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THE MULTI-VALVE FOUR STROKE CYCLE engine design trend is Coward increased engine power and higher fuel efficiency. While a four-valve system is the most common direction, problems occur when the valve area is widened by increasing the cylinder bore for a higher engine output. The layout of four larger valves causes the combustion chamber shape to flatten and the combustion time period to increase. In pursuit of the optimum multi-valve engine we have studied four, five, six and seven-valve per cylinder design. Performance targets and design constraints led us toward the successful five-valve engine technology. This technology develops high engine torque and efficient combustion over a wide range of engine speeds.

In this study, an improvement of fuel consumption by changing the exhaust timing of a two-stroke engine has been made. The study results revealed that a remarkable improvement of fuel economy is possible by controlling the exhaust timing according to the engine speed. The reason for the better fuel economy was clarified through an analysis of exhaust gases, theoretical cycle calculations, and an analysis of combustion pressure. As an example of actual application, the results of tests made on an engine equipped with Yamaha power Valve System (YPVS), which is a variable exhaust timing mechanism using a tabor-shaped valve, will also be discussed.