What do we know about the VII (VM), shear, base-oil viscosity, HTHSV, friction, and wear in relation to each other?
Recently, the combined effect of the base-oil viscosity and viscosity-index improvers [VII, also called viscosity modifier (VM)] on minimum oil-film thickness (MOFT), engine durability (wear), and fuel economy has been under my investigation.
In particular, I developed a simple calculator to attack the problem, called the
high-temperature, full-shear viscosity (HTFSV) calculator. The name high-temperature, full-shear viscosity (HTFSV) was something I coined.
It turns out that this is
state-of-the-art research because high-shear viscometers beyond the shear rate of 1,000,000 (10^6) 1/second were not available until recently because of rapid heating under high shear and the viscosity measurements under temporary shear were not possible beyond this shear rate.
For those who aren't familiar with the terminology, here is a brief summary:
Shear rate is the relative speed of the sliding parts separated by the oil film divided by the distance between them. Its units are inverse seconds (1/second).
Shear stress (force divided by area, units of pressure) equals to the shear rate multiplied by the dynamic viscosity DV [kinematic viscosity KV (in units of cSt) times the density (in units of cP, which is pressure times time)].
Newtonian oils are such oils for which the viscosity doesn't depend on the shear rate.
Non-Newtonian oils are oils for which the viscosity depends on the shear rate.
In practice, monograde oils that do not contain a VII but only contain a base oil and a detergent inhibitor (DI) package are mostly Newtonian other than a small shear of the DI package and in rare cases an even a smaller shear of the base oil.
Multigrade oils that do contain a VII are non-Newtonian. At low-shear rates, they are in the so-called first Newtonian phase, where the viscosity doesn't depend on the shear rate. The first Newtonian phase roughly extends to 10,000 (10^4) 1/second shear rate.
The viscosity then decreases with increasing shear rate, where the VII polymer molecules go under temporary shear:
"Temporary shear thinning is generally believed to result from conformational changes, such as partial alignment of the VII polymer molecules in solution under shear, that reduce the interactions between solvent/polymer and polymer/polymer molecules and thus the blend viscosity. The low-shear-rate viscosity is recovered fully after cessation of shear."
The so-called high-temperature, high-shear viscosity (HTHSV) is measured at a shear rate of 1,000,000 (10^6) 1/second.
Later, a second Newtonian phase is entered and the viscosity no longer decreases with increasing shear. This phase occurs roughly for shear rates greater than 10,000,000 - 100,000,000 (10^7 - 10^8) 1/second, 10 - 100 times higher than at the shear rate at which HTHSV is measured.
It looks like this -- here several oils with the same base oil but different VIIs are shown. The dynamic base-oil viscosity at 120 °C is 3.30 cP, and as the shear rate increases to very high values, the viscosities of all multigrade oils containing this base oil [with no detergent-inhibitor (DI) package] and different VIIs approach this value.
According to the paper that will be presented in the next post here, in the second Newtonian phase, the VII fully shears and has no effect on the viscosity, the only viscosity contributions coming from the base-oil and the detergent-inhibitor (additive) package, the latter of which also goes through some temporary shear.
Note that there is also the permanent shear, where the viscosity loss is never recovered after the cessation of the shear:
"Permanent shear thinning or permanent viscosity loss results from the thermomechanical scission of the VM polymer chains at the high shear stresses present in lubricated contacts and is, as the name suggests, irreversible, resulting in a permanent reduction in viscosity of the lubricant."
These are the shear rates encountered in an internal-combustion engine:
Bearing: 100,000 - 5,000,000 (10^5 - 5 x 10^6) 1/second
Piston rings: 20,000,000 (2 x 10^7) 1/second (peak rate)
Valvetrain: 200,000,000 (2 x 10^8) 1/second (peak rate)
Note again that HTHSV is measured and reported at 1,000,000 (1 x 10^6) 1/second shear rate, which lies somewhere near the middle of the shear-rate range for the bearings and its primary purposes are to correlate with the fuel economy and to serve as a sufficiently high viscosity for the protection of the bearings.
Shear rates are taken from the following paper:
Shear rates in engines and implications for lubricant design
Shear rates in engines and implications for lubricant design
R. I. Taylor (1) and B. R. de Kraker (2)
(1) Shell Global Solutions (UK), Manchester, UK
(2) Shell Global Solutions US Inc., Houston, TX, USA
March 1, 2017
It's been about 25 years since HTHSV has been implemented into the SAE J300 viscosity specifications, starting in February 1991 and then gradually being established in the next several years. Others may have a more concrete historical timeline. Nevertheless, it looks like multigrade oils are still not fully understood when it comes to how their viscosity behaves in different parts of the engine, in other words how they temporarily shear.
As it was said in the beginning, this was partly due to the fact that measuring the viscosity at shear rates beyond 1,000,000 1/second was not possible until recently.
Hence, we have a state-of-the-art paper to discuss here. Thankfully, it's open-access and everyone can read it.
In order to carry out their research, they utilized a new viscometer, which is capable of measuring the viscosity at shear rates up to 10,000,00 (10^7) 1/second. These shear rates had never been studied before.
They tested about 18 different 15W-40 test oils containing different types of VIIs with or without a DI package.
Here are some of their main conclusions:
- Amount of raw (solid) VII (VM) polymers used in oils are only about 1 - 2% for all VII types except the polymethacrylate (PMA) VII, which is 5 - 6%. Note that the VII is often sold as already dissolved in an oil and the percentages for that product will be much higher. Regardless, at the end, the base oil acts as the ultimate solvent that dissolves the VII into a solution.
- Viscosity increases linearly with VII concentration. (This is also what I assumed in my calculator.)
- The first Newtonian phase roughly extends to 10,000 (10^4) 1/second. However, this range increases with the increasing temperature.
- In the second Newtonian phase (at shear rates beyond 10,000,000 - 100,0000,000 (10^7 - 10^8) 1/second, the VII fully shears and the viscosity is solely due to the base oil and detergent-inhibitor (DI) package. In fact, DI also shears to some extent.
- Regarding different VII types, perhaps the only unusual type is the polymethacrylate (PMA) type. It requires a much higher raw (solid) treat rate, which is probably not desirable. However, it differs from other VIIs in that its thickening power, which is the percentage viscosity increase over the base oil, increases with the increasing temperature, wheraas for other VIIs it's either roughly constant or slowly increasing or slowly decreasing. As a result, oils containing PMA VII can have an extremely high viscosity index (VI). I wonder if oils like TGMO 0W-20 SN use PMA VII, which would explain their high VI.
- For all VII types, thickening power is proportional to the lack of temporary-shear stability. In other words, if a VII has a higher thickening power, it goes under more temporary shear. (This is actually good as far as my calculator is concerned because the one and only adjustable parameter in it depends on the ratio of the thickening power to the shear instability, which cancels the effect of varying VII type to some degree.)
- Most important conclusion: HTHSV is not sufficient to describe the viscosity of an oil under shear, as the temporary shear does not stop at 1,000,000 (10^6) 1/second. Both the fuel economy and wear is influenced by both the HTHSV and base-oil viscosity at 150 °C and possibly other factors as well, such as KV100 [kinematic (low-shear) viscosity at 100 °C] and viscosity index (VI).
So, as I have been advocating, base-oil viscosity at 150 °C matters in addition to the HTHSV.
Enjoy the articles (Part I and Part II).
The first one characterizes the viscosity of the oils under shear.
The second one studies friction (fuel economy). I found the second one somewhat inconclusive because the base oil and HTHSV are identical for different oils studied and the only remaining variables are KV100 and VI. As one would expect, a higher VI and lower KV100 results in less friction (better fuel economy).
Shear thinning and hydrodynamic fri...g oils. Part I: Shear-thinning behaviour
Shear thinning and hydrodynamic friction of viscosity-modifier-containing oils. Part I: Shear-thinning behaviour
Nigel Marx (1), Luis Fernández (2), Francisco Barceló (2), and Hugh Spikes (1)
(1) Imperial College, London, UK
(2) Repsol Technology Centre, Madrid, Spain
June 21, 2018
Part II: Hydrodynamic friction of viscosity-modified oils in a journal-bearing machine
Part II: Hydrodynamic friction of viscosity-modified oils in a journal-bearing machine
Sorin-Cristian Vladescu (1), Nigel Marx (1), Luis Fernández (2), Francisco Barceló (2), and Hugh Spikes (1)
(1) Imperial College, London, UK
(2) Repsol Technology Centre, Madrid, Spain
September 6, 2018
By the way, long live Professor Emeritus Hugh Spikes, who has been a pioneer in tribology.