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The two-stroke engines were in the main stream of the outboard motors, but they have been replaced with the four-stroke counterparts reflecting the environmental protection movement in recent years. However, the replacement with four-stroke engines involves increased number of components and additional displacement, and the outboard motors tend to be larger and heavier. This represents an issue, since the maneuverability of the boat is degraded due to the inappropriate weight distribution on the boat. Yamaha outboard motors F300B and VF250A, of which the production started in the year 2009, are equipped with four-stroke engines, and yet achieved the light weight equivalent to their two-stroke counterparts. The production volume of these models reached 20,000 units.

Measurement using a three-dimensional anemometric-tester was made for the gas flow inside the cylinder of a two-stroke engine while the shape of the transfer port was modified. The relationship between port shape and engine performance was investigated for various factors that characterize the flow in cylinder. In this paper, we focused mainly on two engine running conditions: the maximum output at 11750 rpm and the output at 10000 rpm. As a result, we found that the maximum output is most related to the tangential inclination angles of the main transfer port, and the inner vent radius of the main transfer duct.

An original multi-body dynamics simulation program for reciprocating engine system with elastically deformable piston skirt was developed in order to understand and examine the secondary motion of piston. This program uses specialized equations of motion using only the rotational degree of freedom of each components taking the valiation of rotating speed of crank into account. In order to validate the practical use of this program, the calculations were compared with the measurements on the piston motion of a two-stroke engine for motorcycles and a four-stroke engine for automobiles, and good agreements were obtained between them.

A periodic impulsive sound at idle is occasionally described as ‘disagreeable’ in two-stroke engines. The relation between combustion conditions, piston vibrations, and the disagreeable sound is analyzed to clarify the phenomena. Some means to alleviate disagreeable sound are then proposed through stabilized combustion, high rigidity sound transfer systems, and refined skirt profiles. Experimental results are shown for the effects on main three factors evaluating disagreeable sound-loudness, impulsiveness, and frequency characteristics. In addition, piston behavior is measured, and the relation between piston motion and disagreeable sound is discussed in this paper.

In order to obtain more reduction of two-stroke motorcycle engine noise than usual, it becomes necessary to make improvements within the combustion process itself. This study was carried out for two objectives. One is the investigation of the relationship between combustion and noise, and the other one is the development of noise reduction techniques. As the result, it was discovered that there was a significant correlation between engine noise and (dP/dθ)max, called the maximum rate of cylinder pressure rise. Therefore, the reduction of the (dP/dθ) max was recognized to be effective for engine noise reduction. The optimized alteration of combustion chamber shape is the most effective noise reduction technique, because it is able to reduce (dP/dθ) max without any sacrifice of engine power. In fact, the level of noise reduction can be predicted by one of the parameters obtained from the combustion chamber shape.

In this study, an improvement of fuel consumption by changing the exhaust timing of a two-stroke engine has been made. The study results revealed that a remarkable improvement of fuel economy is possible by controlling the exhaust timing according to the engine speed. The reason for the better fuel economy was clarified through an analysis of exhaust gases, theoretical cycle calculations, and an analysis of combustion pressure. As an example of actual application, the results of tests made on an engine equipped with Yamaha power Valve System (YPVS), which is a variable exhaust timing mechanism using a tabor-shaped valve, will also be discussed.

In two-stroke engines, the exhaust port timing has a great effect on engine performance characteristics. If the exhaust port timing can be varied in response to variations in the engine speed, an extensive improvement of performance can be realized. The Yamaha Power Valve System (YPVS) increases engine output with a valve which operates in the exhaust passage; this valve controls exhaust port timing in response to engine speed, despite the extremely high thermal load encountered in the exhaust passage. This literature deals with the construction and operation principles of YPVS and its effect on the increase of engine output. Also discussed is a series of tests and measurements made on the delivery ratio and trapping efficiency in order to elucidate why engine output is increased.

This paper describes aspects of YAMAHA 2 cycle, high speed, high output engines. Generally speaking, in order to obtain good results in developing engine performance, high delivery ratios and high thermal and mechanical efficiencies are essential. In addition to these, the most suitable cooling and lubricating systems must be employed. YAMAHA has developed a separate and automatic lubrication system for 2-cycle gasoline engines, which keeps YAMAHA engines well lubricated.