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