Engine out practice

Morgans wrote:
"Matt Whiting" wrote

The issue with shock cooling isn't the rate of cooling per se, but rather
stress induced by differential cooling. Most engines see far higher
temperature differentials during start-up than they do during cooldown.
Jay, have you timed your engine heat up rate? It would be interesting to
watch how fast your engine heats up from say a 50 degree cold start and
then compare that to the cool-down rate when you pull the throttle for
engine out practice. I'm assuming this would be fairly trivial with your
engine analyzer.

I'm pretty sure that the rate of heating of the metal of the head is not
the big issue, according to the shock cooling proponents.

Instead, it is the heads (and cylinders) cooling more rapidly than the
pistons, and the hot pistons (not able to be cooled as rapidly) against the
cooler cylinders (the cylinders do not have heat instrumentation, so the
closest thing they can do is to measure is the head temperatures) causing a
reduction in the tolerances between the piston and the cylinder walls, thus
causing possible scuffing and abnormal wear.

At least that is my take on what they say.

It seems to me that upon engine start the pistons would heat up much
faster than the cylinders causing the same net affect as cooling down
the cylinders faster once hot. Either way the pistons are hotter than
the cylinders.


Matt

On Oct 13, 8:55 pm, Matt Whiting wrote:

It seems to me that upon engine start the pistons would heat up much
faster than the cylinders causing the same net affect as cooling down
the cylinders faster once hot. Either way the pistons are hotter than
the cylinders.

Matt

At idle or low power settings there is little heat generated. So
little, in fact, that it can take forever to get the CHT warm enough
to carry out the runup when the temps here are -15 or 20°C. The
cylinder has plenty of time to warm up. It's the sudden removal of the
heat source when the atmosphere is really cold that problems might
arise. In Canada we have to think about it a little more than the
pilot in Arizona. Pistons are aluminum and expand at twice the rate of
the steel cylinders, clearances get small during operational temps,
and shrinking a cylinder quickly around a hot piston is asking for
scuffing or seizure.
We run six Lycs in flight training ops. They usually reach TBO
in good condition. They get a lot of rapid throttle movement, even
though I constantly make noises about not abusing the engines. In my
opinion, opening the throttle too fast can do more damage than closing
it too quickly. Cylinder pressures can get high enough with rapid
throttle movement to cause detonation, however briefly, and cracking
of various parts might occur. A pilot who bangs the throttle open is
applying high manifold pressures to an engine at very low RPM, the
definitive extreme oversquare situation.
Closing it quickly in flight will cause afterfiring (lean
mixtures that often don't fire in the cylinder, igniting instead in
the hot muffler). Cracking of exhaust components is a risk there, and
we find that often enough.
Our students get plenty of forced-approach practice. The engine
is throttled back in two or three or four seconds. Transport Canada
tells us that some practice forced landings (PFLs) end in the real
thing when the carb ices up during the glide. The syllabus calls for
an application of power for a few seconds every 1000' of altitude loss
to clear the engine, but since the exhaust system is cool in the
glide, it can take much more than a few seconds to clear any ice
accretion and the engine might not respond when necessary.
For those lucky ones with injection, carb ice is not a problem,
but most of us are stuck with carbs and need to be thinking, when we
check the weather before the flight, about what the atmosphere is up
to. We wouldn't dive into unknown waters without making sure there
weren't hidden rocks or sharks around, and we shouldn't launch without
knowing the temp and dewpoint spread, right?

Dan