At least one well-known engine overhauler paints its engines black because it supposedly improves cooling. It turns out, however, that black is one of the least-desirable colors to paint an engine. For optimum cooling, your engine, accessories, and engine mounts should be painted white, and the inside of the cowling should be painted black. Here's why.
April 9, 1996
|About the Author ...
John Schwaner is AVweb's powerplant expert. John is a world-class
authority on piston aircraft engines, and a specialist in the
engineering analysis of engine failures. John runs Sacramento
Sky Ranch, Inc., a leading distributor of aircraft and engine
parts, and probably the foremost aircraft hose shop and magneto
overhaul facility in the U.S. John and his wife live in Sacramento,
John has also written two superb technical books:
Sky Ranch Engineering Manual
The Magneto Ignition System.
Both can be previewed in and ordered from the
AVweb Online Bookstore.
Some overhaul shops claim beneficial cooling effects of black
paint on their engine. The theory is that black is a better
emitter of infrared (heat) energy. A hot engine in a cold surrounding,
radiates energy from the hot object to the cold. The amount of
radiant cooling (thermal emissions) is generally dependent upon
the temperature of the emitter (engine) and how efficient the
engine is at radiating energy — its emissivity. This derives
from Kirchoff's Law where good absorbers are also good emitters.
Since black is a good absorber of energy (that's why it's black),
it's also a good emitter. A black object emits more radiant energy
(cools faster) than a white or gold object that emits less radiant
energy per unit of time. Painting an engine black increases the
amount of radiant energy emitted by the hot engine to its colder
surroundings and it will be cooler than a non-black engine.
There are fallacies to the above argument. Most heat energy emitted
by the engine is in the invisible infrared portion of the spectrum.
What appears to us as black, white, red, or green in the visible
spectrum is not what we would see in the infrared spectrum. Any
color of paint that uses organic (non-metallic) pigments is black
in the infrared spectrum. For example, white paint emits 90-95%
depending upon pigment type. Black emits 96%, blue 94%, green
92% and red 91%. The best emissivity surface coating is acetylene
soot at 97%. Painting an engine increases radiant cooling but
the color of the paint doesn't.
The second fallacy is that you also have to ask what happens to
the radiant energy being emitted by the engine. Engine cowlings
with bare aluminum on the inside reflect about 95% of the radiant
energy back to the engine where the black paint of the engine
absorbs at the same high efficiency as it emitted. Black paint
has high absorbency and high emittance. Better would be white
paint that has low absorbency and high emittance. An engine contained
within a bare aluminum cowling should be painted white not black
to reflect the radiant energy back to the cowling. Another technique
would be to paint the inside of the cowling black to eliminate
reflection of heat energy. If the aircraft had a plastic cowling
then the engine need not be painted white because of the low reflectance
Most heat from the engine is transferred to the atmosphere by
convection rather than by radiation. Radiant cooling of an engine
is negligible. Engines run quite nicely no matter what color they're
painted. Now when you're at that trade show and the engine shop
salesman is talking performance and how his paint scheme helps
cool the engine just tell him that his black engine is the same
color as a red, white or blue engine — in the infrared spectrum.
Although engines get all the attention, the engine compartment
can be engineered to better manage temperature. The engine compartment
is one of the most heat damaging environments there is. Packed
in the same compartment are glowing exhaust components, rubber
products, electronics, wiring, pumps and magnetos. Little effort
has been made at controlling heat exchange by changing the infrared
emissivity and reflectivity of components. Small changes, such
as the addition of reflective heat shields, can be very effective.
Engine mounts are often corroded where they pass next to an exhaust
pipe. Mounts painted with dark shades of organic pigmented paint
efficiently absorb radiant energy causing the mount to get hot,
burn the paint, and cause corrosion. Black or white paint is
all the same as far as emissivity or cooling a hot object. However,
when keeping an object colder than its surroundings, such as a
mount section next to an exhaust stack, we need to reflect the
radiant energy and not absorb it. In this case white paint reflects
more radiant energy than black paint. A metallic pigmented paint,
such as aluminum, is even better yet.
Another method of keeping objects cool in a hot environment is
by using heat shields that reflect radiant energy. Small shields
made out of aluminum or stainless steel are highly reflective
not only at visible wavelengths but well down into the infrared.
Your mechanic may be able to fabricate these small heat shields
or contact me for pre-fabricated shields.
Aircraft, such as some Aerostar's, use heat shields to keep the
magneto cool. We may be able to forgo the shield and add the reflective
properties of the shield to the surface of the magneto. For this
we want the magneto surface to have high reflectivity to bounce
radiant heat off and we want high emissivity to radiate heat out
of the magneto.
Bare aluminum or a metallic pigmented paint would not be desirable
because it has high reflectance but low emissivity. Black paint,
as most magnetos are painted, also is not desirable because it
has high emissivity but poor reflectance. A white organic pigmented
paint is best because it has both high emissivity and high reflectance.
Painting engines black is more effective as a sales tool than
it is at cooling an engine. However, the concept of regulating
temperature within an engine compartment by controlling radiant
energy can be both effective and easily and inexpensively applied.