Methods and systems associated with free piston compressors. Fuel is ignited within a combustion chamber to increase pressure. A free piston is displaced within the combustion chamber. The free piston acts as an inertial load to high-pressure gas generated by combustion of the fuel. High pressure gas is pumped from the combustion chamber into a reservoir using the free piston. A spring, magnet, or other mechanism engaging the free piston may be used to assist the process. In the case of a spring, the spring assists returning the free piston to its approximate initial position as the spring expands. In the case of a magnet, the magnet may engage opposite ends of the free piston during the process. Two or more free pistons may be used. The system may be used as a power source for equipment such as a robot.