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In recent years, many four-stroke outboard motors have been developed according to the market demand. But four-stroke outboard motor tends to be heavier and larger than conventional two-stroke one due to its more complex mechanism. The outboard motor is required to be light and compact not to disturb the utility of the boat. In addition, boat load requires higher torque at mid engine speed. In such situation, knocking is one of the obstacles against power density. We tried to improve combustion for high power density on a prototype single cylinder engine using up-to-date analyzing and development technology.

For nearly twenty years, DiMethyl Ether has been known to be an outstanding fuel for combustion in diesel cycle engines. Not only does it have a high Cetane number, it burns absolutely soot free and produces lower NOx exhaust emissions than the equivalent diesel. However, the physical properties of DME such as its low viscosity, lubricity and bulk modulus have negative effects for the fuel injection system, which have both limited the achievable injection pressures to about 500 bar and DME's introduction into the market. To overcome some of these effects, a common rail fuel injection system was adapted to operate with DME and produce injection pressures of up to 1000 bar. To understand the effect of the high injection pressure, tests were carried out using 2D optically accessed nozzles. This allowed the impact of the high vapour pressure of DME on the onset of cavitation in the nozzle hole to be assessed and improve the flow characteristics.