Preheating engines: Airplane engines versus auto engines

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, 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.

"Mike Spera" wrote in message
...


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

Mike, the coolant was at the correct winter dilution so I don't think
freezing was a contributory factor. More likely sloppy engineering standards
which similarly and sadly led to the downfall of the UK's motorcycle
industry, I know I owned many UK produced bikes. At least those industries
didn't manufacture aero engines.
Cheers,
Mike

More likely sloppy engineering standards
which similarly and sadly led to the downfall of the UK's motorcycle
industry,

A slight aside: that situation is the end point of all socialized
societies... Go along to get along, and all that...


denny

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"

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

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


Ahhhh...How do you "know" one cold start is going to have long term,
negative effects?

I normally preheat mine and when flying a lot the preheater was always
on with the cowl double wrapped. No condensation and worked great.

OTOH I have gone out on days that were really cold, not 25 degrees and
if I could get it to start I'd go flying without worrying about it.

I think you'll find that an engine that has set a long time and is
really cold takes a while to get oil to the cam. Every thing else gets
oiled just fine.

I'd not be overly concerned about a few cold starts as long as the
engine is given time enough to circulate warm oil.

In the Bo you'll know if it wasn't hot enough as the oil will congeal
in the oil cooler and the prop governor will let you know right away!
:-))

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?

Nearly all the cars we've had in recent years have gone between 100
and 200 thousand miles. None gave engine problems, none were
preheated, and all stayed outside, or in an unheated garage. The
engines stood up far better than the bodies did.

Roger (K8RI)

On Wed, 19 Dec 2007 17:35:50 +0000 (UTC),
(Paul Tomblin) wrote:

In a previous article, "Peter R." said:
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?



Aircraft engines are air cooled, auto engines are liquid cooled.

The following is what I was told when I was driving a Volkswagen Beetle,
and the experts were saying that you needed to let the beast idle for at
least 5 minutes in the winter:

Liquid cooled engines stay in a very narrow temperature range while
operating, so are built with very tight tolerances, but air cooled engines

Careful here. Don't confuse tight tolerances with tight clearances.
Aircraft engines have tighter tolerances than automobile engines.
Automobile engines have tighter clearances, but wider tolerances than
aircraft engines. If my car engine had the same fit as my IO-470N
it'd be considered worn out.

Roger (K8RI)
have more slop because they get both hotter and colder than liquid cooled
engines. Also, they are cooled primarily by the engine oil. Because of
that, you need to preheat the engine enough that the oil is spread around
and everything has warmed enough that the pistons are making good contact
with the cylinder walls.