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Spool valve for fuel metering?

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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Spool valve for fuel metering?

Unread postAuthor: AaronSpuds » Thu Jan 24, 2013 1:33 pm

Hi all, let me just first say I've been lurking ( :P ) these forums for a few weeks, and I fell in love with some of the creations posted here.

Now I have a simple question.

I like the "pump-action" fueling done on UltimateSpudgun's cannons, but don't understand why it takes two pumps to fuel the launchers?

Here's what I was thinking of doing for something similar, although it would only take one press. Sorry for the diagram, but assume that there is a regulated input of propane at the top, and a check valve -> chamber connection at the bottom of the tube:

Image

The volume between the pistons would be the meter volume on a regular setup... pushing the piston back seals off the input and injects the volume at pressure into the chamber. There could be a spring behind the spool to return it to its original position after injection.

Is there something I'm missing or does UltimateSpudgun just use a different method? I would be very interested in using this on the combustion I'm planning on building when I get my tax return, lol.

Thanks in advance for any answers!
~ Aaron
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Unread postAuthor: POLAND_SPUD » Thu Jan 24, 2013 2:44 pm

I see just one problem - your meter has to be purged with fuel first

1.On the first fueling you've got the meter full of air and you inject fuel into it so you end up with a mix of air and fuel
2. When the valve switches that mix of fuel and air that occupied the meter before fueling gets injected into the chamber but some of it stays in it
3. When you fuel the second time you end up with a completely different mix because this time you've got a mix of air & fuel in the meter and you add more fuel into it


There are two ways to solve this:
A) purge the meter first
B) add a spring loaded piston that reduces the volume of the meter to minimum when it's not pressurised

mind you that option B will be rather difficult to implement in this design
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Unread postAuthor: AaronSpuds » Thu Jan 24, 2013 5:34 pm

IIRC, isn't that the case with 'normal' fuel meters? Before the first fueling, the meter pipe has air in it which must be purged (usually by flooding the meter with propane (both ball valves open))?

In any case, I was planning on purging the meter before the first shot (by simply pressing the piston back a number of times). After the first shot, the check valve should keep air from creeping back into the meter volume.

Thanks for the reply!
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Unread postAuthor: jackssmirkingrevenge » Fri Jan 25, 2013 8:50 am

Some details of the launcher you're referring to: http://www.spudfiles.com/forums/cannon- ... 19142.html

You might also want to read up on the Tippmann C3: http://www.spudfiles.com/forums/say-hel ... 23700.html

Also, you look incredibly like Richard Stallman :roll:
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Unread postAuthor: Technician1002 » Fri Jan 25, 2013 6:35 pm

In response to the 2 pumps. It was to keep the size of the meter reasonable. It could have twice the volume for a single stroke, but for size, it was made half size.
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Unread postAuthor: AaronSpuds » Fri Jan 25, 2013 7:32 pm

@Technician1002: That makes a lot of sense. Thanks!

@jackssmirkingrevenge: I was actually referring to the fuel meters on his BL series launchers... the aluminum tubes with the knobs sticking out.

And thank GNU for noticing. :P I am a freedom-hating heathen who uses proprietary software, though. lol
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