The invention describes an internal combustion engine (14) for a vehicle (12) that comprises a fly-wheel (24) placed at the rear part with respect to the internal combustion engine (14), a cooling system (28) placed at the front part with respect to the internal combustion engine, a forced induction group consisting of a turbine (22), placed on the same side as the fly-wheel (24), and a compressor (30), placed on the same side as the cooling system (28), and a system (10) for treating the exhaust gases provided with a conduit for the inlet of exhaust gases, operatively connected to the exit door of the turbine, a conduit for the outlet of exhaust gases and a main body or shell (38) internally hollow for containing a substrate (44) through which the exhaust gases are conveyed so that they are subject to predefined chemical reactions. The system for treating exhaust gases is placed on the head of the internal combustion engine, the main body or shell has a cross-sectional shape, with respect to the driving direction of the vehicle, of an ellipsoid the larger axis of which is oriented according to a substantially horizontal direction and the smaller axis of which is oriented according to a substantially vertical direction, so that the main body or shell has a widened and compressed shape that allows it to occupy the least height-wise possible space with respect to the head of the internal combustion engine, and the conduit for the inlet of exhaust gases and the conduit for the outlet of exhaust gases are placed in fluid communication with a same transverse side wall (46), with respect to the driving direction of the vehicle, of the main body or shell.