Experimental studies of the Homogeneous Charge Compression Ignition (HCCI) combustion have been conducted by using a compact rapid compression machine. The chemiluminescence spectra were obtained sequentially during the HCCI combustion. At the same time, the combustion processes were photographed using a high-speed camera with image intensifier or a color high-speed camera. The image of the cool flame was captured by the high-speed camera with image intensifier, and during the cool flame appearance, the Emeleus cool flame band which originated from the formaldehyde chemiluminescence was observed in the obtained emission spectrum. Then blue flame became evident, its emission spectrum being similar to that of the cool flame with overlapping by HCO bands. During the main heat release duration, the hot flame emission spectrum was observed where the distinct OH emission at 306.4 nm appeared, and it was superimposed on the carbon monoxide continuum. After the main heat release, the broad peak of the spectrum was shifted to the longer wavelength side, and the images of the flames change to red, especially at higher equivalence ratios. This red coloration must be originated from the H_2O vibration-rotation bands (580.7 - 966.9 nm). On the other hand, it has been revealed that Pulsed Flame Jet (PFJ) has a potential for controlling the start of HCCI combustion directly. The chemiluminescence spectra and the color high-speed images of combustion initiated by PFJ were also obtained. In the jet of PFJ, the emission spectrum obtained was composed of the distinct OH emission at 306.4 nm and CH emission at 431.5 nm superimposed on the carbon monoxide continuum.

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