A Novel Laminar Flame Speed Correlation for the Refinement of the Flame Front Description in a Phenomenological Combustion Model for Spark-Ignition Engines

This work focuses on the effects of the laminar flame speed (LFS) and flame stretch on the phenomenological modeling of the combustion process in spark ignition engines. The study is carried out using a 1D model of a small-size naturally aspirated SI engine, equipped with an external EGR circuit. The model, developed in GT-Power™ environment, includes advanced sub-models of the in-cylinder processes. The combustion is modeled using a fractal approach, where the burning rate is directly related to the laminar flame speed. A novel LFS correlation based on 1D chemical kinetics computations is presented and assessed with the experimentally derived Metghalchi and Keck correlation. Moreover, the effects of the flame stretch, evaluated according to an asymptotic theory, are properly considered in the combustion model. In order to verify the consistency and accuracy of the present approach, the model predictions are compared with the results of experimental tests performed at full- and part-load engine operations, with and without the activation of the EGR device. The proposed LFS correlation provides more accurate results than the Metghalchi and Keck one, especially for low engine loads and high EGR rates. The introduction of the flame stretch model leads to a much better prediction of the early combustion stage at low engine loads.