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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.

Variable-valve-actuation mechanism is considered to be one of the most suitable solutions to realize the compatibility between higher power output and performances in the practical speed range. A new variable-valve-actuation mechanism named “Shuttle Cam” was designed and studied. In this mechanism which was applied to a conventional motorcycle engine with rocker arms and gear-train-driven valve system, the cam gears move along the idler gear. And cam shafts simultaneously slide along the rocker-arm slipper surfaces which are concentric with the idler gear. Consequently valve lift varies continuously in accordance with the alteration in the rocker-arm lever ratio and the cam phasing changes simultaneously in accordance with the cam gear rotation. Result of the experiments has confirmed that the mechanism functions accurately even at high speeds up to 10,000 rpm and some improvements were achieved in power output, fuel consumption, idling quality, and exhaust-noise level.

In the previous reports (SAE PAPER 880268, 900223), a relationship between dimensional specifications and total engine loss (Pmf) measured by motoring method under certain conditions was established by analyzing 300 different types of mass-produced engines of which engine speed for maximum power output (Nepsmax) was obtained between 8,000 and 16,000 rpm, and thereby an empirical formula was proposed (Pmf). In this paper, it is proved that brake mean effective pressure (Pme) can be estimated by using the empirical formula for Pmf and modified indicated mean effective pressure (Pmi*) obtained from analyzing various types of motorcycle engines consisting of different cylinder configurations, valve sizes and numbers. At first, the relationship between the characteristic values of engine and Nepsmax is described, and methods to obtain higher power output by increasing engine speed are also discussed.