The Evaluation of Discharge Coefficients in the Cylinders of High Performance Two-Stroke Engines

This paper presents a discussion of the difficulties in evaluating the discharge coefficients of ports in the cylinder wall of high performance two-stroke engines. Traditionally such evaluation requires the knowledge of the area of the port on a chord normal to the direction of flow through the port. However, due to the complex shape of ports in these engines, it is difficult to know the exact flow direction without some kind of flow analysis. Results of a study conducted on various methods of obtaining the port area either by assuming a flow direction or using geometrical information are presented. From the information presented it can be seen that the use of wall area is quite acceptable to determine discharge coefficients. This wall area requires no interpretation by the experimenter and therefore also permits a direct comparison with other ports.

It is also required to know how these discharge coefficients are affected by both area ratio and pressure ratio in both flow directions for use in engine simulation. The paper reports on an investigation of a cylinder having 54 mm bore and 54.5 mm stroke. It has a basic port layout consisting of a bridged exhaust and five transfer ports (two side main, two side auxiliary and one central rear). The results are presented as 3D maps for every port within the cylinder. The influence of flow interaction between ports is also highlighted with results of the two adjacent side transfer ports measured together being presented.