The purpose of this thesis project is to analyse how combustion and HC emissions of a SI engine are affected by an ODE-based phenomenological model for crevice flow simulation implemented in CONVERGE CFD software by means of a user-defined function (UDF) coded in C language. This blowby model has been provided by Politecnico di Torino through MATLAB. CFD (Computational Fluid Dynamics) is a tool for the study of fluid phenomena by means of fluid mechanics, numerical analysis and computer-based simulations. Nowadays, thanks to the thorough developments in informatics of the last decades, the usage of CFD is in every area. There are several computational fluid dynamics simulation software, both commercial and non-commercial.