>Regarding what happens when engine is off, but fan still runs>
The engine and radiator are interconnected by the upper and lower radiator
hoses. When the engine is off, coolant will still continue to circulate. How
does this happen? It is called the thermosiphon effect. This effect happens
anywhere there is a temperature differential in a fluid. The hot fluid rises
and cooler fluid falls (or if you are in the southern hemisphere, the cool
fluid falls and the hot fluid rises). This circulation continues until the
fluid reaches temperature equilibrium. This effect works even with very tiny
temperature differences. The "hot" fluid is slightly less dense compared to
the cool fluid. It is used to cool nuclear reactors. It is used to heat
swimming pools (if you have a solar heater). It is one reason why you have
to have an anti-siphon valve in your system if you have a solar heater. (At
night when the sky is "cold" by thermal radiation standards, your solar
heater will radiate heat to the "sky" and will work in reverse -- that is,
it will cool your pool instead of heat it. The pump is there only to speed
up the thermal energy transfer.
You can see the thermosiphon effect in a glass of water with ice floating on
the water. It is one reason why your home hot water heater doesn't work as
well as it could. You can see this effect when you are waiting for a pot of
water to boil. This effect is one reason there are living fish and stuff in
the oceans and lakes. Engineers have even built a powerplant off the coast
of Hawaii that used the temperature difference between water at the surface
and water at a couple thousand feet deep as the means to drive a generator.
In the car, when the engine is off, the coolant will continue to
thermosiphon cool. If the fan runs, more thermal energy will tranfer out of
the radiator to the ambient air and the coolant will cool somewhat faster
than if the fan did not run. Eventually, the engine block will be cold
sooner than it would have been if the fan had not been running.
In the old Model T, this effect was the way it was for cooling. There was no
fan. The radiator was "supposed to be" big enough to allow for the engine to
be entirely thermosiphon cooled. In fact, you could run any engine this way
even today if you could stand to carry around the very large radiator it
would take to enable the engine to have adequate cooling without a pump or
fan. A typical engine has to dissipate several KiloWatts of thermal energy.
To get away with a tiny radiator (thus allowing a smaller car front end
which in turn enables smaller drag coefficient and in turn better fuel
economy), a water pump and a fan are used along with higher coolant
temperature (which is accomplished by raising the coolant pressure). Because
water contains oxygen and hydrogen, free oxygen combines with exposed metal
to form corrosion products (oxides, i.e., "rust"). To reduce corrosion, a
corrosion inhibitor is added to the water. Because you already are adding
corrosion inhibitor, you might as well use one that, when mixed with water,
results in a boiling point that is higher than water alone and also use one
that prevents the water from freezing. Voila !!! "Antifreeze & Coolant".
Scientists have searched for a long time and have not found many things that
are as cheap as water and have as good thermal properties and don't do bad
things to people or the environment.
Roy
'60 TR3 TS63103LO (in restoration)
techman@metrolink.net
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