Adapting diesel large-eddy simulation spray models for direct-injection spark-ignition applications

Direct-injection spark-ignition engines are a promising technology to achieve high efficiency and low emissions for automotive applications. Robust operation of direct-injection spark-ignition engines requires understanding the local fuel–air mixing process. Large-eddy simulations can capture more details of the local mixing structure than traditional Reynolds-averaged Navier–Stokes methods. Presented in this work are the results of applying a large-eddy simulation spray methodology originally developed for use with diesel injections to direct-injection spark-ignition sprays. Comparisons were carried out over a wide range of ambient temperatures (400–900 K) and densities (3–9 kg/m³). To accurately simulate the large-scale vapor mixing, it was necessary to adjust spray break-up model parameters as functions of the density ratio. After this adjustment to the spray models, the large-eddy simulations matched experimental vapor penetration data and vapor images across the full range of tested ambient conditions. Liquid penetration trends with respect to changing ambient temperature were captured, but the trends with changing ambient density were not fully captured. Simulations using a Reynolds-averaged Navier–Stokes turbulence model at select conditions showed that the liquid predictions were very similar to large-eddy simulation results, but the Reynolds-averaged Navier–Stokes models were unable to accurately capture the large-scale vapor mixing and did not produce accurate vapor results.