A Two-Stroke Cycle Gasoline Engine with Poppet Valves in the Cylinder Head - Part II 920780

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.
In the present paper, for more certainly preventing the short-circuiting of the scavenging gas directly from the scavenging valves to the exhaust valves, shrouded scavenging valves were installed.
As a result, for the case where the scavenging valves are fully open, the shirt-circuiting phenomenon was improved considerably, and the scavenging and power output characteristics were improved markedly.
For example, shrouded valves resulted in a maximum 47% increase in the trapping efficiency and a maximum 27% increase in the charging efficiency. Moreover, the correlation between the in-cylinder scavenging flow and the scavenging performance was examined by a steady-flow visual model test using a transparent cylinder and a real engine cylinder head.


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