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Symptoms of too high or too low motor oil (hot) viscosity?

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby David Redszus » Wed Jan 11, 2017 1:33 pm

For those really interested in the very complex issue of bearing lubrication there is a publication I would
recommend. It is not light reading but very complete regarding virtually all aspects of bearing (several types)
lubrication and lubricants.

NASA Reference Publication 2272, Fundamentals of Fluid Film Lubrication Handbook, by Hanrock.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby David Redszus » Wed Jan 11, 2017 2:29 pm

The determination of correct oil viscosity involves the consideration of the the following variables. Lets insert
typical engine design anf operating variables into Sommerfeld and see what it tells us.

Journal diameter/in...2.25
Bearing clearance/in..0.0025
Bearing length/in......0.875
Load/lbs.................3500
rpm.......................7800
Oil temp/F................300
Oil viscosity @40c/cSt...48.2
Oil viscosity @ 100c/cSt..8.5
Specific gravity............0.861
Oil pressure/psi...........70

Using the above inputs we find:
Clear.........Vis..........Sommerfeld
.0020..........3.77..........0.0436
.0025..........3.77..........0.0279
.0003..........3.77..........0.0194
.0035..........3.77..........0.0142
Our minimum viscosity (3.95) has not been met and we are in danger of metal to metal contact.
The minimum S (0.005) target has been met.

If bearing loading should happen to increase to 4500 lbs, the viscosity is still too low (3.77) but the S value
of 0.0111 is still adequate. If the engine is driven at 3500 rpm, the S value will fall to 0.0050 which
is dangerously low and very close to boundary lubrication.

To retain adequate oil viscosity, we must either reduce oil temperature to 280F, which will raise vis to 4.40,
and S to 0.0058, or use a more viscous oil (30 weight) to obtain adequate vis (4.42) and S (0.0058) values.

Or, use a 10W-30 oil with a controlled temperature (at point of lubrication) of 280F, (vis 5.22, S 0.0069).

Remember that viscosity prevents metal surfaces from touching and the S number represents friction coefficient.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby mekilljoydammit » Wed Jan 11, 2017 3:38 pm

All right, I'm going to chip in here - I spent a couple years working in an R&D lab at a bearing manufacturer, working big fluid film bearings to failure. Glossing over a lot of stuff, we had machines that could work bearings enough to use up 1000hp in heating up the oil.

540Rat has an interesting test procedure for boundary lubrication, which is actually more representative of the sorts of failures he's seeing, which is to say lifters. He and I got into arguing at one point and there's a lot of failures of imagination in his work; he basically says that as long as there's flow you can't fail a bearing with a developed hydrodynamic film. I've done that a bunch of times on test rigs.

I'm going to avoid math here because honestly, for internal combustion engines, it gets really complicated really fast. Look at that graph of friction coefficient vs film thickness - film thickness is *not* the same as bearing clearance! Furthermore, all of the easy bearing calculations involve very long bearings - length long enough compared to diameter you can make a bunch of simplifications. This is very much different from what we see in rod or crank bearings. Booser's Bearing Design and Applications lays out all the governing equations but they don't have a perfect relation to reality either. In short though, engine bearings work because the crank is dragging oil from the unloaded side to the loaded side, the crank is shifted one way inside the bearing and oil is incompressible so it has to force its way out the sides of the bearing. The viscosity determines how much oil is dragged in, and how hard it is to push out the sides - if more is dragged in and its harder to push out the sides, the film is thicker. Oil supply pressure plays basically no part in this whatsoever.

Here's where it gets even worse. There's a benchmark in industry called "bearing unit load"; this is basically force divided by cross section. Crude but it works surprisingly well. So for that bearing David put geometry for, that's about an area of 2 square inches. For that peak force of 3500 pounds, which is pretty reasonable for some peak forces, that works out to around 120 bar bearing unit load. There is basically no statically loaded bearing in service anywhere designed for that sort of load. I know because I was doing development programs on materials far more advanced (and expensive) than we see in engines. The reason it works in engines is you don't see that load for more than, ah, call it 20 degrees of crank rotation to pull a number out of thin air. So temporarily, the bearing is forcing oil out faster than it's drawing it in, but again forcing the oil out is being resisted by viscosity. This effect is called "squeeze film damping" in the industry.

If you want to model this stuff mathematically, you're going to be getting to writing code to do what boils down to very specialized computational fluid dynamics to do even a steady state fluid film bearing, and then you're going to need to build a test rig to correlate it because the math doesn't really describe real bearings very well. And then you're going to need to invent a lot of stuff to correlate it to squeeze film dampers. It might be a fun project if you've got tenure at the one or two universities that have dynamic test rigs. You'll pardon me for not recalling which ones they were.

So, in short, viscosity is hugely important in internal combustion engines, especially ones with especially narrow bearings (I'm looking at you Subaru) or that are highly loaded. Too thick and yeah, there's more friction and heating of oil. To thin though, and you start edging up into mixed lubrication... but worse is that you not only increase friction in that direction, but you have less oil at the most highly loaded parts of the bearing. If the inlet temperature of the oil is around 220F, it doesn't take a heck of a lot of heating (80F is not a lot of heat rise if you're looking at things locally) to get to 300F which is where you start losing babbit.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Wed Jan 11, 2017 6:17 pm

Thanks, David and Killjoy.

I will come up with some numbers later when I'm in front of the right computer. I'll also look at my books and consider ordering that recommended book.

In the meanwhile, let me just ask some very simple (and in my opinion practical) questions, like what a fifth grader would probably ask.

For rod and main bearings, I get the sense that we want to get to the hydrodynamic lubrication regime before we want to put any meaningful load on the bearing. Two of the ways of getting there are either higher bearing speed or higher oil viscosity. Furthermore, we want to be pretty far right from the mixed lubrication regime, because unexpected things can happen. One such unexpected thing is that the engine starts knocking. Is it correct to say that if the engine knocking moderately results in visibly beaten up rod bearings then we're reasonably close to the mixed lubrication regime, with the spike in the load "knocking" (forgive the pun) the bearing to the mixed lubrication regime? It makes sense to reduce knocks, goes without saying, but should I in this situation also consider higher viscosity oil? And if I switch to higher viscosity oil, should I now see higher sump oil temperatures? Can I make some reasonable rule-of-thumb assumptions about how much the oil heats up in the bearings, short of the full temperature equilibrium calculation, to say based on the sump oil temperatures when I'm hitting the limit and can't increase the oil viscosity anymore? I guess one could always see when the oil starts breaking down because of overheating, but that's a little after the fact.

Postscript: My rod bearings are 45% longer than the equivalent crank pin diameter (52mm) Subaru rod bearings. While that's not the same as mathematical infinity, my intuition says that my bearing can hold more than 1.45x the load. But that's just my intuition.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby David Redszus » Wed Jan 11, 2017 8:38 pm

The effect of a change in bearing length is proportional to the change in S value. If bearing length is increased
from .5 to .9 (an increase of 180%), the S value is also increased by 180%.

An increase in nominal oil viscosity of 10% will produce a 13% increase in both hot viscosity and S value.
But oil viscosity is a fleeting thing; it can change with fuel dilution, excess shearing, excess local temperature, aging, not to mention that nominal SAE viscosities vary considerably even with the same package label viscosity.

A reduction in oil temperature of 10% (300F to 270F) will produce an increase of 27% in both hot viscosity and
S value. If there is a single critical parameter that should be addressed to ensure the life of an engine it would be controlled oil temperature. Measured and logged oil temperature at several engine points provides real insight.

Excessively high oil pressure is also to be avoided. High viscosity oils cost power, increases wear, and often
produce oil aeration which reduces the heat transfer property of an oil. Run the lowest oil pressure that you can
without suffering bearing starvation.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Kevin Johnson » Wed Jan 11, 2017 9:05 pm

FFF-2nd.gif


Half Sommerfeld.gif



It is common to see the problems of an aerated oil supply left unmentioned save dissolution at 1 bar. It would be nice if fluids always behaved ideally.
You do not have the required permissions to view the files attached to this post.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Thu Jan 12, 2017 8:04 am

This is all very useful (to me anyway).

Kevin, if I've understood your point about air bubbles in oil correctly, matters to say rod bearings in the following ways. First, the supply pressure needs to be larger because the moving oil film isn't going to pull a quite perfect vacuum as air escapes the oil and thus the oil supply pressure has to fight against some pressure. Second, the load holding capacity of the bearing goes down. Third, oil may not cool as well. aLl bad things.

David, I agree with you that running the lowest oil pressure that supplies oil to all required places with some margin of safety makes sense. So far, I've been taking the oil pressure as a given. To the extent that we need additional oil flow to cool bearings, then I guess that in principle higher viscosity oil might in fact need a higher oil pressure setting to get the flow rate back up. Maybe lower viscosity oil could be run at a slightly lower supply pressure, if there's already enough flow to cool. What's an open question to me is how sensitive say rod bearing oil flow rate is to the supply pressure. If it's mostly just the bearing pumping itself with huge forces, maybe the oil flow is almost completely determined by the bearing geometry, viscosity, and engine speed and any reasonable increase in the supply pressure is just an insignificant drop in the bucket. Anyone know the answer to that question?

In terms of oil temperature, there seems to be a positive feedback mechanism here. Running higher viscosity oil will increase drag and heat. Hotter oil temperature requires one to switch to a higher viscosity grade oil to match the required viscosity at the operating temperature. But this in turn increases drag and heat everywhere.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby mekilljoydammit » Thu Jan 12, 2017 9:55 am

I'm going to basically keep this at a high school level - IE, this is what affects what and why, but leaving out the math. I hate doing appeals to authority, especially when I'm setting myself up as the authority, but when I was doing the R&D stuff we had 3-4 people with doctorates and years of experience in the field doing the simple version of the computer code to figure out this stuff. The code they came up with got increasingly less accurate the closer to the limit things were loaded to, and that's much simpler scenarios (more mechanically complicated designs, sure, but basically static loads) than we're seeing in engine bearings. I may have picked their brains as much as I could, but I don't know nearly as much as they do, so I'm not going to pretend I'm up to coming up with mathematical solutions, you know?

Knock does interesting things actually - I think I've seen examples where the top (piston side) rod bearing was the one damaged! If the bearing has to shift relative to the shaft all of a sudden, there's going to be a sudden pressure drop on the unloaded side where air will come out of suspension and the oil may drop below its vapor pressure - cavitation in other words. Then when things catch up, the oil collapses all those bubbles and the water hammer effect erodes material away... in addition to the possibility of breaking through the film on the other side, of course.

Air entrainment is a huge issue too, but I do really agree that oil temperature is something that needs to be controlled. Too far in either direction on hot viscosity (that is to say, viscosity of what's actually getting to the bearings) can lead to runaway situations - too thick and there's too much heat generation, not enough flow to pull heat away, and localized hot spots kill your bearings, too thin and the film gets too thin, there's excess heat generation and hot spots kill your bearings. Fortunately, in the real world, viscosity of oils that you would put in your engine don't vary enough to produce really disastrous effects - for applications where you're not on the ragged edge, going a bit too thick will just mean that it gets a little hotter, the viscosity comes down and it self corrects, albeit at a little more friction than optimal, too thin may mean that it doesn't heat up quite as much, etc. As a rule of thumb, I'd prefer to err on the side of too thick for stuff that's getting flogged (I come from an amateur roadrace background) because of the next issue.

See, the next issue is rod bearings. First, an ideal bearing looks like a crank setup - fed from the side, everything goes out. In this situation, engine speed has no bearing (ha!) on oil flow, and actually if you have thin oil (thinner than you'll see on any car) you can get away with incredibly small supply pressures if the flow is enough. The problem is rod bearings are fed from the crank. By the nature of the drillings, the oil has to fight centrifugal force to get to the center (most cranks) or a bit in from the journal (Cosworth style drillings) and that reduces the effective pressure. Worse, if the effective pressure at the part of the drilling closest to the center of the crank gets below the vapor pressure of the air/oil mixture you're actually feeding everything, you just starved your rod bearings completely. Intuitively you might think that centrifugal force flinging out to the rod pins would help, but it's kind of a "you can't push a rope" thing - or more scientifically, once you get below the vapor pressure, it's not liquid anymore. This is why, btw, Formula 1 does things like feeds the crank from the nose, and why they were experimenting with hollow cranks with big oil reservoirs in each main.

If I was Roush or Cosworth or someone and was trying to do this, what I'd do is have a bunch of thermocouples embedded in any bearing I could get to, plus in the path of the oil coming out of the bearings, then do endurance testing at high loads - race track simulation on an engine dyno at realistic oil temps ideally. Make sure nothing is getting too hot during the test and then do a test program with teardowns between each rebuild distance and stepping oil viscosity to the minimum that will avoid wear. What I do for race stuff in the real world where I have to pay for it is run somethingW50 and go bigger on the oil cooler.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Kevin Johnson » Thu Jan 12, 2017 10:36 am

Just an aside, the first and second edition of the NASA manual are available online. The second edition deletes several chapters to make room (overall book length) for new material. It is important to check things like that otherwise one might assume the same material is covered.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby David Redszus » Thu Jan 12, 2017 11:51 am

Alright, we've covered the simple stuff.

Now let's look at bearing/journal asymmetry and bearing shell deformation, transient loading and
vibration issues, lubricant degradation,cavitation and corrosion. :D

And then we'll get serious about additives of all sorts.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby mekilljoydammit » Thu Jan 12, 2017 1:13 pm

Kind of amazing these "engine" things work at all when you start looking at it.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Zmechanic » Thu Jan 12, 2017 3:55 pm

David Redszus wrote:Alright, we've covered the simple stuff.

Now let's look at bearing/journal asymmetry and bearing shell deformation, transient loading and
vibration issues, lubricant degradation,cavitation and corrosion. :D

And then we'll get serious about additives of all sorts.


I had heard that with ZDDP zinc is just the carrier for the true lubrication from phosphorus. Sure enough, the book Kevin linked explained it. Zinc is the carrier, it breaks down with heat, and releases phosphorus which can coat metal and provide boundary layer lubrication. Very cool. Even went to say when it was first used it would break down too fast and wouldn't last very long, so they had to experiment to get it to release the phosphorus at a decent rate for it last. Never thought about it being an "consumable" additive! Fascinating stuff. I saved both copies of that book as soon as I found them. That'd be a $300 textbook if you bought it!!
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby statsystems » Thu Jan 12, 2017 4:16 pm

Zmechanic wrote:
David Redszus wrote:Alright, we've covered the simple stuff.

Now let's look at bearing/journal asymmetry and bearing shell deformation, transient loading and
vibration issues, lubricant degradation,cavitation and corrosion. :D

And then we'll get serious about additives of all sorts.


I had heard that with ZDDP zinc is just the carrier for the true lubrication from phosphorus. Sure enough, the book Kevin linked explained it. Zinc is the carrier, it breaks down with heat, and releases phosphorus which can coat metal and provide boundary layer lubrication. Very cool. Even went to say when it was first used it would break down too fast and wouldn't last very long, so they had to experiment to get it to release the phosphorus at a decent rate for it last. Never thought about it being an "consumable" additive! Fascinating stuff. I saved both copies of that book as soon as I found them. That'd be a $300 textbook if you bought it!!



Where is the book Kevin linked?
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby cjperformance » Thu Jan 12, 2017 4:22 pm

David Redszus wrote:The determination of correct oil viscosity involves the consideration of the the following variables. Lets insert
typical engine design anf operating variables into Sommerfeld and see what it tells us.

Journal diameter/in...2.25
Bearing clearance/in..0.0025
Bearing length/in......0.875
Load/lbs.................3500
rpm.......................7800
Oil temp/F................300
Oil viscosity @40c/cSt...48.2
Oil viscosity @ 100c/cSt..8.5
Specific gravity............0.861
Oil pressure/psi...........70

Using the above inputs we find:
Clear.........Vis..........Sommerfeld
.0020..........3.77..........0.0436
.0025..........3.77..........0.0279
.0003..........3.77..........0.0194
.0035..........3.77..........0.0142
Our minimum viscosity (3.95) has not been met and we are in danger of metal to metal contact.
The minimum S (0.005) target has been met.

If bearing loading should happen to increase to 4500 lbs, the viscosity is still too low (3.77) but the S value
of 0.0111 is still adequate. If the engine is driven at 3500 rpm, the S value will fall to 0.0050 which
is dangerously low and very close to boundary lubrication.

To retain adequate oil viscosity, we must either reduce oil temperature to 280F, which will raise vis to 4.40,
and S to 0.0058, or use a more viscous oil (30 weight) to obtain adequate vis (4.42) and S (0.0058) values.

Or, use a 10W-30 oil with a controlled temperature (at point of lubrication) of 280F, (vis 5.22, S 0.0069).

Remember that viscosity prevents metal surfaces from touching and the S number represents friction coefficient.


David, Id like to see this calc run on a bushed/plain bearing solid roller lifter if you dont mind. Looking at 3000 and 7000 crankshaft rpm.
Thanks
Craig.
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