The potential of internal EGR (iEGR) and external EGR (eEGR) in reducing the engine-out NOx emissions in a heavy-duty diesel engine has been investigated by means of a refined 1D fluid-dynamic engine model developed in the GT-Power environment. The engine is equipped with Variable Valve Actuation (VVA) and Variable Geometry Turbocharger (VGT) systems. The activity was carried out in the frame of the CORE Collaborative Project of the European Community, VII FP.The engine model integrates an innovative 0D predictive combustion algorithm for the simulation of the HRR (heat release rate) based on the accumulated fuel mass approach and a multi-zone thermodynamic model for the simulation of the in-cylinder temperatures. NOx emissions are calculated by means of the Zeldovich thermal and prompt mechanisms.As a first step, the model has been calibrated and assessed on the basis of experimental tests carried out on four characteristic operating conditions within the WHSC (World Harmonized Stationary Cycle). The considered engine points have been used to identify the optimal engine parameters on the basis of a DoE approach.As a second step, sweeps simulations centered on the previously considered operating points have been run for iEGR and eEGR in order to study the effects of these latter on combustion and NOx formation processes at steady-state conditions.Finally, a ramp from the WHTC (World Harmonized Transient Cycle) has been reproduced with both iEGR and eEGR modes and the engine performance have been compared in terms of fuel consumption, combustion and NOx emissions.