The high-frequency components of engine block vibration signals during misfire and normal combustion show clear differences of duration and scale by wavelet analysis - continuous wavelet transform (CWT). A new factor, combustion noise intensity (` CNI CWT '), has been defined, which has been derived from the CWT scalogram. It has been shown that CNI CWT can detect misfire more accurately than fast Fourier transform power spectral density. Similarly, it has been found that CNI CWT can also be applied to knock detection. The durations of both knock and misfire detection overlapped each other at the same wavelet scale 8; misfire, normal and knock cycles can be indicated on the same histogram by CNI CWT . Moreover, knock and misfire detection can be combined to one CNI CWT computation.
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Chang et al. (2002) studied this question.