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.
Experimental result showed that indicated thermal efficiency as improved at 1000 r/min from low to high load, and at 1600, 2000 r/min high load by Leaf-shape mechanism. Improvement of indicated thermal efficiency at 1000 r/min was due to the reduction of cooling loss and at 1600, 2000r/min high load due to the recovery of the degree of constant volume. Also it was found that high load operation with compression ratio of 13.7 without knocking was realized using the proposed piston movement mechanism. Auto-ignition occurred at high load conditions even though the proposed piston movement was applied, but auto-ignition was not followed by high frequent in-cylinder pressure oscillation shown in the case of knocking.