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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).

The aim of this research was to investigate the mechanism causing autoignition and the effect of exhaust gas recirculation (EGR) on combustion by detecting the behavior of the OH radical and other excited molecules present in the flame in a spark ignition engine. The test equipment used was a 2-cycle engine equipped with a Schnürle scavenging system. Using emission spectroscopy, the behavior of the OH radical was measured at four locations in the end zone of the combustion chamber. The OH radical plays an important role in the elemental reactions of hydrocarbon fuels. When a certain level of EGR was applied according to the engine operating conditions, the unburned gas became active owing to heat transfer from residual gas near the measurement positions on the exhaust port side and the influence of excited species in the residual gas, and autoignition tended to occur.