Electric Fans

[DRW1000]

Forgive me if this has been covered in the past but I could not find it:

Does anyone know why electric fans on many (if not all) autos equipped with them allow them to run after the engine has been shut off? I am comparing them with the belt drive versions that obviously never run after the engine is shut off. My thinking though is that once the engine is shut off the coolant stops flowing and the coolant near the cylinders now has to absorb more heat as the engine cools. Of course the engine is not producing any more heat. The running fans cool the coolant in the radiator but this will not get circulated at this time and the coolant in the radiator would cool relatively quickly in any event because of the fins (The fan obviously speeds this up).

In summary the running fans after the engine has been shut off cool the coolant in the radiator more quickly but what is the point of this?

[igkyoa]

i'm not sure it makes any difference

[DRW1000]

That is my point. Why are they enabled to run after the ignition is shut off?

[curtis73]

Although the engine is not produing any heat once its shut off, the coolant is still absorbing it. Since water transfers heat to itself very well, cooling the water in the radiator will cool the water in the block. It also provides (neglegible) airflow over the block.

Some smaller engines have a much smaller coolant to surface area ratio. A good example of this is my motorcycle. It only carries .8 gallons of coolant. Idling at rest it will maintain 210 degrees, but as soon as you shut it off, it starts peaking well above 225.

The surface of the water jacket inside can reach several hundred degrees. The temperature of the water is not what's important during operation, its just that it takes heat from one place and puts it somewhere else. Once the coolant flow stops the temperature can spike.

On older cars there were two reasons why they didn't do this; 1) they typically had huge engines (with huge water jackets) and low specific output, and 2) they didn't have a choice since the technology wasn't really refined enough to put an electric fan in there. They sucked too much amperage on a charging system that only had probably a 42 amp alternator anyway.

If you think about it, my 454 runs with a 31 x 19" four-row radiator, five gallons of coolant, and stays cool with about 400 hp. Think about the tiny little radiators in some of today's cars making that same power. They need all the help they can get.

[kcg795]

I never heard of any car where the fans would still run after the car has been shut off. If, however, you have the key in the ON position, but not running, then the fans still might run depending on the temperature of the radiator.

[curtis73]

Many imports do. It was more common in the late 80s to hear the fans actually running since cooling systems were less advanced, but most cars are wired that way. My 96 GM is wired to do that but it never has in 90,000 miles because it didn't need to. I think it only will do that if its something like 230 degrees.

But every so often I've walked by a car in a parking lot with no one in it and the fan is running. Its rare but it happens. No key required

[DRW1000]

I think your response to my original question has to be the best (and most believable) explanation I have ever heard.

I never considered the fact that the thermostat would be open and heat would transfer sans water pump operation.

I owned an Escort in the late 80s and the circuit to turn on the electric fans stopped working. I simply wired it through a switched 12 Volts and it ran like a standard fan. On with the car and off with the car. I never noticed any ill effects.

Thank you

[Speedsteve]

Originally Posted by kcg795

I never heard of any car where the fans would still run after the car has been shut off.

The most "non US-Cars" have this System since over 20 Years!

.

[MagicRat]

Originally Posted by DRW1000

I never considered the fact that the thermostat would be open and heat would transfer sans water pump operation.

Yes, its called 'thermosiphon', where coolant will circulate (slowly) out the top of the engine, through the radiator and into the bottom.

It works better with a top - down radiator than a cross-flow type.

Decades ago, many cars were designed to cool like this with NO waterpump, including about 23 million Model T Fords.