Controller for a fuel injector and a method of operating a fuel injector

1. A controller for controlling the operation of a fuel injector having a piezoelectric actuator, the actuator being operable by an application of a voltage drive profile across the actuator, the controller comprising:

inputs for receiving data relating to one or more engine parameters;

a processor for determining a voltage drive profile for controlling the actuator in dependence upon the one or more engine parameters, the voltage drive profile being arranged to comprise a first pilot injection event, a second pilot injection event, a main injection event, an activating voltage component to initiate the second pilot injection event and a deactivating voltage component to terminate the second pilot injection event, the activating and deactivating voltage components being separated by a time interval T ON ; and

outputs for outputting the voltage drive profile as determined by the processor to the actuator,

wherein the processor is arranged to set the time interval T ON greater than or equal to a predetermined pressure wave time period (T P ) of a pressure wave cycle within the injector.

2. A controller as claimed in claim 1 , wherein T ON >T P .

3. A controller as claimed in claim 1 , wherein T ON =nT P , where n=1, 2, 3.

4. A controller as claimed in claim 1 , wherein the processor is arranged to reduce peak voltage levels within a voltage drive profile as T ON is varied so as to maintain a fixed fuel delivery amount through the injector.

5. A controller as claimed in claim 1 , wherein predetermined pressure wave time period values, in dependence upon the one or more engine parameters, are stored in the controller.

6. A controller as claimed in claim 1 , further comprising a function map of T P in dependence upon engine parameters and wherein the controller is arranged to refer to the function map when setting T ON .

7. A controller as claimed in claim 6 , further comprising a data store for storing the function map.

8. A controller for controlling the operation of a fuel injector having a piezoelectric actuator, the actuator being operable by an application of a voltage drive profile across the actuator, the controller comprising:

inputs for receiving data relating to one or more engine parameters;

a processor for determining a voltage drive profile for controlling the actuator in dependence upon the one or more engine parameters, the voltage drive profile being arranged to comprise a first pilot injection event, a second pilot injection event, a main injection event, an activating voltage component to initiate the second pilot injection event and a deactivating voltage component to terminate the second pilot injection event, the activating and deactivating voltage components being separated by a time interval T ON ;

outputs for outputting the voltage drive profile as determined by the processor to the actuator; and

a function map of a predetermined pressure wave time period (T P ) of a pressure wave cycle within the injector in dependence upon engine parameters, wherein

(i) the processor is arranged to set the time interval T ON greater than or equal to the predetermined pressure wave time period (T P );

(ii) the processor is arranged to set T ON =nT P , where n=1, 2, 3 . . . ; and

(iii) the controller is arranged to refer to the function map when setting T ON .

9. A method of operating a fuel injector having a piezoelectric actuator operable by applying an activating voltage level across the actuator to initiate an injection event and a deactivating voltage across the actuator to terminate an injection event, the method comprising:

determining a voltage drive profile for controlling the actuator, the voltage drive profile comprising a first pilot injection event, a second pilot injection event, and a main injection event;

applying an activating voltage to the actuator so as to initiate the second pilot injection event, and, after a predetermined time interval (T ON );

applying a deactivating voltage to the actuator so as to terminate the second pilot injection event;

wherein the predetermined time interval (T ON ) is selected to be greater than or equal to a predetermined pressure wave time period (T P ) of a pressure wave cycle within the injector.

10. A method as claimed in claim 9 , wherein prior to the first applying step, the pressure wave time period of a pressure wave cycle within the injector is measured on a test rig.

11. A method as claimed in claim 10 , wherein the pressure wave time period is measured for a range of engine operating conditions and the measured pressure wave time periods are stored in a function map.

12. A method as claimed in claim 9 , wherein prior to the first applying step, the pressure wave time period of a pressure wave cycle within the injector is calculated based on the dimensions of the fuel injector and associated fuel injector system.

13. A method as claimed in claim 12 , wherein the pressure wave time period is calculated for a range of engine operating conditions and the calculated pressure wave time periods are stored in a function map.

14. A controller for controlling the operation of a fuel injector having a piezoelectric actuator, the actuator being operable by an application of a voltage drive profile across the actuator, the controller comprising:

inputs for receiving data relating to one or more engine parameters;

a processor for determining a voltage drive profile for controlling the actuator in dependence upon the one or more engine parameters, the voltage drive profile being arranged to comprise a first pilot injection event, a second pilot injection event, a main injection event, an activating voltage component to initiate the second pilot injection event and a deactivating voltage component to terminate the second pilot injection event, the activating and deactivating voltage components being separated by a time interval T ON ;

outputs for outputting the voltage drive profile as determined by the processor to the actuator,

wherein the processor is arranged to set the time interval T ON substantially equal to a predetermined pressure wave time period (T P ) of a pressure wave cycle within the injector.