Does hot oil create more heat? Please Speculate!

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Well ..we seem to be the only two tap dancing here.
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No one else appears to want to play
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You don't happen to have any UOA data on these engines, do you?

I'm surprised that Dr. Haas hasn't checked in here. Although he's no Doc Diesel (strange word strings popping into head: Doc don't do diesel) he's got lots of neat views on low visc states at high temps.


Anyone? Anyone? Bueller? Dueller?
 
If left unchecked, the engine will heat up to 1.21 jiggadegrees and vaporize, along with the driver and all schoolchildren in the vicinity. There's my unchecked speculation.

I've read your posts about cooling on those diesel trucks and it sounds like you're prepping for a lawsuit.
 
If the oil sump temperature is getting up over 340 F regularly the oil will not last long. I would say hours. The oil must be heavily oxidized, thickened. UOA must be done at frequent intervals and the cause of the lack of cooling must be found. The vehicle cannot be working properly.

A way to determine what the oil viscosity contributes is to use several straight wt. oils of a single brand, say Pennzoil, 60, 50, 40, 30 then even a 20 wt. oil. Figure out what oil viscosity gives you the lowest running oil temperature. Dino oil is relatively cheap and the experiment would educate us all. Run each under the same conditions for a week then try another oil viscosity.

aehaas
 
I'll take a stab at it. No, hot oil doens't "create" more heat. In fact just the opposite is true. The hotter the oil, the greater the temperature differential between the cooling medium (oil) and the heat sink (atmosphere). As this differential increases, the ability to transfer heat from the source to the heat sink increases, i.e. the BTUs rejected per hour increase. Basic heat transfer equations will show this to be true. Eventualy an equilibrium will be reached in which the BTUs produced will match the BTUs rejected. You cannot have an imbalance in the heat produced and heat rejected for very long. If more heat is being produced than is being rejected, the oil will continue to heat up until the engine is damaged.

Also, hot oil will have a lower surface tension, which will aid in heat transfer.

However there may be other problems that complicate basic heat transfer equations. What happens to the oil at high temps? You mentioned loss of lubricity. But most surfaces inside an engine are floating on a film of oil so the lubricity probably isn't that important. But if this film breaks down, metal to metal friction will occur and will reduce the available horsepower which will lead to a higher demand for power by the driver to maintain speed, which will lead to burning additional fuel which will lead to additional heat. Once you get into this cycle, the result will be major engine damage.
 
Now, I was under the impression that the hydrodynamic film you refer to, and its thickness is only as good as the oils surface tension. Which we know is severely weakened as temp increases, especially in dino.

I like your logic, especially if the atmosphere was exchanging heat with the oil. But it's not, in most vehicles anyway. It exchanges with the coolant in this specific vehicle, raising coolant temps. Indirectly I see truth in your statement, since coolant heat does shed to the atm.

One other concern I have is the distribution of flow with oil heating up. The oil and lub system channels are designed with specific widths and tolerances, to get the right proportion of total flow into all the areas of importance. Viscosity changes alter that plan, favoring larger passages, starving the thinner veins.

We have all heard about oils optimum "range". Usually quoted as 190-240, somewhere in there. There must be a balance struck between flow rate (Cold oil flows slowly) and lubricity. As viscosity reduces below design points, would lubrication not suffer?

Good discussion Dave.
 
I don't think viscosity alters where the oil goes to, but it does alter how various components use the oil. The oil pump is a positive displacement pump, therefore the same amount of oil will flow each crankshaft revolution regardless of viscosity or temperature. The problem with oil that is too thin (because its too hot) is that it will "leak" out and therefor not support the component. Think of a crankshaft bearing. Pressurized oil is fed to the crankshaft, but oil comes out the sides of the bearing shell. If too much comes out, the crankshaft will not be supported by the oil film, and the crankshaft will contact the bearing. This "leaking" is what causes oil pressure to decrease with increasing temperatures.
 
Also, when I wrote about oil rejecting heat to the atmosphere, it does this directly and indirectly. Directly is accomplished by air flowing over the oil pan, and indirectly by exchanging heat to the coolant, which then exchanges heat to the air.

But the point is that the atmosphere is the ultimate heat sink. For heat transfer purposes it doesn't matter if it happens directly or indirectly.
 
Would the "leaking" be a function of reduced film strength? Surface tension is the mainstay of oil lubrication, Yes?

Clarifying my earlier statement on proportionate distribution. example: A Mcdonalds milkshake with 2 straws, held upside down. One straw with 1 mm diameter, the other with 5 mm, both in parallel. Our pump is gravity. The 5mm straw offloads more of the shake, the 1mm just sits there. Now, melt the shake, now there is some flow from the small straw. The proportions of total combined flow are changing. Our lub systems are setup in parallel also, and see similar changes with warmup IIRC.

This may have little to do with anything on this topic in reality.


Would it be safe to say, that lubrication is almost a pure function of viscosity? Forgetting that oil is also a convective heat transfer medium for a moment. Doesn't metal-metal avoidance depend on it? Is not viscosity, just another measurement of surface tension and film strength?
 
Remember the other side of that heat transfer loop. As the oil heats up, the temp delta between the engine components and the oil is lower, slowing the flow of heat from the engine to the oil. So, the same factor (increasing oil temp) that increases rejection rate at the pan & radiator decreases the absorption rate inside the engine. This only exacerbates the overall cooling problem as the oil is now carrying heat away from the engine more slowly.

KBs; I think I understand what you're trying to get at. Laying that to one side for a moment, perhaps you could disconnect the oil cooler lines from the water radiator and connect them to an air-to-oil cooler radiator on one vehicle and see what happens. Lots of load taken off the cooling system and lots of heat NOT put into the oil from the coolant that way. Could be a bad deal during northern winters, of course, unless you add an oil thermostat.

You noted your speculation of a 650,000 btu engine being served by a 600,000 btu cooling system. I wonder: Was the designer thinking of coming up over the pass from Yuma to San Diego in the summer when they designed it? I mean, it might be a 700,000 btu cooling system when ambient air temps are 90-95F, but only 600,000 btus when they're 125F.
 
All true. And an oil cooler is the plan. Unloading the radiator, the goal. Just trying to learn something about lubrication first.
 
Clarifying my earlier statement on proportionate distribution. example: A Mcdonalds milkshake with 2 straws, held upside down. One straw with 1 mm diameter, the other with 5 mm, both in parallel. Our pump is gravity. The 5mm straw offloads more of the shake, the 1mm just sits there. Now, melt the shake, now there is some flow from the small straw. The proportions of total combined flow are changing. Our lub systems are setup in parallel also, and see similar changes with warmup IIRC.


Well (now that I got something to interact with) ..this should not be true. The 5mm has about 20X the flow capability at the same potential. They should always be proportional (assuming that they aren't orifices).

But I want to offer you a perception that one of our retired members sent my way (he's in self imposed exile just due to the additive nature of BITOG). Are we pumping oil through the bearings ..or are the journals pulling it through? Aren't you effectively loading a hopper and coating a rotating barrel at one limited point? That loading/coating is then spread in a widening "V" from the point of ingress?
 
quote:
"Do increasing oil temps lead to more heat rejection deficit"?
Click to expand...
No.

Increasing oil temps lead to thinner oil films between the bearings and journals and between rings and liners.

Increasing oil temperature also increases the oxidation potential of the oil, which usually thickens it up. The thickened or oxidized oil does not necessarily mean that the oil film will be thicker, since much of the oil film is now composed of polymerized molecules of different sizes.

The only item that limits heat rejection is the size of the oil cooler/heat exchanger.
 
Is there not a point, where metal-metal friction contact occurs with reduced viscosity and film strength?

If not, can't I save money by using non-corrosive water under pressure instead?

(Please forgive the idiocy in my statement, for the point I'm trying to make)

Good discussion.
 
"Would it be safe to say, that lubrication is almost a pure function of viscosity?"

Yes if there were no additives.
No, not in formulated motor oil, as MolaKule says.

Everything depends on the size of the asperities. If there were no asperities then just about any fluid including water (approx 1 cS at all liquid temperatures - a perfect lubricant in a sense) might be able to prevent surface contact. And the pressure in the bearing is irrelevant. Picture a single closed bearing with no flow of oil in or out. If we increased the fluid pressure from ambient to 10,000 PSI there would be no additional force of separation.

aehaas
 
Thanks for the informative responses. There is some magic about 4.5 cSt?

If one had bypass filtration and clean down to, oh, 5u. That make any difference to the hot running vehicle?
 
I'm glad the heavy weights are carrying this thread.
smile.gif


quote:
If one had bypass filtration and clean down to, oh, 5u. That make any difference to the hot running vehicle?
Click to expand...
It will help any engine eliminate larger size particles ..but there is no way it will make this sustained condition tolerable in any engine that is designed for any length of service life.
 
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