Engine out practice

Stefan wrote:
Matt Barrow schrieb:

Thomas offers data and evidence, Lycoming offers anecdote and legend.

Lycoming offers running engines. Thomas offers words.


Lycoming and Continental offer no science whatsoever to back up their
recommendations. There are several companies that can show you hard
scientific data to disprove what the engine manufacturers claim.

"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.
--
Jim in NC

"Morgans" wrote

At least that is my take on what they say.

I should have added that the cylinders heating up faster, on start up,
than the pistons would increase the tolerances, and thus cause no scuffing.
--
Jim in NC

In article ,
Matt Whiting wrote:

Hmmm, thinking about it a bit, any shock cooling should be worse on
the front two cylinders rain or shine.

An analyse of failure rate by cylinder position would be interesting.


Yes, my thought exactly. I wonder if anyone keeps such data? I know
when I owned my Skylane, we never had any unusual issues with the front
two cylinders. This data, if available, would certainly provide some
indication if shock cooling is real or imagined.

My O-320 had 2 cracked cylinders (both on the left side) at the same
time. This was the year after the exhaust stud hold cracked on
the right front cylinder. Shock cooling? More likely due to the fact
that these were cermichromed cylinders with an unknown number
of hours on them before they cracked at 600hours SMOH.

--
Bob Noel
(goodness, please trim replies!!!)

"Matt Whiting" wrote in message
news
Jay Honeck wrote:
The issue with shock cooling isn't the rate of cooling per se, but
rather stress induced by differential cooling.

Actually, I think it is the rate of cooling *and* the differential
cooling -- if it exists at all. Like you, I am skeptical -- but am I
willing to bet $25K on it? Nope.

How does the rate affect things? I have a masters in structural
engineering and work for a materials company so don't be afraid to get
technical. :-)

Matt

By implication, a fast cooling rate would cause *more* differential cooling,
since the cylinders cool from the fins inward. The faster the cooling, the
higher the delta-T between the internal and external surfaces of the
cylinders. The higher the delta, the more internal stresses on the cylinders
due to the different growth between the hot and cold surfaces.

But you already knew that and were just being difficult, eh?

KB

Kyle Boatright wrote:
"Matt Whiting" wrote in message
news
Jay Honeck wrote:
The issue with shock cooling isn't the rate of cooling per se, but
rather stress induced by differential cooling.
Actually, I think it is the rate of cooling *and* the differential
cooling -- if it exists at all. Like you, I am skeptical -- but am I
willing to bet $25K on it? Nope.
How does the rate affect things? I have a masters in structural
engineering and work for a materials company so don't be afraid to get
technical. :-)

Matt

By implication, a fast cooling rate would cause *more* differential cooling,
since the cylinders cool from the fins inward. The faster the cooling, the
higher the delta-T between the internal and external surfaces of the
cylinders. The higher the delta, the more internal stresses on the cylinders
due to the different growth between the hot and cold surfaces.

But you already knew that and were just being difficult, eh?

It is a subtle point maybe, but an important one. It isn't the rate of
cooling that matters. It is a difference in rate between two locations.
Faster cooling doesn't necessarily a greater differential, it all
depends on how the cooling is done.

In some materials and at some temperatures, the rate of cooling can
change the fundamental material properties. That is a different issue
than what is involved with engines.

Matt

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

Morgans wrote:
"Morgans" wrote

At least that is my take on what they say.

I should have added that the cylinders heating up faster, on start up,
than the pistons would increase the tolerances, and thus cause no scuffing.

Why would the cylinders heat up faster? Only the very top of the
cylinder is in constant contact with the combustion heat whereas the top
of the piston is entirely in contact. As you go down the cylinder away
from the head, the cylinder spends less and less time in contact with
the combustion heat and thus will be cooler.

Matt

Jay Honeck wrote:


For you aircraft owners who do this regularly, how slowly do you
retard the throttle to prevent shock cooling? (I know -- does shock
cooling really exist? For purposes of this discussion, I'll pretend
that it does.)


I don't believe it exists so I'll go from a low cruise power setting of
say 20-21" and 2300 rpm to throttle fully retarded in 3-5 seconds.




Given that the power reduction must be incredibly gradual, do you feel
that this exercise is realistic?


Doing it your way? No.




There really is no chance to
simulate how you must "suddenly" find best glide speed (after your
engine has presumably just crapped out), since you're gradually
reducing your speed along with your power. Or do you put the plane
into a shallow dive as you reduce power, so as not to lose airspeed?


Your just going to have to be content that you are unwilling to
realisticly practice this. Find yourself a friends private dirt strip
field and start 5-7 miles away at 3000 AGL and land on it without
touching the power after you have gone to idle. Another thing I do all
the time is pull the power to idle on the downwind and land without
touching the power.

"Matt Whiting" wrote

Why would the cylinders heat up faster? Only the very top of the cylinder
is in constant contact with the combustion heat whereas the top of the
piston is entirely in contact. As you go down the cylinder away from the
head, the cylinder spends less and less time in contact with the
combustion heat and thus will be cooler.

Perhaps they would not. I was going simply on the converse.

My thought is that the cold oil shooting on the piston, and a relatively
weak combustion would keep the piston cooler.
--
Jim in NC