The Schoenner / Parallax(?) engine of the late 19th C used town gas as fuel. Polly Models do a reproduction engine called the Atmospheric Gas Engine. It is possible to run this engine on propane with considerable mods to the ignitor, or acetylene. Problem with acetylene (in Europe at least) is that of obtaining small quantities of the gas at economic prices.
The designer of the AGE, Bruce Davey, suggests using 'carburetted propane' to mimic the approximate ignition range of town gas. Googling for carburetted propane produces no results, but it is basically propane bubbled through petrol with all air excluded.
Apparently, acetylene has an explosive ratio of between 4:1 and 14:1 and propane has an explosive ratio of 4:1. By using carburetted propane, the explosive range is widened to about 7:1, which, it is claimed is good enough to run the AGE.
Dave
The Emerald Isle