Preheating engines: Airplane engines versus auto engines

On Dec 21, 2:39*pm, nrp wrote:
Air doesn't come into the crankcase via the breather - rather
combustion products that leak past the rings vent out thru the
breather. *

It seems to me that the air in the crancase is about 250 deg F
typically, since the pistons and such are hotter than the oil. That
means the partial pressure inside the crankcase decreases as the
engine cools, and the pressure drops, according to the equation Pv=RT.
As the pressure drops inside, the air outside has to enter the
crankcase to equalize the pressure, correct? With the air temp as much
as 200 deg higher inside the engine as outside, that means that a
volume of about half the crankcase of outside air ENTERS the crankcase
during the pressure equalization process. At least that is the way it
seems to me.

Bud

"As the pressure drops inside, the air outside has to enter the
crankcase to equalize the pressure, correct? With the air temp as much
as 200 deg higher inside the engine as outside, that means that a
volume of about half the crankcase of outside air ENTERS the crankcase
during the pressure equalization process. At least that is the way it
seems to me."

But it isn't air in the crankcase during engine operation. It is a
mixture of CO2 and water vapor. Outside air will re-enter only when
the water vapor condenses after shutdown. The amount of water vapor
in the comparatively cool outside air being drawn in is one or two
orders of magnitude less than that in the hot crankcase.

On Dec 21, 11:38*pm, nrp wrote:
*"As the pressure drops inside, the air outside has to enter the
crankcase to equalize the pressure, correct? With the air temp as much
as 200 deg higher inside the engine as outside, that means that a
volume of about half the crankcase of outside air ENTERS the crankcase
during the pressure equalization process. At least that is the way it
seems to me."





But it isn't air in the crankcase during engine operation. *It is a
mixture of CO2 and water vapor. *

Well OK. The gas law of PV=nRT is true for all gases. All of them.
The point is that the engine does inhale (for the lack of a better
word) a significant volume of air from outside as it cools down. This
is a well known process. This air contains moisture. The oxidation
process (rusting) of the engine parts is galvanic corrosion and all it
needs is a molecular thin layer of moisture on the surface of say the
cam lobe. The total amount of water needed to cause this process is
miniscule. What prevents it from rusting your engine parts is the
layer of oil on them. Regular running of the engine replenishes this
layer of oil. This is what is important.

Outside air will re-enter only when
the water vapor condenses after shutdown.

The air enters as the pressure drops when the GAS inside the crankcase
cools. Condensation has nothing to do with it. If there were no
moisture in the gas contained in the crankcase at all, none, at
shutdown, the engine would still injest much more than enough moisture
than necessary to cause problems as it cools down. Condensation is
only only something else that happens along with the heat loss.

The amount of water vapor
in the comparatively cool outside air being drawn in is one or two
orders of magnitude less than that in the hot crankcase.

The point is that you can fly all you want and remove all the water
from the oil that exists. You will still get plenty of moisture inside
the engine as it sits idle just from the outside air it injests as it
cools. Even barometric pressure changes as the weather passes by is
enough to cause engine corrosion.
Many pilots seem to believe that blow-by in the engine is a
normal operating condition. My experience( and opinion) and that of
many other race engine mechanics that I have talked to, is that once
combustion gases begin to leak past the rings, the end of that engine
is imminent and soon. Very soon. Blowby totally destroys the
lubrication of the piston in the area of the blowby, and it shouldn't
take a rocket scientist to know what that means.

regards,
Bud

On Dec 22, 6:15 pm, wrote:

Many pilots seem to believe that blow-by in the engine is a
normal operating condition. My experience( and opinion) and that of
many other race engine mechanics that I have talked to, is that once
combustion gases begin to leak past the rings, the end of that engine
is imminent and soon. Very soon. Blowby totally destroys the
lubrication of the piston in the area of the blowby, and it shouldn't
take a rocket scientist to know what that means.

regards,
Bud

My experience is as an aircraft mechanic. Part of the
inspection process is the differential compression test on each
cylinder, when it's hot after shutdown. All cylinders leak a small
amount past the rings, and when the engine cools the leakage is
considerably worse. All rings have ring gaps, and unless you have
stacked rings (two rings in the same groove) you cannot stop the
leakage. Aircraft engines do not have stacked rings.
The fact that a frozen-shut breather will cause the front seal
to blow out is enough evidence that rings leak.
The volume of the crankcase, as noted earlier, is very small.
The amount of water in a cubic foot of air, even if it's saturated, is
miniscule compared to that which gets past the rings curing
combustion, unless the aircraft is parked for a long time and the
heating/cooling cycles of day/night pump air in and out repeatedly for
a long time. Water, even a small amount, mixes with oil and in the
presence of metal, which acts as a catalyst, breaks the oil down and
creates acids. The thin film of oil on the parts is the first
contributor to this process and is not much protection at all.

A fun science experiment is to put a small amount of water in the
bottom of a 1 gallon rectangular can. Boil it for a few minutes to
displace the air inside, remove it from the heat, and immediately
replace the cap and watch.

For more excitement, repeat with another can only this time cap it and
sprinkle cold water on the outside of the can too.

(Hint - don't plan on using the cans again.....!)

* * * * My experience is as an aircraft mechanic.

Thanks for a great primer, Dan.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"