Investigation of the HCCI/CAI Combustion Process by 2-D PLIF Imaging of Formaldehyde

The paper reports an investigation into the HCCI/CAI combustion process in a single cylinder optical engine. The auto-ignition and combustion processes of primary reference fuels were studied using the two-dimensional PLIF technique as well as heat release analyses. The formaldehyde formed during the low-temperature reactions of HCCI/CAI combustion was visualized by a PLIF system. The formaldehyde was excited by a Nd:YAG laser pumped tunable dye laser at 355nm wavelength and detected by a gated ICCD camera. Both temporal and spatial distributions of formaldehyde were measured during the auto-ignition processes of different primary reference fuels. The results have shown that the formation of formaldehyde and its subsequent disappearance were closely related to the start of the low temperature and high temperature heat release processes, respectively. The formation of formaldehyde was more affected by the charge temperature than by the fuel concentration. But its subsequent burning or the start of main heat release combustion toke place at those areas where both the fuel concentration and the charge temperature were sufficient high. All fuels studied were found having similar formaldehyde formation timings to that of pure n-heptane. This means that the presence of iso-octane did not affect the start of low temperature reactions apparently. However, the heat release during low temperature reaction was significantly reduced with the presence of iso-octane in the primary reference fuels. In addition, the presence of iso-octane retarded the start of the main combustion stage.