There is an urgent need today to improve the thermal efficiency of spark- ignition (SI) engines in order to reduce carbon dioxide emission and conserve energy in an effort to prevent global warming. However, a major obstacle to improving thermal efficiency by raising the compression ratio of SI engine is the easily occurrence of engine knocking.The result of studies done by numerous researchers have shown that knocking is an abnormal combustion in which the unburned gas in the end zone of the combustion chamber autoignites. However, the combustion reaction mechanism from autoignition to the occurrence of knocking is still not fully understood.The study deals with the light absorption and emission behavior in the preflame reaction interval before hot flame reactions. With the aim of shedding some light on this mechanism, the present study focused on light emission and absorption behavior of certain intermediate products of combustion which are known to play a key role in the combustion reactions of hydrocarbon fuels. Emission and absorption spectroscopy were used to measure the behavior of intermediate products of combustion at wavelengths thought to correspond to formaldehyde (HCHO, characteristic spectra of 395.2 and 293.1 nm), which shows cool flame reactions, the OH radicals (306.4 nm) and HCO (329.8 nm), which exhibits blue flame reactions. The light emission behavior measured for the three radicals by emission spectroscopy was introduced into a polychromator and the absorption behavior measured by absorption spectroscopy was introduced into a monochromator for each species individually.The HCHO emission intensity and absorbance waveforms recorded under normal combustion on n-heptane show behavior indicative of the passage and degeneracy of cool flame in the preflame reaction interval. The OH radicals and a HCO also show similar light emission behavior. It was observed that radical behavior differed between normal combustion and knocking operation.