Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Tom, it might help to think of these engines as running just the opposite of a standard internal combustion engine. Instead of an "explosion" of burning gas "pushing" the piston down from TDC, these vacuum engines operate by having the rapid cooling, and thus shrinking, of heated and expanded gas, forming a partial vacuum and "pulling" the piston back up to TDC. Well, actually it is the pressure of the atmosphere "pushing" the piston back up from BDC which happens because there is a partial vacuum in the cylinder, and the pressure differential provides the energy for the movement.