Parametric Study of the Scavenging Process in Marine Two-Stroke Diesel Engines

Large commercial ships such as container vesselsand bulk carriers are propelled by low-speed, uniflowscavenged two-stroke diesel engines. The integral in-cylinderprocess in this type of engine is the scavenging process,where the burned gas from the combustion process isevacuated through the exhaust valve and replaced with freshair for the subsequent compression stroke.

The scavenging airenters the cylinder via inlet ports which are uncovered by thepiston at bottom dead center (BDC). The exhaust gas is thendisplaced by the fresh air. The scavenging ports are angled tointroduce a swirling component to the flow. The in-cylinder swirlis beneficial for air-fuel mixture, cooling of the cylinder liner andminimizing dead zones where pockets of exhaust gas aretrapped.

However, a known characteristic of swirling flows is anadverse pressure gradient in the center of the flow, whichmight lead to a local deficit in axial velocity and the formation ofcentral recirculation zones, known as vortex breakdown. Thispaper will present a CFD analysis of the scavenging process ina MAN B&W two-stroke diesel engine.

The study include aparameter sweep where the operating conditions such as airamount, port timing and scavenging pressure are varied. TheCFD model comprise the full geometry from scavenge receiverto exhaust receiver. Asymmetric inlet and outlet conditions isincluded as well as the dynamics of a moving piston and valve.Time resolved boundary conditions corresponding tomeasurements from an operating, full scale production, engineas well as realistic initial conditions are used in the simulations.The CFD model provides a detailed description of the incylinderflow from exhaust valve opening (EVO) to exhaustvalve closing (EVC).

The study reveals a close couplingbetween the volume flow (delivery ratio) and the in-cylinderbulk purity of air which appears to be independent of operatingconditions, rpm, scavenge air pressure, BMEP etc. The bulkpurity of air in the cylinder shows good agreement with asimple theoretical perfect displacement model.