What does "air cooled" mean?

[CzechItOut]

You often hear people refer to the old Porsche 993 as the last of the "air cooled" Porsches, but what does this actually mean?

My car (Alfa 156) has a coolant reservoir, radiator and a fan that kicks in at a certain temperature. Does this mean that an air cooled car does not have a coolant system?

If so, does this make them more prone to over-heating?

Sorry about the Noddy question!

[CoolC]

The engine is cooled by air flowing over the heads, much the same as older motorcycle engines.

There is on liquid cooling system, no radiator.

[Munter]

CzechItOut said:

You often hear people refer to the old Porsche 993 as the last of the "air cooled" Porsches, but what does this actually mean?

My car (Alfa 156) has a coolant reservoir, radiator and a fan that kicks in at a certain temperature. Does this mean that an air cooled car does not have a coolant system?

If so, does this make them more prone to over-heating?

Sorry about the Noddy question!

Basically yes. An Air cooled car has no coolant system.

More prone to overheating? Not really. Manufacturers end up fitting Oil coolers if it needs more cooling. At which point the oil sort of has a dual role. A)Lubrication and B)Coolant.

[anonymous-user]

All cars are of course "air cooled", but usually there is an intermediate stage that uses the flow of water to convey the heat energy to a radiator, which then releases said heat to the ambient air. On "air cooled" engines, the engine is directly cooled by a flow of air with no intermediate water stage. A large fan literally blows air at high speed over lots of fins (large surface area is required because air has poor thermal capacity as compared to water) cast into the cylinder head and bores. It has the advantage of being lightweight and simple, but the downsides of requiring more power to drive it (normally your water cooled radiator is simply cooled by the air flowing over it from vehicle motion) and it cannot levage the specific heat of vapourisation of water that a water system can use in extremis.

[Sam_68]

It's probably worth mentioning, too that on the majority of more powerful 'air cooled' engines (including the Porsche 911), a lot of the actual cooling is done by the oil which is circulated via an oil cooler (oil radiator). All that is air-cooled, really, is the cylinder barrels.

[Huff]

One more large disadvantage for air-cooled engines: temperature is a very poorly controlled, because of the variances possible oer the range of thermal output over power demand and (road/fan) speed; this doesnt affect a water-cooled engine anyting liek as much. There may be other hidden trade-offs too, like being restricted to lower compression ratios to limit thermal loading in the heads (and so reduce chance of detonation)

The upshot is that an aircooled engine has to have rather larger tolerances for everything, and as a result tends to (1) burn oil (2) use slightly more fuel and (3) not last as long.

[CzechItOut]

Very interesting, thanks all

[Nick Grant]

Also of note is the different noise that they make as they don't have a load of water dampening (pun intended) the engine case. They can't freeze in winter but any damange to the fan can be catastophic to the engine pretty quickly (if they don't have oil cooling, like a stock beetle).

Edited by Nick Grant on Thursday 3rd November 10:55