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mini hybrid win!

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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Unread postAuthor: jackssmirkingrevenge » Mon Nov 15, 2010 7:25 am

The problem as you pointed out here and I discovered in practice with various prototypes is that for a small diameter chamber, the spark prefers to arc externally than across the gap.

One solution would be a plastic chamber, if sufficiently thick walled and small diameter it would probably take a high mix, especially if supported by the breech at the moment of firing, but I'm not too keen on picking out small bits of shrapnel from my vital organs because we all know what my "probably" is worth :roll:

One thing I gleaned from the pneumatic valveless cartridge prototypes is that I would rather have a cartridge ejecting semi-auto with good power than full auto and rather shite power, so tiny cartridges won't do anyway. I want to try a cartridge using the same seal I used for 4.5mm BBs but for airsoft calibre, to see if it will hold pressure for a 15x mix.
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Unread postAuthor: Insomniac » Mon Nov 15, 2010 7:52 am

To reduce external arcing, couldn't you use a plastic insulator, such as a short section of plastic pipe, glued in such a way that it forces the spark to jump a greater distance? It would not be bearing any structural load. I don't have time to make up a diagram but I'm sure you can figure out what I mean.
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Unread postAuthor: inonickname » Mon Nov 15, 2010 7:58 am

Also, ridge the side of the insulator to increase the distance it must arc (an arc will travel across the surface in preference to space, hence why the ceramic section of a sparkplug is ribbed). You just need your mix x times your spark gap in distance, plus a little for security.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 15, 2010 8:34 am

Hmmm... something like this? Maybe ribbed for extra pleasure, this could be done with a file spinning it in a drill...
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Unread postAuthor: inonickname » Mon Nov 15, 2010 8:48 am

Yeah, that'll be fine. Use a small round file and don't make the grooves too large or the current will jump the gap rather than going across the surface. A sparkplug would be a good guide.

You could probably save some time by using clear PVC or silicon tube as an insulator.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 15, 2010 9:27 am

In practical terms I think what I'd make is something like this, it's simple enough to coat the schrader in Araldite as insulation - question is, will a relatively thin layer suffice?
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Unread postAuthor: JDP12 » Mon Nov 15, 2010 10:04 am

Wasn't there a thread awhile back about the conductivity of epoxy?
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Unread postAuthor: SpudBlaster15 » Mon Nov 15, 2010 10:19 am

I've attempted to use epoxy alone as insulation in hybrid spark plugs, but the dielectric strength is insufficient to prevent arcing at even 3-4X mixes. A proper insulator is a must for reliable operation.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 15, 2010 11:40 am

JDP12 wrote:Wasn't there a thread awhile back about the conductivity of epoxy?


It was the hybrid cartridge thread, about the same issue :roll:

I've attempted to use epoxy alone as insulation in hybrid spark plugs, but the dielectric strength is insufficient to prevent arcing at even 3-4X mixes. A proper insulator is a must for reliable operation.


On the other hand, if you look atthe original subject of this thread and practically all my hybrid experiments, the only thing separating the schrader from the chamber is an epoxy plug... perhaps it's the thickness of the epoxy that does the trick?
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Unread postAuthor: ilovefire » Fri Nov 19, 2010 5:18 am

they i knocked this up a couple of days ago and thought you might be interested, its in 10mm ID aluminium (would have gone smaller but the burst disk holder wouldn't fit any smaller tube) whole thing is 9cm long and 1cm epoxy protrusion 4ml capacity and have tested it to 20x with an 8mm x 100cm barrel and has a little bit of a kick to it, downside is that iv made the spark gap to small for a piezo to work but its good with my 9v ignitor
sorry for the bad pic
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Unread postAuthor: Insomniac » Fri Nov 19, 2010 6:43 am

Don't you mean the gap's too large for a piezo?

Either way, it looks nice and clean. Going in any particular project or just an experiment?
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Unread postAuthor: ilovefire » Fri Nov 19, 2010 7:14 am

Insomniac wrote:Don't you mean the gap's too large for a piezo?


i think youll find that the smaller isnt always the better when it comes to the spark gap, i dont think the piezo has enough energy to ignite the mix with the spark gap as small as it is, it has only ignited with my high energy 9v powered ignitor (closest thing i can get to a taser in the AU, and it really gives you a jolt when you accidentally get yourself :lol: )

[/quote]Either way, it looks nice and clean. Going in any particular project or just an experiment?[/quote] :lol:

pretty much just an experiment, maybe if i get it to work with the piezo i might put it on a pistol grip and have it with interchangeable barrels, maybe even see if i can get it to pass 345ms with a long enough barrel and hear that satisfying crack :D
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 7:14 am

Insomniac wrote:Don't you mean the gap's too large for a piezo?


That's what I was thinking. Did you make it as per my last diagram above, if so any external arcing issues? Looks good, you should consider a blowback style shell ejecting action ;)

i think youll find that the smaller isnt always the better when it comes to the spark gap, i dont think the piezo has enough energy to ignite the mix with the spark gap as small as it is


In my experience with a spark gap that's practically nothing (literally you can get a sheet of paper between the electrode and the chamber wall) the highest mix you can ignite with a cigarette lighter piezo reliably is 16x, it will occasionally ignite 17x but less than half the time.
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Unread postAuthor: ilovefire » Fri Nov 19, 2010 7:21 am

jackssmirkingrevenge wrote:
Insomniac wrote:Don't you mean the gap's too large for a piezo?


That's what I was thinking.


well you would think it would be but when the gap is 1/10mm or less (just a guess but i would be surprised if it wasn't of similar size), well its stated above

[/quote]Did you make it as per my last diagram above, if so any external arcing issues? Looks good, you should consider a blowback style shell ejecting action ;)[/quote]
similar to the second last one but i have the wire the other way round, oh i would love to but im sure it will just get added to the pile of already unfinished projects i have spred all over my desk :roll:

edit: after i moded the spark gaps on my carts i could hardly see the spark, and i think this one is even smaller but i will have to do some more tests, maybe even go for a controlled 30x, so if i dont get back to you well.. :lol:
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Unread postAuthor: Insomniac » Fri Nov 19, 2010 7:52 am

Perhaps your particular piezo is just too shizenhousen to get the job done?
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