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Piston Design

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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Piston Design

Unread postAuthor: Moonbogg » Wed May 26, 2010 8:52 pm

Starting a thread about hybrid piston designs. I got a full time job with the people who fabricated my Venom, so there is a slight chance of future projects :lol:
My last attempt at a piston hybrid was a good attempt, considering it was done without machine tools. I am not satisfied and want something more reliable that functions on air rather than O2, so I need a piston with a lot of force behind it that will crack open at around 200psi or so for a 4x mix. I am thinking of using a spring. This will be an inline cannon like the striker (in sig) but with a spring piston rather than a detent based piston. The piston, when pushed back from chamber pressure, will either expose ventilation holes (like tech's pistons) or will be a barrel sealing design. Either way I am thinking of using a spring.

Questions:

Once the piston opens and releases some pressure into the barrel, will the piston shut again until the pressure rebuilds? I don't want a stuttering piston. I suspect that the pressure will build and crack the valve open, and then once the pressure gets into the barrel, the opposing force of the projectile will then aid the piston in opening fully until the pressure falls back down from the projectile leaving the barrel. Also, once the piston opens a little, there will be a lot more exposed area for the pressure to push, but I am concerned the pressure will plummet first, then have to rebuild.
I don't like the idea of left over or unused pressure in the chamber by using a spring loaded piston, but something tells me that by the time the piston ha sa chance to close again, the projectile will be long gone.
Thoughts?
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Unread postAuthor: niglch » Wed May 26, 2010 11:49 pm

I think it is smartest to use an air spring for this. I have never actually built one, but by adjusting the pressure behind the piston you could fine-tune the strength of the "spring" to open at a desired burst pressure. But I guess if you have a strong enough spring then go for it.

Also, once the piston opens a little, there will be a lot more exposed area for the pressure to push, but I am concerned the pressure will plummet first, then have to rebuild.


Not sure what you mean here. The chamber pressure is going to decrease rapidly once the valve opens (hopefully quickly due to the increased piston surface area under pressure). I don't think it would rebuild since the combusting gases cool quickly.

I don't like the idea of left over or unused pressure in the chamber by using a spring loaded piston, but something tells me that by the time the piston ha sa chance to close again, the projectile will be long gone.


Probably yes as long as you're barrel isn't ridiculously long.
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Unread postAuthor: Technician1002 » Thu May 27, 2010 12:00 am

Instead of a spring, why not use a piston ratio so the piston will function properly at any mix?

What I mean is build it like a traditional close ratio barrel sealing pneumatic. It would fire on a sudden rise in chamber pressure due to combustion instead of a sudden drop in pressure in the pilot area. To prevent combustion behind the piston, the chamber would have fuel injected, and then the chamber pressurized through the pilot, purging that area from fuel. With the design, there is no need for a spring and limited narrow pressure ranges.

The desired pressure rise before opening can be designed into it by selecting the piston / seat area ratio.

I hope this makes sense. I have played with this on paper, but have not posted plans yet simply due to the other designs in current build.
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Unread postAuthor: Gaderelguitarist » Thu May 27, 2010 9:44 am

I think Tech is on to something here, Moonbogg. Build the piston so that the slightest increase in pressure will open the piston. You could increase the piston's speed pitential by incorporating a pressure release system behind the piston, so that while the ignition inittially opens it, the vacuum would further pull it back.

This can still also be done with your spring idea, just use a spring that can handle just a bit more pressure than you're filling with. Once you ignite the mix, the pressure will rise, open the piston, and force the round out of the barrel. I could still see the spring closing once the pressure is gone, but the projectile, in theory, would be out of the barrel by then.
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Unread postAuthor: Moonbogg » Thu May 27, 2010 11:02 am

I like tech's idea of adjustable piston resistance for multiple mix functionality and fine tuning for each mix. My concern was that the air pressure would at first be weak until the piston moves back to compress it more, but I could just start off with more air pressure behind it in the first place. Also, a pop off valve in the piston's air cavity could be used to allow the piston to stay open more once it moves back enough. Looks like there are some good options here. I especially like the air pressure idea as there would be no spring to break or weaken.
I also have a concern about the orings being able to remain functional after being subjected to the rapid sliding motion that they will encounter. Tech, how do your o-rings hold up in your piston designs? Does the abrasion wear them down or do you experience unreliable sealing at times?
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Unread postAuthor: Technician1002 » Thu May 27, 2010 12:46 pm

@ Moonbogg, there is no o ring needed in a traditional piston valve. Forget them. The very rapid chamber pressure rise will be maybe an order of magnitude faster than the normal drop in pressure of a pneumatic pilot, so it can tolerate larger leaks. Because of the pilot area air fill, a sealed piston is not desirable.

There is no need for o rings to change mix. The higher the mix, the higher the pilot pressure by default. This is better than a fixed spring with a fixed release pressure.

The pressure behind the piston = chamber pressure prior to ignition.

After ignition, the pilot pressure increases. Air has mass. Both of these factors contribute to the bumper function.

After the projectile exits, the pilot pressure will re-close the piston. The piston will not close at less than the initial chamber pressure. The valve will remain open (unless there is severe rebound) until chamber pressure drops after the shot. This removes the problem caused by a physical spring.

As an added bonus, this air spring is stronger than a mechanical spring and contributes much less mass to the moving piston which = faster speed.
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Unread postAuthor: saefroch » Thu May 27, 2010 6:59 pm

I know this isn't entirely relevant to the current discussion on the thread, but it is however relevant to the thread's purpose.

How well do you guys think this design would function?
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Unread postAuthor: irisher » Thu May 27, 2010 7:07 pm

The metering system would have to be purged before each shot and I think it might turn into a piston valve and vent your fuel/air.
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Unread postAuthor: Technician1002 » Thu May 27, 2010 8:27 pm

My take on the drawing is the outlet piston is IN THE FLOW PATH WHEN OPEN.

By using no o ring or drilling a small eq hole, it could be filled from the rear. Fuel first and then flush the pilot with air to bring it up to the desired mix.

Edit, edited the photo a couple times. Tell me if you like it.
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Attachments
hybridconversion.png
Suggested modification. Piston may retract mostly out of the flow when open.
hybridconversion.png (6.27 KiB) Viewed 1372 times
hybridconversion.png
Edited further to reduce the piston mass (low mass for speed) and permit the piston to further retract out of the flow path.
hybridconversion.png (6.28 KiB) Viewed 1370 times
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Unread postAuthor: saefroch » Thu May 27, 2010 8:43 pm

Well no, it ought not to be. The floating o-ring should be open when filling, but upon venting the meter volume, it should move back and close. Thus, when the chamber mix is ignited, the piston should move back and open under the pressure increase from the chamber... opening the valve. Right? Or are you talking about the need to shorten the span between the front and back of the piston, to allow it to move back and not interrupt flow?

EDIT: Thanks for edited photo, I do like it, that design would work a lot better. I'll have to see if I could create something like that... the only issue for me is all I really have access to is NPT fittings, no welding tools here.
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Unread postAuthor: GalFisk » Wed Jun 16, 2010 10:23 am

It wouldn't require too much work to modify this design into a self-venting valve. My copper hybrid (not finished, and therefore not showcased yet) uses one: at the back of the pilot chamber is a tube with vent holes drilled through the side. Inside the tube is a small piston connected to the main piston via a threaded rod. When the valve is closed, the small piston blocks off the tube before the vent holes. When the piston assembly moves backwards, the small piston moves past the vent holes, and the pressure in the pilot chamber is dumped opening the valve fully. I don't have a spring return yet, but one can easily be incorporated (behind the main piston, or behind the pilot piston).
I can find some of my old drawings/3D models if you wish. This design is based on, and very similar to, the DFTV (darn fast turbo valve) but adapted for hybrid use. I have fired it sccessfully as a hybrid several times, only fueling issues need to be resolved
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Unread postAuthor: Technician1002 » Wed Jun 16, 2010 10:33 am

Look at my drawing above. There is no spring. There is no pilot pressure regulation or release. None is required. Now to crunch numbers..

For a round number let's do a 2X mix.

Chamber pressure = 15 PSI.
Pilot pressure = 15 PSI. Pilot has air only, no fuel.
On ignition the chamber rises to ~200 PSI. Pilot is still at 15 PSI. Valve opens when chamber is 3X the pilot pressure (this can be set using the piston OD to Valve seat diameter ratio). Valve opens, chamber vents, pilot pressure rises, chamber pressure drops below pilot pressure, valve closes. Any questions?

At 10 X mix, Chamber is at 10 Bar. Pilot is at 10 bar. On ignition.. see above but with higher values.
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Unread postAuthor: c11man » Wed Jun 16, 2010 11:35 am

tech you stole my idea! but im glad that someone else though of the same idea. with a close ratio between seat and piston diameter the opening pressure is a function of the starting chamber pressure. i did the math for a 2inch piston sealing on a 1.5inch pipe (iron pipes) and at 10x it would open at 600psi. this isnt the ideal pressure that is around 1000psi but it can be inceased if you have machining capabilites to make a a seat larger than the 1.9inches that is the od of the 1.5inch pipe.

and as tech also mentioned the ratio will allow for firing at all sorts of different mixes because the valve will open at the pressure determined by the chamber preignition pressure.

the problem with this desighn is that the seat has to be very close to the piston diameter like my example of 1.9 and 2inches
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Unread postAuthor: Technician1002 » Wed Jun 16, 2010 12:00 pm

C11, Due to the mass of the piston, there is some mechanical delay due to the mass of the piston. If the piston is given a short nose that extends into the barrel (no o ring) the piston can have a built in time delay. As the piston starts to open it will have low opening force until the chamber pressure can meet the piston face. This is delayed by the piston moving back slowly until the nose exits the barrel and the chamber pressure acts on the entire face area, where it will suddenly increase the force and pop the valve open.

The burn out time is a known. The force lifting the piston is known. The mass of the piston is known. Simple math will give you the length of the nose that is needed to extend into the barrel for a time delay so the valve pops at near burnout.

Think of it as a 2 speed piston. Stage 1 is stopped as the chamber pressure rises. Stage two is low gear as the piston unseats the barrel seal and has low opening force. The chamber pressure continues to rise and then the nose exits the barrel and the piston shifts into high gear.

This would require machining precision on the piston nose for a close fit in the barrel.

Am I crazy or would that finish the piston?


:flower: :geek: :joker:
I feel a little strange trying to design an advanced hybrid when I have not built ANY combustion yet.
:D
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Unread postAuthor: c11man » Wed Jun 16, 2010 12:31 pm

that could certainly help but how much of a preformace would you get for all that work?

but tha kind of seal might be nessecary if you are shooting at very low Xs to make sure the piston seals...
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