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Autoignition

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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Autoignition

Unread postAuthor: daccel » Sun Feb 22, 2009 6:37 pm

This thread seems to determine that propane/air will autoignite at around 15:1. But that was in reference to being compressed by the deflagration. Will the same ratio apply if it was mechanical compression?

I was playing around with a mini butane torch and filling a pipe with it, then quickly sticking on an airsoft barrel with bb, and igniting. It shot virtually every time, so it seems that with no back pressure, the torch can meter fairly accurately. I'm pretty sure I've seen that on some cannons on spudfiles.

Anyways, since I was having troubles accurately metering small volumes manometrically, that got me thinking about using that method but then compressing the fuel/air before igniting. I just don't want it to autoignite prematurely.

So if it is 15:1, what is a good safety factor?

Wait, I was just reading on that other thread that 15:1 would yield 670 psi... which is a 47x mix :shock:. Perhaps the limiting factor will be the pressure rating of parts rather than compression ratio. Or wait... would 15:1 mean 15x, because you are compressing it to 15 times atmospheric... :?
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Unread postAuthor: ramses » Sun Feb 22, 2009 8:01 pm

15x atmospheric, therefore ~1500psi peak.

cheap hardware store metal pipe should work fine. Of course I didn't say that if you get killed and have to wear an electromagnet on your chest like iron man!!!

good idea, though.


EDIT: I'm not even sure that 15:1 is accurate, but if you build it out of metal, you should be able to find out. It probably won't ignite unless you compress it really fast; I think numbers like that have to do with shock heating, which requires shock.
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Unread postAuthor: daccel » Sun Feb 22, 2009 8:25 pm

Ok, 15x makes more sense.

It would be interesting to test, and the difference between rapid and slow compression. Though potentially dangerous without a check valve or if the check valve failed.

Below is the layout that came to mind initially. Breech could be blow forward or attached to the handle. Depending on the size of the chamber and rams, and what mix you were going for, you could either pump once or multiple times.

Doh! Just realized you would need another check valve to be able to move the rams to the left.

I think this idea has the potential to make a compact, high capacity cannon, because you only have to carry the fuel like a combustion, but with the power of a hybrid.
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Unread postAuthor: ramses » Sun Feb 22, 2009 8:45 pm

not really; you could pull vacuum until you get to an equalization hole like rag on his shock pump.

easier to use a check valve, but more dead space (unless you use a floating o-ring)
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Unread postAuthor: daccel » Mon Feb 23, 2009 2:27 am

The equalization hole gives me an idea, see diagram. This would allow you to pump constantly instead of waiting for the fuel to flow through the pump, because the reservoir would fill on the compression stroke.

I should probably search more, I just came across your thread from a few months ago on a semi combustion, as well as a bunch of earlier discussion :oops:.
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Unread postAuthor: Ragnarok » Mon Feb 23, 2009 5:13 am

You're misunderstanding. The whole point of dieseling is that it's compressed very quickly - because the compression generates heat, which then provides the energy needed for ignition.

Propane won't just ignite when it hits 15x - take Larda's 200x hybrid into account here. You have to take the mix, then if compressed sufficently rapidly (i.e. over the course of milliseconds), auto ignition with propane will occur at somewhere between 17:1 and 20:1 compression. (In other words, it's gas density would be that of a 17-20x cannon on ignition)

At this point, the pressure (because of the heat rise) is around 42 atmospheres - hybrid mix is measured not by pressure, but by gas density (some people use fuel density)

After dieseling, the pressure will spike to around 180 atmospheres (~2600 psi) - about what you'd get from a 25-30x mix normally - all of the extra coming from the increased burn efficiency and pre-compression temperature.

You'll lose a lot of this again unless you can keep the piston that compressed the mix from recoiling again under the pressure. Really, you need a very heavy piston that's fired forwards by a high power pneumatic to compress the hybrid fuel, then hope it's mass stops the piston from recoiling back too quickly.

Basically, build a light gas gun with a hybrid mix in the compression side, and use a pneumatic to power the compression piston.
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Unread postAuthor: ramses » Mon Feb 23, 2009 8:59 pm

That seems logical, but I think he wants to avoid dieseling, rather than cause it.
...I just don't want it to autoignite prematurely.
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Unread postAuthor: daccel » Mon Feb 23, 2009 10:19 pm

Yeah, I'm trying to avoid it, not cause it.

I was asking because I seem to remember a warning once about not fueling directly from the pump, but first filling a chamber, due to the risk of auto ignition.

I just wasn't sure how quickly it had to compress to diesel. So you think that a hand powered pump won't approach the speed necessary to be concerned about this?

I wonder if filling the dead space with steel wool would work to suck the heat out of it, like it does in a suppressor.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 24, 2009 1:07 am

Since any internal combustion engine is already a full auto hybrid, have you considered taking some parts from one and adapting them for your use instead of starting from scratch?
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Unread postAuthor: daccel » Thu Feb 26, 2009 3:00 am

Like tapping a barrel into the cylinder? Hmm the work vehicle does run on propane... and it can't need all those cylinders :wink:

Are you thinking things like a carburetor, or externally powering it to use as a pump, or what?
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Unread postAuthor: Ragnarok » Thu Feb 26, 2009 4:09 am

My mistake - I miss one line and it changes the whole context of the conversation...

daccel wrote:So you think that a hand powered pump won't approach the speed necessary to be concerned about this?

It really shouldn't, unless you happen to exceed the norms for human physical performance by a ridiculous amount.
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Unread postAuthor: jimmy101 » Thu Feb 26, 2009 11:52 am

Ragnarok wrote:My mistake - I miss one line and it changes the whole context of the conversation...

daccel wrote:So you think that a hand powered pump won't approach the speed necessary to be concerned about this?

It really shouldn't, unless you happen to exceed the norms for human physical performance by a ridiculous amount.

Like rag said, in a normal setting a person with a pump can't do it fast enough.

It is possible to get "dieseling" if that is what you actually want. A very small diameter piston can probably be smacked down by hand fast enough to ignite diesel fuel. I know it can be done fast enough to ignite kindling. (Google "fire piston"). But if you're talking about a pump suitable for use with a typical spud gun then your talking about mulitple pump strokes to pressurize the chamber. Heat loss will be too fast to get the mixture up to autoignition temperatures (or pressures).
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 26, 2009 11:56 am

daccel wrote:Like tapping a barrel into the cylinder? Hmm the work vehicle does run on propane... and it can't need all those cylinders :wink:

Are you thinking things like a carburetor, or externally powering it to use as a pump, or what?


Why not?

Carburetor for fuel injection, spark plug for ignition and a blow-forward bolt that will move instead of the piston and also serve the function of exhausting the chamber. It would be a significant project, but I don't see why it couldn't be done.
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Unread postAuthor: daccel » Sat Feb 28, 2009 2:43 am

Ragnarok wrote:It really shouldn't, unless you happen to exceed the norms for human physical performance by a ridiculous amount.


ramses wrote:[...] wear an electromagnet on your chest like iron man!!!


Working on that :wink:

Well that's good to know, I'll start some trials with this then.

Jack.. noted, but not really a project I want to take on right now.
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