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

Maybe I'm dense...but I read both of these articles, they both talk about introducing fuel , igniting the fuel with a spark and that pushes the cylinder. ...isn't that   combustion & would that be some kind of compression to push said cylinder ?

Nobody is dense or stupid! They are just not well known in modern times. Non-compression simply means the fuel/air mixture is introduced at atmospheric pressure (0 psi) and ignited without mechanical compression of the mixture before ignition. These engines are not terribly powerful but they do work. They are typically flame ignition.