Press Room - Scaven ging and exhaust valve rotation simulation

	Scavenging and exhaust valve rotation simulation Dr.-Ing. Reiner Schulz, Wartsila Switzerland Ltd
	Sulzer two stroke diesel engines are used as "prime movers" in container ships, bulk carriers and tankers. Such ships need to be in service for as long as possible without stopping for a pre-scheduled overhaul, placing a continuous demand on engine developers in terms of reducing costs and at the same time improving reliability. CFD is a proven tool for minimizing experimental effort during the optimization of receivers, blowers, in-cylinder flow and lubrication systems.

	Sulzer engines are of uniflow scavenging design. Vessel propellers are generally directly driven with top speeds of 60 to 140 rev/min, resulting in stroke to bore ratios from 2.6 to 4.2 for a given mean piston speed slightly above appoximately 8 m/s. The resulting cylinder proportions are depicted in Fig.1 for a piston position at BDC. During scavenging, the burnt and unburnt gases can be kept more or less separate and the development of "plug like" flow is illustrated by three isosurfaces (170, 180, 190 deg CA). To improve the flow behavior even further, an accurate prediction of the flow through the ports (Fig.2) is needed. Experiments are difficult because the flow must be transient and flow visualization techniques in engines of this size are not available. On the other hand CFD can also be applied to calculate flow and pressure losses in inlet and exhaust ports and swirl in the cylinder, as well as the spin of the exhaust valve. Sufficient valve rotation is desirable for achieving long times between overhauls. To achieve this, small vanes are added to the stem that use the exhaust gas kinetic energy for rotating the valve (Fig.3). Fig. 1: Flow visualization of isosurfaces The rotating valve is a simple example of fluid–structure interaction, which was implemented via STAR-CD user coding capability. The rotation angle and a comparison of measured and calculated pressure traces is depicted in diagram 1. Note that friction in the valve train is not known and was not taken into account in the calculation.	Fig. 2: Close up view of inlet ports Fig. 3: Close up view of valve Diagram 1 – Comparison of experimental and calculated results
	In conclusion, we found that the residual gas content, the ratio of trapped to delivered charge and the pressure trace can be calculated accurately and even the rotation of the valve is close to measured results. Thus we have a tool for design optimization of the whole scavenging cycle, helping to reduce design and testing times. For further information contact: reiner.schulz@wartsila.com