A New Mobile Fluid Power Source Based on Hydraulic Free Piston Engine

Abstract

Fluid power is widely used nowadays due to its many advantages, and many of these applications are mobile, where the power source has to be carried on board. However, the energy efficiency for such systems is relatively low, mainly due to the inefficient power source architecture, whose centralized architecture and slow response introduce significant loss. To tackle this issue, we propose to use the hydraulic free piston engine (HFPE) as a power source to provide the actuator with demanded flow at desired pressure in real time. To achieve this goal, a novel operation scheme of HFPE is proposed in this study, where the HFPE is essentially used as a digital pump. In the proposed system, within each cycle, while the piston travels full strokes, only a part of the output flow is directed to the load, and the rest is dumped back to low pressure by controlling the valve system. This operation scheme is validated through simulations, while a systematical method is proposed accordingly to find the optimal operation parameters so as to achieve maximum overall efficiency. Simulation results show that the proposed system can provide the desired flow rate at any load pressure with very fast response time as well as a very high fuel-to-hydraulic energy efficiency. To ensure robust operation of the proposed system, an in-cycle robustness reinforcement method and a cycle-to-cycle robustness reinforcement method are proposed. The former employs the idea of trajectory tracking to regulate the piston motion and improve robustness, while the later improves system robustness by detecting and recovering the system from misfires. Both methods are experimentally validated, with results showing significant robustness improvements. To summarize, this study is the first one that combines a digital pump with a HFPE. The relationships among the HFPE operation parameters, HFPE efficiencies and working conditions are clearly revealed. Key issues for robust operation are addressed. Performance of the proposed system is demonstrated through simulations and experiments, showing its feasibility and benefits. Together with the HFPE’s intrinsic advantages, the proposed solution can provide highly efficient, fast responding, compact and modular power sources for mobile hydraulic applications.