On Wed, 11 Jun 2014 19:50:20 +1200, Gregory Ewing wrote:

> Chris Angelico wrote:
>> So, let me get this straight. A CPU has to have a fan, but a car engine
>> doesn't, because the car's moving at a hundred kays an hour. I have a
>> suspicion the CPU fan moves air a bit slower than that.

I'm not sure where Chris' message comes from, I can't see the original, 
so I'm guessing the context.

Air cooled cars don't just cool the engine when they are travelling at 
100kmh. Some air-cooled engines used a fan to blow extra air over the 
cooling fins, but many did not. Normal air flow is sufficient to keep 
them in a safe operating temperature, the hot engine warms the air, which 
flows away and is replaced by cooler air.

It's possible to design CPUs to work the same way. My wife is using a PC 
right now with a 1.66GHz Atom CPU and no CPU fan. Even though the power 
supply fan died, the machine is still running perfectly, with two laptop 
HDDs, and no overheating. 1.66GHz is plenty fast enough for web browsing, 
word processing, email, etc.

Go back 30 years, and I don't think that the average PC needed a CPU fan. 
Possibly not even a case fan. Just the normal air flow over a small heat 
sink was enough. And of course, your mobile phone has no room for a heat 
sink, unless it's tiny, and no fan. And people expect it to keep working 
even when shoved in their pocket.

> If the car were *always* moving at 100km/h, it probably wouldn't need a
> fan.
> In practice, all cars do have fans (even the ones that aren't
> air-cooled), for the occasions when they're not moving that fast.

That may be true of water-cooled engines *now*, but it's not a law of 
engineering. Many air-cooled engines do not (did not) require a fan, or 
only needed the extra cooling when stuck idling for long periods in hot 
weather. E.g. Beetles didn't use a fan. (A great idea for Germany, not so 
much for hot and dusty Southern California, as my wife can tell you.)

> (BTW, so-called water-cooled engines are really air-cooled too, just not
> by air flowing directly over the engine block. (Although marine engines
> may be an exception.))

Yes, technically water-cooled engines are cooled by air too. The engine 
heats a coolant (despite the name, usually not water these days) which 
then heats the air.

Steven D'Aprano

Reply via email to