Light emission and absorption spectroscopic measurements of chemical species were made to investigate the effects of the fuel octane number, residual gas state and intake air temperature on the ignition characteristics of a Homogeneous Charge Compression Ignition (HCCI) engine. The results revealed that the activity level of low-temperature oxidation reactions and the tendency for autoignition to occur can be ascertained by making simultaneous spectroscopic measurements of light emission and absorption at wavelengths corresponding to those of formaldehyde (HCHO). The measured results showed that light absorption attributable to HCHO and faint light emission occur coincidentally with the cool flame. Once the cool flame degenerates, light absorption and emission are moderated. Absorbance declines sharply when autoignition occurs, indicating the consumption of HCHO. Increasing the octane number, raising the intake air temperature and increasing the residual gas fraction all have the effect of weakening the absorbance and light emission intensity of HCHO at the time the cool flame occurs, thereby lengthening the interval from the cool flame to autoignition.

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