Light Emission and Absorption Spectroscopic Study of HCCI Combustion

In this study, light emission and absorption spectroscopic measurement techniques were used to investigate the Homogeneous Charge Compression Ignition (HCCI) combustion process in detail, about which there have been many unclear points heretofore. The results made clear the formation behavior and wavelength bands of the chemical species produced during low-temperature reactions. Specifically, with a low level of residual gas, a light emission band was observed from a cool flame in a wavelength range of 370–470 nm. That is attributed to the light emission of formaldehyde (HCHO) produced in the cool-flame reactions. Additionally, it was found that these light emission spectra were no longer observable when residual gas was applied. The light emission spectra of the combustion flame thus indicated that residual gas has the effect of moderating cool-flame reactions. In addition, with a low level of residual gas, a strong light absorption band attributable to HCHO was observed in the ultraviolet region from approximately 260 nm to 320 nm concomitant with heat release from a cool flame. Increasing the residual gas fraction or fuel octane number reduced the absorptance level. This implies that cool-flame reactions were moderated and that the production of HCHO decreased. However, strong light absorption occurred just prior to ignition. The results of an analysis made on the basis of a chemical kinetic simulation revealed that the ignition temperature under conditions nonconducive to cool-flame reactions was higher than that under conditions conducive to the reactions. Corresponding to that, the period from the onset of the cool flame to autoignition became longer. This indicates that the activity level of low-temperature reactions can be ascertained by emission and absorption spectroscopic measurements. The data obtained with these measurement techniques made it clear that the residual gas moderates low-temperature reactions, thus changing the ignition characteristics.