and allow the actuators to be more compact results from a simulation study using mathematical models that compared a conventional non hybrid displacement controlled hydraulic system with hybrid displacement controlled systems 10 of this invention demonstrated that the rated engine power of an engine of an excavator for example fig1 could be reduced by approximately half with the hybrid systems 10 without losing any performance from the digging functions of the excavator furthermore the simulation predicted that excavators equipped with the hybrid systems 10 may consume about 20 less fuel than the simulated non hybrid displacement controlled systems based on a high power cycle when the excavator was operated by an expert operator and even greater fuel savings were predicted if the excavator were operated by a novice operator on a low power cycle simulations also quantified benefits of hybrid hydraulic systems 10 of the invention particularly in terms of engine power energy and fuel consumption in addition to excavators the invention can be implemented on a variety of heavy mobile hydraulic machines such as wheel loaders and other similar material handling machines having both linear and rotary actuators suitable pumping capacities of the pump motors 24 26 and 28 and the energy storage pump 30 and suitable operating pressures and capacities for the high pressure accumulator 36 and low pressure accumulator 42 will depend on the particular application while the.