This paper compares two different rule-based power management (PM) strategies, in terms of their resultant fuel consumptions, through a simulation study as applied to a hybrid hydraulic multi-actuator displacement controlled (DC) system. Specifically, the system analyzed is a mini-excavator, wherein the digging functions are powered using four variable displacement pump/motors - these units are also shared by the auxiliary functions. In addition, the on-board hydraulic energy storage device, or accumulator, is charged or discharged using an additional pump/motor, called the storage unit. A parallel architecture is used for the hybrid system wherein the additional pump/motor is on the engine shaft, running at the same speed as the engine (and the other four pumps).
An aggressive and fast, digging cycle was used to size the storage unit and accumulator, as well as to compare the performance of the two different strategies. The motivation behind this simulation study was to investigate additional fuel savings over the non-hybrid DC system, while also demonstrating feasibility of engine downsizing. The first PM strategy presented operates the downsized engine at the maximum speed possible while commanding the displacement of the storage pump in such a manner that the engine operates at constant torque. The second strategy operates the engine at the minimum allowable speed at every instant (pre-computed offline), while commanding the displacements of the storage pump so as to keep engine operation along a constant power line, in a high-efficiency area of the engine fuel map, whenever possible. As a result of the minimum-speed strategy, the displacements of the pump/motors supplying the actuator also tend to stay higher than the ones resulting from the maximum-speed strategy.