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

Abstract

A controller for controlling the operation of a fuel injector having a piezoelectric actuator which is operable by the application of a voltage drive profile across the actuator, the controller comprising inputs for receiving data relating to one or more engine parameters and 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 is arranged to comprise an activating voltage component to initiate an injection event and a deactivating voltage component to terminate the injection event, the activating and deactivating voltage components being separated by a time interval TON. The controller further receives 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 TON greater than or equal to a predetermined pressure wave time period (TP) of a pressure wave cycle within the injector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Reference has already been made to FIGS. 1, 2, 3 and 4 which show, respectively, a piezoelectric injector having associated control means, a known drive voltage profile for applying to the injector and corresponding ideal and actual injection delivery rate profiles corresponding to the known drive voltage profile. The invention will now be described, by way of example only, with reference to the following drawings in which:

FIG. 5 is a graph of the difference in fuel delivery volume between pilot injection events (hereafter ‘delivery error’) against temporal separation of pilot voltage discharge pulses;

FIG. 6 is a voltage discharge profile for first and second pilot injections according to an embodiment of the invention; and

FIG. 7 is a delivery rate profile of first and second pilot injections corresponding to the voltage discharge profile in FIG. 6.

DETAILED DESCRIPTION

Referring to FIG. 5, it has been observed that varying the temporal separation of the pilot injection voltage discharge pulses results in a cyclical variation in the delivery error between injections. The cause of this phenomenon is the pressure wave effects within the injector 2 as the valve needle 6 is disengaged and re-engaged with the valve seat 8 during an injection event. When the valve needle 6 is disengaged from the valve seat 8 to initiate a pilot injection, a pressure wave is generated that travels up the internal fuel passages within the injector 2. The pressure wave then reflects back down the injector 2 to its tip. If a high pressure wave front coincides with the valve needle 6 lifting from the valve seat 8, the effect is to increase the delivery of fuel through the nozzle outlets 10 during the second pilot injection. Conversely, if a low pressure wave front coincides with the valve needle 6 lifting from the valve seat 8 the effect is to reduce the volume of fuel delivered through the outlets 10 during the second pilot injection.

The Applicant has identified that it is possible to compensate for the pressure wave effects in the injector 2 and guard against substantial variation between pilot injections by modifying the pilot injection voltage discharge waveform.

The proposed solution is to minimise the delivery volume variation to control two aspects of the discharge profile:

• • i) reduce the magnitude of peak voltage discharge level for both pilot injections; and
  • ii) increase the time interval between the start of discharge and the start of charge (hereinafter “injector on time” T_ON) so as to be greater than or approximately equal to a pressure wave time period.

The above aspects are shown in FIGS. 6 and 7, which show the voltage discharge profile for pilot injections P1 and P2, and the corresponding fuel delivery rate.

As a result of the above steps, during the second pilot injection P2, the valve needle opening duration is approximately equal to the time period for a single pressure oscillation. Thus, the fuel pressure at the nozzle outlets increases to a relatively high pressure and a relatively low pressure during the same pilot delivery period. The result is that the area under the second pilot injection delivery profile (Area B) is substantially equal to the area under the first pilot injection delivery profile (Area A). Put another way, the total delivery volume is substantially unaffected by the standing wave set up in the injector nozzle and the pilot injection separation.

The above voltage discharge waveform is applicable to a ‘de-energise to inject’ injector. However, it should be appreciated that the invention is also applicable to a so-called ‘energise to inject’ injector. In such an injector, an injection event is initiated by applying a voltage charge pulse to the actuator rather than a voltage discharge pulse.

In other words, in the “de energise to inject” case the “activating voltage component” of the voltage drive profile is a voltage discharge pulse and the “deactivating voltage component” is a voltage charge pulse. In the “energise to inject case” the “activating voltage component” of the voltage drive profile is a voltage charge pulse and the “deactivating voltage component” is a voltage discharge pulse

It is to be appreciated that that the injector on time T_ON need not be selected to be equal to the pressure wave time period. In another embodiment, the injector on time T_ON may be selected to be greater than the pressure wave time period.

It is noted that the effect of the present invention will be to reduce the delivery error as depicted in FIG. 5. In other words, once the method and controller of the present invention are activated the peak amplitudes of the cyclical variation of FIG. 5 will reduce.

The pressure wave time period may be calculated with reference to the geometry and dimensions of the fuel injection system or alternatively can be measured on a test rig. In either case, the pressure wave time period for a given engine operating parameter may conveniently be stored in a function map 30 within the controller 20 (as indicated in FIG. 1). As an alternative the function map 30 may be stored in a data store 32 either in the ECU 22 or elsewhere within the vehicle.

It will be understood that the embodiments described above are given by way of example only and are not intended to limit the invention, the scope of which is defined in the appended claims. It will also be understood that the embodiments described may be used individually or in combination.

Claims

1. A controller for controlling the operation of a fuel injector having a piezoelectric actuator, the actuator being operable by the 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 an activating voltage component to initiate an injection event and a deactivating voltage component to terminate the injection event, the activating and deactivating voltage components being separated by a time interval TON;outputs for outputting the voltage drive profile as determined by the processor to the actuatorwherein the processor is arranged to set the time interval TON greater than or equal to a predetermined pressure wave time period (TP) of a pressure wave cycle within the injector.

2. A controller as claimed in claim 1, wherein TON>TP.

3. A controller as claimed in claim 1, wherein TON=nTP, where n=1, 2, 3 . . . .

4. A controller as claimed in claim 1, wherein the processor is arranged to reduced peak voltage levels within the voltage pulse profile as TON 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 TP in dependence upon engine parameters and wherein the controller is arranged to refer to the function map when setting TON.

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 the 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 an activating voltage component to initiate an injection event and a deactivating voltage component to terminate the injection event, the activating and deactivating voltage components being separated by a time interval TON;outputs for outputting the voltage drive profile as determined by the processor to the actuatora function map of a predetermined pressure wave time period (TP) of a pressure wave cycle within the injector in dependence upon engine parameters.wherein(i) the processor is arranged to set the time interval TON greater than or equal to the predetermined pressure wave time period (TP);(ii) the processor is arranged to set TON=nTP, where n=1, 2, 3 . . . ; and(iii) the controller is arranged to refer to the function map when setting TON

9. An engine control unit for a vehicle comprising a controller according to claim 1.

10. 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: applying an activating voltage to the actuator so as to initiate an injection event, and, after a predetermined time interval (TON);applying a deactivating voltage to the actuator so as to terminate injection;wherein the predetermined time interval (TON) is selected to be greater than or equal to a predetermined pressure wave time period (TP) of a pressure wave cycle within the injector.

11. A method as claimed in claim 10, 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.

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

13. A method as claimed in claim 10, 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.

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

15. A carrier medium for carrying a computer readable code for controlling a controller or engine control unit to carry out the method of claim 10.