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Color and Engine Cooling
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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