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This paper deals with an investigation of the scavenging and power performances of a two-stroke cycle gasoline engine having poppet valves in the cylinder head but no cylinder ports. In a previous experiment, a model test apparatus was used to carry out the experimental analysis of scavenging performance. We found as a result of this analysis that better scavenging efficiency could be obtained by improving the flow pattern of scavenging gas in the cylinder by means of scavenging valves with shroud than by perfect diffusion scavenging. The present experimental study using a real engine was conducted according to this finding. The basic engine is a four-stroke motorcycle engine, one cylinder, 250 cm3, twin-cam, four-valve. The engine was modified into a two-stroke cycle engine by altering the camshafts so as to have twice the engine speed of the four-stroke cycle engine, and the valve timings were adjusted by a pin that connected the camshaft and sprocket.

This paper deals with the continued investigation of The scavenging and power characteristics of a two-stroke cycle gasoline engine with scavenging and exhaust valves in the cylinder head. Since this engine is reconstructed from a four-stroke cycle engine, its intake valves are used as the scavenging valves, and the scavenging of the cylinder consists of U-type flows. In the previous paper (SAE paper No. 901664), by setting up a deflector near the valve seat for each scavenging port, the study to prevent the scavenging gas short-circuit effect was conducted. As results in those test, by setting up the deflector within each scavenging port the scavenging effect and power characteristics were improved considerably. However, as the scavenging valve lift was increased, the effectiveness of the deflector decreased. The difference with or without the deflectors was only 10∼14% in scavenging efficiency, and only improvements of 5∼10% in charging efficiency were obtained.

Improvements in the engine performance, efficiency, scavenging, and emission characteristics of a modified two-stroke-cycle engine with the poppet valves and gaseous-fuel direct-injection system are described. In previous papers, SAE 901664 [1]* and 920180 [2], in which the scavenging characteristics were mainly investigated, the modified two-stroke-cycle engine was based on a production four-stroke-cycle engine. The base engine intake valve was used as scavenging valve, with a shroud installed for preventing the short-circuiting charge loss. As Shown in the previous results, considerable improvement of the scavenging efficiency was obtained. While a gasoline fuel carburetion system was used in the earlier engine [1], [2], the present engine used a propane gas direct-injection system, with the purposes of further decreasing fuel short-circuiting loss and also improving combustion preparation characteristics in terms of fuel atomization, vaporization and mixing with air.