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 dual fuel mode has demonstrated to be effective for the reduction of particulate matter and nitric oxides emissions from this type of engine. In particular, methane is injected into the intake manifold to form a premixed charge with air, while a reduced amount of diesel oil is still directly injected to ignite the mixture inside the cylinder. As a matter of fact, the liquid fuel burns following the usual diffusive combustion, so activating the gaseous fuel oxidation in a premixed flame. Clearly, the whole combustion process appears to be more complex to be described in a CFD simulation, mainly because it is not always possible to select in the 3-dimensional codes a different combustion model for each fuel and, also, because other issues arise from the interaction of the two fuels. In this work, the Autoignition-Induced Flame Propagation model, which is included in the ANSYS Forte® tool, is applied since it represents the most appropriate model to describe the dual fuel combustion. Indeed, this model uses the G-equation to track the position and the propagation of the premixed turbulent flame, but the flame activation source is represented by the autoignition kinetics reaction scheme for the n-dodecane. The results discussed in this paper refer to experimental tests carried out on an optically accessible research engine whose real geometry and mesh were reproduced with the K3PREPW tool. Through the use of a system of sensors and optical diagnostic, the combined numerical - experimental study allows a deeper investigation of phenomena that take place in real dual fuel operations characterized by different engine speeds, 1500 and 2000 rpm, load levels, 2 and 5 bar of BMEP, injection timing and a premixed ratio between 86 and 89%.