Application of Exhaust Pipe Restriction Technique to a Small Two-Stroke Engine

A simple method to reduce HC emissions caused by fuel short-circuiting in a two-stroke engine is to apply a certain restriction to the exhaust gas flow in the exhaust pipe. This technique, developed for motorcycles, vehicles and outboards, has been applied to a small off-road two-stroke engine with a constant speed operating condition. In order to assess the feasibility and effectiveness of this technique for the reduction of HC and CO emissions in such an engine, a butterfly valve was installed in the exhaust pipe next to the exhaust port. Based on test results using the butterfly valve, a cylindrical restrictor was developed to generate various restrictions to the gas flow in the exhaust pipe and consequently reduce the fuel short-circuiting. The effectiveness of the restrictor on emission reduction was further improved by leaner air/fuel mixture. The experimental results showed improved engine performance including the best 24% reduction of total weighted HC and 25% reduction of total weighted CO when the exhaust pipe restriction was 50%. Additionally, Mota, a two-stroke engine simulation program, was used to simulate the engine performance with reduced diameter of the exhaust pipe. The outcomes of this simulation support and explain the experimental results.