Numerical study on fan spray for gasoline direct injection engines

In gasoline direct injection engines, it is important to optimize fuel spray characteristics, which strongly affect stratified combustion process. Spray simulation is expected as a tool for optimizing the nozzle design. Conventional simulation method, however, cannot predict the effect of various nozzle geometries on the spray characteristics, because they are based on the experimental data and/or the empirical laws regarding spray boundary condition at the nozzle exit. In Japan, a fan spray injected from a slit type nozzle has been adopted recently for gasoline direct injection engines. This paper proposes a computational model for the fan spray. The structure of two-phase flow inside the nozzle was numerically analyzed using the volume of fluid (VOF) method in a three-dimensional CFD code based on the nozzle geometry. Then, applying the results of these analyses to the linear instability theory, the mean diameter of fuel droplets at the nozzle exit was calculated. These results leaded to the boundary condition at the nozzle exit for the spray simulation instead of the experimental data and/or the empirical laws. Then, the discrete droplet model (DDM) and many sub-models were used for the spray calculation. Spray tip penetration, Sauter mean diameter (SMD), and spray mass flow rate distribution were verified for various pressures and nozzle geometries.