fuel mixing

[ramses]

First, the relevant specs of my cannon:

• 2x12" chamber plus a lot of fittings
  manometric metering
  ignition: camera flash+ignition coil (tested to work at pressure)

(see picture for configuration of spark and fuel)

Anyway, the cannon won't fire reliably, but does fire occasionally. It does fire reliably if, after I fill the chamber up with air, I pull off the hose and vent some of the air/propane to atmosphere. I guess this mixes the gasses. My question is whether or not I can do something simple and cheap to mix the gasses better without venting any. I read in another thread about adding a venturi, but no one else has needed anything like this to make theirs work.

thanks

[Technician1002]

I guess this mixes the gasses. My question is whether or not I can do something simple and cheap to mix the gasses better without venting any. I read in another thread about adding a venturi, but no one else has needed anything like this to make theirs work.

thanks

I have seen items left loose in the cannon to mix gasses much like a spray paint can has a marble inside. I have seen a length of PVC plastic chain, superball, or other related shake to mix items. Be sure the mixing agitator won't knock your spark gap out of whack.

Try not to use something that displaces a lot of space as this will mess up your mix. I have toyed with using a practice wiffle ball. It is the hollow ball with lots of holes. One of those shaken back and forth a few times should do a fine job mixing the contents.

[ramses]

Thanks, I may do that, but the future 3" steel barrel will make the shaking part rather difficult. It won't screw up my mix because I use manometric metering.

I have control over how fast my chamber is filled with air (while I am at my compressor) should I dump the air in as fast as possible, or just crack the valve and let it in slowly?

Does anyone think that making a nozzle to accelerate flow would be worth while?

[Lentamentalisk]

Ok, two things here. Make sure you put the fuel in fist, otherwise it ends up all concentrated on one side. 2nd, the faster the air is flowing when it enters the chamber, the more venturi effect it will have, so the better it will mix. there are two ways of doing that, either dumping in the air as fast as possible, on the compressor side, or sticking a tiny nozzle on the end, to make it go really fast.

Good luck.

[jimmy101]

Tech's suggestion are pretty good. A practice golf wiffle ball might work pretty well and is small enough. People have also used a length of chain.

Back to Ramses original question...
The easiest way to trouble shoot your problem is to simply let the gun sit a couple minutes after fueling and before firing. If the problem is that the fuel isn't well mixed then letting the gun sit for a couple of minutes (perhaps with occasional inversion) will let the gases mix fully. If the gun fires consistently after setting, then the problem is mixing. If it doesn't, then the problem is either the mixture or the ignition.

[ramses]

I will attempt the nozzle, as I intend to use a bike pump for firing in the woods. I am putting the fuel in first, and like I said before, the gun will have a big heavy barrel on it, so shaking it around won't be practical. I wouldn't be able to put anything large in the chamber anyway, because I don't have a big enough vice at home.

Thanks for your help, everyone!

[daccel]

Jimmy's suggestions for a troubleshooting process make sense, my first thought was that perhaps your mix is off and when you vented it was bringing the % back into line.

[Technician1002]

I am putting the fuel in first,
Thanks for your help, everyone!

With the 1/2 inch T, there is an excellent possibility the plug area is mostly air and too lean to fire. After filling with air, venting a tiny bit to expell the air and draw mixture to the plug may be helpful.

[ramses]

I added a venturi type thing and still have problems! here are some pictures. Even after waiting 3 min, it won't fire. would the configuration shown in the first picture be a better configuration than the one in my first post?

Thanks

[GalFisk]

Your spark plug is a bit "out of the way", so you may have a problem getting a good mixture at the spark (possibly in addition to general mixture issues). I have had similar problems with my hybrid. You could try placing the spark in the middle of the chamber to see if you get more reliable ignition.

[Willdebeers]

You could always add a fan (brushless preferably) to aid mixing.