A new physical model to calculate the fluid-dynamic behaviour and energy conversion in turbo-charger turbines for internal-combustion (I.C.) engines is presented. The model has been developed to be used as a boundary condition in wave action models. The model uses data from the turbine characteristic curves and it is based on the idealization of the turbine by simple elements: ideal nozzles and an intermediate volume. Two typical turbine types are discussed here: a single entry turbine and a twin entry turbine. In order to validate the model, a comparison between calculated and measured engine running variables related to the turbine behaviour is presented.