Combustion gas-powered fastener driving tool

A portable fastener driving tool comprising a housing, a main cylinder in said housing, a piston in said main cylinder and movable from a driving to a driven position, a driver attached to said piston, a magazine for supplying fasteners into position to be driven by said driver, a combustion chamber formed within said housing and having said piston as one wall thereof, a fan in said combustion chamber and controls therefor to operate same to cause turbulence in said chamber, a valve means controlling the flow of gases into and out of said combustion chamber, a work sensitive probe assembly secured to said valve means whereby the combustion chamber is not closed off until the tool is in position to drive a fastener into a workpiece, means for providing fuel into said combustion chamber and igniting same for driving said piston to drive a fastener, and trigger operated cam means responsive to the movement of said valve means whereby the tool cannot be fired until the combustion chamber is closed and the chamber cannot be opened until the trigger is released, and means for facilitating the return of said piston to its driving position after the tool has been fired.