Modeling of Pressure-Swirl Atomizers for GDI Engines

A new simulation approach to the modeling of the whole fuel injection process within a common-rail fuel injection system for direct-injection gasoline engines, including the pressure-swirl atomizer and the conical hollow-cone spray formed at the nozzle exit, is presented.

The flow development in the common-rail fuel injection system is simulated using an 1-D model which accounts for the wave dynamics within the system and predicts the actual injection pressure and injection rate throughout the nozzle. The details of the flow inside its various flow passages and the discharge hole of the pressure-swirl atomizer are investigated using a two-phase CFD model which calculates the location of the liquid-gas interface using the VOF method and estimates the transient formation of the liquid film developing on the walls of the discharge hole due to the centrifugal forces acting on the swirling fluid. Parametric studies reveal the effect of various nozzle operating and design parameters, such as the injection and back pressure, the needle lift and the radius of curvature of the discharge hole both at its inlet and exit, on the development of the liquid film. The nozzle CFD calculations are extended outside the injection hole in order to predict the initial development of the cone angle of the hollow-cone spray formed by the pressure-swirl atomizer. The nozzle flow exit characteristics are then used as inputs to a liquid sheet atomization model which estimates the size of the droplets formed after the disintegration of the injected liquid. Images obtained with a CCD camera of the spray structure, as a function of the injection pressure, close to the injection hole confirm that the proposed computational approach can simulate the near-nozzle spray development as a function of the geometric and operating characteristics of the fuel injection system and the pressure-swirl atomizer itself, thus providing the necessary initial conditions for spray predictions in the engine cylinder.