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Hybrid Piston and Pilot valves

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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Hybrid Piston and Pilot valves

Unread postAuthor: MashedPotatoes87 » Thu Jan 27, 2011 12:02 am

Ok, forgive me because I've been looking around the forums here and I cant find anything that really explains exactly how exactly a piston hybrid works. I've made numerous combustion spud guns (2 spay and prays and 1 metered propane) and really want to try making a hybrid because I've seen some really innovative designs out there. I can and will be be able to machine the piston and find o-rings and thick rubber, as those are obvious things found in a good piston. Now just to be clear, as far as I understand it, the piston is sealed airtight against the barrel with a calculated amount of pressure behind it so you can pressurize the chamber with your gases and it wont open, until you ignite the mixture and then?... the increase in pressure overcomes the pressure behind the piston and forces its way into the barrel? Thats my understanding thus far and I dont even know whats up with the pilot valve etc... I'm proficent with stiochiometry and gas laws etc... and have done a fair amount of research, but the only thing holding me back is how the heck the piston/piolt valve hybrid works. (I understand the burst disk) I have a feeling it's alot simpler than I'm making it seem but I would REALLY appreciate it if somebody could help or point me in the right direction. :)
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Unread postAuthor: Hotwired » Thu Jan 27, 2011 12:34 am

Do you know how a barrel sealing piston valve works?

Assuming yes:

You would then understand that the piston moves back because the chamber pressure is higher than the pilot pressure.

The difference between a pneumatic and a hybrid piston valve is that the pneumatic piston moves because the pilot side pressure is lowered and a hybrid piston moves because the chamber pressure is raised (on ignition).
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Unread postAuthor: Technician1002 » Thu Jan 27, 2011 1:02 am

To fully understand how it works, it needs to be broken down into the forces on the piston before and after combustion, and the forces on the piston after the barrel seal opens.

The forces are relatively easy to calculate. The area X pressure on each end of the piston will give the force.

The Pilot area is the diameter of the piston.

The chamber side has two components. The full area is used when the barrel seal has broken and the chamber can apply pressure to the entire face of the piston. Before the valve opens the chamber area on the piston is much smaller. It is the area of the face MINUS the area not under pressure sealed off by the barrel.

As an example, a Piston with an area of 2 sq inches at 100 PSI can have 200 lbs pushing it closed against the barrel.
A Chamber at 100 PSI but with 1 sq inch sealed from the chamber by the barrel will have only 100 lbs trying to open it. The difference in forces will show the piston will lean on the barrel with 100 LBS of force.

When the mix is ignited and the chamber pressure rises, when 200 PSI is reached, the force trying to open the piston is now 200 LBS which matches the pilot force holding it closed with 200 lbs of force.

As the chamber pressure rises, the opening force rises and the barrel seal opens. Now the chamber pressure can act on 2 square inches instead of 1 square inch. The opening force rises due to the larger exposed area to 400 lbs. From there the piston pops open very quickly.

Your piston diameter and barrel seat diameter will vary from this example. You can do the math for your piston diameter and valve seat diameter to find the force on the piston before and after ignition.
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 27, 2011 2:47 am

you seem to have understood it already and wrote:the piston is sealed airtight against the barrel with a calculated amount of pressure behind it so you can pressurize the chamber with your gases and it wont open, until you ignite the mixture and then?... the increase in pressure overcomes the pressure behind the piston and forces its way into the barrel?


That's pretty much it ;)
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Unread postAuthor: MrCrowley » Thu Jan 27, 2011 5:58 am

In my opinion the piston valve setup SpudBlaster15 (link) and I (link) used on our hybrids is the best option to go with unless you can think of a different design.

They can be a bit tricky to fine tune to find out the pressure required in the pilot volume for each mix, stop the piston bouncing and to stop the piston being damaged every shot but once that's all sorted out they're fairly trouble free.

Like the others have said, if you understand piston valves you shouldn't have trouble with piston hybrids. The only difference with the designs linked above is that the pilot volume is vented once the piston starts to move back which means the pilot valve requires a significant amount of flow to vent the pilot fast enough and prevent piston bounce.
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Unread postAuthor: MashedPotatoes87 » Thu Jan 27, 2011 2:45 pm

Thanks for the help everyone. I think I get it now. But the part that is still confusing me is the back of the piston where or seems that the spool connects and goes through a hole on the back of the chamber which is sealed between the spool with an o ring or rubber seal. Then there's another hole used to pressure the piston against the barrel with a quick disconnect valve? Also, how is the pilot valve vented? As I said I'm new to this design, so is the pilot supposed to be vented at firing and repressurized after each shot? Thanks again guys this is really good information and I read alot about mistercrowleys and spudblaster15s designs and think those should be a good example to base mine off.
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Unread postAuthor: MrCrowley » Thu Jan 27, 2011 3:32 pm

If you still struggle with the concept of piston valves, start with looking at pneumatic piston valve cannons as they are slightly more simple and will help you understand the unbalanced forces which keep the valve closed and then open it.

is the pilot supposed to be vented at firing and repressurized after each shot?

Yup.

Try to look at this problem by thinking of what the piston needs to achieve to make this launcher work.

For a hybrid you need to have a pressurized mixture in the chamber, so you need pressure behind the piston to keep it closed and stop the mixture leaking out the barrel. This is when it starts to get a little different from normal piston valves. In a hybrid, the pilot volume (the area behind the piston) needs to be sealed off from the chamber otherwise when the mixture is ignited in the chamber it could ignite the mixture behind the piston, which we don't want.

So now the piston has o-rings on it and seals the pilot area off from the chamber. This means we need a quick-connect in the pilot area so we can pressurize it with air. Once that is done, you then fill the chamber with the fuel mixture. Now if we ignited the mixture, it would force the piston back but the air in the pilot volume has no where to escape and will compress. This will limit the flow of the valve and quickly close it once chamber pressure drops.

Now we've established we need some way of exhausting the pilot volume as soon as the chamber mixture is ignited and the piston starts to move back. The best way to do this would be to attach a spool to the back of the piston which, when the piston is in the closed position against the barrel, seals against a hole and keeps the pilot volume air tight. So the mixture in the chamber is ignited and this time the piston starts to move back, since the spool is connected to the piston it to starts to move back and will unseat from the hole allowing the pressure in the pilot volume to escape.

Behind the piston there is now little to no force since the pilot air has all been vented and the ignited chamber mixture can force the piston completely back without problem. The valve is fully open allowing maximum flow and the combustion gases go out the barrel and accelerate the projectile.
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Unread postAuthor: MashedPotatoes87 » Thu Jan 27, 2011 4:53 pm

Thanks mrcowley, that's exactly what I wanted to know. :D When I finish my cannon which should be in the next month hopefully haha, I'll post some pictures and hopefully videos. Oh and should an aluminum 40 barrel be strong enough(1.5 inch id)? For a ten to fifteen mix? And as far as venting, what's the best method you guys have found? Fan or pressurized air injected? And as a final note, had anybody every tried using a hpa tank on a handheld cannon?
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Unread postAuthor: MrCrowley » Thu Jan 27, 2011 5:29 pm

Oh and should an aluminum 40 barrel be strong enough(1.5 inch id)? For a ten to fifteen mix?

Hmm not sure, that stuff is quite thin isn't it? Think you should go for something thicker, like at least 3mm walls maybe?

And as far as venting, what's the best method you guys have found? Fan or pressurized air injected?

The fan would have to be external because if it were in the chamber it would be destroyed. I think both SpudBlaster15 and I prefer to use pressurized air, I can even use a few pumps from a bike pump to vent my chamber.
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Unread postAuthor: MashedPotatoes87 » Thu Jan 27, 2011 5:55 pm

The fan would have to be external because if it were in the chamber it would be destroyed

    Yeah I knew that, I've seen pictures of destroyed ones haha

    Ok, and I just realized I spelled your name wrong both times MrCrowley haha sorry about that. Also, at least until I think of something else, is an aluminum cam lock coupling strong enough or do I need a stainless steel one? I've seen cannons with the aluminum and some with the stainless steel but the steel ones are pretty darn expensive and I don't want to pay like 60 dollars for just a coupling. Just checking.
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