The process of homogeneous charge compression ignition (HCCI) combustion was compared with stratified charge compression ignition (SCCI) and standard spark ignition (SI) using local spectrum analysis, with a transparent cylinder and piston. The engine was operated at 1000 rpm. The combustion characteristics were analyzed using the heat-release rate, burned-mass fraction, and time-series spectra. Local emission spectra were measured using Cassegrain optics, which have a measurement volume 0.1 mm in diameter and 0.8 mm in length. Regular gasoline was used for the SI engine and a 50:50 blend of iso-octane and n-heptane was used for the HCCI and SCCI engines.
Detailed spectra were measured and compared to demonstrate the differences in the combustion process with time. The combustion characteristics of HCCI were distinguished from those of standard SI combustion using the emission spectrum, which measures the cool-flame region in front of the main hot flame. Cool-flame regions were found in both HCCI and SCCI. The cool-flame region in HCCI was about 12Π crank angle in front of the main hot flame, while the two cool-flame regions in SCCI were 10 and 5Π crank angle in front of the main flame. These cool flames have the spectral structure of formaldehyde bands regardless of combustion type. The characteristics of CO-O* emissions were well correlated with the ratio of heat release (ROHR), and chemiluminescence measurement was confirmed as a good method for analyzing engine combustion.