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Propane Injection 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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Propane Injection question.

Unread postAuthor: talknspud » Mon Feb 09, 2009 6:03 pm

My buddy and I have been making guns for awhile and we are working on some propane gun designs. When using propane, what is the best way to get the propane from the injector to the chamber? I have seen starman's guns with the air fittings but I wanted to use a direct shot from the injector. We were going to try a quarter inch hose directly into the chamber but didn't know if that was the best way to go.
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Unread postAuthor: ramses » Mon Feb 09, 2009 6:09 pm

you could open up the back and just open a valve to let it in like a spray and prey.

if you wanted you could go with the hose, just get a 1/4" NPT hose barb, and tap it into the chamber, that has been done before.

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Unread postAuthor: talknspud » Mon Feb 09, 2009 6:13 pm

Do you have to have like a valve at the chamber so pressure does not go back into the hose? I didn't know if that would make a difference.
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Unread postAuthor: Combustion Monkey » Mon Feb 09, 2009 6:20 pm

For the sake of repeatability I recommend some type of meeter with a pressure regulator. This can be either onboard or a pod/syringe setup like starman and others use. Works great for those of us that cant stop tweaking and fine tuning, and all the cool kids do it :lol: not that I am one of the cool kids (kicks a bit of loose sod).
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Unread postAuthor: talknspud » Mon Feb 09, 2009 6:52 pm

I have the meter and the regulator built and ready to go. I was wondering the best way to get the fuel from that to the chamber without loosing pressure or fuel in the transition.
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Unread postAuthor: Combustion Monkey » Mon Feb 09, 2009 7:03 pm

For onboard , most run a 90 deg elbow to the meter pipe and mount it to the chamber. For an off board setup I would recommend a airline quick connect with a ball valve for dumping the charge. Without the ball valve I have found that it's possible to have significant pressure loss when connecting the fittings.
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Unread postAuthor: Radiation » Mon Feb 09, 2009 8:27 pm

Check out my sig. As combustion monkey said, a 90 elbow two ball valves and a nipple for your meter is the base of the system. My gun has an onboard metered propane setup. I have since upgraded to a Husky pressure gauge/regulator and now the whole setup self regulates the exact PSI my gun needs to operate without me having to do anything other than open the first ball valve. It is very efficient, fast, and accurate. Hope that helps!
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Unread postAuthor: starman » Mon Feb 09, 2009 8:31 pm

talknspud wrote:I have the meter and the regulator built and ready to go. I was wondering the best way to get the fuel from that to the chamber without loosing pressure or fuel in the transition.


You need a valve of some sort. That could take the form of many things...it just so happens that ball valves are cheap and reliable and tend to dominate usage on fuel meters.

You can use a 3 way solenoid valve like is used here...

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...a little more complication and expense but very cool.

Do any of these comments answer your question?
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Unread postAuthor: talknspud » Mon Feb 09, 2009 9:11 pm

i have the metering devise built and regulated. What is the best way to get the propane from the meter to the comb. chamb.? A lot of people tapped the chamber to accept male NPT. do we have to do that or can i just attach a hose with superglue?
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Unread postAuthor: Radiation » Mon Feb 09, 2009 9:28 pm

No. No hose and definately no super glue. Take time do it right.

NPT tapped into the double thick section of the PVC.
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Last edited by Radiation on Mon Feb 09, 2009 9:34 pm, edited 1 time in total.
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Unread postAuthor: starman » Mon Feb 09, 2009 9:34 pm

OK now I get you. I'm not sure why you would want to super glue a rubber hose in a hole. If you think you must go down this road, use a lot of epoxy, not super glue. Even then, I would consider it a bush league thing to do.

Why would you not want to tap and fit an appropiate threaded brass piece..?..pvc is very easy to tap...a 7/16" drill bit and a 1/4" npt tap and you're good to go.

BTW, welcome to Spudfiles.

Edit: Yeah, and what radiation said.... 8)
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Unread postAuthor: jimmy101 » Tue Feb 10, 2009 1:16 pm

For PVC you don't really need the tap, PVC is soft enough that the fitting will do a decent job of tapping the hole. Get a piece of scrap PVC and drill a couple slightly different size holes. Tap the holes with the fitting to figure out what bit size works best.

A tap is a better way to go but a 1/4" NPT tap will probably cost about $5 (if you can find one).
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Unread postAuthor: talknspud » Tue Feb 10, 2009 3:51 pm

Will any of the combustion or pressure go back up the hose once fired? Does a person disconnect the hose from the fitting on the chamber before you fire? Do you guys use Schedule 80 at all or does 40 do the trick for a powerful propane gun?
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Unread postAuthor: Hubb » Tue Feb 10, 2009 3:55 pm

When I installed mine, I simply used the fitting to cut the threads (as jimmy mentioned). It went through two layers of PVC, just to be sure (which was Sch40). I also used teflon tape.

As far as keeping the pressures from going back into the meter, most meters have a ball valve that separates the meter from the chamber.

My combustion is on here, somewhere, (Edit: It's right here) and I believe I got a decent pic of the metering system. It is, however, pretty standard. Just search through the Combustion Cannon Showcase and find one. It will give you a good idea of what you want.
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Unread postAuthor: starman » Tue Feb 10, 2009 5:27 pm

talknspud wrote:Will any of the combustion or pressure go back up the hose once fired? Does a person disconnect the hose from the fitting on the chamber before you fire? Do you guys use Schedule 80 at all or does 40 do the trick for a powerful propane gun?


Whatever is open to the chamber at the time of ignition will experience the same pressure spike as the chamber. The meter's chamber side ball valve typically halts that spike back into the meter, at least with an on-board meter. A quick connect will seal itself.

Sched 40 is fine for 1x combustion. Use pressure rated pipe (no cellular core) and fittings whenever possible. Drill holes in 2 layers of PVC.
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