Shock *Heating*?

In another thread (about cold starting) it was mentioned that Alaskan bush
pilots routinely drain their oil, take it inside, and heat it on the stove
before starting in the morning.

While this guarantees a warm(er) engine, wouldn't this risk damage to the
engine from shock heating (assuming there is such a thing), by suddenly
introducing boiling hot (or nearly so) oil to a cold-soaked engine?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:
In another thread (about cold starting) it was mentioned that Alaskan bush
pilots routinely drain their oil, take it inside, and heat it on the stove
before starting in the morning.
While this guarantees a warm(er) engine, wouldn't this risk damage to the
engine from shock heating (assuming there is such a thing), by suddenly
introducing boiling hot (or nearly so) oil to a cold-soaked engine?

Your engine operates at what temperature?
Think about the operating temperature of the oil. I doubt someone
knowledgeable would heat their oil up to the full operating temperature
and risk breaking down the oil. There is sufficient mass in the engine
to absorb the "warm" oil without damaging the case. I suspect that the
temperature of the oil by the time it has all been added will have
dropped many degrees.

In article QdQZb.376650$na.567800@attbi_s04,
Jay Honeck wrote:
While this guarantees a warm(er) engine, wouldn't this risk damage to the
engine from shock heating

Look at a graph from one of those data-capturing engine monitors some
time. The steepest lines on the graph will always be at takeoff. Even
pulling power and diving can't cool the engine as fast as the takeoff
roll can heat it up.

--
Ben Jackson

http://www.ben.com/

Look at a graph from one of those data-capturing engine monitors some
time. The steepest lines on the graph will always be at takeoff. Even
pulling power and diving can't cool the engine as fast as the takeoff
roll can heat it up.

So would this be proof that damage from "shock cooling" is a myth?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

john smith wrote:

I suspect that the
temperature of the oil by the time it has all been added will have
dropped many degrees.

It will have dropped many degrees just carrying the can out to the plane.

George Patterson
A diplomat is a person who can tell you to go to hell in such a way that
you look forward to the trip.

It is up to those that assert there is such a thing as damage from shock
cooling and shock heating to prove it. So far all anyone has ever offered is
a small amount of anecdotal evidence: "I knew a pilot once who did this and,
sure enough, his engine quit, although it was for some other reason."

"Gene Kearns" wrote in message
...
On Sun, 22 Feb 2004 02:18:40 GMT, "Jay Honeck"
wrote:

Look at a graph from one of those data-capturing engine monitors some
time. The steepest lines on the graph will always be at takeoff. Even
pulling power and diving can't cool the engine as fast as the takeoff
roll can heat it up.

So would this be proof that damage from "shock cooling" is a myth?

No. The offender is thermal shock... heating or cooling.....


It's not clear to me that the stresses in shock heating should be
the same as for shock cooling. Just because people don't
(apparently) suffer from the effects of shock heating doesn't
by itself preclude the possibility of shock cooling.

On take-off, the equilibrium is disturbed by heat being applied
uniformly around the cylinder bores. Any distortions result in
forces being applied to metals at low temperature. Speed is low,
so cooling (and hence cooling distortions) are minimal.

In cruise and at least the initial part of the descent, metal temperatures
are higher. Whether this makes metals more or less susceptible to
cracking, I've no idea - but if shock cooling is a fact and shock
heating isn't, we should expect metal to be more fragile at higher
temperatures. Also, since the initial airspeed is higher, changes in
cooling (speed-up during descent, closing cowl flaps) will alter
temperatures (and introduce differentials) on the 'cooling' side which
are far less uniform than during the initial climbout.

In summary, take-off -- low metal temps, less cooling, similar
'outer surface' temperatures, faster temperature change, descent --
high metal temps, more cooling, distorted 'outer surface'
temperatures, slower temperature change. Quite different
régimes.

"Gene Kearns" wrote in message
news
On Sun, 22 Feb 2004 17:31:16 GMT, "Tony Cox" wrote:

It's not clear to me that the stresses in shock heating should be
the same as for shock cooling. Just because people don't
(apparently) suffer from the effects of shock heating doesn't
by itself preclude the possibility of shock cooling.


Agreed, but I think the point not to lose sight of is that it is the
rapid *change* in temperature that causes the problem...
high-low...low-high... does it really matter?

The problem is induced stress, caused by different temperatures
at different locations in the metal of the engine. Temperature rate
of change observed on (say) the cylinder head is just one factor
which might predict the temperature variations throughout the engine;
my point is that there are other factors too. These factors are
different on take-off and landing. So even if the temperature rate
of change in one régime is higher than in another, this doesn't
necessarily indicate that the maximum stress in the engine is
higher. So no, I don't agree that rapid change (as measured by
some CHT) is the cause of the problem.

High-low and low-high *do* matter if the stress limit of metal
is a function of temperature, even if the internal stress fields are
similar for shock heating and cooling. Assuming that maximum
stress occurs at the instant that take-off power is applied (or
when power is reduced during descent) one would expect the
engine materials to be subject to this stress when they were
relatively cool (or hot, in descent). Stress limits of metals at
different temperatures are probably different.

{snip}
Whether this makes metals more or less susceptible to
cracking, I've no idea - but if shock cooling is a fact and shock
heating isn't, we should expect metal to be more fragile at higher
temperatures.

The word "fragile" is not used much in reference to metallurgy and
doesn't convey a lot of meaning to me. Remember, we are talking about
the forces involved in incidents akin to pouring a cool liquid into a
warm punch bowl.... not a pretty sight.... and is common to all
crystalline substances (amorphous or not).

OK, more likely to crack then. (and if you're keen to be pedantic,
crystals are never amorphous).


Also, since the initial airspeed is higher, changes in
cooling (speed-up during descent, closing cowl flaps) will alter
temperatures (and introduce differentials) on the 'cooling' side which
are far less uniform than during the initial climbout.


No, remember here, this is about transfer of (heat) energy. There is
more capacity of rushing (relatively cooler) air at descent, thus the
greater energy is cooling..... just the opposite is true at climb,
where the combustion process holds the upper hand and can cause
temperatures to soar in non-uniform ways.

You'd really have to do a finite element analysis to verify this.
But you can reasonably assume that if the airspeed is higher, then
the *variation* of the airspeed over different parts of the engine
is higher too. This means that substantially different amounts of heat
are being transferred away from different parts of the engine, which
(other things being equal), will mean that there'll be a temperature
variation over the surface which could itself lead to cracks.

The point is not to quantify this one way or the other - as I've said,
you need to do some complex modelling to be sure - rather, it is
to show that you can't predict the stresses in different modes of
flight by simply looking at the CHT rate of change.

C J Campbell wrote:

It is up to those that assert there is such a thing as damage from shock
cooling and shock heating to prove it.

With the cost of engines the way it is, I suggest that the situation is exactly
the other way 'round. Until you can prove conclusively that shock cooling does
not exist, baby the engine.

George Patterson
A diplomat is a person who can tell you to go to hell in such a way that
you look forward to the trip.

With the cost of engines the way it is, I suggest that the situation is
exactly
the other way 'round. Until you can prove conclusively that shock cooling
does
not exist, baby the engine.

That's good advice -- with any engine.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"