On Fluid Compressibility in Switch Mode Hydraulic Circuits—Part I: Modeling and Analysis

Abstract

Fluid compressibility has a major influence on the efficiency of switchmode hydraulic circuits due to the release of energy stored in fluid compression during each switching cycle and the increased flow rate through the highspeed valve during transition events. Multiple models existing in the literature for fluid bulk modulus, the inverse of the compressibility, are reviewed and compared with regards to their applicability to a switchmode circuit. In this work, a computational model is constructed of the primary energy losses in a generic switchmode hydraulic circuit with emphasis on losses created by fluid compressibility. The model is used in a computational experiment where the system pressure, switched volume, and fraction of air entrained in the hydraulic fluid are varied through multiple levels. The computational experiments resulted in switchmode circuit volumetric efficiencies that ranged from 51% to 95%. The dominant energy loss is due to throttling through the ports of the highspeed valve during valve transition events. The throttling losses increase with the fraction of entrained air and the volume of fluid experiencing pressure fluctuations, with a smaller overall influence seen as a result of the system pressure. The results of the computational experiment indicate that to achieve high efficiency in switchmode hydraulic circuits, it is critical to minimize both the entrained air in the hydraulic fluid and the fluid volume between the highspeed valve and the pump, motor, or actuator. These computational results are compared with experimental results in Part II of this two part paper series.