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Newbie combustion cannon problem(s)

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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Newbie combustion cannon problem(s)

Unread postAuthor: Quantumbit » Sat May 18, 2013 2:58 am

Hi

So... This is my third combustion cannon, because the first one wasn't very powerful, and the second one was made of too thin metal, so it became buckled, this one has a thicker metal combustion chamber and (I hope) it should be solid enough. However, I seem to have a lot of problems in getting the gas to ignite:

The chamber volume is about 600ml, so I theorise that 20ml of butane should be about ideal.

I'm using a spark plug at the back of the chamber to ignite it.

I've tried both butane and propane, in different mixtures (10-30ml)
I've used different sources of electricity for the spark incase the spark was too weak
I've tried injecting the gas into different places (Seems to make a difference) E.G. in the end of the barrel instead of in the back of the chamber
And yet, after so many attempts it has only ignited twice, which produces flames about 1 meter long, so quite effective... When it ignites :P


I thought that maybe the mixture is wrong, but I've tried to mix the gas with air by adding 20ml of gas to a syringe, and pulling the plunger out fully to fill it with air (100ml syringe) I figure this should mix it sufficiently?

I also tried to inject the gas directly without mixing, which didn't work ofcourse

Unfortunately there is no room for a chamber fan as the neck of the bottle (that I used for the chamber) is a 3/4" female thread, so not really any room :P

The temperature is well above -0.5C, so that shouldn't be a problem

The black thing pictured is a pump to remove the combustion gases or whatever its called

Yellow thing is butane canister

Thing with red wires attached is transformer, (mains transformer)
Circuit to the right is a camera flash circuit (I discharge camera flash cap into mains transformer to produce quite a high voltage pulse on the secondary)

Black tube at the right should connect to butane canister

Valves and syringe are at the other side of the gun

Spark plug is visible on the left



Any ideas how I can make it work better?

P.S. I know that the chamber is a little too big compared to the barrel, but I wanted to make it fire bullet type objects rather than large things.

P.S.S Sorry about the blurred picture

Thanks in advance, and for your time spent reading this XD
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Last edited by Quantumbit on Sat May 18, 2013 4:58 am, edited 1 time in total.

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Re: Newbie combustion cannon problem(s)

Unread postAuthor: jackssmirkingrevenge » Sat May 18, 2013 4:52 am

Quantumbit wrote:I've tried both butane and propane, in different mixtures (10-30%)


Far too much mate.

[youtube]http://www.youtube.com/watch?v=EoVDuh3ziNg[/youtube]
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Re: Newbie combustion cannon problem(s)

Unread postAuthor: Quantumbit » Sat May 18, 2013 4:59 am

jackssmirkingrevenge wrote:
Quantumbit wrote:I've tried both butane and propane, in different mixtures (10-30%)


Far too much mate.

[youtube]http://www.youtube.com/watch?v=EoVDuh3ziNg[/youtube]


*Facepalm*

Sorry I meant 10-30ml XD
I have edited it now
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Unread postAuthor: jackssmirkingrevenge » Sat May 18, 2013 5:14 am

An actual spark plug is probably not a good idea, try a 1/4" or so spark gap.
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Unread postAuthor: Quantumbit » Sat May 18, 2013 5:19 am

jackssmirkingrevenge wrote:An actual spark plug is probably not a good idea, try a 1/4" or so spark gap.



Thanks. Will try it :)
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Unread postAuthor: Quantumbit » Sun May 19, 2013 12:20 pm

jackssmirkingrevenge wrote:An actual spark plug is probably not a good idea, try a 1/4" or so spark gap.


Problem solved, it now ignites almost every time!

Thanks again
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Unread postAuthor: dewey-1 » Sun May 19, 2013 1:10 pm

Care to indulge on your solution so other newbies can learn?

To many people answer solved to their own problems without a giving a relative answer. This is the main purpose of forums like this!
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Unread postAuthor: jackssmirkingrevenge » Mon May 20, 2013 12:50 am

I'm assuming he widened the spark gap :)
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Unread postAuthor: Quantumbit » Mon May 20, 2013 12:54 pm

dewey-1 wrote:Care to indulge on your solution so other newbies can learn?

To many people answer solved to their own problems without a giving a relative answer. This is the main purpose of forums like this!



Yes, thanks for pointng that out, I shall do so from now.

I widened the spark gap across the spark plug from around <2mm to around 5mm, and replaced the transformer ignition with a piezo igniter.

The conclusion of which is that the <2mm hot spark is less effective than the 5mm blue (Lower current) spark - contrary to what I had expected
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Unread postAuthor: jackssmirkingrevenge » Mon May 20, 2013 1:01 pm

Remember that an automotive spark plug is designed to ignite a compressed mixture of fuel and air, something like 5 times atmospheric pressure is a typical figure if memory serves me well, meaning that within a given gap, there are a lot more fuel and air molecules for the spark to interact with.
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