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Nichrome Ignition

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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Nichrome Ignition

Unread postAuthor: Hubb » Tue May 17, 2011 7:24 pm

I was looking at Larda's hybrid yesterday. The ignition system for this beast uses a thin wire connected to a low voltage source to ignite the launcher. The wire, of course, heats up to the ignition point of the fuel, causing it to ignite.

The method actually seems a lot less bulky, a bit easier, and a bit cheaper to incorporate than the conventional methods of ignition. It would also seem that this source creates a much larger flame front all at once in the chamber rather than one or two or three.

A little bit of research and I've learned that propane (which is the most commonly used fuel) will ignite itself without flame or spark in temperatures of 920 to 1020 degrees F. A little more research has shown that Nichrome wire (22 gauge) will heat up to 1000 degrees F with using 5.6 amps.

I suppose some experimenting should be conducted to test this, but does it not seem feasible to use it on something more than just the greatest hybrid ever built?

Thoughts?
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Unread postAuthor: Lockednloaded » Tue May 17, 2011 7:29 pm

JSR used the stuff in his clear combustion experiments, he was pretty successful IIRC
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Unread postAuthor: saefroch » Tue May 17, 2011 8:14 pm

You could always attempt to calculate the temperature curve for a given current... or more usefully, calculate the temperature curve for a given voltage. I might try that...

I've no doubt that this will work, by <sup>V</sup>/<sub>R</sub> = I, you can calculate the current in the wire pretty easily. Without doing any calculations, I bet you'd be able to make this work on just batteries...

As my ignition system is finicky, this may be my way out. Thanks for bringing up the idea. Also of note is that it'd be able to ignite over a massive area with short nichrome wire sections distributed over the entire chamber.
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Unread postAuthor: Hubb » Tue May 17, 2011 8:19 pm

The way I see it, if this ignition source works like I'm predicting (primarily based on the success of Larda's hybrid) then it may just be able to replace conventional ignition sources. It's worth a shot anyway - it's about time for a little something different in this hobby.
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Unread postAuthor: clemsonguy1125 » Tue May 17, 2011 8:33 pm

Would a 9 volt battery with it be able to produce that amount of heat and would their be a delay between trigger it and it actually firing? But it should work.
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Unread postAuthor: ramses » Tue May 17, 2011 8:46 pm

The 9 volt would create enough heat, but it wouldn't last too long.

There may be perceivable delay. In JSR's video, if it took two seconds to ignite at 300 fps, would have added about 1/15 of a second to lock time. In firearms terms, that is unacceptable, but it may not impact you.
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Unread postAuthor: saefroch » Tue May 17, 2011 9:04 pm

Capacitor dump through a nichrome wire? Or other heating filament material?

I have a camera flash circuit hooked up, if it's possible for me to quickly acquire some nichrome wire, I could at least test it in a spray 'n' pray...
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Unread postAuthor: Technician1002 » Tue May 17, 2011 9:28 pm

Start with a long length of wire and gradually shorten it until a cap discharge lights it orange. Too long won't get hot enough. Too short will quickly overheat and burn in two.

An ideal length will suddenly light up and dim out.
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Unread postAuthor: Hubb » Tue May 17, 2011 9:30 pm

The capacitor dump crossed my mind as well, although I'm not as, um "electronically inclined" as I once was. One thing I do know, however, is that the heat of the wire is going to depend more on the wattage (the voltage AND the amperage) rather than just one or the other.
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Unread postAuthor: PVC Arsenal 17 » Tue May 17, 2011 9:35 pm

Would there be any benefit to using a supercapacitor? It could simplify the charging circuit some.

And what about r/c glow plugs rather than wire?
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Unread postAuthor: Technician1002 » Tue May 17, 2011 9:52 pm

A supercap will work, but they are often in large values at low voltage, so the heat cycle will be longer.

The advantage of the photoflash is very fast heat times and lower total power draw. Instead of taking several seconds to heat the wire, it is heated in milliseconds instead, then allowed to cool. The supercap requires much longer to heat due to the lower voltage and current.

If the voltage is too high because the wire is too short, you risk burning out the wire.
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Last edited by Technician1002 on Tue May 17, 2011 10:02 pm, edited 1 time in total.
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Unread postAuthor: irisher » Tue May 17, 2011 9:58 pm

A while back I ordered some nichrome for this exact same purpose. I do have to warn you though. It must be supported and not knocked around because when it is red hot it is very fragile.
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Unread postAuthor: saefroch » Tue May 17, 2011 9:59 pm

650 FARADS???? HOLY SHITE that's a lot of capacitance. So far as I can tell, a supercapacitor is just a really high capacitance capacitor... lots of charge can be stored per volt used to charge it. However, Wikipedia says they can only usually be charged at 2 or 3 volts... :?

EDIT: Could probably encase most of the nichrome in a feed-through of some sort...
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Unread postAuthor: PVC Arsenal 17 » Tue May 17, 2011 11:21 pm

How about 3000? Tech, would you suggest adding additional caps to a photoflash bank or should the single one be enough?
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Unread postAuthor: saefroch » Wed May 18, 2011 2:25 pm

That depends on the dimensions of the wire. Most likely, yes.

Does anyone know if the filament used for igniting model rocket engines would be a suitable replacement?
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