Combined CFD - Experimental Analysis of the In-Cylinder Combustion Phenomena in a Dual Fuel Optical Compression Ignition Engine

Methane supply in diesel engines operating in dualfuel mode has demonstrated to be effective for thereduction of particulate matter and nitric oxidesemissions from this type of engine. In particular, methane isinjected into the intake manifold to form a premixed chargewith air, while a reduced amount of diesel oil is still directlyinjected to ignite the mixture inside the cylinder. As a matterof fact, the liquid fuel burns following the usual diffusivecombustion, so activating the gaseous fuel oxidation in apremixed flame. Clearly, the whole combustion processappears to be more complex to be described in a CFD simulation,mainly because it is not always possible to select in the3-dimensional codes a different combustion model for eachfuel and, also, because other issues arise from the interactionof the two fuels. In this work, the Autoignition-Induced FlamePropagation model, which is included in the ANSYS Forte®tool, is applied since it represents the most appropriate modelto describe the dual fuel combustion. Indeed, this model usesthe G-equation to track the position and the propagation ofthe premixed turbulent flame, but the flame activation sourceis represented by the autoignition kinetics reaction schemefor the n-dodecane. The results discussed in this paper referto experimental tests carried out on an optically accessibleresearch engine whose real geometry and mesh were reproducedwith the K3PREPW tool. Through the use of a systemof sensors and optical diagnostic, the combined numerical -experimental study allows a deeper investigation of phenomenathat take place in real dual fuel operations characterized bydifferent engine speeds, 1500 and 2000 rpm, load levels, 2 and5 bar of BMEP, injection timing and a premixed ratio between86 and 89%.