Method for controlling at least one piezoelectric actuator of a fuel injector of an internal combustion engine

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.