Numerical Investigation Of The Scavenging Process On A Low-speed Two-stroke Marine Diesel Engine

For large marine two-stroke diesel engines,it is of great significance to optimize scavenging process to improve engines’ power output and NOx emissions.This paper is based on a marine two-stroke diesel engine and conducts numerical simulations to optimize its transient scavenging process,which can provide useful information for new marine diesel engines’ design and engineering application.At the beginning,a complete working cycle of a large marine diesel engine was simulated by using the 3D-CFD software CONVERGE.The model was validated by mesh sensitivity test and experiment data.Based on this calibrated model,the influence of scavenge ports’ structure and exhaust system on the in-cylinder flow characteristics and scavenging process were investigated.The results show that,the swirl orientation angle can affect in-cylinder swirl ratio,tangential velocity and axial velocity field significantly.For the scavenge ports with a single inclined angle,a larger or smaller SOA will go against the scavenging performance.For optimizing the scavenging process,this paper invented combined scavenge ports.The combined scavenge ports have a satisfactory scavenging performance,whose distance between the vortex core and Z-axis is also lowest.The overall inclined scavenge ports can reduce the peak of in-cylinder swirl ratio and improve the scavenging performance only in small inclined angle.The analysis of exhaust system shows that adjusting the exhaust valve open(EVO)and close(EVC)timings has great impacts on the scavenging performance,in-cylinder temperature and pressure,thus affecting the engine’s power output and NOx emissions.The diameter of exhaust valve mainly affects the in-cylinder flow characteristics,but has little effects on the scavenging performance.