In a message dated 11/11/2007 9:31:25 PM Central Standard Time,
mdporter@dfn.com writes:
> 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.
>
This may be a condition common at your altitude but I've never seen it in
action down here at sea level. And most all the car's I've experienced with
electric fans do swithch off the fans with the key so it doesn't seem to be a
concern to some car designers.
Besides, I don't see how this applies to an LBC which is running at more
conservative temperatures.
I have seen stationary diesel's the run on neucleate cooling as a normal mode
of operation so utilizing boiling water as a method of cooling an engine is a
workable system if the design is right. But not practical in a vehicle.
Cheers
Dave
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