Spray Characteristics of a Fuel Injector: A CFD Study

Abstract

Main function of a fuel injector used in internal combustion (IC) engines is to properly atomize liquid fuel for vaporizing and mixing with air. In order to achieve good vaporization and mixing, location of the fuel injector inside the combustion chamber is very critical especially in gasoline direct injection engines. In automotive engines, vehicle layout possesses main constraint to mount the fuel injector at a particular location and orientation. In the present study, a conventional carburetor fitted engine was operated with port fuel injection to meet the future emission standards for a two-wheeler application. In general, for gasoline port injection engines, straight cone angle fuel injectors are mainly employed. The direction of fuel spray (cone angle) should be targeted to minimize the wall wetting, which in turn affects the performance and emission characteristics of the engine. Therefore, it is important to study the fuel spray characteristics in these engines. In this study, a CFD analysis has been carried out on a fuel injector to understand the effect of cone angle (8 and 18°) on fuel penetration, droplet size, and evaporation characteristics. In order to carry out CFD analysis, a fuel injector commonly used for Indian two-wheeler application is considered. The geometric model of the injector is generated using ProE software. The model is meshed with polyhedral cells and surface refinement is done at injector and intake pipe regions. The meshed model has a grid density of 0.2 million cells. Analysis has been carried out with inlet air velocity (at the outlet of throttle body) and pressure outlet boundary conditions (cylinder pressure at bottom dead center). Outer surfaces are considered as walls with no-slip boundary condition and intake temperature used was measured from an actual engine, which is used as the boundary conditions. In this study, wide-open throttle position is selected for detailed numerical analysis. Out of the two cone angles considered, 8° is found to be better in terms of lower sauter mean diameter (SMD), fuel evaporation and penetration. However, higher cone angle is found to be better, due to larger spread of fuel and higher probability of getting energy from incoming air so that the size of droplet can be smaller and mixing with the air will be faster, which will enhance the fuel evaporation. At wide-open throttle position, due to higher air velocities, air–fuel mixing is better due to higher evaporation rate with a lesser particle diameter. The CFD results have been compared with steady state measurements in a test bench and the predicted results found to match with experimental results reasonably well with a maximum deviation less than about 6 %.