The Stirling Engine has the highest theoretical efficiency compared to other heat engine cycles (it is equivalent to that of the Carnot Cycle). Simply it depend

The Stirling Engine has the highest theoretical efficiency compared to other heat engine cycles (it is equivalent to that of the Carnot Cycle). Simply it depends on the temperature difference between the hot and cold side. If you drive it it becomes a refrigeration machine. The problem was and is materials for practical applications. It needs highly efficient and compact heat exchangers and also the porous nature of the re-generator is a problem. Helium is a better gas to use, but being so light at high pressures you get leakages. I spent more than 5 years of my life (PhD) designing, building and simulating one (with only one moving part) and that got me in a major way into computing:) in the late 80's-90's. I did not touch the field for many years, but lately it has been at the back of my mind, if there are any applications for it in nano-machines.