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This math look correct to you guys?

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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This math look correct to you guys?

Unread postAuthor: Bangotwago » Wed Jan 16, 2013 7:06 pm

Built a prototype already. Ended up with 1:5-1 and Im not completely satisfied with power. Second unit I'm going to learn from my mistakes. Wanted you guys to check my metering math please.
Going for 1:2-1
Chamber volume is 2297 ml
140.10 cu in
Need 5.60 cu in of propane for 4.2%
40psi * x=14.7psi*5.60 cu in

2.06 cu in needed @40 psi
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Unread postAuthor: pneumaticcannons » Thu Jan 17, 2013 1:26 am

I don't know how you got 5.6 in^3
140.1 * 0.042 = 5.88. Not 5.6
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Unread postAuthor: jimmy101 » Thu Jan 17, 2013 3:20 pm

I believe you need to take into account the fuel left in the meter. Looks to me like you are under fueling.
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Unread postAuthor: mark.f » Thu Jan 17, 2013 3:36 pm

The fuel left in the meter is accounted for by the fact he is using 14.7 PSI on the left hand side, but using "40 PSI" as is on the right hand side as gauge pressure.

If you wanted to match units, the "40 PSI" on the left would be PSIA, which would be 25.3 gauge PSI. Using the extra 14.7 PSI accounts for leaving an atmosphere of fuel in the meter.

I made a write-up for this on my website, which is down. I could copy-paste it here if you'd like.
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