Dynamics of cycle-to-cycle variations in a natural gas direct-injection spark-ignition engine
The dynamics of cycle-to-cycle variations (CCV) was investigated in a natural gas direct-injection spark-ignition engine. The method of continuous wavelet transform was used to analyze the time series of the indicated mean effective pressure (IMEP) and other combustion variables. The dominant oscillatory modes in the CCV were identified, and the engine cycles over which these modes may persist were delineated. Results were obtained for four compression ratios: CRÂ =Â 8, 10, 12 and 14, at two engine speeds of 1200 and 1800Â rpm. The results reveal that the CCV exhibit multiscale dynamics with fluctuations occurring at different timescales. At the engine speed of 1200Â rpm, the spectral power of CCV for CRÂ =Â 12 was found to be significantly reduced at the different timescales compared to the CCV at other values of CR. At the higher engine speed of 1800Â rpm, this reduction was less pronounced. In addition, cross wavelet transform was used to explore the relationships between the CCV of IMEP and those of flame development duration, main combustion duration and total combustion duration. Strong interdependence was found to exist between the IMEP and main combustion duration as well as total combustion duration, over a wide range of frequencies and engine cycles.
