Efficiency analysis of an electro-hydraulic drive for mobile lifting machines

Environmental issues have become a critical topic at all levels and the Fluid Power industry is no exception. One of the most important research trends is aimed at improving efficiency through electrification, the availability of new electric motors and drive solutions is expected to promote the introduction of speed-controlled pump systems in mobile applications, nowadays dominated by conventional valve-controlled solutions. The paper will firstly present a collection of tests carried out on a lifting application, powered by an inverter-controlled electric motor moving a reversible gear pump/motor. The lowering phase is carried out in two alternative modes, the first one is the electric regenerative mode, the second one is the throttle-controlled mode. The analysis of the results will highlight different energy efficiencies of the system at different operating conditions. In particular the losses of volumetric unit, piping, throttle elements, hydraulic cylinder and electric components can change dramatically depending on operating conditions. In the second part of the paper, a lumped parameters model will be presented, using a commercial software. Firstly, the model will be able to catch the behavior of the test bench and secondarily it will be used to predict the expected performances of alternative design choices. Particular attention will be focused on low speed operation. In fact, there is more uncertainty and much less well-established knowledge on the behavior of the volumetric units at low speed, concerning the large body of work available for the nominal speed conditions. Finally an outlook will be drawn of expected advantages and limitations arising from the implementation of variable speed pumps on mobile machines. Such benefits will be presented with practical figures of merit such as the energy recovery performance, the increase of cycles and the cooling system size reduction.