How-to: Syringe Fuel Metering

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
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jackssmirkingrevenge
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Sat Feb 05, 2011 6:28 am

I made a video tutorial for those who might have difficulty grasping the concept, or think it's somehow difficult or complicated :roll:

[youtube][/youtube]

edit: in case it's blocked in your country due to the audio, here it is without sound.

edit 2:

For hybrids:

Same calculation, just multiply the resulting fuel volume by whatever mix number you want to achieve.

It's important that you consider the dead space after the check valve of your pump to be part of the chamber volume when making your calculations, especially for small chambers.


The calculation is as follows:

(ideal fuel % / 100) x (chamber + pump dead volume) x mix number


In the case of this Beto shock pump, the dead volume is 1.75mL. For a 10mL chamber using butane to say 5x, the calculation is as follows:

3/100 x (10+1.75) x 5 = 1.76mL of butane

Had we not considered the pump dead volume, the result would have been 1.5mL of fuel, an error of 15% which would most likely prevent ignition. If it were a 100mL chamber on the other hand, the error would have been of 1.5%, and likely the mix would have still ignited.

After the fuel is injected, all you have to do is pressurise with your pump to the following pressure (assuming you're using psi):

(mix number - 1) x 14.7

In the case of our 5x mix, the calculation is as follows:

(5-1) x 14.7 = 59 psi

If your gauge can read bar, simply pressurise to (mix number - 1) bar, so 4 bar in this case.
Last edited by jackssmirkingrevenge on Tue Nov 26, 2013 2:37 pm, edited 3 times in total.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Technician1002
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Sat Feb 05, 2011 9:34 am

Nice video. I love the SF plug at the very end. :D
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jackssmirkingrevenge
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Sat Feb 05, 2011 9:38 am

It's only there because I am loath to answer youtube comments ;)

Also, I seem to have misspelled "concentration." Bother.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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al-xg
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Thu Apr 28, 2011 8:34 am

Mmm, this doesn't work as well for atmospheric pressure shots.

Injecting the gas must raise the pressure inside the chamber, either venting air past the projectile if it isn't air tight, or expelling some of the propane out the back when the syringe is removed. Having a check valve and an airtight projectile would stop that, but then there would be too much air inside the chamber. Obviously on a hybrid pressurised after fuelling this isn't a problem.
I guess filling from the bottom of the chamber and having air bleed out through the top could correct that with the propane being denser and all that.

And to be honest I'm surprised at how pedantic I'm being :)

The shot obviously still goes off fine.

I've just got hold a a syringe you see, as part of my exam revision procrastination program. Just earlier I had to stop myself from dismantling a transformer for the step up coil. :roll:
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Technician1002
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Thu Apr 28, 2011 9:53 am

Both sealed and unsealed combustion will work. Use GGDT for the amounts to use for both sealed where the added fuel raises pressure and unsealed where some air is displaced out the barrel.
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jackssmirkingrevenge
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Thu Apr 28, 2011 4:04 pm

al-xg wrote:And to be honest I'm surprised at how pedantic I'm being :)
You are :)

The projectile doesn´t have to be completely airtight, you´ll be surprised how long it takes the gas to leak out. but yes, it is a good idea to fuel holding the chamber upright due to the density of fuel.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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MrCrowley
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Thu Apr 28, 2011 5:59 pm

Posted as a sticky because I like it so much :wink:

Somehow I had missed it until now.
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Technician1002
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Thu Apr 28, 2011 6:14 pm

Syringe fueling worked fine in my first combustion. It was fueled and fired with no projectile. There was time to fuel, set the camera focus, start the timed exposure and fire it. Diffusion rate out the barrel is relatively low.
Image

The calculated metering volume is a starting point. You can try slightly smaller or larger charges of fuel to find the best result for an unsealed chamber. Due to the unsealed chamber in this shot, I fueled slightly rich to ensure ignition for the shot. 3.5 cc for the 75 cc chamber is 5%.
Last edited by Technician1002 on Fri Apr 29, 2011 9:39 am, edited 2 times in total.
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al-xg
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Thu Apr 28, 2011 6:20 pm

Yeah, I was only commenting on the theoretical accuracy of the metering. Can't realistically complain, compared to spray and pray.

Just had first my experience of capacitor discharge through arm, still a bit stiff. It was bound to happen someday :roll:
I've had worse jolts than that, but always a shock when you're not expecting it.
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jackssmirkingrevenge
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Fri Apr 29, 2011 1:19 am

Technician1002 wrote:my first combustion.
does that mean we're going to see bigger and better from you :)

And cheers MrC :)
Just had first my experience of capacitor discharge through arm, still a bit stiff
Happens to the best of us, al least it was your arm ;)
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Technician1002
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Fri Apr 29, 2011 9:21 am

jackssmirkingrevenge wrote:
Technician1002 wrote:my first combustion.
does that mean we're going to see bigger and better from you :)
I have a full auto on the drawing board. :D
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mark.f
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Fri Apr 29, 2011 9:29 am

Feel free to borrow from this design, as I'm never going to get around to building it. :roll:
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jackssmirkingrevenge
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Fri Apr 29, 2011 7:50 pm

mark.f wrote:...as I'm never going to get around to building it. :roll:
... because it's a tad too complex. Simplify and add lightness!
hectmarr wrote:You have to make many weapons, because this field is long and short life
shardbearer
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Mon May 09, 2011 9:29 pm

I had an idea for those who want a bit more control and finished product.

Propane Tank---Check Valve---Tee going to syringe---Check Valve---Combustion chamber

Both check valves go left to right. Dashes are hose
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jackssmirkingrevenge
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Mon May 09, 2011 11:48 pm

That would certainly be a good way of "automating" the system for an on-board fuelling setup, good thoughts.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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