US Patent 7292933 Engine misfire detection

A method of detecting an engine malfunction such as a misfire includes determining engine speed values at each of a plurality of measurement angular positions, heterodyning the engine speed values with sine and cosine functions indexed in the angular domain, passing the heterodyned results through a low pass filter, and computing the resulting magnitude from the resulting two vectors. An apparatus for detecting an engine malfunction, such as a misfire, includes an engine speed analyzer, a multiplier, and a low pass filter.