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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.

Four-, five- and six-valve engines having two, three and four intake valves, respectively, were fabricated. Fifteen types of output characteristics were then investigated using these engines by varying the intake specifications to examine the relationship of the intake valve mean effective area and the number of valves needed to attain the required maximum output. Improvement of combustion in the light load range at low speeds was also examined by comparing the cylinder flow of four- and five-valve engines. As a result, it was found that the maximum output of multi-valve engines is generated at essentially the same mean intake Mach number regardless of the number of valves, and proportional to the intake valve mean effective area for an identical single cylinder volume. In addition, the result of five-valve engines with the larger intake valve mean effective area than four-valve engines showed that they could attain both the satisfactory mean flow coefficient and tumble.