Spark Ignition Engine Simulation Using a Flamelet Based Combustion Model

Three-dimensional Computational Fluid Dynamics (CFD) has become an integral part in analysing engine in-cylinder processes since it provides detailed information on the flow and combustion, which helps to find design improvements during the development of modern engine concepts. The predictive capability of simulation tools depends largely on the accuracy, fidelity and robustness of the various models used, in particular concerning turbulence and combustion. In this study, a flamelet model with a physics based closure for the progress variable dissipation rate is applied for the first time to a spark ignited IC engine. The predictive capabilities of the proposed approach are studied for one operating condition of a gasoline port fuel injected single-cylinder, four-stroke spark ignited full-metal engine running at 3,500 RPM close to full load (10 bar BMEP) at stoichiometric conditions. The combustion model employs pre-calculated unstrained laminar flamelet libraries parametrised by the reaction progress variable, its variance, the unburnt mixture temperature and pressure. The look-up tables are generated using both a skeletal (29 species and 49 reactions) and a detailed (1,389 species and 5,935 reactions) mechanisms for iso-octane - air combustion to study the influence of chemical kinetics. A simple"energy deposition" approach has been adopted as a first step to model spark ignition and its impact on the results is discussed. The sensitivities of the results to the calculation of the unburnt mixture temperature and the temperature increment used in the look-up table are also investigated. The predicted pressure variations show fair agreement in view of the first application of the model to IC engines and a comparison of this result with those obtained using the level-set approach is discussed. Despite the observed sensitivities to the ignition treatment, the unburnt mixture temperature calculation and the chemical kinetics, this first application shows considerable promise of the proposed flamelet approach to model premixed combustion in IC engines