Preheating engines: Airplane engines versus auto engines

Air doesn't come into the crankcase via the breather - rather
combustion products that leak past the rings vent out thru the
breather. Burning hydrocarbons generate CO2 and water. The net dew
point of combustion and blowby products is about 180 degF. The water
will condense in cooler sections of the crankcase. It is this water
that causes most corrosion - especially after combining with nitrogen
oxides and sulfur oxides which make acid.

Crankcase condensation happens from engine operation - not from just
sitting around. The real trick is to ventilate these residual
combustion products from the crankcase immediately after shutdown
before they all condense. Systems are now starting to be sold which
actively do this.

A lot of this moisture accumulation problem would go away if aircraft
engines had a positive crankcase ventilation (PCV) system like car
engines now do, but they don't for whatever reason. I suspect part of
the reason car engines now last so much longer is due to the PCV
system.

On Dec 21, 1:39 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. Burning hydrocarbons generate CO2 and water. The net dew
point of combustion and blowby products is about 180 degF. The water
will condense in cooler sections of the crankcase. It is this water
that causes most corrosion - especially after combining with nitrogen
oxides and sulfur oxides which make acid.

Crankcase condensation happens from engine operation - not from just
sitting around. The real trick is to ventilate these residual
combustion products from the crankcase immediately after shutdown
before they all condense. Systems are now starting to be sold which
actively do this.

A lot of this moisture accumulation problem would go away if aircraft
engines had a positive crankcase ventilation (PCV) system like car
engines now do, but they don't for whatever reason. I suspect part of
the reason car engines now last so much longer is due to the PCV
system.

Part of the problem is the water mixed with the oil; it's
reluctant to evaporate when it's like that. Running the engine long
enough to give it time to boil out is the best thing, and a PCV system
would surely help.
Proof of water as a combustion byproduct can be noted in
colder climates. If the breather tube is not drilled with a relief
hole partway up from its exit, it's liable to freeze up in cold
weather as the moisture that's constantly leaving the tube freezes at
the exit and plugs it. Then the pressure builds in the case and blows
the front seal out, scaring the daylights out of the pilot as oil
covers the windscreen. Some operators insulate that tube as well to
keep the gases hot enough to keep that exit open.
When we bring the airplanes in after operating in cold
weather, oil and water emulsion will be found on the floor under the
breather tube the next morning. That water wasn't sucked into the
engine as it cooled off. The engine's internal volume might be two or
three cubic feet, and if the air in there contracts by even 30%, that
little bit isn't going to pull in much moisture. It becomes a bigger
problem in wet climates and repeated warming/cooling cycles, as an
airplane sits outside for months on end and gets warm in the sun and
cools off at night. The same phenomenon puts water in your fuel tanks.

Dan

There's probably no more than about 1 cu ft of volume in a typical
crankcase. Even so that will contain on the order of half to one shot
glass full of water on shutdown. This will almost entirely condense
out as the crankcase is cooled to room temperature.

There is a slight amount of in-out-in of surrounding atmospheric
humidity with temperature, but the amount of water contained in that
air is trivial compared to that generated or left over by the products
of combustion.

When we bring the airplanes in after operating in cold
weather, oil and water emulsion will be found on the floor under the
breather tube the next morning.



I suspect that the puddles have more to do with the fact that breather
outlets tend to be on the top of the engine and are connected to a 3
foot tube running straight down than any gasses purging out of the
crankcase at shutdown (or what the temperature was outside). The tube
walls are coated with a water/oil mix from flight and slowly this drips
down to cause the puddle.

An interesting test would be to remove the breather tubes completely
after flight and see if anything accumulates.

Good Luck,
Mike

On Dec 22, 6:43 am, Mike Spera wrote:
When we bring the airplanes in after operating in cold
weather, oil and water emulsion will be found on the floor under the
breather tube the next morning.

I suspect that the puddles have more to do with the fact that breather
outlets tend to be on the top of the engine and are connected to a 3
foot tube running straight down than any gasses purging out of the
crankcase at shutdown (or what the temperature was outside). The tube
walls are coated with a water/oil mix from flight and slowly this drips
down to cause the puddle.

An interesting test would be to remove the breather tubes completely
after flight and see if anything accumulates.

Good Luck,
Mike

That's what I meant. It's not any gases coming out after
shutdown; it's the thickened oil, containing water, that hangs in that
cold breather tube and drips out slowly overnight in the heated
hangar. The water in the emulksion came from combustion blowby.

Dan

I suspect that the puddles have more to do with the fact that breather
outlets tend to be on the top of the engine and are connected to a 3
foot tube running straight down than any gasses purging out of the
crankcase at shutdown (or what the temperature was outside). The tube
walls are coated with a water/oil mix from flight and slowly this drips
down to cause the puddle.

An interesting test would be to remove the breather tubes completely
after flight and see if anything accumulates.

Good Luck,
Mike


That's what I meant. It's not any gases coming out after
shutdown; it's the thickened oil, containing water, that hangs in that
cold breather tube and drips out slowly overnight in the heated
hangar. The water in the emulksion came from combustion blowby.

Dan

Sorry Dan, That thread was so screwed up I could not tell if you were in
the "it blows out after shutdown" or the "it sucks in after shutdown"
group. I'm not sure the breather tube needs to be in the cold. My
Cherokee drips whether it is 0 or 90 outside.
Mike

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.