A Study on Optimization of an Over-Expansion Cycle Gasoline Engine with Late-Closing of Intake Valves

This study presents the possibility of realizing better thermal efficiency in a spark-ignition engine with over-expansion cycle. The test engine with the displacement volume of 649cc was used together with four kinds of expansion ratios (geometric compression ratio) from 10 to 25, and four sets of intake valve closure timings from 0 to 110 ° C.A. ABDC. In previous studies, the indicated thermal efficiency reached 48% However, there was a problem that the maximum output was reduced to almost half compared with the conventional engine, since the effective displacement volume was decreased with decreasing the substantial compression ratio (ε_c). As a method of solving this problem, supercharging was applied by using compressed air supplied from an external compressor. As a result, BMEP could be vastly expanded under most conditions, and the BSFC could be improved by 22% at the maximum comparing with the original condition with the compression ratio of 10 and the intake valve closure timing of 0deg. C.A. ABDC.