GTL = higher PVC = a lot less wear says Chevron

[turtlevette]

Originally Posted By: Shannow
Originally Posted By: turtlevette
But I thought you guys, the core group here, say oil is oil and minimum oil film thickness is proportional to viscosity and nothing else.


Are you being deliberately disingenuous, or just troll baiting ?



I'd love to see you defending a paper you're presenting at a conference. I guess you'll accuse the questioning attendee of being a troll. That only works when you're sitting behind a computer.

We won't hold our breath on the paper

 

[Shannow]

Read what I was responding to turtle, a deliberate snipe, on a tangent that had nothing to do with the topic of EHD.

So by making the snipe, you either:
* had no idea of the difference between EHD and hydrodynamics;
* were being deliberately disingenuous; or
* were trolling.

You're a professional, so I assumed that you DID know the difference between them, so had to guess at the last two.

As to if you asked the same question in an auditorium...you'd look like a fool.

 

[DeepFriar]

I would appreciate continuing the discussion here about the unsuitability of ester based lubricants due to their lower PVC qualities mentioned by the OP in an earlier thread. I was having difficulty, still am, squaring 20 years of real world experience with those lubricants and the demonstrated weakness of esters in the PVC arena as opposed to the HTHSv metric.

 

[Jake777]

so i take it none of you feel the film strength test done by RAT has anything to do with PVC strength?

 

[Shannow]

Originally Posted By: Jake777
so i take it none of you feel the film strength test done by RAT has anything to do with PVC strength?


Could be a mix of additives/EHD, but the reason that it gets slagged is it's not representative of any operational mode seen in an IC engine...it doesn't tell you anything about what oil is going to make your Honda run 200 or 300,000 miles.

For cam wear, the industry has the (expensive) sequence IV wear test, where the oil is deliberately held at a temperature which is poor for additive activation, and the oil has thinned some...if the Rat test gave the same repeatable data, they'd be all over it like coconut on lamingtons.

As it is, the labs DO use similar testing to detemine which lubes make it to the formal test bench...so he's "prescreening" oils that have already been pre-screened and finally tested.

And a lot of the discussion in the paper is incorrect.

 

[Jake777]

ok so don't pick my oil based on his tests? I like MSHM but was thinking of switching to Maxlife for the great results on his tests.

 

[Gokhan]

Originally Posted By: Jake777
ok I am going to ask the question. But first let me say I don't understand 10% of what you guys are saying but I do hear you saying film strength and it just so happens that I heard that same statement from the RAT guy who does his oil testing and shows what he says are the film strengths of various oil as tested on his rig. NOW many people on here said he was full of it even though I said if indeed film strength was stronger from one oil to another why is that not a good thing and not an indication of a strong point in the oil. Can anyone who has commented so far please help me out with this? Am I hearing that film strength is important and that as such tests like what RAT did showing various film strengths could have some actual usfal info?

for ref... https://540ratblog.wordpress.com/2013/06/20/motor-oil-wear-test-ranking/

I am not going to read that long blog, but basically oil-film strength is directly related to the viscosity under pressure and at high temperature. The temperature part is easy -- it's inferred from the HTHSV viscosity, which is not only measured at a high temperature (150 C) but also viscosity drop due to high shear rate is included.

The difficult part is the PVC (pressure - viscosity coefficient), which is an exponential coefficient that determines the pressure dependence, and we are talking about pressure reaching several thousand times the atmospheric pressure when the engine parts slide against each other.

Unfortunately, PVC is neither well-measured nor well-reported. However, we know that Group IV base stocks as well as some esters have the worst (smallest) PVC. Group III is the second worst. Group II used to be the best and the uncommon naphtene base stocks (that fall under the diverse Group V) are phenomenally good. Now, Chevron is claiming that GTL is the best among common base stocks, even better than Group II.

One can think that the high wear Mobil 1 experienced at some point was due to their use of Group IV in the past, which had very low PVC. One can also want to believe the Chevron patent and hope that the GTL PVC is great and it therefore greatly reduces wear. At the end of the day, they don't report PVC numbers and we are left with indirect information from papers and patents here and there as well as the UOA results.

 

[Garak]

Originally Posted By: Jake777
ok so don't pick my oil based on his tests?

Well, maybe if you plan on being within a millisecond of wiping a bearing or something like that. Even then, I'm not so sure.

 

[Garak]

Originally Posted By: Gokhan
I am not going to read that long blog....

You're not missing much.

 

[SigShooter]

Gee wiz dude you have a 30 year old Corolla and you are wondering about using the latest oil concoction, why?

 

[LubeLuke]

To protect the engine components.

 

[SigShooter]

You can use Supertech for a 30 yr. old car and it will work fine.

 

[OVERKILL]

Originally Posted By: Gokhan
and the uncommon naphtene base stocks (that fall under the diverse Group V) are phenomenally good.


*snip*

Originally Posted By: Gokhan
One can think that the high wear Mobil 1 experienced at some point was due to their use of Group IV in the past, which had very low PVC.


Are you talking about the "Katrina incident" or the gossip on here about UOA's? As we know Mobil confirmed with the API that the oils were "within spec" with regards to the claims made by Ashland (which were believed to be Katrina batches that had dodgy Seq. IVA performance).

Anyways, the reason I grabbed these two sections is that as far back as Tri-syn (that's quite a while ago) the base oil blend was, as the name implied, a combination of PAO, Esters and AN's. So even if PAO was potentially an "issue", which I don't think we can assume it actually is, and even if we assume the esters are an issue, which I also don't think we can assume actually are, the AN's would have had an opposing effect.

Ultimately none of this amounts to a hill of beans compared to the OEM testing protocols which actually measure and analyze wear but it certainly is interesting so thanks for sharing

 

[Shannow]

Originally Posted By: Garak

Well, maybe if you plan on being within a millisecond of wiping a bearing or something like that. Even then, I'm not so sure.


Pretty sure that the test is ferrous/ferrous, so nothing can really be gleaned on mixed metallurgy like white metals and ferrous

 

[Garak]

Originally Posted By: Shannow
Pretty sure that the test is ferrous/ferrous, so nothing can really be gleaned on mixed metallurgy like white metals and ferrous

Well, even better then.

And I'm sure differing AW and EP additive levels make those results a mishmash, too.

 

[Gokhan]

Originally Posted By: Gokhan
Originally Posted By: lubricatosaurus
Originally Posted By: Gokhan
This very interesting Chevron patent claims that GTL base stocks have a higher pressure - viscosity coefficient (PVC), ......

The Chevron patent didn't claim that at all.

"More particularly, the present invention employs base stocks exhibiting low temperature sensitivity of the Pressure- Viscosity-Coefficient (PVC) to obtain improved wear properties." from the patent indicates they only have observed a lack of sensitivity to temperature in their GTL basestocks, not that it has a higher PVC.

That's because PVC decreases with temperature. (See, for example, this reference.) They are trying to find base stocks with higher PVC at higher temperatures so that they will have thicker EHL oil films at higher temperatures that various parts of the engine experience during normal or severe operation. By keeping the ratio of PVC100/PVC40 close to 1, they are ensuring that they have a small variation of PVC with temperature and have a high PVC at high temperatures.

It's like a "PVC index" analogous to the viscosity index. The latter ensures that the viscosity stays high and the oil-film thickness stays large when the temperature increases and the former ensures that the PVC stays high and the oil-film strength stays high when the temperature increases.

The following excerpt from the patent explains this more:

"[0008] The present lubricant composition provides greatly improved valve train wear control due to the selection of base stocks with specific temperature sensitivity characteristics. The base stocks of choice exhibit less temperature sensitivity of the Pressure-Viscosity- Coefficient (PVC) than conventionally used base stocks, thus minimizing the reduction of contact film thickness with increasing temperature, relative to 'typical' base oils. Such base stocks have been found to exhibit higher PVC at the temperature levels experienced in the valve train's wear contacts of an internal combustion engine. As a result of this higher PVC, these base stocks behave more solid-like under extremely high contact pressures, greater than 0.5 GPa. By measuring the PVC of a base oil at 40°C and 100°C, the temperature sensitivity of the PVC can be determined by the ratio of 40°C PVC/100°C PVC. As this ratio approaches 1.00, the base stock PVC is less sensitive to the temperature. Base stocks showing this behavior have been found to provide improved wear protection. Such base stocks surprisingly cover a broad range of viscosity indices (VI), indicating that the characteristic of interest is not necessarily correlated with VI or any of its associated parameters such as the VII content or the lubricant's VII dosage related shear behavior."

(Note: Authors refer to PVC100 divided by PVC40 as the PVC40/PVC100 ratio; so, it's sloppy terminology on their side, as it should be referred to as PVC100/PVC40.)

What's interesting here is that this PVC effect is not being indirectly caused by the VI effect (sensitivity of atmospheric-pressure viscosity to temperature) but it's a separate effect where the "PVC index" (sensitivity of PVC to temperature) is playing a role. Some GTL base stocks tend to have very high VIs, but this patent claims that they also tend to have high "PVC indexes," resulting in high viscosity under extreme pressure and at high temperature, which results in a thicker EHL film (higher oil-film strength) at high temperature.

 

[Garak]

The real question is will this gain them any advantage in current oil certification regimes, even if they are firmed up? Or, would such benefits be only realized if there is different testing thanks to lower HTHS lubricants coming down the pipe?

 

[Jetronic]

Originally Posted By: Gokhan
Originally Posted By: Jetronic
from the data I've seen, doubling the KV also doubles the oil film thickness under EHD regime, when using newtonian fluids.

The point is that in the EHD regime, pressures are much higher, and the KV that applies is not the atmospheric KV but the high-pressure KV, which is determined exponentially by the former and PVC. This is also a low-shear quantity and has nothing to do with non-Newtonian effects (VIIs, HTHSV, etc.), and it's a Newtonian base stock effect. It applies to all (Newtonian) base stocks.


My point is that with the tested viscosities the EHD film also doubled in thickness for a given pressure. These where all newtonian fluids though. We're talking sub-micron thickness though, sometimes only a few nm..

 

[Jetronic]

Originally Posted By: Gokhan
Originally Posted By: Jetronic
from the data I've seen, doubling the KV also doubles the oil film thickness under EHD regime, when using newtonian fluids.

The point is that in the EHD regime, pressures are much higher, and the KV that applies is not the atmospheric KV but the high-pressure KV, which is determined exponentially by the former and PVC. This is also a low-shear quantity and has nothing to do with non-Newtonian effects (VIIs, HTHSV, etc.), and it's a Newtonian base stock effect. It applies to all (Newtonian) base stocks.


My point is that with the tested viscosities the EHD film also doubled in thickness for a given pressure. These where all newtonian fluids. We're talking sub-micron thickness though, sometimes only a few nm..

but yes, temperature increase means thinner film under EHD.

Did the patent state what film thicknesses were measured, as groupIII oils are amongst the worst for PVC. I mean: what does it matter if the oil film thickness stays at 17nm for a large temperature range if the surface roughness is a lot more than that in a machine...


edit time out..

 

[Gokhan]

Originally Posted By: Jetronic
My point is that with the tested viscosities the EHD film also doubled in thickness for a given pressure. These where all newtonian fluids. We're talking sub-micron thickness though, sometimes only a few nm..

I'm not sure what you mean. In order to double the atmospheric viscosity, you need to go from xW-20 to xW-60. It's not practical. PVC effect is a lot larger.

Quote:
Did the patent state what film thicknesses were measured, as groupIII oils are amongst the worst for PVC. I mean: what does it matter if the oil film thickness stays at 17nm for a large temperature range if the surface roughness is a lot more than that in a machine...

GTL is not really Group III, despite currently being classified there for lack of a new API group. According to the patent, GTL even beat Group II in PVC, which has the highest PVC among common non-GTL mineral and synthetic base stocks.