Method and system for variable displacement engine diagnostics

1. A method for an engine, comprising:

actuating, via a solenoid, an electric latch pin of a cylinder valve deactivation mechanism coupled to a valve of a cylinder at a first time while engine speed is less than a threshold; and

indicating degradation of the cylinder valve deactivation mechanism based on inferred latch pin movement during the actuating,

wherein the actuation of the latch pin is independent of engine load; and

wherein, to actuate the latch pin, a magnetic force generated by the solenoid moves the latch pin.

2. The method of claim 1 , further comprising inferring latch pin movement based on an electrical current signature of the solenoid, the electrical current signature including one or more of a position of peaks and valleys of an electrical current of the solenoid, and a slope of the electrical current.

3. The method of claim 2 , wherein the actuating includes unlatching and then latching the latch pin to a rocker arm of the cylinder valve deactivation mechanism, and wherein the solenoid is energized by applying voltage of a first polarity to unlatch the latch pin and by applying voltage of a second polarity to latch the latch pin.

4. The method of claim 2 , wherein the indicating is further based on a cam timing of the actuating.

5. The method of claim 4 , wherein the indicating based on the cam timing of the actuating is further based on whether a cam coupled to the cylinder valve is at a base circle position or at a lift position.

6. The method of claim 5 , wherein the indicating includes:

indicating degradation responsive to an absence of latch pin movement during the actuating when the cam is at the base circle position; and

indicating degradation responsive to a presence of latch pin movement during the actuating when the cam is at the lift position.

7. The method of claim 6 , further comprising:

responsive to the indication of degradation when the cam is at the base circle position, maintaining the cylinder active; and

responsive to the indication of degradation when the cam is at the lift position, selectively deactivating the cylinder when actuating the electric latch pin after the first time.

8. The method of claim 2 , wherein actuating at the first time includes actuating independent of engine load, the method further comprising actuating the electric latch pin after the first time based on engine load to selectively enable or disable the cylinder valve deactivation mechanism.

9. The method of claim 8 , further comprising:

inferring a plurality of latch pin movement parameters based on the electrical current signature of the solenoid during the actuating at the first time; and

adjusting the electrical current applied to the solenoid during the actuating the electric latch pin after the first time based on the inferred plurality of latch pin movement parameters.

10. The method of claim 9 , wherein the inferring the plurality of latch pin movement parameters includes learning a response time of the solenoid based on the electrical current signature, and wherein the adjusting includes adjusting a timing of energizing the solenoid based on the learned response time.

11. The method of claim 10 , wherein the threshold speed is determined as a function of the learned response time.

12. The method of claim 9 , wherein the inferring the plurality of latch pin movement parameters includes learning an impact velocity applied by the latch pin to a final position during the latching, and wherein the adjusting includes adjusting a pulse-width signal commanded to energize the solenoid based on the learned impact velocity.

13. A method for an engine, comprising:

after starting an engine, while engine speed is at or below an idling speed and independent of engine load, diagnosing a valve deactivation mechanism coupled to a cylinder valve based on latch pin movement, the movement inferred while electrically actuating the latch pin; and

after the diagnosing, electrically actuating the latch pin to selectively operate the valve deactivation mechanism based on the engine load.

14. The method of claim 13 , wherein during the diagnosing, the latch pin is sequentially electrically actuated from a latched position to an unlatched position and then from the unlatched position back to the latched position, and wherein the diagnosing is based on latch pin movement inferred while the latch pin is actuated to the unlatched position and back to the latched position.

15. The method of claim 14 , wherein after the diagnosing, the latch pin is either maintained in the latched position to operate the valve deactivation mechanism in a state where the cylinder valve is active, or electrically actuated to the unlatched position to operate the valve deactivation mechanism in a state where the cylinder valve is deactivated.

16. The method of claim 13 , wherein electrically actuating the latch pin includes energizing a solenoid, the method further comprising inferring the latch pin movement based on an inductive current signature of the solenoid, the signature including one or more of a position of peaks and valleys of an inductive current of the solenoid, and a slope of the inductive current.

17. The method of claim 13 , wherein the diagnosing is further based on whether a cam coupled to the cylinder valve is at a base circle position or at a lift position at a time of electrically actuating the latch pin, the diagnosing including:

indicating that the valve deactivation mechanism is degraded responsive to an absence of latch pin movement during the actuating when the cam is at the base circle position;

indicating that the valve deactivation mechanism is not degraded responsive to a presence of latch pin movement during the actuating when the cam is at the base circle position;

indicating that the valve deactivation mechanism is degraded responsive to a presence of latch pin movement during the actuating when the cam is at the maximum lift position; and

indicating that the valve deactivation mechanism is not degraded responsive to an absence of latch pin movement during the actuating when the cam is at the maximum lift position.

18. An engine system, comprising:

an engine cylinder including an intake valve;

a cam mounted on a camshaft for opening and closing the intake valve;

a valve deactivation mechanism coupled to the intake valve, the valve deactivation mechanism including a rocker arm assembly and a latch pin, an inner arm of the rocker arm assembly coupled to the cam via a cam follower and coupled to a stem of the intake valve, an outer arm of the rocker arm assembly coupled to the latch pin, the inner arm engagable to the outer arm via the latch pin;

an electric solenoid coupled to the latch pin;

an engine speed sensor; and

a controller with computer readable instructions that when executed cause the controller to:

after starting an engine, while engine speed is below a threshold, and independent of engine load over a drive cycle,

energize the solenoid to actuate the latch pin from a latched position where the inner arm is engaged to the outer arm via the latch pin, and where the intake valve can lift via concerted movement of the outer arm and inner arm of the rocker arm assembly, to an unlatched position where the inner arm is disengaged from the outer arm and the intake valve cannot be lifted;

measure an induction current generated by the solenoid upon energization;

infer presence or absence of movement of the latch pin between the latched position and the unlatched position upon the energization based on the measured induction current; and

indicate degradation of the valve deactivation mechanism responsive to the inferred presence or absence of movement of the latch pin relative to an expected movement, the expected movement based on cam timing.

19. The system of claim 18 , wherein the controller includes further instructions that when executed cause the controller to:

maintain the latch pin in the latched position over a remainder of the drive cycle responsive to the indication of degradation of the valve deactivation mechanism when the cam is at a base circle position; and

selectively actuate the latch pin to the unlatched position in response to a lower than threshold engine load over the remainder of the drive cycle responsive to the indication of degradation of the valve deactivation mechanism when the cam is at a lobe position.

20. The system of claim 19 , wherein the controller includes further instructions that when executed cause the controller to:

energize the solenoid to actuate the latch pin back to the latched position in response to the inferred presence of movement of the latch pin when the cam is at a base circle position;

infer a plurality of latch pin movement parameters based on the induction current, the plurality of parameters including a response time of the solenoid and a latching force applied by the latch pin; and

adjust an energization current applied to the solenoid during the selectively actuating based on the inferred plurality of latch pin movement parameters, a timing of current application advanced as the response time decreases, a magnitude if the current application decreased as the latching force increases.