With helium readily available through part balloon shops, I would certainly seriously consider that as the best working fluid.
As it's inert and not eco-unfriendly, those are also on the plus side, and as the engine can be designed to be a sealed system, there would be the minimum top-up requirements. There will be some, because helium will diffuse through anything eventually
(Hands up who remembers Graham's Law of diffusion - or was it Dalton's (chair) )
Geof - just to put one idea to rest, is it deffinitely best to use a gas as the working fluid, rather than a liquid ? I feel there may be pro's and cons, but I'm out of my depth to be honest. :drowning:
John