Method of operating a piezoelectric actuator

1. A method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, using a discharge apparatus to reduce the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection.

2. The method as claimed in claim 1 , further comprising increasing the voltage across the actuator after it has been reduced at the second rate, prior to a further subsequent injection.

3. A method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, reducing the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection;

including reducing the voltage across the actuator at the second rate as a function of time that has elapsed since the initial injection.

4. A method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, reducing the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection:

including reducing the voltage across the actuator at the second rate a predetermined time after the initial injection.

5. The method as claimed in claim 1 , wherein the initial injection is an initial injection of an injection sequences and the subsequent injection is a subsequent injection of the same injection sequence.

6. The method as claimed in claim 4 , wherein the initial injection is a pilot injection of the injection sequence, and the subsequent injection is a main injection of the same injection sequence.

7. The method as claimed in claim 4 , wherein the initial injection is a pilot injection of the injection sequence, and the subsequent injection is a further pilot injection of the same injection sequence.

8. The method as claimed in claim 4 , wherein the predetermined time is selected to be greater than the time period for the injection sequence to complete.

9. A method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, reducing the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection;

including reducing the voltage across the actuator at the second rate as a function of the voltage across the actuator.

10. A method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, reducing the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection;

further comprising reducing the voltage across the actuator at the second rate through a resistance associated with the injector.

11. A method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, reducing the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection;

further comprising reducing the voltage across the actuator at the second rate through a resistance across terminals of the actuator.

12. A method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, reducing the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection;

further comprising reducing the voltage across the actuator at the second rate actively under the control of an engine control system.

13. A computer program on a computer readable memory or storage device for execution by a computer, the computer program comprising a computer program software portion that when executed, is operable to implement a method of operating an injector having an injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate an initial injection,

increasing the voltage across the actuator in order to terminate the injection, and

once the initial injection has terminated and before a subsequent injection is initiated, using a discharge apparatus to reduce the voltage across the actuator at a second rate, that is lower than the first rate, so as to de-energize the actuator but without initiating an injection.

14. A data storage medium having the computer program software portion of claim 13 stored thereon.

15. A microcomputer provided with the data storage medium of claim 14 .

16. A method of operating a fuel injector having an injector valve needle, a valve needle seat engageable with the injector valve needle and a piezoelectric actuator for controlling movement of the injector valve needle relative to the valve needle seat, the method comprising:

reducing the voltage across the actuator at a first rate in order to initiate a first fuel injection event;

increasing the voltage across the actuator in order to terminate said first fuel injection event; and

once the first fuel injection event has terminated and before a second fuel injection event is initiated, using a discharge apparatus to reduce the voltage at a second rate, said second rate being too slow to initiate a fuel injection event, and wherein said second rate is a function of the voltage across the actuator.

17. The method as claimed in claim 16 , wherein said first fuel injection event and said second fuel injection event are not consecutive fuel injection events of the same fuel injection sequence.