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On 06/04/2014 02:37 PM, Nigel Graham wrote:
> Your experimentation and the experience of coolant failure of
> others begs an interesting question, if the Rotax air cooled
> barrels do not offer any increased endurance in the event of
> coolant loss - why use air-cooled barrels? Water cooled barrels
> would offer far better heat distribution and thus less localised
> distortion and allow tighter tolerance pistons that in turn would
> develop greater power with less noise.
The barrels don't need to radiate much heat. Over 90% of the heat of
the combustion is transferred to the cylinder heads. This is where the
combustion takes place. Once the piston is moving downwards the
combustion has already ended and because of the expanding space
temperature drops rapidly.
The fins on the barrels are misleading. They look like the fins on the
barrels of truly air cooled airplane engines, and make people think
that the Rotax engine is partially air cooled. This is not so. True, a
little bit of air helps to get rid of the residual heat of the
barrels, but not much can be gained by increasing air flow over them.
The barrels are also cooled by the cylinder heads and engine core. One
end is water cooled, the other end is oil cooled.
In my airplane I have closed off the gills AND the round openings at
the front of the cowling. Air flow is guided with two hoses from a
single opening which also feeds an oil radiator, and the air exits via
the cowl flap which is shared with the radiator exits and is usually
almost closed. With tuning the system I was amazed how little air flow
is needed to keep the barrels within a healthy temperature range.
In the original Europa setup, the rear most barrels have almost no air
cooling, without adverse effects.
Loss of coolant will almost certainly melt the engine. Although it
seems that while the engine is running the softening barrels and heads
will remain enough in shape by the pistons to keep power production up
fow quite a while.
Frans
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