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Fueling - Butane torch and pump

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Fueling - Butane torch and pump

Unread postAuthor: daccel » Sun Mar 01, 2009 4:03 pm

This fueling system uses a small butane torch to mix the fuel/air, an intake chamber, and a small pump consisting of a piston, equalization or intake hole, and a schrader valve.

It is a workaround for fueling small hybrids that are hard to meter without very precise gauges. It also does not require a regulator. If you initially measure the number of pumps required to reach each mix number, you can precisely control the combustion pressure without a gauge on the finished cannon. This makes it easier to design compact cannons.

Because it uses a threaded schrader valve as a check valve, you can use common steel pipe for all the high pressure parts of the cannon, making for a simple and safe build.

Here it is in action. This was the third firing, and the chamber was returned to 0 psi after each, but not vented.

[youtube]http://www.youtube.com/watch?v=zFCRHuq_3hM[/youtube]

This is a fairly simple build. The most challenging part is the pump, but I had a piston already made for something else, so in this case all I had to do was drill a few holes and glue a few things together.

Edit: added quick diagram.

I thought it would be more clear for searching and future reference to start a new thread. If not, could a mod please merge this with the initial thread, below.
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Unread postAuthor: iemand » Sun Mar 01, 2009 5:33 pm

Good idea, but wouldn't it be better to let the pump piston move back because of the pressure from the torch(in other words: let the torch fill the pump). Then ofcourse push the pump rod down and fill the chamber.

Because pumping as fast as you did in the video is imo screwing the mixture :roll:
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Unread postAuthor: jonnyboy » Sun Mar 01, 2009 6:14 pm

Nice build. Did you get that idea from the sponsor thread? I made a post in that thread about something similar except no reservoir.
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Unread postAuthor: daccel » Sun Mar 01, 2009 7:01 pm

Thanks guys.

Using the torch pressure to draw back the piston wouldn't work because if there is too much back pressure, the butane will just flow out of the air inlet holes on the torch.

As long as the flow of the torch is adequate, it doesn't matter how fast you pump. The pump just compresses the fuel and air which has already been mixed at the correct ratio in the torch. Though I am considering some sort of baffles to enhance the mixing.

Also, given butane's narrow flammable range, the consistent ignition I'm getting would seem to confirm the theory.

No, I came up with it independently, though I don't claim to be the first to think of it. You were talking there about using all fuel, not mixed with air right?
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Unread postAuthor: jonnyboy » Sun Mar 01, 2009 8:32 pm

No, I came up with it independently, though I don't claim to be the first to think of it. You were talking there about using all fuel, not mixed with air right?

Yes there was no air, given it was in the context of a 1x mix there wouldn't need to have air.

How do you measure mix numbers with your design?

To open the shreader you need 70 psi correct? How do you get that with such a little stroke?
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Unread postAuthor: ramses » Sun Mar 01, 2009 9:19 pm

I think his pump rod was depressing the little pin thing in the middle of the schrader.
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Unread postAuthor: daccel » Sun Mar 01, 2009 10:21 pm

To measure the mix you need a temporary gauge on the chamber to see how many pumps it takes. In this case the pressure rises 15 psi every 11 pumps. Once you know that, and assuming the volume doesn't change much taking off the gauge, you can measure the mix fairly accurately by counting pump strokes and dividing by the number you found it took to reach 15 psi.

The rod is depressing the pin. But I think, given the minimal dead space, that it probably passes 70 psi anyway.

Edit: Just had a thought - if you are not putting in fuel for the air that is already there, or previously combusted gases, and you charge to a 2x pressure, then there is only 1x's worth of fuel. So what would be the peak pressure? 1x plus 15 psi, so 115 instead of 100? Or would the inert gases expand with the heat to give a peak around 200 anyway?
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Unread postAuthor: Ragnarok » Tue Mar 17, 2009 10:30 pm

Daccel, got your PM. Figured I'd drop it in here rather than PM back, so other people can get an answer.

daccel wrote:So what would be the peak pressure?

It's worth using the program GasEq for questions like this.

Bear in mind, these results assume adiabatic conditions. I'll be doing the calculations for propane, but the same should apply for butane.

Propane, 1x ignition: peak ~10 atmospheres. (147 psi)
Propane, 1x fuel/air + 1x combusted gas mix: peak ~11.8 atmospheres (174 psi)
Propane, 2x fuel/air: Peak ~20 atmospheres. (294 psi)

That is, you gain around 25 psi peak from inserting 15 psi of inert gas.
Adding inert gases into the combustion mixture increases the end result pressure more than the basic gas pressure added, but not as much as the same pressure of combustible gas mixture.

However, I should note that it does sacrifice combusted gas temperature. Where the pure fuel/air mixtures have combustion temperatures of around ~2700 K, the mix with inert gases only reaches ~1700K
This also makes the SOS in the 50% inert mix fall to slightly above 800 m/s, compared to 1000 m/s in the pure fuel/air mixes. Not too noticeable most of the time, but in high speed cannons it might have an effect.
However, the loss of gas temperature and increase in the masses of the gases will of course reduce heat loss.

I have proposed the technique of using a light inert gas such as helium in combustion mixtures to allow gas density to fall sufficiently that the loss of this temperature is actually offset by the reduced gas density. In the right percentages of helium, one could boost SOS in the gas to over ~2000 m/s, making mile per second velocities within reach.
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Unread postAuthor: inonickname » Wed Mar 18, 2009 2:46 am

I like it- i assume the torch has a venturi to mix the fuel with air. It will be accurate, but beware- as the torch is most likely designed for a longer flame, the mix will be a little richer than standard.

I will give this a go, even if for only practice and implementation- hopefully I could build a gun (again) for something like this.

good work
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Unread postAuthor: daccel » Wed Mar 18, 2009 3:20 am

Thanks for the thorough explanation Rag, I will check out GasEq.

inonickname, thanks. Yes there is a venturi (I think? .. two small holes in the tube :P).

I did notice through the clear epoxy i used for my spark plug that it was orange not blue, and I think that means it's burning rich which would confirm that.

If you're going to try this, check out my how-to on using a shock pump for this, here. Thought I would try to open it up, as not everyone is going to build their own pump. And now that I think about it, why reinvent the wheel.
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