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Combustion experiments were conducted with an optically accessible engine that allowed the entire bore area to be visualized for the purpose of making clear the characteristics that induce extremely rapid HCCI combustion and knocking accompanied by cylinder pressure oscillations. The HCCI combustion regime was investigated in detail by high-speed in-cylinder visualization of autoignition and combustion and emission spectroscopic measurements. The results revealed that increasing the equivalence ratio and advancing the ignition timing caused the maximum pressure rise rate and knocking intensity to increase. In moderate HCCI combustion, the autoignited flame was initially dispersed temporally and spatially in the cylinder and then gradually spread throughout the entire cylinder.

Behaviors of shock waves in knocking phenomena were observed in detail and influences of low temperature reaction on the flame and spontaneous ignition of end gas were investigated through experiments using high-speed direct and schlieren photography. As a result, it was found that light emission of shock waves, that is an indicator of pressure, rose when the shock waves collided with the cylinder wall and that pressure waves arose by low temperature reaction before knocking occurrence. Flame oscillation was caused by pressure waves. It is presumed that pressure waves influence spontaneous ignition.

Homogeneous Charge Compression Ignition combustion in homogenized temperature fields was investigated experimentally using a super rapid compression machine. Temperature fields before a blue flame occurs are thought to control the burning process. The time of blue flame occurrence, burn rate and ignition delay time were measured. Temperature homogeneity before a blue flame occurred was controlled by two means. One was by the piston shape which controlled the roll-up vortex and the other was by the heat release of low temperature reactions that has a homogenizing effect. It was found that homogenized temperature fields contribute to the occurrence of a homogeneous blue flame while inhomogeneous fields produce an uneven occurrence.