"Superheated" again, in this context means simply the use of a vapor (working fluid) heated above it's boiling point in a heat engine. At least through some portion of the cycle.
In actuality any "hot air engine" is operating in a "superheated" condition (above the boiling point of Liquid Air)
This thread is mainly intended for investigating "alternative" working fluids with boiling/condensation - phase change temperatures closer to ambient.
These fluids are frequently used as refrigerants or working fluids in heat pumps and the like.
In browsing around for information, It is apparent that engines of this type running on what amounts to a refrigerant, are very frequently employed as the working fluid for modern geothermal energy plants.
Historically attempts have also been made to use such working fluids with boiling (phase change) points closer to ambient in ordinary reciprocating engines, often with disastrous results due to containment, flammability and toxicity issues.
The Douglas Self website has a great deal of information on the history of such engines, using unusual working fluids including "liquid air".
http://www.douglas-self.com/MUSEUM/POWE ... d.htm#isob
There are a number of additional pages, some devoted entirely to a specific working fluid.
Working fluids covered include: mercury, Ammonia, Petrol and Alcohol, Compressed Air, Mercury, Freon, Carbonic Acid, Ether & Chloroform, Carbon Disulphide, Potassium vapour, Bitumenous vapours,
Sulphur dioxide, Diphenyl oxide, Nitrogen dioxide, Aluminium bromide, Nitrosyl chloride, Gallium iodide, Rubidium, and Isobutane.
Throughout the site, frequent mention is made of the "Carnot Rule", suggesting that use of such alternative working fluids is a largely futile endeavor.
As such alternative fluids have been successfully employed, both historically as well as in modern times, I consider this additional reason to call the "Carnot Rule" itself into question.
Again, I believe the Carnot theory, generally, is largely responsible for holding back research and advancement in this area, and in the area of heat engines generally.
These Douglas Self pages at least are useful for revealing the various possibilities for alternative working fluids as well as some of the requirements, problems and dangers.