A Spectroscopic Analysis of a Homogeneous Charge Compression Ignition Engine

Homogeneous Charge Compression Ignition (HCCI) combustion offers the advantages of high efficiency and low emissions of pollutants. However, ignition timing control and expansion of the stable operation region are issues remaining to be addressed in this combustion process. Detailed analyses of ignition and combustion characteristics are needed to resolve these issues. HCCI combustion of a low octane number fuel is characterized by two-stage heat release attributed to a cool flame and a hot flame, respectively. In this study, spectroscopic techniques were used to investigate the effect of exhaust gas recirculation (EGR) on ignition and combustion characteristics using a low octane number fuel, which is apt to give rise to a cool flame. The reaction mechanism of a cool flame produces formaldehyde (HCHO). Measurements were made of spontaneous light emission and absorption at wavelengths corresponding to the light emitted at the time HCHO was produced. The light emission intensity and absorbance measured with the spectroscopic techniques revealed clear differences in the cool flame magnitude depending on changes in the EGR rate. It was found that the cool flame magnitude in the HCCI combustion process varied depending on the EGR rate and thus ignition characteristics differed considerably. An investigation was also made of the respective influence on HCCI combustion of high-temperature internal EGR and low-temperature external EGR. The results indicated that external EGR was more effective for retarding the ignition timing, owing to its larger mass per unit volume than that of internal EGR.