Abstract:  A new free piston expander-linear generator ( FPE-LG) prototype for small-scale organic Ｒankine cycle ( OＲC) waste heat recovery system was developed，and a new valve train was designed to control the intake and exhaust process of working fluid． The motion characteristics，output performance and indicated efficiency of the FPE-LG were investigated based on a compressed air test rig． The results show that the intake duration and exhaust duration have a great influence on the motion characteristics and output performance of the FPE-LG． The stroke and velocity of free piston assembly increase with the increase of the intake duration，and decrease with the decrease of the exhaust duration． the valve timing optimization can significantly improve the output power of linear generator． The maximum value of voltage is 13 V and the maximum value of peak power is 18. 6 W when the intake angle is 60°and the exhaust angle is 180°，and the intake pressure，operating frequency and the external load resistance are 1. 9 ×105 Pa、2 Hz and 9 Ω，respectively． The velocity and stroke of the free piston assembly show an increasing tendency，and higher peak voltage and power output，when the external load resistance increases． The indicated efficiency shows a downward tendency when the external load resistance increases． The indicated efficiency is 66. 2% when the external load resistance，operating frequency and the intake pressure are 3 Ω、3 Hz and 1. 9 ×105 Pa，respectively． 

Key words: free piston expander-linear generator, ORC waste heat recovery system, valve timings, output performance, motion characteristics, indicated efficiency