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The current work presents a recent development of the Extended Coherent Flamelet Model (ECFM) for 3D combustion modeling in spark-ignited gasoline engines. The reference-based ECFM model, originally published in 2003, computes the conditional unburned and burned gas species mass fractions from both real species and species tracers. This current work is motivated by two limitations of the reference-based model. First, the difference between convection of species tracers and convection of real species leads to small discrepancies between the two, due to high velocity gradients during gas exchange. This can lead to inaccurate estimation of the progress variable and consequently to negative conditional mass fractions in the burned gases after ignition. Second, the reference-based ECFM model assumes implicitly that the unburned and burned states correspond to the same mixture fraction. This assumption is valid for low stratification cases, but it can lead to substantial conditioning errors for highly stratified systems like gasoline direct injection (GDI) engines. To address these shortcomings, a new species-based ECFM (SB-ECFM) implementation is presented. In this species-based model, the unburned and burned gas states are entirely defined by the transported species in each zone. It is shown that SB-ECFM more reliably defines conditional quantities and the progress variable. This enhancement allows the use of a second-order central scheme in space when using full decoupling of auto-ignition and premixed flame progress variables as proposed in Robert et al., Proc. Comb. Inst, 2015, while the reference model is limited to the first-order upwind scheme in this case. Finally, simulations of a GDI engine are presented at different loads and rpm conditions. It is shown that, with the higher order scheme, SB-ECFM demonstrates very good agreement with measured pressure. |
