The results of a detailed study of energy conversion within a radial turbocharger turbine stage are presented in this paper. A 2.5D simulation is conducted near the optimum running point in order to construct the local aero thermodynamic flow field and the associated mechanical energy production within the rotor. Then, a complementary analysis of the flow led within each of the turbine sub-blocks is summarized. The simulation uses models of losses which are studied locally with a multi-dimensional approach and then reduced in 1D description (models reduction) allowing establishment of a global turbine performances map. The evaluation of losses appears to be particularly troublesome within such small machines. Losses due to flow-blade profile mismatch which represent a large source of energy dissipation are modeled by way of the entropy balance. The results are compared and validated in accordance with the experiments that were conducted on special turbocharger tests rigs and on a Diesel truck engine test bench.