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A semi-empirical model for the scavenging process in a crankcase-scavenged two-stroke cycle engine is proposed. The model is based on the assumption that, typically, the time variation profile of the exhaust gas purity, β, exhibits a sigmoid-type curve while a rounded protuberance usually appears on its back, thus forming a ‘crest’ profile. The hump has been interpreted as the occurrence of an extensive short-circuiting. An exponential function of the form of is suggested to fit this curve, from which the scavenging efficiency has been derived. Two parameters, namely the mixing degree, b, and the short-circuiting degree, c, are to be calibrated. However, it was found that the selection of the best fitting values for b and c do not depend on the engine speed or on the engine load. Rather, b and c are inherent properties of the engine design. For modern engine design a value of 0.57 is recommended for b and a value of 0.81 for c/c-max. Calculations of the scavenging efficiency were found to be in excellent agreement with predictions of a detailed computer program for three small Schnürle-type different make engines. The present model does not consider a specific mechanism for the gas exchange process, and as such it is believed that it is applicable to other engine types (cross, loop or uniflow) to the same extent.

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