The principal issues of Homogeneous Charge Compression Ignition (HCCI) combustion that must be addressed include ignition timing control and expansion of the stable operation region. Detailed analyses of ignition and combustion mechanisms must be undertaken to resolve these issues. In this study, spectroscopic technique was used to investigate the effects of the air-fuel ratio (AFR) and residual gas state on ignition and combustion characteristics. Spectroscopic measurement was made of light emission spectra. The results revealed that the distribution profile of the light emission intensity of the hot flame differed substantially depending on the air-fuel ratio (AFR). In high AFR condition, a continuous spectrum attributed to carbon monoxide-oxygen (CO-O) glow is seen between 300–500 nm, but there is no strong light emission such as that from the OH radical. However, decreasing the AFR, a strong light emission is seen a little after the CO-O glow in a wavelength range from visible light to the near-infrared region (500–850 nm). It is also clear that increasing the internal EGR declined the cool flame magnitude, which substantially altered the ignition characteristics of HCCI combustion. Specifically, when a low-octane fuel was used, the heat release rate waveform for HCCI combustion showed two-stage ignition, which was clearly observed in the light emission spectra.

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