Investigation of the effect of ignition position on combustion behavior and knock characteristics by using an optically accessible engine

Abstract

It is generally known that the temperature and mixture concentration in the combustion chamber influence knock intensity. It is assumed that the distribution of temperature and mixture concentration in the end-gas region of a two-stroke engine differs depending on the ignition position. Therefore, this study investigated the effect of the ignition position on knock behavior in a spark-ignition engine. A two-stroke air-cooled single-cylinder gasoline test engine was used to investigate autoignition and abnormal combustion behavior based on in-cylinder pressure analysis, in-cylinder visualization of combustion and frequency analysis of in-cylinder pressure waveforms. Six ignition positions were defined, considering the distribution of temperature and gas concentration in the cylinder. The results revealed that the ignition position influenced localized autoignition development behavior and knock intensity owing to the related distribution of temperature and fuel concentration. It was observed that knock was relatively weak when a large quantity of high-temperature residual burned gas was present in the end-gas region because it suppressed a sharp pressure rise. Knock was also relatively weak in the presence of a large quantity of fresh charge in the end-gas region because the flame was able to propagate sufficiently. Strong knock occurred when the fresh charge and residual gas were both present in the end gas-region because the high-temperature residual gas easily autoignited and autoignition developed rapidly in the new charge that readily burned. Accordingly, it was found that conditions tending to induce strong knock were present when a distribution of high-temperature areas conducive to autoignition and a distribution of high fuel concentration conducive to combustion occurred in the end-gas region. In addition, the dominant knock vibration mode was the 1,0 mode regardless of the ignition position, although it was observed that the resonant frequency varied depending on the end-gas distribution.