Improving Engine Cooling by Painting
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.
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 of plastic.
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.