Performance characteristics of compressed air-driven free-piston linear generator (FPLG) system – A simulation study

•An integrated model of PMLG coupled with the free-piston engine is set up.•The accuracy of model is verified by comparison with experimental results.•Influence of piston pressure, mass, intake position on motion characteristics are found.•Output power and efficiency with pressure of FPLG under different loads are found. The integrated model of the two-stroke free-piston engine system driven by compressed air is established using MATLAB/Simulink software and the relationship and the influence of the reciprocating motion, friction force, cylinder process and electromagnetic characteristics of permanent magnet linear generator are fully investigated. The piston displacement, velocity, acceleration, thrust, air pressure in cylinder and generator voltage and their relationship are simulated and analyzed, and the motion performance, the characteristics and interaction relationship of the system are obtained. The experimental platform of air driven free-piston linear expander system is tested. Simulation results are compared with experimental data and the whole model is validated. The developed model is then used to study the influence of piston assembly mass, the timing of air intake and exhaust on piston motion characteristics. The peak voltage output, voltage harmonic distortion and efficiency of permanent magnet linear generator under different air pressure and the output power and efficiency with pressure of FPLG under different loads are obtained. The results show change rules of piston displacement, velocity and frequency changes with air pressure, piston assembly mass. Finally from the simulation result, the air pressure and air intake position of highest efficiency of the linear generator are found.