The role of oil in engine cooling

[RotaryWingBob]

Lycoming's Key Reprints contains a couple of lines about the idea that a secondary function of the engine oil is to help cool the pistons and cylinders.

Does anybody have a reference that discusses whether oil quantity has an effect on the ability of oil circulation to provide cooling? In other words, let's say the minimum oil level is 6 quarts. If the oil is very low, let's say at 3 quarts, does that impact cooling? My gut says yes, but my gut isn't always righht

 

[ScottM]

I would guess no, that the oil quantity, as long as it is in the min to max range, does not affect cooling.

I say this because the primary way that oil is cooled is through the oil cooler not by coming into contact with other oil in the sump. Therefore, whatever amount of oil is in the sump will basically be about the same temp and not being cooled in the sump to affect engine temp.

If the amount of oil were to drop to too low, well below the oil minimum level, I would then say that you are right and it would adversly affect temps. But that is just because the volume of oil going through the cooler would be a lot less than what normally goes through there. There should be other changes in the engine guages and performance as well.

At least that is my guess at it.

 

[rmciottijr]

Bob

I would say the oil volume would have an effect on the oil temperature. The more volume, the more heat displacement/absorption. I don't think a quart low would make a noticeable increase in temperature but it will increase.

I believe Nascar cars use a lot of oil to reduce heat. I am not a Nascar fan but I think they have a ton of oil in a tank in the back of the car.

Bob

 

[Cap'n Jack]

My educated guess is that the amount of oil does play a role in the amount of cooling. The oil not circulating in the engine is in a sump someplace being cooled. With more oil, the portion of oil sits in the sump longer being cooled. If the oil level is low, the oil get recirculated through a hot engine more times causing faster degradation.

IIRC, about 1/3 of the engine cooling is done via the oil.

I'm sure an A&P will jump in and explain this much better than I have done here.

Edited for spelling

 

[mikea]

I always thought that you did need a minimum volume of oil for cooling. I dunno where I read that early on.

 

[Tom-D]

Lycoming's Key Reprints contains a couple of lines about the idea that a secondary function of the engine oil is to help cool the pistons and cylinders.

Does anybody have a reference that discusses whether oil quantity has an effect on the ability of oil circulation to provide cooling? In other words, let's say the minimum oil level is 6 quarts. If the oil is very low, let's say at 3 quarts, does that impact cooling? My gut says yes, but my gut isn't always righht

If you answer this you will answer your own question.

If oil doesn't have any thing to do with engine cooling, why would you place an oil cooler in the system?

Or If the oil doesn't absorb heat from the engine, why does it get hot?

 

[Nav8tor]

I suspect that once you have the minimum required amount in the crankcase you acheive the maximum cooling benefit from the oil. Any additional oil just fills the crankcase higher. The lubrication channels in the engine can only hold a given volume and I expect the oil temp is somewhat (for a given OAT) constant regardless of oil volume as long as it is at or above the minimum requirement.

 

[Steve]

Below a certain volume the oil will heat up faster than the oil cooler can reject the heat due to inadequate flow through the cooler. Determining what that volume is would require a heat balance calculation across the oil circulation loop. I suspect the oil pump would cavitate before the minimum theoretical cooling volume was achieved.

It would be hard to distinquish between damage due to loss of lubrication vs. lack of cooling as loss of lubrication necessarily allows localized heating to occur.

The IAR has two recommended oil volume operating ranges. One for aerobatics with a minimum of 6 qts. and one for "normal" flight with a minimum of 9 qts. The engine capacity is 13 qts. I put in 11 qts. at oil change and try to maintain 10 qts. between oil changes. Any more than that ends up on the belly in short order. This is an IO-540 Lyc.

 

[gismo]

Below a certain volume the oil will heat up faster than the oil cooler can reject the heat due to inadequate flow through the cooler. Determining what that volume is would require a heat balance calculation across the oil circulation loop. I suspect the oil pump would cavitate before the minimum theoretical cooling volume was achieved.

Seems to me that as long as the oil is circulating at the same rate, the temp drop in the cooler would be the same regardless of the level in the sump. And while the dwell time in the sump would indeed decrease with lower oil level, the actual heat transfer from oil to sump might even be higher with less oil since some of the oil is insulated from the sump by the rest.

 

[Steve]

At some point in the sump level the net positive suction head (NPSH) for the oil pump would be less than rated, thereby lowering the flow rate, even for a positive displacement pump. Drop it low enough the pump will cavitate. Don't have the vapor pressure tables for 50W oil handy.

Besides oil cooling, there's direct air cooling and exhaust cooling as paths of heat rejection. A heat balance for the engine to determine the percentages would be fun to do empirically, but major engine manufacturers have the instrumentation (and the skill) to do it.

 

[Shipoke]

Well as a Truck mechanic, my opinion is yes the amount in the sump does effect cooling only because as air passes around the sump pan it disipates heat.and steve you are also right at a lower amount pump will cavitate,cause if it gets to near the pickup it will cause a funnel effect in the oil just like water down the drain. And air does not cool or lube very well in a engine and will produce higher temp readings.
Dave G

 

[gismo]

Well as a Truck mechanic, my opinion is yes the amount in the sump does effect cooling only because as air passes around the sump pan it disipates heat.and steve you are also right at a lower amount pump will cavitate,cause if it gets to near the pickup it will cause a funnel effect in the oil just like water down the drain. And air does not cool or lube very well in a engine and will produce higher temp readings.
Dave G

If air enters the oil pump, a rise in oil temp would be the least of your problems because the pump cannot generate much pressure with air in the gears and even if it could, air isn't much of a lubricant is it?

But we were talking about the difference in heat export capacity between a full crankcase and one with the oil level reduced a quart or two but still within the manufacturer's low oil limit. And in that case I'm pretty certain that the difference is minscule, plus as I said it may be that full oil would actually cause a slight decrease in the ability to shed heat from the oil (due to the insulating effect of a layer of oil on the bottom of the pan). In any case, if being a quart down causes the oil temp to rise too high, there are more serious problems underfoot.

 

[Skip Miller]

But we were talking about the difference in heat export capacity between a full crankcase and one with the oil level reduced a quart or two but still within the manufacturer's low oil limit. And in that case I'm pretty certain that the difference is minscule,

Since the engine is certified to run at the low oil limit, I am with Lance on this one. The difference can't be huge.

The primary reason that the engine carries more oil than that is to satisfy a different condition: to be able to fly with a certain size leak or certain high oil consumption for a certain time... allowing the pilot to complete the flight and (hopefully) catch the low oil/leak/high consumption on the next preflight. (This last sentence is hearsay, I'd love to read this in some authoritative source.)

In any case, if being a quart down causes the oil temp to rise too high, there are more serious problems underfoot.

Agreed!

-Skip

 

[Steve]

Reread the original post.

The question involved operation below the "minimum oil level".

Let's change the initial conditions to satisfy a conclusion.

If air enters the oil pump, a rise in oil temp would be the least of your problems because the pump cannot generate much pressure with air in the gears and even if it could, air isn't much of a lubricant is it?

But we were talking about the difference in heat export capacity between a full crankcase and one with the oil level reduced a quart or two but still within the manufacturer's low oil limit. And in that case I'm pretty certain that the difference is minscule, plus as I said it may be that full oil would actually cause a slight decrease in the ability to shed heat from the oil (due to the insulating effect of a layer of oil on the bottom of the pan). In any case, if being a quart down causes the oil temp to rise too high, there are more serious problems underfoot.

 

[Skip Miller]

Reread the original post.

The question involved operation below the "minimum oil level".

On reviewing the first post, you are right

Let's change the initial conditions to satisfy a conclusion.

No, let's just read quickly and not understand the question. Anyhow, apologies from me, regardless of why you think we went astray.

I also would think that oil below the minimum could well have an effect on temperature. In the marine environment, with big block chevys converted to marine use, we could definitely see a correlation between low oil quantity and high oil temperatures.

-Skip

 

[GaryO]

I suspect that once you have the minimum required amount in the crankcase you acheive the maximum cooling benefit from the oil. Any additional oil just fills the crankcase higher. The lubrication channels in the engine can only hold a given volume and I expect the oil temp is somewhat (for a given OAT) constant regardless of oil volume as long as it is at or above the minimum requirement.

While I am no mechanic I can say definately that this statement is dead wrong!
While it is true that oil temperature will remain in the green when the oil quantity is at/above the minimum level, I can judge my oil quantity by the oil temp.

My Lyc 235 has a min/max of 4/6 quarts. If I fill it to 6 it will go down to 5 on the first flight. When it drops below 4 3/4 I notice an increase in oil temp.

 

[Stache]

As an A&P/IA I can say oil does play a very big role primary engine cooling and secondary engine lubrication. Oil keeps moving parts from touching each other causing less friction and picks up heat from the parts and takes it to an oil cooler or sump to cool off. All engines are designed to run with a minimum about of oil. Engines are certificated to run at least one quart low with no problems as long as oil pressure is maintained you will not hurt the engine. The oil also aids in forming a seal between the piston and the cylinder wall to prevent leakage of gases from the combustion chamber. This is what turn oil BLACK in color. Reference is AC65-12 Powerplant Handbook, Chapter 6 Lubrication and Cooling Systems.

“Aviation engine lubricants that are referred to as straight mineral oils contain paraffinic base oils plus very small amounts of pour point depressant and antioxidant. These additives lower the temperature at which the oils will flow and increase the temperature that they can be used without breaking down. The oils contain no cleanliness or anti-wear additives.

Straight mineral aviation oils are available in different viscosity grades. Viscosity grade refers to the oil’s thickness or resistance to flow say’s Ben Visser”. Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.

Stache

 

[Tom-D]

As an A&P/IA I can say oil does play a very big role primary engine cooling and secondary engine lubrication. Oil keeps moving parts from touching each other causing less friction and picks up heat from the parts and takes it to an oil cooler or sump to cool off. All engines are designed to run with a minimum about of oil. Engines are certificated to run at least one quart low with no problems as long as oil pressure is maintained you will not hurt the engine.

I'm wondering why Lycoming says 2 quarts is the minimum oil level for the 0-320 series?

 

[RotaryWingBob]

Reread the original post.

The question involved operation below the "minimum oil level".

Let's change the initial conditions to satisfy a conclusion.

Thank you Steve! I'd like to get this discussion back on track. What bothers me is that folks are offering opinions, as opposed to something which can be substantiated...

 

[gismo]

Reread the original post.

The question involved operation below the "minimum oil level".

Let's change the initial conditions to satisfy a conclusion.

You're right. I think I must have had the second post stuck in my mind, although I might have been confused by the 6 qt minimum in the first post. I don't think the "minimum oil level" (the level below which the pump might pick up air) in any 4 or 6 cylinder modern aircraft engine is anywhere near 6 quarts even if that's the lowest reading on the dipstick. Certainly if you get down to 2-3 quarts, you could have serious lubrication issues and there's little doubt that the oil temp would go up, if for no other reason than the extra heat due to poor lubrication of the bearings it's supposed to be flowing through.

 

[RotaryWingBob]

You're right. I think I must have had the second post stuck in my mind, although I might have been confused by the 6 qt minimum in the first post. I don't think the "minimum oil level" (the level below which the pump might pick up air) in any 4 or 6 cylinder modern aircraft engine is anywhere near 6 quarts even if that's the lowest reading on the dipstick. Certainly if you get down to 2-3 quarts, you could have serious lubrication issues and there's little doubt that the oil temp would go up, if for no other reason than the extra heat due to poor lubrication of the bearings it's supposed to be flowing through.

Lance, that's what what my gut says, but I'm looking for something substantative. As I said before, my gut isn't always right

 

[Keith Lane]

Here's a little something I found that is related.
http://www.erc.wisc.edu/publications/thesis/thesis/thesis_Dembroski.html

Dembroski, Terry Joseph - MS

Piston Heat Transfer in An air-cooled Engine


Thermal energy flow within the piston of an air-cooled engine was investigated by measuring the heat flux in the engine cylinder liner in the region of travel of the piston rings. Engine sump oil temperature and oil viscosity were varied to change the piston ring heat flow by altering the oil film between the cylinder wall and piston ring. Engine load was varied to change heat flow into the piston. Also, the crankcase oil splash cooling of the piston was varied to change heat flow leaving the piston.