Painting and air cooled motor?

[CitizenX]

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.

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

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

[jls]

"CitizenX" > wrote in message
. 170.85...
> 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.
>
> the other popular school of thought is that unless it is an insulated
paint
> then it will not affect the cooling of the motor. 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.
>
> 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

SAE data shows a light coat of flat black paint enhances cooling. Check it
out in Smoky Yunick's book or the typical racecar books.

[x]

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

[Ebby]

I see several engine/cylinder overhaul outfits offer the option of paint or
cadmium plate on cylinder barrels. There is something about a freshly
painted aircraft engine that stirs the senses. But I have a gut feeling
cadmium plating offers the corrosion resistance needed with minimal
insulating effect and therefore would be the preferred method of
anticorrosion.

Ebby

[Gordon Arnaut]

Nicely done, Mr. X.

A well-developed, factually-supported argument that shed some light on an
interesting question.

Regards,

Gordon Arnaut
Ontario, Canada


"x" > wrote in message news:AYiEc.130630$HG.50890@attbi_s53...
> 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
>
>

[Brian Whatcott]

On Tue, 29 Jun 2004 11:53:14 -0400, " jls" >
wrote:

>
>"CitizenX" > wrote in message
. 170.85...
>> 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.
>>
>> the other popular school of thought is that unless it is an insulated
>paint
>> then it will not affect the cooling of the motor. 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.
>>
>> 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
>
>SAE data shows a light coat of flat black paint enhances cooling. Check it
>out in Smoky Yunick's book or the typical racecar books.
>

Well, a thin, dark coat anyway! :-)

Brian W