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Speedy Squirrel Speedy Squirrel is offline
Rocket Scientist
Join Date: Jun 2008
Location: Detroit
Posts: 898
I do measure temperature on the lowest (closest to the cylinder) fin on my cylinder heads as a surrogate for coolant temperature for my EFI system. I measure it on the back side (closest to driver) of cylinder 3. I use an RTD epoxied directly to the fin.

I have never seen higher than 250.

I don't see what it has to do with maximum power limit for air cooled engines though. It is the internal surface temperature that limits power due to pre-ignition and structural concerns. The fin temp is something else entirely.

We use laser induced florescence and visible light tomography to identify "hot spots" inside a running engine now. I think 450 isn't a bad number to use. It depends on effective compression ratio, and mixture turbulence too. The hot spot is usually on the edge of the exhaust valve (see why we have sodium filled valves?). Obviously, this stuff has been raised to a high science in this, the 111th year of the Otto cycle.

The bulk gas temperature in the cylinder is not the same as the surface temperature either. A boundary layer forms close to the surface, and this actually limits heat transfer to the inner metal.

Usually, when you see melted metal in the combustion chamber, it is an effect of detonation. Detonation destroys the boundary layer, and allows heat fluxes an order of magnitude higher to the chamber walls. That will melt aluminum in a hurry.
Old 08-03-2009, 08:28 AM
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