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.

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 Wed, 19 Dec 2007 12:21:28 -0500, Peter R. wrote:

Why is it that here in the Northeast US seemingly no one preheats their
automobile engine before start-up in very cold temperatures? Is the long-term
damage the same for both autos and aircraft engines? If so, why do you
suppose auto owners don't typically do this? Is it because that most auto
owners do not keep their cars very long?

No because it is entirely unnecessary.
--
Remove numbers for gmail and for God's sake it ain't "gee" either!

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

"Peter R." wrote in message
...
This time of year here in the Northeast US I always preheat my Bonanza's
IO520 engine with a Tanis heater and an insulated cowling/prop cover as it
sits in an unheated t-hangar. The result is that the oil temperature at
startup is around 105 degrees F, even if the outside air temperature is as
low as -15 degrees F.

Monday night I arrived at my t-hangar to discover that at some point
during
the day the line person accidentally pulled out the plug connecting the
Tanis
heater to the small extension cord I use to extend the plug to the outside
of
the cowling cover, so the aircraft had not been preheating. Outside and
inside temperatures were both a cold 25 degrees F.

Given any other day, I would have plugged the aircraft back in and
scrapped
the flight but in this case I had an Angel Flight patient waiting in
another
city for my arrival and I was already late. Thus I made the painful
decision
to start up the aircraft and allow it to low idle until the oil heated
thoroughly. A small consolation is that the engine had been recently
filled
with fresh Exxon Elite oil. To my relief the aircraft started right up.

I know what I did has negative long term repercussions on my engine's
health
and I have already derived a tool to lock the two cords and prevent this
accidental unplugging from happening again. However, this leads me to
question the differences between aircraft engines and auto engines:

Why is it that here in the Northeast US seemingly no one preheats their
automobile engine before start-up in very cold temperatures? Is the
long-term
damage the same for both autos and aircraft engines? If so, why do you
suppose auto owners don't typically do this? Is it because that most auto
owners do not keep their cars very long?

--
Peter

I've been more cautious about rapid heating up of engines since I
unfortunately split the engine block of my Jaguar XJ6 4.2 litre that I
conclude was caused by immediate rapid driving from -5c being late for an
appointment. Guess that expensive incident will always stay in my mind...

Mike

Mike Gilmour wrote:
"Peter R." wrote in message
...
This time of year here in the Northeast US I always preheat my Bonanza's
IO520 engine with a Tanis heater and an insulated cowling/prop cover as it
sits in an unheated t-hangar. The result is that the oil temperature at
startup is around 105 degrees F, even if the outside air temperature is as
low as -15 degrees F.

Monday night I arrived at my t-hangar to discover that at some point
during
the day the line person accidentally pulled out the plug connecting the
Tanis
heater to the small extension cord I use to extend the plug to the outside
of
the cowling cover, so the aircraft had not been preheating. Outside and
inside temperatures were both a cold 25 degrees F.

Given any other day, I would have plugged the aircraft back in and
scrapped
the flight but in this case I had an Angel Flight patient waiting in
another
city for my arrival and I was already late. Thus I made the painful
decision
to start up the aircraft and allow it to low idle until the oil heated
thoroughly. A small consolation is that the engine had been recently
filled
with fresh Exxon Elite oil. To my relief the aircraft started right up.

I know what I did has negative long term repercussions on my engine's
health
and I have already derived a tool to lock the two cords and prevent this
accidental unplugging from happening again. However, this leads me to
question the differences between aircraft engines and auto engines:

Why is it that here in the Northeast US seemingly no one preheats their
automobile engine before start-up in very cold temperatures? Is the
long-term
damage the same for both autos and aircraft engines? If so, why do you
suppose auto owners don't typically do this? Is it because that most auto
owners do not keep their cars very long?

--
Peter

I've been more cautious about rapid heating up of engines since I
unfortunately split the engine block of my Jaguar XJ6 4.2 litre that I
conclude was caused by immediate rapid driving from -5c being late for an
appointment. Guess that expensive incident will always stay in my mind...

No, that was caused by driving a Jaguar...

Matt

I've been more cautious about rapid heating up of engines since I
unfortunately split the engine block of my Jaguar XJ6 4.2 litre that I
conclude was caused by immediate rapid driving from -5c being late for an
appointment. Guess that expensive incident will always stay in my mind...


My guess would be that the very cold temps caused the block to split due
to the coolant freezing over night. If it was -5C when you started it,
chances are it was much colder at some point in the evening.

Common in the upper Midwestern US was to see 70's era cars blow out
freeze plugs and/or crack blocks by running no (or too weak) antifreeze.
A cold start and immediate run up to high speed should not cause
catastrophic failure of a block. Being a Jag, it may have had a badly
cast block since new. Their QC ain't the best.

Good Luck,
Mike