Hi Michel.
You wrote:
< But Platinum is a bad electrical conductor, almost
5 times worse than copper. And on some plugs the electrode is thinner
resulting in lower spark current. So the spark energy is lower than
a normal copper plug. I think that this is why some car manufacturers
are specifying larger gaps.>
The electrical conductivity of the electrode material is almost completely
irrelevant in spark plugs. Resistor type plugs are built with an internal
resistance of 10,000 ohms (typically) so the odd few milliohms difference
caused by the electrode material is just not an issue.
The thermal conductivity is a much more significant factor. The spark plug
is designed so that the thermal resistance from the electrode tip to the
threaded barrel (where the heat is transferred to the watercooled cylinder
head) is sufficient to keep the spark plug electrode tips at no greater
than 800 deg C, and no lower than about 500 deg C. The electrode tip is
right out in the mddle of the petrol explosion and heated every power
stroke.
if the thermal resistance is too high - ‘hot’ plug - the heat won’t get
away quickly enough (like a bath with a small plug hole) and it will get
too hot and either a) Melt, b) erode very quickly, c) crack the ceramic
insulator nose or d) cause pre-ignition (dieseling). All these effects are
potentially fatal for an engine. (The bath analogy is that it will
overflow if you leave the taps running).
If the thermal resitance is too low - ‘cold’ plug - the heat gets away too
fast (like a bath wit a HUGE plug hole) and it will not get hot enough and
simply get ouiled and sooted up and eventually stop working. (The bath
analogy is that it will never get deep enough for a bath even with the taps
running)
Plugs keep clean by running at temperatures high enough for deposits, oil
etc., to burn off - that’s why they are using Alumina ceramic insulators
and hard alloys.
Thermal resistance is controlled by both material and geometry. Long
copper cored electrodes have the same thermal resistance as shorter
Nichrome electrodes. Long fat electrodes have the same thermal resistance
as short thin electrodes (this is the typical Platinum form) and so on.
Copper cored electrodes have no particular advantage, the thermal
characteristics are identical to any other type. Platinum erodes much less
at high temperatures, but have the particular disadvantage that they have a
catalytic action and are more prone to pre-ignition. For this reason they
tend to be run colder and with a domed insulator construction that is less
prone to fouling.
Gap geometry is to permit a reliable spark available with modern ignition
components and systems, the actual spark plug voltage is set by the plug
gap, any excess voltage is just lost in the ignition system and plug
internal resistor (like a bath not being fillable over the level of it’s
sides). If the gap is too big, the plug simply flashes down the outside
surface of the insulator. Modern low/non lead fuels mean lower compression
ratio’s, and therefore lower cylinder pressures. This allows larger gaps
to be used without the danger of flashover. Engines will work perfectly
well with a 20 thou gap or a 10 thou gap, all it does is sets off a flame
front, once the flame has started the spark is quenched and the spark
current short circuited by the ionized gas in the high temperature flame.
Just a few pence worth of self opinionated humbug, hope it’s interesting.
Chris Dupr�s
'88 Sovereign.
Surrey, UK.
