Multiple ignition points?

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
speedster
Private
Private
United States of America
Posts: 4
Joined: Mon Oct 30, 2017 6:10 pm

Sun Jun 14, 2020 6:15 pm

Im thinking about making a spark strip, but I was just wondering if there is really any evidence to support that miltiple ignition points are better than one? If I do decide to only do one ignition point, is it best to have it in the middle, or back of the chamber? I understand the theory behind multiple ignition points, and putting the ignition point in the middle if there is only one, but I was just wondering if there is evidence for these. Thanks in advance!
User avatar
Moonbogg
Staff Sergeant
Staff Sergeant
Posts: 1593
Joined: Mon Oct 13, 2008 10:20 pm
Location: SoCal
Has thanked: 38 times
Been thanked: 17 times

Mon Jun 15, 2020 11:23 pm

I saw some chrony data that Latke did showing an average velocity with 2 sparks at about 360fps. With 3 sparks it was about 390fps. I haven't seen data between 1 spark and 2 sparks. The thread is around here somewhere, but it's old and I don't want to spend 30 minutes searching for it to dig it up.

I believe the order of importance goes something like this:

1st: fuel/air ratio
2nd: fuel/air mixing
3rd: C:B ratio
4th: multiple spark gaps

If I was doing one spark only, I'd put it in the back so the flame can just move forward through the cannon. I used to think putting it in the middle was best, but I have my doubts now. If using two sparks, try to divide the chamber evenly. Run the fan while firing for maximum turbulence.
KannonKing
Private
Private
United States of America
Posts: 16
Joined: Fri Apr 24, 2020 10:12 am
Has thanked: 5 times
Been thanked: 8 times

Wed Jun 17, 2020 9:21 pm

http://www.burntlatke.com/strip.html

He is right about the veracity of the data, since his standard deviation from 10 shots is bonkers high due to the russet factor, but his data does bear out what we all know instinctively to be true, more sparks (well distributed down the chamber) = more higher initial increase in pressure at the projectile and thus more velocity, certainly for you high C:B ratio boys...

While we are wondering about number of sparks and where I have an untested suspicion that a sequenced sparker that fires first at the rear, then progressively fires multiple sparks down the chamber right ahead of the pressure wave would do something ungodly. Definitely not a pvc project...
User avatar
Moonbogg
Staff Sergeant
Staff Sergeant
Posts: 1593
Joined: Mon Oct 13, 2008 10:20 pm
Location: SoCal
Has thanked: 38 times
Been thanked: 17 times

Thu Jun 18, 2020 12:03 am

I wonder if the benefit of 2 gaps vs 1 is greater than the benefit of 3 vs 2. I'd imagine diminishing returns would be kicking in at 3 gaps. As you mentioned above, the variance in velocity from potato to potato is often very significant anyway. It might make any real benefit of additional gaps kind of get lost in that variance. But, I'd rather have that baseline advantage from at least 2 sparks myself.
Regarding your rapidly sequential firing idea; I'm wondering how that would compare to simply using multiple sparks and letting them all go at once. I don't think it would do much. Even if you triggered a DDT event somehow, I don't think it would be very catastrophic. Maybe a pressure spike of 1000psi or something like that. But yeah, bad for any cannon really, but especially PVC if that happens.

I'm under the impression that DDT is actually pretty difficult to bring about on purpose using fuel/air mixtures like we use around here. I've read that using pure 02 increases the burn rate so much that DDT becomes more likely to occur, but even then it seems to be a rare thing. I guess it takes place most often in long tubes, but has been known to happen even in open-air environments with sufficient obstacles around which forces the flame front to increase in surface area and burn faster, sometimes leading to a detonation. Wish I understood it, but I'm not about to go get a PHD just to build a safer spud gun.
Post Reply