When the designer's target is the optimization of a composite system, the analysis of the interactions between the different elements of the system becomes a crucial topic. As a matter of fact, in some cases, the effect of these interactions can become more important than the behavior of each individual component. In the area of fluid power, this problem is very common. In particular the case of hydraulic powered transmission for mobile applications can be considered a paradigm of these problems. This paper presents an original numerical approach to study and design of a hydrostatic transmission: the target is the optimization of the system as a whole, taking into account the characterization and the interaction among all parts. First, the system and the application are presented; the attention is focused on the analysis and modeling of its hydraulic parts (pumps, motors, valves). The potentials of the hydraulic model are enhanced showing how they are used for achieving an optimal design. Several solutions are compared in terms of performance (torque vs. speed) and efficiency, on the basis of numerical results. In the second part of the paper, the hydraulic model is integrated with the model of the combustion engine. In this case, the performance of the system can be calculated from a wider point-of-view, considering also the fuel consumed over user-defined duty cycles. In particular, the interaction between pump (primary unit) and combustion engine has a relevant effect on the system efficiency. Finally, the sub-system given by Internal Combustion Engine (ICE) and the primary unit is analyzed and optimized, considering engine speed and pump displacement as variable parameters.
