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Automatic Combustion Chamber Idea?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Automatic Combustion Chamber Idea?

Unread postAuthor: Doctore » Tue May 21, 2019 9:18 pm

Hello!

Again, one of those hypothetical scenarios, it might sound silly but here is the idea.

The device would be composed of two chambers connected with a hammer valve. The first chamber is the reservoir in which pure Alcohol (Ethanol) and pressurized air would be stored. The idea is also to heat this chamber externally (via gas stove) to force alcohol to evaporate and homogeneously mix with the pressurized air. I have no clue is this even safe! It would be calculated how much of liquid alcohol could be poured into this chamber. I know that a liquid evaporates harder when pressure is increased.

The second chamber is the combustion chamber in which the ignition system would be installed. This chamber may or may not have a piston at the exit. One way valve would be between this chamber and a hammer valve.

I was thinking about two types of ignition systems. First one could be a regular taser, or a device that produces spark oscillations constantly. As for the second type of ignition, I was thinking about some material that has electrical resistance and glows red hot while electric current passes through it. Perhaps a modified car cigarette lighter. It would have to operate for a longer time and not fail after a few seconds.

After the ignition system is switched on and the reservoir is filled with Alcohol and pressurized air, you would fire this device by activating the hammer valve which is supposed to spray this Alcohol/Air mixture at the ignition system in the second chamber. Hopefully there would be a reaction.

As for the reservoir, honestly I am not sure if this thing could explode. Pressurizing a flammable liquid plus heat sounds like a bad day.

Here is the illustration.

Image
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: D_Hall » Wed May 22, 2019 12:10 am

There would be no need to heat your reservoir. Ethanol is sufficiently volatile to result in a combustible mixture in confined spaces even at room temperature.

Note: If your hammer valve leaks during firing, your reservoir just turned into a bomb. In other words, this is a Bad Idea.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: mrfoo » Wed May 22, 2019 1:05 am

As D_Hall says, this is a really bad idea, safety wise. The idea of having a reservoir full of a (presumably stoichiometric) mix of fuel and air, ready to go, is, on the face of it, appealing. But look a bit more closely, and you have a fuel-air bomb. And even the military don't keep the fuel and oxidiser together in their *designed for purpose* fuel-air bombs - it's mixed "on site".

Technically, there's a couple of issues, too.

Firstly, ethanol is a bad choice of fuel. If you must go with an alcohol, methanol has a far wider range of flammability limits, and you're not wasting good drinking material. Alcohols, however, are not energy-dense, you'd be better off with, in order, fuel oil, diesel, or gasoline (which, as a bonus, are far easier to lay your hands on).

Regardless of fuel and the dangers of an explosive mixing tank, evaporation of a pool of fuel is a pretty lousy way of mixing. You end up with layered mixtures, many of which will be above or below the flammable limits. You can get around that by shaking, I guess. But on top of that, it's really damn hard to meter, too. How can you be sure all the fuel has evaporated? The technology to do what you want exists, and has done for over a hundred years - precise metering and good mixing can be achieved for liquid fuels using either a carburettor or direct fuel injection.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: jackssmirkingrevenge » Wed May 22, 2019 6:14 am

miniature-cannons/topic26232.html

combustion-cannon-database/topic1278.html

Have a look through Rambo's posts, he was the forum's foremost exponent of this type of launcher.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: hectmarr » Wed May 22, 2019 2:07 pm

To eliminate the danger of this design, you can consider including the fuel, in part of the explosion chamber. I've seen in San youtube, which includes some absorbent material impregnated with ethyl alcohol. This foam should occupy only 30 p 40% of the total volume of the combustion chamber, to make room. When you inject the air, it evaporates the alcohol, and you can achieve, by means of tests, a certain prolixity in the mixture, in terms of stoichiometry. I do not know how long the foam sponge can last, but surely it could be used in many shots. A matter of trying and correcting until it works.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: Doctore » Wed May 22, 2019 4:05 pm

So my suspicions are confirmed, definitely not a safe design. Perhaps the only possible way would be to fill the majority of the reservoir with alcohol and use as little pressurized air as possible.

mrfoo wrote: And even the military don't keep the fuel and oxidiser together in their *designed for purpose* fuel-air bombs - it's mixed "on site".


Exactly. I thought this would be somewhat safe since 80% of air is nitrogen, an inert gas.

hectmarr wrote:To eliminate the danger of this design, you can consider including the fuel, in part of the explosion chamber.


Yes, I have seen this mini cannon made by Rambo and I am very impressed.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: hectmarr » Thu May 23, 2019 4:12 pm

It can be a simple option, introduce the pulverized fuel and mix it with the air in your explosion chamber. The compressed air should be regulated in quantity and time, so that it does not vary much between each shot. The amount of fuel, alcohol, that enters its combustion chamber along with the compressed air, is what can vary with the regulating ajuga, exactly equal to the carburation system that the small glow engines have, of aeromodelling.
Ethyl alcohol, in cold days, less than 15 Cº, will not evaporate. To start you must warm a little, up to about 30Cº your camera. The fact that the alcohol is pulverized, facilitates this, without doubts. I recommend DO NOT use methyl alcohol, because it is very toxic. Their vapors are dangerous, no doubt.
It is necessary to eliminate the residual burnt gases from your combustion chamber, before discharging. A little air alone, and then the mixture load. Its stoichiometry can be excellent, if it keeps the amount of air for each shot always the same, and the same, with the amount of alcohol.
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Re: Automatic Combustion Chamber Idea?

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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: mrfoo » Fri May 24, 2019 1:04 am

I recommend DO NOT use methyl alcohol, because it is very toxic.

Sure, if you drink the stuff. It metabolises into formaldehyde, and then to formic acid, and it will cause liver damage and send you blind. Hence the term "blind drunk", which comes from drinking extremely carelessly distilled alcohol. It's so terribly toxic that it's used to denature ethanol (meths), as part of windscreen washer fluid (that sweet smell is the methanol vapour), in bottles of HEET (US based fuel line antifreeze, pretty much pure methanol), forms ~85% of the fuel for model IC engines, etc.

Their vapors are dangerous, no doubt.

Every time you wash your windscreen hilst driving, you're ingesting methanol vapour. Methanol, even 100% pure methanol, is far less dangerous by ingestion than petrol. Same goes for the vapours. Methanol poses no known cumulative health hazard and is not classified as carcinogenic, mutagenic or teratogenic. etc etc.

Methanol is far more forgiving than ethanol in terms of flammability limits. Ethanol will burn from 3.3 to 19% in air, methanol from 6.9 to 36%. If you can handle mixing for proper combustion of ethanol, you're better off going for diesel, which has far more energy density.

But diesel is poisonous!
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: hectmarr » Fri May 24, 2019 8:31 am

Yes, I know methanol well, I have about 30 years of manipulating it. I used to sell a mixture of glow model aircraft, in the past. I have always had a lot of caution in his handling. My accidents, (fortunately without serious consequences for my health), with this fuel was always due to "unforeseen", splashes in the face, spills of the container by falling to the ground, (the boats were glass, large, about 3, 8 litos, etc., more expensive and difficult to obtain than ethanol, which you get at any pharmacy.It can be used for combustion weapons but one has to be much more tedious and cautious in its handling, without doubts.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: Doctore » Fri May 24, 2019 5:51 pm

hectmarr wrote:It can be a simple option, introduce the pulverized fuel and mix it with the air in your explosion chamber. T


How would you vaporize alcohol, like a quick injection? Sounds great to be honest. Anyways, do you think it would be safe to fill the majority of the tank with alcohol and use less volume of pressurized air. That way you would minimize the amount of oxidizer. I am sure that it would be more stable that way.

Edit -

mrfoo wrote:
Methanol is far more forgiving than ethanol in terms of flammability limits. Ethanol will burn from 3.3 to 19% in air, methanol from 6.9 to 36%. If you can handle mixing for proper combustion of ethanol, you're better off going for diesel, which has far more energy density.

But diesel is poisonous!


Methanol is not an option for me currently, the simple reason is availability. I can easily get pure ethanol, but I am not sure where to find methanol.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: hectmarr » Fri May 24, 2019 6:46 pm

In my combustion weapons, in winter, I heat them a bit, (the aluminum walls of my combustion chamber), with a hair dryer. I live near the tropics, and I have a warm climate almost all year round, but on cold days, I proceed like this. Once hot, the explosion chamber is maintained by the action of the shots.
The "serious" option is to wind copper wire to the explosion chamber, on the outside, as an electrical resistance, and to pass a little electric current from a battery on board the weapon, coupled to a simple thermostat. It would be necessary to design this topic well, to maintain an ideal temperature. I calculate according to my experience, that with 40 Cº enough.
On the other hand, speaking of your first idea, if you increase the amount of liquid alcohol and lower the air pressure, it is equally dangerous. You never know if the alcohol is running out, or how much it evaporates according to the ambient temperature ... a PUM can happen !!! :roll: just when you're playing with your burning weapon, and in the best part of the game, it's the law of murphy :shock: If you want a stoichiometric mixture does not work out well, if you do not want it because of the danger of explosion, it will form perfectly ...
Keep in mind that the amount of alcohol vapor may be independent of the amount of liquid alcohol you have accumulated. I do not like to run that risk, having other, safer options.
I leave you a video of an old combustion gun that I have. Use a small manual air pump to introduce air alone, to clean the burned gases and to spray the alcohol. It works like a common spray. You can impersonate the air pump by compressed air, I've tried it and it works well, just because of my preference, and to make it more practical, I prefer to pump and shoot.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: Moonbogg » Fri May 24, 2019 10:54 pm

OMG I love that pump mechanism! That thing is so easy to use it's crazy. The convenience of use is really a great thing to see. Seems to pack a punch as well. It's hitting that target pretty hard from what I can tell.
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Re: Automatic Combustion Chamber Idea?

Unread postAuthor: hectmarr » Sat May 25, 2019 1:53 am

Fun toy, my first homemade "weapon" :D
It does not have much energy because it is not compressed, and the ammunition is small, but I always liked it. And in the future, make a well-finished version. For playing with hybrids, never make this type of weapon again. I'm glad it's to your liking.
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