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In view of the incoming laws concerning the emission limits during test cycles, it is increasingly important reducing emissions at low and medium loads by using exhaust gas recirculation (EGR) techniques. The development of control strategies can be greatly improved by simulation models able to predict accurately the behaviour of the engine and its emissions in the entire field of operation, and specifically at low loads when the turbocharger is operated at low rotational speed and in the vicinity of the surge line. Often the maps of compressor and turbine made available by the manufacturer are not extended to low rotational speeds and low pressure ratios. Methodologies have been therefore developed in order to extend the available characteristic curves to other working points, taking into account the geometry and the rotational speed of the turbocharger. For the compressor, a technique based on the fluid dynamic similitude is proposed.