Internal viscous damper monitoring system and method

1. A method for operating an internal combustion engine of a vehicle traveling on the road, the method comprising:

damping engine vibration with a damper coupled to the engine; and

when said damping is reduced, preventing engine speed from rising above a threshold value, during a first engine or transmission state, to reduce degradation of the engine that would otherwise occur, and permitting engine speed to rise above the threshold during a second engine or transmission state.

2. The method of claim 1 wherein said damping is reduced when an amplitude of vibration increases.

3. The method of claim 2 wherein said amplitude is an amplitude of a sixth order harmonic of engine firing frequency.

4. The method of claim 3 wherein a change in said amplitude is used to identify said reduction in damping.

5. The method of claim 4 wherein said a limit of said change is varied with engine operating parameters.

6. The method of claim 2 further comprising identifying said damping is reduced by measuring engine speed fluctuations.

7. The method of claim 6 wherein said measuring is performed under selected operation conditions.

8. The method of claim 1 , wherein said transmission state is at least a gear ratio or a torque converter state and wherein said engine state is at least a misfire state, where the first and second states include engine and transmission states.

9. The method of claim 6 wherein said engine speed is processed using a discrete Fourier Transform.

10. The method of claim 1 wherein said damper is an internal viscous damper.

11. The method of claim 1 wherein said threshold value varies with engine operating conditions.

12. The method of claim 1 wherein said prevention is performed by adjusting a position of an electronically controlled throttle coupled to an intake manifold of the engine.

13. A system for an engine, comprising:

an internal viscous damper coupled to a crankshaft at a front end of the engine, the damper having a fluid located therein having a viscosity selected to reduce engine speed oscillations; and

a controller configured to maintain engine speed below a threshold value, at least under selected operating conditions based on a transmission state, when viscosity of a fluid in said damper is reduced, where said threshold value is reduced when said viscosity is reduced.

14. The system of claim 13 wherein said controller is further configured to adjust a throttle position of the engine to maintain engine speed below said threshold value, and wherein said selected operating conditions include at least an engine state, wherein said engine state is at least a misfire state.

15. The system of claim 14 wherein said reduction in viscosity is identified based on at least one of a frequency and amplitude of engine speed oscillations.

16. The system of claim 14 wherein said controller identifies said reduction in viscosity based on an increase in amplitude of engine speed oscillations at a selected speed range.

17. The system of claim 16 wherein said controller is further configured to perform discrete Fourier Transform of engine position data to identify said reduction in viscosity.

18. The system of claim 13 wherein engine is coupled to a transmission of a passenger vehicle traveling on a road.

19. The system of claim 13 wherein said controller is further configured to set a diagnostic code when viscosity of a fluid in said damper is reduced.

20. A method for operating an internal combustion engine of a vehicle traveling on the road, the method comprising:

damping engine vibration with a damper coupled to the engine;

when said damping is reduced, maintaining engine speed below a threshold value, at least under selected operating conditions based on a transmission state, to reduce degradation of the engine that would otherwise occur; and

setting a diagnostic code indicating degradation of said damper.