One time I had to write white paper or a green paper or something about a Stirling engine for a job interview but I still didn't really understand how they work. Something about expansion and contraction of gas and...oh, wait. Upon further reading it seems this engine isn't even a Stirling engine is it? It's a two-cylinder V-twin. Well. I guess I'm even less surprised I didn't get that job now than I was then.
The working model of a two-cylinder V-twin gas powered engine has 2 "injecting carburetors" and a large brass flywheel to facilitate startup with a Piezo crystal ignition system (no batteries required) and a butane or propane gas supply. A turn knob on the engine's gas tank flicks the flywheel and sounds the extended exhaust pipes for an accurate V-engine-type sound at startup. The know then adjusts to bring the engine to your desired speed as the sound carnival continues.
V-twin moving pistons, crosshead, and valve rods fire in opposite directions to balance the engine and produce an interesting visual display of interacting mechanics. This model engine arrives assembled and ready to run.