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This report describes the engineering challenges in the development of 4 stroke 50ps outboard motor. The goal was set to attain the top speed and acceleration performance comparable to the existing 2 stroke versions, while providing the advantages of 4 stroke engine, including the alleviated exhaust odor and operational noise, as well as consistent performance with stable rpm. Emphasis was also given to the trolling at low rpm (at 650rpm). Our engineering challenges have been embodied into the 4 stroke outboard motor which is “clean” and yet allows similar application as existing 2 stroke versions.

A unique combustion system using an energy cell was designed during the course of this two-stroke, spark-assisted DI diesel engine development program. The system generated high gas turbulence and significantly increased fuel-air mixing rate during combustion. Using the spark-assisted diesel engine concept, this system allowed modification of a production, loop-scavenged, two-stroke gasoline engine to increase fuel tolerance, decrease fuel consumption to levels close to diesel engines, and produce a power-to-weight ratio comparable to a gasoline engine. The experimental engine was constructed and developed by Southwest Research Institute and the project was funded by Sanshin Industries. This paper summarizes the results of the project.

A loop-scavenged, two-stroke, spark-assisted DI diesel engine was developed by modifying an outboard marine gasoline engine to operate on diesel fuel with high fuel efficiency similar to a diesel engine, yet retain the two-stroke engine advantages of low cost, light weight, and high power-to-weight ratio. Engine modification was concentrated in the area of the combustion system, including transfer port design to generate air swirl in the cylinder, and combustion chamber design to generate air squish and turbulence. Bore and stroke (84 × 72 mm) remained the same as those of the base engine. The experimental engine used the production engine's piston, crankshaft, connecting rod, bearings, and cylinder block. The transfer port design was optimized using a flow test bench for best swirl and air flow pattern with a simple flow visualization technique. The best combustion chamber geometry, compression ratio, and fuel injection spray pattern were determined through engine experiments.