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Ignition

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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Ignition

Unread postAuthor: Isomer » Sat Jan 05, 2008 2:08 am

My question is mainly about the spark gap "contacts". Is the best way to setup a spark gap to drill two holes and insert two conducters (im thinking steel wire?) that are insulated? My strategy is to make two "schrader valve" spark plugs (from SB15 i think) and tap them into my chamber very close together?
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Unread postAuthor: BigGrib » Sat Jan 05, 2008 5:48 am

well apparently you are making a combustion, what kind of material are you using? as far as the contacts i use #6 x 2" machine screws taped in 2 inches apart around the circumference which brings them pretty close to 1/4 inch gap. and what the hell are you talking about "schrader valve" spark plugs that makes no damn sense to me. anyone else confused by that?? just drill your screws in and hook your igniter wires to it and bingo there ya go. please search the forum next time too
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Unread postAuthor: spanerman » Sat Jan 05, 2008 6:19 am

if your chamber is metal you can use 1 electrode and spark to the chamber wall....make sure its well insulated or you could weld a spark plug in there
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Unread postAuthor: mark.f » Sat Jan 05, 2008 11:18 am

BigGrib, please study a little about what the user is posting about before flaming them.

A schrader spark-plug would work fairly well. The threaded inside presents a fairly large surface area, so filling the interior with insulative epoxy around the contact, would work well. If you're using a metal cannon, (or even if you're not), just hook one electrode to the center contact, and another electrode to the brass body, or the cannon body if it's conductive. If you're using high-frequency AC for ignition, (not to common, but it's an option), insulate your cannon body or make sure not to touch it. :wink:

Also, one of my ideas for a spark plug was to take a 1/4"-20 nylon machine screw, (look at Home Depot and you should be able to find them), and drill a hole through the center and tap to #6-32 threads. Thread a #6-32 brass machine screw through and secure with nuts. Then, tap a hole in your chamber to 1/4"-20 threads and thread into place with a small washer and gasket.
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Unread postAuthor: Isomer » Sat Jan 05, 2008 4:30 pm

Not to be mean but seeing as how this is the hybrid cannon forums, i insinuated that everyone knew it would be a hybrid. Also, the cannon im planning will have a steel chamber so the conductive chamber + spark plug idea you posted (markfh11q) would work great! thanks a ton! Just to make sure i have it: I could make one (1) spark plug, tap it into the chamber, connect one contact to the spark plug and one to the conductive chamber? Will this ignite high mixes (up to 8x; also im planning on using a transformer flyback of some sort to power the circuit-if that makes sense) and if it works as i said ^above^ this design would rid me of the spark gap dilema, correct?
**On a side note: yes, i know it seems quite easy to flame someone with a 3-post count but please at least be open-minded that not everyone starts out on spudfiles as a ball valve noob (nothing against ball valves, i love them but they are the simplest valve for a pneumatic cannon so i thought it would be a good example)
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Unread postAuthor: Isomer » Sun Jan 06, 2008 12:17 am

Another ignition question:
Is it best to ignite the chamber from the back, middle, or front?
Simple as that
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Unread postAuthor: mark.f » Sun Jan 06, 2008 12:23 am

As the air density becomes greater, the resistance increases. It is generally stated that this makes the effective spark distance inversely proportional to density.

So, if you had an ignition source which could create a maximum cold spark of 1" in air, in those same conditions, at 2 atmospheres, you could get a maximum cold spark of 1/2", for 3 atmospheres pressure, you could get a maximum of 1/3", 4 atmospheres > 1/4" maximum spark, etc.

So, if you use a gap of around 1/16" in your hybrid, and you wanted to use a maximum mixture of 4:1 compression in your hybrid, then 1/16 * 4 = 1/4", your ignition source would have to push a cold spark of 1/4" at atmospheric pressure. A flyback generally creates in the neighborhood of 30 kV, so you should be good there.

EDIT: Flame fronts propagate faster in spherical mode, but it would take some tricky manipulation to get a spark in the middle of the chamber. So, halfway down the chamber is a good position for your situation.
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Last edited by mark.f on Sun Jan 06, 2008 12:26 am, edited 1 time in total.
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Unread postAuthor: benstern » Sun Jan 06, 2008 12:26 am

Can schrader valves handle the pressure?
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Unread postAuthor: mark.f » Sun Jan 06, 2008 12:28 am

Ben, it's only the Schrader body which is taking the pressure. There's no valve core to lower the working limit of the valve, so you're only putting a rating on the brass body, which is quite thick. Any type of pressure rating on a Schrader valve bought off an industrial supply is mostly for the benefit of the weaker valve core.
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Unread postAuthor: Isomer » Sun Jan 06, 2008 12:29 am

umm, sorry mark, i couldn't decipher that text front, middle, or back
LOL
@ Bernstein: im not literally talking about a schrader valve but a modified onee with inards removed and a steel wire w/ insulation in its place
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Unread postAuthor: Novacastrian » Sun Jan 06, 2008 1:11 am

benstern wrote:Can schrader valves handle the pressure?


Yes they can, i use schrader valves on both my Hybrids :)
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