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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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Ignition choices

Unread postAuthor: broken_system » Mon Nov 23, 2009 9:49 pm

Hey guys,
I posted a topic before about my idea for a fairly high rate of fire propane combustion cannon however I have run into a snag.

I checked the wiki and read about my readily available options for an ignition system and the only one that would be legal/simple to do would be the camera flash system. This poses the problem of that mandatory charge up period between shots.

My question for you guys is if you have heard or have used/designed a series of batteries and capacitors all connected to one sparker so that while say the first of a series of three capacitors can be discharged, the other two are ready for firing while the first charges, and so on.

I am aiming for a minimum cycle rate of about 8-10 shots per second.

Any ideas?
Thanks guys!
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Unread postAuthor: jmadden91 » Mon Nov 23, 2009 9:53 pm

Where are you from mate?
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Unread postAuthor: broken_system » Mon Nov 23, 2009 9:57 pm

I'm from Ontario, Canada so the stun gun ignition would be very hard to obtain/ wouldn't be legal, otherwise that would probably be my best bet.
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Unread postAuthor: jmadden91 » Mon Nov 23, 2009 10:05 pm

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

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Or check ultimatespudgun (our sponsor) and see if they ship their spark circuit to canada
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Unread postAuthor: broken_system » Mon Nov 23, 2009 10:13 pm

Wiked I'll check it out for sure thanks jmadden91!
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Unread postAuthor: Technician1002 » Mon Nov 23, 2009 10:25 pm

If you use an ignition coil with a camera flash, replacing the flash capacitor with a 300 volt mylar or polyester cap of about .5 MFD will work fine and drastically cut the cycle time. (CDI ignition) Cutting the cap from about 50 mfd to .5 mfd cuts the cycle time by 100.

Many electronic ignition modules do this internally.

http://www.bella-audioparts.com/capacitors.htm

0.47 at 630 volt are not too hard to find. They are sold for high performance audio applications such as speaker crossovers.
http://www.chokes.com/crossovers_explained.html

Their clarity at passing high frequencies with little loss also works for delivering a good spark current to a coil with little loss. :D
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Unread postAuthor: D_Hall » Mon Nov 23, 2009 10:47 pm

Vera is running a conceptually modified automotive ignition system that fires every 0.004 seconds. That fast enough for you?

Assuming so, there's your answer: An automotive ignition system.
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Simulation geek (GGDT / HGDT) and designer of Vera.
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Unread postAuthor: kjjohn » Mon Nov 23, 2009 10:48 pm

try a car ignition coil
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Unread postAuthor: kjjohn » Mon Nov 23, 2009 10:55 pm

For camera ignition, lowering the farad rating obviously increases charge time, but because of this, could one use a very high voltage capacitor with a very low farad rating and still have a short charge time? Or maybe the camera circuit would not be able to produce high enough voltges to charge an HV cap - from what I have experienced, they only reach around 450v max.
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Unread postAuthor: jimmy101 » Tue Nov 24, 2009 3:50 pm

There are a lot of ways to get the cycle rate you want. It all depends on how much work you want to do.

A battery piezo or "HV Supply" (gutted stun gun) are probably the cheapest, simplest and most reliable.

A photoflash board is probably going to be more trouble than it is worth. It can be cycled that fast if the cap is small enough (probably could just use the trigger cap and omit the 120uF cap).

A old Ford "buzz coil" setup would work if you really want ghetto/old school.
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Unread postAuthor: broken_system » Tue Nov 24, 2009 5:57 pm

Sweet thanks for the input guys!
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Unread postAuthor: Technician1002 » Tue Nov 24, 2009 8:30 pm

FYI, Disposable cameras charge to about 250-300 volts. To match the energy in a CDI car ignition, changing the cap and charging it the same will produce the desired result. And yes the cycle time is much faster. :D

In this online schematic, a 2 uF cap is used and charged to about 340 volts. A cap around 1/2 uF works OK to produce a spark like an electronic BBQ igniter. The 2 uF at about 300 volts will often spark over the coil tower to the primary connections if a spark plug wire is not inserted.

http://www.circuitdb.com/circuits/id/164

This other schematic I just found does use a cap in the range of a 1/2 uF.
Image

My first home kit built CDI I used on my first car was similar to this. I built one in the late 1970's.
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Unread postAuthor: SpudBlaster15 » Tue Nov 24, 2009 11:01 pm

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.
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Unread postAuthor: broken_system » Tue Nov 24, 2009 11:52 pm

That does seem to be my best bet. I would just need some way of timing the spark as to not set off the gas before it is sealed in the combustion chamber. I was thinking about blinking lights that you see on some signs and was wondering if maybe I can incorporate their timing devices into my ignition setup.

Withought going into too much detail I'll explain my firing process:
1) trigger pulled, gas releases while the trigger continues to travel to its final stage of discharging the ignition system.

2) expanding gas pushes a pressure controlled valve to open it and allow the gas to rush forward, while also pushing the bolt forward and sealing the round in the barrel.

3) when the round has fired and there is no more pressure from the expanding gas, the bolt and pressure controlled valve return to starting position via springs. Also during this stage the combustion chamber is vented.

4) the next round is pushed up via magazine, or dropped down through a hopper system and ready for the next pull of the trigger/firing sequence.

So really my biggest obstacle with the ignition system is reliability. My cannon will use propane as its fuel, and I would ideally like to be able to fire a decent amount of shots as to not be required to change the battery every so many shots.

My other idea was to not be reliant on batteries and use the push-click bbq lighters however I want to somehow lower the amount of force required to produce a spark, say using the aid of a spring, then returned by the bolt cycling forward. This would be fairly important to allow for a decent rate of fire.
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Unread postAuthor: jimmy101 » Wed Nov 25, 2009 12:03 pm

If you do decide for something like a CDI system you can probably down grade the output voltage a fair amount. That'll help a lot with preventing sparks where they are not wanted. The CDI is designed to ignite a ~10X mixture using an ~1mm gap. Since you probably won't be using this on a hybrid the spark voltage can be considerably less.

Still, a battery piezo or $15 gutted stun gun really is pretty much a no brainer.
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