Abstract
This paper presents a further investigation into the effect of more-expansion cycle in a spark-ignition engine. On the basis of the results obtained in the previous studies, several combinations of late-closing (L.C.) of intake valve and expansion ratios were tested using a single-cylinder production engine. A large volume of intake capacity was put into the intake manifold to simulate multi-cylinder engines. With a large intake capacity, L.C. can decrease the pumping loss and thus increase the mechanical efficiency. Increasing the expansion ratio from 11 to 23.9 with L.C. application can produce about 11 % improvement of thermal efficiency which was suggested to be caused by the increased cycle efficiency. The decrease of compression ratio from 11 to 5.5 gives little effect on the thermal efficiency if the expansion ratio could be kept constant. Thus, the expansion ratio is revealed to be a determining factor for cycle efficiency, while compression ratio is no more important, which suggests the usefulness of controlling the intake charge with intake valve closure timing. These were successfully explained by simple thermodynamic calculation and thus the mechanism could be verified by the estimation.