A Study of HCCI Combustion Characteristics Using Spectroscopic Techniques

The principal issues of Homogeneous Charge Compression Ignition (HCCI) combustion that must be addressed include ignition timing control and expansion of the stable operation region. Detailed analyses of ignition and combustion mechanisms must be undertaken to resolve these issues. In this study, spectroscopic techniques were used to investigate the effects of the fuel octane number and residual gas state on ignition and combustion characteristics. Spectroscopic measurements were made of light emission intensity and of absorbance at a wavelength corresponding to HCHO (formaldehyde) which plays an important role in the low-temperature reaction (i.e. cool flame). The results revealed that varying the fuel octane number and the residual gas state changed the cool flame magnitude and the duration of the low-temperature reaction period, which substantially altered the ignition characteristics of HCCI combustion. Specifically, when a low-octane fuel was used, the heat release rate waveform for HCCI combustion showed evidence of a cool flame, which was clearly observed in the light emission spectra and light absorption behavior. The level of absorbance at a wavelength corresponding to the cool flame reaction decreased when the fuel octane number was increased. Additionally, the level of absorbance also decreased when residual gas was applied to HCCI combustion of a low-octane fuel and approached that seen for HCCI combustion of a high-octane fuel. With the application of residual gas, the low-temperature reactions showed less reactivity when low-octane fuels were used, and the reactions were less sensitive to changes in the octane number of the fuel. As a result, the ignition timing tended not to change in relation to variation of the fuel octane number.