Effect of Over-Expansion Cycle to a Larger Gasoline Engine with Late-Closing of Intake Valves

Abstract

This paper presents an investigation into the effect of voer-expansion cycle with late-closing of intake valves on the engine performance in a gasoline engine. A larger single-cylinder test engine with the stroke volume of 650cc was used with four kinds of expansion ratio (geometrical compression ratio) from 10 to 25 and four sets of intake valve closure timings from 0 to 110 deg C. A. ABDC. Late-closing has an effect of decreasing the pumping work, but higher expansion ratio increases the friction work. Combining the higher expansion ratio with the late-closing determines the mechanical efficiency on the basis of these two contrastive effects. The indicated thermal efficiency is mostly determined by the expansion ratio and is little affected by the nominal compression ratio. The value of the indicated thermal efficiency reaches 48% at most which is almost comparable with the value of diesel engines. The brake thermal efficiency is determined by both the mechanical efficiency and the indicated efficiency.