Current demands for high fuel efficiency and low emissions in automotive powerplants have drawn attention to the two-stroke engine configuration. The present study measured trapping and scavenging efficiencies of a firing two-stroke spark-ignition engine by in-cylinder gas composition analysis. Intermediate results of the procedure included the trapped air-fuel ratio and residual exhaust gas fraction. Samples, acquired with a fast-acting electromagnetic valve installed in the cylinder head, were taken of the unburned mixture without fuel injection and of the burned gases prior to exhaust port opening, at engine speeds of 1000 to 3000 rpm and at 10 to 100% of full load. A semi-empirical, zero-dimensional scavenging model was developed based on modification of the non-isothermal, perfect-mixing model. Comparison to the experimental data shows good agreement.Until recently, two-stroke engines had been considered unsuitable for automotive use under current and pending emission regulations. With a gasoline injection system applied directly in-cylinder, however, the unacceptable short-circuiting of raw fuel during the scavenge period can be avoided. Thus, renewed interest in exploiting the potential advantages of a two- stroke engine-higher power to weight ratio, smaller package size, and fewer parts-is warranted.The ported two-stroke requires open exhaust ports during the entire scavenge period. Some fresh air is inevitably lost to the exhaust during the scavenge process, and is not trapped in the cylinder. To attain stoichiometry in the exhaust, so that a three-way catalyst could be employed, a direct-injection (DI) two-stroke engine would require a rich trapped charge to compensate for the air which bypasses the combustion process. As this inefficiency cannot be tolerated, three-way catalysts cannot be relied upon. Instead, emissions and fuel consumption are controlled by operating lean with high levels of residual, and a knowledge of combustion gas composition is crucial to a description of the engine operating condition.A consequence of the short-circuiting of fresh air into the exhaust is that the air-fuel ratio during combustion cannot be determined by mass measurements of the fuel and air supplied to the engine or by overall exhaust chemical analysis. Although the two-stroke charge may be stratified, an understanding of its emission characteristics is enhanced by knowledge of the over-all air-fuel ratio of the trapped charge. Further, it is desirable to determine the fraction of the in-cylinder mixture comprised of residual gases. This paper describes a measurement of the scavenging parameters of a DI two-stroke engine, from which trapped air-fuel ratio and residual fraction are calculated.