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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 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 general, two-stroke engines have a tendency to develop irregular combustion under light load. Under this condition, two-cycle motorcycles have inferior riding qualities. This paper discusses the operating conditions of the engine under which irregular combustion takes place. To analyze irregular combustion, the gas scavenging process is simulated and the adiabatic flame temperature is introduced as a criteria. Auto ignition, which is apt to occur under light load and high speed, is also investigated and is discussed in association with gas temperature and adiabatic flame temperature at the moment of ignition.

The 1979 U.S. EPA emission standards on larger-sized two-stroke cycle engine motorcycles were very strict. This paper discusses the butterfly exhaust valve system installed on the YAMAHA RD400 Daytona Special which is the only two-stroke cycle engine motorcycle in the controlled 400cc class. The paper also refers to the structure of the valve system and the principle of valve operation as well as its effects on reducing exhaust emissions, improving brake specific fuel consumption, and increasing engine power at part-throttle. In addition, the butterfly exhaust valve system has a great effect on reducing engine surge due to the cyclic variations in combustion inherent in two-stroke cycle engines. The reduction ratio of engine surge is expressed quantitatively in our unique method. The above effects were investigated by means of exhaust gas velocity measurements and by analysis of combustion pressure.