Bertie the Bunyip wrote:
Matt Whiting wrote in news:foeQi.309$2n4.18956
@news1.epix.net:

Stefan wrote:
Matt Whiting schrieb:

And Lycoming benefits if your engine lasts fewer hours.
So avoiding shock cooling actually lowers its life span? Wow.
You have no evidence that following Lycoming's recommendations avoids
the mythical shock cooling demon or that it lengthens engine life. My
experience is that the engines that are run the hardest also last the
longest. I'm basing this on everything from chainsaws to lawnmowers
to
motorcycles to cars to trucks to off-road heavy equipment (dozers,
skidders, etc.) to airplanes (trainers, air taxi operations, cargo).

I'm personally not convinced that Lycoming's recommendations lengthen
engine life.

Matt



Shock cooling isn't mythical. It's a fact. It's a physical law.

A physical law, eh? I've had 8 years of engineering school and haven't
seen this law. Can you provide a reference to the law of shock cooling?
I searched for the "law of shock cooling" in Google and came up empty...


Any component subject to heating is subject to this law. If you take a
piece of metal and heat it rapidly on one side, that side will expand
more rapidly than the other. This gradient of temp will cause a
difference in physical size one side to the other. The elastic stress
induced by this is cyclically compounded and the resultant locked stress
points that build up in the material, particularly if it's a brittle
material like cast iron, will eventually fail, given time.
The speed at which these stresses are imposed are critical. Speed
because if you introduce the heat gradually (decrease the speed of the
overall temp change), it's given a chance to get to the other side and
expand the other side at a rate not quite so dramatically different as
the side the heat is applied to. Simple eh?
The quicker you insert heat on one side of the material, the greater the
load on the opposite side and the more likely minor damage events
(cracks on a near molecular leve) are occuring. These tiny bits of
damage will become stress risers for the next time th ematerial is
loaded and the cracks will continue to expand until a failure of the
component occurs.

Yes, I'm well aware of thermal expansion and its affects. When an
engine is pulled to idle, the cylinders and heads are getting cooled
from both sides, the outside via airflow and the inside via airflow
through the engine. The far greater differential is under full throttle
during the first take-off when the engine has not yet reached thermal
equilibrium and you are heating it intensely on the inside and cooling
it on the outside.

If people wanted to talk about shock heating, then I'd be much more
willing to believe them and this fits the physics a lot better in my
opinion. Shock cooling is much less an issue from both a physics
perspective and an experience perspective.

Matt