[TR] Fan Hookup

[Michael Porter]

Dave1massey at cs.com wrote:
> In a message dated 11/11/2007 5:20:13 PM Central Standard Time, 
> david.brister at wanadoo.fr writes: 
>   
>> Is there a guru with a definitive answer to whether it's better to have an 
>> electric fan wired to run whenever the thermal switch says so?
>> ISTR that I've owned a Volvo and a VW, both of which had fans that would run 
>>
>> with engine off and ignition key out, so there must be an argument for 
>> this..
>>
>>     
> Sure.  You can save a $5.00 relay that way.  (actually, the cost to the car 
> manufacturer for that relay is under $2.00)  Since fans pull a healthy current 
> you don't want it powered off the keyswitch so switching it off with the 
> switch requires a relay.
>
> Is there a functional benefit?  No.  As Jim pointed out the engine is off and 
> the waterpump is stopped so there is no movement of coolant through the 
> radiator.  Also, since the engine is off there is no more heat being produced so 
> there is no more need to run the fan.
>
>   
Umm, this is sort of true, and sort of not.  The question originally was 
if the car makers are doing it, what's the reason?  I think it's quite 
simple. Ever since the thermostats on most production engines were 
raised to 195-degree items (the higher temps were necessary to get 
higher fuel effiicencies and better cold-weather heater performance), 
engines are running hotter, and aluminum heads mean that heat is 
transferred through the thermal mass more quickly.  So, when the engine 
is shut down, the water pump is no longer pressurizing the coolant and 
accumulated heat in the engine causes nucleate boiling to begin.  This 
raises the gas volume in the cooling system and forces coolant out of 
the overflow.  If this is allowed to go on unimpeded, eventually the 
overflow bottle fills up and coolant is released to the environment.  
That's the reason for the fan running after shutdown--to minimize 
groundwater contamination with ethylene glycol. 

Running the fan after the engine stops gets heat out of the system in 
exactly the same way as it does when the engine is running.  Even though 
the water pump is not turning, coolant thermosiphons through the system 
as long as there's enough energy in the system to cause the coolant to 
move, or the coolant temp drops to the point where the thermostat 
closes.  Of course, when the coolant volume goes down, vacuum pulls 
coolant back into the radiator, except for the reserve in the overflow 
bottle.

The reason is mostly time-dependent.  Running the fan after shutdown 
ensures that the now-hotter-running engine cools more quickly than it 
would statically, thus reducing the chance of coolant overflow.


Cheers.

-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....