Method for monitoring the condition of a piezo injector of a fuel injection system

1. A method for operating a fuel injection system of an internal combustion engine, the method comprising:

testing a piezo injector during a switch-on test routine, a switch-off test routine, or an overrun phase of the internal combustion engine, the test including:

injecting fuel during an injection cycle that includes a charging phase, a holding phase, and a discharging phase,

determining a leakage resistance of the piezo injector based on characteristics of the piezo injector measured during the holding phase,

comparing the determined leakage resistance to a predetermined threshold, and

switching off injection by the piezo injector during normal operation of the internal combustion engine if the determined leakage resistance is less than the predetermined threshold and providing an indication that the piezo injector should be replaced.

2. The method of claim 1 , comprising:

charging the piezo injector to a predetermined voltage during the charging phase using a voltage source,

measuring a voltage at the piezo injector at a beginning of the holding phase and at an end of the holding phase, and

calculating a difference value from the measured voltages at the beginning and end of the holding phase.

3. The method of claim 2 , comprising calculating the leakage resistance from the difference value, a duration of the injection operation, and a capacity of the piezo injector.

4. The method of claim 1 , comprising:

measuring further voltage values during the holding phase, and

calculating a straight line based on the measured voltage values that indicates a drop in voltage that occurs during the holding phase.

5. The method of claim 4 , comprising applying a mean determining process to a plurality of measured voltage values to determined mean values, and calculating the straight line from the mean values.

6. The method of claim 5 , comprising calculating a gradient of the straight line based on a quotient calculated from a time difference and a difference of the mean values.

7. The method of claim 1 , comprising calculating the leakage resistance using the equation R=U0/I, wherein U0 is the voltage that is measured at the commencement of the holding phase and I is a mean leakage current.

8. The method of claim 7 , comprising calculating the mean leakage current using the equation I=ΔQ/t, wherein ΔQ is an amount of charge that has been lost and t is a time difference.

9. The method of claim 8 , comprising calculating the amount of charge that has been lost using the equation ΔQ=C·ΔU, wherein C is a capacity of the piezo injector and ΔU is a difference value indicating a difference of voltages measured at a beginning of the holding phase and at an end of the holding phase.

10. A fuel injection system for an internal combustion engine, the system comprising:

a combustion chamber;

a piezo injector to inject fuel into the combustion chamber during injection cycles;

wherein the injection cycles include a charging phase, a holding phase, and a discharging phase;

a controller directing the operation of the piezo injector, and

a monitoring system for monitoring the condition of the piezo injector, the monitoring system comprising computer instructions stored in non-transitory computer-readable media and executable by a processor to test the piezo injector during a switch-on test routine, a switch-off test routine, or an overrun phase of the internal combustion engine, the test including:

determine a leakage resistance of the piezo injector based on characteristics of the piezo injector measured during the holding phase, and

compare the determined leakage resistance to a predetermined threshold;

and wherein the controller switches off injection by the piezo injector during normal operation of the internal combustion engine if the determined leakage resistance is less that the predetermined threshold.

11. The system of claim 10 , wherein the piezo injector is charged to a predetermined voltage during the charging phase using a voltage source, and wherein the monitoring system is configured to:

measure a voltage at the piezo injector at a beginning of the holding phase and at an end of the holding phase, and

calculate a difference value from the measured voltages at the beginning and end of the holding phase.

12. The system of claim 11 , wherein the monitoring system is configured to calculate the leakage resistance from the difference value, a duration of the injection operation, and a capacity of the piezo injector.

13. The system of claim 10 , wherein the monitoring system is configured to:

measure further voltage values during the holding phase, and

calculate a straight line based on the measured voltage values that indicates a drop in voltage that occurs during the holding phase.

14. The system of claim 13 , wherein the monitoring system is configured to apply a mean determining process to a plurality of measured voltage values to determined mean values, and calculate the straight line from the mean values.

15. The system of claim 14 , wherein the monitoring system is configured to calculate a gradient of the straight line based on a quotient calculated from a time difference and a difference of the mean values.

16. The system of claim 10 , wherein the monitoring system is configured to calculate the leakage resistance using the equation R=U0/I, wherein U0 is the voltage that is measured at the commencement of the holding phase and I is a mean leakage current.

17. The system of claim 16 , wherein the monitoring system is configured to calculate the mean leakage current using the equation I=ΔQ/t, wherein ΔQ is an amount of charge that has been lost and t is a time difference.

18. The system of claim 17 , wherein the monitoring system is configured to calculate the amount of charge that has been lost using the equation ΔQ=C·ΔU, wherein C is a capacity of the piezo injector and ΔU is a difference value indicating a difference of voltages measured at a beginning of the holding phase and at an end of the holding phase.