A 3D-CFD Methodology for Combustion Modeling in Active Prechamber SI Engines Operating with Natural Gas

Active prechamber combustion systems for SI engines represent a feasible and effective solution in reducing fuel consumption and pollutant emissions for both marine and ground heavy-duty engines. However, reliable and low-cost numerical approaches need to be developed to support and speed-up their industrial design considering their geometry complexity and the involved multiple flow length scales. This work presents a CFD methodology based on the RANS approach for the simulation of active prechamber spark-ignition engines. To reduce the computational time, the gas exchange process is computed only in the prechamber region to correctly describe the flow and mixture distributions, while the whole cylinder geometry is considered only for the power-cycle (compression, combustion and expansion). Outside the prechamber the in-cylinder flow field at IVC is estimated from the measured swirl ratio. A flame area evolution model combined with a deposition ignition model is used to describe the flame propagation process. Experimental data from a single-cylinder engine were used to validate the proposed approach. Different operating conditions were considered, to evaluate the effects of an increasingly lean mixture inside the main chamber, as well as the impact of a load variation. Comparison between computed and experimental data of in-cylinder pressure and heat release rate allowed to assess the capability of the modelling strategy in capturing the different phases of the combustion process originating by turbulent jet ignition.