it wont fire!

[jackssmirkingrevenge]

You do know this story. I've just found the original topic on Spudtech, and a certain "fullmetaljacket" was one of those who responded.

Right, I had forgotten about that one.

Doesn't really matter. 1mm seems like a good spark gap to me.

In my experience, it does matter.

[psycix]

In my experience, it does matter.

Any arguments to back that up?
A spark ignites a mix, and short or long sparks don't perform differently, ignition is ignition. However, the shorter gap is more reliable.
The only thing I can imagine is when the fuel isn't mixed properly, the spark will need to cover more space in order to increase the chance to hit a flammable mix.

[jackssmirkingrevenge]

The only thing I can imagine is when the fuel isn't mixed properly, the spark will need to cover more space in order to increase the chance to hit a flammable mix.

I think that's the logic, it seems to be an accepted rule here that shorter spark gaps don't ignite reliably expect for hybrids, and from what few combustions I've made, I found it to be true.

[jimmy101]

I've also noticed that a too small spark can be a problem. My long necked popper is particularly sensitive to the length of the spark.

I don't think it has anything to do with the spark having to "find" a combustible mixture. I suspect it has to do with the spark energy as a function of the spark gap width. A small gap breaks down to soon and gives a lower energy spark (which may last longer). A larger gap doesn't break down until the piezo has reached a higher voltage.

I believe typical piezos are pretty marginal in terms of how much energy they produce. It takes perhaps 0.5mJ to ignite propane in and air. Typical piezos don't generate all that much more energy than that.

[psycix]

Ah ok, but that only applies to piezos.
Kinda comparable to piston diameter, a smaller piston diameter (but same porting) opens later, but when it opens it opens faster.
The larger gap breaks down later, but when it breaks down there is a larger amount of energy available, causing a bigger pulse.


So when using stungun ignition (or camera flash or any other similar thing) the gap should be as short as possible, but with piezo's you will need a significant gap?
Instead of a longer gap you could always use multiple gaps in series.

[baardkaas]

yaay! thnx for the help
i did to much in it. even when i tought i was spraying too little.. THNX!