Re: Colors under the hood, longish.

[Don Hoefer <hoefer@eng.pko.dec.com>]

Martin Pajak states:

> Emissivity (Ratio of the radiation emitted by a surface to the 
> radiation emitted by a blackbody at the same temperature) is 
> dependent on the wavelength and the objects temperature.
> For example at 300K Solar emissivity of human skin is the same as a 
> black painted metallic surface (0.97). When comparing emissivity of 
> stainless steel @ 1000K the highly oxidized has a value of 0.67 
> compared to 0.23 for highly polished.  So the surface finish has a 
> great impact on it as well.  My book does not list different paints 
> at high temperatures if someone has access and can compare emissivity 
> for different colors it would probably show that flat black surfaces 
> have the best emissivity.

Correct.

Mr. Daniel P. Grady says:

> Painting things black to absorb/radiate heat sounds neat. But, once you
> close the hood there is no light so isn't everything black? Won't heat
> transfer depend more on the actual type of paint and thickness of paint?
> I'm really not a science guy so I just keep every thing clean and avoid
> chrome parts.

Well, err, correct but no.  Absence of light does not make things "black"
in the infrared, but rather the ability of the surfaces to convert heat
to invisible infrared radiation and visa versa.  To paraphrase what Martin
said, we are all black in the infrared.  Skin color makes no difference.
Paint is surprisingly similar.  Many paints (excepting aluminum paints)
have emissivities in the high .8s to the high .9s.  My book does not have
high temperatures either, but high temperatures will manifest their
effects on emissivity by degrading the material with time and changing the
optical characteristics of the surface (a high temperature difference within
or between materials will have a greater reflectivity than a low temperature
difference) impeding the radiation of heat.  Closing the hood adds one
more barrier to impede the flow of heat from your engine, slowing the
transfer of heat.  Convection via air flow is what cools the engine the most.

Obviously, the thicker the paint, the slower the heat will convect from the
base material underneath.  Paint is a good radiator but poor conductor of
heat.  Metal is a good conductor but poor radiator of heat.  

Matte black paint or lacquer has an emissivity of .97.  Gray paint has
the lowest emissivity, .87.  If you want to minimize the radiation of
heat from the exhaust manifold, chrome plate them.  Polished metals with
the exception of iron and stainless steel are generally well below .1.
To maximize it, paint them flat black.  These numbers are from "The
Infrared Handbook", Office of Naval Research, for 100 deg C.

I once did an experiment with a solar powered radio.  Held the solar
collectors above different cars in the parking lot so that the reflected
sunlight powered the radio.  There was a slight loss when it was held
over a black car vs the other colors.  It didn't work at all when it
was held above a black vinyl top.

Don Hoefer
'82 Coupe

Massachusetts, USA