The main objective of this thesis project is to give a contribution to the definition of the optimal CFD simulation methodology for NG engine investigation, so as to support the design process of innovative engines featuring a low environmental impact. The first part of this study presents the effect of hydrogen addition and EGR dilution on two different SI engines with different turbulence characteristics. The focus of the research has been to analyze the effect of different fuels and mixture composition by comparing experiments and corresponding numerical models. Natural gas has been a focus of research as an alternative fuel due to its chemistry which helps in reducing the CO2 emissions and its high knock tolerance.