Effects of Fuel-Induced Piston-Cooling and Fuel Formulation on the Formation of Fuel Deposits and Mixture Stratification in a GDI Engine

Fuel deposits in DISI engines promote unburnt hydrocarbon and soot formation: due to the increasingly stringent emission regulations (EU6 and forthcoming), it is necessary to deeply analyze and well-understand the complex physical mechanisms promoting fuel deposit formation. The task is not trivial, due to the coexistence of mutually interacting factors, such as complex moving geometries, influencing both impact angle and velocity, and time-dependent wall temperatures. The experimental characterization of actual engine conditions on transparent combustion chambers is limited to highly specialized research laboratories; therefore, 3D-CFD simulations can be a fundamental tool to investigate and understand the complex interplay of all the mentioned factors. The aim is pursued in this study by means of full-cycle simulations accounting for instantaneous fuel/piston thermal interaction and actual fuel characteristics. To overcome the standard practice, based on the adoption of time-independent wall temperatures, solid cell layers are added onto the piston crown. In particular, thermal boundary conditions on the lower face of the piston portion are derived from a complete CHT simulation, thus considering both the actual piston shape and the point-wise cooling effect by the oil jets, the friction contribution and the heat transfer to the cylinder liner and the connecting rod. Furthermore, the use of a simplified fuel model based on a single-component formulation is compared to a more realistic hydrocarbon blend. The methodology is applied to a currently produced turbocharged DISI engine operating at full load peak power and maximum torque regimes; the piston thermal field is completely resolved in space and time during the engine cycle, and its effects on spray guidance, fuel impingement and liquid film formation are carefully analyzed.