Since Kawasaki introduced solo standing type watercraft "Jet Ski" in 1974, Jet Ski has been well-accepted by young generation because of its dynamic sports riding. There are new type Jet Ski that can accomodate two or three persons and more customers now can enjoy sporty and comfortable riding with their family members on the new type. The engine of Jet Ski has been improved in order to meet more sporty riding and higher reliability for the various models. For example, the earliest Jet Ski engine performed 15kW and the latest one for super sports model brings up to 107kW at peak. For the effective mean of increasing the power, engine displacement has been enlarged and exhaust system has been improved. Jet Ski is an exciting new type of recreational watercraft with many features such as compact and lightweight, closer contact with water, great feeling in speed and acceleration, and provision of water jet propulsion. Jet Ski has applied two stroke cycle engine as an effective solution to meet the features. There are many features in the construction and component parts on the Jet Ski engine such as ① lightweight of adapting aluminium for many parts, ② all positional operation by utilizing non float carburetor, ③ good unti-immersion of special water tight design, and ④ good corrosion resistance. From the current enviromental view point, conventional two stroke engine on present Jet Ski has to be improved. As possible alternative for future engine, four stroke engine and two stroke engine with direct fuel injection system are considered. Four stroke engine is inferior at both weight/power and size/power. The engine is also difficulties of lubrication and water worthness when Jet Ski rolls over. Two stroke engine with direct fuel injection system looks effective to improve the weakpoint of conventional two stroke engine because the engine with direct fuel injection system can decrease THC emission and mis-fire in the light load range.
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