M20 Air/Oil separator

John_F wrote:

On Thu, 02 Sep 2004 16:56:03 -0400, Matt Whiting
wrote:


John_F wrote:


There are two terms in this temperature equation.

One is steady state thermal transfer rate. If you generate a
quantity of heat say one BTU then it will raise one quart of oil X
degrees F. If you add two times as much heat to the same oil it will
raise the temperature of the quart of oil 2X degrees F.
At any given RPM the oil pump will pump "Y" quarts of oil per minute
whether you have 2 quarts or 10 quarts in the sump since the pump is a
gear positive displacement pump. This means that the oil will carry
of the SAME amount of heat per minute if the temperature delta is the
same. If you want to get rid of more heat then you have to pump the
oil faster or heat the oil hotter to get a larger delta temperature
difference. This is the steady state condition. This is the condition
the engine is in when the temperature gage quits moving up.

Or you start with cooler oil in the sump. This is what, I believe, will
happen when you have more oil in the engine. The oil has a longer
residence time in the sump and contacts more surface area of the sump
through which it may dissipate heat.


You always START with COOL oil. It just does not stay that way long.

I didn't mean start as in first starting the engine. I meant start as
in the oil pump is starting out with cooler oil at its intake if there
is more oil in the sump and it has resided there longer to dissipate its
heat to the sump.



Let's run a "thought" experiment at the limits. Let's assume that the
oil level is so low that no oil is ever in the sump. The oil pump pulls
it out just as fast as it comes in, just short of the point of sucking
air. I realize this isn't possible in the real world, but that is why
this is a thought experiment.


This is done all the time. It is called a DRY sump and is used on many
aircraft. As far as I know ALL radial use a dry sump.

You are really showing your ignorance now. A dry sump system doesn't
mean that there is no reservoir of oil, it just isn't in the sump, but
is remotely located. This actually gives better oil cooling typically
as the oil reservoir often isn't even connected to the engine. You may
want to read a little at the following link. It not only explains
different oiling systems, but is also states that oil serves a cooling
function in an engine.
http://www.geocities.com/zuuba_2000/Aeromat.html



In this case, the oil will get very hot
as it is constantly being circulated through the heads which are one of
the hottest parts of most engines. The oil has very little opportunity
to reject heat in the coolest part of the engine, the sump. The
equilibrium temperature will be rather high.


Nope! That is what the oil cooler is for.

Heat doesn't care where it's at, it always moves from a hotter location
to a cooler location. Sure, the oil cooler is made to cool the oil, but
the oil will still lose heat anywhere in which the oil temperature is
higher than the surroundings. This can be in the sump or can be in a
remote oil tank as with dry sump systems.



Most oil sumps on Lycoming engines are flat bottomed with vertical
sides. Look at the ratio of the surface area of the bottom to the
sides. It is at least two to one. Aluminum conducts heat MUCH
better than oil so any oil that runs across the bottom of the sump
will get cooling via the bottom and by conduction through the aluminum
up the sides. Most of the heat does NOT go out through the sump but
through the oil COOLER. If you have enough oil in the sump to cover
the oil intake it will be cooled by the bottom surface of the sump.
If you do not believe that cover up your oil cooler and see what will
happen to the oil temperature.

I never said that most of the heat goes out through the sump. I said
that SOME heat does. And I said that MORE heat will be rejected as the
contact area between the oil and the sump increases. Sure the bottom
has a lot of area in contact, but if you raise the oil level it will
lose even more heat due to contact with the sides of the sump. More
heat loss equals lower oil temperature and greater engine cooling by the
oil.



Now take the other extreme. The oil sump has infinite capacity so the
oil starts out at the same temperature regardless of how hot the hot
parts of the engine are. The oil will enter the oil pump relatively
cool and pick up heat, but will never again get circulated through the
engine so it has "forever" to dissipate its heat.


If you have an infinite oil cooler that will cool the oil back to the
original temperature it still does not matter how much oil is in the
sump.

True, what is your point?


A real engine is somewhere in between these to limit cases, therefore it
is reasonably logical to expect some slope that connects the
steady-state oil temperature of the one limit with the other. I don't
think it reasonable to believe that both steady-state temperatures will
be the same and thus have a zero-slope line in between. This is what
would have to be the case for the oil temperature to be completely
independent of the amount of oil in the engine.


There may be a small slope but it may be positive not negative since
the extra oil gets tossed around more and heats up due to mechanical
splashing.

Wrong. If any significant splashing occurs due to contact with the
crank, the oil will aerate, the oil pump will cavitate and the engine
will fail shortly thereafter.



Applying some simple math:
Since 90 % of the heat that is carried by the oil is lost in the oil
cooler not the sump you would expect that if the quantity of heat
carried away by the sump doubled the oil temperature would only drop
50% of 10% which is 5%.

What is your source for the 90% figure? 5% is still 5% and will result
in increased engine cooling as the oil level increases. It may well be
a small number, but any extra cooling at all is significant as the life
expectancy of oil drops dramatically with increasing temperature.


Matt