Oil Temperature

[sszuch]

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?

 

[patprimmer]

UMMM

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.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sszuch]

Thanks for the expedient replies.

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.

 

[sszuch]

Thanks for the expedient replies.

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.

 

[dgallup]

Many engines have piston cooling jets that squirt oil on the undersides of the piston crown to cool them. The oil flow to air cooled cylinder heads is also generally much higher than needed for lubrication. Most air cooled engines, especially anything that makes significant horsepower, are really air/oil cooled engines. Oil makes a poor substitute for water cooling but in many traditional engine layouts like BMW flat twins it is the best solution available. Why do you think they have big oil coolers?

 

[patprimmer]

Even the humble VW Beetle had an oil cooler since it's inception. As the factory increased power with model development, they also increased oil cooler size. When hotrodding Beetles, the power/durability relationship was very dependant on oil cooling capacity. Also, very good oil cooling allowed a slight increase in compression ratio before detonation set in.

Regards

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[sszuch]

Although motorcycle engines did come up in the conversation, along with traditional V8 water cooled engines, and recip aircraft engines, we specifically concentrated on aircooled VW engines. The oil coolers are small in comparison to newer motorcycle engines and they do not have oil squirters on the back side of the pistons, which was also brought up. It seems at that point, piston temperature was ruled out because some how it wasn't the head, and the head temperature/oil temperature relationship became the focus. I was wondering if I was incorrect to associate the head/piston temperature relationship in relation to the oil sump temperature, and that oil temperature was not due to friction only? Additionally, I had brought up that oil spash to the back side of the piston and cylinder would be scraped back to the sump by the rings, but I wasn't sure if this would make a significant difference in oil temp.

 

[patprimmer]

Contact with hot engine surfaces is a major source of heat input to engine oil.

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.

Regards

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[RossABQ]

I'm not sure I agree that the VW oil cooler is small in comparison to modern motorcycle coolers. Maybe physically smaller, but I'd bet the BTU's exchanged are pretty similar. The VW cooler has airflow through it at all times (changes with RPM, but always has convective cooling). Motorcycles don't always have airflow (stoplights, etc) and have to contend with rocks, bugs and other debris. All that leads to larger apparent area.

 

[tbuelna]

"Is it safe to assume the piston temperature would be the same as the head temperature?" No. Peak piston temps will be much higher than cylinder head temps. That's why aluminum cylinder heads can get by with lesser materials like sand cast A356, and pistons require better, hypereutectic alloys like 2618. Piston mid crown temps can quite easily reach 500degF. The only way for pistons to transfer heat are through contact at the skirt and rings, and during the intake stroke.

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.

 

[Lou Scannon]

Pistons in many high output engines of all sizes are also cooled by dedicated oil jets directed at the crown bottom or into a cast in oil gallery in the crown.

 

[patprimmer]

Even without dedicated jets, a considerable amount of oil is thrown off the big end journal onto all the piston surfaces that are seen from underneath the piston.

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.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drwebb]

Numerous studies have been done to experimentally determine oil/friction effects in different engine components as well as entire engines by driving the engine at appropriate rpms unfired. Might be a good source to research if you're interested in definitive data. If memory serves these universally employ an outboard oil heater to maintain a realistic working temperature . . .

 

[turbomotor]

In modern high BMEP diesel engines, cooling the piston top ring land with oil has become a major enabling technology. High performance light diesels (mercedes, audi, bmw) have recently used salt cores to make oil cooling channels in the piston. Mahle now has a piston with a cast-in-place oil cooling gallery that has been production released in some engines concurrent with a large rated power and torque increase (such as the Duramaz LLY engine).

 

[evelrod]

Just a note to lighten up the conversation...

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

 

[sszuch]

I would like to thank all of you again for the replies. Some went into more detail than the scope of my discussion, while others hit the nail on the head at what I was trying to get at.

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!

 

[kilrbee]

Yes, in some vehicles, oil is every bit the coolant that water is, naturally VW is a good place to find an example. I did an article recently on this as it applies to the duramax motor.

http://members.cox.net/beekiller/GMC Light Duty/OIL COOLING.doc

 

[kenre]

Oil can absorb heat quickly, but its not great at getting rid of the heat.

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....

http://www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

 

[drwebb]

sszuch- when I said research I meant 'in the library' and not the garage. The answer is out there; I've seen it before but can't remember exactly where. SAE papers are a good place to start, since the index at least is available for free.

 

[sszuch]

kilrbee,

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".