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Old 04-22-2003, 08:34 PM
psfred psfred is offline
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
There are several "types" of spark produced by spark plugs, and while I'm sure I don't understand all the physics involved, the different types (gap bridging, surface, and mixed) can all ignite an air fuel mixture. Most jet engine ignition systems use surface spark -- the electrodes are flush with a ceramic surface -- for the simple reason that they would probably burn off otherwise. In automotive use, they would also tend to foul in a manner not very conducive to self cleaning ( which mostly means, I think, that the fouling material will fracture off nicely with temperature changes).

Platinum has a much lower (or higher, I don't remember) specific ionization, meaning that a spark will form much more easily and reliably on the platinum compared to any other metal. It is also one of the very few metals that does not "plate off" from the formation of the spark (a funtion of the ionization potential being what it is). Doesn't take much for it to work it's magic, even a fairly thin coating or wire will last much longer than the ground electrode.

Multiple electrode plugs DO work better, in that a usable spark can be obtained under harsher condtions than a single ground electrode, and they do not require indexing in engines senstive to it. Plugs with surface spark design (always in combination with gap bridging spark in automotive use) are much less suseptible to "blowout", and can self clean carbon fouling better, since the spark path gets so hot the carbon burns off.

Fouling is more complicated, since it is deposit of either insulating, or worse, conductive, materials on and around the electrodes. Zinc oxide from the zinc stearate in lubricating oil will cover the electrodes well enough to prevent mixture ignition, even though there is sufficient spark, and at high load with overly lean mixture can even "glaze". Lead oxides would melt, shorting out the plug until the engine cooled off and the salts re-cyrstalized and stopped conducting.

Carbon fouling is always a result of over-rich mixture or ignition breakdown, it's not "normal" for any gasoline engine to carbon foul plugs of the correct heat range.

Of course, the response of any particular engine to any particular plug isn't fully predictable -- what works in one engine/fuel/oil/condition/driving condition combination may and may not work well in another.

If any engine fouls plugs, the source of the fouling probably needs to be fixed -- modern engines simple burn so clean fouling is almost always an indication of wear of a malfuntion. Leaking valve guide seals can ruin plugs, for instance, and it doesn't matter what type you use, the fouling will be different, but still there.

Oh, and Bosch plugs have a reputation (well deserved, I think) of not clearing from carbon fouling very well, it at all. I have no explaination, but it took about six weeks of drving to get the 280 cleaned up and running well.....

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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