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

[4sfed]

. 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.

How much temperature rise would you expect as the oil passes through a bearing (hopefully something similar to our engines)? You define the parameters ... diameter, width, clearance, viscosity, RPM.

[mekilljoydammit]

I'm going to be honest and say that I don't want to guess - the stuff I was doing professionally was (in engine terms) huge with good length per diameter, running on cold, thin oil, and loaded statically. In other words, nothing comparable enough to an engine for me to want to give authoritative sounding bad information. In the stuff I was doing... being vague enough to avoid getting in trouble, let's say that this is machinery that's big enough you can control the inlet temperature to whatever you want, so it's much colder than automotive oil, and one bearing manufacturer I know of is touting on their website bearings that have operating temperatures in the 300-450F range. Although sometimes that's just because the shaft they're riding against is hot.

Sorry to not be more exact; I don't own that test equipment nor data, you know? I'm trying to be careful to not go into any performance data not available by doing web searches, because all that performance data cost a heck of a lot of money to generate.

You could make a guess by looking at oil temperature if you have a sensor in the gallery going to the mains and a decent history of "this temperature means the engine goes kaput" with thick enough oil where the failure will be due to babbit going away instead of breaking through the film.

*edit*

http://www.waukbearing.com/en/technical ... documents/ Here's a link to some publicly available PDFs I came across from one manufacturer; the Material for Plain Bearings one has some pressure profiles of rod bearings and mentions them a bit. I haven't read through all of them, but there might be more useful data. A word of caution though, this is one of those things where the stuff that gets published isn't necessarily the whole story.

[David Redszus]

Using the engine data set below, I have calculated some oil flow volumes under various conditions.

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

Using the above inputs at 70psi, various clearances and temperature, we find:
Clear.........250F..........275F..........300F
.0020.........0.3756........0.4689.......0.5714
.0025.........0.6561........0.8191.......0.9982
.0003.........1.0348........1.2920.......1.5744
.0035.........1.5213........1.8993.......2.3145
Flow units are in gallons per minute. Note that oil flow volume will increase with
temperature and bearing clearance. But as oil increases in temperature, its ability
to cool the bearing diminishes and we need increased oil volume to compensate.
If high oil temperatures are unavoidable (bad design) then increased clearances are
needed to increase oil flow and cooling.

Now let's use a fixed bearing clearance and vary the oil pressure at various temperatures.
Using the above inputs at a bearing clearance of 0.025" and various temperatures, we find:
Press.........250F..........275F..........300F
40 psi........0.3749........0.4681.......0.5704
60 psi........0.5624........0.7021.......0.8556
80 psi........0.7498........0.9361.......1.1408
100 psi.......0.9373.......1.1701........1.4259
Flow units are in gallons per minute. From an oil flow volume perspective, bearing
clearance is much more productive than oil pressure. Up to a point where excessive
bearing clearances cause their own set of problems. Note the large difference in flow
volume as a result of oil pressure changes, Now we know why cornering/braking forces
that cause oil pick-up problems and pressure drops have an adverse affect on bearing life.

The oil being used is a SAE 5W-20 grade. We know the oil becomes less viscous with
increased temperature; and it becomes more viscous with lower temperature. Consider
the above engine with 0.025" bearing clearance being started when the ambient temperature
is 32F. At 60 psi the oil flow rate becomes 0.0078 gal/min, and at 120 psi it becomes a
whopping 0.0156 gal/min. Bearings do not like cold starts. Where the hell are the anti-wear
additives when they are needed?

Bearing problems are often not a result of oil quality, temperature, or flow volume, but
rather due to poor journal/bore geometry. A bearing bore that is not round and straight
will require a larger clearance. The same is true of rough surface finishes that allow
metal asperities to make contact.

[cjperformance]

Suppose that I use either too low or too high (hot) viscosity oil in an engine. What are the symptoms of the two wrong choices? What problems should I experience in either case in terms wear, failures, or oil degradation?

The 1st 2 symptoms you will experience are.

1-Too high a hot oil vis will noticably reduce engine response and hp. Waaaaay before any bearing damage will occur from the hot vis being too high. If as a result of the extra friction your oil is getting too hot, it if course break down quicker than normal. At least it will boil any moisture from condensation out!

2-Too low a hot oil vis will give a very low hot idle oil pressure that will pick back up by typically around 1500/2000 rpm. Once you get the vis this low that the pump is struggling to hold any pressure (there is still flow) at idle your hydro lifters 'may' begin to rattle until rpm and pressure pick back up. But you are probably still a way off of actual bearing damage as you have plenty of flow with some rpm.

Note that load (tq/hp) on the bearings will govern what happens next.

[4sfed]

Sorry to not be more exact; I don't own that test equipment nor data, you know? I'm trying to be careful to not go into any performance data not available by doing web searches, because all that performance data cost a heck of a lot of money to generate.

It was really an impossible question because there are so many variables. Problem is that David's' calculations aren't that useful until the temperature in the bearing is known.

I suspect the rod bearing gets the hottest, but is the also most difficult to measure.

[midnightbluS10]

I'd trust a lot of the guys at bobistheoilguy.com before Rat540. They've got people there that are in the oil industry. Guys that are chemists and chemical engineers and shit. People with the intelligence, ability, and proper tools to test different oils and get honest, repeatable results from those tests, whatever they may be.

[David Redszus]

Problem is that David's' calculations aren't that useful until the temperature in the bearing is known.

Absolutely correct. Various points of lubrication, cam lobes, lifters, rods, mains, chains, gears, all have different
temperatures. Therefore the oil viscosity that is proper for one location may not be optimal for other locations or conditions.

Yet, how many engine builders do you know that have fitted thermocouples to various engine locations in an attempt to measure and understand local temperatures? Some new cell phone apps enable infra-red images that graphically show local temperatures very accurately.

Yet we blabber on and on about additives, oil brands, viscosities, and regurgitated marketing hype.

Bearing manufacturers and oil companies have vast experiences and data regarding component temperatures.
So does every engine OEM.

[4sfed]

Yet we blabber on and on about additives, oil brands, viscosities, and regurgitated marketing hype.

Bearing manufacturers and oil companies have vast experiences and data regarding component temperatures.
So does every engine OEM.

The oil and bearing manufacturers have been very helpful, but the only information I've received from the OEM's is through personal small number of contacts I have in their engineering departments ... so we bring our questions here, hoping to find someone with the necessary knowledge or experience. Thanks for your contributions.

[peejay]

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.

I am wondering now why Mazda and "old guard" racers recommended straight 40 or even 50 weight motor oil in the rotaries, which have the benefits of much cooler oil and bearing width measured in inches, along with (I would think) lower bearing loads since the combustion cycle is 50% longer and less "peaky".

I'm all but convinced that oil supply is the main problem with Subarus, not viscosity... either the oil staying stuck in the head and the pickup running dry, or the operator failing to ensure there is actually oil in the engine. Oil doesn't do any good if it's still in the bottle.

[mekilljoydammit]

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.

I am wondering now why Mazda and "old guard" racers recommended straight 40 or even 50 weight motor oil in the rotaries, which have the benefits of much cooler oil and bearing width measured in inches, along with (I would think) lower bearing loads since the combustion cycle is 50% longer and less "peaky".

I'm all but convinced that oil supply is the main problem with Subarus, not viscosity... either the oil staying stuck in the head and the pickup running dry, or the operator failing to ensure there is actually oil in the engine. Oil doesn't do any good if it's still in the bottle.

The first, I have a theory - you have a babbitted bearing in direct contact with something that sees combustion temperatures. I mean yeah, it's at the end of a bunch of relatively spindly vanes, but direct contact, man. So I'm wondering if all the excess oil (and yeah, the bearing unit loads on rotaries are going to be tiny, comparatively) and everything is all geared to keeping the babbitt in the rotor bearings - jack up the viscosity to keep the oil in the main bearings, jack up the pressure to get flow up further, open the rotor bearing clearances, voila.

Just a theory. May try some temperature sensitive paint on rotors next motor I do though, for grins. I'm Kenku on the RX7club forum, btw.

The second, Subarus have all sorts of problems, to be quite honest. Bad oil feeds not doing a good job lubing the rods (seriously, the stock feed system is crap) problems with oil drainback, oil having to be forced to the center of the crank... and *THEN* the journals are all too narrow. Even if you use one of the good cranks that fixes the supply issues and fix the oil return issues (... somehow...) the journals are still all too narrow. I'm kind of amazed they work as well as they do.

[peejay]

Just a theory. May try some temperature sensitive paint on rotors next motor I do though, for grins. I'm Kenku on the RX7club forum, btw.

No shit? If you're also on the Grassroots Motorsports forum, you're on pretty much every forum I visit. (This is Pete from Rallyanarchy... I use a different username on different forums...)

[mekilljoydammit]

Just a theory. May try some temperature sensitive paint on rotors next motor I do though, for grins. I'm Kenku on the RX7club forum, btw.

No shit? If you're also on the Grassroots Motorsports forum, you're on pretty much every forum I visit. (This is Pete from Rallyanarchy... I use a different username on different forums...)

Yeah, I put that together from the common cars. Small world - not on the GRM forum though, I think I'm at pretty much the upper limit of places I can sign up and still be productive.

[ptuomov]

This was a good thread. Reread it and it makes a lot of sense still.

Here’s another question. By my understanding, the oil flows into the rod bearing when the oil hole in the rod journal is aligned with the gap side of the bearing. By the gap side of the bearing I mean the opposite side of the squeezed side. I’m thinking that the oil hole location can influence the flow when one works out the forces acting on the rod and crank over different strokes. Any thoughts on this?

Another follow up question is how much will the rod oil flow increase with cross drilled rod journals? Does it almost double as in double the time that the gap side is aligned with the oil hole? Or is the squeeze side the only mechanism the flows oil out of the bearing, and thus the cross drilling of the rod journals doesn’t meaningfully increase oil flow?

[ptuomov]

Reading this now because can’t sleep with jet lag/time dine difference: https://scholarworks.rit.edu/cgi/viewco ... ext=theses