To solve problems of ignition and combustion based micro free-piston engines, the combustion characteristics and extension of the ignition limits of the micro free-piston engine were investigated. A single-shot visualized experiment was conducted for a micro free-piston engine with a combustion chamber diameter of 3 mm. The results reveal the micro free-piston engine performance concerning combustion pressure, temperature, power density, mass loss etc., especially three ignition types for micro free-piston engine with a mixture gas of methane and air under different compression ratios. Methods for extending ignition limits of mixture gas preheating, catalyzing and hydrogen addition were investigated by numerical simulation. The results reveal that the ignition time of mixture gas could be advanced, and the maximum pressure and temperature declined down in the micro-combustion chamber under certain conditions; also the flammability limits can be extended to improve the combustion stability for micro free-piston engine. (C) 2019 Elsevier Ltd. All rights reserved.