The development of new generations of Diesel engine focuses on the engine downsizing, fuel economy and performance. The new engines must also satisfy the new legislation, regional and state requirements in term of NOX, CO2, particle emissions. A new compact and high performance cooling system, named Air Intake Module (Valeo’s Air Intake Module wins a 2013 Automotive News PACE Award ) has been developed to satisfy and achieve these objectives. VALEO’s compact AIM uses a charge air inter cooler fully integrated into the intake manifold in order to minimize air pressure losses and reduce turbo lag during acceleration and also manages the Exhaust Gas Recirculated (EGR) introduction. During the development of AIM, several types of simulation have been deployed to optimize the performance and validate its integration in engine before launching prototypes. These simulations have played an important role to validate this innovative cooling system and reduce its development cycle. The purpose of this paper is to present some main CFD applications and in particular the air and EGR gas mixing simulation for optimizing the EGR rate balancing in the engine’s cylinders: Co simulation STAR CCM+ (CFD) and GT-POWER (1D).