Unusual Working Fluids.

Updated: 16 Oct 2008
Minor corrections
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The Power section of the Museum of Retrotech contains many unusual engines- for some of them, their interest is in the choice of working fluid. Steam and air (the latter as in the internal combustion engine) reign supreme today, for very practical reasons. Both are chemically stable, non-poisonous, and cheap. However, in the past, several other fluids have been tried. Some of these have their own galleries in the Museum; this now includes mercury:


Petrol and Alcohol

Compressed Air


Carbonic Acid

Ether & Chloroform

Carbon Disulphide

It was reported in the French journal Nature for 21 July, 1888, that petrol, alcohol, ether, chloroform, and carbonic acid had all been tried as working fluids. As you can see from the pages listed above this is certainly true, though I have so far only been able to sniff out only the sketchiest details of what appears to be the only attempt to build a chloroform engine. Such an engine would have been interesting to work with; chloroform may be non-flammable, but it is poisonous as well as a (dangerous) anaesthetic.

Liquids that vapourise easily, when used in a stand-alone cycle, are not more efficient than water as a working fluid. Quite the reverse. See: Carnot's Law on the thermodynamics page.

Helium has also been used as a working fluid in Stirling-cycle engines; it improves efficiency but is expensive. Hydrogen would be cheap and even more efficient but the very small molecules are hard to contain- hydrogen will diffuse through solid metal- and hydrogen embrittlement of the metal parts is also a problem. Hydrogen is also highly inflammable if it leaks out.

This gallery, however, is devoted to the truly weird amongst working fluids. Now read on...


The boiling point of sulphur dioxide is -10 degC. It is definitely a minority choice when it comes to working fluids. It is poisonous, and in combination with water forms corrosive sulphurous acid, H2SO3. (not to be confused with sulphuric acid, H2SO4) So far I have found only one example of its use.

Henry E. Willsie was a pioneer of solar energy. A Willsie installation had large flat-plate solar collectors that heated water, which could be kept warm overnight in an insulated basin. Liquid sulphur dioxide was passed through pipes immersed in this basin, and it duly boiled to give a high-pressure vapour capable of driving an engine. The sulphur dioxide was then condensed for reuse; quite how, given that the boiling point of sulphur dioxide is -10 degC, is not currently known. Possibly the whole system was pressurised so that condensation could occur at ambient temperature.

In 1904 Willsie built two solar/SO2 power plants. One was a 6-horsepower installation in St. Louis, Missouri, and the other a 15-horsepower system in Needles, California.

Willsie believed that the ability to store energy so power could be generated at night made his system a commercial proposition. Unfortunately he was wrong; no buyers came forward. The long-term cost analysis might have looked good, but it appears potential purchasers were doubtful of the machine's durability; very possibly they thought corrosion from the sulphur dioxide would be a problem. They were also put off by the large amount of machinery required for a tiny power output, and the big initial investment it required. His solar energy company, like other pioneering efforts before it, disappeared.


An exotic working fluid that once had considerable hopes invested in it is diphenyl oxide, a very stable organic chemical with a boiling point of 258 degC at atmospheric pressure. It was explored as a way of getting heat into a thermodynamic cycle at a higher temperature, but nothing came of it. Oliver Lyle (author of The Efficient Use of Steam) wrote "Diphenyl oxide has been suggested as being more suitable than mercury." in 1958, which sounds as though it had not been tried in practice at that date.

Diphenyl oxide has the structure C6H5-O-C6H5. At room temperature it forms colorless crystals, with a smell of geraniums and phenol; in fact it is sometimes called "geranium crystals". When molten it is a colourless liquid. It is insoluble in water. It is relatively non-toxic as organic compounds go, but I wouldn't sprinkle it on my cornflakes.

Diphenyl oxide has lots of names: diphenyl ether, phenyl ether, 1,1'-oxybisbenzene, geranium crystals, oxydiphenyl, phenoxybenzene, and phenyl oxide are all the same chemical.

Diphenyl oxide is still used in specialised heat transfer applications at high temperatures, sometimes mixed with plain diphenyl; diphenyl is also called Biphenyl, Lemonene, Phenyl Benzene, Bibenzene, and Xenene, and has the structure C6H5-C6H5.


Some thoughts from Sadi Carnot, who knew a thing or two about thermodynamics:

"As to the other permanent gases, they should be absolutely rejected. They have all the inconveniences of atmospheric air, with none of its advantages. The same can be said of other vapours than that of water, as compared with the latter. "


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