If you want, go to yahoo or google and type in "heat transfer conduction
radiation convection" and look at the sites for universities that come up
(the ones that end in .edu).

The discussion/disagreement on the painting of air cooled motors has come
up in a motorcycle discussion group and I am interested in knowing what
the
pros and cons are.

IMHO the paint on the surface of an air cooled motor is making a barrier
between the metal and the air thus delaying the transfer of heat.


Heat conductivity is a measure of how well heat transfers through a
material. Metals have very high heat conductivity. Paint has a lower heat
conductivity than metal, but air has a very poor heat conductivity in
comparison to both of them.

Heat is transferred from place to place by conduction (direct contact),
radiation (light of any wavelength, not just IR), and convection (not really
a different process, but a combination of the first two - when heat is
transferred to a gas or liquid by conduction or radiation, the gas or liquid
can then circulate away from the object and be replaced by cooler gas or
liquid. I think radiation is more important than conduction here, but I'm
not sure).

So lets look at each type of heat transfer in the case of painted vs.
unpainted metal to air.

CONDUCTION:
The conduction of heat from metal to air is poor.
The conduction of heat from metal to paint is not great, but it is better
than metal to air.
The conduction of heat from paint to air is poor.

Both the metal and the paint can give up heat (by conduction) faster than
the air can take it, so the conductive transfer is the same in the painted
and unpainted cases. You were correct to think that the paint transfers
heat more slowly, but the limiting factor is the air, so the low heat
conductivity of the paint is made unimportant by the much lower heat
conductivity of the air.

RADIATION:
The radiation of heat to and from dark materials is high.
The radiation of heat to and from light materials is low.

For conduction, it mattered what the material being conducted to was. For
radiation, it doesn't matter where the radiation is going, it just matters
what the color (really, the emissivity) of the hot object is. So painting
the engine black will increase the amount of heat radiated, as long as the
conductive heat transfer from the metal to the paint can keep up. Painting
the engine white would actually decrease the amount of heat radiated.
Polishing the fins to a mirror finish will reduce the radiation even more.

CONVECTION:
Air that has been heated by either conduction or radiation won't pick up any
more heat when it's at the same temperature as the metal. If that air is
moved away and cooler air brought in, then conduction and radiation can dump
more heat.

So, painting the engine black doesn't hurt conduction, but it increases
radiation, which then increases convection.

the other popular school of thought is that unless it is an insulated
paint
then it will not affect the cooling of the motor.

That's a red herring. The fact that insulated paint exists doesn't bear on
our problem at all.

Also since it is a high
temp engine paint then it may assist the cooling acting similar to thermal
paste on CPU's and other such electronics.

Another red herring. Thermal paste has nothing to do with painting an
engine.

Any input on either side of the disagreement? I assume there to be several
engineers in this group that could lend insightful data as well


N8