Rear engine/motor AIR COOLING

[Frank Lee]

This thread is whack

[dcb]

Originally Posted by Arragonis

- As mentioned better heat transfer from using water vs air

I think there is something to it being possible to tweak a bit more efficiency out of an optimally air cooled engine, but not with the bolt on fan.

The "as mentioned" bit is whack, because you still have a metal to air interface with water cooled, only now it is at the radiator.

and yah, there are conveniences with water, that isn't the point

[Frank Lee]

Think of all our engines being air cooled, with the exception of marine engines. "Water" is just another xfer medium for air to do the ultimate cooling.

As one with several rear-engined "air-cooled" cars, I have seen that:
they are more compact
they are lighter weight
but...
they are built looser and are thus emissions dirtier
HVAC not as good.

IMHO my "ultimate" engine would be a water pumper...

[dcb]

my "ultimate" engine wouldn't need any cooling ;P

[Peter7307]

Winfield1990,
You might want to consider the Citroen GS which had an aircooled engine and front wheel drive.
The logic was the air intake was in the high pressure area thus reducing pumping losses for the fan and allowing a minimum of ducting since the body work created a low pressure area just behind the engine , which was very compact and sat ahead of the front wheels.
The engine was a flat four with SOHC and 1100 cc capacity and increased slightly later.

This allowed air to flow from a high pressure area to a low pressure area and the fan simply added some momentum and allowed for low speed cooling.

The car itself was an aerodynamic four door , four seat sedan with a decent sized boot / trunk , four wheel disc brakes and self leveling suspension. All quite innovative for the mid 1970's when it came onto the market.

Keep in mind too many air cooled engines are designed with reliability as a prime requirement so efficiency does tend to take a lower priority.

Finally if you are looking at air cooling have a look at the racing Porsche engines of the 908 / 917 era. The fan size barely increased but the power more than doubled which meant of course the percentage of engine power delivered to cooling was actually decreased. Quite an achievement.

Peter.

[cujet]

A little bit off topic, but since this is an efficiency forum,,,,,

Air cooled engines sometimes have an advantage in efficiency. They reject less heat into the cooling system, generally by design. This is due to the surface area of the cooling fins and the fan's capacity. Sure, we could move massive quantities of air over fine pitched fins to try to mimic liquid cooling. But, generally, air cooled engines (at power) run with cylinder head temperatures in the 300 to 400 degree F range.

This results in a slight decrease in BSFC.

However, there is more to the story. Air cooled engines function well with 2 valves per cylinder. It's somewhat difficult to get 2 exhaust valves to cool properly (yes, it can be done). So, most are 2 valve engines. This can result in lower valve train frictional losses. But, the issue at hand is piston ring swept area. The lower output of air cooled 2 valve engines, generally means that more displacement per cylinder is needed for equivalent output. This results in larger pistons and lower RPM's. Piston ring swept area is greatly reduced in this situation. Therefore, BSFC is greatly reduced when some air cooled engines are compared to liquid cooled "modern" engines doing the same work.

Of course, this does not apply to all situations and all engine applications. But, it holds true for many modern air cooled engines. Consider two examples. The modern, well tuned V-twin, such as a Harley or clone. Compared with liquid cooled bikes of equal power, the Harley engine achieves a better BSFC number. Modern piston aircraft engines such as the Lycoming IO-540 can achieve BSFC numbers better than nearly all similarly powerful modern liquid cooled engines (generally 0.38-0.39 Lb/fuel/HP/Hr). In other words, your 350 small block Chevy won't make 300HP and do any better than 0.55.