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We have investigated the cycle-to-cycle pressure fluctuations in a natural gas engine under lean burn conditions. In particular, we have examined the dynamics of the indicated mean effective pressure (IMEP) variations for four different values of the equivalence ratio. For each equivalence ratio, we used a continuous wavelet transform to identify the dominant spectral modes and the number of cycles over which these modes may persist. Our results reveal that when the mixture is not so lean, the IMEP undergoes persistent low frequency oscillations together with high-frequency intermittent fluctuations. For leaner mixtures, the low frequency periodicities tend to be less significant, but high-frequency intermittent oscillations continue to be present. When the mixture is made sufficiently lean, high-frequency oscillations become persistent, together with weak low frequency variations reflecting weak combustion. These results may be useful for understanding the long-term variability of the pressure fluctuations. They can also be used to develop effective control strategies for improving the performance of natural gas-fired engines under lean burn conditions.
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