hths vs kinematic viscosity

[fpracha]

Originally Posted By: 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.
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.
+1.
The hths at 150 C is relating to the VII at very high temperatures, much more than the oil itself.

 

[CATERHAM]

G-Oil 5w30 is another recent example of why you cannot rely on the KV100 spec' to access how thick an oil will be at operating temp's. It has a KV100 of 9.84cSt but performs like a much heavier oil as was discovered in a members Ford F150 fitted with as oil pressure gauge as compared to Mobil 1 5w30; see below:
https://bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2341220#Post2341220

And sure enough, we've learned from G-Oil that it's HTHSV is 3.5cP, which confirms what member tj90 suspected based on his oil pressure readings.

 

[fpracha]

Originally Posted By: CATERHAM
G-Oil 5w30 is another recent example of why you cannot rely on the KV100 spec' to access how thick an oil will be at operating temp's. It has a KV100 of 9.84cSt but performs like a much heavier oil
Ok. Does anybody know the top 3 reasons for this behavior of Mobil 1 5w30?

 

[A_Harman]

Why does there have to be 3 reasons? Maybe it's just due to the presence of VII's in M1 that give a high KV100 reading in the falling-ball viscometer, but suffer temporary breakdown under shear when subjected to the HTHS test.

 

[edhackett]

HTHS viscosity is directly linked to the kinematic viscosity of the base oil(s) used to formulate the oil. A 5w30 oil formulated with a thin base oil plus VIIs will have a lower HTHS viscosity than an oil formulated with a thicker base stock that has a naturally higher VI.

Ed

 

[CATERHAM]

The chemistries of different base oils have varing pressure-viscosity coefficients. Many esters tend to have higher coefficients than mineral and PAO base oils; hence the higher HTHSVs relative to the KV100 spec's in their finished oils.

 

[JAG]

When I last looked into the pressure of oil of the in the HTHS test, I concluded it was not high enough to cause significant increases in dynamic viscosity. Oils with a large amount of esters tend to be more dense and since KV is gravity driven, a more dense oil will flow faster, causing a lower KV than another oil of lower density but same dynamic viscosity at very low shear rate. HTHS is not a gravity driven test so there is not that same effect of density on the measures viscosity that occurs in KV measurements. I don't know for sure but there may also be intermolecular forces between polar ester molecules and/or ester-metal forces that raise the dynamic viscosity of ester oils in the HTHS test. Also many of the ester oils we look at have none to little VIIs which raises the HTHS viscosity relative to their KVs.

 

[Jeff_in_VABch]

Originally Posted By: jaj
[quote=GMorgYou're almost there. HTHSV is simply not a factor in this. HTHSV is measured at a shear rate of 10^-6 seconds. To put that rate of shear in physical terms, 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.


Im not real sure what your point is, but no part of an internal combustion engine actually moves very fast in land speed terms.

Example:

A 2" crankshaft journal has a circumference of 6.2832 (6.3)"
Each engine revolution travels 6.3" X 7,000 RPM = 44,100 in/min or 3,675 ft/min.

3,675 ft/min X 60 = 220,500 ft/hr or 41.76 MPH.

The camshaft is driven at half of crank speed so all the speeds are reduced by half.

 

[jaj]

HTHS testing measures viscosity in oil that's sheared at 10^-6 seconds of shear. So what does "10^-6 seconds" of shear mean?

Well, it's either a 2" crank journal turning 7000RPM with an oil film 0.00075" thick, or it's two surfaces 50 times further apart (one millimeter) and moving 50 times faster (2100mph).

 

[cp3]

OK! Three things!
1. I will not comment on the actual content of this thread as I think I am barely following along with everyone's thoughts as this is far beyond anything I learned in Grade 10 science class. I am however proud that I am even doing that.

2. This should be mandatory reading for members, not based on content but on how the discussion has proceeded. How multiple points of view and opinions can be presented and discussed in a civil manner with out resorting to the debating tactics of children in a school yard.

And 3. This brought a huge smile to my face and actually made me chuckle when I re-read the thread (really, my daughter looked at me funny), nice to see in such a complex discussion.

Originally Posted By: Shannow

I've designed and troubleshot bearings inn some very big, very high power spinny stuff....


Thanx and Cheers gentlemen.

 

[GMorg]

Well, I too am glad the thread didn't turn into a fight.

I have some additional curiosity not yet quenched. We've discussed the likelihood of HTHSV as the critical parameter of OP across a range of RPMs, but what about across temperatures. Is HTHSV irrelevant at -5F(-20C)? Is KV the major/only parameter that deserves our concern at cold start-up. Or, is OP higher with high HTHSV even when KV is constant? Similarly, if the KV is same at low temp between to oils and HTHSV is equal, is the OP different between the different oils or does the temporary shear in the bearings still drive OP? (Assuming equal RPM, of course.)

 

[MolaKule]

Quote:
A 5w30 oil formulated with a thin base oil plus VIIs will have a lower HTHS viscosity than an oil formulated with a thicker base stock that has a naturally higher VI.



Well, almost.

Interesting discussion gentlemen.

What the tribologists and lubrication chemists are doing to attain high HTHS properties with low viscosity oils is to use three or more base oils of varying viscosities and types, and or specialized polymers.

One example (not necessarily the exact ratios for obvious reasons) for say a 5w20 high HTHS oil: Mix a base oil of about 70% 4.0 cSt, 25% of an 8 cSt, and 5% of a high viscosity PAO, ester, or special polymer of say 150 cSt or higher. The special polymer is NOT a VII polymer. A separate VII polymer may be used to correct the overall KV.

 

[fpracha]

Originally Posted By: A_Harman
Why does there have to be 3 reasons?
well there are just 3 top reasons, as evident from the many above replies by several memebers here!

Originally Posted By: A_Harman
Maybe it's just due to the presence of VII's in M1 that give a high KV100 reading in the falling-ball viscometer, but suffer temporary breakdown under shear when subjected to the HTHS test.
yes but not all shearing is hths based because there is shearing at cold temps too... and not all shearing is temporary, or at least not for too long periods of engine operation!

 

[fpracha]

Originally Posted By: MolaKule
Quote:
oil formulated with a thin base oil plus VIIs will have a lower HTHS viscosity than an oil formulated with a thicker base stock
Well, almost.
One example for say a 5w20 high HTHS oil: Mix a base oil of about 70% 4.0 cSt, 25% of an 8 cSt, and 5% of a high viscosity PAO, ester, or special polymer of say 150 cSt or higher. A separate VII polymer may be used to correct the overall KV.
Molakule, yes, you have nailed this one!

 

[JAG]

Originally Posted By: GMorg
I have some additional curiosity not yet quenched. We've discussed the likelihood of HTHSV as the critical parameter of OP across a range of RPMs, but what about across temperatures. Is HTHSV irrelevant at -5F(-20C)? Is KV the major/only parameter that deserves our concern at cold start-up. Or, is OP higher with high HTHSV even when KV is constant? Similarly, if the KV is same at low temp between to oils and HTHSV is equal, is the OP different between the different oils or does the temporary shear in the bearings still drive OP? (Assuming equal RPM, of course.)

HTHS is irrelevant at -5F simply because HTHS is measured at a much higher temperature BUT high shear rate IS relevant at all oil temperatures. That's why the CCS test is done on multigrade oils, which induces much higher shear rates than the MRV and pour point test and tests that measure KV do. I can't comment on your oil pressure questions other than to say that temporary shear will always occur to some degree with polymeric VII-containing oils in a running engine.

Check out the following link starting on page 17. It has some interesting data that ice never seen before. http://books.google.com/books?id=chx8Swz...utput=html_text

 

[fpracha]

Originally Posted By: CATERHAM
The chemistries of different base oils have varing pressure-viscosity coefficients. Many esters tend to have higher coefficients than mineral and PAO base oils; hence the higher HTHSVs relative to the KV100 spec's in their finished oils.
and let us not assume that the pressure-viscosity coefficients are static over the wide extremes of "temperature ranges" under discussion, in addition to the differences among different base oils and formulation chemistries!