Power distribution method for a parallel hydraulic-pneumatic hybrid system using a piecewise function

Abstract The Benedict-Webb-Rubin model or the ideal gas equation of state alone cannot accurately describe the load model of a parallel hydraulic-pneumatic hybrid system. This paper proposes a load model of the energy recovery system based on a piecewise function considering the load components switching problem under high energy regenerative braking and cruise charging conditions. This model can express the influence of the pre-charge pressure of the hydraulic accumulator and the air compressor speed on the system load power. This is accomplished by detecting the hydraulic accumulator state of charge, the transient temperature of nitrogen, and the transient temperature of the air in the reservoir. The load power of the energy recovery system can be estimated under various working conditions and used to guide the adjustment of the hydraulic pump/motor displacement to adjust the power distribution of the energy recovery system. This can solve the problem of ineffective power distribution and overspeed conditions, as well as fluctuation of the air compressor operation caused by changes in mechanical input power. The method proposed in this paper provides a new technical approach for the configuration of parallel hydraulic-pneumatic hybrid power systems.