SpudFiles

Spark plugs for ignition - how many of you use them ?

I just realized that the spark plug is causing me some problems - I measured the volume, calculated the amount of butane needed and yet I get roughly 50% or less success rate... first I thought that my measurements might be a bit off, then I tried waiting for a minute or so to let the gasses mix better - that didn't help...

So I've just replaced the spark plug with a homemade spark gap (just an insulated lenght of wire and some fittings) and I suddenly get close to 90% success rate

WTF I am doing wrong ??
should I use a different spark plug ? how much should it protrude into the chamber??



Would I get better results with a longer spark plug (ie one that threads all the way into the fitting and protrudes more)?

Um sorry for the slight off-topic, but how do you keep it airtight/ok with pressure with a homemade spark gap?

Wire enters the chamber through a lenght of 4mm OD polyamid tubing, which in turn is forced into rubber tubbing of 6mm OD... that goes though a 6mm push to connect fitting

I improvised but so far it works great


People normally epoxy a lenght of wire through a reduction of some sort and thread that in the chamber

How much pressure can it hold? (either method)

And where does it spark to---unless you wire two wires in?

I'd love to use this on my hybrid if it's better than standard spark plugs.
Sorry for the nooby questions...

my method works ok for 6 bars - I haven't tried higher pressures yet

as far as epoxy goes jsr has used it on 28X hybrids IIRC so I guess it's more than you need... I am sure he can offer more info and advice on this stuff than me

Ah ok thanks. I think I'm just gonna use a normal spark plug, and hope it works

I haven't had a chance to try out my new spark plug yet so I can't comment on that. I used to use a spark gap on my first burst disk hybrid, which used volumetric metering, and that thing had a far from perfect ignition rate and I could never figure out why. Except for that one spark plug I tried a few months ago and couldn't get to work, I've never tried sparkplugs since using manometric metering so it's hard to say whether my change to manometric metering or change to homemade electrodes was the key to the higher ignition rates I get now.

The problem with homemade electrodes is first insulation and then fixing them in place. I find that epoxy struggles to secure thin electrodes (2-3mm in diameter) and they can end up turning and changing the distance of the spark gap. Even when at a state where they can be turned, they are still held in place and don't leak at 10x pre-ignition pressures.

My advice would be to make electrodes that, if the epoxy fails, can rotate without changing the spark gap distance and are protected from being bumped. I made the mistake of making little 'knobs' at the top of my electrodes to secure the electrode wires but if the cannon rolled on its side these knobs would give leverage that could break the epoxy seal and rotate the electrodes. If I had used electrodes that were a just a straight length of rod, there wouldn't be any leverage to rotate the electrodes if they were bumped or had forced applied to them.

what kind of spark plug did you use ? a standard one or a longer one ?


either mine did not protrude deep enough into the chamber or there was something else wrong with it

can the spark gap be too small ? I tried different gap spacing and I got it to arc across the electrodes but for some reason it didn't work 100% of times


then I try to recall a spark plug on any of the hybrids here and I realized I could even find one cannon that used them - so that got me thinking

I replaced the spark plug with a custom one and I now get consistent ignitions. I don't even have to be very precise when metering fuel (lol 100ml syringe). Well anyway it would be interesting to see what other ppl have to say on this

Try a more advanced spark stalk design!

Essentially a coaxial design - there are a few methods for ensuring a seal. DYI went with a threaded design where the coaxial pieces mate by common thread. I chose a somewhat simpler method and went with resin injection. You have to ensure near 100% injection or it won't seal and that makes it somewhat difficult - creating the seal and injection port.

Here's my simple 1 spark gap that I used to test the concept of fabrication.



I later made a 5 spark stalk that places each ignition point at a spheroid vertex location along the combustion cylinder.

It's about 15" long - so injection was even harder - but with patience you'll get it.



It's been working great and never a misfire. I used silicone HV wire and it's even held up to no-burst situations where the combustion chamber get's insanely hot for a relatively extremely long period.

It was a concept I needed to test before I went to higher pressure pre-ignition concepts because I hate the idea of having to reload an ignition source.

VERA uses a plane ol' sparkplug.

How many, just one?

PaperNinja wrote:How many, just one?

Ten.

POLAND_SPUD wrote:what kind of spark plug did you use ? a standard one or a longer one ?

either mine did not protrude deep enough into the chamber or there was something else wrong with it

can the spark gap be too small ? I tried different gap spacing and I got it to arc across the electrodes but for some reason it didn't work 100% of times

I think I have a longer one but I got it to protrude far in to the chamber by epoxying it inside a bushing from the bottom rather than the top.

I'm still yet to test it as my stungun (which looks familiar to MC's) stopped working last week and hasn't worked since. Since I got someone like sgort87 or BCPneumatics to send it over to me from the U.S. a few years ago, I haven't been able to find a replacement yet. Maybe I need to look at ignition coils and flyback transformers.

oh the way you wrote it seemed like it was one, or few which I thought was kinda weird

Spark gap needs to be tiny, tiny tiny.

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So small.