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1 There is an increasing demand to improve the fuel economy of motorcycles in recent years. The lean-burn combustion is one effective means of accomplishing this. However, it is difficult to introduce the lean-burn concept while maintaining the high output and the quick response these are important for application to motorcycles. In this research, we have developed a fuel supply system that realizes both the quick response and the stable lean combustion using a conventional injector targeted for small motorcycles. This system intensifies the in-cylinder flow and helps the fuel droplet atomization, much better than conventional system due to air flow generated by the additional the passage. Therefore these improve the air-fuel mixture formation and flame propagation. As a result, it was possible to make the air-fuel mixture leaner while improving the throttle response by shortening the distance over which the fuel is carried.

A construction of the technology concerning fuel consumption improvement is an important problem not only for the four-wheeled vehicle but also for the motorcycle in recent years when petroleum resources are depleted rapidly. Yamaha originally developed a new fuel injection system (YMJET-FI) and applied the system to a single cylinder, water-cooled and small-displacement engine. In this paper, we would introduce the results of improving the fuel economy with keeping high performance. Improvements were noted in three matters, namely, in the lower load range, 1.Strengthening of in-cylinder flow, 2.Atomization of fuel spray, and 3.Reduction of wall film quantity.