Emission Control of Two-Stroke Motorcycle Engines by the Butterfly Exhaust Valve

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.