Carbonic Acid Engines.

Updated: 18 June 2008
Railway signal article added
Back to Home PageBack to The Museum

Now and again, in the very early history of motor cars and aeroplanes, you will encounter references to a "carbonic acid motor", usually quickly followed by a statement that the machine in which it was installed was a total failure. Information on this method of deploying power is very hard to come by, but here are a few gleanings.

Carbonic acid is an old name for carbon dioxide. A carbonic acid motor (or engine) is driven by pressure which may simply come from a reservoir of compressed gas, or be produced by evaporating liquified carbon dioxide. A liquified gas takes up much less space than its compressed gaseous equivalent, giving the possibility of greater energy storage. Carbon dioxide was used for the liquid version because it is fairly easily liquified, unlike the so-called "permanent" gases such as nitrogen and oxygen, which require more energy and more complex machinery to liquify, and are much harder to store for any length of time.
All gases have a critical temperature; below this temperature, the gas can be liquefied by the application of pressure alone. Carbon dioxide has a critical temperature of 31.1 C, and so can usually be liquefied just by compression. The critical temperatures of oxygen and nitrogen are -118C and -146C respectively, and they are therefore much harder to liquify, as considerable pre-cooling is needed as well as compression.

Calling carbon dioxide "carbonic acid" is an obsolete usage. Nowadays carbonic acid refers to H2CO3, a weak dibasic acid formed when carbon dioxide dissolves in water; this acid exists only in solution.


In 1884 or 85 the US Navy exhibited a Lay-Haight torpedo, powered by a reservoir of compressed carbonic acid that worked a Brotherhood engine connected to the screw. The torpedo was wire-guided and exploded from the shore, so was presumably intended for coastal defence only.


Lilienthal holds an honoured position as one of the pioneers of flight. Like several others before him, Lilienthal never quite abandoned the idea that flapping wings was the key to motion. In 1893 and again in 1896, he built gliders with flapping wings in the ornithopter fashion. Each machine had a lightweight carbonic acid engine that produced about two horsepower (1.5 kilowatts). The engine was supposed to make the wing tips flap up and down and move the aircraft forward. Neither model was successful.


Left: An article on railway signals powered by carbonic acid.

An intriguing reference to carbonic acid, but with no real information at all. Was the gas piped to each signal, or carried there in bottles? Presumably it was in liquid form to increase the power storage density, otherwise they might just as well have used compressed air.

I have never heard of this system of signalling before- it cannot have made much impact.

From Model Engineer & Electrician for 26 May 1904, p489


In 1906 the Romanian aeronautical experimenter Trajan Vuia was living in Paris and testing a small bat-like monoplane with a tractor propeller. The pilot sat well below the wing on a framework with a four-wheeled undercarriage. It was driven by a carbonic acid motor, of which no details have so far been found, except that it was of inadequate power. (A later monoplane was powered by 24hp Antoinette petrol engine) Some short hops from level ground were made, the longest being 24 metres. Proper flight was not realised, but the machines are considered to be the immediate ancestors of the monoplanes which appeared in Europe before World War 1.


This sort of motor must be quite obsolete, no? No. Certainly, typing "carbonic acid motor" or "carbonic dioxide motor" into a search engine will (at the time of writing) yield nothing. The result is quite different if you use "CO2 motor". You will find that modern carbonic dioxide motors are used to power model aeroplanes. Advantages include low noise output and no flammable fuel.

A small metal reservoir is charged with either liquid or gaseous carbon dioxide under pressure. This powers a motor that looks very much like a small glowplug engine. There is a valve at the top of the cylinder usually operated by a protrusion from the crown of the piston. Exhaust is via a uniflow port uncovered by the piston at the bottom of its stroke. Fins are provided, just as on a glowplug engine, but here the purpose is not cooling but to stop the engine cooling down too much as the CO2 expands.

Left: Two CO2 motors for model aircraft.

One pipe runs from a non-return filler valve to the reservoir, and the other takes the CO2 to the engine.

Image taken from the website of Davis Diesel Development. (with permission, I might add) See link below.

CO2 motor links: Do not miss the historical overview in the "CO2 motors" section. which shows some fine CO2 engines, including a V12!

Back to Home PageBack to The Museum EntranceTop of this page