A aviation & planes forum. AviationBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » AviationBanter forum » rec.aviation newsgroups » Home Built
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Painting and air cooled motor?



 
 
Thread Tools Display Modes
  #1  
Old June 29th 04, 03:02 PM
CitizenX
external usenet poster
 
Posts: n/a
Default Painting and air cooled motor?

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
  #2  
Old June 29th 04, 04:53 PM
jls
external usenet poster
 
Posts: n/a
Default


"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.


  #3  
Old June 29th 04, 07:57 PM
x
external usenet poster
 
Posts: n/a
Default

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



  #4  
Old June 30th 04, 02:27 AM
Ebby
external usenet poster
 
Posts: n/a
Default

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


  #5  
Old June 30th 04, 02:43 AM
Gordon Arnaut
external usenet poster
 
Posts: n/a
Default

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





  #6  
Old June 30th 04, 12:47 PM
Brian Whatcott
external usenet poster
 
Posts: n/a
Default

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
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT +1. The time now is 10:21 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AviationBanter.
The comments are property of their posters.