On the Application of Hybrid Turbulence Models for Fuel Spray Simulation in Modern Internal Combustion Engines

In order to satisfy the increasingly restrictive EU norms on air pollutant emissions, most of the world-leading passenger car manufacturers are currently forced to apply different engine electrification solutions to a large part of their model portfolio, ranging from Mild-Hybrid Electric Vehicles (MEHVs) to Plug-in Hybrid Electric Vehicles (PHEV5). Nonetheless, the efficient design of the thermal engine part still plays a fundamental role in the overall fuel consumption and polluting emissions reduction. Both gasoline-fueled and diesel-fueled modern engines rely on finely tuned direct fuel injection strategies, in order to simultaneously optimize primary energy consumption and particulate matter and/or gaseous emissions. Therefore, it is of foremost importance to develop robust and reliable multidimensional numerical tools, to support engineers during the injection system design and testing processes. In that sense, turbulence modeling is a key point for the accurate description of fuel spray evolution and mixture formation, due to the very high injection pressures (in diesel-fueled engines) or the complex spray patterns and severe flow cyclic variability (in downsized, turbocharged, gasoline-fueled engines). In the present paper, we evaluate the usage of hybrid URANS/LES turbulence modeling techniques for fuel spray simulation, based on the current scientific literature on this topic and on some recent computational studies from the authors. Aspects such as the comparison with URANS and standard LES models are discussed, and strengths and weaknessess of the analyzed hybrid approaches are pointed out. The authors assume that this work could pave the way for further debates on the potential vs. actual benefits of mixing statistically-derived (URANS) and scale-resolving (LES) turbulence modeling options for engine flow simulation.