Patent Number: 
Section: claims

1. A method of detecting a malfunction of an engine, the engine having a rotating member and an angular position correlated with an angular orientation of the rotating member, the engine having an engine speed, the method comprising:determining the engine speed at a plurality of angular positions;multiplying the engine speed by a sine function of the angular position to provide a first product;passing the first product through a first low pass filter to produce a first output; anddetecting the malfunction of the engine using the first output. 2. The method of claim 1, wherein the engine speed is determined a number of times per engine revolution, the number of times being substantially independent of engine speed. 3. The method of claim 2, wherein the engine speed is determined at least three times per engine revolution. 4. The method of claim 3, wherein the plurality of angular positions are separated by an angular interval,the angular interval being substantially independent of engine speed. 5. The method of claim 4, further including the steps of:multiplying the engine speed by a cosine function of the angular position to provide a second product;passing the second product through a second low pass filter to provide a second output;combining the first output and the second output into a magnitude term, the magnitude term being correlated with a magnitude of an engine speed variation; anddetecting the malfunction of the engine using the magnitude value. 6. The method of claim 5, wherein the engine speed variation has an angular period,the sine function and cosine function both being a function of angular position, and both having the angular period. 7. The method of claim 5, wherein the low pass filter is a first order infinite impulse response filter. 8. The method of claim 1, wherein the sine function is stored in a look-up table. 9. The method of claim 1, wherein the engine malfunction is an engine misfire, and the sine function has an angular period equivalent to two engine revolutions. 10. The method of claim 1, wherein the engine malfunction is an engine misfire, and the sine function has an angular period equivalent to one engine revolution. 11. The method or claim 5, wherein detecting the malfunction from the magnitude of engine speed variation includes comparing the magnitude of engine speed variation with a threshold value, the malfunction being detected if the magnitude of engine speed variation is greater than or equal to the threshold value. 12. The method of claim 6, wherein the angular period is selected from a group consisting of 360 degrees, corresponding to first order, and 720 degrees, corresponding to half order. 13. The method of claim 11, wherein the threshold value is a multiple of baseline value, the baseline value being an engine speed variation for a non-misfiring engine. 14. The method of claim 13, wherein the threshold value is at least twice the baseline value. 15. The method of claim 13, wherein the threshold value is between approximately 2 and 10 times greater than the baseline value. 16. The method of claim 1, wherein the engine speed determining step comprises receiving a train of pulses from an engine speed sensor as the engine rotates and determining the engine speed from the elapsed time between pulses received from the engine speed sensor.