Analytically Predicted improvements in the Scavenging and Trapping Efficiency of Two-Cycle Engines

The effects of using trapezoidal shaped protuberances over the squish region of the piston bowl on the characracteristics of the air exchange process of a uniflow, two-cycle diesel engine is investigated analytically. The CONCHAS multidimensional code is used in this analysis. The engine geometry and boundary conditions simulates the operation of an experimental engine at maximum load and speed.

The predicted air breathing characteristics of the engine with a base case protuberance are compared against those without a protuberance. The effect of protuberance height is studied by repeating the calculations with protuberance heights above and below the base case. The effects of the radial position of the protuberance are also studied by comparing similar calculations with protubeances at larger and smaller radius than those of the base case. The results are presented in terms of cylinder total, fresh air and residual gas mass prediction plots, as well as flow rate predictions at the valves and ports for each case. The detailed features of the flowfield are represented in terms of velocity vector plots, fresh air concentration contour plots and zero vertical velocity vector plots at selected crank angles. Predictions are evaluated by comparing the results against the base reference case.

In review, the predictions show that the engine scavenging and trapping efficiencies are influenced considerably by the use of a protuberance. With proper choice of the location and height of the protuberance, it is possible to improve the volumetric, trapping and scavenging efficiency of the engine at the same time. Basic physical reasons for these improvements are described through evaluation of the predictions.