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Combustion/Hybrid Fuels...

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Combustion/Hybrid Fuels...

Unread postAuthor: saefroch » Sun Jun 13, 2010 9:41 pm

I was musing about my hybrid/combustion plans after reading over the thread about using isopropanol as a fuel, when I began to wonder about other fuels for combustion/hybrid cannons. The real question I have here is this: What is the relationship between molecular weight of other alkanes (not sure if that's the right IUPAC designation for those) and actual pressure spike output? I've thought about using 2,2,4-trimethylpentane, but I'm not sure if it would work well in a spudgun. I know I tried it once in my spray 'n' pray and was quite terrified by the output... but that wasn't exactly a controlled experiment.

And I'm fairly sure isopropanol and propanol are lower-output fuels than propane, but not fully sure, anyone confirm or deny?

Any anecdotal evidence is appreciated, but calculations and numbers are preferred.
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Unread postAuthor: inonickname » Mon Jun 14, 2010 3:40 am

For your simple alkanes I believe lighter is better. So, methane will be better than propane, which is generally better than butane. Methane is the simplest alkane and considered one of the best fuels for this purpose (conventional CLGG guns use hydrogen or methane). However, the longer the chain is, the more energy is released as more bonds are broken. I believe the lightest alkane you can use would still be the best.

Hydrogen is an option as well. As far as other propellants go I can't really comment.
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Unread postAuthor: spudtyrrant » Mon Jun 14, 2010 7:37 am

Good Ole 100% nitro-methane with NO2 should do the trick........jk as ino said, generally lighter is better, as as far as iso-octane goes it shouldn't be too much more powerful than gasoline as far as pressure generated goes. which for iso-octane should be right around 85-90psi
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Unread postAuthor: saefroch » Mon Jun 14, 2010 9:43 am

So.... why would a lighter alkane be a better fuel if it releases less energy per mole than a heavier? I can see the obvious benefits of rate of diffusion with methane versus octane which is certainly a factor in spudguns, as well as measurement resolution with metering more moles of gas, but still...

Can anyone offer me numbers on this? I'm going to try to use my high-school chemistry skills to calculate the different pressure spike yields, but I'm a little rusty, and it's still somewhat beyond my skills.

EDIT: I'm clearly incapable of doing these calculations, as I keep getting results saying I'll get a max pressure spike of about 18 psi off an octane mix.
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Unread postAuthor: 12332123 » Mon Jun 14, 2010 10:39 am

Lower Mw alkanes are better because they produce light combustion gasses. The longer the carbon chain the lower the H/C ratio so the higher the proportion of high MW CO2.
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Unread postAuthor: saefroch » Mon Jun 14, 2010 2:07 pm

So are you saying incomplete combustion is more likely with larger alkanes, or that producing a higher proportion of water to carbon(IV) oxide yield a larger pressure spike?

EDIT: And also that such incomplete combustion is the only reason that one would chose the lighter alkanes?
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Unread postAuthor: ramses » Mon Jun 14, 2010 7:37 pm

Once you factor in the nitrogen in the air, the number of molecules doesn't change much at all. For propane, you start with roughly 31 moles, and end up with around 32.

The real power comes from the temperature increase. It really has to do with the heat of combustion of the fuel, along with the heat capacity of the products, combined with the nitrogen that is also in the air. The simplest way to look at that would be the adiabatic flame temperature.

One must also look at flame front speeds, so that the fuel is burned in a timely manner, but doesn't DDT.
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Unread postAuthor: saefroch » Mon Jun 14, 2010 7:53 pm

Okay... but is there even any difference in the heat output between completely combusted methane and octane?

And what's wrong with DDT? Wouldn't that be a good thing to get reliable DDT in a combustion or hybrid?
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Unread postAuthor: spudtyrrant » Mon Jun 14, 2010 8:50 pm

saefroch wrote:Okay... but is there even any difference in the heat output between completely combusted methane and octane?

And what's wrong with DDT? Wouldn't that be a good thing to get reliable DDT in a combustion or hybrid?
yes methane produces about 50 MJ/kg trimethylpentane is about 44MJ/KG, its a delicate balance if you go weight for weight hydrogen is the best but energy/mole acetylene is the best, but as far a common fuels go methane is about the most powerful fuel you will find. roughly 10% more powerful than propane(unless you count mapp but thats a little bit on the acetylene side), and as far as the other question goes, no DDT is a bad thing , it would damage most galvanized chambers beyond repair
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Unread postAuthor: ramses » Tue Jun 15, 2010 3:56 pm

Like I said, one should focus on the adibodic flame temperature. The temperature in the chamber can be assumed to rise to that temperature, and the pressure will follow. One should also consider how the flame front velocity, which impacts how quickly the temperature (and thus pressure) increase occurs. A faster combustion is like a faster valve in a pneumatic, but DDT is bad, unless you prepare for it.

Since you are not larda, DYI, Ragnarok, rp181, D_Hall, etc. it is bad.

CLGG's use certain fuels because of vapor pressure, SOS, etc. They actually push things fast enough and with enough pressure that those factors become actual considerations.
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Unread postAuthor: jimmy101 » Tue Jun 15, 2010 6:06 pm

The heat of combustion per mole of oxidizer is virtually identical for all simple hydrocarbons. Methane, ethane, propane, isopropane, butane, ... hexane .. gazillionane all have energy contents that are within a couple percent of each other.

(Yes, that is the proper measure, the heat of combustion per mole of oxidizer. Your chamber geometry determines the number of moles of oxidizer you have available. The fuel ratio is calculated to completely use that amount of oxidizer.)

There is a small difference in the ratio of water to CO2 produced by the various hydrocarbons but that difference isn't enough to affect the performance of a spudgun. (Figure that even a well operated, precisely fueled gun with a constant ammo probably has at least a 5% shot-to-shot variability in muzzle energy.)

Take a look at the spudwiki page on common fuels and you'll see little difference between the common fuels. Certainly not enough difference to actually choose one fuel over another.

There are a few "outliers" but they are not simple hydrocarbons. Hydrogen and acetylene are unusual. In practice it is mostly because they burn faster than most other fuels and not because their energy content is all that much greater. (MAP is between the regular hydrocarbons and acetylene.) Hydrogen and acetylene contain about 10% more energy per mole oxidizer than do the hydrocarbons.

Hydrogen reaches about 10% lower peak pressure than most hydrocarbons but burns roughly 7X faster. Acetylene gives ~5% higher pressure than propane and burns about 3X faster.
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Unread postAuthor: jimmy101 » Tue Jun 15, 2010 6:10 pm

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Last edited by jimmy101 on Thu Jun 17, 2010 4:36 pm, edited 1 time in total.
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Unread postAuthor: D_Hall » Tue Jun 15, 2010 6:10 pm

LOL... Hell, I *am* D_Hall and I would avoid DDT. It's not that it can't be controlled/contained. Clearly, it can be. But the PITA factor for dealing with DDT outweighs the PITA factor of simply building a larger gun with better behaved fuels.


As a bit of an aside.... I honestly don't grok why so many people around here like to play with "odd" fuels. Ether for the spray and pray crowd. Propane or MAPP for the rest of us. Very few of us have pushed the envelope with Propane/MAPP; so why the desire for odd fuels? If you want a gun that bangs better, why not just use the Propane/MAPP in a more effective manner? Why go into uncharted territory if you don't have to?
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Unread postAuthor: saefroch » Tue Jun 15, 2010 7:05 pm

I was hoping to investigate using different fuels to possibly squeeze a bit more yield out of a mix, and for interest in ease of storage and injection.

My questions have duly been answered, thanks to you all.
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