Alcohol Motors.

Updated: 16 Oct 2008
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This page deals with engines that use alcohol vapour as a working fluid, rather than simply as a fuel to burn. Some references are so bound up with The Petrol-Vapour Motors that they remain on that page.

Alcohol vapourises easily, but that does not mean it is more efficient than water as a working fluid. Quite the reverse. See: Carnot's Law on the thermodynamics page.

ALCOHOL VAPOUR ENGINES

The French journal Nature, (21 June, 1890) has this to say: "Artwright proposed and constructed, in 1797, an alcohol-vapour machine which worked well enough, but which did not survive, very probably because of the very imperfect construction of the time; substantial leaks of a relatively costly vapour would have made the system very uneconomic."

This is a tantalising glimpse into the past which I shall try to follow up. I strongly suspect that "Artwright" should be "Arkwright".

Left: A toy steam-engine driven by alcohol vapour, with the exhaust combusting.

This scheme comes from the book "Engines" by E N da C Andrade, published G Bell & Sons in 1928. (A very engaging book that manages to deal with the complexities of thermodynamics in a most approachable way)
The author points that the alcohol vapour exhaust can be lit, though it would be wise to make sure that all the air has been driven out of the plumbing before trying it. This underlines that a non-condensing (total-loss) alcohol vapour engine is a grotesquely inefficient proposition. I hasten to add that the author was not claiming otherwise, but goes on to deal with the concept of Carnot efficiency.


BINARY VAPOUR ENGINES

A binary vapour engine boils a mixture of liquids. It doesn't appear to be a good idea, and I have found only one account of people trying it:

Above: An experiment with a binary working fluid- in this case water and methyl alcohol.

This is taken from the British journal Engineering for 9th Jan, 1885.

Note that the board of US engineers had grave (and entirely justified) doubts about the thermodynamics of low-boiling-point liquids as working fluids.

The major problem with the later large-scale test seems to have been leakage; not good when the vapour leaking is toxic, inflammable and relatively expensive. Whether methanol really does leak out more easily than steam I have not so far been able to determine; presumably it depends on the size of the vapour molecule.
Another pitfall was clearly the tendency of this binary mixture to change its composition when boiled, so that the condensed liquid in the hotwell was much richer in methanol than the liquid in the boiler. If the behaviour of ethanol-water mixtures is anything to go by, the vapour would not boil off in the same proportions as in the liquid in the boiler unless the methanol concentration was a lot higher than 15%; ethanol-water forms a constant-boiling mixture at a concentration of 95.6% ethanol.

To learn more about constant boiling mixtures see here. (external site)

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