More oil == cooler engine temps?

[focal_plane]

Many times I have seen it said that having more oil in the crankcase leads to lower oil temps. I can understand that rationale during warmup since more oil would have more thermal mass. But once equilibrium is reached, the same volume of oil should be moving through the oil cooler and rejecting the same amount of heat regardless whether you have 50%, 75% or 100% of capacity. Not talking about dreadfully low oil level where volumetric flow gets impeded. What am I missing?

 

[MRC01]

Here's one possible explanation for this notion. I don't know whether it is correct, but it seems intuitive to me. As each drop of oil travels through the engine it gets hot. Once it passes through the engine and falls back into the oil sump, it cools because the the sump is not as hot as the engine. It sits in the sump for some length of time before it reaches the bottom, gets sucked up by the pump and pushed through the engine again.

When the hot oil fresh out of the engine falls into the sump it can't change temperature instantly. The longer it sits in the sump, the more time it has to cool to the temperature of the sump. When you add more oil to the sump, it doesn't change the amount of oil that is in the engine at any given time. It only increases the amount of oil in the sump, which increases the average duration that each drop of oil spends in the sump before the pump sucks it up again and sends it back into the engine. It also increases the thermal mass of the sump, which promotes that temperature change.

 

[Dana]

The oil cooler isn't the only place where oil sheds heat. Many engines don't have an oil cooler at all, so all the heat is lost via conduction through the crankcase and sump. The more oil in the engine, the more time it stays in the sump, shedding heat.

 

[kyleb]

Many times I have seen it said that having more oil in the crankcase leads to lower oil temps. I can understand that rationale during warmup since more oil would have more thermal mass. But once equilibrium is reached, the same volume of oil should be moving through the oil cooler and rejecting the same amount of heat regardless whether you have 50%, 75% or 100% of capacity. Not talking about dreadfully low oil level where volumetric flow gets impeded. What am I missing?

You are correct. As long as the sump has some oil in it and the oil cooler is getting its fill, you're just in a heat exchange situation. More oil means it takes slightly longer for the engine to reach equilibrium temperature, but doesn't change the ability of the engine fins, oil cooler, sump, etc. to shed heat.

 

[MRC01]

You are correct. As long as the sump has some oil in it and the oil cooler is getting its fill, you're just in a heat exchange situation. More oil means it takes slightly longer for the engine to reach equilibrium temperature, but doesn't change the ability of the engine fins, oil cooler, sump, etc. to shed heat.

Consider a hypothetical oil sump that is infinitely large. Every drop of oil the pump draws from the sump and pushes through the engine starts at ambient temperature, much cooler than the hot temperatures in the engine. As that drop of oil passes through the hot engine, it absorbs some of that heat and gets much hotter. Then it falls back into the sump and eventually cools to ambient temperature, shedding the heat it absorbed from the engine.

Now consider a hypothetical engine with no sump at all. As soon as each drop of oil passes through the hot engine, the oil pump picks it up and pushes it through the engine again. The oil does not cool before re-entering the engine.

In the first case the engine runs cooler than the second case because some of its heat is shedded away. So theoretically, the oil sump definitely does help cool the engine, and a bigger sump cools it more. So it's really just a question of degree: how much difference does the size of the sump actually make? For example is 33% more oil in the sump (like 6 quarts to 8 quarts) enough to make a measurable difference in engine operating temperature?

I'll refine my answer to say, yes certainly more oil in the sump should help the engine run cooler. But whether it actually makes any measurable difference is a different question. The answer may vary from "no difference" to "significant difference" from one engine to the next.

 

[kyleb]

I'll refine my answer to say, yes certainly more oil in the sump should help the engine run cooler. But whether it actually makes any measurable difference is a different question. The answer may vary from "no difference" to "significant difference" from one engine to the next.

Here's the way I see it. The crankcase, sump, and oil cooler are where the engine rejects any heat that isn't dissipated by the cylinder fins. Since all of those things are aluminum, they all have tremendous heat conduction and rejection capabilities, so the entire assembly has roughly the same surface temp (and therefore radiates heat via delta T and airflow). A quart or three of oil doesn't change any of that. It doesn't change the amount of heat created, it doesn't change the amount of heat rejected. Now, if you make the SUMP (a heat transmitter) larger, that changes things since you now have more surface area to dissipate heat. But that's a physical change to the engine, not a change in the oil level...

 

[MRC01]

Here's the way I see it. The crankcase, sump, and oil cooler are where the engine rejects any heat that isn't dissipated by the cylinder fins. Since all of those things are aluminum, they all have tremendous heat conduction and rejection capabilities, so the entire assembly has roughly the same surface temp (and therefore radiates heat via delta T and airflow). A quart or three of oil doesn't change any of that. It doesn't change the amount of heat created, it doesn't change the amount of heat rejected. Now, if you make the SUMP (a heat transmitter) larger, that changes things since you now have more surface area to dissipate heat. But that's a physical change to the engine, not a change in the oil level...

So far so good. Now consider that the amount of heat that the sump itself (the metal tub or oil pan) can absorb and shed or radiate, depends on how much oil is in it. Suppose you have a large sump that is only 10% full, most of its surface area is wasted. It will not absorb and shed as much heat as that same sump when it is 90% full. When the sump is 90% full of oil, 2 things happen. (1) it sheds more heat, and (2) each drop of oil spends more time in the sump before getting pulled back into the engine.

PS: even if it's true that the sump remains at the same temperature regardless of how full it is (not sure whether that's true, but suppose it is), that temperature is cooler than the engine, so more thermal mass (oil) at that same lower temperature, means more total heat radiated. For example if the sump is always 20* cooler than the engine, when it has 8 quarts of oil in it instead of only 1 quart, that's more thermal mass being cooled to the same temperature. That means more heat radiated.

 

[kyleb]

Now consider that the amount of heat that the sump itself (the metal tub or oil pan) can absorb and shed or radiate, depends on how much oil is in it.

In my world, the entire engine (less cylinders) is at 230F (an arbitrary number for point of discussion) because of the heat conductivity of aluminum. It is stabilized. Steady state. The amount of heat the crankcase and sump can reject is determined by that 230F, the ability of aluminum to shed heat to 80F air, and the amount of airflow around the engine. Having more (or less) 230F oil in the sump won't change things.

 

[Dan Thomas]

More oil will run cooler. Experience and geometry says so. More oil occupies more of the tank or sump walls, and it spends more time there. It can't help but run cooler.

 

[Chip Sylverne]

Oil is a heat sink, but I can tell you from experience the old saw about oil temp rising as an indicator of oil loss is not reliable. I once started a long cross-country with 7 qts in an O-320 and experienced a loss of oil pressure on final about 4 hrs later. Pulled the mixture and after landing found 2.5 qts on the stick. Oil temp guage never increased a lick.

Apparently the ring gaps lined up on one of the pistons and blew the oil out the breather tube all over the belly.

 

[Clip4]

You are flying an air cooled engine. The longer the oil dwells in the sump, the more it cools. Your oil cooler only gets hot oil. In cold temps, the oil cools enough in the sump it never reaches the cooler.

If your engine were to suffer a substantial leak, you may see the oil temps rising before the oil pressure gauge indicated you had an issue.

 

[AV8R_87]

The effect might be more noticeable in aircraft without an oil cooler and a vernatherm that regulates the flow to maintain a given temperature.
I can't say I've noticed a difference in temperature between having 7.5 and 5.5 qts in the engine, but I only have an analog temp gauge.

 

[Clip4]

The effect might be more noticeable in aircraft without an oil cooler and a vernatherm that regulates the flow to maintain a given temperature.
I can't say I've noticed a difference between in temperature between having 7.5 and 5.5 qts in the engine, but I only have an analog temp gauge.

You would if you had a turbo charged engine on a hot day.

 

[Dan Thomas]

Oil is a heat sink, but I can tell you from experience the old saw about oil temp rising as an indicator of oil loss is not reliable. I once started a long cross-country with 7 qts in an O-320 and experienced a loss of oil pressure on final about 4 hrs later. Pulled the mixture and after landing found 2.5 qts on the stick. Oil temp guage never increased a lick.

That would depend on the environmental temperature. If the air was cool, the Vernatherm won't be fully closing the oil cooler bypass, and as the oil is lost and the remaining oil's temp rises, the Vernatherm closes more and directs more oil through the cooler, and the temp gauge won't move much until the Vernatherm is fully closed.

 

[William Pete Hodges]

I am going to look at this a little differently than most of you. The original question is posed in reverse. More oil = cooler oil temperature is backwards. LESS oil (if too low) WILL lead to higher oil temperatures, due to oil foaming or cavitation. But if there is enough oil to keep the oil flowing smoothly and covering all parts as needed then additional oil above that will not reject additional heat.

Enough oil keeps the engine cool, too little oil makes it run hot.