Series, Parallel and Hybrid Series-Parallel Hydrostatic Transmission Architectures for Ground Locomotion

Ground locomotion of wheeled vehicles, in all-wheel drive configuration, is subject to unique, generally highly variable and not predictable, loading conditions on the driveline subsystem. Any sensible design must cope at least with the effects induced by slope changes, asymmetrical losses of adherence and cornering maneuvers. When severe constraints on the driveline layout make the implementation of standard mechanical transmissions unfeasible, a typical option is given by compound hydrostatic transmission architectures, with multiple dedicated motor-wheels. Requirements are then defined on two different levels: at vehicle level, traction functionality must never be lost; at hydraulic circuit level, all components must work within their nominal operating ranges and hydraulic stresses must be limited. Common standard topologies for motor connection, viz. series and parallel, come with different strengths and weaknesses, often requiring the implementation of auxiliary highly dissipative compensation components and/or complex electronic control, while a hybrid series-parallel concept, derived from a patented application, based on three-port motors allows the implementation of an effective, purely hydraulic system. In the present work, a general comparison of the above-mentioned architectures is provided, by means of numerical simulation, over a wide set of virtual experiments. Each architecture is analyzed: its specific features are described and the correlation between hydraulic performance/specifications and vehicle traction performance is pointed out.