Fourier-based misfire detection strategy

1. A method for misfire detection in an internal combustion engine system, comprising the steps of:

producing a first signal corresponding to rotational characteristics of an engine crankshaft taken at selected times, said first signal including a measure of variability corresponding to characteristics unique to the internal combustion engine system irrespective of misfire;

converting the first signal into a frequency-domain misfire detection metric having components selected from an engine cycle frequency and harmonic orders thereof;

normalizing said misfire detection metric using a non-misfire metric corresponding to a non-misfire operating condition of the internal combustion engine system to remove said measure of variability;

detecting a misfire condition when the normalized misfire detection metric satisfies a predetermined threshold;

identifying one of (i) an individual cylinder and (ii) multiple cylinders that misfired when said misfire condition is detected as a function of a calculated phase.

2. The method of claim 1 further including the step of:

selecting the first signal from the group comprising a crankshaft speed signal, a crankshaft acceleration signal, an inverse of the crankshaft speed signal and an inverse of the crankshaft acceleration signal.

3. The method of claim 2 wherein the first signal comprises the inverse of the crankshaft speed signal defining a delta time.

4. The method of claim 2 wherein the first signal comprises the speed signal.

5. The method of claim 4 wherein said step of producing a speed signal includes the sub-steps of:

recording said selected times at predetermined, known angular positions of said crankshaft;

developing said speed signal in accordance with said known, angular positions and said recorded times.

6. The method of claim 1 wherein said internal combustion engine system includes a target wheel configured for rotation with said crankshaft, said target wheel being configured for use with a crankshaft sensor for sensing an angular position of said crankshaft, said target wheel including a plurality of radially-outwardly projecting teeth separated by intervening slots, said measure of variability including deviations due to tooth errors and engine variations.

7. The method of claim 6 further including the step of:

establishing said non-misfire metric as a function of engine speed and load on a per harmonic order basis.

8. The method of claim 7 further including the steps of:

determining engine operating speed and load parameters;

retrieving values for said non-misfire metric as a function of said determined engine operating speed and load parameters.

9. The method of claim 8 wherein said step of determining said speed and load parameters includes the sub-step of measuring a manifold absolute pressure (MAP) value associated with said internal combustion engine system.

10. The method of claim 8 wherein said step of determining said speed and load parameters include the sub-step of measuring a mass air flow (MAF) value associated with said internal combustion engine system.

11. The method of claim 8 wherein said step of determining said speed and load parameters include the sub-step of measuring a throttle position value associated with said internal combustion engine system.

12. The method of claim 6 wherein said converting step includes the sub-step of:

defining a sampling interval during which said first signal is defined in N samples;

selecting at least one harmonic order;

applying a Discrete Fourier Transform (DFT) to the first signal comprising N samples to obtain said misfire detection metric for the selected harmonic order.

13. The method of claim 12 wherein said step of selecting a harmonic order includes selecting a plurality of harmonic orders.

14. The method of claim 13 wherein said misfire detection metric comprises a real portion and an imaginary portion, said step of selecting at least one harmonic order comprising the sub-step of:

selecting one of (i) individual cylinder and (ii) multiple cylinder misfire detection;

defining said at least one harmonic order as the engine cycle order when individual cylinder misfire detection is selected;

defining said at least one harmonic order as the engine rotation order when multiple cylinder misfire detection is selected.

15. The method of claim 14 wherein said multiple cylinder misfire corresponds to at least one of an adjacent pair cylinder misfire, opposing pair cylinder misfire and at least a plurality of cylinder misfiring.

16. The method of claim 14 further comprising the step of:

defining said sampling interval as two engine cycles;

selecting the number of samples N during said sampling interval from a range of between about 80 and 240 and preferably 80.

17. The method of claim 14 wherein said sampling interval is a first sampling interval and said step of applying a DFT is a first DFT, said method further including the steps of:

defining a second sampling interval over which a second DFT is performed;

staggering the first and second sampling intervals by a predetermined amount for detecting random or low level misfire.

18. The method of claim 17 wherein said predetermined amount of staggering corresponds to one crankshaft revolution, and said first and second sampling intervals correspond to one engine cycle.

19. The method of claim 18 wherein said step of normalizing said misfire detection metric is performed for each of the first and second sampling intervals.