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Old 04-20-2015, 10:47 PM   #25 (permalink)
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Originally Posted by Old Tele man View Post
...iridium is great for longivity, and the small "needle-point" electrode area enhances "arc/spark" creation.
To bump the thread and expand on this point a bit: the higher melting amd oxidation points of the iridium coating (and to a lesser extent platinum) allows a smaller diameter electrode - with copper, the extra heat from concentrating the spark like this would cause the electrode to erode.

The smaller diameter electrode has the desirable effect of concentrating the charge at the tip (see schoolphysics ::Welcome::). This increases the strength of the electric field at the electrode for the same electric potential (voltage), and just as with a lightning rod, this increased electric field strength causes the dielectric (in this case, air-fuel mixture) to break down at a lower electric potential.

AFAIK the main benefits of this for engine operation are
(1) More reliable timing - the spark is created at a lower voltage, so there's less uncertainty about what point in the voltage upswing the dielectric breaks down and the spark is formed.
(2) More stable spark - again, the higher electric field strength for a given voltage means the intensity and duration of the spark is more predictable.
(3) Potential for longer spark - the charge concentration effect also means in theory you could also use a longer spark gap and get the same electrical spark performance as a copper plug with a 'standard' gap - or trade off by using a slightly longer gap while still improving spark formation and stability slightly. Given the cost of 'precious metal' plus though (and the issues with re-gapping), I'm not particularly inclined to experiment with this aspect myself though .

One other advantage not related to the electric field is that the smaller area of the electrode means less heat is wicked away from the first bubble of burning fuel by the center electrode.

Bear in mind that even taken together, all these effects basically add up to slightly improved consistency of ignition timing, so the effects may not be huge. Also, other technologies like high energy ignition and coil-on-plug systems address similar issues. As with many things, there are real advantages, but YMMV
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