Barnyard BOb -- wrote:

In SOME, but not all engines...
The hot spark causes the cold moist air WITHOUT FUEL
in the cylinder to CONDENSE across the electrodes as
ice... if it's 32 or colder.

Why? How?

Bill

Decades ago, when I first grabbed an ice cold mug out of the
refrigerator for a brewski....a coat of ice formed almost instantly.
Ambient heat and humidity at work on the frozen mug, right?

Since then, I have repeated the experiment many times.
Might work with Chardonnay, too. Check it out. g

Apparently, the same phenomena occurs inside some
engines when the heat of an intense spark meets the
freezing cold spark plug and moisture rich air in the cylinder.

Make sense?
If so, hoist one fer me.

If not....
hoist one fer me.


Unka' BOb --


Bob, your theory is all wet (frosted?). It isn't the spark + moist
air that causes a frosted plug. Rather, it is simply the water vapor
in the cylinder condensing on the subfreezing plug -- the spark plays
no direct role in the process. The water vapor in the cylinder can
come from air being pumped through the cylinder before the engine
fires (see 1) or, more commonly, from the combination of ambient water
vapor and water vapor liberated through combustion in a failed start
(see 2). Although the spark does play a role in the latter case, it
is a decidedly indirect one.

As for your frosty mug, when frost forms on a subfreezing mug it is
because the dew point of the ambient air is above the temperature of
the mug. When the ambient air meets the mug, the air is cooled below
its dew point and water vapor condenses out and freezes on the mug.
The only role your "ambient heat" plays in the process is enabling an
ambient dew point that is higher than the temperature of the mug. Of
course, the dew point of the ambient air does not necessarily have to
be higher than the temperature of the subfreezing mug, but often is.

David O -- http://www.AirplaneZone.com

1: For this case, the dew point of the air must be above the
temperature of the subfreezing plug for frost to form.

2: There is a lot of H2O liberated in the combustion of avgas, chiefly
through the reaction 2 C8H18 + 25 02 = 16 CO2 + 18 H2O.