Piston Hybrids, a couple of questions

Harness the power of precision mixtures of pressurized flammable vapor. Safety first! These are advanced potato guns - not for the beginner.
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advancedspuds
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Thu Dec 05, 2013 9:32 am

I am getting back into spudguns and really like the hybrid piston concept. They look fairly straight forward, but I do have a few questions though.

Do the pistons have a equalization hole through the piston, connecting the pilot and chamber area? I would be concerned about either losing a bit of fuel from the pilot opening before it sealed, losing a lot of it out of the pilot when fired, or the effects of having fuel compressed when fired.

Has the "fernco" pipe adapters pretty much became the go to for bumpers on these?

I like the threaded rod pistons but how well do they hold up after use?

Is there someone on the forum who can machine aluminum pistons if I decide to go that route?

Are there any nice build logs or prototype post other than the Spudchukker III build log? It was very helpful but I would still like to see some more concept designs and such. No one can do too much research. I did some searching but not too much luck.
CannonCreations
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Thu Dec 05, 2013 3:09 pm

I too am just getting into piston hybrids as well. From the research I have done, I can tell you that there should be no equalization hole in the piston between the pilot and the chamber. If there were a hole there, you could risk accidentally igniting a fuel mixture behind the piston as well as in front of it. If you were using PVC for this I would expect it to explode at this point, which I highly recommend that you don't do. It might work that way, but none of the hybrids I have seen here have an equalization hole. The main idea for having the two separate, is that you can pressurize the pilot to open once the chamber reaches a certain pressure, if that makes sense. The pilot is essentially acting as a spring, and once the fuel is ignited, there is more force on the chamber side of the piston, which forces it to move back, and opens the pilot. Another thing that could happen if there is an equalization hole is that you could end up with flame shooting out the back of the cannon if the pilot volume where to ignite.
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advancedspuds
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Thu Dec 05, 2013 6:11 pm

CannonCreations wrote:I too am just getting into piston hybrids as well. From the research I have done, I can tell you that there should be no equalization hole in the piston between the pilot and the chamber. If there were a hole there, you could risk accidentally igniting a fuel mixture behind the piston as well as in front of it. If you were using PVC for this I would expect it to explode at this point, which I highly recommend that you don't do. It might work that way, but none of the hybrids I have seen here have an equalization hole. The main idea for having the two separate, is that you can pressurize the pilot to open once the chamber reaches a certain pressure, if that makes sense. The pilot is essentially acting as a spring, and once the fuel is ignited, there is more force on the chamber side of the piston, which forces it to move back, and opens the pilot. Another thing that could happen if there is an equalization hole is that you could end up with flame shooting out the back of the cannon if the pilot volume where to ignite.
Thanks for confirming! I imagine if you were to get right down to it, an equilization hole wouldn't effect the valve popping, but it sure would be trouble if the fuel/air mix behind there ignited. I would think it would still pop but you would get a nice jet of flames spewing out the back of your gun at several times atmospheric pressure! That could be a bad day indeed.
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ramses
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Thu Dec 05, 2013 6:17 pm

If I recall, both of the 2" piston hybrids built "recently" vent the pilot with what is essentially a quick dump valve attached to the piston. I believe that they do use some kind of equalization hole, but probably a check valve as well.

So although there might be a stoicheometric air/fuel ration behind the piston, it should never ignite because it is never compressed significantly (because it is vented) and it is never exposed to the combustion process because of the check valve.

That said, I had similar fears when designing mine, and constructed it almost exactly as Cannon Creations described. Be advised, though, that the piston slam closed on the seat fairly hard, potentially wearing or destroying the sealing face.
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jackssmirkingrevenge
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Fri Dec 06, 2013 9:04 am

I only made a mini piston hybrid: http://www.spudfiles.com/miniature-cann ... 22465.html

I built it as a normal pneumatic piston cannon, performance was less than spectacular but then again, it was only a mini.
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