[TR] Fan Hookup

[Dave1massey at cs.com]

In a message dated 11/11/2007 7:05:57 PM Central Standard Time, 
DLylis at aol.com writes: 
> I have owned many cars over the years in which the cooling fan continued to 
>  
> run after the key was shut off.  Now, someone who knows a lot more  about 
> thermodynamics (?) may be able to tell me if this is correct or  not. If my 
> 3A is 
> running (when it was running, anyway) and the temp  was in the normal range, 
> 
> when I shut off the car the temp would rise before it  would fall.  I 
> assumed 
> this was because the coolant stopped circulating and  carrying heat away 
> from 
> the block, and I think that is correct.  That would  seem to me to be a 
> reason 
> for the electric fan to continue to run after the  motor is shut off even 
> though it is only cooling the fluid left in the radiator,  and any 
> circulation 
> that is created by the temperature gradient.   Obviously the water pump is 
> not 
> running.   
> 
When the engine is running the internal temperatures are not uniform in that 
the temperature of the core of the cylinder head is higher than the coolant 
and the metal in direct contact with the coolant.  When you shut down the 
coolant stops moving the heat away and the temperatures equalize.  This means the 
hot center of the head cools down and the metal in contact with the coolant (and 
the coolant itself) heats up.

But running the fan does nothing for this.  The effect it has on the head 
directly, even if the fan is blowing on the head, is insignificant.

Dave