Integrated Analysis of the Scavenging Process in Marine Two-Stroke Diesel Engines

Abstract

Large commercial ships such as container vessels and bulk carriers are propelledby low-speed, uniow scavenged two-stroke diesel engines. An integralin-cylinder process in this type of engine is the scavenging process, where the burned gases from the combustion process are evacuated through the exhaust valve and replaced with fresh air for the subsequent compression stroke. The scavenging air enters the cylinder via inlet ports which are uncovered by the piston at bottom dead center (BDC). The exhaust gases are then displaced bythe fresh air entering the cylinder. The scavenging ports are cut with an angle to introduce a swirling component to the ow. The in-cylinder swirl is benecial for air-fuel mixture, cooling of the cylinder liner and minimizing recirculation zones where pockets of exhaust gas are trapped. However, a known characteristic of swirling ows is an adverse pressure gradient in the center of the ow, which might lead to a local decit in axial velocity and the formation of central recirculation zones, known as vortex breakdown. Ever more stringent emission legislations over the last 10-15 years have changed the engine lay out diagram in the pursuit of an engine which is both fuel effective and within the current emission legislations. To achieve this goal, a fundamental understanding of the in-cylinder processes, and the interactions between them are needed. This thesis aims at providing in-depth knowledge of the scavenging process and to identify the parameters that governs its performance.This thesis will present a CFD model that is tested and validated with quantitative data obtained from a dedicated test engine and during engine commissioning on location at the shipbuilder. The CFD model comprises the full geometry of a single cylinder from scavenge receiver to the exhaust receiver fora two-stroke diesel engine. Time resolved boundary conditions corresponding to measurements obtained from an operating engine as well as realistic initial conditions are used in the simulations. The CFD model provides a detailed description of the in-cylinder ow from exhaust valve opening (EVO) to exhaust valve closing (EVC). A string of studies are included in this thesis. An engine load sweep isincluded to evaluate the scavenging process as function of engine load. The engineload sweep follows the propeller curve, where the engine speed varies with the engine load. This implies that the pressure in the scavenge and exhaust receivers increase while the scavenge port exposure time, tscav, decrease. Further the scavenging pressure is varied while the engine speed is kept constant. From the perspective of the scavenging process this will resemble a load sweep following a generator curve. The scavenge port angle is varied to investigate the influence of in-cylinder swirl. A total of 7 port angles is applied; α = 0°,α = 10°, α = 15°, α= 18°,α = 20°, α = 25° and α= 30°.The CFD analysis shows that the bulk purity of air in the cylinder is proportional to the volumetric flow rate (mass flow rate divided by the air density) of scavenge air through the cylinder. The volumetric flow rate decreases with density for a given mass ow rate. When the engine load is increased, both the mass flow rate and the scavenging pressure is increased due to the turbo charger response. It is shown in this thesis that the increased density of the scavenge air, in conjunction with the reduced port exposure time, actually decrease the volume ow rate of air in the cylinders. This impairs the scavenging process at high engine loads. The CFD model also shows that the scavenging process consist of two sub processes. The volumetric scavenging, where the scavenge air displace the exhaust gas. And the push out process, where the piston displace the scavenge air and exhaust gas mixture between inlet port closing, IPC, and exhaust valve closing, EVC. The port angle study shows that the scavenging process is unaected by the changes in the in-cylinder swirl. Visualization of a passive scalar shows some influence of the in-cylinder distribution of scavenge air and exhaust gas, but volumetric displacement is the prime mover in the scavenging process.The CFD simulations is in good agreement with a simple perfect displacement model proposed by Sher (1990). The perfect displacement model is used as the basis for a simplied scavenging model in conjunction with a model topredict the contribution from the push out process. The model is modied to tthe CFD results to account for mixing between the scavenge air and the exhaustgas and can therefore only be considered as a preliminary model. However, this model shows that it is possible to obtain a simple model which can be used to ensure adequate scavenging based on turbocharger characteristics and exhaust valve lift proles.The CFD model described in this Ph.D. thesis is used to investigate the response of key parameters on the scavenging process and gives detailed andprofound insight to an integral in-cylinder process in the two-stroke diesel enginecycle. Further, the results from the CFD model is a valuable part of the R&Dstrategy of "full cycle CFD modelling" where the scavenging CFD model shallbe coupled together with a combustion CFD model to simulate the completeengine cycle.