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Old 03-09-2012, 06:34 PM   #28 (permalink)
gone-ot walking...
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Typical 'minimum' spark ignition energy is 30 millijoules for a stoichiometric gasoline air-fuel mixture of 14.7:1 and 9-10:1 compression ratio.

Any inductive (coil) ignition system that provides that much energy will work fine.

However, having MORE than the minimum energy is OK with today's high-EGR engines, because the extra energy permits a LONGER (1,000 microsecond) "arc" duration to exist, AFTER the initial spark, which greatly improves "ignition probability"--the odds of igniting ALL the A/F-mixture that's swirling around within the combustion chamber--remember, EGR is not combustive!

But, it ALSO means the spark plug "electrode" AND ground terminal are FAR MORE LIKELY to get "eaten" away by physical metal loss...which is why noble metals are used; they're not as good electrical conductiors but their "hardness" to both heat and combustion mean they LAST far longer, but only "work" with high-energy discharge systems.
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