Homogeneous Charge Compression Ignition (HCCI) is focused on as an innovative combustion method for the internal combustion engines. Homogeneousness of the charge may still leave discussions, but, at a particular operating condition of gasoline engines, HCCI demonstrates that it is a high-efficiency, extremely low-NOx and soot-free combustion process. The chief concern of the researchers would be the combustion control. Firstly, methodology for inducing this combustion, then controlling its onset timing and combustion speeds. Eventually its applicable zone should be sufficiently wide for a practical use. In case of gasoline engines, from viewpoint of the compatibility with a higher power output, HCCI should be combined with spark ignition (SI) at the higher load. Thus it would not be a very good solution for inducing HCCI, if the engine were given an extraordinary high compression ratio or heated intake air. As one of the most promising methods, the gas exchange of two-stroke engines is discussed. It is introduced into four-stroke engines by using a sophisticated variable valve system. However, a much more practical way to benefit from this combustion should be the use of a two-stroke engine itself. Prototype two-stroke engines were developed. These engines are equipped with an exhaust valve, which controls the gas exchange in order to induce the controlled auto-ignition, namely, Activated Radical (AR) combustion. And, again, a pneumatic direct injection (PDI) system makes the engines free from the new mixture short circuit, so that the AR combustion effect could be directly evaluated. These engines successfully showed very good efficiency and the purity of the exhaust emissions. It was also revealed that the two-stroke gas exchanging method could induce and control the HCCI very widely in the engine's speed and load range. Moreover, thanks to the large amount of residual gases in the combustion chamber, the combustion process was not extremely rapid but it was well controllable. This paper will explain the mechanism of the AR combustion and discuss the possibility of controlling auto-ignition for internal combustion engines.
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