The goal of this thesis is to validate through a series of steady-state tests a GT-SUITE model of a 3.0L diesel engine. The starting point is the comparison between two model versions of the same engine. The first model is equipped with After Treatment System (ATS), while the second model does not have the latter. The main differences between them are highlighted and one of them, in particular, is the adopted model of the heat convection exchange in the exhaust pipes. The first engine model exploits a map of coefficients of convective exchange, which do not have a physical consistency due to their values too far from the typical real ones.