This paper presents our simulation model of Rankine-Hirn cycle. This model is developed for sizing and for the working fluid choice of a test bench of Rankine-Hirn cycle for engine waste heat recovery. The heat source characteristics were measured on a diesel tractor engine during work in field. The simulation tool allows to study some of static operating points. An iterative calculation is used to find the best parameters as pressure or working fluid flow rate according to some constraints such as heat exchanger surface or maximum pressure. We identify that each fluid is adapted for a limited range of temperature heat source. For our application, ethanol and water are the adapted working fluids. These results define the future control laws of the waste heat recovery system.