[SEL] Thermo-syphon Question

Skip Cleveland skipcleveland at bellsouth.net
Sat Jan 20 08:02:02 PST 2007


Thermosyphoning and Heat Soak Effects
by Paul Rollins

      Q:

On some cars the electric cooling fan continues to run on even after the 
engine is shut off. What's the point? If the engine isn't running, the water 
pump isn't circulating coolant, so the fan will only lower the temperature 
of coolant in the radiator and won't affect engine temperature.
      A:

This is not correct. It is indeed beneficial to have the fan continue to run 
after the engine is turned off. Many early automobiles did not have water 
pumps. They relied upon the process of thermosyphoning, where the hot 
coolant in the engine, being less dense than the cooler fluid in the 
radiator, rises to the outlet and flows into the radiator. As the coolant in 
the radiator gives up heat to the air and cools, it moves downward and back 
into the engine. This cycle continues as long as there is a temperature 
differential between the radiator and engine (and the thermostat is open), 
whether the engine is running or not. Technically, even contemporary engine 
cooling systems are pump-assisted thermosyphon systems.
The other phenomenon that is involved is heat soak. This occurs because 
there is a temperature differential (really a gradient) between the engine 
metal that is not in close contact with the coolant and that which is 
farther away. When the engine is off and there is not a high airflow through 
the radiator, the thermosyphon cooling effect is low. This allows heat from 
these remote areas to flow to areas that are much cooler when the engine is 
running, resulting in fuel percolation, evaporation, flooding, and coolant 
boiling.

To take advantage of thermosyphoning and reduce heat soak effects, many 
modern cars use fans that run after engine shutdown.



I found this on google. Doesn't say anything about the water having to be 
above either radiator or tank opening. Could this be wrong?

Skip

----- Original Message ----- 
From: "peter ogborne" <jopeter at omninet.net.au>
To: "The SEL email discussion list" <sel at lists.stationary-engine.com>
Sent: Saturday, January 20, 2007 7:04 AM
Subject: Re: [SEL] Thermo-syphon Question


>I think percolate was a bloody good description of what happens . 
>Percolate....intrans [lit and fig] To filter,ooze,drip gradually through. 
>You got it JB!!!
> ----- Original Message ----- 
> From: <jbcast at charter.net>
> To: "The SEL email discussion list" <sel at lists.stationary-engine.com>
> Sent: Saturday, January 20, 2007 7:40 PM
> Subject: RE: [SEL] Thermo-syphon Question
>
>
>> ---- Ray Freeman Portable Line Boring <plb at iinet.net.au> wrote:
>>> Well we have all been informed Edd! I hope you have learned from your
>>> inexperience (something this list is so good as correcting) I now know 
>>> that
>>> my tank cooled Corbett Williams has a thermo siphon system, that is 
>>> until
>>> the water level drops .001 below the pipe then it's a percolating system 
>>> !
>>> Ray Freeman
>>> Perth WA
>>>
>> Have you ever syphoned water from a container? Hold the hose .001 above 
>> the water level and see how much you can syphon. If everything isn't 
>> under water it's not a syphon. Percolating is a term I used because I 
>> don't know the correct one, but I know what it isn't.
>> J.B. Castagnos
>> Belle Rose, LA
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>>
>>
>
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