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Spherical Hybrid Combustion Chamber?

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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Spherical Hybrid Combustion Chamber?

Unread postAuthor: JohnnyBOOM » Tue Dec 23, 2008 2:05 pm

I've read in several posts that the optimal shape to prevent detonation would be a sphere. So that got me to thinking, if you had access to a CNC Mill what would be your conception of the perfect spherical hybrid combustion chamber for very high X combustion? I was doodling in MSPaint and came up with this rough concept.
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Unread postAuthor: ramses » Tue Dec 23, 2008 2:34 pm

that could be done, but it would be impractical and expensive. the max mix for hybrids using propane and butane is 320x. the corresponding pressure is over 4500 psi. the unbalanced pressure that you rely on to trip the valve will be applying craploads of force against the spring. It could be done with a pneumatic spring, but the pressure would have to be finely tuned. if the valve failed to trip, there would be very little left of the chamber. I would rather make something like Larda's hybrid, and use some beastly strong burst disk.

good design, though. the valve mechanism has been used on a certain, kick ass cannon
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Unread postAuthor: ALIHISGREAT » Tue Dec 23, 2008 2:36 pm

i would use a burst disc too... its just much simpler and there is less to go wrong...
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Re: Spherical Hybrid Combustion Chamber?

Unread postAuthor: Ragnarok » Tue Dec 23, 2008 2:45 pm

JohnnyBOOM wrote:hybrid combustion chamber for very high X combustion?

Higher than Larda's 200x hybrid?
Even that didn't detonate (although he suspects the 40x oxy/propane mix may have done), and got away fine with a cylindrical chamber.
Beating the pre ignition pressure of that would take some serious work (and in fact it's not really possible to go much further with propane), so you'd already have more challenges to face before you need to worry about special chamber shapes.

That said, a spherical chamber does indeed have the greatest volume for a given diameter, and would therefore be able get the most out of something within a given detonation distance, but that doesn't mean that it has to be needed, as it seems detonations are a much less likely event than older spudding wisdom would have had it.

As Larda's hybrid is unlikely to see an equal or better in pressure for a very long time (excepting of course anything Larda may have planned), and that I believe the runner up on pre-ignition pressure is barely more than a tenth of what the HyGaC20 uses, I doubt we need to be looking at spherical chambers yet, except when using other fuel combinations which have not yet seen popularity, such as my proposed propane/Butane hydrocarbon blend (to boost pre-ignition limits to 320x - but based on the results of 200x, it isn't likely to detonate anyway) or the He/H<sub>2</sub>/O<sub>2</sub> idea (which I proposed as a ultra-high velocity option) - or just anything oxygen enriched in general, which has a faster flame propagation rate, so is more likely to undergo DDT.
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Unread postAuthor: psycix » Tue Dec 23, 2008 6:54 pm

It would be a waste of material to machine a spherical chamber.
I'd stick to cylindrical if I were you.
If you want to prevent long distances of flame front (possible DDT runup) simply invest into multiple ignition points.
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Unread postAuthor: starman » Tue Dec 23, 2008 7:09 pm

psycix wrote:If you want to prevent long distances of flame front (possible DDT runup) simply invest into multiple ignition points.


Yep I agree, the sphere would be way too esoteric to be practical. Multi-gaps is really no big deal to implement in a cylinder.
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Unread postAuthor: JohnnyBOOM » Tue Dec 23, 2008 9:52 pm

Cost was really not a consideration, which is why I stipulated having free access to a CNC mill. I guess when this popped out of my head I had a couple scenarios in mind.

1) Extreme strength - For very high pressures, safety and durability.
2) Possible Semi/Auto fire - That's why no burst disk.

Has anyone thought about metering liquid propane into a chamber using a positive displacement pump or a fuel injector? My thinking was that it is already liquid in the cylinder and If you injected it you could fuel the chamber after it was already pressurized with air.

Just thinking out loud really. /shrug
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Unread postAuthor: CasinoVanart » Tue Dec 23, 2008 10:29 pm

I like the concept but think it would be somewhat limited in terms of practical mix numbers. I don't see why something like this could not be made to operate reliably at say 20x . Loads of fun and not burst discs to change :)
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Unread postAuthor: starman » Tue Dec 23, 2008 11:33 pm

JohnnyBOOM wrote:Has anyone thought about metering liquid propane into a chamber using a positive displacement pump or a fuel injector? My thinking was that it is already liquid in the cylinder and If you injected it you could fuel the chamber after it was already pressurized with air.


I've been kicking around the idea of using a fuel injector but with the propane in pressurized gasous form...just haven't followed through with anything. Would need to fiddle with activation control, timing, etc. but would be very cool if worked out.

Considering all the associated air pressure you'll need, you would typcially fuel first then slam the air in behind it.
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Unread postAuthor: psycix » Thu Dec 25, 2008 7:15 am

2) Possible Semi/Auto fire - That's why no burst disk.

Dont get too ambitious. We all know that building a 200+ X mix hybrid OR building a (semi)-automatic hybrid is a great job already.
Dont do both.
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Unread postAuthor: JohnnyBOOM » Thu Dec 25, 2008 8:59 am

Dare to dream? :twisted:
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Unread postAuthor: psycix » Sat Dec 27, 2008 10:37 am

Daring to dream, yes.
...knowing you can't.... no.

Look, a 200+ X mix hybrid is hard to make, but it is possible.
A (semi)-automatic hybrid is hard to make, but it is possible.
Combination of both is practically impossible to make.

I dont dream of something I know I will probably never be able to build.

For me, dreaming is the thing that comes just before designing.
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Unread postAuthor: D_Hall » Sat Dec 27, 2008 3:05 pm

starman wrote:I've been kicking around the idea of using a fuel injector but with the propane in pressurized gasous form...just haven't followed through with anything. Would need to fiddle with activation control, timing, etc. but would be very cool if worked out.

Considering all the associated air pressure you'll need, you would typcially fuel first then slam the air in behind it.


And congratulations, you probably just experienced pre-mature ignition. "Slamming" that air in there will result in localized adiabatic compression of the air/fuel that's already in the chamber.... and just like that you've reinvented the diesel engine.
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Unread postAuthor: JohnnyBOOM » Sun Dec 28, 2008 2:55 pm

D_Hall wrote:.... and just like that you've reinvented the diesel engine.


The mother of re-invention? :lol:
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Unread postAuthor: starman » Sun Dec 28, 2008 4:23 pm

D_Hall wrote:And congratulations, you probably just experienced pre-mature ignition. "Slamming" that air in there will result in localized adiabatic compression of the air/fuel that's already in the chamber.... and just like that you've reinvented the diesel engine.


Since most low X hybrids typically load the fuel before the air, primarily for convenience of low pressure fuel metering, are you suggesting on high x hybrids the fuel should go in after the air...because of a higher potential of spontaneous combustion?
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