Numerical Analyses of Spray Penetration and Evaporation in a Direct Injection Engine

Numerical analyses of the liquid fuel injection and the subsequent fuel-air mixing in a high-tumble, long-stroke direct injection engine at operation conditions of 2000 RPM are presented. The Navier-Stokes equations are numerically solved with a finite-volume method for compressible flow based on a hierarchical Cartesian mesh. The solid wall boundaries are represented by a conservative multiple cut- and split-cell method, where a semi-Lagrange level-set solver is used to track the location of the individual moving boundaries. To determine the fuel vapor before ignition, a two-way coupled large-eddy simulation of the turbulent flow field with the spray droplets is conducted. Due to the large number of spray droplets, a Lagrangian Particle Tracking (LPT) algorithm is used to predict the liquid spray penetration and evaporation. The hierarchical Cartesian mesh ensures a highly efficient usage of high performance computing platforms by applying solution adaptive mesh refinement combined with dynamic load balancing. The simulations are based on meshes with approximately 170 million cells and 1.5 million embedded spray parcels. The influence of the tumble motion on fuel distribution at the start of ignition is analyzed for several injection timings and ethanol and methanol bio-hybrid fuels. Injection at 60 CAD shows a strong influence of the fuel jets on the in-cylinder flow field and the tumble motion is deteriorated. A 7-hole modification of the spray-G injector is introduced, which shows improved fuel-air mixing by sustaining tumble motion. When methanol fuel injection is compared to ethanol, a disadvantageous fuel distribution towards the end of the compression stroke is observed, caused by the longer methanol injection duration.