Computational Analysis of Internal and External EGR Strategies Combined with Miller Cycle Concept for a Two Stage Turbocharged Medium Speed Marine Diesel Engine

摘要

In this work different internal and external EGR strategies, combined with extreme Miller cycles, were analyzed by means of a one dimensional CFD simulation code for a Wartsila 6 cylinder, 4 strokes, medium speed marine diesel engine, to evaluate their potential in order to reach the IMO Tier 3 NO_x emissions target. By means of extreme Miller cycles, with Early Intake Valve Closures (up to 100 crank angle degrees before BDC), a shorter compression stroke and lower charge temperatures inside the cylinder can be achieved and thanks to the cooler combustion process, the NO_x specific emissions can be effectively reduced. EIVC strategies can also be combined with reductions of the scavenging period (valve overlap) to increase the amount of exhaust gases in the combustion chamber. However, the remarkably high boost pressure levels needed for such extreme Miller cycles, require mandatorily the use of two stage turbocharging systems. Despite two stages turbocharging, combined with extreme Miller timings, may allow up to 50% NO_x reduction compared to a conventional, single stage turbocharger architecture, further NO_x emissions reductions are necessary to meet the IMO Tier 3 NO_x limit. Higher EGR percentages, which could be achieved by means of external circuits, were therefore also evaluated. However, it should be pointed out that, although the external EGR technology is well established for automotive and heavy duty diesel engines, it is not yet state of the art for marine diesel engines, and its application to a highly boosted engine using extreme Miller timings is not straightforward. Several different complex EGR routes were thus investigated, and for the preliminary assessment of their NO_x emissions abatement potentialities, the use of numerical simulation allowed a detailed and extensive evaluation of the effects on engine performance, fuel consumption, NO_x emissions and thermal and mechanical loads on engine components of the combination of different intake valve profiles, intake valve closure timings and scavenging periods with different external exhaust gas recirculation solutions. Percentages of exhaust gases recirculated in the combustion chamber up to 20% were evaluated that combined with extreme Miller timings (up to 100 crank angle degrees before BDC) allowed up to 90% NO_x reduction compared to a conventional, single stage turbocharger architecture, with only moderate BSFC increase.