This is a gas engine my grandfather made, although it was or still is perhaps a well engineered model I have never understood the workings of a "gas engine". On the left of the flywheel there is a pipe inlet and a lever valve. I can only imagine that some sort of gas pressure was introduced into the cylinder to push the piston down or up when the valve was opened and so operating the crankpin to turn the flywheel. This engine was in a glass case similar to the steam engine.

Write to me if you know of this engine