Non-intrusive measurements are always desirable in flame research, particularly in the study of internal combustion engines where intrusive measurements are usually not applicable. With the use of digital image processing and color analysis, the imaging system can be turned into an abstract multi-spectral system to determine the characteristics of flame emission. First this study conducts a precise calibration to make up a spectral correlation between the camera spectrum responses and the radical emissions of an ethanol diffusion flame. The color model of HSV is used to represent the camera spectrum responses. The actual wavelength of each radical of the diffusion flame has also been examined using a spectrograph. Subsequent experiment is the application of the spectral correlation into a direct injection spark ignition optical engine to research the combustion behavior. Two fuel injectors, different in nozzle configuration, were utilized and tested individually. The high-speed imaging system films hundreds of engine combustion cycles, and each cycle covers the propagation from the flame ignition stage towards the end of combustion. In those cycles, the presence of radicals of interest was captured and represented by Hue degree.

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