Posted by Steve@Advance on October 27, 2020 at 20:54:32 from (66.169.147.211):
A few posts down, Flying Belgian asked:
"Why would a plug fail with more compression? It's just a gap between a positive and ground electrode."
The question got danced around, Pete 23 gave some answers, but no one ever got down to the real "why" this happens.
There is no doubt it happens. A weak spark will idle, but fail under increased throttle, which means more cylinder pressure somehow cancels the spark.
The old spark plug testers proved the same, with a known good spark, put in a good plug, it would continue to arc as the regulated air pressure was increased. Put in a worn plug and the arc would fail under pressure.
So, lets try to figure out WHY this happens!
It has to be strictly pressure related, because it happens with the plug tester, which only introduces air, no fuel, no heat.
I tried searching for an answer, really didn't find any good answers, just verification that it happens.
So I went the other way, "Can an electric arc occur in a vacuum?"
Most of the answers went over my head, but the general response was it is more difficult to establish an arc in a vacuum. My reasoning, the presence of "nothing" has the highest possible resistance.
So, wouldn't the opposite apply, increasing the pressure would reduce the resistance making the arc easier to establish? Wouldn't the various gasses in air, along with some moisture, have "just a little" conductivity? Cram a lot in the gap, shouldn't it be easier to jump the same distance?
Remember, we are only dealing with air pressure, just as in the tester. Yes, there is fuel present, and heat, no fire yet...
Some of what I observed with the plug tester, a dirty partially fouled plug would begin arcing down the inside of the insulator instead of jumping the gap. I blame that on increased voltage looking for a place to go. But again, why? Why did increasing pressure make the gap more difficult to jump to the point the charge bled off through impurities stuck to the insulator?
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