Preheating engines: Airplane engines versus auto engines

On Thu, 20 Dec 2007 08:21:32 -0800 (PST),
wrote:

On Dec 20, 6:04 am, Denny wrote:
The biggest killer of aircraft engines is dry starts
from weeks/months of sitting between starts...


That's one, but there's another bad one: Short flights,
especially in cold weather. byproducts of combustion include water
vapor, and some of that squeezes past the pistons and rings when the
engine is cold (some when it's hot, too, but much less so) and this
vapor condenses in the crankcase and ends up in the oil. If the engine
doesn't get hot enough for long enough, the water isn't boiled off and
will mix slowly with the oil, breaking it down and combining with
sulfur and chlorine and nitrogen to form sulfuric, hydrochloric and
nitric acids. These don't belong in your engine. The stuff that's left
from these reactions forms sludge and clogs up hydraulic lifters and
cakes on the inside of the case and soon enough breaks off and shows
up as scary black guck in the filter. The acids cause dissimilar metal
corrosion between the crank and cam and their bearings, between the
aluminum piston and the steel cylinder and rings, and on valve stems.
Bad. Corroded valve stems break and the engine tries to eat the valve
heads and gets indigestion.
The oil in my little old Continental doesn't get above
120°F on cold days. There's a tank blanket that I need to buy or make
to get it up. I just finished rebuilding the thing to fix corroded
bearings and seized valve lifters.

Dan

as a matter of deep religious significance, once your engine is warm
start the clock and dont land until an hour is up at least.

in the depths of winter remember that some parts of the world need
global warming :-)

now to be serious...
squeezing past the rings probably contributes a millionth of one
poofteenth of a percent to the problem.
when the crankcase cools there is moist external air sucked into the
cavity of the case through the crankcase breather. the moisture
condenses onto the cold internal surfaces of the engine.
can you please factor that into the alchemy above? it probably does
the damage occurring between the poofteenth of your scenario and the
100%.

now if you really want scarey black gunk try taking off the oil
reservoir and cleaning it out.

honestly you must lay awake sleepless at night :-)

Stealth (the O-200 is king) Pilot

On Dec 21, 4:45 am, Stealth Pilot
wrote:

squeezing past the rings probably contributes a millionth of one
poofteenth of a percent to the problem.
when the crankcase cools there is moist external air sucked into the
cavity of the case through the crankcase breather. the moisture
condenses onto the cold internal surfaces of the engine.
can you please factor that into the alchemy above? it probably does
the damage occurring between the poofteenth of your scenario and the
100%.

Nope. We have taken rocker covers off engines immediately
after a runup of a brand-new engine and found copious amounts of water
in them. The blowby of any cold engine is significant. If we briefly
run up an engine that has sat all night in a heated hangar and in our
very dry winter climate, we will find water on the dipstick every
time, with the engine at any point in its life. And the dipstick was
dry beforehand.
We operate on the western Canadian prairies where the air is
drier that where I grew up in south-central BC, which is the northern
tip of the Sonora Desert. We get little rain and snow here. Temps
reach -40C, more typically -20C, no fog and clear skies most of the
winter. It's REALLY dry, and any air sucked into these engines after
shutdown doesn't have enough moisture to make a couple of tears.

Dan

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.