I’ve heard people restoring cars say time and time again that painting an engine bay black with attract heat away from the engine and help the engine keep cool. This seems ridiculous to me for two reasons. First, any difference would be made would be so minimal that it wouldn’t make any difference whatsoever. Secondly, since most cars have the hood down while running, wouldn’t everything be completely in shadow anyway?
Along similar lines, most heat sinks are black.
At the very least, it won’t reflect as much heat as a brighter/metallic color. I would think the only time this would make a more then negligable difference is when the car isn’t moving. Any other time, there’s so much air moving through the engine compartment, I think that would blow more heat out then the color of the paint.
But still, wouldn’t a bright color in shadow still be “black,” thus cancelling out the reflection of heat?
No, Charger, because heat is radiant energy, just like light.
Charger -
I agree that the total “bonus” from painting the inside of the compartment probably isn’t large, but if you have to paint it (to look nice) you might as well paint it a color that won’t actively retard heat absorption.
I guess I should elaborate my above post.
“Colour” is a product of perception.
What’s “really” there is an object’s property of absorbing or reflecting energy of different frequencies. Something looks black to us if it absorbs most of the radiant energy in the part of the spectrum that’s visible to us.
The idea is that many materials that absorb energy in the visible spectrum will also absorb energy at frequencies below what are visible to us. Ever wonder why a “black” heating element turns red when it gets hot? Most of the “heat” is invisible to us, but it’s also putting out energy that’s just visible to us, in the “red” part of the spectrum.
So a white housing will reflect a lot of energy, while a black housing will absorb a lot of energy. The engine is itself a “light” source-- our eyes just aren’t sensitive to frequency of energy that it radiates, unless something’s gone terribly wrong.
All radiant energy is heat, simply because energy is heat. From gamma rays on down to long-wave radiation, and including things like alpha and beta rays, everything involving moving particles carries heat from one place to another. (And you can think of photons as being particles in this instance.)
I’m not sure that’s quite true. Heat is the random vibration of molecules. Energy induces these vibrations but is not, in itself, heat.
IR light is used to warm things because longer wavelength EM radiation more readily induces vibrations in molecules. This phenomena peaks with microwaves and their ability to give water molecules a good hard shaking. Shorter wavelength radiation is less able to induce such vibrations.
Not to mention the active cooling by the pumped coolant through the carefully designed radiator and fan mechanism…
No, let’s do mention that. The engine radiates so little heat (relatively speaking) into the sheet metal surrounding it that I can drive 20 minutes before the frost even melts off the hood over the V8 under my hood.
I think it’s perfectly acceptable to call radiation heat, even if it hasn’t induced vibrations in anything yet. After all, photons can be considered particles for the purposes of a wide range of experiments, and heat is defined as the motion of particles.
Semantic disputes aside, I wanted to emphasize that all radiation carries energy, and that it’s highly artificial to call any one wavelength `heat energy’.
I have never heard that the color of the paint on the walls of the engine compartment had any effect on temperature under hood.
Why auto restoration people use black, I would bet, it is due to the fact that factories originally painting the bays black,( and that was probably more of an economic choice than an engineering one) not becasue of any heat advantage.
There is a reason the radiator is painted black, IIRC,and that is related to the heat transfer properties involved.
I just ran a quick search to see if my last statement was still valid, and I found information that supports the radiator benefiting, but not the bay, from black paint.
With respect to the color of the radiator,
[(this is a commercial site)](http://car-restoration.e-shopping- network.com/Radiator_Black_Paint_Aerosol_12_oz.html)
Apparently, paint can be an insulator. My opinion, if black paintworks at all, it would only amount to a few degrees, after the engine was turned off. Otherwise, it just looks good.
Assuming you would rather that your engine bay runs significantly hotter, and your engine runs insignificantly colder, then yes, this makes sense.
Personaly, if the color of my engine compartment was the only thing keeping my engine from overheating, I would fix my cooling system.
Well, it is black to visible light, meaning it absorbs it. But the radiant heat from the engine is IR so you would have to find out how well the interior of the compartment absorbs IR.
If it absorbs the engine IR, then the walls of the enclosure would get hotter. That higher temperature would transfer more heat to the air blowing over the hood - maybe - athough most hoods are double walled over much of their expanse. On the other hand, the increased temperature of the walls would also result in more IR from the walls which would be directed back toward the engine in large part, or to the other walls. I don’t see how it could help much in cooling. Much better to keep the radiator in good shape.
The formula for radiative transfer between two parallel plates is
dQ/(dT dA) = s (T2[sup]4[/sup] - T1[sup]4[/sup]) / ((1/e1) + (1/e2) - 1)
where Q is heat, A is area, t is time, T1, T2 are temperatures of each surface, and e1 and e2 are emissivity (absorption/emission coefficient) of each surface, and s is the Stefann-Boltzmann constant. If you use an area and using 6 square feet of surface area (1x1x1 ft cube-shaped engine), T1=20C, T2=100C, e1=0.5 and vary e2 from 0 to 1, the heat transfer rate varies from 0 to 180 watt. I think it’s safe to say that this is a negligibly small fraction of the heat produced by the engine. (I don’t have the numbers for that, but if an engine is producing 50 horsepower at 50% efficiency, that’s a waste heat of about 18,000 watt.)
Not correct. It’s a widespread misconception.
The “Heat” stored inside an object has two components: mechanical vibration and electrical vibration, also called “phonons” and “photons.” Part of the energy inside a warm object is in the form of photons flying between molecules, and part is in the form of hypersonic white-noise sound waves moving between molecules.
And about the engine compartment… nearly all types of paint look black in longwave IR light. White paint is black. Black paint is black. If you don’t want the surface of the engine bay to absorb so much “heat” (longwave IR,) then sand down to smooth bare metal.
On the other hand, if your engine is so hot that it’s glowing red or orange, in that case the black paint is again a better absorber than white.
Hmmm. Metallic paint? MAYBE some ‘silver’ paint reflects long IR better than the white paint.