Visualization of the homogeneous charge compression ignition/controlled autoignition combustion process using two-dimensional planar laser-induced fluorescence imaging of formaldehyde

The paper reports an investigation into the homogeneous charge compression ignition/ controlled autoignition (HCCI/CAI) combustion process using the two-dimensional planar laser- induced fluorescence (PLIF) technique. The PLIF of formaldehyde formed during the low-temperature reactions of HCCI/CAI combustion was excited by a tunable dye laser at 355 nm wavelength and detected by a gated intensified charge-coupled device (ICCD) camera. Times and locations of the two-stage autoignition of HCCI/CAI combustion were observed in a single-cylinder optical engine for several fuel blends mixed with n-heptane and iso-octane. The results show, when pure n-heptane was used, that the initial formation of formaldehyde and its subsequent burning were closely related to the start of the low-temperature heat-release stage and the start of the main heat-release stage of HCCI combustion respectively. Meanwhile, it was found that the formation of formaldehyde was more affected by the charge temperature than by the fuel concentration. However, its subsequent burning or the start of main heat-release combustion took place at those areas where both the fuel concentration and the charge temperature were suffciently high. As a result, it was found that the presence of stratified residual gases affected both the spatial location and the temporal site of autoignition in an HCCI/CAI combustion engine. All studied fuels were found having similar formaldehyde formation timings with n-heptane. This means that the presence of iso-octane apparently did not affect the start of low-temperature reactions. However, the heat release during the low-temperature reaction was significantly reduced with the presence of iso-octane in the studied fuels. In addition, the presence of iso-octane retarded the start of the main combustion stage.