This paper describes a detailed analysis of the unsteady flows in the intake and exhaust systems of a modern four-cylinder, turbocharged Diesel engine with different EGR circuits, by means of a research 1D thermo-fluid dynamic tool and detailed experimental data. Firstly, a detailed simulation has been carried out to predict the pressure pulses, average pressures and temperatures in several cross-sections of the pipe systems for different speeds, loads and EGR rates, considering the complex geometry of the air filter, the intake manifold, the intercooler, the EGR-cooler and the exhaust manifold. An extensive set of measured data has been used to validate the model and refine the 1D schematic for the simulation. Secondly, the model has been used to study the different response of the two main lay-outs of external EGR systems: the high pressure and the low pressure circuits, also known as short and long EGR routes respectively. (C) 2013 Elsevier Ltd. All rights reserved.