Thanks Raphael, in all the excitement I forgot the word... yes, it displaces the air. Some displacers are filled with wire wool so the air passes through it as well round the outside. This increases the engine power as the heat is held then released by the wire woold on each stroke. This give more efficient heat exchange. The thing many forget or don't know, is these engines were before petrol engines and electric motors!!! So despite low power, they were the domestic work horse.
Yes, it is a water cooled sterling engine. Now wish I'd said that!!! But I'm just a hobbyist, there is so much information, types and configurations to mention. I just wanted to introduce folks to a great engineering piece of history..... and wish I'd said Sterling engine...
