Free-piston compressed-gas engine variant and engine pinion with several decompression stages and return springs

A free-piston compressed-gas engine of several decompression stages, the engine comprising a compressed-air tank, with an exhaust valve, pipes and valves that link said tank to motor pistons located each at its respective cylinder, the rod of each cylinder being connected to a chain linked at its other end to a spring, said spring being, at its other end, fixed to a point, each chain of each piston being connected to a pinion located in a same axis that is the motor axis of a device where the engine is to be applied, the engine further comprising an opening and closing command system arranged to open and close the valves in due time and form.