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optimal operating oil temprature?

Kalibr

Well Known Member
I?ve been researching oil cooling for my setup and came across the following post by DanH (logical and informative, as always):

Barely gets to 190? Ain't picking on 'ya, but I detect some wives tails in the air.

Lycoming recommends 180F. The standard vernatherm, if it meets specification, starts closing at 150F, and is fully seated and directing all oil flow to the cooler between 183F and 187F. The oil barely reaches 190 because the system is working perfectly.

180 is better, but there is nothing terribly wrong with 160F oil. If you're thinking "But what about boiling out the water?", consider that in the context of water removal in an aircraft engine, 212F has no relevance at all.

Water evaporates just fine at sea level standard temperature. It merely evaporates faster at higher temperatures. By 160F vapor pressure is already 18x the sea level standard. It's 29x at 180.

We don't care about boiling. Boiling point is merely the temperature at which vapor pressure exceeds local pressure at the point of bubble formation in a liquid. That temperature point drops with reduced atmospheric pressure (same vapor pressure, less local pressure). Water boils in the low 190's at 10,000 feet, not that it matters. It's just telling us we can run a lower oil temperature for the same rate of evaporation. Personally I run a case evacuator, pull 2" Hg, and at 10K my oil thinks it is at 13K, which increases water removal without decreasing oil viscosity. BTW, boiling point would be about 188F, again not that it matters.

The shutter design seen here blocks a good bit of flow when open, so I'd expect higher oil temperature in hot weather if installed. And it's pretty much for low performance parallel valve engines with no squirters, if needed at all. Angle valve motors and parallels modified for piston squirters shed a higher percentage of their waste heat via the oiling system.

I understand and agree with that post. And wouldn?t worry about water contents if oil operating temprature stays at around 180f degrees. What makes me wonder is why the modern car engines operate with the normal oil operating temprature at around 235-245f degrees with oil temprature redlines of 300f+ degrees while Lycoming specifies the optimal temprature of 180f degrees with the redline of 235/245f degrees? Especially while the oil viscosity specs for car engines are significantly less than those spec-ed for the aircraft engines.... if the SAE -20, -30 oils can run all day long at 235/245f degrees for hundreds of hours in car engines, SAE -50 oil certainly can handle tempratures well in access of the 235/245f redline specified by aircraft engine manufactures. Why such a big difference in oil operating tempratures between moder car engines and the aircooled aircraft engines? And should we really be concerned about our oil tempratures climbing to the 235/45f degree redline once in a while?
Granted, it is prudent to follow the engine manufacture?s guidlines, but perhaps these guidlines are a bit outdated taking into account the advances in motor oil developments, etc. There must be good reasons why the car engines are designed to run at operating oil tempratures significantly hotter than those we consider normal for aircraft engines...
 
I'm not an engine guy, but my first thought is that car engines are water cooled, and the oil is not required to provide cooling. Our engines are air cooled and the oil provides a substantial amount of cooling, so it has to be kept at a lower temperature.

Could be totally off base, but????
 
I think another might be that modern auto engines run rather tight clearances which work just fine at the lower oil viscosity that is the result of the higher oil temps.
Lycomings by comparison are quite a bit looser in bearing clearance dimensions, etc. That in itself makes lower oil viscosity less desirable.
 
Temperature location

Maybe where we take the temperature is 180F but the oil sees a much higher temp somewhere in the path through the engine?
 
I actually thought of exactly these two reasons as well (a) greater tolerances requiring higher oil viscosity and (b) higher ?local? tempratures, in particular in the cylinder walls area where water cooled engines would have substantially lower tempratures. Is there anything else? The reason I am still questioning this is because there must be a good reason for auto engines to run those higher oil tempratures as I?d think that it would be quite easy for a car engine designer to lower the oil temperatures. Yes, fuel economy might be a factor (and this I understand is the main driving force behind the recent switch to 0w20 oils), but there must be something else as the engines running -30 and -40 viscosities still normally run in the 230-250f range. All obviously running synthetics, which are much much more tollerant (in terms of retaining viscosity, lubricity and resisting thermal breakdown) than mineral oils.
 
What makes me wonder is why the modern car engines operate with the normal oil operating temprature at around 235-245f degrees with oil temprature redlines of 300f+ degrees while Lycoming specifies the optimal temprature of 180f degrees with the redline of 235/245f degrees?

I'd ask two questions before going further.

The "180F is best" figure is straight from Lycoming engine manuals. Does the 235~245 figure also come from a manufacturer's manual, and is it the recommended temperature, or a maximum?

The 180F figure for a Lycoming is measured at the oil filter housing, after passing through the cooler, and before entering the main lube galleries...in other words, in the coolest place possible. Specifically where is that auto engine temperature measured?

If a Lycoming has a CHT of 350, the small quantity of oil in the head is probably 300F.
 
I'd ask two questions before going further.

The "180F is best" figure is straight from Lycoming engine manuals. Does the 235~245 figure also come from a manufacturer's manual, and is it the recommended temperature, or a maximum?

The 180F figure for a Lycoming is measured at the oil filter housing, after passing through the cooler, and before entering the main lube galleries...in other words, in the coolest place possible. Specifically where is that auto engine temperature measured?

Good point. I do not know where the oil temprature prob is normally placed on an auto engine. I just assumed that the indicated tempratures are comparable to the indicated tempratures on a lycoming engine. This assumption might be wrong. The 235-245 tempratures I was referring to came from my looking at the oil temprature gages in the cars I was driving, most recently a BMW (with an oil cooler) It is also consistent with the oil tempratures mentioned elsewhere when people talk about operating oil tempratures in auto engines. I?ll try to find out where the oil temp prob is typically located on an auto engine. But I suspect that it is measuring the oil temprature at the sump and not post-oil cooler as many car engines actually do not have oil coolers.
 
Okay, it looks to me that auto engine oil probs are installed to measure the temprature within the engine (either in the sump, a galley within the engine block or the oil filter housing). So, I was trying to compare apples and oranges as the oil temprature in a lycoming engine is taken post oil cooler. My guess is that when the indicated post oil cooler temprature is around 180f, the normal operating oil temprature in the sump of a lycoming engine would be quite a bit higher...
This should further ease the worries of those trying to raise oil tempratures to boil off the water in their engine sumps.
 
My understanding is that auto mfgrs have actually been raising operating temps over the last couple of decades, searching for efficiency improvements. Running at higher temps is supposed to improve efficiency. Google (I think) 'adiabatic engine'. Improved metallurgy plus improved lubricants have allowed higher operating temps.

Kinda makes one wonder if those ~400 degree head temps are one of the reasons traditional a/c engines can be as efficient as they are, given all their other design limitations....
 
Okay, it looks to me that auto engine oil probs are installed to measure the temprature within the engine (either in the sump, a galley within the engine block or the oil filter housing). So, I was trying to compare apples and oranges as the oil temprature in a lycoming engine is taken post oil cooler. My guess is that when the indicated post oil cooler temprature is around 180f, the normal operating oil temprature in the sump of a lycoming engine would be quite a bit higher...
This should further ease the worries of those trying to raise oil tempratures to boil off the water in their engine sumps.

The auto engines that I'm familiar with (as far as oil temp probe location goes) have the probe after the oil cooler, if there's one installed, just like a/c engines. But the oil isn't picking up heat from 400 degree heads, like an a/c engine, as others have pointed out. You're right; sump temps in an a/c engine are much higher than the temp probe indicates, since it's after the cooler.
 
All on highway engines have been attempting to push oil temps for 4-5 decades due to emissions and fuel economy. Actually oils have not changed all that much, but the engines have, resulting in much better additive packages.

OTB - oil -to-bearing temps are important to the engine and sump temps (including peaks like in the ring groove) are important to the oil. A bit ( lot actually) of research will yield the parameters of limits of oils and the application parameters that drive them. I might not say the the operating clearances are much tighter, but the precision of variability (6 sigma) tolerances are reduced by an order of magnitude over the same decades. Liquid cooling and lower load factor of the on-highway and autos in particular make extending the general experience to air-cooled, high power density, continuous power aircraft engines unwise without some working knowledge of the underlying design issues.

DanH put his finger on it best. It is fun and educational to revisit engine design, but use the Lycoming numbers and recommendations until more facts for allowing changes are proven.

my 2 cents
 
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