Traction Coefficient of different oil formulations

[JAG]

Credit for this link goes to unDummy who posted it in another thread. Have a read. It is a big mythbuster in my opinion and shows the complex behavior of motor oils.
http://www.valvoline-technology.com/upload/dynamic/Diesel Engine Cam Galling.pdf

 

[Tempest]

I didn't get a chance to read the entire article, but this link seems pertinent.
http://www.machinerylubrication.com/article_printer_friendly.asp?articleid=661

 

[buster]

Quote:

---

While Group I and Group II base oils are prevailing in
today’s commercial market, Group III and Group IV base
oils are gaining popularity as the basis for high
performance, more expensive, engine oils. These high
performance oils may inadvertently cause cam problems
due to their very low traction coefficients.

---

Interesting links, thanks guys.

 

[JAG]

Yes, Figure 2 in that link gives Traction Coefficients for PAO, POE, Diester, paraffinic mineral oil, and some funky non-motor oil basestocks. In that test at least, lower traction correlated with thinner film thickness.
Traction Coefficient in increasing order were:
PAO < POE < Diester < mineral oil

My note of caution is keeping in mind the lubrication regime being tested. Such as ball bearings vs camshafts. Results in one regime may not correlate well in another regime. I was thrilled to find a camshaft study. I do need to read it more carefully at home.

 

[JAG]

one more note: we've been hearing some old school gearheads say that they prefer mineral oils over synthetics in their old school valvetrains. I used to think that makes little sense but I stayed on the fence about it since who am I to disagree. The Valvoline link seems to give credence to those gearheads that have valvetrains that need oils with enough traction coefficient in order to avoid galling that can lead to massive failure.

 

[Laminar Lou]

Thanks for the links. I read both and I think I understood them. I have a few questions though:
Why did they use a lighter weight for synthetic in the Diesel test? It appears that the HTO1 oil (5W30 synthetic) still has a lower traction coefficient than the HTO oil (15W40 Group I). Forgive me if this is a dumb question, but does the traction coefficient relate to the viscosity?

 

[JAG]

They said the reason for that choice was to try to find out how viscosity and inherent oil traction of the basestock affects failure in the test. I'm not sure they answered the viscosity question (with same basestock) but by comparing LTO and HTO1 that were both 5W-30 (same viscosity) that had very different traction coefficients, they certainly proved that they type of synthetic basestock affects traction. That traction coefficient dependence on the type of basestock is also proven by the comparison of the factor of two difference in traction of Group 2 HDEO and Group 1 HTO that were both 15W-40 with same additive package.

It would be interesting to see same basestock but different viscosities tested. We'd all probably guess higher viscosity gives more traction coefficient.

 

[Terry]

Much less of a problem in real world, especially since 2001 than implied by the title and links in fully formulated motor oils.

From the first valvoline article linked above;

"The high-pressure rheological properties of VHVI
and synthetic base-stocks have to be carefully
optimized in a fully formulated lubricant in order to
minimize wear as demonstrated in the N14 engine
cam wear test."

bruce, molakule, any experienced oil formulator will tell you that proper formulation is the key, not ONE aspect of a chemistry like base oils.

This discussion is about appropriate formulating not base oils.

 

[JAG]

Hi Terry. Comparison of Figures 7 and 8, which are for pure basestocks and fully formulated oils show almost identical trends (order of Group 1-4 basestocks) in traction coefficients. Additives affected the values, but not really the trend or order. The only change in trend was swap between Group 3 and 4 when additives were added.

Basestock type does matter for traction coefficient. It probably goes without saying that additives matter too but they can't change the laws of physics dictated by the basestock choices.

 

[Laminar Lou]

Quote:

---

They said the reason for that choice was to try to find out how viscosity and inherent oil traction of the basestock affects failure in the test. I'm not sure they answered the viscosity question (with same basestock) but by comparing LTO and HTO1 that were both 5W-30 (same viscosity) that had very different traction coefficients, they certainly proved that they type of synthetic basestock affects traction. That traction coefficient dependence on the type of basestock is also proven by the comparison of the factor of two difference in traction of Group 2 HDEO and Group 1 HTO that were both 15W-40 with same additive package.

It would be interesting to see same basestock but different viscosities tested. We'd all probably guess higher viscosity gives more traction coefficient.

---

What is interesting is that Group III and Group IV traction coefficients are reversed when they are formulated with the same add pack as the HDEO. (after I posted I noticed that Jag made the same comment)

I still don't understand that they used higher viscosities for the Group I and II oils if they would impact the traction coefficient. That was why I asked the question, because how can you be sure in the test that the viscosity isn't playing a role in the traction coefficient. Unless it is well known that it does not have any impact (which is counter intuitive to me, but I am still learning here).

 

[JAG]

LL, I found it. Page 4 in Discussion section. Viscosity is indeed one factor in the traction coefficient in a given system. The other factor is the pressure-viscosity coefficient. I guess with that you can predict the traction coefficients of thicker synthetics, assuming same basestock type, given the coefficients of the thinner versions and the relative viscosities of each. Twice as viscous would give twice the coefficient assuming the viscosity dependence is linear. The viscosity that matters here may not be KV at 100 C, however. Maybe it is more related to HTHS which is at certain high shear rate and 150 C. I dunno.

 

[Laminar Lou]

Thanks Jag. I see it now. I am going to have to read this thing a few times to understand it. Neat stuff though.

 

[AEHaas]

This is yet another example of just how hard it is to formulate motor oil. It also supports the notion that there is no single test that will tell how an oil will perform in an engine.

It think it supports my belief that the best formulation is actually a mineral / synthetic oil blend.

aehaas

 

[oilyriser]

The strange grey streaks I saw on my car's camshaft might have been galling. But I've only occasionally used synthetics. Maybe 20w20 would be ideal.

Mobil 1 has PAN. That could be one of those with the high traction coefficient.

 

[mdocod]

this is scary..

1 minute it's oils don't have enough ZDP for one type of valvetrain, next minute they don't have enough traction for another valvetrain...

all the while, I'm wondering.. when you build an oil for one, do you destroy it for the other, and what about the rings that are always "sliding"... would a high traction oil make things better or worse in a situation where the parts are supposed to "slide."

bah, this is too confusing.

 

[JAG]

I'm quite sure that some engine designs (mostly newer) would do great on a very low traction oil and that same oil would be death to some other engines (older styles and some modern diesel engines). Such an oil could be a thin highly friction modified PAO synthetic with near API SM minimum ZDDP levels. Other studies have found that higher ZDDP concentrations can increase traction coefficients. That bodes well for the valvetrains that need high traction oils and lots of ZDDP since extra ZDDP adds more of what they need for both scuffing protection and higher traction coefficient.

I once read what piston rings prefer as far as resistance posed by the oil on the cylinder walls but I honestly don't remember what the answer was.

These tests have shown that PAO and Group 3 don't have as high pressure-viscosity coefficients as mineral oils which causes them to supposedly have thinner oil film thicknesses in the Elastohydrodynamic lubrication regime. So it seems logical that if using synthetics with mostly Group 3 or 4 basestocks and valvetrain wear is a concern, going to a higher viscosity than one would use with mineral oil will make up part or all of the loss in oil film thickness in the elastohydrodynamic regime that is caused by such basestock types. The thicker synthetic will still be suitable for reasonably low temperatures.

 

[jmac]

It was these papers that I was thinking of when a couple of months ago I questioned the oft stated maxim that synthetic base oils have higher film strength than mineral, I was confusing the traction with film strength. Now I am confused though. How can a basestock have higher film strength in EHD but have less film thickness in that realm than a mineral counterpart?
Was the question of what is the actual definition of film stregnth ever resolved?
This is counter-intuative to me.
As a side, while I have been known to mix certain oils, the conclusions fo this paper are why I am very leary of adding anything to certain VHVI products, especially when used in a diesel engine, such as Rotella-T-S. Especially after one user reported getting a big glop of extra viscous stuff on drain after adding VSOT to the RTS, whether that is unfounded or not.
In a high pressure rolorized engine, and maybe this extends to gearsets?, a careful balance of the traction coefficient is one of the most important propeties of the lubricant?
All relating to various ongoning threads on this and other forums such as VW PD engines and 505.01 oils, why do certain gearshops only recommend use of mineral oil, best oil for use in a 7.3, Cummins, LY7 etc. motor....

 

[jmac]

Jag, my read of it was yes, higher HTHS is beneficial for higher trac. coef, and is my thinking why certain engines spec HDEO's along with other properties with higher HTHS.

 

[JAG]

Good questions. I can't really answer them well. I can't recall actual data from "film strength" tests, but if any found that PAO or Group 3 had higher film strength of same viscosity as Group 2, then I'd look to see if the lubrication regime or the oil temperature in those tests differed significantly than from the tests in the two links above. If not, I'd be unable to make sense of the discrepancy. I guess I'd check for repeatability of the data in the different tests. Poor repeatability tests would be tossed out the window. Do you have links to such film strength data?

 

[JAG]

jmac, just saw your second post. I was wondering how much fluid pressure is on the oil in the HTHS test vs the pressure in the EHD lubrication regime. I'm thinking the latter has much higher fluid pressure which will bring out the importance of the pressure-viscosity coefficient. If so, HTHS alone wouldn't fully predict fluid film thickness in the EHD regime. If they are similar pressures, then it could. But on a practical level I still think HTHS viscosity is a reasonably good indicator and fortunately we do have data for the different oils. It has been shown to predict it well in some bearings in engines. I just don't know about how it does for high pressure valvetrains.

Ok, I found this on the HTHS test.
"If desired, variable shear rates (± 30 percent of the shear rate specified in ASTM D 5481) may be obtained by altering the test pressure (variable from 75 to 500 psi)."
http://www.cannoninstrument.com/HTHS.htm

So the pressure is somewhere between 75 and 500 psi in the HTHS test.

For EHD lubrication regime, this site http://www.engineersedge.com/lubrication/elastohydrodynamic_lubrication.htm
indicates pressures can be up to 5000 psi in roller bearings. So I think EHD regime and HTHS test are not comparable in terms of pressures in the oil.