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+20x 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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+20x piston hybrid

Unread postAuthor: Gunner » Wed Oct 31, 2012 3:05 pm

As you can see the title, Im designing high mixture hybrid and one thing that is the "problem" is this:

What kind of piston area ratios you guys use? I know that the area backside of the piston must be much bigger than the area of frontside of the piston which seals the barrels(the little area that is between barrel and piston edge, my english is so bad that I cant explain that but I think that you guys understand what I mean :wink: )

If I have understand right, ratio 1:5(barrel miniarea:piston backside area)
means that the valve opens five times bigger pressure than the initial pressure, for example, in 20X mixture the valve opens something like 100bar?

And how about the "spool valve" behind the piston? Do I need it? Is it too dangerous to use the air as bumber and if the dead volume ignites, its not good :D

This is like reading hieroglyfs but :D
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 31, 2012 4:17 pm

The smaller the ratio, the higher pressure it will take for the piston to open, and the better the performance. Also, smaller piston equals less weight so less stress on the launcher.
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Unread postAuthor: MrCrowley » Wed Oct 31, 2012 4:50 pm

The smaller the ratio, the higher pressure it will take for the piston to open, and the better the performance
Assuming a constant pilot pressure? If you want to use less pressure in the pilot for a given mix, you would make the piston area the combustion gases act on quite small relative to the area the pilot pressure acts on the piston. For a given mix, the combustion gas pressure is fixed, but the ratio can change. If the ratio changes, the pressure required in the pilot would change accordingly.

That being said, I would also suggest a small ratio. I can't remember the exact specifics, but I think my seat is 46mm O.D and my piston sealing face is about 50mm O.D., giving the combustion gases that 50mm diameter, 4mm wide, ring.

You don't have to use a spool, might be easier not to on a 20x design. That being said, you have to make your piston and seat strong enough to handle piston bounce. You might also see a drop in performance if the piston closes too soon.
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Last edited by MrCrowley on Wed Oct 31, 2012 10:35 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Wed Oct 31, 2012 10:27 pm

jackssmirkingrevenge wrote:The smaller the ratio, the higher pressure it will take for the piston to open, and the better the performance.


If the ratio is too small, the combustion will not raise the pressure enough to unbalance the valve. A ratio needing a 5:1 pressure raise is one the small seal area is 0.8 of the large seal. Note this ratio is AREA, not DIAMETER. The diameter ratio is much smaller.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 01, 2012 1:23 am

MrCrowley wrote:Assuming a constant pilot pressure?


Assuming you're using an unsealed piston and the pressure in the chamber is that in the "pilot" area.

If the ratio is too small, the combustion will not raise the pressure enough to unbalance the valve.


A typical hybrid will raise pressures roughly by an order of magnitude. shouldn't be an issue.
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Unread postAuthor: Gunner » Tue Nov 06, 2012 5:18 am

OK, thanks for good answers. What is max ratio(piston area:"thin area") that is safe to use? I think that 6 sounds ok, but 7-> sounds risky because if the piston is too loose, it leaks so much that the piston wont open?

If think that Im gonna use spool valve, because Im afraid that the dead volume ignites so it can went to "backport" :DD
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