A comprehensive CFD methodology for the simulation of Spark Ignited Engines

In this work, a Computational Fluid Dynamic methodology for the simulation of the charge formation process in Gasoline Direct Injection engines is presented. The aim of the work is to develop a methodology suitable in an industrial environment to drive and support the development process of modern GDI engines. A big emphasis is placed on the comparison of the proposed CFD models with experimental data obtained using a single-cylinder optical engine. Chapter 1 describes the working context and sets the aim of the work. After a brief recall of the theoretical background of CFD in chapter 2, an overview of the optical techniques interesting for Internal Combustion Engine applications is presented in chapter 3, and the basic principles of spray atomization theory are reviewed in chapter 4. In chapter 5 the CFD simulations for the charge motion in-cylinder are described. Two different engines were investigated, and the effect of different turbulence models and numerical schemes are analyzed, comparing the results with optical experimental data. The standard k-eps model, together with the MARS numerical scheme, showed the better capability to reproduce the charge motion and turbulence pattern in-cylinder, and therefore they were used for the remaining part of the work. In chapter 7 the injection model used is discussed. Despite a traditional Lagrangian-Eulerian approach, the model presents an innovative procedure capable to reproduce also the liquid core. After that the effects of the use of the liquid core and a bi-component fuel are analyzed, the in-cylinder injection results for the two investigated engines are presented. The injection model shows its capability to correctly reproduce the spray shape and penetration in different operating conditions and for different injector types, using a reduced amount of calibration parameters. Finally, chapter 8 presents some "diagnostic indexes" capable to resume the results of the CFD simulations in a reduced number of parameters. In particular, some indexes to assess the quality of the mixture and the wall impingement tendency are proposed, allowing to use the CFD simulations to address these crucial aspects in the choice of injector targeting and actuation strategy. The proposed methodology allows to use CFD simulations to support the engine development process, and was successfully applied to many different spark ignited engines.