Only the first two are Stirling engines, which derive their power from the air being heated to produce pressure which acts on the front of the power piston, and then the air is cooled to produce a partial vacuum, so that atmospheric pressure presses on the back of the power piston. The third engine is a vacuum engine, commonly know as a “flame licker” which derives its power from hot gases being sucked into the cylinder, then the valve shuts, the gases cool down and atmospheric pressure presses on the back of the piston to push it back down the cylinder, producing power only on the return stroke.