Analysis on operation characteristic and output performance of free piston expander-linear generator based on displacement control strategy

In this paper, a free piston expander-linear generator (FPE-LG) test rig is established and the control strategy based on displacement is applied. The operation characteristic, output performance, and energy conversion efficiency of the FPE-LG are analyzed. The coefficient of equilibrium oscillation (C) is identified to evaluate the stability of the free piston assembly (FPA). Experimental results show that the FPA has best stability when fixed position point (FPP) is between 20 - 75 mm. The actual stroke and operation frequency increase with the intake pressure and load resistance. The variations of voltage and power output are in accordance with velocity. The average power output increases with intake pressure and FPP, but reduces with the advanced exhaust duration. The maximum average power output can be achieved when the load resistance is 20 Ω. The energy conversion efficiency first increases with intake pressure and then remains constant. The energy conversion efficiency reaches up to 17.5 % when intake pressure is 0.9 MPa, FPP is 40 - 55 mm, advanced exhaust duration is 0 ms, and load resistance is 20 Ω. The lower the coefficient of equilibrium oscillation is, the higher the average power output is.