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Hybrid Design

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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Hybrid Design

Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 3:21 am

This is the hybrid that I am designing for and end of year science project.

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there is actually a valve between the O2 and the rest, not shown in the diagram.

I plan on using electrolysis to separate water for my O2. I found an empty helium tank lying around, so I will fill it up with my O2. If I am really lucky I will find another tank, and use that to store the H2 that I will also be getting off the water.

I am going to get a very sensitive gauge so that I can just fill up the main tank with my gases, rather than need a separate chamber for that. The ball valve there is to keep the explosions from destroying my precious gauge.

I dont know what I will be shooting, or what scale I will make the whole thing. I am leaning towards the small end of the range, as it may take a lot of time to separate my H2 and O2.
I am aiming for a good range of power, from maybe 3x to 8x, and would like to be able to do some impressive demos in class at very low power, though if I don't want to get arrested I may want to stay away from that.


Any and all suggestions are welcome!
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Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 11:35 am

Sorry to bump, didn't want to clog up my post any more.

On the ammo front, I have a grinder, so with a metal rod I can make some fierce stuff. The question is what size should I be going for. Like I said, I want it to be on the small side to conserve fuel and O2. Any genius ideas?
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Re: Hybrid Design

Unread postAuthor: rcman50166 » Sat Jan 26, 2008 12:05 pm

There is a problem with the electrolysis part. I've done research and experimentation on electrolysis to find that unless you have a reversable fuel cell, it will take an extremely long time. An example would be to fill up a test tube with oxygen from electrolysis. With a nine volt battery it would take close to 8 hours! If you have designs for the seperator itself I would like to see them. The main problem with trying to make it is trying to keep the two chemicals seperated. The initial containers from electrolysis have to be ambient pressure. After collecting the two you would have to send them through a compressor. The volume you're looking for would take close to 2 hours with a homebuilt electrolysis machine. That's using 120v 15A DC; or the same power as the wall outlet.

Oh. BTW if you find a solution to all those problems, you should just use hydrogen and oxygen as the fuel since hydrogen is also a product of electrolysis. You wouldn't even need an ignition source! It's the same fuel combo used on the space shuttle's main engines.
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Unread postAuthor: DYI » Sat Jan 26, 2008 12:11 pm

For one, just use the O2/H2 mix that the electrolysis outputs. As long as you don't give it more than a few inches of runup distance, it shouldn't detonate.

And good luck getting any volume of gas out of the electrolysis. I set up a very well built rig yesterday, powered by a car battery and using large stainless steel electrodes, and it couldn't even move a 0-100 psi gauge off of the stop after running for an hour. Unless you're using an arc welder or something to split the water (which I may just try if I can't get my current rig to work), you have almost no chance of even filling a tiny chamber to 2x with it.

So, in conclusion, use an arc welder for the power source :D Either that, or a V8 engine hooked up to a homebuilt, 1kA generator...
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Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 12:47 pm

@ rcman: It is very easy to keep the H and O separate. One collects on the + side, and the other on the -

@DYI: is it the voltage, amperage, or both that needs to be high? It cant be too power intensive, otherwise the hydrogen fuel cell cars would never run. Maybe if I get one of the little kits that they have and use that to separate my water...
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Unread postAuthor: DYI » Sat Jan 26, 2008 12:58 pm

The total energy discharged through the water is what determines the amount that splits into oxygen / hydrogen, but voltages over 12 volts or so are completely unnecessary, as you start getting unwanted reactions of the electrolyte.
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Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 1:02 pm

shouldn't it be only 218 (or what ever the heat of formation is) Jules per mol of water separated?
I dont know my J to DC current conversions, but thats not very much energy, why does it take so much power?
edit: 286KJ per mol, but that is still not that much...
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Unread postAuthor: jimmy101 » Sat Jan 26, 2008 2:24 pm

You will have a heck of a time getting the pressure generated in an electrolysis cell to actually pressurize its receiver. And it is very dangerous to try to do it that way. If the cell ruptures you'll be spraying hot electrolyte around. An efficient cell probably uses NaOH or KOH, at high concentration, as the electrolyte. So spraying it about would be bad.

Usually, you collect at atmospheric and then pressurize the gases after you are done collecting them. Water displacement is the most common method of collecting the H2 and O2.

A moderate sized electrolysis cell should be able to make hundreds of cc's per hour, if not a lot more.

A small and extremely well made hydrolysis rig, drawing ~9 amps, generated 430mL total gas per minute. That would be 140mL of O2 and 290mL of H2 per minute.

Unfortunately, the web pages describing the cell appear to be gone;
Dean Wheeler's Mr. Splitter which used to be at the www.et.byu.edu site, and
http://polyplex.org/cjh/rockets/other/r ... ctrolysis/
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Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 2:33 pm

Of course I would be pressurizing it by hand, with a specially made hand pump, not getting the machine to do it for me.
You seem to be fairly experienced in this, so I was wondering what you used for your electrodes. Wikipedia recommends platinum honeycombs, but neither do I have Platinum, nor do I have a way to shape it into honeycombs, so what would you recommend?
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Unread postAuthor: jimmy101 » Sat Jan 26, 2008 5:45 pm

Since $$$ is a always an issue, especially when you start talking about platinum ($Pt > $gold) ....

Just about any old hunk of metal will work if you use NaOH or KOH for the electrolyte. People often use drain cleaner for this but some drain cleaners (like Drano) also contain aluminum shavings. Read the label.

Stainless steel is good if you can get it. Galvanized steel is OK. You can even chop up a metal coffee can if you want to really keep things cheap.

You want the largest surface area possible for the electrodes and you want them fairly close togther. Ten square inches (or more) of electrode seperated by 1/4" to 1/2" would be a good starting point.

It would be best if you had a 3~5V power supply that can source 5~10 amps. Since that type of supply isn't very common, people often use a car battery or car battery charger. The voltage is too high with that setup but there isn't much that can be done unless you build three or four cells and wire them in series. (If you know the resistance and how much current the electrolysis cell draws you could try to find a power resistor with about twice the resistance of the cell that'll handle the power. A 50 or 100 watt light bulb might also work.) It is best if the cell is provided with no more voltage than it needs. At higher voltages you get unwanted electrolysis reations.

Be careful. A car battery (or charger) can put out enough current to really knock you on your ass. Concentrated solutions of NaOH and KOH are hazardous.
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Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 5:58 pm

Thanks so much for all of the help. I just built a tiny crappy little electrolysis machine out of a Gatorade bottle and some copper wires, just to play around with it a little and get the hang of things. If I am using a strong acid or base as an electrolyte, do I need a glass contraption, or could it be done with plastic? I dont want to have to be replacing destroyed plastic bottles all the time...
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Unread postAuthor: rcman50166 » Sat Jan 26, 2008 6:57 pm

Well plastic is surprisingly corrosive resistant. It repels the forces of nature so well that it is considered non biodegradable by environmentalists. Plastic, in one case, was placed over a raft to be used in a crater lake of a volcano, which is extremely acidic, for research of the lake itself. Glass is the same way but is breakable. Your choice in the end though.

I learned electrolysis the same way you are about to. The current needs to be high but not to high to boil of the water. As DYI said anything above 12v is unneccessary. But adds energy to the equation. You'd really have to experiment to find out. However, for the amount of H2 and O2 you wanna produce I'd use a reversable fuel cell. I have two myslef and the produce H2 and O2 5x faster than I could with the same amount of batteries. But you shouldn't ever use anything except absolutely distilled water for the fuel cells.
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Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 7:14 pm

how do i go about getting a fuel cell, and do they not need electrolytes? Also, what size fuel cell would we be talking about?
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Unread postAuthor: rcman50166 » Sat Jan 26, 2008 7:33 pm

Well the fuel cell I have I used in the Northeast Regional Science Bowl. It is small; mostly for demo cars. I'll look for it online but I have a picture.
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A double membrane fuel cell that has an operating voltage of 4 volts either way. It runs you up for about $100 but its extremely useful. (H2, O2, electricity, rechargeable)
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Unread postAuthor: Lentamentalisk » Sun Jan 27, 2008 4:13 am

Since I am doing this project for my AP Chem class, I think I am going to stick with making my own electrolysis machine rather than a fuel cell. (I'll do anything for extra credit in that class)

Many question on the electrolytes:
1) Are certain acids/bases better for electrolysis than others?
2) If so, what makes one better than another (is it just which is stronger, or is there a desired strength)?
3) Do I need to keep on adding more into the water as it is split and I need to add more, or will it be reused?
4) Does it matter how much I put in the water? ie: can I just pour a ton in just to be safe, or should I measure it out?

On the electrodes:
What should I make them out of?
I have heard that graphite is good, though I have no idea how to get very much surface are out of that (if I'm aiming for 10sq. in. like Jimmy suggested)
I have no way of getting enough platinum, (and there is no way I would use it in a spudgun even if I did,) which Wikipedia seems to think is the best.
As for stainless steel, I can't think of a place to get thin stainless steel strips, to make the honeycomb shape of electrode that Wikipedia showed. I was thinking maybe go to Goodwill and see if they have any thin stainless steel pots and cut those up...

thanks so much for the help


Edit: money is not a huge issue, I would be willing to put at least $50 into it, and though I dont have access to a lathe or other heavy machinery, I have the motivation to do lots of manual labor on this.
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