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Experimental studies of homogeneous charge compression ignition (HCCI) combustion have been conducted by using a compact rapid compression machine. The image of the cool flame was captured by a high-speed camera with image intensifier, and during the cool flame appearance the Emeleus cool flame band was observed. 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 appeared, and it was superimposed on the carbon monoxide oxidation 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 changed to red, especially at higher equivalence ratios. This red coloration must originate from the H 2 O vibration-rotation bands (580.7-966.9 nm). On the other hand, it has been revealed that a pulsed flame jet (PFJ) has the potential for directly controlling the start of HCCI combustion. In the jet of PFJ, the emission spectrum obtained was composed of distinct OH and CH emissions superimposed on the carbon monoxide oxidation continuum.

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