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To avoid knocking phenomena, a special crank mechanism for gasoline engine that allowed the piston to move rapidly near TDC (Top Dead Center) was developed and experimentally demonstrated in the previous study. As a result, knocking was successfully mitigated and indicated thermal efficiency was improved [1],[2],[3],[4]. However, performance of the proposed system was evaluated at only limited operating conditions. In the present study, to investigate the effect of piston movement near TDC on combustion characteristics and indicated thermal efficiency and to clarify the knock mitigation mechanism of the proposed method, experimental studies were carried out using a single cylinder engine with a compression ratio of 13.7 at various engine speeds and loads. The special crank mechanism, which allows piston to move rapidly near TDC developed in the previous study, was applied to the test engine with some modification of tooling accuracy.

A new combustion method of high compression ratio SI engine was studied and proposed in order to achieve higher thermal efficiency of SI engine comparable to that of CI engine. Compression ratio of SI engine is generally restricted by the knocking phenomena. A combustion chamber profile and a cranking mechanism are studied to avoid knocking with high compression ratio. Since reducing the end-gas temperature will suppress knocking, a combustion chamber was considered to have a wide surface at the end-gas region. However, wide surface will lead to high heat loss, which may cancel the gain of higher compression ratio operation. Thereby, a special cranking mechanism was adopted which allowed the piston to move rapidly near TDC. Numerical simulations were performed to optimize the cranking mechanism for achieving higher thermal efficiency. An elliptic gear system and a leaf-shape gear system were employed in the simulations.