The 3rd oldest operable gas engine in the world. The 2 oldest are both in Europe.
The engine was designed to run on "town gas", ie very poisonous coal gas. Due to the explosive nature of this gas, it couldn't be used to power a crank directly. Instead, the gas explosion is used to compress air above the piston. The pressurized air then drives the piston down, thus turning the main shaft via a gear and overrunning clutch.
It's not a very efficient design, this big engine delivers a thumping 1/2 (ie 0.5) Hp.
Coolspring put on a wonderful Flame Ignition show, which brought in rare engines from many parts of the country. The show was in honor of a massive, beautiful new book by one of the members, on the history of flame ignition engines, and the various ingenious designs.Coolspring Power Museum