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Ignition choices

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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Unread postAuthor: broken_system » Wed Nov 25, 2009 3:49 pm

I wonder if it's possible to open up a piezoelectric clicker, steal the crystal, then make a trigger system that releases a spring-tensioned hammer to strike the crystal.

This would make the initial trigger pull easy then I could harness some of the expanding gas to reset the hammer, ready for the next trigger pull.

The only negative side of this setup would be heavy reliance on the one-time spark actually working and setting off the combustion which would reset the hammer. Otherwise I would have to include a way to manually reset the trigger incase of failure.

Anyone have any experience with piezoelectric crystals? Or for that matter piezoelectric igniters, and can give some input about whether or not you think my idea could work?
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Unread postAuthor: Coodude26 » Wed Nov 25, 2009 4:29 pm

Piezoelectrics are very weird, and are a somewhat precise science. It takes a fair bit of force to get a spark from them. While I'm sure some of the people on this forum are experienced enough to get it working in a tight little package like that, I'm not sure it's exactly worth it. I would think that a small electric switch wired to a tazer mounted somewhere else on the gun would be a good idea.

Also, PE sparkers start to wear down after 1000 sparks or so, but you can easily replace the batteries on a tazer.
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Unread postAuthor: Pookydarts » Wed Nov 25, 2009 4:48 pm

Keep the unit whole and actuate the button with a spring powered hammer... 8)
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Unread postAuthor: kjjohn » Wed Nov 25, 2009 5:21 pm

my advice - stay away from piezos. They're unreliable, can't produce much current, and are not good for rapid cycling. Use a stun gun, or if that isn't an option where you live, use an automotive ignition coil
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Unread postAuthor: broken_system » Wed Nov 25, 2009 6:53 pm

Kk thanks guys

@ Pookydarts: Yeah thats was i was trying to think of, have like a hammer that swings down to hit the crystal and produce the spart, then gets returned to an upright position when the combustion happens (by using some of the expanding gas to push a pin forward, driving the hammer back into position where it locks in place ready for the next shot).

Anyone familiar with the legality of ordering the parts to or making the icb & parts for a tazer in Ontario Canada?
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Unread postAuthor: broken_system » Wed Nov 25, 2009 11:39 pm

I've looked into a couple things and here's what I've come up with so far:

After some thought and some helpful peers on here, I think a spark plug/ignition coil system is my best bet. I did some searching around and have only found fairly large setups (meant for vehicles). Ideally I would like the whole ignition system to only be roughly a couple inches by a couple inches (so fairly small), and correct me if I am wrong but the systems for cards look pretty hefty.

So I started thinking about smaller sources, and at first thought of the ignition systems used in fuel-powered model rc planes, however they use a system that wouldn't work for me. Then I thought of small, engine powered things; such as a whipper snipper (or weed-whacker).

I came across this from a website on how a 2-stroke whipper snipper engine works:

"The motor is started by means of a pull cord, which forces the piston to the top of the cylinder, where the spark plug ignites the compressed fuel. This ignition pushes the piston to the bottom of the cylinder. This turns the crankshaft. Exhaust created by the ignition leaves out the exhaust port because of the greater pressure within the chamber than in the surrounding environment. This pressure imbalance doesn't equalize, but instead causes a vacuum to form within the combustion chamber. This vacuum pulls fuel in from the inlet, causing the reed valve to hinge inward and allowing more fuel from the carburetor to refill the reservoir sitting around the combustion chamber, waiting for the next stroke. As all this is happening, the momentum from the crankshaft turning causes the piston to rise back to the top of the cylinder, where the spark plug ignites again, causing the process to repeat itself."

My question is if this ignition system purely relies on input to produce the spark output (i.e the pull cord initially, then the revolutions of the piston), or does it contain a battery/spark generator that might be small enough for my purpose?
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Unread postAuthor: roboman » Thu Nov 26, 2009 12:11 am

It probably contains some sort of alternator that relies on the turning of the driveshaft. Unless there is a battery you have to charge, in order to make it work, it's most likely some an alternator.

So I started thinking about smaller sources, and at first thought of the ignition systems used in fuel-powered model rc planes, however they use a system that wouldn't work for me.


Why not? It seems to me that glow plugs would be perfect; low voltage input, high heat, reliable, cheap, small...Besides, if you threaded one into the rear of your chamber, you could just unscrew and replace it when it finally died.
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Unread postAuthor: broken_system » Thu Nov 26, 2009 12:21 am

True, I would just have to figure out a way of timing the glow plug somehow. From what I understand, glow plugs work by staying red hot and the gas/fuel ignites when it hits the appropriate temp. If I were trying to fire around 8-10 rounds a second (max), I wonder how I could time each explosion. Maybe these things work faster than I am imagining though.

I was also reading into CDI's, which apparently are what is used on small motors (chainsaws, etc). There are 2 types, AC-CDI which relies on an alternator, and DC-CDI which is battery powered.

The main problem I am reading about the battery powered DC-CDI (which would be my choice), is that the spark is far too short in duration to guarantee ignition, however it does fire very fast which could make the downside negligible. I just wonder how long a 9v, or series of them, would be able to keep firing (i.e how many shots am I going to get out of it before I'd need new batteries).
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Unread postAuthor: roboman » Thu Nov 26, 2009 12:52 am

8-10 rounds/second? I could see 8-10+ rounds/minute, but 8-10 RPS seems a little unreasonable for a combustion...

How do you plan to refill the chamber between shots?

I've never actually messed with glow plugs, so I'm not exactly sure how long they stay hot enough to ignite something, but I would imagine a creatively-placed fan would cool it off enough to refill the chamber without igniting the fuel/air mix.
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ilovetoblowthingsup wrote: The yellow/orange back fits super snugly, very nice in fact, but the head does not fit.
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Unread postAuthor: kjjohn » Thu Nov 26, 2009 1:11 am

Check out this site:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G17350

It's legal and I have a feeling it will solve a lot of your problems.
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Unread postAuthor: broken_system » Thu Nov 26, 2009 1:14 am

@ roboman- This is gonna be a smaller scale cannon, firing 3/4" projectiles. Ive drawn some plans, just ironing out details, then going to build a prototype.

@kjjohn- that is an awesome find!!! Thanks!!!! :D
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Unread postAuthor: broken_system » Thu Nov 26, 2009 1:27 am

Actually for that matter I could probably find a good deal on some battery powered bug zappers where I live, that way I would have all the connections for batteries, etc.

Anyone ever try using a bug zapper circuit as an igniter?
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Electric BBQ

Unread postAuthor: Spudnyk » Wed Jan 27, 2010 10:18 am

I used a electric bbq igniter and it works great very consistent east to wire runs off a AA battery
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Unread postAuthor: Spudnyk » Thu Jan 28, 2010 9:55 am

SpudBlaster15 wrote:
jmadden91 wrote:Yeh thats what I was going to suggest. Your next best option is probably an electronic BBQ igniter like this:

Image



I second this. I have a bunch of similar igniters that I bought at a department store for $5 each, and they work well in almost any combustion application I have tried. They produce about 10 sparks per second with a good battery, and are capable of igniting up to 10x hybrid mixes.

Unless you require very high voltages for a high mix hybrid, camera flash/ignition coil setups are overly complex and unnecessary.

I third this lol this is all I use and it runs off a AA batery
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Unread postAuthor: McCoytheLesser » Fri Jan 29, 2010 12:55 am

If your feeling froggy you could try the 555 timer Ignition circuit. It works and gets a decently long, hot arc BUT you need some sort of higher current source, like an R.C. battery or two. For simplicity's sake, I'd just stick to what everyone else is telling you and use either the electric BBQ ignitor or a stun gun (if it's legal).
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