Trends in Shear Stability of Automotive Engine Oil

[Ducked]

I've linked to this in another post, which is probably bad practice, but I'm thinking if it hasn't already been discussed on here (If so I missed it) then this might be a better place for it.

http://www.api.org/~/media/Files/Certification/Engine-Oil-Diesel/Publications/4-Trends-in-Shear-Stability-of-Automotive-Engine-Oils.pdf

Its a presentation, so a bit short on detail, but they seem to be saying that viscosity for quite a lot of multi's increases after shearing.

I know it isn't news that viscosity starts to increase with degraded oil due to polymerisation of oxidation products, but they seem to talking about something new involving VI's.

"This behavior of viscosity increase with shear was observed primarily on 5W-30 oils, and primarily those purchased in Europe.

This trend has been growing over the last several years and is spreading to the other regions of the world."

Of one of the other papers mentioned, they say "VM chemistry does play a role in the viscosity increase phenomenon, although the authors were not sure why."


(Low-Temperature Rheology of Engine Lubricants Subjected to Mechanical Shear: Viscosity Modifier Effects, M. Covitch, J. Weiss, I. Kreutzer;Lubrication Science 11-4, August 1999).

Anyway, thought it was a bit surprising. Straight weight oil is looking better all the time.

 

[Jetronic]

Note: while the KV100 increased, kv150 dropped... just to make it even more of a headscratcher.

What happens at lower temperatures? Will they stiil behave like a 5w-30 when it's very cold?

It looks like some low VI, high viscosity products are formed, can't readily see another explanation.

 

[UltrafanUK]

Interesting as I've only ever seen the viscosity figures most UOA decrease and that has been the norm even when there is no fuel contamination involved. Once the active detergents run out, the blowby products then thicken the oil up.
Oddly enough I have seen some petrol job UOA results recently that show the oil thickened up slightly for no obvious reason, such as a low TBN.
Most compression ignition safer fuel fans I know use Xw40 grades (Mostly 5w40's), so I wonder if this new thickening effect means that some 5w30's are using different VI's, although I would point out that diesels don't seem to shear their oil as much as petrol jobs.

 

[Popsy]

I've seen viscosity increase due to soot contamination on neglected diesels, but that's a different story.

 

[Ducked]

Originally Posted By: Jetronic
Note: while the KV100 increased, kv150 dropped... just to make it even more of a headscratcher.



They point that out, though they offer no explanation.

They also say "Based on the performance of the oils, the stay-in-grade requirements appear to an important criteria for the formulators (particularly the 5W-30 oils in Europe)"

This may be intended to imply that this behaviour is deliberately engineered "self healing" of shear induced viscosity loss, which has "overshot" a bit.

Its not clear to me whether this is likely to be harmful, but there seems to be some disaster potential in extreme conditions/over-extended OCI's.

It may also add an additional element of uncertainty when mixing oils, which has been the topic of some recent discussion/argument.

More worryingly for me, as a kicker they add:-

"California has now put legislation in place to ban obsolete oils. " I didn't know that.

I rather like obsolete oils (straight-weight oils look better all the time) and, while I couldn't care less what happens in California, it does tend to drive the global automotive marketplace.

 

[Shannow]

Originally Posted By: Ducked
"California has now put legislation in place to ban obsolete oils. " I didn't know that.

I rather like obsolete oils (straight-weight oils look better all the time) and, while I couldn't care less what happens in California, it does tend to drive the global automotive marketplace.


They are (I believe) defining obsolete as non current certification...e.g. SA through SG and maybe beyond, presumably to stop people getting caught out by no spec oils.

 

[SonofJoe]

I can't say for sure but I'd hazard a guess that what the Institute Of Materials is seeing (ie increases in the viscosity of used oils) is not because of oil oxidation or strange VII interactions but because of engine oil 'distillation'.

Anyone who's read my posts on BITOG before will have heard this story many times over but if you move in the direction of FE/Emissions driven engine oil formulations, the penalty you pay is higher Noack volatility. What this really means in engineering terms is that you have engine oils which exert a greater vapour pressure. It's the way this higher vapour pressure interacts with other mechanical trends (higher compression ratios, more and hotter blow-by, GDI with associated fuel dilution, higher power density engines, more complex crankcase ventilation requirements) that causes the lightest fraction of the engine oil to preferentially evaporate into the blow-by and ultimately be burnt. If you remove the lightest fractions of the oil, by definition, what gets left behind in the sump must get heavier and MORE VISCOUS!

 

[zeng]

Quote:
http://www.api.org/~/media/Files/Certification/Engine-Oil-Diesel/Publications/.............pdf.

I'm afraid this paper is in the context of HDEO and not PCMO.

 

[SonofJoe]

Zeng,

Sadly the Noack arguments apply equally to HDDO as they do to PCMO.

There was a time when 15W40 was synonymous with low volatility (Group I 15W40 is typically around 7%). However if you at look the way HDDO specifications have evolved over the years, the emphasis has been on ever cleaner pistons. This has usually been achieved by increasing amounts of ashless dispersant in oil and the physics of oil blending mean that this increases finished oil Noack.

Take a look at the Noacks of 15W40 CJ-4 oils on PQIA. Would you say they are low or high? If CK-4 allows 10W30 & 5W30, where do you see Noacks heading, up or down?

 

[Ducked]

Originally Posted By: zeng
Quote:
http://www.api.org/~/media/Files/Certification/Engine-Oil-Diesel/Publications/.............pdf.

I'm afraid this paper is in the context of HDEO and not PCMO.


Well, it isn't a paper, its a presentation, so there's not enough detail to tell, but it seems to be quoting studies on both.

Quoting the study that they say specifically mentions VM chemistry effects, they refer to a taxi-fleet, while the abstract of that study refers to "a passenger car fleet test". I suppose I was assuming they'd be US taxi's and using petrol (er..gasoline), but can't be sure.

Their account of the Institute Of Materials data just refers to 5-30 engine oils. If it was restricted to HDEO I'd expect them to say so.

 

[Ducked]

Originally Posted By: SonofJoe
I can't say for sure but I'd hazard a guess that what the Institute Of Materials is seeing (ie increases in the viscosity of used oils) is not because of oil oxidation or strange VII interactions but because of engine oil 'distillation'.



Again, its unclear because its just a presentation, but I'm not sure that "increases in the viscosity of used oils" is an accurate or complete description.

As I understand it, they are claiming an effect of shear applied in a bench test, so its a before and after shearing/with or without shearing test applied to both new and used oils.

I'd have to assume this is adequately controlled for distillation effects during the bench test shearing and subsequent viscosity measurements.

There isn't any comparison of new and used oils in the presentation, since the samples aren't identified.

The only specific reference to VM chemistry is where they say:-

"VM chemistry does play a role in the viscosity increase phenomenon, although the authors were not sure why." referring to another study

(Low-Temperature Rheology of Engine Lubricants Subjected to Mechanical Shear: Viscosity Modifier Effects, M. Covitch, J. Weiss, I. Kreutzer;Lubrication Science 11-4, August 1999).

 

[SonofJoe]

I had a more thorough read of the IoM presentation.

It sounds like all the work they did was in the lab on fresh oils so I retract what I said before.

It made me wonder if the behaviour they were seeing was simply the result of better dispersion of the VII making it a little more effective? Typically you add VII to oil as a liquid concentrate and you mix it in with a simple paddle stirrer.The operation is very low tech and low shear. Certain VII polymers (Shellvis 50 comes readily to mind) have a tough time getting properly solublised and many a bit of higher shear blending is just what they need to operate to the max.

 

[Ducked]

Originally Posted By: SonofJoe


It sounds like all the work they did was in the lab on fresh oils so I retract what I said before.



Not quite. They do say:-

"Samples include fresh and degraded oils"

Originally Posted By: SonofJoe


It made me wonder if the behaviour they were seeing was simply the result of better dispersion of the VII making it a little more effective?



Reporting on the results, they say

"The sheared or degraded oil had higher viscosity results than the fresh oil at 100 °C"

That implies they looked at used oil, but its irritatingly ambiguous/incomplete.

It wouldn't be especially surprising/interesting if the viscosity of degraded oil rose.

It would be of interest, and surprising, and would relate directly to your explanation, if used (not necessarily degraded) oil responded to experimental shear treatment with an increase in viscosity, because we could probably assume that the VM's in used oil were as dispersed as they were going to get.

That MAY be what they mean, i.e. that the effect of experimental shear treatment was greater on used oil, but its unclear.

Since the samples aren't identified in the presentation its not possible to compare the effect of experimental shear treatment on used and fresh oils for ourselves.

 

[Ducked]

Originally Posted By: SonofJoe
I had a more thorough read of the IoM presentation.

It sounds like all the work they did was in the lab on fresh oils so I retract what I said before.

It made me wonder if the behaviour they were seeing was simply the result of better dispersion of the VII making it a little more effective? Typically you add VII to oil as a liquid concentrate and you mix it in with a simple paddle stirrer.The operation is very low tech and low shear. Certain VII polymers (Shellvis 50 comes readily to mind) have a tough time getting properly solublised and many a bit of higher shear blending is just what they need to operate to the max.




If that's what's happening, these results just seem to suggest a trend to lower solubility VM's, especially in 5-30 oils.

Do you know if this is likely?

 

[friendly_jacek]

Quote:
Key Conclusions
•
The low shear results on several 5W-30 oils revealed some
interesting behavior. The sheared or degraded oil had higher
viscosity results than the fresh oil at 100 °C. Both the dynamic
viscosity and the kinematic viscosity results showed the same
trend.


while this is surprising, the magnitude seems low (based on eyeballing the charts).

 

[CT8]

Originally Posted By: zeng
Quote:
http://www.api.org/~/media/Files/Certification/Engine-Oil-Diesel/Publications/.............pdf.

I'm afraid this paper is in the context of HDEO and not PCMO.
I would agree.

 

[Ducked]

Originally Posted By: CT8
Originally Posted By: zeng
Quote:
http://www.api.org/~/media/Files/Certification/Engine-Oil-Diesel/Publications/.............pdf.

I'm afraid this paper is in the context of HDEO and not PCMO.
I would agree.


I wouldn't. See above