Flame Sucker 1

Flamesuckers are so-named because they work by opening a valve and allowing the vacuum formed behind the moving piston to suck hot gas from a flame into the cylinder.  The valve shuts and the hot gas quickly loses heat to the finned cylinder which causes it to condense and form a partial vacuum inside the sealed cylinder.  External atmospheric pressure on the exterior of the piston is greater than the pressure inside the cylinder and the piston is forced to move by this pressure differential.  Properly, these are called atmospheric engines for that reason.