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In-cylinder visualization of the entire bore area at an identical frame rate was used to investigate knocking conditions under spark ignition (SI) combustion and under Homogeneous Charge Compression Ignition (HCCI) combustion in the same test engine. A frequency analysis was also conducted on the measured pressure signals. The results revealed that a combustion regime accompanied by strong pressure oscillations occurred in both the SI and HCCI modes, which was presumably caused by rapid autoignition with attendant brilliant light emission that took place near the cylinder wall. It was found that the knocking timing was the dominant factor of this combustion regime accompanied by cylinder pressure oscillations in both the SI and HCCI combustion modes.

This study was conducted to investigate the influence of cooled recirculated exhaust gas (EGR) on abnormal combustion in a Homogenous Charge Compression Ignition (HCCI) engine. The condition of abnormal HCCI combustion accompanied by cylinder pressure oscillations was photographed with a high-speed camera using a 2-stroke optically accessible engine that enabled visualization of the entire bore area. Exhaust gas was cooled with a water-cooled intercooler for introducing cooled EGR. Experiments were conducted in which the quantity of cooled EGR introduced was varied and a comparison was made of the autoignition behavior obtained under each condition in order to investigate the influence of cooled EGR on abnormal HCCI combustion. The results revealed that cylinder pressure oscillations were reduced when cooled EGR was introduced. That reduction was found to be mainly ascribable to the effect of cooled EGR on changing the ignition timing.