Let’s Discuss Viscosity Index, Again!

[Blue99]

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BTW, thanks Tom NJ for your post. As the saying goes, "nuggets of wisdom" appear in threads on this forum, mixed in with all the myths, conjecture and mis-statements!

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[Blue99]

My comment was:

"Within the same grade, 5w30 for example, a 151 VI versus a 156 VI simply indicates that the 156 has a higher spread..."

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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.

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Yep, I just really blew that statement

What I had in mind was a higher VI index number generated by one oil being slightly thicker at 100C than similar grade oil.

Here’s a good example:

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Pennzoil Citgo

5w30 5w30



Viscosity 100C 10.5 10.8



Viscosity 40C 63.9 64



VI Index 158 161

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The Citgo oil is already thicker than Pennzoil at 100C, so it gets thick at a lesser rate from 100c to the 40C value of 64 cSt.

And this example actually fits our definition, that the higher the VI index number, the less change in viscosity, or less thickening as the oil cools down.

 

[bruce381]

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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. 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.

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So if a given multi grade is heated from 40C to 100C and vis is measured say every 10 C going up curve will be diferent than cooling even at same temp measureing points?

Do you have example?

bruce

 

[Stooge]

The example is fact for most complex mixtures, solid, liquid and gasses. One of the reasons why chemical composition of oils doesn't tell the whole story; its how they perform in the particular engine via reaction kinetics.

 

[bruce381]

Ok lets see if I understand if I heat a given oil and measure the vis at every 10C from 40C to 100C when I turn off the heat and run chill water into the vis bath
to cool the oil and then measure the vis at the same temp I will get a different viscosity?

bruce

 

[Ron AKA]

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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

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I've heard of hysteresis in metals, but not oils.

 

[Ron AKA]

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So if a given multi grade is heated from 40C to 100C and vis is measured say every 10 C going up curve will be diferent than cooling even at same temp measureing points?

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I would think we are talking many cycles here not just one, and this is just another way of saying that the VII's breakdown over time. I would suggest it is more likely due to shearing during operation than startup shutdown cylces. Or, in another way high cycle fatigue of the polymers, and not low cycle, or thermal cycle.