Scavenging efficiency relates directly with the power delivery capability in a conventional 2-stroke cycle engine. However, the high scavenging efficiency at high load is unavoidably accompanied by a large short-circuiting of fresh charge resulting in the loss of fuel economy and the increase in the unburned HC emission. Since the scavenging efficiency can be measured only by an accurate collection of completely burned exhaust gas, there were several unsuccessful attempts to construct the sampling valve. As a next best alternative completely mixed exhaust gas of the burned and the unburned was sampled in an exhaust surge tank under various operating conditions. The gas composition was analyzed in a gas chromatograph from which trapping efficiency could be estimated by O2 concentration measurement instead of scavenging approximation. There were several noticeable findings in the relationships between the trapping efficiency and the variables such as delivery ratio, exhaust port opening time and HC concentration in the exhaust gas.
