Predicting Temporary Shear Stability - Part 2

[JAG]

Here, Shear Stability, is a useful thread from 2004, that I think may be good to share with newer members. The calculation, KV100/HTHS, that TooSlick proposed for the inverse of the temporary shear stability has the benefit of being the most simple. I prefer HTHS/KV100, which is a measure of the shear stability rather than its inverse. I use it when I want a quick estimate.

TooSlick’s calculation can be improved and this helps the most when the oil has unusually low or high density (ex: Redline). One improvement is to use dynamic viscosity at 100C (DV100) instead of KV100, using the equation DV100 = KV100 x density. Ideally, the density is measured at 150 C, but whatever is in the PDS is close enough for most purposes.

An even more accurate shear stability estimate is DV150/HTHS, since both of those are at the same 150 C temperature. This has been known on our forum as the A_Harman index because that member popularized it. It is generally less than 1 and the more shear stable the oil is, the closer to 1 it is. I am baffled why I could not find his posts when I just searched. Calculating DV150 involves using a viscosity calculator than provides KV150 from KV100 and KV40 or KV100 and VI. Then multiply by density to get DV150. Member Gokhan later did a lot of work on the viscometrics subject. Here is a spreadsheet he made: Link

 

[Jetronic]

Yes, I do that to determine what the maximum realistic HTHS viscosity is of an oil that doesn't have it listed, or when I don't trust what is listed.

calculate KV150, multiply by density + a correction factor for loss of density at 150°C and that give you approximately the no shear DV at 150°C.

 

[JAG]

It might not be apparent why the dynamic viscosity should be used instead of the kinematic viscosity. That’s because the force that causes the fluid motion in kinematic viscosity tests is gravity. Viscosity resists the motion. A denser oil will have a greater gravitational force causing the motion, so the kinematic viscosity is biased by the density and is not a measure of purely the resistance to motion. Dynamic viscosity tests involve using forces other than gravity.

Kinematic viscosity measurements are at very low shear rates, so when converting to dynamic viscosity, know that it is also at very low shear rate. HTHS is also a dynamic viscosity measurement, at 150 C and 1e6/s shear rate.

 

[JAG]

Yes, I do that to determine what the maximum realistic HTHS viscosity is of an oil that doesn't have it listed, or when I don't trust what is listed.

calculate KV150, multiply by density + a correction factor for loss of density at 150°C and that give you approximately the no shear DV at 150°C.

What density correction factor(s) do you use?

 

[JAG]

High shear rate is a partial cause of temporary shear thinning, as most know. But the more complete cause is high shear stress. Shear stress is shear rate x viscosity. At lower temperatures, viscosity is higher, so at the same shear rate, the shear stress is higher. That means that oils with VIIs undergo more temporary shear thinning at cold temperatures than at higher temperatures. This is one of the benefits of VIIs in terms of gas mileage while oil is cold. When the oil is cold and much more viscous than ideal, the dynamic viscosity at relatively high shear rates in the engine is even lower than what you might think based on the kinematic viscosity alone. Example: Oil A with VI of 200 and kinematic viscosity at 0 C of X and Oil B with VI of 140 and same kinematic viscosity of C at 0 C. Assume Group III base oil and same additive package except for VII percentage in both oils. At 0 C, Oil A with its large VII percentage will undergo quite a bit of temporary shear thinning while Oil B will not, at higher shear rate parts of the engine. The kinematic viscosities being equal at that temperature provides no indication of the actual dynamic viscosities at those high shear rates.

 

[Jetronic]

What density correction factor(s) do you use?

x0.88 seems to be the ballpark to go from 15°C to 150°C

 

[TurboZedd-R]

Does this apply to something that is shear stable such as HPL or Ravenol?

 

[OVERKILL]

Does this apply to something that is shear stable such as HPL or Ravenol?

It starts to fall apart when you get away from the base oil behaviours the calculations are based on. Some good examples, the HPL "No VII" oils, which, as the names would suggest, have no VII's in them (last column is estimated VII content):

Another are the EVOLVE bio-based synthetics:

 

[JAG]

I remember that Gokhan knows that dispersants tend to cause temporary shear thinning but I can’t remember if he factored that into those calculations. If he did not or did not compensate for it properly that could cause those HPL no-VII predictions. Even without VIIs, modern motor oils typically experience temporary shear thinning, but it is not as dramatic at VIIs cause.

The physical and chemical phenomena that occur in oils are very complicated, so it’s impressive that simple models like described in this thread predict as well as they do. Every different type of VII behaves differently and they even behave differently depending on the base oil(s) they are in because those affect the VII solubility. An example of weirdness is the finding that some motor oils that are sheared, experience an increase in low shear rate viscosity at 100 C. If that’s not weird enough, they do not low shear rate viscosity at 150 C. I wish they had tested viscosity at a high shear rate.
https://www.api.org/~/media/files/c...shear-stability-of-automotive-engine-oils.pdf