Detection of faults in an injector arrangement

1. A method of detecting faults in an injector arrangement in an engine, the injector arrangement comprising at least one fuel injector having a piezoelectric actuator, and the method comprising:

charging the piezoelectric actuator during a charge phase (t C );

attempting to recharge the piezoelectric actuator during a test phase (t T ), the test phase (t T ) commencing after a time interval (Δt) following the end of the charge phase (t C );

sensing a current (I S ) that flows through the piezoelectric actuator ( 16 a , 16 b ) during the test phase (t T ); and

generating a short circuit fault signal if the sensed current (I S ) reaches a first predetermined threshold current (I SC ) which is indicative of a short circuit in the piezoelectric actuator.

2. The method of claim 1 , further comprising:

generating a first control signal ( 40 ) during the test phase (t T ), the first control signal ( 40 ) being variable between a first state and a second state in response to the sensed current (I S );

chopping the first control signal between the first state and the second state if the sensed current (I S ) reaches the first predetermined threshold current (I SC ); and

generating the short circuit fault signal when a chop occurs in the first control signal during the test phase.

3. The method of claim 1 , further comprising:

discharging the piezoelectric actuator during a discharge phase (t D );

sensing the current (I S ) that flows through the piezoelectric actuator during the discharge phase (t D ); and

generating an open circuit fault signal if the sensed current (I S ) during the discharge phase (t D ) does not reach a second predetermined threshold current (I 2 ,I OC ).

4. The method of claim 3 , further comprising:

generating a second control signal during the discharge phase (t D ), the second control signal being variable between a first state and a second state in response to the sensed current (I S ) during the discharge phase (t D );

chopping the second control signal between the first state and the second state if the sensed current (I S ) exceeds the second predetermined threshold current (I 2 ,I OC ); and

generating an open circuit fault signal if a chop does not occur in the second control signal during the discharge phase (t D ).

5. The method of claim 4 , wherein the open circuit fault signal is generated if a chop has not occurred in the second control signal after a predetermined time interval following the start (t 1 ) of the discharge phase (t D ).

6. The method of claim 1 , wherein the time interval (Δt) depends on an angle of rotation of a crankshaft of the engine.

7. The method of claim 1 , wherein the time interval (Δt) depends on an engine speed.

8. An apparatus for detecting faults in an injector arrangement, the injector arrangement comprising at least one fuel injector having a piezoelectric actuator, and the apparatus comprising:

charge arrangement (C 1 ) for charging the piezoelectric actuator;

current sensing arrangement for sensing a current (I S ) through the piezoelectric actuator; and

control arrangement arranged to cause the charge arrangement (C 1 ) to connect to the piezoelectric actuator during the charge phase (t C ) and re-connect to the piezoelectric actuator during a test phase (t T ), the test phase (t T ) commencing after a time interval (Δt) following the charge phase (t C );

wherein the control arrangement is further arranged to generate a short circuit fault signal if the sensed current (I S ) during the test phase (t T ) reaches a first predetermined threshold current (I SC ).

9. The apparatus of claim 8 , further comprising an arrangement for generating a first control signal which is chopped between a first state and a second state when the sensed current (I S ) during the test phase (t T ) reaches the first predetermined threshold current (I SC ), wherein the control arrangement is arranged to generate the short circuit fault signal if a chop occurs in the first control signal during the test phase (t T ).

10. The apparatus of claim 8 , further comprising:

discharge arrangement (C 2 ) for discharging the piezoelectric actuator during a discharge phase (t D ), wherein the control arrangement is arranged to generate an open circuit fault signal if the sensed current (I S ) during the discharge phase (t D ) does not exceed a second predetermined threshold current (I 2 ,I OC ).

11. The apparatus of claim 10 , further comprising an arrangement for generating a second control signal which is chopped between a first state and a second state if the sensed current (I S ) during the discharge phase (t D ) exceeds the second predetermined threshold current (I 2 ,I OC ), wherein the control arrangement is arranged to generate the open circuit fault signal if a chop does not occur in the second control signal during the discharge phase (t D ).

12. The apparatus of claim 11 , wherein the control arrangement is arranged to generate the open circuit fault signal if a chop has not occurred in the second control signal after a predetermined time interval following the start (t 1 ) of the discharge phase (t D ).