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Not Sure if it's been done...

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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Not Sure if it's been done...

Unread postAuthor: jeepkahn » Thu Feb 26, 2009 12:29 pm

Or doable for that matter, I'm just throwing this out there, combustibles and hybrids aren't my thing so , NO, I haven't searched it...

Has anybody built or designed a pneumatic/hybrid, that instead of using a burst disc, uses a piston to release the pressure???

I was thinking along the lines of a coax or even a t-valved chamber that is filled with (I'm guessing here) a 7x aka 100psi mixture that either the piston slamming back hits a trigger button for the ignitor, or could the ignitor be in the barrel behind the projectile so that it's sparking before the piston opens, and when the piston releases the mixture it starts the combustion process???

I don't have any intention of building something like this but I figured if it hasn't been done some of you guys may want to check into it...

What would the stumbling blocks be? What materials would be needed to support something like this, would you be able to gain performance over standard hybrids or pneumatics by combining the 2?

It's just one of those things that popped into my head while trying to go to sleep....
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Unread postAuthor: Ragnarok » Thu Feb 26, 2009 12:34 pm

Yes, it's been done before.... here!

Anyway, I have to go shopping.
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Unread postAuthor: jeepkahn » Thu Feb 26, 2009 1:36 pm

Ragnarok wrote:Yes, it's been done before.... here!

Anyway, I have to go shopping.


What I was thinking was slightly differant in the fact that I was thinking of the piston being open when ignition takes place?? Would the flame propagate fast enough to cause an increase in performance, and if the piston is already open, could ddt actually improve performance without turning the cannon into a bomb???
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Unread postAuthor: Fnord » Thu Feb 26, 2009 1:47 pm

I sent a PM to someone a while ago explaining some things... here:

I'd add info about the do's and don'ts of pistons to the wiki but I can't log in for some reason.


_Fnord wrote:I don't pilot the valve. The pressure behind the piston keeps the valve shut until the chamber reaches a certain pressure, then it opens rapidly, with the pilot air then acting as a cushion after that.

Letting the valve open at the exact moment of ignition will cause much unburned gas to be lost out the barrel.

The "hybrid 2.0" you see has kind of a bad valve in terms of opening pressure, because it was built before I really knew "what I was doing".
There is a version 3.0 in the works that uses all the experience I've gained and should have comparable power to a good burst disk.

Once you have your design drawn, pm me and I'll give any advice I can.
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Unread postAuthor: jeepkahn » Thu Feb 26, 2009 2:34 pm

Since I'm not a hybrid fan ,per se, what typically is the limiting factor for hybrids? the burst disc???
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Unread postAuthor: ramses » Thu Feb 26, 2009 7:14 pm

the compressor, then the pipe, then limits on ear protection. After all that propane qualification(solved by mixing with butane), after that, limits on scuba tank pressures (max is 4500psi)

burst disk sealing usually gets harder with thicker disks, but one could just use sheet steel and a small orifice.
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Unread postAuthor: jeepkahn » Thu Feb 26, 2009 7:33 pm

ramses wrote:the compressor, then the pipe, then limits on ear protection. After all that propane qualification(solved by mixing with butane), after that, limits on scuba tank pressures (max is 4500psi)

burst disk sealing usually gets harder with thicker disks, but one could just use sheet steel and a small orifice.


I guess I should have clarified why I was asking...

I was thinking more along the lines of something that advanced users might employ.

something along the lines of a press fit projectile and a somewhat reusable "shell casing" so to speak, imagine a bullet shell casing with the bottom removed, not the flange, but the actual bottom where the cap goes, and then a crimped in projectile, upon reaching whatever pressure it would obviously overpower the crimp and proceed post haste out the barrel...

Yes it would be a little expensive, but I would imagine you could reach much higher release pressures than with a typical disc...
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Unread postAuthor: D_Hall » Thu Feb 26, 2009 7:44 pm

ramses wrote:burst disk sealing usually gets harder with thicker disks, but one could just use sheet steel and a small orifice.

Scored aluminum. It's been used around here for a generation. Easy to seal. Breaks when you want it (within reason). Trivial.
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Unread postAuthor: jeepkahn » Sat Mar 14, 2009 9:57 am

Crowleys hybrid got me thinking about this again....

How abour a larger version of fnords piston hybrid, except make it a toolie style chamber/valve...

by using high pressure 300-400psi popoffs as the pilot for the piston you could effectively have a controllable high psi release valve for the chamber....

Maybe I'll go Hybrid with my steel gun....
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Unread postAuthor: Fnord » Sat Mar 14, 2009 10:27 am

except make it a toolie style chamber/valve...


That's brilliant.
Maybe not for practicality, but for power-to weight ratio.
I never gave much thought to that valve style, but I bet it would work well.
If it's ok with you I might actually try a hybrid with a toolie valve eventually, but I think it'll need a lot of tweaking to get it functioning correctly.

The pop-off won't necessarily make it controllable, that depends largely on the valve dimensions. (I can't seem to see why everyone insists on pop-offs...)
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Unread postAuthor: jeepkahn » Sat Mar 14, 2009 10:34 am

To clarify for me, Doesn't the popoff act as the pilot release for the piston that sets the piston actuation pressure, or is it just a safety precaution?

And since we are talking combustion/hybrid having more than the normal opening distance(for a pneumatic) for the piston I think would be in order...

And I would think that for practicallity it would be easier...
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Unread postAuthor: Fnord » Sat Mar 14, 2009 11:13 am

A pop-off IMO will never be a safety feature because your chamber should already be able to contain the blast without damage. If it can't, I wouldn't use it.

The opening pressure normally depends on the difference between the seat diameter and the piston diameter (or in the toolie valve, the "disk" diameter).
If the pop-off is rated to a lower pressure than this, the valve will open at the lower pressure. If the pop-off is rated to a higher pressure, the valve will still open, but the pop-off won't do anything.

Obviously, the opening pressure also depends on the mix pressure you use.

A high-opening pressure valve should have a seat diameter that is 80-90% of the piston diameter. This presents a flow problem in a toolie valve(sealing face diameter is too big for gas to flow around!), but that can be corrected by reducing the overall valve diameters. Sleeve some smaller pipe inside the chamber for the disk(back) to rest in, then reduce the porting diameter appropriately.

Clear as mud?

Note to mods and PCguy: I would really, REALLY like to put all this piston-related crap in the wiki, but I still can't login due to the 'bad password' error.
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Unread postAuthor: jeepkahn » Sat Mar 14, 2009 12:18 pm

I get it, the popoff is more or less to keep the pilot volume from becoming a pneumatic spring....

As far as sealing face and disc size ratio, one could make a chamber similar to this diagram relatively easy, and not have to do a bunch of sleeving...
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Attachments
hybrid toolie.jpg
red is barrel, green is chamber, blue is toolie valve, and pink is pilot port...
hybrid toolie.jpg (7.57 KiB) Viewed 1058 times
Last edited by jeepkahn on Sat Mar 14, 2009 3:26 pm, edited 1 time in total.

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Unread postAuthor: Brian the brain » Sat Mar 14, 2009 3:06 pm

As this thread started out being about a new method of ignition...
Has anybody tried shock heating ignition..

In other words...who has a smallbore metal pneumatic and is willing to spray some starter fluid down the barrel before loading a projectile..?

As we have all discussed long ago, the sudden compression of the dead space behind the projectile caused a rapid rise in temperature, wich should be enough to ignite the ether, wich in turn keeps the temperature of the " chambervolume" gas up.

If it works, hybrids got easy all of a sudden.If it doesn't...you've wasted a single spray of starter fluid..


@ Psycix...you feel up for it?
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: D_Hall » Sat Mar 14, 2009 4:14 pm

To get auto ignition you're generally talking compression ratios on the order of 20:1. I doubt you'll see that in the dead space on a pneumatic.
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