Internal combustion engines is actually one of the most important source of pollutants and greenhouse gases emissions. In particular, on-the-road transportation sector has taken this environmental challenge and worldwide governments set up regulations in order to limit the emissions and fuel consumption from vehicles. Among the several technologies under development, an ORC unit bottomed exhaust gas seems to be very promising, but it still has several complications when it is applied on board of a vehicle (weight, encumbrances, backpressure effect on the engine, safety, reliability). In this paper, a comprehensive mathematical model of an ORC unit bottomed a heavy duty engine, used for commercial vehicle, has been developed. The model is completed with the sizing of the two exchangers involved in the ORC plant: the heat recovery vapor generator (HRVG) and the condenser. The first has the important role to transfer thermal energy from the exhasut to the woorking fluid, but not only thermal efficiency and pressure drops on the working fluid side are important, but also the backpressure realized at the engine exhaust, which affects engine fuel consumption. The second is realized with the similar technology of a radiator, because it has to reverse the condensing energy through the environement, and it is not negiglible in tems of vehicle frontal area occupation. Also weights have been calculated. Thanks to the model, a wide analysis of the recoverable power and the exchanger sizing has been performed on the ESC-13 engine working points. Once a unique sizing point has been chosen, the analysis has been reported in terms of ETC cycle where the effect of weights, backpressure and frontal area has been evaluated in terms of fuel consumption and compared to the energy recovered by the ORC.