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Hybrid fueling chamber or chamber + meter?

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 fueling chamber or chamber + meter?

Unread postAuthor: pat123 » Tue Jun 01, 2010 2:15 pm

I've made hybrids before but I forgot if you count the meter as part of the chamber when you find the chamber volume since the gas in the meter will expand to the chamber and stay in the meter? most formulas that I have seen to find the correct amount of propane to use don't add the volume of the meter to the chamber, but it seems like you should since all of the propane does not leave the meter.

Thanks,
Pat
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Unread postAuthor: jrrdw » Tue Jun 01, 2010 4:05 pm

Don't you seal the meter off by closing a valve before mixing and firing? Common sense tells me if your blocking off the meter it don't get added in...
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Unread postAuthor: spudtyrrant » Tue Jun 01, 2010 4:11 pm

jrrdw wrote:Don't you seal the meter off by closing a valve before mixing and firing?
yes, but when you open the meter valve it equalizes the pressure in the meter and the cannon, so you do have to take the meter volume into account.
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Unread postAuthor: jrrdw » Tue Jun 01, 2010 4:19 pm

spudtyrrant wrote:
jrrdw wrote:Don't you seal the meter off by closing a valve before mixing and firing?
yes, but when you open the meter valve it equalizes the pressure in the meter and the cannon, so you do have to take the meter volume into account.


What is your fueling sequence?

If you primed the meter then released the dump valve to the chamber then closed the meter back off then pressurized the chamber, would you still count the meter volume?
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Unread postAuthor: spudtyrrant » Tue Jun 01, 2010 4:29 pm

jrrdw wrote: If you primed the meter then released the dump valve to the chamber then closed the meter back off then pressurized the chamber, would you still count the meter volume?
yes, the only thing of real importance is the fuel to air ratio, as i said before when you open the meter to allow gas into the chamber it equalizes(same f/a ratio with the valve open and closed, regardless of separation of the chamber and meter) which means if you don't take the meter into account, you will have too lean an air mixture likely resulting in misfires the
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Unread postAuthor: jrrdw » Tue Jun 01, 2010 4:41 pm

If you made your calculations on the chamber volume only, wouldn't you have to add what would be left in the meter, 'extra' to the chamber to keep it from being to lean?

Is that why you answer yes?

So you would measure a chambers worth, then add another meters worth???
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Unread postAuthor: saefroch » Tue Jun 01, 2010 5:00 pm

The way I see the issue, the V1 is the meter volume, V2 is your meter and chamber volume... I just run the gas laws for these calculations. They're pretty simple. Boyle's law and the Ideal Gas Law equation.

So yes, jrrdw, you would have to add a chamber's worth and a meter's worth, keeping in mind also though how some air will diffuse into your meter and change the amount you've injected into the chamber as well.
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Unread postAuthor: MrCrowley » Tue Jun 01, 2010 5:24 pm

On a setup like mine, you do have to include the meter volume.

Same calculation, you just add the meter volume with the chamber volume.
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Unread postAuthor: ramses » Tue Jun 01, 2010 6:08 pm

It depends on whether or not you inject the air through the meter.
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Unread postAuthor: saefroch » Tue Jun 01, 2010 8:14 pm

Correct me if I'm wrong MrCrowley, but does not your meter still hold some MAPP after injection, along with maybe some air?
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Unread postAuthor: MrCrowley » Tue Jun 01, 2010 8:23 pm

saefroch wrote:Correct me if I'm wrong MrCrowley, but does not your meter still hold some MAPP after injection, along with maybe some air?

That's correct and that's why you include it in the calculation of the chamber volume. If you inject 4.2% of propane in to your chamber and it is mixed perfectly, cut the chamber in half and each half still has 4.2% of propane to air in it. That's the same principle here. Since the meter is closed off from the chamber, you need to include it in the chamber volume measurement so that when it is closed off the chamber still has the correct amount of fuel to air in it.

edit: Though some of the fuel lingering in the meter will be pushed in to the chamber with the air injection, so perhaps that would make the mixes slightly rich. Just a thought.
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