oil level effects oil pressure

I've noticed that if I run my engine lower on oil than usual (but still
well above the min) my pressure runs lower. I"m trying to understand
how oil level effects oil pressure. If there is enough oil to hit the
sump, wouldn't the pressure be uneffected?

-Robert
IO-360-A3B6 engine.

You are correct in thinking that as long as the oil pickup has a
constant supply of oil the pressure should remain the same no matter
the amount of oil in the sump. If you look closer you will probably see
that the oil temp is higher. Since you have less oil it circulates
through the motor faster and that gives it less time in to loose heat.
Hotter oil = lower oil pressure, it's that viscosity thing ya know....

Ben

wrote in message
ups.com...
You are correct in thinking that as long as the oil pickup has a
constant supply of oil the pressure should remain the same no matter
the amount of oil in the sump. If you look closer you will probably see
that the oil temp is higher. Since you have less oil it circulates
through the motor faster and that gives it less time in to loose heat.
Hotter oil = lower oil pressure, it's that viscosity thing ya know....

Ben

No doubt that higher oil temperature would result in lower oil pressure.
However, unless the engine is running with insufficient oil, I don't see
how the oil level would drive oil temperature.

In flight, the oil eventually reaches a temperature equilibrium. It cycles
through the engine, into the pan, then through the oil cooler. At a given
power level, the engine adds X calories/minute (or joules, or whatever) to
the oil and the cooler removes Y calories/minute. Eventually, the oil
temperature stabilizes at a temperature where X = Y. Oil level doesn't
change that temperature.

Sure, an airplane with 4 quarts in the sump will reach operating temperature
faster than one with 8 quarts in the sump, but the difference isn't that
great, and in the end, both engines should end up with the same oil
temperature...

KB

No doubt that higher oil temperature would result in lower oil
pressure.
However, unless the engine is running with insufficient oil, I don't
see
how the oil level would drive oil temperature.

In flight, the oil eventually reaches a temperature equilibrium. It
cycles
through the engine, into the pan, then through the oil cooler. At a
given
power level, the engine adds X calories/minute (or joules, or whatever)
to
the oil and the cooler removes Y calories/minute. Eventually, the oil
temperature stabilizes at a temperature where X = Y. Oil level doesn't

change that temperature.


Sure, an airplane with 4 quarts in the sump will reach operating
temperature
faster than one with 8 quarts in the sump, but the difference isn't
that
great, and in the end, both engines should end up with the same oil
temperature...


KB


///////////////////////////
KB, First let me say you explained yourself well. My thoughts are this,
if the oil pump can move 4 quarts a minute through the motor and the
motor has 4 qts in it then the time spend in the sump/oilpan is
minimal. The sump/pan has intake tubes in it and a large surface that
dissapates heat as the air passes over the surfaces. If the oil level
is low the hot oil coming out of the motor is recirculated right back
before it can shed alot of its heat. If there is more oil in the
system, then it can spend a little more time in the sump/pan which then
will let it loose extra heat. Think of it like this, suppose you had a
solar heater for a swimming pool, the thing moves 10 gallons a minute
and raises the temp of the liquid passing through it 20 degrees. If the
pool holds 100 gallons then it will rise in temp pretty quick, if it
held 30,000 gallons by the time all the water is circulated, the
surface area will dissapate heat faster then the solar panel can keep
heating it. I agree if the pool was perfectly insulated then your
temperature equilibrium concept is valid. Am I missing something here??

Normally I wouldn't think oil level would affect pressure either.
Might the oil pickup tube (I think your engine has one) be leaking in
air where it is attached to the accessory cover, or maybe the oil pump
have an inlet leak in it?

How about it you engine guys.........

I used to have an engine that burned a lot of oil. The book said
minimum level was 2 quarts. However, on a couple instances, I got down
to about 4 1/2. There was a distinct and consistant drop in pressure at
that point. It wasn't gradual, but a distinct drop at a certain level.
Always wondered about that too.
--
Gene Seibel
Gene & Sue's Aeroplanes - http://pad39a.com/gene/planes.html
Because we fly, we envy no one.

wrote in message
oups.com...



No doubt that higher oil temperature would result in lower oil
pressure.
However, unless the engine is running with insufficient oil, I don't
see
how the oil level would drive oil temperature.

In flight, the oil eventually reaches a temperature equilibrium. It
cycles
through the engine, into the pan, then through the oil cooler. At a
given
power level, the engine adds X calories/minute (or joules, or whatever)
to
the oil and the cooler removes Y calories/minute. Eventually, the oil
temperature stabilizes at a temperature where X = Y. Oil level doesn't

change that temperature.


Sure, an airplane with 4 quarts in the sump will reach operating
temperature
faster than one with 8 quarts in the sump, but the difference isn't
that
great, and in the end, both engines should end up with the same oil
temperature...


KB


///////////////////////////
KB, First let me say you explained yourself well. My thoughts are this,
if the oil pump can move 4 quarts a minute through the motor and the
motor has 4 qts in it then the time spend in the sump/oilpan is
minimal. The sump/pan has intake tubes in it and a large surface that
dissapates heat as the air passes over the surfaces. If the oil level
is low the hot oil coming out of the motor is recirculated right back
before it can shed alot of its heat. If there is more oil in the
system, then it can spend a little more time in the sump/pan which then
will let it loose extra heat. Think of it like this, suppose you had a
solar heater for a swimming pool, the thing moves 10 gallons a minute
and raises the temp of the liquid passing through it 20 degrees. If the
pool holds 100 gallons then it will rise in temp pretty quick, if it
held 30,000 gallons by the time all the water is circulated, the
surface area will dissapate heat faster then the solar panel can keep
heating it.

If you had a 10 gallon pool with the same heat rejection ability as your
30,000 gallon pool, you'd get to the same temperature as your 30k gallon
pool. It ain't the mass of the fluid, it is the heat added to or rejected
from entire system. No doubt the 10 gallon pool would stabilize at the
equilibrium temperature faster, tho...

I agree if the pool was perfectly insulated then your
temperature equilibrium concept is valid. Am I missing something here??

I would say that regardless of oil level, the sump rejects Z calories of
heat per minute (due to the excellent heat conduction properties of
aluminum), so it really doesn't matter whether there are 4 or 8 quarts in
the sump. If you don't change the radiating area of the sump or the amount
of calories you need to reject via the sump the dwell time of the oil in the
sump really shouldn't matter in a meaningful way.

KB

I would say that regardless of oil level, the sump rejects Z calories
of
heat per minute (due to the excellent heat conduction properties of
aluminum), so it really doesn't matter whether there are 4 or 8 quarts
in
the sump. If you don't change the radiating area of the sump or the
amount
of calories you need to reject via the sump the dwell time of the oil
in the
sump really shouldn't matter in a meaningful way.

KB


///////////////////

Hmmmm. I think you just answered your own question. Aluminum is a great
conductor of heat, we all agree on that. If you have more oil in the
sump the you have a greater area of liquid in contact with the aluminum
sump because of a higher oil level. A larger wetted area results in
greater heat dissapation...No ?????