Matt Whiting wrote:
Bertie the Bunyip wrote:
Matt Whiting wrote in

It is the same if the same delta T is present, but my point is that it
is easier to heat something quickly than cool it quickly. Even at 250
C, you are only 523 degrees above absolute zero. So, this the
absolute largest delta T you can induce for cooling, and it is very
hard to get absolute zero, so you are more likely to have a cool temp
closer to 0 C yielding a delta T of only 250 degrees.

On the hot side things are more open-ended. It isn't too hard to get
450 C exhaust gas temperatures. For an engine that is started at say
20 C ambient temperature, you now have a delta T of 430 degrees which
is much greater than the 250 likely on the cooling side of the cycle.

That is one reason why I suspect that "shock heating" is more likely
to be an issue than "shock cooling." I suspect you can induce a
higher delta T during a full-throttle initial climb than you can
during an idle descent from a cruise power setting.


Right, I'm with you now. yeah, I can buy that. Froma strictly clinical
viewpoint it absolutely makes sense. My experience with damage says
otherwise, though I can offer no explanation why that should be the
case. Years ago I towed gliders with Bird-dogs and we cracked a lot of
cylinders when we just closed the throttle after release. When we
moved to gradual reduction to ultimately 1500 RPM the problem
disappeared completely. Later, when I flew big pistons,the procedures
for cooling down the cylinders on the way down. You were almost
gaurunteed a crack if you yanked the taps closed. Can't see how we
went from cold to hot any more than you would just starting up and
taking off. I've just bought an aerobatic airplane with a Lycoming.
We're not expecing to get to TBO with the engine because we'll be
doing aerobaics with it, but of course we're prepared to live with that.
I suppose the point I'm making is that even if shick cooling is over-
rated, it certainly does no harm to observe trad practices as if it did.

I suspect, as with most "real world" problems, that there is more in
play than delta T induced stress. Probably geometry and other factors.
Maybe having the thin fins on the outside vs. thick metal on the inside
is making a big difference in the stress profile.

I've not had experience with the larger engines or with radials.
However, my experience with O-470 and smaller engines is that shock
cooling just isn't an issue and many folks are paranoid for nothing.

Operating the engine as if shock cooling was an issue is probably not a
problem in most cases, but if it causes you, as it has with Jay, to not
practice essential emergency procedures, then I disagree that it causes
no harm. This may be very harmful should Jay experience an engine
failure for real.

Matt

Shock cooling damage is merely the effect of different rates of thermal
dimensional change between the aluminum cylinder head and the steel
valve seats and possibly between the head and the barrel where its
threaded on. When contracting, the head shrinks faster than the valve
seats and barrel and huge tension stresses are built up in the area of
the head between the valve and the nearest hole, which is usually the
spark plug. Most shock cooling damage is a crack between the exhaust
valve and the nearest plug hole.

It's not a problem when heating because the head expands faster than the
steel parts so the stress effects are reversed. This is why the
manufacturers have no problem with going from idle to full power as soon
as the engine will take it without stumbling. There is no such thing as
shock heating...

Shock cooling is generally a problem when at the extremes, going from
full power to idle. You won't get enough stress to cause damage going
from cruise power to idle, so for airplanes not used for towing, or
aerobatics or some flight training scenarios, it's not a problem.

John