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Liquid fueled 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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Liquid fueled hybrid

Unread postAuthor: daccel » Thu May 14, 2009 12:31 am

I was thinking about using methanol in a hybrid, but would the liquid not evaporate under pressure? Or if you heated it to evaporate would it condense when it cooled to room temperature?

Would all liquid fuels act the same in this regard?
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Unread postAuthor: psycix » Thu May 14, 2009 7:27 am

I was thinking about using methanol in a hybrid, but would the liquid not evaporate under pressure?
That depends on the pressure/temperature characteristics of the liquid you use. Under a high enough pressure and/or low enough temperature, it will stay below its boiling point and thus remain liquid.
Or if you heated it to evaporate would it condense when it cooled to room temperature?
Yes.
Would all liquid fuels act the same in this regard?
Besides having different boiling temperature/pressure curves, yes.
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Re:

Unread postAuthor: jackssmirkingrevenge » Thu May 14, 2009 7:31 am

daccel wrote:Liquid fueled hybrid


Isn't every internal combustion engine a liquid fueled hybrid? You have over a century of technological development to draw on ;)
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Re:

Unread postAuthor: jeepkahn » Thu May 14, 2009 8:31 am

jackssmirkingrevenge wrote:
daccel wrote:Liquid fueled hybrid


Isn't every internal combustion engine a liquid fueled hybrid? You have over a century of technological development to draw on ;)


NOPE, propane engines are a gas hybrid...

Careful how you use words like EVERY...lol...
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Unread postAuthor: daccel » Thu May 14, 2009 9:16 am

Well it's the pressure/temperature characteristics I'm trying to understand. The boiling point of methanol is 65 deg, but it will slowly evaporate at room temperature. So would the same still happen with more pressure, just more slowly or incompletely?

I found this vapour pressure curve. But I don't think you have to stay below that pressure at that temperature, because propane hybrids can reach 200x? which is outside of its curve?
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I know you could spray in the fuel like in an engine, but it's much easier to measure the liquid with a dropper or syringe and then vaporize via time/heat/fan than build a fuel injection system. And there's the time factor, because an engine ignites momentarily after injecting the fuel and compressing, but here there would be a delay giving the fuel time to condense?
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Unread postAuthor: jackssmirkingrevenge » Thu May 14, 2009 9:40 am

Damn semantics :P
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Unread postAuthor: Ragnarok » Thu May 14, 2009 9:50 am

daccel wrote:I found this vapour pressure curve. But I don't think you have to stay below that pressure at that temperature, because propane hybrids can reach 200x? which is outside of its curve?

Not quite. Liquefication due to vapour pressure relates to PARTIAL pressure.

That is, total system pressure times molar percentage of gas. Because propane is used in a 4.2% mix, that means it's partial pressure is just 4.2% of the system whole.

Which is why, propane can be used at 200x, because it's partial pressure is not high enough to liquefy it.
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Unread postAuthor: psycix » Thu May 14, 2009 11:14 am

daccel, note that every gas/liquid has its own curve. Gasoline, methanol and propane are not the same.
A curve graph like that is worthless if it doesn't say what liquid it is about.
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Unread postAuthor: Ragnarok » Thu May 14, 2009 11:23 am

psycix wrote:A curve graph like that is worthless if it doesn't say what liquid it is about.

It's the vapour pressure curve of methanol.
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Unread postAuthor: daccel » Thu May 14, 2009 12:25 pm

Sorry didn't label it, yes that graph is for methanol.

So then the approximate limit for methanol at room temperature would be 2x? 14.7 psi x 14% (fuel mix) = 2.1 psi partial pressure, which is ~ the vapour pressure at 20 deg.

And the only way to increase that is heating?

If you used something like a fuel injector to spray the fuel in, would it stay suspended long enough to be practical in a cannon?

Or what about using this idea to inject the fuel, then pressurize with air?
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Unread postAuthor: Ragnarok » Thu May 14, 2009 12:39 pm

daccel wrote:So then the approximate limit for methanol at room temperature would be 2x?

No, because partial pressure equations need to be done with absolute pressures. In other words, the limit here is pretty much 1x, assuming your % figure is correct (I haven't bothered to check).

Now, there has been talk here occasionally about suspended fuel cannons - those that use a solid or liquid fuel with a gaseous oxidiser. They're not always useful because of lots of "OMG, solid fuel is against the rules" stuff. No, reading it properly dictates that solid or liquid oxidisers are against the rules.

Anyway a couple of links:
http://www.spudfiles.com/forums/flour-c ... 14856.html
http://www.spudfiles.com/forums/sugar-a ... t4082.html

Those talk more about powders than vapours, but they might give ideas.
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Unread postAuthor: daccel » Thu May 14, 2009 11:38 pm

Interesting links, hadn't read before.

Here's a good one on liquid fuels.

I was initially interested in methanol because of its wide flammability range (6%-36%, compared to propane at 2.5%-9.5%) but it seems that for hybrids it would end up being more work/cost more than an accurate propane meter, negating that advantage.
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Unread postAuthor: jimmy101 » Fri May 15, 2009 1:20 pm

Some highlights from the earlier dust explosion discusion.
Most dusts will form explosive mixtures in air. Heck, the dust doesn't even have to be flammable in the normal sense. In certain situations even iron dust will give a dust explosion.

Folks use coal dust, flour, powdered sugar, corn starch, non-dairy coffee creamer, and a bunch of other things. The most common material is lycopodium, a dried moss. The tricky part is that you have to get the dust particle sizes just right. And, you have to get the dust well dispersed in the air. And, you have to get within the materials flammability limits (just like with propane). Flour and sugar are very sensitive to the moisture level, too humid and it won't properly disperse. The lycopodium is supposed to be particularly easy to get to a combustable suspension.

The energy produced in a typical dust explosion isn't all that much different than in a natural gas explosion. On a pound fuel basis it isn't even all that much different than "real" explosives. (The same can be said about propane etc.)

Heck, I'm surprised you can still get non-dairy coffee creamer on airlines. Why hasn't TSA banned it because it is a good material for creating explosions? (only partly kidding)


* Energy content of flour, coal dust etc. is about the same as propane.

* The burn rate in most dust explosions is a fair bit slower than for propane + air.

* The ignition energy for most dusts is perhaps 1000x more than for propane + air. It is unlikely that a typical sparker will have enough oomph to ignite a dust mixture. A simple ignition system might be a "poor man's squib". Basically a 10 ohm (or so) 1/4 watt resistor attached to say a car battery. The resistor will pretty much explode (or at least burst into flames) when power is applied.

* The peak pressure in a dust explosion should be higher than for propane+air. Perhaps as much as 20% higher in a closed chamber?

* An efficient, high velocity chamber fan is probably required.

It would be really cool if someone would actually give this a try. Just place a small amount of coffee creamer on the fan of a combustion gun and ignite with a resistor?

I think I'll add some of this info to my spud gun fuel pages.
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Unread postAuthor: SPG » Sat May 16, 2009 4:12 am

Why is a fuel injection system so hard to make? You're already dealing with pressurised air after all? Given the huge combustion range methanol has I'd have thought you could pressurise your chamber to say 90% of the pressure you're seeking, then use pressurised air to blow your liquid fuel through a fine spray nozzle and ignite.

Surely this would produce enough vapourised methanol in the chamber to start the combustion process?
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Unread postAuthor: daccel » Sat May 16, 2009 10:13 am

Yes, that could work, and I'd thought about dumping all the air pressure through the spray nozzle to maximize mixing.

But I wasn't able to see a feasible way to do that which allowed for relatively quick repeat shots or semi-auto, which is what I'm more interested in. Because you'd have to measure out more methanol each time to behind the nozzle.

The only possibility I saw was to have a small tank of methanol and inject the air up through it, past a check valve, and into the chamber. The air would possibly pick up enough vapour as it moved through the tank, but could also flood the chamber come to think of it unless you slowly bled the air in.

But whether that would get you in the right range reliably I'm not sure, and the cannon would have to remain perfectly level all the time.

I think I remember a combustion cannon on here that used one of the alcohols just sitting in the bottom of the chamber with a constant spark, and fed the air in, and it would fire when the mix reached flammable range. But I don't think that would work for hybrid because it would fire before pressurizing, and if you delayed the spark the pressure would prevent vapourization of enough fuel.
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