I could see both sides in the discussion on painting engines. So I decided to look up the emissivity coefficient, how well it radiates heat, of paint.
For everyone not familiar, a "black body" both absorbs and radiates heat most efficiently. An "emissivity coefficient" of 1.0 is assigned to black bodies. Everything else will be less a less efficient radiator and will have a an ε value (emissivity coefficient) less than 1.0. Shiny surfaces are less efficient radiators than dull. Of course there are paints with special insulating properties that slowdown the transfer of heat energy, but we're not talking about those special cases.
Emissivity does vary some with temperature, but here are a couple of websites with emissivity coefficients for 300 degrees Kelvin, approximately 81 F:
roliv.com/emissivity-values-definition-and-coefficients
engineeringtoolbox.com/emissivity-coefficients
As you can see, the emissivity coefficients of aluminum varies from as low as .039 for highly polished aluminum up to .31 for heavily oxidized aluminum. Interestingly, anodized aluminum has a coefficient of .77, but that is a surface treatment.
Black paint has, as expected, has values in the .80 to .89 range, but I don't think anyone was planning on painting their engine black.
Bottom line - as long as a clear coat is as relatively transparent to infrared as it is to the visible spectrum; it should not cause any excess heat build-up over a bare engine.
Jim
paint emissivity coefficient
Re: paint emissivity coefficient
Hi Jim, very good info, too high tech for me :)!
I also see both sides.
I machine heat sinks for a military application. Some are machined and some are cast. According to tests, the cast is the best heat sink because the surface is rougher and has lots of pits, peeks and valleys, meaning more surface area. To keep the aluminum from oxidizing they clearcoat anodize the surface, I have never seen them painted.
My racing Go-Kart engine is as cast, no polishing, no coatings, but kept clean and free of any dirt and oxidation. None of the top teams coat the fins.
As far as my Honda Coupe engine I rebuilt, I cleaned it well, but did not paint it.
It does not look as nice as Bill's engine, it's discolored and kind of dirty looking, from baked on grease and oil. And Bill's engine will stay cleaner and look better for years.
If I was going to paint it, I would use a paint like Bill used.
High temp, which has lots of aluminum/metal particals in it. A thin coat so it doesn't fill in all the cast roughness. A thin coat will probably not affect heat transfer much, can it even be measured?
But, I still think the as cast, uncoated is a better heat sink.
But, then Bill will beat me at a car show.
:(
Lyle
I also see both sides.
I machine heat sinks for a military application. Some are machined and some are cast. According to tests, the cast is the best heat sink because the surface is rougher and has lots of pits, peeks and valleys, meaning more surface area. To keep the aluminum from oxidizing they clearcoat anodize the surface, I have never seen them painted.
My racing Go-Kart engine is as cast, no polishing, no coatings, but kept clean and free of any dirt and oxidation. None of the top teams coat the fins.
As far as my Honda Coupe engine I rebuilt, I cleaned it well, but did not paint it.
It does not look as nice as Bill's engine, it's discolored and kind of dirty looking, from baked on grease and oil. And Bill's engine will stay cleaner and look better for years.
If I was going to paint it, I would use a paint like Bill used.
High temp, which has lots of aluminum/metal particals in it. A thin coat so it doesn't fill in all the cast roughness. A thin coat will probably not affect heat transfer much, can it even be measured?
But, I still think the as cast, uncoated is a better heat sink.
But, then Bill will beat me at a car show.
:(
Lyle
--- In
2cylinderhondas@yahoogroups.com
, "usaf_res_ret" wrote:
> I could see both sides in the discussion on painting engines. So I
> decided to look up the emissivity coefficient, how well it radiates
> heat, of paint.
> For everyone not familiar, a "black body" both absorbs and radiates heat
> most efficiently. An "emissivity coefficient" of 1.0 is assigned to
> black bodies. Everything else will be less a less efficient radiator and
> will have a an ε value (emissivity coefficient) less than 1.0.
> Shiny surfaces are less efficient radiators than dull. Of course there
> are paints with special insulating properties that slowdown the transfer
> of heat energy, but we're not talking about those special cases.
> Emissivity does vary some with temperature, but here are a couple of
> websites with emissivity coefficients for 300 degrees Kelvin,
> approximately 81 F:
> roliv.com/emissivity-values-definition-and-coefficients
> engineeringtoolbox.com/emissivity-coefficients
> As you can see, the emissivity coefficients of aluminum varies from as
> low as .039 for highly polished aluminum up to .31 for heavily oxidized
> aluminum. Interestingly, anodized aluminum has a coefficient of .77, but
> that is a surface treatment.
> Black paint has, as expected, has values in the .80 to .89 range, but I
> don't think anyone was planning on painting their engine black.
> Bottom line - as long as a clear coat is as relatively transparent to
> infrared as it is to the visible spectrum; it should not cause any
> excess heat build-up over a bare engine.
> Jim