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Using natural gas as a fuel in internal combustion engines is a promising way to obtain efficient power generation with relatively low environmental impact. Dual fuel operation is especially interesting because it can combine the safety and reliability of the basic diesel concept with fuel flexibility. To deal with the greater number of degrees of freedom caused by the interaction of two fuels and combining different combustion regimes, it is imperative to use simulation methods in the development process to gain a better understanding of the combustion behavior. This paper presents current research into ignition and combustion of a premixed natural gas/air charge with a diesel pilot spray in a large bore diesel ignited gas engine with a focus on 3D-CFD simulation. Special attention was paid to injection and combustion. The highly transient behavior of the diesel injector especially at small injection quantities poses challenges to the numerical simulation of the spray.

The introduction of the so called Emission Controlled Areas within the IMO Tier III legislation forces manufacturers of maritime propulsion systems to adherence to stringent emission thresholds. Dual fuel combustion, which is characterized by the injection of a small amount of fuel oil to ignite a premixed natural gas air mixture, constitutes an option to meet this target. At high diesel substitution rates and very short pilot injection events, the injector is operated in the ballistic regime. This influences spray penetration, mixture formation and ignition behavior. In the present work, a seven-hole dual fuel injector was measured in a combustion chamber to provide data for the generation of a CFD model using the commercial code AVL FIRE®. The liquid and the vapor phase of the fuel spray were quantified by Mie-scattering and Schlieren-imaging technique for different chamber conditions.