The Effects of Engine Speed on the Scavenging Characteristics of a Two-Cycle Engine

The effects of engine speed on the fresh air breathing characteristics of a uniflow, two-cycle engine were studied by using a multidimensional computational code for in-cylinder flows. Computed results are presented describing both global and detailed features of the flow field during the air breathing and compression stroke, at four different engine speeds, namely, 400, 700, 900, and 1000 rpm. Global features are presented including the variations of cylinder pressure, mass, angular momentum, turbulence kinetic energy, and the exhaust mass as a function of crank angle. Detailed features of the flow field are presented in terms of the velocity vector plots, fresh air concentration contour plots, and swirl velocity contour plots at certain crank angles. The volumetric scavenging, and charging efficiency decreases, but the trapping efficiency increases with increasing engine speed. A simple scavenging model (correlating the fraction of fresh air in exhaust to the fraction of fresh air in cylinder) suitable for use in engine performance algorithms is presented. Predictions indicate that the effect of engine speed on such a model is not negligible. The use of similar models would be questionable and limited in range, if they are insensitive to engine speed. The residual gas concentration at TDC is shown to be relatively uniform at low engine speeds, but at high speeds, the residual gases are more concentrated at the squish region and the fresh air is more concentrated near the bowl center.