Originally Posted By: Shannow
Originally Posted By: Festiva_Man
so, are they saying that oil temps at the bearing were as high as 300f or that just the shell of the bearing got that hot and the oil heated up to something in between as it didnt take on all the heat because its going through there too fast?
just want to know if i let my bulk oil temp reach 240f if my oil is actually reaching 295f at the bearings under full load/high rpm. if thats the case i may be more wary of letting it get that hot
I've posted on BITOG a number of times (met with criticism, but it's a fact), that most of the heat that the oil is dealing with in the bearings is self generated, but the lubricant within the bearings shearing, being worked on by the engine.
Each bearing is creating a couple hundred watts of heat, and that heat is related to an exponential function of engine speed, not engine load, or fuel burned per se.
The big end oil temperature will be 20-30C more than the bulk oil temperature.
It's not what 101 has about flow carrying away temperature, it's thermodynamics of oil under shear.
I'll give an example, on my 3.8 supercharged Holden.
I can place a type K thermocouple down the dipstick hole, and get the oil temperature of what's raining off the crank (immediately after shut-off.
On a typicla 15 mile highway commute, it's 105-110C (230)...heat gun on the sump is around 90C (195F).
However, if I hold the tranny in "2", and do the same trip at 4,000RPM (not 1750 like in drive), I've got oil temperatures of 135C in the drain down areas...275F...and that's still a mix of the bearings and the coolant temperature oil that has drained down the valley.
So yes, the oil in your bearings could easily be 300F, the exit temperature of the oil is the average of the supply temperature and the shell temperature.
As an aside, it's my view that this hot atomised spray of oil, into an environment of highly reactive blowby gasses is where the process of varnish formation takes place.
ok, do you mean " that most of the heat that the oil is dealing with in the bearings is self generated, by (or but?) the lubricant within the bearings shearing"? why would anyone argue that? its common sense. rub your hands togeather and call them molecules and you proved it. i believe thats how a microwave works, it excites water molecules which heats them up...
"It's not what 101 has about flow carrying away temperature, it's thermodynamics of oil under shear." sorry, what?
i have a oil temperature guage connected at the oil filter sandwich adapter. i can confirm that. at 100km/hr (2550rpm) on my civic on a long drive the oil will stabilize at say 210f. speed up to 110km/hr (2800rpm) and it will stabilize at 215f. speed up to 220-230km/hr (3000-3300ish rpm) and it will run 220-230f. all on the same drive, same terrain, same outside temps, and same load on the car.
if after an hour drive i stop and measure and record my oil pressure readings at different rpms, so stop, put it in first and write down what pressure is at 1k, 2k, 3k... by the time i get to 6k rpm the oil has gone from say 212f to 220f. i thought that had more to do with the oil around the piston rings being heated faster at higher rpms but it makes sense that it would come from all the components as they all turn faster.
prior to now i assumed that while oil can take away a large amount of heat that it did not heat up quickly like coolant does. i thought that because coolant might take 2 minutes of driving to reach 180f (op temp) and oil takes 18-20 min to reach 180f and 25-30 min to reach 212f (op temp) on the same drive all while being closer in contact with heat sources that meant it heated up slower. so i thought for example that oil at 212f passing very quickly through a bearing at 280f would only heat up to maybe 230f, fall back to the pan and cool off to 212 before being sucked back up to my temp guage. and i thought just the large volume of oil passing through the bearing-each particle not heating up much- kept the bearing cool enough.
from what your saying i take it that that assumption is wrong and the oil picks up and looses heat very, very fast. if thats the case can you please tell me why it takes so long for oil to heat up compared to coolant when it would seem that the oil comes in contact with a lot more heat? i have heard tell that the valves get up to 900f... if thats true wouldn't the oil boil as it got near there if it heated up that fast? or is it different when it shears compared to just flowing around the valves? i presume the area the oil sees near the rings is very hot as well.
thanks!
and interesting idea, but that spray is all in the pan and most varnish happens in the valve cover right? or does just as much occur in the pan and i have just taken off more valve covers than pans?