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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<div class="section abstract"><div class="htmlview paragraph">Engine knock is the one of the main issues to be addressed in developing high-efficiency spark-ignition (SI) engines. In order to improve the thermal efficiency of SI engines, it is necessary to develop effective means of suppressing knock. For that purpose, it is necessary to clarify the mechanism generating pressure waves in the end-gas region. This study examined the mechanism producing pressure waves in the end-gas autoignition process during SI engine knock by using an optically accessible engine. Occurrence of local autoignition and its development process to the generation of pressures waves were analyzed under several levels of knock intensity. The results made the following points clear. It was observed that end-gas autoignition seemingly progressed in a manner resembling propagation due to the temperature distribution that naturally formed in the combustion chamber. Stronger knock tended to occur as the apparent propagation speed of autoignition increased. It is particularly notable that a condition was observed in which the apparent propagation speed of autoignition through the end gas clearly exceeded the speed of sound. Exceptionally strong knock occurred at that time. The measured results for the actual development of autoignition made it clear that extremely strong knock, such as what occurs under high-speed, high-load operation and also that induced by low-speed pre-ignition (LSPI) in a supercharged downsized engine under low-speed, high-load operation (Super-Knock), is presumably caused by such phenomena. In other word, extremely strong knock is caused by detonation phenomena produced in autoignition process.</div></div>

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