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Name: Alan
Status: Student
Grade: Other
Location: AZ
Country: United States
Date: August 2006


Question:
Black air cooled engines radiate heat better than white engines. If the water jackets of a water cooled engine are painted black internally, would they radiate the heat to the water better than a white painted water jacket? I believe the absence of light inside the engine would have no effect on the radiated heat.



Replies:
Hi Alan,

Actually, your first statement is not true at all. Air cooled engines do not (for the most part) dissipate their heat by radiation. Almost 100% of their heat is carried away by forced convection to the air that flows through their fins. This is the same "mechanism" that causes your soup to cool when you blow on it. In addition, even if their heat WAS lost as a result of pure radiation, black paint is only black in the visible spectrum. It is in the infrared spectrum that heat is radiated, and to effectively radiate heat, a body must appear "black" to infrared radiation. What color it is to your eyes (in the visible spectrum) is irrelevant. But as I said, an air cooled engine dissipates almost all of its heat in contact with the air that flows through its fins. The color of the fins makes no difference at all.

To answer your question, the color of the water passageways in a water cooled engine will have no effect whatsoever on heat dissipation. There are several reasons for this. First, remember that heat is radiated by emitting infrared radiation. Water is transparent, so any radiated energy would pass right through the water without being absorbed. Second, any radiated energy would be simply absorbed again when it hit the opposite side of the cooling water passageway. Third, there can be no radiation anyway, because the water and the metal walls are all at almost the same temperature, and radiation only occurs when there is a difference in temperature between two bodies.

So inside a water cooled motor, radiation plays no part at all in carrying away the heat. Just like in an air cooled motor, heat is carried away by forced convection. The only difference is that the fluid removing the heat is now water, not air. Heat is first transferred to the water from the metal, in the same way that heat is transferred to your finger when you touch a hot object... that is, by conduction. This makes the water warmer, and the water pump carries the water away.

As a general statement, thermal issues like these are exceedingly complex and not intuitive at all. They are usually the source of great confusion even amongst university physics students.

Regards,

Bob Wilson.


It is not true that black surfaces radiate heat better than white surfaces. Thermal radiation of heat depends on the emissivity, and emissivity depends on the wavelength (temperature) of the material. White paints often have equal or higher emissivity than black paint at and above room temperature.

Bob Erck



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