Oil Temperature
Oil Temperature
(OP)
Some collueges and I were in a discussion the other day about the influence of cylinder head temperature on oil temperature. In this case, we were specifically discussing an aircooled engine. The two sides of the disagreement are:
Oil temperature was only influenced by the friction of the rotating parts internally, and
Splash from the oil in the cylinders and the back side of the piston carry heat back to the sump.
I am under the impression that heat generated in the combustion process will transfer to the piston and cylinder. Is it safe to assume the piston temperature would be the same as the head temperature? I realize that friction from the piston rings will add to the oil temperature in the cylinder. Any suggestions?
Oil temperature was only influenced by the friction of the rotating parts internally, and
Splash from the oil in the cylinders and the back side of the piston carry heat back to the sump.
I am under the impression that heat generated in the combustion process will transfer to the piston and cylinder. Is it safe to assume the piston temperature would be the same as the head temperature? I realize that friction from the piston rings will add to the oil temperature in the cylinder. Any suggestions?





RE: Oil Temperature
You pass oil over a surfaces that is hotter than the oil, the oil gets hot.
You burn air fuel mixture in contact with a surface with lower temperature than the burnt compressed gas and the surface gets hot.
Except in the most extraordinary circumstances, the cylinder head in the oil wetted areas, the bores and the piston bottom sides will be somewhat hotter than the oil in the sump, especially in an air cooled engine.
The piston temperature will vary independantly of head temperature as each has a different heat sink, surface area exposed to heat and surface area exposed to different cooling media. These differences vary greatly in magnitude depending on engine design and operation.
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RE: Oil Temperature
We did not get into the details of the combustion process itself, but rather "normal" driving, either steady state (60 mph highway) or a quasi-equilibrium (around town). At first I tried to tie the oil temperature to head temperature, and since we were getting nowhere, I tried to use piston temperature since it is on the other side of the head in the combustion process. I knew there would be a temperature differential in the piston, but considered it negligible in our discussion and basically thought of it simply as a heat transfer problem. Obviously, head temperature will vary with relatively quick load changes and the oil temp will not have time to react, which became the "heat" of our discussion. I the opposing party seemed steadfast that oil temperature was influenced by friction only.
RE: Oil Temperature
We did not get into the details of the combustion process itself, but rather "normal" driving, either steady state (60 mph highway) or a quasi-equilibrium (around town). At first I tried to tie the oil temperature to head temperature, and since we were getting nowhere, I tried to use piston temperature since it is on the other side of the head in the combustion process. I knew there would be a temperature differential in the piston, but considered it negligible in our discussion and basically thought of it simply as a heat transfer problem. Obviously, head temperature will vary with relatively quick load changes and the oil temp will not have time to react, which became the "heat" of our discussion. The opposing party seemed steadfast that oil temperature was influenced by friction only.
RE: Oil Temperature
RE: Oil Temperature
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RE: Oil Temperature
RE: Oil Temperature
Better oil cooling does reduce detonation by cooling the head and piston. For the oil to cool the head and piston, it must itself get hotter, unless it boils away while doing so. This is basic laws of physics.
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RE: Oil Temperature
RE: Oil Temperature
There are two issues you should be concerned about with regards to oil temperatures in a recip piston, IC engine.
The first is temperature rise at the EHD oil film in the crank and rod journal bearings. If the oil temp rise at the bearing EHD film exceeds the flash temp capability of the oil (usually about 350degF), the oil will rapidly fail along with the bearings. The mass flow of (cooling)oil through the bearings is also critical to maintain bearing temperatures within the allowable limits of it's materials. In fact, in a N/A liquid cooled engine, most of the heat flow to the oil is due to journal bearing losses.
The second import issue is the temperature of the boundary layer oil film on the cylinder liner walls, left behind by the passing of the oil control ring set. The piston compression rings rubbing against the liner wall (due to combustion gas pressure) operate in a precarious boundary lubrication condition, especially around TDC. The function of the oil control ring is to leave behind a very light layer of lubricant, especially about the TDC portion of the liner. Just enough so that the compression rings don't seize, but not so much that it produces high HC emissions. Since the upper portion of the liner surface (with its oil coating) is exposed to high temp combustion gas during the piston's expansion downstroke, the coolant jacket heat transfer capability must be such that the inner, upper liner surface temperature never exceeds the flash temp capability of the oil. Otherwise, the oil film will fail in the area where the compression rings are most highly loaded.
RE: Oil Temperature
RE: Oil Temperature
High performance engines typically have larger oil pumps to supply more oil some of which is then thrown onto the piston, effectively increasing cooling of bearings and piston.
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RE: Oil Temperature
RE: Oil Temperature
RE: Oil Temperature
Drive a vintage Mini and forget about "cooling the undersides of the pistons"....forever! ;o)
Someone of our Mini cadre tried to spin up a gearbox, sans engine, to see how much oil would be pumped by the rotating gears...LOTS AND LOTS!!! Apparently the oil covered the ceiling rather well!!!
Rod
RE: Oil Temperature
drwebb,
I made a similiar point to pull the plug wires and turn the engine over at a specific RPM, to see if the oil temp would reach operating temperature without ignition (and minus the oil heater, obviously).
tbuelna,
While not considering peak piston temperatures, I would think that at steady state operation there would be temperature gradient in the piston, which would still be higher than the sump temp and contact splash oil from the rotating parts, inducing some heat transfer.
With these examples and the ones I've posted earlier for my arguements, all were passed off as "smoke and mirrors" and I was beginning to wonder if I was loosing my mind. Although these were not engineers that I was talking to, my basic approach was what patprimmer had pointed out earlier. I'm still new in the engineering world and I appreciate all of the feedback everyone has provided. This proves that one can never stop learning!
RE: Oil Temperature
http://me
RE: Oil Temperature
Evelrod, Ive seen a similar thing with to a Farm Tractor, cab and gearbox top removed, oil everywhere!
Ahh Minis!! one oil change does engine, gearbox and diff!
They will also run for a very long time with 10psi hot oil pressure....
www.retallickeng.com.au
Was told it couldnt be done, so
i went and did it!
RE: Oil Temperature
RE: Oil Temperature
Thank you for the informative paper. Again, I made very similiar arguments without going through all of the calculations. Somehow, everyone decided that a typical water cooled engine could not be compared as it was not the same as as a VW!? DRwebb, I did find a couple of SAE papers, but again they were dismissed because they tested a water cooled V8's. One of the people I was talking to does his own testing, and quite a bit I might add, and this was his reply:
"I have welded the tips of pushrods completely literally shutting all oil off tto the heads with DLC coated components literally trying to cause failures and the heads didn't run any notable higher temperature!
tHE ONLY QUESTION i'LL ASK NOW IS WHERE IS your DATA The only question I ask is where is your data and how have you come to these conclusions that are so firmly burned into your brain? Guess what, articles, papers and other forms of reference material don't argue with direct comparisons.
Spend less time reading and more time "doing" and you'll learn the same things I have time and time again, day after day. Drive it and pay attention, all it takes is a logbook, some good instruments and a stretch of road that you can maintain speed on...
Drop the hammer in 3rd gear, remove load and increase revs and you'll see that the oil temp climbs and the heads cool down as friction and RPM increase. Shift into 4th, lug the engine and watch the head temps rise while the oil temp remains steady or slightly reduces it's temp...
Then take away 3-4 degrees of timing, increase EGT by doing so and watch head temps drop, EGT rise and oil temp increase while head temps drop...Then try to figure out why all that occured- when you do this you have basically experienced what i do most everyday for 12+ hours straight..."
Again, not coming from an engineer. I tried to explain that the heat transfer to the oil under a load, as in klrbee's paper takes time, and is not instantaneous. Somehow this point was missed, or more probably ignored, becuase I mentioned it several times. I guess it would be easier not to argue with people that are not from a technical background. It is disheartening that people in my hobby are not inclined to be more open minded. Besides, I've heard and read their opinion of "engineers".