Improvements of 3D-CFD simulation of high-performance multi-hole injectors

The increasingly stringent regulations in terms of pollutant emissions have lead the automotive industry to concentrate its resources in the research of new methods to improve internal combustion engine’s efficiency while at least keeping the same performance. Particular attention has been given to the development of more precise and efficient fuel injection systems combined with suitable injection strategies. One of the most powerful tools in the investigation of injection and mixture formation process is the 3D-CFD simulation, which can provide a very precise and reliable reproduction of thermo-fluid-dymanical phenomena inside the engine, with consequent savings in time and financial resources related to the not anymore necessary test campaigns. The subject of this work is the study and the research of possible improvements of the injection model for 3D-CFD simulation, in the particular application of highperformace multi-hole injectors for Gasoline Direct Injection (GDI) applications. A set of experimental tests will provide the necessary support for the validation of a new, more precise injection model developed for the 3D-CFD tool QuickSim. The 3D-CFD tool QuickSim was developed at FKFS and the IVK of the University of Stuttgart, on the base of the 3D-CFD commercial software Star-CD. Its peculiarity is the adoption of specifically designed models for combustion, heat transfer, etc. which lead to a consistent reduction in computational time, hence the possibility of performing full engine simulations over successive cycles in a time sufficiently short to allow the utilization of 3D-CFD simulation directly in the engine development process. The starting point of this investigation is the analysis, by means of a developed imaging tool, of a set of experimental tests of fuel injection with a multi-hole injector at high injection pressure, which provides data about the spray formation and propagation inside a constant volume test chamber under different conditions of injection pressure and chamber pressure. The obtained data are then exploited in the process of calibration of the injection initialization parameters of the 3D-CFD simulation. In fact QuickSim’s injection model does not include the simulation of internal nozzle flow and droplets primary breakup, and an initialization domain is present in which the properties of the injected droplets (velocity, diameter, target direction) are defined. The dependence of the simulation quality on the initialization parameters will be discussed, in order to provide a methodology for the selection of the best set of parameters. Furthermore a sensitivity analysis of the injection simulation to the injection conditions is performed, with particular focus on how the different models implemented for the description of secondary breakup and vaporization of the injected fuel influence the final results. The validation of the developed methodology is performed again exploiting the experimental data obtained in the first phase of the work, and by means of the application of the new injection model to a case of full engine simulation. In summary, this work is dedicated to the implementation of a set of guidelines for the definition of an injection model able to provide more precise and reliable injection simulations without affecting the computational effort required by the 3D-CFD software, taking into account its final application in the virtual engine development process.