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Manifolddesigner (Automotive) (OP)
5 Jan 11 11:48
Any recommendations for oil temp in a motor?
Gas engine?
Race vs street?  
Diesel? (especially diesel!)
My knee jerk reaction would be to aim for slightly over 212F to boil the water out.

  Sources? Books/articles?

Jason
 
IRstuff (Aerospace)
5 Jan 11 12:31
There seems to be plenty of answers on the web:
http://www.elephantracing.com/techtopic/oiltemperature.htm

Note that:
> there shouldn't be water anywhere in the first place
> you do not need anything close 212°F to evaporate water, 180°F will do it; it just takes longer, but your engine will also last much longer

TTFN

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Helpful Member!  evelrod (Automotive)
5 Jan 11 15:48
While oil temp can become a problem in street cars in the areas where I live, it is not normal to see a great many failures.  Today's oils are really pretty good at handling high temps. Modern cars have larger oil capacity than, say, a '57 SBC @ 5 qts.  A good long run across the Arizona desert in the summer would kill that dude whereas a new Chebby V8 will do just fine, even running the AC full blast.  They don't make em like the used to...thank goodness.

Given all that---In our race cars, I shoot for about 220f in the sump and ~180/190f after the oil pump/filter/cooler and back into the main oil gallery.  It does not always work out that way, but using good $ynthetic 40 or 50 race oil temps as high as 250 in the sump cause no problems.  Of course, oil is changed after each race weekend.

In my old Dodge/Cummins with it's 13 qt. capacity and a factory heat exchanger,  I have never seen oil temps above about 190f, even after a long pull at high speed when the outside temp was hovering in the 100f+ range.  At somewhere between 300k and 400k miles (the odo broke at 186k about ten years ago), she still performs as she did when new.  What can I say?  Still gets ~20 mpg, too.

Rod
Helpful Member!  ivymike (Mechanical)
5 Jan 11 16:08
on some large diesels we regulate to 85C oil return from the cooler/filter ("oil to bearing temp").  forced shutdown would be ~98C if equipped.

on some smaller diesels we set an upper alarm limit (derate) on oil from the cooler at 110C.  

A good rule of thumb for max allowable cylinder wall temperature at top ring turnaround is ~180C.  That'd be about as hot as the oil can get at that location without oxidizing too quickly.

@IR: there will always be water present if oil temps aren't kept high enough - about 1-2% of the combustion products travel through the crankcase (gasoline or diesel), and there is a great deal of water vapor in the combustion products.  
 
IRstuff (Aerospace)
5 Jan 11 16:20
Manifolddesigner (Automotive) (OP)
5 Jan 11 16:27
Great info!  Thanx guys,  Part of the curiosity, actually i'm as worried about oil too cold as anything else.  I've got a john deere diesel in a 59 ford pickup and the oil temp after an hour of pulling hard won't even go over 170f.  It takes 1/2hr on the freeway to get it over 140f.  my oil pressure is 65psi on freeway 2200rpm.  I'm wondering if i need lower my viscosity?  (15w40).  The thought being that hotter oil is lower viscosity and therefore if it's running too cold, lowering the viscosity would be desirable.
Changing to a hotter thermostat (195 vs 170) has crossed my mind as well.
I'm considering using a racing engine diaper as an oil pan insulator as well.
...but if cold is fine, then it's all good.

Jason.
IRstuff (Aerospace)
5 Jan 11 16:43
I would be leery of changing anything since your temperature sensor is only in one spot, yes?  So there may be portions of the engine where you can't measure oil temp, and it may be considerably higher there.

TTFN

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Manifolddesigner (Automotive) (OP)
5 Jan 11 17:05
it's in the pan near the middle of the sump.  it's oil temp.  13qt diesel.   
IRstuff (Aerospace)
5 Jan 11 18:18
That's exactly my point, it's not measured anywhere near the pistons or cylinders, where it would be considerably hotter.

TTFN

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Helpful Member!  patprimmer (Publican)
5 Jan 11 20:24
If you run a good synthetic oil, I would suspect you can go a grade lighter and a fair bit hotter. If nothing else it will give you more power and economy and it will keep oil going to the bearings rather than an oil water blend that may well boil the water out at the bearing surface.

Regards
Pat
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Manifolddesigner (Automotive) (OP)
5 Jan 11 20:28
Thanx Pat, you're my hero.    
ivymike (Mechanical)
5 Jan 11 21:54
you can always do a pair of oil analyses too, and compare wear metal levels after say 2000mi on the oil.  If there is substantially more wear going on you should see it in the metals (lead/copper/tin for the bearings, al+si for the pistons - but si can also be from dust in the intake).  A single analysis after the oil+temp switch won't likely tell anything, since you won't know what "normal" levels are for your engine.  
 
MikeHalloran (Mechanical)
5 Jan 11 23:58
IF you have a separate circuit (not full-flow) oil cooler, you could put a thermostatic valve in it to bypass the cooler until the oil is up to some nominal temp.  I wouldn't bother.

 

Mike Halloran
Pembroke Pines, FL, USA

patprimmer (Publican)
6 Jan 11 2:16
Mike

That is exactly what I did on my old hot rodded air cooled VW engines with added on external oil coolers. Together with a change to Mobil 1 it gave a dramatic increase in engine life from maybe 10,000 miles to over 80,000 miles.

Regards
Pat
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RangerGress (Mechanical)
7 Jan 11 20:44
Generally when a person asks about oil temps, they're ultimately comparing their engine with someone elses.

I race an '87 BMW and have collected a lot of data on oil over the years.  One of the things I learned is that oil temps can vary more widely than would would expect, depending on exactly how you measure it.

As a result, one has to be cautious when they compare oil temps between cars.  Just because an oil temp sensor seems to be "in the flow" doesn't mean it really is.  Just because the temp sensor in the pan "ought to be submerged", doesn't mean that it always is.  And finally, the local temp of an engine block will vary wildly depending on how far you are from the water jacket.

 
RangerGress (Mechanical)
7 Jan 11 21:25
And another thing....

There's no ideal temp, but there's an ideal range.  Here's some variables to consider.

Optimum viscosity at temp.  An oil's "Weight" XwY describes it's cold (0degC) and hot (100degC) viscosity.  X is cold and Y is hot.  The curve of visc vs. temp is non-linear.  Temp differences around 0deg create big differences in visc and temp differences around 100deg don't make much difference.  So the visc difference of an oil at 0degC vs. 10degC is large, and the visc difference between 90degC and 100degC oil isn't very big.  Therefore in terms of viscosity it doesn't matter much if your oil is 10deg this way or that.

And if visc was a problem, just go to a thicker or thinner oil.

Note that those XwY values are actually a band of viscosities and one 20W50 can vary from another 20W50 by easily 20%.

Boiling out impurities.  Oil doesn't heat up in an engine nearly as fast as coolant.  My commute is 22min and as a result my oil doesn't get to operating temp.  Oil needs to get hot to boil out fuel and water.  If it doesn't get hot the fuel and water accumulates and it doesn't take much of each to either cut the visc of the oil in half or turn it into chocolate milk.  

Engine Management.  Racers spend a lot of time at the dyno.  In my class we are not allowed to mess with our engine managegment.  Therefore we spend a lot of time trying to understand how that "black box", the car's ECU works.  One of the things we learned is that the ECU seems to start dialing back power when the coolant temp approaches 210deg F.  Sure, a cool intake tract equals power.  But your engine management may decide that a cool engine means power.  Therefore there would be advantage in keeping your oil cool.

Thin hot oil is slippery oil.  Not necessarily.  Oil is complicated.  Oil OEMs do all sorts of interesting things with oil additives to improve oil characteristics.  Some publish their data and some don't.  Every race oil I know of is thin.  But some relatively thick oils seem to beat relatively thin oils at the dyno and in various tests designed to measure oil efficiency.  So gaining efficiency and power from oil is more complicated than simply choosing a thin oil.
Manifolddesigner (Automotive) (OP)
7 Jan 11 21:28
hmm...that begs a lot of questions, but gives few answers.  Appreciate the info though.   
RangerGress (Mechanical)
7 Jan 11 22:13
Air cooled guys don't get excited about 250degF oil.  If my oil hit 250, I'd put the car on the trailer and head for the beer cooler. There's a range for what is acceptable, and as long as you're within reasonable limits you're probably ok.

I've configured my oiling system a lot of different ways.  When my oil was running hot it was 220-230deg.  Last year it was 185-195degF. As I began to understand that there was a power advantage in keeping the engine cool I worked harder to cool it.

There is no generic answer.  Just keep your oil temp within a reasonable range.  If you start getting obsessed with this stuff you'll just turn into me.

Re. oil pressure.  There's no generic "right" OP. Every model engine is different, and it can be terribly difficult to find folks that really know what they are talking about.  The Internet is full of folks who aggresively assert "My car loves <brandX>" as if they tear their engine down each year, do oil analysis after each event, and have a pile of SAE mags in the bathroom.
RangerGress (Mechanical)
7 Jan 11 22:36
I read the Elephant Motorsports article that was linked.  It's simplistic.  There's 3 kinds of lubrication in the engine, hydrodynamic (bearings), thin film (cam and rockers), and combo of both (rings).  The article deals with hydrodynamic only, and then doesn't allow for the fact that you can choose oils to meet your needs.

There's various ways to measure and test how well an oil will provide hydrodynamic lubrication and I'm no specialist. But the common measure is HTHS...High Temp Shear. All else being equal, thick oils have high HTHS. So in the case of that article linked, all the owner needed to do to restore adequate hydrodynamic lubrication in the face of high temps was to choose an oil with higher HTHS.
tbuelna (Aerospace)
8 Jan 11 21:44
RangerGress gets it partially correct.  In a typical engine the three contact regimes are hydrodynamic, mixed, and boundary.  The various engine component interfaces can operate in different contact regimes, under different operating conditions.  Crank bearings operate with hydrodynamic contacts.  Piston skirts, wrist pins and rings operate with boundary contacts about TDC and BDC, and with mixed or partial hydrodynamic contact through mid-stroke.  Cam lobe and follower are hydrodynamic contacts. Rocker pivots and tips, and valve guides are mostly boundary contacts.

The temperature of the oil can affect its viscosity, which in turn can affect the fluid film thickness in hydrodynamic contacts such as crank bearings.  If the fluid film thickness in the contact becomes less than the surface asperity heights (lambda ratio), friction and heating will increase and cause rapid deterioration of the bearing and journal surfaces.

The temperature of the oil can also affect the heat transfer rate of components that rely on oil flow for cooling.  Many materials in an engine can lose fatigue life if exposed to temperatures above their design limits.

The extreme upper temperature limit for engine oils is usually determined by their flash point.  Flash point can be an issue for lubrication of the upper cylinder liner surfaces.  The compression ring briefly operates in boundary lubrication conditions around TDC, which also coincides with peak gas pressure loads and temperatures during the combustion cycle.  Even though the outer surface of the liner is liquid cooled, if the inner surface of the liner gets hot enough to flash the oil film, then ring scuffing is likely to occur.

The only benefit to using very high oil temps that I can think of is that a smaller oil heat exchanger can be used.
drwebb (Automotive)
10 Jan 11 9:22
Flash points are determined at atmospheric pressure, so how on earth could they predict oil film thickness at peak combustion pressure?????  
 
ivymike (Mechanical)
10 Jan 11 9:39
I must've missed something - why does the flash point matter when calculating (simulating) oil film thickness in a bearing?  

 
patprimmer (Publican)
10 Jan 11 19:11
Well if your using methanol or acetone as a lubricant it might.  winky smile

Regards
Pat
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ivymike (Mechanical)
11 Jan 11 8:46
I know it's important in a bearing simulation, if you intend for your calcs to be mass-conserving (not all useful formulations are), to have some representation of cavitation behavior.  This is in some instances as simple as an input "cavitation pressure" below which the particular cell (calculated entity) has no mass or load-bearing capability.  

Non-mass-conserving bearing simulation schemes gave "close enough" answers to design successful engines for a decade or two... and the simple "cavitation" models are an improvement in accuracy.  I'm not sure there is a practical need for a mechanistic or even table-based vaporization representation (perhaps that was what was meant by "flash point" above?)
 

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