Dave is exactly right. When I have some time, I will
put some pictures of hot vs cold plugs so you can see
the shorter conduction path of the colder plug.
Mike
--- "Perron, Dave C" <[EMAIL PROTECTED]> wrote:
> Thanks, Mike. I experienced some raised eyebrows
> when I read this passage,
> too. A spark
> plug has just a (relatively) small copper wire down
> the center, which would
> act as just about the sole conduction path for heat
> for the plug, as
> compared with just more cylinder head. Where does
> this wire (which by the
> way is a lovely heat conductor) lead? To the spark
> plug wire. Which is NOT
> a good heat conduction path. The rest of the plug
> is typically iron or
> steel which doesn't conduct even as well as your
> aluminum cylinder head.
>
> So I have to say the plug likely conducts very
> little heat. The only heat
> conduction path that has any potential puts the heat
> right into your
> distributor, which is not where you want it to go.
> So, I'm going to make a
> stretch here and say the idea that your spark plug
> acts as a heat conductor
> is totally incorrect.
>
> What is more likely is that your spark plug sticks
> out into the combustion
> environment and so gets a lot of heat loading. This
> plug will eventually
> come up to an equilibrium temperature that is a
> function of its thermal
> resistance to the prime thermal sink in that part of
> the engine, which is
> the cylinder head (and from there into the water
> jacket). So a "hotter"
> plug will likely have a higher thermal resistance
> and will have a higher
> equilibrium temperature. So the function of the
> plug is NOT to conduct heat
> out of the combustion chamber, but part of the
> function of the plug is to
> conduct heat out of ITSELF so that it operates at a
> temperature that is both
> efficient and not conducive to pre-ignition.
>
> This is just an engineer's assessment. As always I
> have to defer to Mike
> when it comes to how things REALLY work.
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