Exhaust Systems of Two-Stroke Engines

Abstract

Crankcase-scavenge two-stroke engines have always been fitted with a large expansion chamber immediately outside the exhaust ports, but this is by no means essential, as such engines will operate very satisfactorily with a plain exhaust pipe. The length of this pipe is most important and has a controlling influence on the scavenging of the cylinder and the performance of the engine. Even when an expansion chamber is used in the exhaust system, the length of exhaust pipe still has a very marked effect on engine performance. It has been found that certain arrangements of this combination of expansion chamber and pipes completely upset the performance of the engine, whilst others improve the performance, but it is possible to calculate the “equivalent length” of any system of this nature, and so arrange matters that the exhaust system is a help rather than a hindrance to the engine. Though certain results have not been satisfactorily explained, the tests carried out do give a fairly clear indication of the way in which the engine performance is affected by the variations in pressure in the exhaust system. The concluding section gives a description of the principle on which the self-induction engine works, and indicates how use is made of the pressure variations or oscillations in the exhaust pipe to scavenge the engine cylinder, resulting in the complete elimination of the air pump on a two-stroke engine.