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HYBRID FUELING 101

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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Unread postAuthor: Gunner » Wed Jul 16, 2008 9:45 am

OK...what is the volume ratio because I have LP gas, can I use 0.96:0.04?

(Edit: No problems anymore :wink: )
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Unread postAuthor: LCTChamp » Sun Oct 19, 2008 6:22 pm

i understand the how much propane to put in its the air thats confusing me. For a 2x mix you would just add 14.7psi of air?
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Unread postAuthor: LikimysCrotchus5 » Sun Oct 19, 2008 6:58 pm

Yes but you would also add twice as much propane so that your mixture is combustible.
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Unread postAuthor: fogus » Tue Nov 11, 2008 6:18 pm

OK, I believe I have a recipe for filling a hybrid combustion tank in my head.

It is:

1. Decide on what --X (compression ratio) you want to end up with.
2. Assuming your chamber is filled with 14.7PSIa of air:
3. Raise the pressure of the chamber to 14.7 + 0.62 = 15.32 PSIa using propane. (I will consider this a nominal 1X mix).
This will read 0.62 PSI on a gauge.
4. Continue to raise the pressure of the chamber using propane, until it reaches 15.32 + 0.62*(X) PSIa, where X is the compression ratio you are using.
This will read 0.62*(X+1) PSI on your gauge.
5. Using air now, raise the pressure to 15.32 + 0.62(X) + (14.7-0.62)*(X) = 15.32 + (X)*(14.7) PSIa. This will read 0.62*(X+1) + (14.7-0.62)*(X)
= 0.62 + 14.7(X) PSI on your gauge.

Did I screw up anywhere?
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Unread postAuthor: POLAND_SPUD » Tue Nov 11, 2008 7:04 pm

uhmmm yes you did....

1x mix is a 'proper' mix of air and propane at atmospheric pressure
2x mix is a mix of air&propane at 2atm and so on
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Unread postAuthor: fogus » Tue Nov 11, 2008 7:38 pm

POLAND_SPUD wrote:uhmmm yes you did....

1x mix is a 'proper' mix of air and propane at atmospheric pressure
2x mix is a mix of air&propane at 2atm and so on


Yes, that's why I called it a nominal 1X mix. It is 0.62 PSI above a true 1X mix, but it makes it easier to do the calculations. A my starting nominal 1X actually about a 1.042X mix.
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Unread postAuthor: FishBoy » Mon Dec 08, 2008 4:26 pm

For manometric metering, how accurately should the chamber volume be measured? (I am planning to fill it with water and then pour the water into graduated cylinders)
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Unread postAuthor: Ragnarok » Mon Dec 08, 2008 4:44 pm

FishBoy wrote:For manometric metering, how accurately should the chamber volume be measured?

Your error can be as large as all the oceans in the world, and it still wouldn't matter.

You don't need to know any chamber volume for a manometric meter. You could take one off a cannon of one size, then bolt it to one of any other size, and you wouldn't need to adjust a single thing.
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Unread postAuthor: FishBoy » Mon Dec 08, 2008 4:58 pm

I understand that, I just had a brain lapse/poor wording combo. Anyway, for the mix calculations, how accurately does the chamber volume need to be measured?
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Unread postAuthor: Larda » Tue Dec 09, 2008 1:22 am

FishBoy:
You can use the same numbers for any chamber size.
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Unread postAuthor: trigun » Tue Feb 10, 2009 5:27 pm

I built my hybrid i cant get it to fire. My gap is .01 of an inch. I know as a fact it is firing due to small circuit gap installed by the firing switch. Idea from one of PCguys guns. My chamber is 61.023 cubic inches. Meter is 5.858 cubic inches. I am using manometric metering for air and filling it to 3 bar for a 4x mix. Does that sound correct. Also with 26.494 psi in the meter.
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Unread postAuthor: starman » Tue Feb 10, 2009 6:22 pm

I'm guessing your total mix is rich.

I did a quick calc on it and it's tad rich but I think in the ball park. Are you injecting your fuel first in a freshened atmosheric pressure chamber? That's going to add about 2.5 psi to your chamber. After that, you'll need to run the total pressure with air up to about 46.6 - 47 psi. This is all assuming you're still getting a good spark...the gap sounds plenty small.

If still no success, step back a bit and try a much smaller mix, 2x or even smaller. Ignition troubles won't show up as much then. Your chamber is smallish but not out of the typical hybrid realm. However, small chambers require more precise fueling.

Edit: OH.. did you burp your meter well first...meaning, it's originally full of air. Loading it with fuel mixes the fuel and 1 atm of air making for a lean overall shot. After the meter shot, 1 atm of a fuel air mix is left behind.

Burping means just filling and shooting the meter several times to where the gas left behind in the meter is all fuel and no, or very little air.

Edit 2: And another...since your meter is a fairly low pressure model, you may need to compensate for the fact that the chamber pressure rise to 2.5 psi will also be the final resting pressure in the meter, therefore not allowing all of the fuel to release from the meter. However, since your fuel load looks a tad rich you may have already compensated for that. Just something to check.
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Unread postAuthor: trigun » Tue Feb 10, 2009 6:57 pm

Thanks starman i actually found out i made a very critical error in my fuel meter volume. I was off by about a 3 cubic inches which adds up really fast ! thanks for the suggestions i did burp by meter actually.

Your peanut apprentice, trigun
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Unread postAuthor: ramses » Tue Mar 03, 2009 5:13 pm

noob question, but would this be acceptable for manometric metering. Apparently it is "accurate to 1.0 psi." it reads to .1psi, so it is probably more precise than that.

I am hoping that the seller knows about the difference between accuracy and precision, so that once I know by experimentation how much it is off, I can adjust accordingly. As a bonus, it has a built in regulator.
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Unread postAuthor: fogus » Tue Mar 03, 2009 5:48 pm

It almost seems like a mistake. He might have meant to say, "precise to within 0.1 psi".

I'd ask.
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