Method of operating a fuel injector

1. A method of operating a fuel injector having a piezoelectric actuator operable by applying a drive pulse thereto, wherein the drive pulse has a frequency domain signature, the method including;

determining at least one resonant frequency of an injector installation in which the injector is received, in use; and

modifying the drive pulse such that a maximum of the frequency domain signature thereof does not coincide with the determined at least one resonant frequency of the injector installation, wherein modifying the drive pulse includes

determining a demanded fuel volume to be delivered during the drive pulse based on an engine operating condition,

determining a tuned injector on time value (TON_TUNED) based on the demanded fuel volume and the at least one resonant frequency of the injector installation,

determining a peak discharge/charge current amplitude value (ITUNED) and a discharge time period (TDISCHARGE) based on the demanded fuel volume and the tuned injector on time value (TON_TUNED), and

operating the fuel injector according to the determined values of the tuned injector on time value (TON_TUNED), the peak discharge/charge current amplitude value (ITUNED), and the discharge time period (TDISCHARGE).

2. The method of claim 1 , wherein, in order to reduce the volume of fuel delivered by the injector during a first series of successive injection events, the method includes reducing the tuned injector on time value (TON_TUNED) to a predetermined injector on time threshold value (TON_ 6 ) and, for subsequent reductions in fuel delivery volume, holding the tuned the injector on time value (TON_TUNED) substantially constant and thereafter reducing the discharge time period (TDISCHARGE).

3. The method of claim 2 , wherein, for a subsequent series of successive injection events, the method further includes holding the discharge time period (TDISCHARGE) substantially constant and reducing the peak discharge/charge current amplitude value (ITUNED) to a predetermined peak current threshold value (I 5 ).

4. The method of claim 1 , wherein, in order to reduce the volume of fuel delivered by the injector during a first series of successive injection events, the method includes reducing the tuned injector on time value (TON_TUNED) to a predetermined injector on time threshold value (TON_ 6 ) and, for subsequent reductions in fuel delivery volume, holding the tuned injector on time value (TON_TUNED) substantially constant and thereafter reducing the peak discharge/charge current amplitude value (ITUNED) to a predetermined peak current threshold value (I 5 ).

5. The method of claim 4 , wherein, for a subsequent series of successive injection events, the method further includes holding the injector on time period substantially constant at the predetermined injector on time threshold value (TON_ 6 ), holding the peak discharge/charge current amplitude value (ITUNED) at the predetermined peak current threshold value (I 5 ), and reducing the discharge time period (TDISCHARGE) in order to further reduce the volume of fuel delivered by the injector, in use.

6. The method of claim 1 , wherein the step of determining the tuned injector on time value (TON_TUNED) includes referring to a first data map relating an injector on time period (TON_DEMAND) to the tuned injector on time value (TON_TUNED).

7. The method of claim 6 , wherein the step of determining the discharge time period (TDISCHARGE) includes referring to a second data map relating the injector on time period (TON_DEMAND) to the discharge time period (TDISCHARGE).

8. The method of claim 6 , wherein the step of determining a peak discharge/charge current amplitude value (ITUNED) includes referring to a third data map relating the injector on time period (TON_DEMAND) to the peak discharge/charge current amplitude value (ITUNED).

9. A method of operating a fuel injector having a piezoelectric actuator operable by applying a drive pulse thereto, wherein the drive pulse is defined by two or more drive pulse characteristics including a discharge time period (TDISCHARGE), an injector on time period (TON), and a peak discharge/charge current amplitude (I), and has a frequency domain signature, the method including;

determining at least one resonant frequency of an injector installation in which the injector is received, in use;

modifying at least one of said characteristics of the drive pulse such that a maximum of the frequency domain signature thereof does not coincide with the determined at least one resonant frequency of the injector installation,

wherein, in order to reduce the volume of fuel delivered by the injector, the method includes initially reducing the injector on time period (TON) to a predetermined injector on time threshold value (TON_ 6 ) and, for subsequent reductions in fuel delivery volume, holding the injector on time period substantially constant and thereafter reducing the discharge time period (TDISCHARGE).

10. A method of operating a fuel injector having a piezoelectric actuator, the method comprising:

determining at least one resonant frequency of an injector installation in which the injector is received, in use,

applying a drive pulse to the actuator, the drive pulse comprising first, second and third injection drive pulses and having a frequency domain signature; and

selecting a separation time period between the first injection drive pulse and the second injection drive pulse and/or a separation time period between the second injection drive pulse and the third injection drive pulse, and modifying the drive pulse so as to modify the frequency domain signature of the drive pulse such that a maximum of the frequency domain signature does not coincide with the determined at least one resonant frequency of the injector installation, wherein modifying the drive pulse includes determining a demanded fuel volume to be delivered during the drive pulse based on an engine operating condition, determining a tuned injector on time value (TON_TUNED) based on the demanded fuel volume and the at least one resonant frequency of the injector installation, determining a peak discharge/charge current amplitude value (ITUNED) and a discharge time period (TDISCHARGE) based on the demanded fuel volume and the tuned injector on time value (TON_TUNED), and operating the fuel injector according to the determined values of the tuned injector on time value (TON_TUNED), the peak discharge/charge current amplitude value (ITUNED), and the discharge time period (TDISCHARGE).

11. A computer program product comprising at least one computer program software portion which, when executed in an executing environment, is operable to implement the method of claim 1 .

12. A data storage medium having the or each software portion of claim 11 stored thereon.

13. A microcomputer provided with the data storage medium of claim 12 thereon.

14. A computer program product comprising at least one computer program software portion which, when executed in an executing environment, is operable to implement the method of claim 9 .

15. A data storage medium having the or each software portion of claim 14 stored thereon.

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

17. A computer program product comprising at least one computer program software portion which, when executed in an executing environment, is operable to implement the method of claim 10 .

18. A data storage medium having the or each software portion of claim 17 stored thereon.

19. A microcomputer provided with the data storage medium of claim 18 thereon.