Extension of a thickened flame model to highly stratified combustion—Application to a spark-ignition engine

An extension to the dynamic thickened flame model for large-eddy simulations (DTFLES) is presented, which allows modeling highly stratified combustion and accounts for spatial variations of flow and combustion metrics. The extension, called the stratified DTFLES (or SDTFLES) relies on a local thermodynamic formulation to calculate combustion parameters that are required for the thickened flame model, more specifically the wrinkling factor Ξ. The advantage of this method is that model parameters are calculated locally without information from outside the computational cell. The extension is developed using user-defined functions in STAR-CD v4.2, and tested first in a baseline parametric study and then in a computational four-valve direct-ignition spark-ignition engine operating in a highly-stratified late-injection mode. The large-eddy simulation (LES) results are compared to experimental results for validation purposes. The comparison covers the spatial variation of combustion parameters, qualitative flame progression, and quantitative in-cylinder pressure, indicated mean-effective pressure (IMEP), and heat release. The model extension is able to reproduce the in-cylinder flame behavior, wrinkling, and burn rate.