Mapping the Efficiency for a Hydrostatic Transmission

Efficiency maps have long been used by engineers to understand the topographical behavior of their machinery. Most commonly, efficiency maps have been generated for internal combustion engines, where the torque-speed curve for the engine shows the wide-open-throttle line with constant efficiency lines drawn beneath this maximum operating limit. From such maps, engineers have been able to determine the most efficiency operating point for the engine, given a desired output power (torque and speed). There currently exists a great interest in using hydrostatic transmissions for improving the operating efficiency of an internal combustion engine. However, efficiency maps for hydrostatic transmissions, similar to efficiency maps for internal combustion engines, do not exist in the literature and therefore it is difficult to assess the overall efficiency gains that are achieved when using a hydrostatic transmission in these applications. This paper proposes a method for generating efficiency maps for hydrostatic transmissions, and presents a typical set of maps that may be used as a first approximation for assessing transmission efficiency. The results of this paper are nondimensional and are generalized for a transmission of any size. As shown in this research, there are regimes of transmission operation in which the efficiency is nearly independent of either the output torque, or the output speed. Furthermore, it is shown that maximum operating efficiencies typically exist at high output speeds, and mid-to-high output torques.