I have just been reading a brief description of these ancient gas engines in a 1950 model engineering magazine and wonder if any still exist. The Dawson engines had an odd mechanism for rotating the piston to open ports in the cylinder walls, thereby avoiding any valve gear. They were made in 1894/1895 by the Paris Singer Syndicate in Clapham, and running on town gas were supplied to run workshop machinery or generate electricity.
All helpful info. The patent search has got me to patent 12097 of 1895 which is primarily to do with the gubbins to fuel an oil engine but refers back to patents 6407, 9865 and 6952 in relation to using the rotating piston to open and close the valves, allowing a normal 4- stroke cycle. So thats the correct Dawson. However I can't locate these earlier patents in the database, maybe I am not using the correct search term.
The article in ME Vol102, Edition 2546 has a photo of a Dawson engine, and an account from someone who worked on their production. They were
4 inch bore and water or air cooled, and they also made a 2.5inch bore engine that they experimented with in a bicycle frame using liquid fuel. Almost certainly the first British motorcycle - albeit an unsuccessful experiment.
If anyone has a copy of the Model Engineer for Sept 1st 1949, that apparently triggered the article I have, as someone discovered one of these engines and requested information. There was also an article on them in The Motor magazine on October 28th 1942 - but I don't expect that to be easy to find.