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Hybrid Entropic Fuel Meter Formulas

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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Hybrid Entropic Fuel Meter Formulas

Unread postAuthor: mark.f » Sun May 11, 2008 9:08 pm

Alright, before I get into any math or numbers, let me clarify a little on the scenario.

These fueling formulas are for what I call an "entropic meter". That's just a fancy way of saying the meter operates in the following manner:

1.) You fill the chamber to a certain pressure with compressed air.
2.) You fill the meter to a certain pressure with compressed propane.
3.) You open the valve connecting the two and allow the gases to fully diffuse, ideally with a chamber fan, or other mixing device.
4.) You fire, burp the meter, vent the chamber, and repeat.

I decided to make a set of formulas that you could simply plug in given variables to obtain chamber pressure and meter pressure. The following formulas should work iff:
1.) The scenarios above are met.
2.) The fuel is propane and the oxidizer is air.

Here are the formulas:

<div align="center">Image</div>

<div align="center">Image</div>

Where:
P<sub>m</sub> = Meter pressure (absolute scale)
V<sub>m</sub> = Meter volume (cubic units)
P<sub>c</sub> = Chamber pressure (absolute scale)
V<sub>c</sub> = Chamber volume (cubic units)
P<sub>s</sub> = Desired final mixture pressure (absolute scale)

For inputting information into these formulas, you MUST use consistent units, and your pressures should be on an absolute scale (easiest to use is atmospheres, or you could use PSIA, etc.)


Right now, I know it may look like I just pulled those formulas out of my ass, but I have three sheets of looseleaf paper going through the derivation of these formulas step-by-step, which I will photocopy and upload tomorrow. Right now, I am tired. By all means: please post any comments, what have you. Criticism is welcome if it leads to improvement.

EDIT: Added the derivation pages. Read if you are very bored. The file size is 2.88 mB so be warned if you're on dial-up.

http://www.markfh11q.net/Derivation.zip
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Last edited by mark.f on Mon May 12, 2008 12:58 pm, edited 1 time in total.
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Unread postAuthor: Lentamentalisk » Sun May 11, 2008 9:51 pm

I dont really see the advantage of doing this over filling it up with a certain pressure of propane, and then a certain pressure of air. It is the same, but in your version you get absolutely no mixing what so ever, because you can't put a chamber fan in a hybrid.
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Unread postAuthor: D_Hall » Sun May 11, 2008 10:24 pm

Lentamentalisk wrote:you can't put a chamber fan in a hybrid.

I'm curious... Why do you say this?
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Unread postAuthor: Lentamentalisk » Sun May 11, 2008 10:27 pm

because of many times that chamber fans have been blown to pieces. You your self didn't want to put a chamber fan in your "Pipe Dream." if I'm not mistaken that was why we got into the whole Venturi pump discussion.

Edit: to specify I meant above any mix worth calling a hybrid, ie: 4x
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Unread postAuthor: D_Hall » Sun May 11, 2008 10:33 pm

I didn't want to put an electric-powered chamber fan in because I knew the safety folks would never buy off on it.

The venturi-based system was a nod towards system simplicity(*) (or "belt and suspenders engineering" depending on when it was being discussed).

But to simply say that chamber fans can't be used in hybrids simply because the typical muffin fan used in combustions can't handle it? I don't buy that at all.


(*) Note that there's no reason the system described above couldn't use a venturi system.
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Unread postAuthor: Lentamentalisk » Sun May 11, 2008 10:59 pm

Ok, ya, I have been thinking the same thing lately. Why not just put a super strong steel or aluminum or what not, brushless fan in there. If it was at the back of the chamber I guess it would do all right.


I revoke my previous comment about chamber fans in combustions. They are possible, just they generally would need to be higher quality than your standard computer fan, if you are going to use high mixes.
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Unread postAuthor: mark.f » Mon May 12, 2008 10:20 am

I've used a 12 VDC micro fan from Radio Shack in my 2" steel hybrid I stopped working on a few months ago. It didn't fail.

And even without a chamber fan, you get a little mixing because air flows from high pressure to low pressure, and the chamber pressure is usually higher than your meter pressure, depending upon the design and mix used. This would allow air into the meter, and simple diffusion could do the rest, but a chamber fan would be much better.
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Unread postAuthor: drex » Mon May 12, 2008 5:05 pm

i'm 90% sure that simple diffusion would inshore mixing of the fuel and air assuming the you're not doing any thing extreme (sorry dhall) but i could be wrong.
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Unread postAuthor: jimmy101 » Tue May 13, 2008 11:28 am

markfh11q wrote:And even without a chamber fan, you get a little mixing because air flows from high pressure to low pressure, and the chamber pressure is usually higher than your meter pressure, depending upon the design and mix used. This would allow air into the meter, and simple diffusion could do the rest, but a chamber fan would be much better.

I think the operative phrase here is "a little mixing". The volume of fuel is so small compared to the volume of the chamber that the mixing you'll get from injecting the fuel is going to be pretty minimal. Furthermore, the restriction between the meter and the chamber is going to really slow down the rate of diffusion.

Switch the fueling order and inject the propane first. You'll inject a much large volume of air, which will give you much better mixing.

Of course, if you have a fan that'll survive that's the way to go. Even with a fan though, any restriction between the meter and the chamber is still going to make mixing very slow. If you are counting on equilibrating the mix in both the chamber and the meter you'll probably have to go with some type of recirculation loop.

drex wrote:i'm 90% sure that simple diffusion would inshore mixing of the fuel and air assuming the you're not doing any thing extreme (sorry dhall) but i could be wrong.

Simple diffusion is an extremely slow way to mix fuel. There's a good chance it'll take at least several minutes in a typical sized gun. And that's to get to a combustable mix at the spark gap. To get complete mixing will take even longer.
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Unread postAuthor: mark.f » Tue May 13, 2008 12:56 pm

Well, the point of this was to simply fill both the meter and chamber to their respective pressure, and then just open the valve between them to mix. Simple to calculate and simple to understand, but I guess it's back to the drawing board.
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Unread postAuthor: dongfang » Wed May 14, 2008 6:16 am

Hi,

Why not just put a super strong steel or aluminum or what not, brushless fan in there. If it was at the back of the chamber I guess it would do all right.


I tried that once, in a combustion with 40% oxygen. It survived 1 shot, then died. No visible damage, just dead. I suppose a copper wire in there got torn.

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