Let’s Discuss Viscosity Index, Again!

[Blue99]

Here are my statements regarding the definition of VI index number, and as all you veteran Bitoger’s are aware, I claim no professional expertise – I’m just an enthusiast!

Per the definition of Viscosity Index in the BobisTheOilGuy glossary, VI index “reflects the viscosity/temperature relationship between temperatures of 40 C and 100 C.”

It is the slope of the viscosity/temperature graph line between 40 C and 100 C.

The VI index number, for a base oil, is used as a basic yardstick to label conventional/mineral base oils as Grp I, II, II+, or Grp III.. (Note that levels of aromatics and saturates also separate the categories.)

VI index is also a common number listed on a motor oil product data sheet. It reflects the viscosity of the finished formulation, which includes additives and carrier oils, fluid additives, and, of course, the VI improver additive.

Q What is the VI index number on a product data sheet used for?

-A It provides an indication of the rate of viscosity change as the oil gets thicker while it cools down from 100C to the cold temp xW rating.

Straight weight SAE 30’s are typically about 105 VI index and get thick relatively quickly as the oil cools down. 0W-30’s or 5W-40’s can be 175 VI index and do not get thick as fast as an SAE 30 as temps cool down.

Q Does the VI index number indicate the amount of VI improver in the formulation?

A Yes and No, the VI index of a finished oil reflects the blended viscosity of pour point depressants, base oils, fluid synthetic additives, additives + carrier oils and VI improvers.

Q What is the significance of comparing the VI index of brand X & brand Y motor oils?

A Within the same grade, 5W-30 for example, a 151 VI versus a 156 VI simply indicates that the 156 has a slightly higher spread between the low end 5W rating and the 100C hot end.

Here’s a good GF-4/SM product data sheet example to consider:

Chevron Supreme 10W-30….10.8 cSt….5800 cP @ -25C….135 VI index

Phillips Trop Artic 10W-30…10.9 cSt….3500 cP @ -25C….157 VI index

Note that both oils are a very similar viscosity at the hot end and the VI difference is due to the better cold temp properties of the Trop Artic 10W-30.
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Now go ahead & fire away with your comments!

 

[FD777]

First off, nice thread. This is something I wish I coulda read when I first joined. Now for the comments

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Within the same grade, 5W-30 for example, a 151 VI versus a 156 VI simply indicates that the 156 has a slightly higher spread between the low end 5W rating and the 100C hot end.

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Doesn't a higher VI offer a smaller spread? In other words, the higher the VI, the smaller the slope with respect to Vicsosity vs Temp.

For example, I just used a VI calculator:
Given that an xW-30 oil with a viscosity of 11 cst @ 100c:
VI = 175, 60.9 cst @ 40c
VI = 150, 69.2 cst @ 40c

A higher VI implies the oil doesn't thicken too much as it cools down. (Which is a good thing and why we don't use straight weights anymore.)

Also, IIRC, using the VI to calculate viscosity isn't accurate at cold temps.

Just my 2 cents.

 

[Tom NJ]

Hi Blue 99,

In a nutshell, all organic liquids will thin out when heated and thicken up when cooled, but they don't all do so at the same rate. Viscosity Index is simply a scale to compare the rate of viscosity change with temperature between different fluids. A fluid that thins more upon heating and thickens more upon cooling has a lower VI than one that thins and thickens less. Or put another way, higher VI oils change their viscosity less when the temperature changes. This can be a good property for lubricants that are used in a wide temperature range and low shear rates.

The VI scale was originally established by assigning a value of "0" (zero) to the worse known base oil at the time, and "100" to the best. The theory was that all other base oils would then fall between these end points. Apparently they didn't anticipate synthetics or hydrocracked mineral oils back then.

The VI of a base oil is dictated by its chemical structure. Linear hydrocarbons have the best VI, but they also exhibit a freeze point when cold (waxes). The next best VI comes from branched hydrocarbons and the worse comes from ring compounds, such as aromatics, naphthenics, and cyclic hydrocarbons.

Group I base oils are a mix of linear and ring shaped hydrocarbons and usually have a VI around 90-100. By hydrotreating the Group I stock (reacting with hydrogen) you break a portion of the ring compounds into branched hydrocarbons, and this raises the VI and makes a Group II oil. Severely hydrotreating (called hydrocracking) the Group II opens more rings and raises the VI further (120-125), making a Group III oil. Since the VI is very sensitive to the degree of hydrotreating/cracking, it is used as a measure in defining the Groups.

PAOs are pure branched hydrocarbons and have even higher VIs, usually 125-140. Esters are hydrocarbons that also contain oxygen atoms, and can come in different shapes and sizes, so their VIs can run from 50 to 200, but most esters used in motor oils have VIs of 130 to 150. ANs are ring compounds and have lower VIs (75-105).

Viscosity Index can be important in applications that operate in a wide temperature range where the oil is expected to flow under its own weight (low shear), but there are other and better measures of performance over temperature. In motor oils, the oil is pumped (high shear) at cold start-up and the flow properties are better defined with the Cold Crank Simulator and MRV tests. Even at the high temperature end, HTHS viscosity is a better measure because it simulates the shear rates in the bearings. Hence the VI of a motor oil is not very useful in telling you how the oil will behave at start-up or at full temperature. About all its good for is giving an indication of the amount of VI Improver in the oil, but that too is deceptive.

People tend to think that less VI Improver the better, but that depends on the type of VI Improver used. Some are much more shear stable than others, and a higher quantity of a shear stable VII may be better than a lower quantity of a non-shear stable VII. In addition to permanent viscosity loss cause by breaking (shearing) the large VII molecules, they also exhibit temporary viscosity losses under high shear, and this affects the HTHS viscosity.

If only oil chemistry followed simple rules we would all be experts, but there is a lot of chemistry going on in the engine, and trying to predict or rank motor oils by comparing basic physical properties like VI, Kinematic viscosity, pour point, and flash point is pretty much a waste of time. What matters is how well they work, and since that depends on the engine design, driver, and environment, what really matters is how well they work for YOU.

 

[bruce381]

Tom is correct, also.

"""Q What is the VI index number on a product data sheet used for?""
""A It provides an indication of the rate of viscosity change as the oil gets thicker while it cools down from 100C to the cold temp xW rating.""

VI is not a 100% indicator of any viscosity at LOW temps it will not predict a CCS vis or even a pour point.

As posted above a higher VI indicates a SMALLER vis spread.
bruce

 

[427Z06]

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If only oil chemistry followed simple rules we would all be experts, but there is a lot of chemistry going on in the engine, and trying to predict or rank motor oils by comparing basic physical properties like VI, Kinematic viscosity, pour point, and flash point is pretty much a waste of time. What matters is how well they work, and since that depends on the engine design, driver, and environment, what really matters is how well they work for YOU.

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Best post I've seen here in quite some time.

 

[Oil Changer]

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Best post I've seen here in quite some time.

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I agree. It took me a lot of reading and studying to be able to understand half of what TomNJ posted. You should be advanced to a Group II+ or something for sharing that knowledge.

 

[Ron AKA]

I would basically agree with what you say, except for the slight mis-wording re spread and higher VI. But, I think you knew the concept and just slipped up on the choice of words.

I also think and will stand corrected on this that CCS or MRV, and I believe it is CCS is simply the viscosity at that temperature. So, I think you could calculate the wider range VI based on that viscosity and the 100C one to get a better view of what the real wide range VI is. That one is more important for the cool/cold weather startup issues.

 

[G-MAN]

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Esters are hydrocarbons ...

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What?

 

[Tom NJ]

No - I said they are hydrocarbons with oxygen atoms. I was trying to explain chemistry in more of a laymans style so as to not lose the non-chemists.

 

[G-MAN]

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No - I said they are hydrocarbons with oxygen atoms. I was trying to explain chemistry in more of a laymans style so as to not lose the non-chemists.

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Yeah, but esters used in motor oils are not HYDROCARBONS at all.

 

[Nebraskan]

aboard Tom of New Jersey! Toasts a little wine this evening to catch the subtle nuiances of those esters. *S*

 

[Tom NJ]

Strictly speaking, hydrocarbons are molecules composed of only hydrogen and carbon atoms. Esters are composed of mostly hydrogen and carbon atoms, but also contain some oxygen atoms. Hence my statement that they are hydrocarbons with some oxygen. My intent was to compare and contrast ester molecules to hydrocarbon molecules so the non-chemist can understand the similarity and the difference.

The most common ester used in motor oils is TMP C8C10 and the formula is C32 H61 O6, i.e. 32 carbons, 61 hydrogens, and 6 oxygens.

Tom

 

[Tom NJ]

Thanks, Nebraskan. Wine tasting has been a hobby of mine for some 32 years, although it's more like wine drinking these days

I did not know there were plantings in Nebraska - learn something new everyday. Do you grow vinifera or hybrids?

Tom

 

[Nebraskan]

I started helping to develop vineyard for folks in Nebraska, back in 1999 and took a job as Associate Winemaker at Fenn Valley Vineyards near Fennville, Michigan. We grow Riesling, Gewürztraminer, Pinot Gris, Pinot Noir, Chardonnay, Cabernet Franc, Syrah, Vignoles, Vidal, Chambourcin, Seyval, Chardonel, and Chancellor to name a few. And yes, Nebraska is growing more and more wine grapes, though they are mostly hybrids.

Now since this is about viscosities, is there a reasonable belief that one should not use an oil with too high a VI when one uses a bypass filter system that goes to less than 3 µ if fine filtration is your ultimate goal?

 

[bruce381]

why is it because a high VI oil may have more VII in it which maybe filtered out? if so no worry unless it is a polymer that was/is not correctly all dissolved.
bruce

 

[G-MAN]

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Strictly speaking, hydrocarbons are molecules composed of only hydrogen and carbon atoms. Esters are composed of mostly hydrogen and carbon atoms, but also contain some oxygen atoms. Hence my statement that they are hydrocarbons with some oxygen. My intent was to compare and contrast ester molecules to hydrocarbon molecules so the non-chemist can understand the similarity and the difference.

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Non-chemist...that would be me.

I was thinking more along the lines of PAO being a "synthetic hydrocarbon," whereas esters are synthetic, but not "hydrocarbons." But I see your point.

 

[Tom NJ]

Filtration capability/efficiency is a function of the oil viscosity and solids content. If the oil viscosity at the filtration temperature is too high, filtration will be slower, but this should not be an issue if the filter is properly sized to the application. And if the oil has suspended solids, they may clog the filter. VI is not an issue in a well made oil as the VI Improver should be fully soluble throughout the temperature range.

Some of my favorite grapes there, especially Riesling, Gewürztraminer, and Pinot Noir!

Tom

 

[Stooge]

Also of note should be that the viscosity/temperature "curve" when a motor oil is heated is different than the "curve" when the motor oil cools back down, and does not reach the same point at each cycle. Better oils tend to limit this effect and put the oil back to the same point more precisely. Low quality oils "cycle out" more quickly than high quality oils with better additive packages. Thus, its not only chemistry but reaction kinetics as well that says "this is a good or bad oil" - a point sometimes missed in the discussion.

 

[Tom NJ]

Hi Stooge,

Interesting. I know that the log/linear plot curves upward, off the linear, at very cold temperatures, but I never heard of temperature cycling shifting the plot. Could it be oxidation, or thermal degradation of the VI Improver?

Tom

 

[Blue99]

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Also of note should be that the viscosity/temperature "curve" when a motor oil is heated is different than the "curve" when the motor oil cools back down, and does not reach the same point at each cycle.

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Huh, Really!

So what you are saying is that the rate at which the long chain viscosity improvers are molecularly "uncoiling" while heating up is different than the rate exhibited by the oil cooling down?

I hadn't considered this.

And I'm assuming the ASTM D2270 Viscosity index test is run by heating the oil to 100C and then cooling down?

- I think from a practical standpoint, we are more interested in the heating up data points, which represents the crankcase oil as the engine is warming up to full operating temperature.