An Analysis of Light Emission Intensity Behavior Corresponding to Intermediate Products in Different Places of the Combustion Chamber

Knocking is one phenomenon that can be cited as a factor impeding efforts to improve the efficiency of spark-ignition engines. With the aim of understanding knocking better, light emission spectroscopy was applied in this study to examine preflame reactions that can be observed prior to autoignition in the combustion reaction process of hydrocarbon fuels. Attention was focused on light emission behavior at wavelengths corresponding to those of formaldehyde (HCHO), Vaidya's hydrocarbon flame band (HCO) and the OH radical in a forced progression from normal combustion to a knocking state. Light emission behavior was measured simultaneously in the center and in the end zone of the combustion chamber when the engine was operated on two different test fuels. The test fuels used were n-heptane (0 RON) and a blended fuel (70 RON) consisting of n-heptane (0 RON) and iso-octane (100 RON). With n-heptane (0 RON) as the test fuel, behavior corresponding to the passage and degeneracy of a cool flame was observed. When the blended fuel (70 RON) was used, a tendency was observed for the preflame reaction interval of HCHO to become longer as combustion progressed from a normal state to trace knocking. As a result of analyzing the experimental data, the degree to which the preflame reactions observed in the end zone of the combustion chamber were influenced by flame propagation was confirmed quantitatively.