This paper presents a scheme to obtain large output power from a hydraulic reciprocating pump in which the diaphragm is driven by a multilayer piezoelectric element (MLPE). The scheme is summarized as follows : (i) reduction of the spring constant of the diaphragm for elongation of the MLPE, (ii) application of static pressure on the working liquid in the hydraulic circuit, and (iii) connection of pumping valves and pressure chambers with pipes having the same resonant frequency. When the MLPE is driven at a frequency near the resonant frequency of the pipes, the pressure behind the exhaust valve fluctuates violently such that it decreases at the exhaust stroke. Similarly, the pressure in front of the intake valve increases at the intake stroke. Then, due to the elasticity of the MLPE, the flow rate of the pump increases considerably compared with the case of no resonance. To illustrate the effectiveness of the scheme, it was applied to improve the performance of a pump (max. power 34 W, efficiency 27%) which was driven by the MLPE of 22 mm diameter, 55.5 mm length and 170 g mass. Consequently, its maximum power and efficiency were increased to 62 W and 35%, respectively. Since the efficiency of the valves was only 51% in this experiment, further improvement should be achieved in the future by using better valves.
