voltage being applied during the main injection.
6. The method as claimed in claim 1, wherein the instructing the closure of the injector includes an application of a first electric discharge of the piezoelectric actuator up to the nominal control charge thereof, followed by a second electric discharge of the actuator until the valve means is closed.
7. The method as claimed in claim 6, wherein the first and second electric discharges of the piezoelectric actuator are separated over time.
8. The method as claimed in claim 3, wherein the instructing the closure of the injector includes an application of a first electric discharge of the piezoelectric actuator up to the nominal control charge thereof, followed by a second electric discharge of the actuator until the valve means is closed, and the first electric discharge of the piezoelectric actuator up to the nominal control charge is obtained by application of a first electric discharge current which reduces the voltage at the terminals of the piezoelectric actuator as far as the nominal charge voltage, and wherein the second electric discharge of the actuator is obtained by an application of a second electric discharge current.
9. The method as claimed in claim 8, further comprising applying the voltage applied to the piezoelectric actuator over time during an electric discharge thereof, wherein a discharge level is provided in the development of the voltage applied to the piezoelectric actuator over time.
10. The method as claimed in claim 9, wherein the discharge level in the development of the voltage applied to the piezoelectric actuator over time during the electric discharge thereof is located at a voltage which is lower than the nominal charge voltage in order to take into account a phenomenon of hysteresis of the piezoelectric actuator.
11. The method as claimed in claim 1, wherein the polarization charge is deactivated above a predetermined value threshold for torque or rotation speed of the engine.
12. The method as claimed in claim 3, wherein an increase of electrical voltage between the nominal charge voltage and the polarization voltage is between 5 and 40 volts in order to reach a maximum polarization voltage value of 140 volts.
13. The method as claimed in claim 2, wherein the first electric charge and second electric charge are obtained by the respective application of first and second electrical voltages that are a nominal charge voltage and a polarization voltage, respectively, the second electrical voltage being greater than the first electrical voltage.
14. The method as claimed in claim 4, wherein the instructing the closure of the injector includes an application of a first electric discharge of the piezoelectric actuator up to the nominal control charge thereof, followed by a second electric discharge of the actuator until the valve means is closed, and the first electric discharge of the piezoelectric actuator up to the nominal control charge is obtained by application of a first electric discharge current which reduces the voltage at the terminals of the piezoelectric actuator as far as the nominal charge voltage, and wherein the second electric discharge of the actuator is obtained by an application of a second electric discharge current.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Other features and advantages will be appreciated from the following description of two examples of an embodiment of a method according to the invention, together with the appended drawings, the examples being given by way of non-limiting illustration.
(2) FIGS. 1a and 1b, which have already been discussed above, show two synchronized schematic graphs of an example of a method for controlling an example of a piezoelectric actuator, according to the prior art, relating to, respectively: a profile example of the voltage at the terminals of the piezoelectric actuator in accordance with the time for which the injector is opened, an example of a charge/discharge current intensity profile passing through the piezoelectric actuator as a function of the time.
(3) FIG. 2a shows two synchronized schematic graphs of the voltage profile at the terminals of the piezoelectric actuator as a function of the time for which the injector is opened, according to a first example (broken line) and a second (continuous line) example of an embodiment of a control method according to the invention for a piezoelectric actuator.
(4) FIG. 2b shows two schematic graphs synchronized with FIG. 2a, of a first (broken line) and a second (continuous line) intensity profile of the charge/discharge current passing though the piezoelectric actuator as a function of time, corresponding to the first and second voltage profile examples of FIG. 2a, respectively.
(5) FIG. 3a shows a schematic graph of the voltage profile at the terminals of the piezoelectric actuator as a function of the time for which the injector is open, according to a third example of an embodiment of a method for controlling a piezoelectric actuator according to the invention.
(6) FIG. 3b shows a schematic graph which is synchronized with FIG. 3a of the intensity profile of the charge/discharge current passing through the piezoelectric actuator as a function of the time corresponding to the third voltage profile example of FIG. 3a.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(7) FIGS. 2a and 2b will now be described in relation to the first embodiment example (broken line).
(8) It should be noted that this first example in FIGS. 2a and 2b uses the same basic schematic graphs as FIGS. 1a and 1b according to the prior art, respectively, to which the first example of a method according to the invention has been applied, as will be described in greater detail below. The corresponding basic profile examples of electrical voltage and current intensity corresponding to the opening and closure of the injector are therefore identical in FIGS. 2a and 2b and in FIGS. 1a and 1b, respectively, setting out the same first basic example for controlling a piezoelectric actuator in order to open and close an injector. These basic signals for instructing a piezoelectric actuator will therefore not be described in greater detail in the context of the description of the first example of an embodiment of the method according to the invention with reference to FIGS. 2a and 2b, the reader being referred back to the description given above of FIGS. 1a and 1b for these basic signals. In FIGS. 2a and 2b, two vertical broken lines 5 and 6 have been added across both FIGS. 2a and 2b and correspond to two different times t.sub.5 and t.sub.6 on the timescale t, respectively, each of these two times t.sub.5 and t.sub.6 being the same for the two graphs 2a and 2b, the times t.sub.5 and t.sub.6 being between the times t.sub.2 and t.sub.3 as explained in detail below.
(9) Furthermore, the electronic control system of a piezoelectric actuator is also known to the person skilled in the art and will not be described in greater detail here. For the application of the method for controlling the piezoelectric actuator according to the invention, the electronic control system as known may be suitable. The method for controlling the piezoelectric actuator according to the invention can be implemented using control software which will be implemented in the engine control unit of the vehicle.
(10) The first method example schematically illustrated in FIGS. 2a and 2b is an example of a method for controlling at least one piezoelectric actuator for injecting fuel for an internal combustion engine of a vehicle, the at least one piezoelectric actuator acting on a valve means in order to open or close the injector, permitting or stopping the injection of fuel into a combustion chamber of the engine, respectively. It is specified in this instance that the expressions opening and closure of the valve means of the injector and opening and closure of the injector are considered to have an equivalent meaning. It should be noted that only the signals for controlling voltage and charge current intensity applied to and passing through the piezoelectric actuator have been illustrated in the Figures, the method according to the invention being able to be applied to an internal combustion engine of known type which is provided with injectors also of known type, which have therefore not been illustrated.
(11) The control method according to FIGS. 2a and 2b is applied from an engine control unit ECU (not illustrated) of known type which is on-board the vehicle during operation, for example, using an item of control software which is implemented in the engine control unit, and comprises the following steps involving: applying in known manner to the piezoelectric actuator a first nominal electric charge Qc which is required to open the injector, called the nominal control charge Qc, between the times t.sub.1 and t.sub.2, as a function of the torque requested and the engine speed, for example, according to a predetermined conventional voltage gradient in the engine control unit, in order to open the valve means of the injector for an injection of fuel into the combustion chamber, as illustrated in FIG. 2b, applying according to the invention to the piezoelectric actuator on the nominal control charge Qc, from a time t.sub.5 which follows t.sub.2, after the application thereof and before the step involving instructing a closure of the injector, therefore before the time t.sub.3, a second electric charge Qp, called a polarization charge Qp, in addition to the nominal control charge Qc, in order to polarize the piezoelectric actuator during an opening phase of the injector and during the injection of the fuel into the combustion chamber, as illustrated in FIG. 2b, then instructing the closure of the injector at the time t.sub.3 in order to stop the injection of fuel, by applying an electric discharge Qdc1, Qdc2 to the piezoelectric actuator between the times t.sub.3 and t.sub.4, for example, as illustrated in FIG. 2b.
(12) As illustrated in FIG. 2b, the polarization charge Qp, defined by a corresponding current profile applied to the piezoelectric actuator, is advantageously decoupled from the nominal control charge Qc, which can be seen in the example in that the end of the control charge Qc and the beginning of the polarization charge Qp are separated by a time t.sub.5t.sub.2 not equal to zero.
(13) As illustrated in FIG. 2a, the first electric charge Qc and second electric charge Qp are, for example, obtained by means of the application of a first electrical voltage Uc and second electrical voltage Up, called nominal charge voltage Uc and polarization voltage Up of the piezoelectric actuator, respectively, the electrical polarization voltage being greater than the electrical nominal charge voltage Uc.
(14) It should be noted that, in the examples of embodiments according to FIG. 2a, there is a level 7 in the development of the charge voltage applied to the piezoelectric actuator over time, this level 7 preferably being located in the region of the first electrical voltage Uc or nominal charge voltage. This voltage level 7 which represents the period of time which elapses between the end of the application of the electric charge Qc for instructing the opening of the actuator and the beginning of the application of the polarization charge, that is, a time which is equal to t.sub.5t.sub.2, may be between 0 (excluded) and a few microseconds, or may form a more marked level in the order of several microseconds, for example, from 10 to 100 s, in accordance with the conventional time available for applying a polarization charge during the opening of the injector, given by the engine control unit. The minimum time is preferably defined so that the charges Qc and Qp are decoupled, that is to say, they are separated over time.
(15) Furthermore, the voltage gradients applied to the terminals of the piezoelectric actuator, between the times t.sub.1 and t.sub.2, on the one hand, for instructing the opening of the injector and after the time t.sub.2, on the other hand, in order to polarize the actuator, are illustrated in FIG. 2a as having the same value or substantially the same value. However, it should be noted that these gradients may be different from each other.
(16) It should be noted that FIGS. 2a and 2b illustrate, for example, a main fuel injection, given that the method according to the invention may be applied to a cycle which comprises multiple injections which are in particular carried out several times, comprising at least one main injection, for example, at least one pilot injection and at least one main injection, the charge Qp or polarization voltage Up preferably being applied in this instance during the main injection.
(17) The polarization voltage Up at the terminals of the piezoelectric actuator remains constant, in general as a result of the fact that the piezoelectric element, since it behaves as a capacitor, retains the voltage applied to the terminals thereof until the electric discharge of the actuator in order to close the injector or until the electric discharge of the polarization, that is to say, until the time t.sub.3 for the first instance (second example) as set out below in detail.
(18) According to the first example of an embodiment illustrated with broken lines in FIGS. 2a and 2b, the step involving instructing the closure of the injector involves the application of a first electric discharge Qdp of the piezoelectric actuator up to the nominal control charge Qc thereof, or substantially this nominal charge Qc, followed by a second electric discharge Qdc1 of the actuator until closure of the valve means, as illustrated in the broken line portion of FIG. 2b.
(19) In this first example, the first discharge Qdp is applied before the time t.sub.3, or before the closure of the injector, so that the first electric discharge Qdp and second electric discharge Qdc1 of the piezoelectric actuator are decoupled as illustrated in FIG. 2b. In the example, the decoupling of the discharges Qdp and Qdc1 can be seen by the existence of a time period which is not equal to zero between the time t.sub.6 corresponding to the end of the polarization discharge Qdp and the subsequent time t.sub.3 which corresponds to the beginning of the control discharge Qdc1 for closure of the injector.
(20) As illustrated in FIG. 2a with broken lines so as to correspond in a synchronized manner to FIG. 2b, the first electric discharge Qdp of the piezoelectric actuator as far as the nominal control charge Qc is advantageously constituted by a first electric discharge current which reduces, for example, the voltage at the terminals of the piezoelectric actuator as far as the nominal charge voltage Uc, the second electric discharge Qdc1 of the actuator being constituted by a second electric discharge current until the piezoelectric actuator returns to the initial length thereof which brings about the closure of the injector. The first and second electric discharge currents can, for example, be obtained by short-circuiting the terminals of the piezoelectric actuator.
(21) It should be noted in FIG. 2a and from the broken line of the first example that there is a level 8 in the development of the voltage applied to the piezoelectric actuator over time during an electric discharge thereof, this level 8 being, for example, located in the region of the nominal charge voltage Uc, as illustrated in FIG. 2a. This voltage level 8 illustrating the period of time which elapses between the time t.sub.6 of the end of the application of the electric polarization discharge Qdp of the actuator and the subsequent time t.sub.3 at the beginning of the application of the control discharge Qdc1, that is, a time which is equal to t.sub.6t.sub.3, may be between 0 (excluded) and a few microseconds, or may form a more marked level in the order of a few microseconds, for example, from 10 to 100 s in accordance with the conventional time available for applying the control discharge for closure of the injector, given by the engine control unit which imposes the opening time of the injector. The minimum time is preferably defined so that the electric discharges Qdp and Qdc1 are decoupled, that is to say, they are separated over time.
(22) Furthermore, the gradients of voltage drop applied to the piezoelectric actuator in FIG. 2a (broken lines), for the discharge of the polarization, on the one hand (before time t.sub.3), and for the discharge of the actuator in order to close the injector (from time t.sub.3), are illustrated in FIG. 2a as having the same value or substantially the same value. However, it should be noted that these gradients may be different from each other. Furthermore, the discharge gradient(s) may be different from the charge gradients.
(23) FIGS. 2a and 2b will now be described in relation to the second embodiment example (continuous line).
(24) It should be noted that this second example shows in FIGS. 2a and 2b a portion common to the first example described above, which involves the control of the opening charge Qc of the injector and the control of the polarization charge Qp, as illustrated. The difference involves a different method for controlling the discharge of the piezoelectric actuator in order to close the injector, after having applied the polarization charge Qp. This difference more specifically involves the absence of a voltage level in the discharge of the actuator, resulting from an electric discharge Qdc2 of the actuator in this second example which is produced in one step from the time t.sub.3 to the time t.sub.4, as illustrated in FIG. 2b. In FIG. 2a, it can be seen that the discharge voltage drops between the times t.sub.3 and t.sub.4, in order to reach a value zero at the time t.sub.4, corresponding to the closure position of the injector, according to a constant gradient. In this second example, the first discharge Qdp of the first example therefore does not exist, and only the discharge Qdc2 is applied in this instance at the time t.sub.3 from the polarization voltage Up and illustrates the closure of the injector in a single instruction from the polarization voltage Up.
(25) The application of this second embodiment is dependent on the time available for opening the injector and where applicable the level of noise permissible for the closure of the injector. This second embodiment where applicable allows a longer period of time to be made available for maintaining the actuator at the polarization voltage Up.
(26) FIGS. 3a and 3b will now be described in relation to the third preferred example of an embodiment of a method for controlling a piezoelectric actuator according to the invention.
(27) It should be noted that this third example shows in FIGS. 3a and 3b a portion which is common to the first example described above with reference to FIGS. 2a and 2b, which involves control of the opening charge Qc of the injector and control of the polarization charge Qp, as illustrated.
(28) This third example is also similar to the first example in FIGS. 2a and 2b with regard to the discharge of the piezoelectric actuator in that this discharge also comprises in this third example a level 8 in the development of the discharge voltage, as illustrated.
(29) In FIGS. 3a and 3b, the reference numerals used are identical to those of FIGS. 2a and 2b, first example, for means, elements or events which are similar.
(30) The difference from the first example illustrated in FIGS. 2a and 2b involves a different method for controlling the discharge of the piezoelectric actuator in order to close the injector, after having applied the polarization charge Qp. This difference involves more specifically a discharge level 8 which is not located in the region of the control charge voltage Uc
(31) The discharge voltage level 8 according to this third example is advantageously located at a voltage level which is lower than the level of the control charge voltage Uc in order to take into account a hysteresis phenomenon of the piezoelectric actuator.
(32) This is because, according to the first example, an iso-voltage discharge level 8 is carried out at the terminals of the piezoelectric actuator, with the charge level 7; that is to say, the length of the piezoelectric actuator at the discharge level is not the same as that corresponding to the charge level 7, taking into account the extension hysteresis of the piezoelectric actuator, which is dependent on the actuator itself. This length at the discharge level is greater.
(33) The preferred third example in FIGS. 3a and 3b carries out a discharge level 8 with iso-displacement of the piezoelectric actuator with the charge level 7 corresponding to the control charge Qc for opening the injector. This means that the voltage level of the discharge level 8 according to this third example is less than the iso-voltage discharge voltage level of the first example, by a value given by the actual hysteresis line of the piezoelectric actuator used. This characteristic of a discharge level 8 with iso-displacement with the charge level 7 allows the time to be improved for the piezoelectric actuator to return to its original size which it had before the control charge Qc was applied and therefore allows the control of the closure of the injector to be improved, in particular when the method according to the invention is interfaced with an existing control mapping of the injector. In this manner, this characteristic of a discharge level 8 with iso-displacement allows the original mapping of the injector implemented in the engine control unit not to be modified and enables the method according to the invention to be implemented using an additional item of software which will be implemented in the engine control unit without modification of this mapping which already exists.
(34) In practice, the instruction of the discharge of the piezoelectric actuator from the polarization voltage Up will advantageously be carried out by means of learning of the hysteresis line as follows: a first discharge Qdp3 of the actuator will be instructed, from the knowledge of the hysteresis line thereof so that the piezoelectric actuator returns to a length which is equal or substantially equal to the length which corresponds to the control charge Qc, a second discharge Qdc3 will then be instructed so that the piezoelectric actuator returns to the initial length before the control charge Qc was applied, this second discharge advantageously corresponding to the normal or nominal electric discharge of the piezoelectric actuator for the electrical time applied to the actuator.
(35) With the exception of the voltage level of the discharge level 8, what has been set out above relating to the discharge level 8 in relation to the first example applies to the discharge level 8 of this third example.
(36) In FIG. 3b, the distribution of the electric discharges Qd can be seen, more specifically the polarization discharge Qdp3 and control discharge Qdc3 which are applied to the piezoelectric actuator. In the example illustrated, the polarization discharge Qdp3 is greater than the polarization discharge Qdp of the first example (FIG. 2b) and the control discharge Qdc3 is lower than the control discharge Qdc1 of the first example (FIG. 2b). This control discharge Qdc3 advantageously corresponds to the discharge programmed in the initial control mapping of the injector in accordance with the electrical control time of the injector.
(37) Preferably, the polarization charge is applied in a permanent and continuous manner during the operation of the engine in the vehicle in order to ensure a unique polarization voltage over the range of torque/rotation speed values of the engine. Alternatively, the polarization charge can be deactivated above a predetermined threshold for a value of torque or rotation speed of the engine corresponding to control voltage values of the piezoelectric actuators close to the polarization voltage.
(38) By way of example, the increase of electrical voltage between the nominal charge voltage Uc and the polarization voltage Up may be between 0 (exclusive) and 40 volts, in order to reach a maximum polarization voltage value Up in the order of 140 volts, for example, the range used of control voltages Uc for the piezoelectric actuator in accordance with the engine speed and the engine torque required substantially being between 100 and 140 volts in the example.
(39) The polarization charge as described with reference to FIGS. 2a and 2b, 3a and 3b, is, for example, advantageously applied in a continuous manner to all the injectors of the engine of the vehicle, each time an injector is opened for a main injection of fuel.
