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In this study, knocking over a wide range of engine speeds was visualized using an optically acssessible engine. In addition, knock under a high compression ratio and supercharged, lean combustion was investigated. The results revealed that under high-speed knock, the flame propagation velocity declined when low-temperature oxidation reactions occurred. Subsequently, autoignition began locally and expanded gradually. Eventually, it was observed that a highly brilliant autoignited flame appeared and propagated through the unburned end gas at a high speed of approximately 1700-1800 m/s. This suggests that high-speed knock causes "developing detonation" in which combustion proceeds at a supersonic speed while pressure waves and the reaction front mutually interact. It was also found that strong knock occurred under supercharged, ultra-lean conditions (Compression Ratio: CR=14, Equivalence Ratio:  =0.5, Intake Pressure: Pin = 140 kPa). In addition, the application of exhaust gas recirculation markedly reduced strong pressure oscillations.

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