Fuels such as hydrogen, natural gas, and propane are great for
internal combustion engines, because of their high octane and low
pollution exhausts.
But I heard that Acetylene makes a poor fuel, not because it’ll
make the engine melt, but because the fuel is so explosive, it’s
octane rating is below that of gasoline!
True?
Acetylene might not be a poor fuel, but rather an unpractical one.
Similar to the way that diesel will self-combust when placed under ~20 atmospheres of pressure, acetylene will do the same, but at much lower pressure. IIRC, 3 atmospheres of pressure is sufficient to set it off.
That means you can’t store it in a conventional pressure vessel like natural gas or hydrogen.
It also wouldn’t be a suitable alternative fuel for conventional internal combustion engines, again due to the relatively low pressure with which it self-ignites. Typical compression ratios in vehicle engines are around 10:1, so the air/fuel mixture would ignite way, way, too early in the compression stroke.
Don’t you mean that an acetylene/air mixture would ignite at 3 atmospheres? I don’t see why pure acetylene cannot be stored in conventional pressure vessels. Otherwise how are they stored?
Acetylene is stored by dissolving it in acetone.
As the device uses the gas, it percolates out of the acetone just like the CO2 bubbles in carbonated fizzy drinks.
The maximum recommended “safe” pressure for acetylene is 15 psi- over 18 and it’s possible that it can self-ignite. As in inside the system, not necessarily needing a fuel/air mixture.
Anyway, since the “gas tank” would have to be filled with acetone, and the correspondingly low energy density in a tank of such weight, and the rate at which the gas would have to percolate out of solution to feed a fair sized engine, all combine to make it a rather poor choice for automotive use.
Doc.
It’s also stored in the form of Calcium Carbide, which is used in carbide lamps (Una, the ex-caver, still has a can or two of carbide laying around…). The carbide chunks are put into a bottom reservoir, and water is dripped slowly from a top reservoir. The water reacts with the carbide to produce acetylene gas, which is then directed through a tip to be ignited.
What’s all this about hydrogen and methane having a high octane? Hydrogen (H[sub]2[/sub]) and methane (CH[sub]4[/sub]) have zero octane (C[sub]8[/sub]H[sub]18[/sub]). Octane is not a property of fuels, it’s a specific chemical substance, which happens to be a major component of gasoline.
IAAEngineer and I have a couple of corrections-
Octane is a specific chemical compound.
Octane rating is be a property of fuels (usually gasoline, which is a mixture of several hydrocarbons, not only octane), and can be measured using a complicated procedure involving calibrated engines.
And, unless there was some acetone hiding itself inside the tanks for my cutting torch, it is perfectly OK to store acetylene under high pressure, as long as it is not mixed with air or any other source of oxygen.
Longhorn- If you ask at the shop where you get the tanks refilled/replaced, you’ll find that they do indeed contain acetone.
From what I understand, that is the only real safe way to contain it at higher pressures other than in solid form as Anthracite mentions.
I know one person who laid the tanks down in the bed of his pickup to do a remote jobsite project, and thus ruined his Victor torch reg- the acetone eventually degraded the seal(s).
Doc.
Yes, I is be an engineer, and English really is be my first language 
.
DocI stand corrected. From the MSDS for acetylene:
I didn’t notice the liquid sloshing around, since it is suspended in what is basically a sponge.
Ya learn something new everyday.
I’m curious about why this is necessary. It can’t be that it will explode on its own (in the absence of an oxidizer). Is it because it is a pretty unstable molecule and several common things (like “copper, brass, copper salts, Hg Hg salts, K, Ag and Ag salts, and HNO3”) will catalyze its breakdown into elemental carbon and hydrogen?
They don’t contain octane, but they have an octane rating.
Fuels, with Research and Motoring Octane Numbers (RON and MON, respectively) from one source I have lying here:
Fuel RON MON
----------------------------------------------
Methane 120 120
Propane 112 97
Ethanol 129 ?
I have no idea what the RON or MON of acetylene is. Anyone else?
Ya learn something new every day. So just what does the octane rating measure, then?
This is a subject of endless and often misleading debate, but a good place to get more of the straight dope is from the Gasoline FAQ.
From my reading of that FAQ: Would it be accurate, then, to say that a higher-octane fuel will ignite more uniformly under given conditions? So, when hydrogen, say, or octane itself, reaches a certain temperature and/or pressure, it all ignites at once?
Chronos: I wouldn’t say that higher octane fuels ignite more uniformly per se. Instead, I’d say that higher octane fuels can ignite as uniformly as lower octane fuels, but can do so at higher pressures. So in a low-compression Ford engine, both fuels ignite uniformly. In a high-compression Mercedes engine, only the high octane fuel ignites uniformly.
As for the instant ignition of all the fuel if it were a pure fuel, I don’t think that it will because there will be hot spots within the cylinders – early ignition will likely occur there first and the flame fronts will likely get to the bulk of the fuel before the bulk of the fuel hits autoignition.