Yet another topic begging for help: Metered propane edition

[madmarcs]

Several weeks ago I set out to build my first metered propane cannon and up until actually trying to fire it things were going well. Every system works: the meter pipe holds pressure and seems to vent into the chamber, the fan turns on, the igniter (made from a stun gun) can be clearly heard arcing, but nothing happens. Between every misfire I removed the barrel and flushed the chamber before giving it another go with the same results. I have two theories as to why this is, but before I go messing things up I figured it would be good to get more expert advice.

Theory 1: Due to faulty math I ended up making the meter pipe volume 58 in^3 (a ridiculously large number, I'm aware of this). I redid all the math following a guide on these forums using Boyle's law (p1v1=p2v2) and discovered that with my chamber volume of 188 in^3 that my meter pipe would only need to be at 2 psi. The only gauge I could find read from 1-100 psi so getting to two psi is a challenge and I imagine that its not particularly accurate. Based on this inaccuracy have I been flooding the chamber each and every time? If so should I make the meter pipe smaller so that higher psi is required and an accurate reading can be obtained?

Theory 2: Considering the fact I'm testing this thing in my back lawn, no projectiles have been loaded into the barrel when firing. I did put a giant wad of paper towels all the way to the bottom of the barrel to try and block up the chamber but could the gas be escaping? I figured that a completely 100% perfect seal would not be necessary due to the low pressure of the gas and the nature of the cannons operation, is this misinformed?

Any help regarding this problem would be greatly appreciated.

[jimmy101]

The paper towels should work fine to keep the fuel in the chamber for a minute or so.

If you have a chamber fan then the paper towels might be leaking to much fuel. If you don't have a chamber fan you'll want to wait tens of seconds or so after fueling to insure that the fuel is fully mixed with the air in the chamber.

The real problem is the meter pipe. It needs to be smaller. The meter pipe volume should be less than or equal to about 4% of the volume of the chamber. If you set the meter pipe to be exactly 4% the volume of the chamber then you would pressurize the meter to 1 ATM gauge pressure (that's 2 ATM absolute pressure) and when you vent to the chamber you'll get one meter pipe volume at the proper pressure. (Don't forget to "burp" the meter pipe a few times to get the air out.)

Or, get a syringe. For a 188 ci chamber you need 7.5 ci (123 ml) propane at atmospheric pressure. You can usually get 70 ml syringes for a buck or so at farm supply stores.

[madmarcs]

Thank you for the advice. During the time between me posting my initial query and now the cannon actually did fire on accident one time when I was working on the ignition system. Thankfully nothing was in the barrel except for paper towels and I am not dumb enough to look down the barrel. This led me to assume that the combustion chamber did not have any leaks and that I would have to re work the meter pipe. The more I look at it the more I realize it was ridiculously over sized. I will cut it down and try again. As always, I greatly appreciate the help.

[jackssmirkingrevenge]

I say this every time but metering fuel with a syringe at atmospheric pressure is also a viable option with less variables.