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MAPP gas question

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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MAPP gas question

Unread postAuthor: Budarooski » Mon Jul 06, 2009 12:16 pm

I just finished my propane canon and it requires 75psi in the meter for the correct 4.03% mixture.

If I use MAPP gas, what is the correct % mix I need? If I know that, I can figure out the required psi.
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Unread postAuthor: niglch » Mon Jul 06, 2009 1:50 pm

Check the wiki and seach for MAPP gas. I'm pretty sure it is posted there.
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Unread postAuthor: Moonbogg » Mon Jul 06, 2009 2:40 pm

http://www.thehalls-in-bfe.com/HGDT/installation.html

If I could have this program etched in my brain id do it. Say no more.
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Unread postAuthor: niglch » Mon Jul 06, 2009 3:02 pm

Hmm... I just took a look at HGDT and it seems to disagree with the percentage given on the wiki. HGDT calculates a ~2% mix (by volume) while the wiki says 4.6%.
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Unread postAuthor: Moonbogg » Mon Jul 06, 2009 3:51 pm

niglch wrote:Hmm... I just took a look at HGDT and it seems to disagree with the percentage given on the wiki. HGDT calculates a ~2% mix (by volume) while the wiki says 4.6%.


I actually wondered about that as well. I tried a different program and it called for MAPP pressure much closer to propane while HGDT was way less.
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Unread postAuthor: jimmy101 » Mon Jul 06, 2009 4:00 pm

The proper fuel ratio for MAPP is a bit of a problem. MAPP is a mitutre of things, primarily methyl acetylene, propadiene and propane. The exact mixture is ...?

Methyl acetylene and propadiene have the same fuel ratio (since they are chemical isomers and have the same molecular formula); 4.98% if the fuel displaces some of the air in the chamber and 5.24% if the fuel doesn't displace any air.

Methyl Acetylene (C3H4): CH3—C≡CH
Propadiene (C3H4): CH2=C=CH2

The fuel ratio then depends on the amount of methyl acetylene/propadiene versus propane in the mix.

The 4.6% (additive, not displacive) quoted in the Wiki MAPP page appears to be based on the mixture being about 67% propane.
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Unread postAuthor: Moonbogg » Mon Jul 06, 2009 4:22 pm

So either way, more mapp is needed than propane, right? I found mapp to be very forgiving for some reason. I used it for 2x shots and tried 80psi in the meter pipe as well as 50psi and both worked about the same from what I could tell without a chrony.
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Unread postAuthor: Budarooski » Mon Jul 06, 2009 4:26 pm

jimmy101 wrote:The proper fuel ratio for MAPP is a bit of a problem. MAPP is a mitutre of things, primarily methyl acetylene, propadiene and propane. The exact mixture is ...?

Methyl acetylene and propadiene have the same fuel ratio (since they are chemical isomers and have the same molecular formula); 4.98% if the fuel displaces some of the air in the chamber and 5.24% if the fuel doesn't displace any air.

Methyl Acetylene (C3H4): CH3—C≡CH
Propadiene (C3H4): CH2=C=CH2

The fuel ratio then depends on the amount of methyl acetylene/propadiene versus propane in the mix.

The 4.6% (additive, not displacive) quoted in the Wiki MAPP page appears to be based on the mixture being about 67% propane.


I was wondering about that, because on this link http://www.spudfiles.com/spud_wiki/index.php/Common_Fuels_for_Combustion_Spudguns it shows propane to be 4.02% and MAPP to be 4.98%, but then you go to this link http://www.spudfiles.com/spud_wiki/index.php/MAPP and it says 4.2% and 4.6% respectively, so what is the correct value?

I have the spudtool.xls file that will tell me the correct PSI based on my cannon's dimensions and desired %, but whe Wiki contradicts itself I don't know what to use.
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Unread postAuthor: D_Hall » Mon Jul 06, 2009 5:20 pm

LOL... Ya know, you folks may have something there...

When I wrote HGDT I concentrated on the propane stuff but included logic for MAPP. I threw in some values for various MAPP-related things, but I'm not 100% sure I ever went back to correct all of it. I know I did the flame propogation rate... But fuel mixture? I'm honestly not sure.
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Unread postAuthor: jimmy101 » Mon Jul 06, 2009 5:23 pm

Budarooski wrote:I was wondering about that, because on this link http://www.spudfiles.com/spud_wiki/index.php/Common_Fuels_for_Combustion_Spudguns it shows propane to be 4.02% and MAPP to be 4.98%, but then you go to this link http://www.spudfiles.com/spud_wiki/index.php/MAPP and it says 4.2% and 4.6% respectively, so what is the correct value?

I have the spudtool.xls file that will tell me the correct PSI based on my cannon's dimensions and desired %, but whe Wiki contradicts itself I don't know what to use.

Actually, if you look closely at the MAPP entry in the Common_Fuels_for_Combustion_Spudguns Wiki page you'll notice that no values is given for MAPP. There are values for methyl acetylene and for propadiene but not MAPP itself.

Here is the MSDS for MAPP. There are six things in it and the exact percentages aren't stated, just ranges. <table><tr><td> Name<td>Chemical Formula</td><td>Percentage Range</td></tr> <tr><td> Propyene</td><td>CH3-CH=CH2</td><td>40 - 50</td></tr> <tr><td> Methyl Acetylene</td><td>CH3-C≡CH</td><td>27 - 33</td></tr> <tr><td> Propadiene</td><td>CH2=C=CH2</td><td>13 - 15</td></tr> <tr><td> Isobutane</td><td>CH3-CH(CH3)-CH3</td><td> 2 - 5</td></tr> <tr><td> n-Butane</td><td>CH3-CH2-CH2-CH3</td><td> 2 - 5</td></tr> <tr><td> Propane</td><td>CH3-CH2-CH3</td><td> 1 - 5</td></tr></table>

When calculating the fuel percentage Methyl acetylene and propadiene are the same. n-Butane and isobutane are the same.
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Unread postAuthor: Moonbogg » Mon Jul 06, 2009 6:15 pm

Screw it. Start with 5% then tweak from there.
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Unread postAuthor: mark.f » Mon Jul 06, 2009 6:25 pm

I remember working this out in Junior year on a sheet of notebook paper and getting an additive percentage of around 4.89%. Can't remember where I got my figures but I have been using my figure in both combustion and hybrid cannons since then with success. Guess it's time to sit down and work it out again to see if I get the same number.

Although I doubt the small difference effects final mixture much unless using a volumetric meter at high compressions.

EDIT: I remember now. The MSDS I used (from BernzOmatic's site, since I use their gas) quoted 56% LPG and 44% methylacetylene, which I then extrapolated further using the average fuel percentages for LPG.
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Unread postAuthor: starman » Mon Jul 06, 2009 6:45 pm

My working rule of thumb has been to add 20% more psi for MAPP on your fuel meter. Sooo if you build a meter that requires 50 psi for propane, bump it to 60 psi for MAPP.

I too have found MAPP to have a much larger margin for error.
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Unread postAuthor: mark.f » Mon Jul 06, 2009 7:28 pm

Well, I reworked it using 56% liquefied petroleum gas and 44% methylacetylene/propadiene. Since I don't know what "blend" of LPG the fuel producers are using, I simply treated the LPG as propane. Got an additive ratio of 7/152 or around 4.6%.

I guess the real source of variability of the constituting fuels is that the companies use 56% LPG and 44% methylacetylene/propadiene mixture, but the makeup of those two gases is variable since they probably get them from outside sources.
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Unread postAuthor: Budarooski » Mon Jul 06, 2009 9:03 pm

Moonbogg wrote:Screw it. Start with 5% then tweak from there.


That's funny...that's what I was thinking. :)
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