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Combustion/burst valve/piston hybrid?

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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Combustion/burst valve/piston hybrid?

Unread postAuthor: Daltonultra » Sat Nov 29, 2008 9:49 pm

I've been tossing this one around in my head for a couple days. I don't have the resources to do it myself, so I'll throw it out for anyone who thinks they can pull it off.

One caveat: If this thing works like I think it will, it would be incredibly powerful. Even welded steel pipe wouldn't be safe at high fuel concentrations. It would probably have to be machined out of billet.


Basically, it's a coax, but instead of firing by exhausting the back chamber, it would ignite a fuel air mix in the main chamber, over-pressuring the main chamber and BLOWING the piston back, popping a burst valve in the back chamber to keep it from blowing the case. At the same time, it would increase firing pressures astronomically...

Whadaya think?

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Unread postAuthor: FordGtMan » Sat Nov 29, 2008 10:08 pm

I have thought of this before, but i dont think that a coaxial setup does well with combustion though. I really dont think that it would have as much performance as a burst disk combustion, but i could be wrong.
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Unread postAuthor: Daltonultra » Sat Nov 29, 2008 10:11 pm

You know, I think I may have spent the last two days pondering a truly stupid design. I could get the same effect out of a suitably thick burst disk in a conventional hybrid.

Nevermind.

EDIT, actually, I may have spoken to soon. The piston/burst disk combination COULD slow down the release, allowing more time for the pressure to build...I don't know.
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Last edited by Daltonultra on Sat Nov 29, 2008 10:14 pm, edited 1 time in total.
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Unread postAuthor: Sticky_Tape » Sat Nov 29, 2008 10:13 pm

I think that that would be a hybrid hence the burst valve it holds in pressure. seriously 2 seconds after shut up. ok don't edit your post.
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Unread postAuthor: jonnyboy » Sat Nov 29, 2008 10:27 pm

I really don't see what is happening. When you fill the piston will be pushed back allowing air to escape to the barrel. So basically if filled from the correct side all it does is allow more pressure to build before firing?

The energy would be better spent with the barrel on the other side of the burst disk. Then it would be like a compressor. I'm not sure what kind of ratio that would make but it would be an interesting concept.
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Unread postAuthor: Fnord » Sat Nov 29, 2008 11:23 pm

It's called a piston hybrid, and I've done it. Twice now, but I haven't completely finished the second one. I think a guy by the name of galfisk also might have a semi-working version.

It's a better idea to not let the pilot 'pilot' with a burst disk; keeping air behind the piston will act as a cushion and not let it slam into the back so hard.

Also, as I found out from the first version, a piston will almost ALWAYS open at low pressures if it isn't specifically designed not to. You need a fairly high seat-diameter-to-piston-diameter ratio to get it to open at a high pressure (think big barrel porting, small piston housing).
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Unread postAuthor: Daltonultra » Sun Nov 30, 2008 12:36 am

Yeah, I just stuck the air valve in the wrong spot.

And wouldn't a SMALL barrel and a LARGE piston face make for faster opening? Make the pressure differential as close to even as possible?

I suppose the burst disk could be done away with. I'd be worried about the rebound trashing the piston face.
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Unread postAuthor: FordGtMan » Sun Nov 30, 2008 3:40 am

seriously 2 seconds after shut up. ok don't edit your post.


What the hell, does that even make sense? If i am thinking right, why is it wrong to edit a post soon after the original post? He realized something and wanted to fix it.

The good thing about these piston hybrids would be that you dont have to bother with those burst disks. Someone should try the coaxial design to see how it effects performance too.
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Unread postAuthor: GalFisk » Sat Jan 10, 2009 7:16 am

_Fnord wrote:It's called a piston hybrid, and I've done it. Twice now, but I haven't completely finished the second one. I think a guy by the name of galfisk also might have a semi-working version.

Hey, people remember me :D
Yes I still have the cannon, the valve itself works perfectly, it's only the fueling system that has given me grief. I haven't worked on cannons for a long while now, but I will pick up again when the time is right.
That's a lovely valved hybrid you've got there _Fnord. I understand the pilot is vented via a pop-off? My hybrid valve is self-venting, it has a small piston connected to the main piston, which opens the pilot to the atmosphere when the assembly starts moving backwards. Evidently it's not neccessary to do things as complicated as this, but I built it mostly as a proof of concept. It works great if used as a straight pneumatic.
Here are a couple of images for your inspiration (click for large image):
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Unread postAuthor: inonickname » Sat Jan 10, 2009 7:47 am

Looks too complex for it's own good.. I don't see whats wrong with the standard hybrid.. (I suppose I'm not one to talk with the project I'm working on at the moment..)
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Unread postAuthor: starman » Sat Jan 10, 2009 12:49 pm

Galfisk, very nice drawing you have put together there!

I couldn't help but notice you're using what looks like 70v speaker line as your HV leads... :shock: and the extra is all wrapped up nice and tidy... I'm surprised you are getting any spark at all to your gap.

Maybe you were looking for a little extra induction coil boost...??... :wink:
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Unread postAuthor: GalFisk » Sun Jan 11, 2009 9:52 am

Thanks, maing the drawings beforehand is part of the fun :) It also lets me see many problems before even ordering the parts.
The wrapped up wire is for 9-12V - underneath the tape is a flyback transformer circuit, you can see the HV wire going of to the right. The long wire is so that the cannon can be fired somewhat remotely, from around a corner or behind a rock. It's been dry-fired at 5x and works perfectly. Yes it's complex, but when it's finished it's simpler to use than a burst disk cannon, and doubles as a very fast pneumatic.
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Last edited by GalFisk on Sun Jan 11, 2009 12:39 pm, edited 1 time in total.

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Unread postAuthor: iemand » Sun Jan 11, 2009 11:47 am

What if, you would keep a constant airpressure coming in at the back of the piston(so that the piston is shut), and a constant fuel/air mixture sprayed in? Maybe someone could use this to make a full auto hybrid.

Well, there's also gonna be a problem with the ammo :?
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Unread postAuthor: Fnord » Sun Jan 11, 2009 1:22 pm

I understand the pilot is vented via a pop-off? My hybrid valve is self-venting, it has a small piston connected to the main piston, which opens the pilot to the atmosphere when the assembly starts moving backwards.


Actually, no, Not anymore. I'll explain;

originally the idea of the pop-off was to keep the pressure inside from getting too high. After I did a test and found that sch40 steel could hold in a 11x mix without being damaged, I didn't need to worry about this in my cannons.

Now the idea is to keep the valve shut as long as possible, letting the pressure build and all the gasses burn before opening.
Here is an equation for finding a valve's opening pressure


VS = Valve seat area
P = Pre-ignition pressure
PD = Piston diameter

VS * P = CF (force keeping valve closed)
(VS - PD) = A (area ignition pressure will act on)

Then you just need to find what number, when multiplied by A, will be greater than CF. Add your pre-ignition pressure to this and you'll have your absolute opening pressure.


For example:

1.388 * 132 = 183.216 (CF)

2.00- 1.388 = 6.12 (A)

Through trial and error, I found that A * 300 psi = 183.6 (just greater than CF). Add mix pressure and my valve opening pressure is 432psi.

It should be noted that the example uses 1" steel for porting and 1.5" steel for a piston housing (same as my unfinished hybrid). Since you used the metric system I didn't feel like calculating yours.
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Unread postAuthor: starman » Sun Jan 11, 2009 1:50 pm

GalFisk wrote:The wrapped up wire is for 9-12V - underneath the tape is a flyback transformer circuit, you can see the HV wire going of to the right. The long wire is so that the cannon can be fired somewhat remotely, from around a corner or behind a rock.


Ahh I see. That HV wire looked suspiciously like the trigger wire. Yeah, being remote and behind a big rock is good thing!!
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