Modeling of Scavenging and Plugging in a Twin-Cylinder Two Stroke Engine Using CFD

This paper presents a study on multi-dimensional CFD modeling of scavenging and plugging in a twin-cylinder two-stroke engine. A general-purpose CFD code, KIVA, was extended to track an arbitrary number of moving pistons. The code was also modified to allow piston snapping through complicated transfer ports. Thus, a multi-cylinder simulation together with a full exhaust manifold to fully account for the interaction between scavenging and plugging becomes possible. The developed code is intended to be a numerical tool for exhaust-manifold design and optimization. The studied engine is a five-port loop scavenged twin-cylinder engine with a cylinder displacement of 432 cc. The computed exhaust pressure was compared with measured data, and reasonably good agreement was obtained. The results were also compared with those from a one-dimensional gas dynamics model, which over-predicts the plugging intensity while under-predicting the pressure loss in the exhaust manifold. It is also suggested that a simulation that only includes a partial exhaust manifold may not be adequate to predict details of the in-cylinder air/fuel mixing when a uniform boundary condition is applied at the exhaust port because the predicted in-cylinder swirl level is significantly different from that of a full exhaust manifold simulation. The effect of plugging pulse on the flow pattern and the air-fuel mixing was also examined. It is shown that the plugging pulse not only increases the trapped mass, but also enhances the in-cylinder tumble motion and promotes air/fuel mixing.