I was brought to the conclusion I stated through several opportunities to learn. Each from a different direction.
In Automotive trade school (Vincennes University I am a Hoosier by birth), we were taught on Sun Diagnostic scopes. On a non race passenger car, the spark lines initial peak voltage (plug gap jumping point) is highest at idle, maybe 12 to 14kv. If the throttle was flipped open, the breakdown voltage of the gap dropped to maybe 8k and as the engine revved from idle speed to the "Danger do not rev an unloaded engine higher" point the spark line proceeded to climb back to high values, near idle but not quite. As the throttle was closed some (still racing at high RPM) to avoid over speeding the engine, the vacuum readings went up as the swept volume was restricted by the reduced throttle. If the throttle was released, the spark line jumped a little over idle range, then settled back to the 13kv area. I"ve watched this happen on every car/tractor truck engine I"ve had on a scope. We were told/informed that the reason was that at high vacuum the spark had little in the way of ionizable gasses between the electrodes, and that a vacuum was a great insulator. As the vacuum dropped (opening the throttle) the air/fuel in the cylinder went from 20" of Hg, to atmospheric, to pressure in tenths of a second. The result was more molecules of hydrocarbon, oxygen, and nitrogen between the electrodes. These molecules were far closer together and much hotter than idle rare air. (Compression Heating) and as such they were conductors (willing or not) for an arc initiation, stripping from them multitudes of electrons, and turning them into a plasma. Plasmas are very conductive, and the plug fires multiple times across this plasma bridge spraying heat and ignited atoms into the combustion chamber igniting a flame front which then travels outward to the quench boundaries of the chamber where cold metal squelches the flame.
The need for higher voltage ignition, and longer duration spark discharge lines on a race engine has to do with four factors:
Arc blow under turbulent extremes in a high compression engine at the top of its RPM range can literally make the ionization path blow out of the gap.
Probable use of wider gapped plugs to present longer arc length to the fuel.
Far more difficult fuel to burn. Alcohol and high octane fuels are much harder to ignite from anti-knock additives or chemistry.
And the shortening of the time available to get the fire started (extreme engine RPM)
A hot high voltage snap across the gap initiates ignition more reliably than a puny wisp of yellow.
That is the essence of my belief in this techno topic. Thanks for asking. Jim
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