hths vs kinematic viscosity

[JOD]

Originally Posted By: CATERHAM

An OP gauge directly measures bearing viscosity in a running engine.


I don't want to parse your words, but this isn't really true. Oil pressure measured at the sending unit is simply a proxy for the viscosity at the bearings, it's certainly not a direct measurement of it. While it may seem like an unimportant distinction, I think it lends credence to the belief that higher hths automatically leads to thicker films, when in certain cases the inverse is true.

I agree that as a proxy for viscosity at the bearings, oil pressure works pretty well.

 

[CATERHAM]

JOD I'm simplifying.
In my captioned thread I elaborated:
"The best proxy for the operating viscosity is oil pressure as determined by an oil pressure gauge. The higher the oil pressure reading (it actually measures back pressure) the higher the viscosity of the oil."

Originally Posted By: JOD
Originally Posted By: CATERHAM

the belief that higher hths automatically leads to thicker films, when in certain cases the inverse is true.

I think this is off topic although an interesting point.
What cases do have that thicker oils as measured by the HTHS method do not correlate to higher oil film thickness? Are you referring to the chemistry of different oils?

 

[fpracha]

Originally Posted By: GMorg
However, I do not agree that loss of VIIs performance occurs only under those conditions.

As the fluid begins to flow, the effective viscosity can drop dramatically at even low flow rates and low pressures.

The exception would be whenever the fluid also contains molecules that have strong interactions with the sliding surface. In this case, there is a counter force that orients the surface-interacting molecules across the flow.

+1.
And this is really more relevant when the oil has actually thickened from its designed viscosity range, or in other words the oil now contains varnish and sludge formations (or at least these have begun to form)!
These varnish and sludge have strond interactions with the sliding surfaces...!

Otherwise, by definition the 15w40 and 20W-50 oils have such a higher hthsv than the 0W-20 oils, that it should seize the engine!

 

[fpracha]

Originally Posted By: JOD
While it may seem like an unimportant distinction, I think it lends credence to the belief that higher hths automatically leads to thicker films, when in certain cases the inverse is true.

Higher hths should practically imply a higher film-strength, due to the inherent nature of the overall lubricant design, and not just the base oil considerations. i will repeat that higher hths = higher protective film-strength and not necessarily a significantly thicker oil film!

 

[jaj]

Originally Posted By: GMorg
I agree that HTHSV is measured under the conditions that are described above. However, I do not agree that loss of VIIs performance occurs only under those conditions.

In fact, any time that laminar flow exists, linear molecules begin to orient with the flow. I work with some of nature's longest water soluble polymers. The solutions act like a gel when not in motion. As the fluid begins to flow, the effective viscosity can drop dramatically at even low flow rates and low pressures. In fact, for a while after the flow is stopped, it can be restarted without much effort because the molecules are so large that are not significantly displaced by brownian motion or diffusion- they are still oriented with the historical flow.

In other words, the phenomena observed during an HTHSV measurement can occur at other pressures and temperatures too. I would argue that in spinning bearings the effective viscosity is always lower then the KV whenever long, linear molecules are in solution - even at room temperature. The exception would be whenever the fluid also contains molecules that have strong interactions with the sliding surface. In this case, there is a counter force that orients the surface-interacting molecules across the flow.


I guess the question of the moment is "what shear rate is needed to reduce the effectiveness of a VII?" I wonder if the additive companies publish that. It would be interesting to know.

 

[Shannow]

Originally Posted By: jaj
it's two surfaces a millimeter apart (0.040") moving at 1000 meters per second relative to each other. That's 2,237 miles per hour, or Mach 2.9! I doubt that speeds even 1/100th of that arise in your testing manifold.


I've never seen two surfaces operating under hydrodynamic lubrication that are anywhere near 1mm (0.040") apart.

Even a 24" diameter turbine bearing would have a diametrical CLEARANCE of about 30 thou, with the film thickness in the hydrodynamic wedge orders of magnitude smaller.

And I.C. engines are maybe 0.002" CLEARANCE tops ???

 

[GMorg]

I think that the answer to jaj's question will depend on the VII and the base fluid. However, given that the data in the linked thread shows a nearly perfect correlation between oil pressure and HTHSV at 90C and 6500 RPM, and given CATERHAM's assurance that similar data could have been collected at lower RPMs, then I think we have to conclude that whatever the shear rate is, it is found in an engine at operating temps at most RPMs. In other words, the KV is irrelevant most of the time (at least at operating temps).

It sounds like for the cost of the oil, a standardized engine could be used as surrogate device for HTHS. Just do an oil change, get the engine up to temp, and read the OP. Except for the standardization (calibration) and washes between tests and labor and storage and fuel, it's just the cost of the oil...

 

[jaj]

Originally Posted By: Shannow
Originally Posted By: jaj
it's two surfaces a millimeter apart (0.040") moving at 1000 meters per second relative to each other. That's 2,237 miles per hour, or Mach 2.9! I doubt that speeds even 1/100th of that arise in your testing manifold.


I've never seen two surfaces operating under hydrodynamic lubrication that are anywhere near 1mm (0.040") apart.

Even a 24" diameter turbine bearing would have a diametrical CLEARANCE of about 30 thou, with the film thickness in the hydrodynamic wedge orders of magnitude smaller.

And I.C. engines are maybe 0.002" CLEARANCE tops ???


It was just an example to illustrate what 10^-6 seconds of shear would mean if you ever saw it at a human scale. From what I've read, the loaded and rotating shaft runs off-center in the bearing shell, with a very thin layer of oil holding the pieces apart where the load is actually carried. If a 2.5" bearing is turning at 6000 RPM, the speed is about 20 meters per second, so a film thickness of 20 microns or 0.00079" yields a shear rate of 10^-6.

 

[JOD]

Originally Posted By: CATERHAM

I think this is off topic although an interesting point.
What cases do have that thicker oils as measured by the HTHS method do not correlate to higher oil film thickness? Are you referring to the chemistry of different oils?


My initial thought is "yeah, this is way off topic", but when you think about it a little more, it's relevant to the topic at hand. After all, no one really cares ultimately about an oil's thickness (or at least they shouldn't), they care about the oil's lubrication properties (among other things), and oil thickness is ultimately used as a proxy for film strength. Thickness alone doesn't really mean much.

I'm referring specifically to fact that multigrade oils demonstrate an elasticity under load that results in film thicknesses which aren't directly related to viscosity alone. This certainly is nothing new, Okrent measured oil viscosity at the bearings 50 years ago. More recently, Moore measures the results of oil films at the piston during operation in this paper.

The relevant results are summarized in this paper. The chart listed as fig. 20 tells the story: thicker viscosity oils do not always make for thicker films.

Originally Posted By: fpracha

Higher hths should practically imply a higher film-strength, due to the inherent nature of the overall lubricant design, and not just the base oil considerations. i will repeat that higher hths = higher protective film-strength and not necessarily a significantly thicker oil film!


I think above the above papers, which include direct measurements of the film thicknesses of different viscosity oils during operation, suggest that higher hths doesn't necessarily= higher protective film strength. I think it's an open question, with the answer being "it depends".

 

[jaj]

But, you see, I ran the same experiment as Caterham did and my results were very different. I used my BMW E39 M5 with the S62 V8 engine. I ran the test with four different oils: GC 0w-30, Motul 300V 5w-40, Motul 300V 10w-40 and Castrol TWS 10w-60. HTHS and kinematic viscosities were quite different from one oil to another.

For the four oils, I recorded curves of rising oil pressure every 500 RPM from idle (600 RPM) to 6,500 RPM at roughly constant oil temperatures. The curves overlaid pretty much exactly; at no point did the highest reading at a specific RPM differ from the lowest reading at that RPM by more than 5 PSI. There was no consistency either - sometimes thicker oils were higher, sometimes thinner were higher. The differences were noise.

So, my results differ from CATERHAM's because I used a different engine. While my results don't contradict CATERHAM's conclusions, they don't support them either. Hence, I remain skeptical.

 

[JAG]

I have hard copies of really good studies showing viscosity vs. shear rate curves, but obviously can't post them easily. However, Figure 7 in this link shows some representative curves.
http://priuschat.com/forums/attachments/...cal-article.pdf
The curves are flat and then start to drop down and then go flat again at some higher shear rate. The curves at 100 C temp. Start dropping around 10,000/s shear rate. At 150C, they start dropping around 100,000/s shear rate. I guess the polymers' shape varying with temperature affects temporary shear.

 

[jaj]

That's exactly what I was looking for, JAG, thanks!

So, for shear of 10^-4 seconds or less, you're looking at kinematic viscosity. Viscosity drops as shear increases (the exponent becomes more negative) and the fluid is pretty much sheared by the time it gets to 10^-6 seconds where HTHSV is measured.

 

[CATERHAM]

Originally Posted By: jaj
But, you see, I ran the same experiment as Caterham did and my results were very different. I used my BMW E39 M5 with the S62 V8 engine. I ran the test with four different oils: GC 0w-30, Motul 300V 5w-40, Motul 300V 10w-40 and Castrol TWS 10w-60. HTHS and kinematic viscosities were quite different from one oil to another.

For the four oils, I recorded curves of rising oil pressure every 500 RPM from idle (600 RPM) to 6,500 RPM at roughly constant oil temperatures. The curves overlaid pretty much exactly; at no point did the highest reading at a specific RPM differ from the lowest reading at that RPM by more than 5 PSI. There was no consistency either - sometimes thicker oils were higher, sometimes thinner were higher. The differences were noise.

So, my results differ from CATERHAM's because I used a different engine. While my results don't contradict CATERHAM's conclusions, they don't support them either. Hence, I remain skeptical.

Your sample isn't of much use.
Both the KV100 and HTHS viscosities of Motul are higher than GC, and both viscosity measures for TWS are higher than for Motul which is as one would expect.

What you need to do is compare the oil pressures of two oils with the same HTHSV but widely different KV100 values.
A good example is pretty much any Red Line Oil product.
Anyone who has run there oils experiences higher OP than what they expected for the grade. Not surprisingly that's why RL suggests dropping a grade when switching to most of their oils.

 

[jaj]

If oil pressure curve doesn't vary with a wide variety of HTHSV's and KV's, then the shape of the curve is a function of the engine, not the oil.

 

[GMorg]

I found what appears to be an Infineum presentation that is related to this discussion. Their data suggests a correlation between KV and OP to be 0.92 (R^2). The URL is very long. You may have to copy/paste to get the link to work.

http://www.google.com/url?sa=t&rct=j&q=hths%20viscosity&source=web&cd=6&ved=0CFUQFjAF&url=http%3A%2F%2Fwww.astmtmc.cmu.edu%2Fdocs%2Fdiesel%2Fhdeocp%2Fminutes%2F2001%2Fhdeocp.2001-05-25%2F052501ATT12.PDF&ei=7MIcT8yJOO6rsALWsfm6Cw&usg=AFQjCNEtK-1amjj-tg2yW69AEZoBxXYfTw&cad=rja

 

[Shannow]

Originally Posted By: jaj
If oil pressure curve doesn't vary with a wide variety of HTHSV's and KV's, then the shape of the curve is a function of the engine, not the oil.


The oil squirters in your engine also introduce another parallel path, with significant quantities of oil to the traditional passage of oil being predominantly through the bearings.

 

[jaj]

My opinion is that for any engine, more oil flows through low-intensity paths than flows through the main and rod bearings. To me, pressure curves can only tell you whether the oil pump is keeping up with demand. In the M5, it was.

 

[GMorg]

The link below is an Infineum work that discusses lowering of HTHS standards. It includes some graphs relevant to the discussion.

http://www.google.com/url?sa=t&rct=j&q=hths%20viscosity&source=web&cd=10&ved=0CG8QFjAJ&url=http%3A%2F%2Fwww.infineum.com%2FDocuments%2FCrankcase%2520Technical%2520Papers%2FSAE%2520J300%2520viscosity%2520grades%2520below%252020%2520grade-SAE%2520Powertrains%2520Fuels%2520and%2520Lubricants-2010.pdf&ei=7MIcT8yJOO6rsALWsfm6Cw&usg=AFQjCNEF8VL_wwNCllsd59OY_Sy399IMHg&cad=rja

 

[Shannow]

Originally Posted By: jaj
If a 2.5" bearing is turning at 6000 RPM, the speed is about 20 meters per second, so a film thickness of 20 microns or 0.00079" yields a shear rate of 10^-6.


Yep, that's about right.

 

[fpracha]

Originally Posted By: JOD
The chart listed as fig. 20 tells the story: thicker viscosity oils do not always make for thicker films.

Originally Posted By: fpracha

Higher hths should practically imply a higher film-strength, due to the inherent nature of the overall lubricant design, and not just the base oil considerations. i will repeat that higher hths = higher protective film-strength and not necessarily a significantly thicker oil film!


I think above the above papers, which include direct measurements of the film thicknesses of different viscosity oils during operation, suggest that higher hths doesn't necessarily= higher protective film strength. I think it's an open question, with the answer being "it depends".


Well these only reconfirm what i have said all along above!
Just add following: higher hths = higher film strength at higher temperatures.
This is true because of the way hths is derived/deducted.

The hths value actually corresponds to "operational film strength of the oil" at the higher temperatures and not at the lower colder temperatures. At least this is the intention or reasoning behind the hths, as i understand.

Shearing occurs both at low temperatures and high temperatures. Low temperature shearing is really devastating to the oil. This is exacerbated when coupled with fuel dilution for longer OCIs.

And yes i agree - "it depends" on the motor in question, oil pumping volume per stroke, temperature of the oil, RPMs, etc.