A_Harman index - base oils

[OVERKILL]

What does it look like if you run the regular SpectraSyn backwards?

And I'll reach out to Mobil and see if they have KV150 and what they say about the HTHS difference.

 

[Gokhan]

Originally Posted by OVERKILL
What does it look like if you run the regular SpectraSyn backwards?

These are the A_Harman indexes calculated with the Exxon-Mobil-provided density correction:

4.1 cSt: 0.981
5.8 cSt: 1.042
8.0 cSt: 0.985

3.6 cSt Plus: 0.847
3.9 cSt Plus: 0.859
5.9 cSt Plus: 0.906

SS4 and SS8 are somewhat reasonable but SS6 overestimates HTHSV by 4%. So, the backward VI would for it = 164 vs. the actual reported VI = 138. SS+3.6, SS+4, and SS+6 all grossly underestimate the HTHSV and backward VI. We are getting less sensible results for the A_Harman index for the base stocks than for the finished oils using the Exxon-Mobil-provided HTHSV.

The concept of the viscosity index VI, which the ASTM D341 two-point viscosity extrapolation relies on, shouldn't be this flawed, as it's used so widely for different purposes, including inferring the base-oil quality and API base-oil groups from it.

Thanks for the reach. Let's hear what they will say.

 

[edhackett]

Originally Posted by Gokhan
A_Harman indexes you calculated:

Originally Posted by OVERKILL
SpectraSyn 4: 1.011
SpectraSyn 6: 1.077
SpectraSyn 8: 1.017

SpectraSyn Plus 3.6: 0.874
SpectraSyn Plus 4.0: 0.886
SpectraSyn Plus 6.0: 0.936

What I calculated using better density correction factors listed above:

4.1 cSt: 0.981
5.8 cSt: 1.042
8.0 cSt: 0.985

3.6 cSt Plus: 0.847
3.9 cSt Plus: 0.859
5.9 cSt Plus: 0.906

Now, we don't expect these low-viscosity "neat" base stocks to temporarily or permanently shear significantly. It's true that their high-viscosity PAO (100 cSt or greater) shears to some degree with increasing viscosity and their 1000 cSt can shear like a VII, which is not unexpected as it's practically plastic. However, I would expect negligible shear for these low-viscosity PAO neat (no additives or VII, pure) base stocks.

Then, what is going on? Two possibilities:

(1) The Widman operational viscosity calculator doesn't reliably work.

(2) We are having a garbage in, garbage out (GIGO) situation by being provided bad data by Exxon Mobil.

Given the scatter in the numbers, could it be that (2) is the case, namely garbage in, garbage out?

According to ASTM D341 the values obtained from the viscosity curves can only be applied to pure hydrocarbons. This makes the application of the Widman viscosity calculator and any index based on it invalid for fully formulated motor oils. Esters, organometalic additives, inorganic additives, any molecule that contains anything other than carbon and hydrogen affects the linearity of the viscosity curve.

Originally Posted by ASTM D341

3. Technical Hazard
3.1 Caution
...It should also be emphasized that fluids other than hydrocarbons will usually not plot as a straight line on these charts.

https://www.scribd.com/document/356620101/ASTM-D341-pdf

As for number 2, all of these indexes are based on the assumption that pure base oils don't shear. What if they all shear to some degree? The HTHS viscosity includes that shear. As OVERKILL has already stated, SpectraSun and SpectaSyn Plus are different classes of chemicals. I have no problem thinking that the longer, straighter molecules of the Spectra SynPlus will exhibit more temporary shear that the shorter, more branched molecules of the SpectraSyn.

Ed

 

[JAG]

I agree with Ed that the Spectrasyn Plus PAO results may be due to them exhibiting temporary shear. That's actually what I think is the most likely cause of the HTHS and kinematic viscosity data oddity. I think some blenders would consider temporary, but not permanent, shear of a base oil to be an advantage from a fuel economy standpoint.

 

[Gokhan]

We're discussing pure (neat) base stocks here. So, you would think ASTM D341 would apply well in that respect.

Regarding shear, these are low-viscosity PAOs. Their high-viscosity PAOs do shear to some degree but I strongly doubt that you would see more than about 0.1% shear with either SS or SS+ low-viscosity PAOs. SS Elite 300 cSt mPAO permanently shears only 1%. It looks like they got rid of their SS Ultra line, of which the 300 cSt base stock permanently sheared about 15 - 20%. Moreover, one of the SS viscosities appeared to thicken with shear, obviously some artifact in the data or calculation.

Hopefully, Exxon Mobil will reply to OVERKILL.

 

[JAG]

I forgot to include a link to a study that shows simulation results of PAO having temporary shear at very high shear rates. It is far from proof of my belief but it is evidence that might be true.
https://aip.scitation.org/doi/am-pdf/10.1063/1.4986552

 

[Gokhan]

Originally Posted by JAG
I forgot to include a link to a study that shows simulation results of PAO having temporary shear at very high shear rates. It is far from proof of my belief but it is evidence that might be true.
https://aip.scitation.org/doi/am-pdf/10.1063/1.4986552

Thanks, yes, those shear rates are several orders of magnitude more than a million per second. They are interesting in the boundary and mixed lubrication regimes.

Let's see what Exxon Mobil will say. I don't see any reference of them mentioning shear for their low-viscosity PAOs. 15% temporary shear of a pure base oil is a lot if that's happening.

 

[JAG]

15% does require some major explaining.