Alcohol Engines

Updated: 28 Sept 2011

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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.


AN ALCOHOL VAPOUR ENGINE DEMONSTRATION

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 and expensive proposition. I hasten to add that the author was not claiming otherwise, but goes on to deal with the concept of Carnot efficiency.


THE HISTORY OF 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."

I have long suspected that "Artwright" should be "Arkwright", but actually it looks as though the name was Cartwright; the entry below refers to Dr Edmund Cartwright, but it only says he suggested an alcohol engine, not that he actually constructed one.

Left: The entry on alcohol engines from Knight's Dictionary

With a description of Howard's alcohol engine, patented in 1825.

From Knight's Practical Dictionary of Mechanics pub Cassell & Co, 1884.

Above: Howard's alcohol engine: 1825

Howard appears to have been fully aware of the need to minimise the losses through leakage of an expensive working medium. He used hot oil as a liquid piston. (Early experiments seem to have beeen done with mercury, but the cost of this for a large engine would have been crippling. Breathing the mercury vapour would probably have been crippling too) In the diagram above a flat dish D can be seen floating on the hot oil A in the middle cylinder. The oil was kept hot by the four Argand burners underneath. Alcohol was sprayed into the hot dish by the narrow pipe just above it, propelled by pump R, and flashed into vapour. This pushed down the oil and pushed up the piston in cylinder B on the right.
On the return stroke the alcohol vapour was pushed into the condenser on the left via pipe S. The condenser tubes U were wrapped in wet flannel, with air blown over them by a fan mounted on vertical shaft Z.

This engine, with its complexity and its interesting combination of alcohol and hot oil, seems wholly impractical. No wonder that according to Knight, it "...wearied out the patience or the means of its inventor...".


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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