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Inefficient piston

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Inefficient piston

Unread postAuthor: Biopyro » Wed Jun 04, 2008 10:36 am

My piston doesn't have an o-ring and has a large-ish sized pilot volume 250cm^3 for a 1.5" valve, but according to GGDT it doesn't need reducing or a larger pilot valve (currently 1/2"). The thing is it seems very inefficient. When I fire there is a long blast of air through the pilot valve, often about 2-4x as long as in the video, which is aactually pretty good. I haven't used any sort of piston valve before, and it seems less powerful than my combustion.
Is this normal, is GGDT wrong about it or does my piston just need more optimisation?

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Unread postAuthor: Carlman » Wed Jun 04, 2008 11:05 am

yea there is definetly a big lag which is deacresing performance a bit.

try a modded sprinkler as pilot, or fill pilot volume with hot glue.
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Unread postAuthor: MaxuS the 2nd » Wed Jun 04, 2008 1:36 pm

I go with the modded sprinkler valve as a pilot, a 1" version should do fine.
Shame even 1" versions cost $50 here instead of $5 over in good ol' america.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 04, 2008 2:25 pm

The ideal pilot volume for a 100% efficient 1.5" piston shouldn't be more than 20cm<sup>3</sup>, I would look into ways of reducing that volume before increasing the pilot valve size.
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Unread postAuthor: MrCrowley » Wed Jun 04, 2008 2:35 pm

I agree with JSR. Cut out a big piece of a kneeling board used for gardening, the foam ones, and use that as a bumper. They're quite thick so it gets rid off a lot of pilot volume.

Also check the tightness of the piston. How easy does it move inside the pipe? If you stuck it in 1.5" pipe and looked down the pipe, can you see much light coming from around the piston?

A 1/2" ball valve should be fine for this, but going bigger will improve performance.
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