Philosophy of Modern Oil Formulation: The Environmental and Economic Importance of Mixed and Boundary Friction

[naav]

I found a another really interesting open access paper from 28 April 2024 you guys may enjoy: https://www.mdpi.com/2075-4442/12/5/152

This article is probably as close as it gets to the philosophy of modern oil formulation.

The article discusses the challenges of reducing fuel consumption by going to thinner oils especially 0w-8 which is studied in detail and how and where oils need to change to meet wear requirements and reduce friction losses.

 

[naav]

Mixed lubrication, where some of the contact load is carried by the liquid lubricant and some of the load is carried by the surface asperities, is generally assumed to take place provided 1 < λ < 3, and boundary lubrication (where all the contact load is carried by the asperities) is assumed to occur if λ < 1

What I don't understand is if modern cars operate in mixed/boundary lubrication at lower RPM (due to aggressive gearing) why should it matter much for fuel efficiency to use a low viscosity oil since we aren't reaching into the higher rpm regions (where the engine operates mostly in the hydrodynamic regime)?

 

[naav]

Found the smoking gun for the solution to mixed/boundary lubrication buried deep in another paper quoted below: Experimental and Numerical Study of the Mixed Lubrication Considering Boundary Film Strength (Jan 2023) https://www.mdpi.com/1996-1944/16/3/1035

As the viscosity increases, the percentage load carried by asperities decreases from 26.5% to about 9% and the area of asperity contacts decreases from 9.6% to about 3.1%, as shown in Figure 15b. The variation of the friction force and its components with viscosity can be seen in Figure 15c.

When the viscosity is below 0.07, the friction force is 4500 N, mainly provided by the boundary film and dry friction. The dry friction component disappears as the viscosity increases, and the total friction decreases continuously.

Interesting that past a certain viscosity dry friction completely disappears and it does so suddenly.

n (Pa.s) = viscosity (horizontal axis)
Figure 15: https://www.mdpi.com/materials/mate...eploy/html/images/materials-16-01035-g015.png

From the above analysis, low speed is not conducive to the formation of oil film and cannot ensure a good lubrication state. An appropriate increase in speed can avoid the breakdown of the boundary film and the direct contact of asperities, reducing friction to improve mechanical efficiency and lubrication performance.

Therefore, under the current working conditions, improving viscosity can significantly
improve the lubrication performance. It should be noted that, in practice, high viscosities
can cause increased friction within the lubricant, leading to increased energy consumption
and the oxidative deterioration of the lubricant

In conclusion: why are we told to use thinner motor oils if we aren't operating our cars between 3000 and 6000 rpm all the time where mixed and boundary lubrication is hardly an issue? Longevity cannot be a factor. IMO, it must be efficiency at the expense of a calculated acceptable damage rate if we put extenuating circumstances like very cold weather cold starts aside.

 

[twX]

What I don't understand is if modern cars operate in mixed/boundary lubrication at lower RPM (due to aggressive gearing) why should it matter much for fuel efficiency to use a low viscosity oil since we aren't reaching into the higher rpm regions (where the engine operates mostly in the hydrodynamic regime)?

I just skimmed the studies, but they seem a bit heavy on theory and modelling, and light on engine testing. They don't really put into context just how thin oil needs to be for this increase in friction to occur in an engine or how significant it is.

Here's a 2007 study by Toyota that measured engine friction with different oil grades, from 0W-8 to 0W-20, in a 1.5L engine.

If we look specifically at friction in the valvetrain (Figure 5), thinner oil grades do result in higher friction at low rpm.

However, valvetrain friction is only a small portion of total engine friction. If we look at total engine friction (Figure 2), there's little difference in friction between grades at low rpm, but thinner grades have less friction at 1,000 rpm and above.

With the oil at a lower temperature of 45°C, the thinner grades reduce valvetrain friction at all engine speeds, and to an even greater extent than with warmer oil.

Another factor that needs to be considered is friction modifier, which the study you linked doesn't seem to address. All of the figures above were from tests with oil that had no friction modifier. Friction modifier is especially effective in the boundary regime.

Figure 7 below shows the effect of MoDTC on reducing valvetrain friction. It has a large effect at low rpm. Had the tests from the figures above been done with oil containing friction modifier, the thinnest grades may have resulted in lower total engine friction even at very low rpm. In fact, that is shown in a similar study done by Honda, SAE 2011-01-1247.

That Honda study also showed no increased wear when using 0W-8 at 130°C in a Honda Fit engine that had a recommended grade of 5W-20 (though they only measured bearing wear and the wear metal content of the oil). Several other studies show no increase in engine wear unless the oil is multiple grades thinner than the recommended grade, and even then only at very high oil temperatures. Automakers are not necessarily sacrificing engine wear at all by recommending a thinner grade of oil.

 

[naav]

Your charts prove that low viscosity motor oils reduce total engine friction but this does not address wear at all because friction and wear are sometimes not very correlated in the context of internal combustion engines.

More support for higher viscosity directly from your linked Toyota study:

One possible factor contributing to the increases infuel consumption is the expansion of boundary lubrication areas due to decreases in viscosity specifically after engine warm-up.

It is already proven that mixed and boundary lubrication regime applies at low speed higher load such as those frequently encountered when cruising at low RPM (due to transmission gearing optimized for fuel efficiency).

For preventing wear in mixed and boundary conditions, where metal-to-metal contact is more likely, the viscosity of the motor oil is crucial. In these conditions, a higher viscosity oil is generally more suitable because it forms a thicker lubricating film that can better protect surfaces from wear.

Therefore, between two otherwise identical motor oils with a 0w rating but different viscosities, the one with the higher viscosity would be more suitable for preventing wear in mixed and boundary conditions. This is because the thicker oil film can more effectively keep surfaces separated and reduce direct metal-to-metal contact, thereby minimizing wear.

In a sense these low viscosity motor oils are better suited for saving fuel with good wear prevention at higher speed freeway operation because at higher RPM hydrodynamic lubrication regime applies.

Over the past 20 years, the clearances for Toyota's journal bearings have remained relatively consistent, reflecting standard practices in engine design and manufacturing. The typical range for main bearing clearances in modern Toyota engines is approximately 0.0012 to 0.0025 inches. For example, the 2AZ-FE engine, widely used in various Toyota models, specifies main bearing oil clearances ranging from 0.0007 to 0.0016 inches.

In specific rebuild contexts, such as the 1ZZ-FE engine, rod bearing clearances have been measured around 0.002 inches, which aligns with the general guidelines of maintaining clearances within a narrow and precise range to ensure optimal performance and longevity.

Overall, while there have been some minor adjustments and refinements in bearing materials and manufacturing precision, the fundamental specifications for bearing clearances in Toyota engines have not seen significant changes over the last two decades

If this is all true, then the main consideration for using low viscosity motor oil in passenger car engines which operate at low speed and higher load must be primarily fuel efficiency and not longevity.

Somewhat surprisingly it seems to me that these low viscosity motor oils are superior for racing applications where high speed engine operation means the more of the engine is operating in the hydrodynamic lubrication regime.

 

[twX]

Your charts prove that low viscosity motor oils reduce total engine friction but this does not address wear at all because friction and wear are sometimes not very correlated in the context of internal combustion engines.

I agree, and this is addressed in other studies, like the Honda study I mentioned. You can also check out these studies:

Toyota R&D - Low Friction Gasoline Engine Oil - Effects of Lower Viscosity and Friction Modifiers
SAE 892154

These studies show that oil viscosity needs to get very low before wear starts to increase.

More support for higher viscosity directly from your linked Toyota study: "One possible factor contributing to the increases infuel consumption is the expansion of boundary lubrication areas due to decreases in viscosity specifically after engine warm-up."

That quote is in relation to the only test in the study that showed increased fuel consumption with thinner oil; 0W-8 oil without friction modifier in a turbodiesel engine. The same test with MoDTC added to the oil showed a reduction in fuel consumption.

This is an engine that specs a Euro 5W-30 with an HTHS of >3.5 cP, so a 0W-8 with an HTHS of 1.7 cP is a huge reduction in viscosity, yet friction and fuel economy were still improved so long as the oil was fully formulated. Of course, wear would probably increase, which is why Toyota doesn't recommend 0W-8 in these engines.

 

[naav]

I see your point.

After reading SAE 892154 I think you're still missing the bigger picture. I say this because you're still referring to HTHS viscosity which that publication makes abundantly clear is not a good metric. A very clear key finding in your paper is that minimum oil film thickness (MOFT) is the most important. And interestingly, excessive viscosity damaged their test engines from local overheating and insufficient flow (their speculation with regards to the 30 vs 50 weights they tested).

Per your paper, a critical value of minimum oil film thickness (MOFT) exists, below which catastrophic wear occurs in journal bearings. For the studied engine, this critical minimum MOFT is approximately 0.8 mm at specific operating conditions (2000 rpm, 45 N.m, 100°C). Above this critical minimum, increasing oil viscosity can paradoxically increase wear due to potential reduction in oil flow, leading to oil starvation and overheating.

Direct measurement of MOFT in an operating engine provides a more precise and practical assessment of lubricant performance compared to traditional viscosity measurements.

MOFT measurement correlates better with bearing wear than either kinematic or HTHS viscosity, which can be misleading due to differences in shear rates and pressure conditions in actual engine bearings versus those in viscosity tests

Different VI improvers affect the MOFT/HTHS viscosity correlation, implying that setting a minimum HTHS viscosity does not guarantee consistent bearing performance.

Oils with lower than average HTHS viscosity can still achieve satisfactory MOFT values due to the specific properties imparted by different VI improvers.

Routine assessment of lubricants should prioritize direct MOFT measurements during engine operation over bench viscosity tests. This approach would enable more accurate predictions of journal bearing performance, supporting the optimization of lubricants for both fuel efficiency and durability.

 

[naav]

I don't have evidence of it yet but it seems that these sub-20 weight oils are able to avoid catastrophic engine damage because they are able to maintain adequate MOFT.

In light of everything just posted above, the only way I see 0W-16 and 0W-8 oils achieve adequate MOFT is through the use of advanced synthetic base oils like PAO and Esters, coupled with highly effective additive packages.

These formulations must be designed to ensure that, despite their low viscosity, they provide the necessary protection and lubrication to prevent wear and maintain engine performance. This could be achieved through the superior film-forming properties of synthetic base oils, the use of shear-stable viscosity index improvers, and the inclusion of advanced anti-wear and friction-modifying additives.

PAOs have a high viscosity index (VI), which means their viscosity changes less with temperature. This property helps maintain oil film thickness across a wide temperature range.

Esters have polar molecular structures that adhere strongly to metal surfaces, forming a robust oil film that enhances MOFT.

MoDTC creates a low-friction surface layer that helps maintain MOFT by reducing the shear stress on the oil film.

Additives like boron nitride nanoparticles fill in microscopic asperities on metal surfaces, creating a smoother surface and enhancing load distribution, which contributes to maintaining a consistent MOFT.

 

[naav]

I still want to see more evidence that PAO and Esters are able to help maintain MOFT in these super light oils

 

[ZeeOSix]

I don't have evidence of it yet but it seems that these sub-20 weight oils are able to avoid catastrophic engine damage because they are able to maintain adequate MOFT.

Those sub-20 oils typically use a more robust AF/AW additive package because the MOFT is reduced due to the lower viscosity. When the film thickness isn't fully adequate, then the AF/AW package needs to help mitigate friction and wear. Also, engines specifically specifying those sub-20 grade oils have design aspects that use the lower viscosity take into account, like wider journal bearings, different materials and coatings to mitigate friction and wear.

 

[naav]

I doubt that the additive package is that much different from what had been used in the recent past. I suspect the PAOs and Esters are doing the heavy lifting. No evidence either way. Someone posted an article from Nissan in the recent articles section that hinted at this too.

 

[ZeeOSix]

I doubt that the additive package is that much different from what had been used in the recent past. I suspect the PAOs and Esters are doing the heavy lifting. No evidence either way. Someone posted an article from Nissan in the recent articles section that hinted at this too.

There are less VIIs in those sub-20 grade oils too, which helps reduce the temporary shearing which helps keep the film thickness from decreasing, comparing no-shear to full shearing, percentage wise. But since the viscosity while not under shear is lower to start with, there is less headroom in the change in film thickness under shear.

Keep in mind that parts that are mainly in the boundary lubrication realm, like cams and followers, rely more on the AF/AW additives and the composition and hardness of those materials than they do the film thickness of the oil between them. Those parts without proper material and heat treatment won't last long regardless of what oil is used. A higher HTHS viscosity oil will help some, but the wear mitigation on those parts is heavily relied on the AF/AW tribofim.

 

[naav]

There are less VIIs in those sub-20 grade oils too, which helps reduce the temporary shearing which helps keep the film thickness from decreasing, comparing no-shear to full shearing, percentage wise. But since the viscosity while not under shear is lower to start with, there is less headroom in the change in film thickness under shear.

Keep in mind that parts that are mainly in the boundary lubrication realm, like cams and followers, rely more on the AF/AW additives and the composition and hardness of those materials than they do the film thickness of the oil between them. Those parts without proper material and heat treatment won't last long regardless of what oil is used. A higher HTHS viscosity oil will help some, but the wear mitigation on those parts is heavily relied on the AF/AW tribofim.

Why even bother with 0w16 and 0w8 then? We can go straight to 0w3 if the AW additives are already perfected for boundary lubrication.

Also you wrote about lesser headroom in low viscosiry motor oil, but it should be irrelevant if the base oil is able to resist dipping lower than the engine's MOFT

 

[ZeeOSix]

Why even bother with 0w16 and 0w8 then? We can go straight to 0w3.

Give it some time, lol. They probably could if the journal bearings were wide enough to still maintain adequate hydrodynamic lubrication. But engine wear studies still show that the lower the HTHS viscosity (all other factors held constant), the more wear there is over the long run. Oil viscosity will always be the main factor that keeps moving parts separated to minimize wear.

 

[RDY4WAR]

There are less VIIs in those sub-20 grade oils too, which helps reduce the temporary shearing which helps keep the film thickness from decreasing, comparing no-shear to full shearing, percentage wise. But since the viscosity while not under shear is lower to start with, there is less headroom in the change in film thickness under shear.

Keep in mind that parts that are mainly in the boundary lubrication realm, like cams and followers, rely more on the AF/AW additives and the composition and hardness of those materials than they do the film thickness of the oil between them. Those parts without proper material and heat treatment won't last long regardless of what oil is used. A higher HTHS viscosity oil will help some, but the wear mitigation on those parts is heavily relied on the AF/AW tribofim.

Correct. In testing flat tappet camshafts, it makes no difference if the oil is a 5W-20 or 20W-50. The cam and lifter wear tracks with ZDDP content regardless of viscosity and also regardless of temperature. However, friction is a different story. There's a lot of variables when it comes to friction. With some formulas, the CoF keeps improving the hotter the oil gets. HPL's Euro/Supercar oils don't see peak CoF until 305°F.

 

[naav]

Give it some time, lol. They probably could if the journal bearings were wide enough to still maintain adequate hydrodynamic lubrication. But engine wear studies still show that the lower the HTHS viscosity (all other factors held constant), the more wear there is over the long run. Oil viscosity will always be the main factor that keeps moving parts separated to minimize wear.

That's not true. Read what I just posted earlier about HTHS. In essence, HTHS is something relatively easy to measure but no longer very informative as a metric because it has huge problems. I posted a ton about it above summarizing a paper recently posted by twX

 

[ZeeOSix]

That's not true. Read what I just posted earlier about HTHS.

I said with all other factors held constant. If oil A has more HTHS viscosity than oil B, then in the same journal bearing at the same rotational speed at 150C, the MOFT will be greater with oil A. When you start shearing more than 1M/sec at 150C, then it's possible that the film thickness would not follow the HTHS viscosity.

 

[ZeeOSix]

However, friction is a different story. With some formulas, the CoF keeps improving the hotter the oil gets.

Maybe due to the AF/AW additives used, since some take a lot of heat and pressure to activate fully.

 

[naav]

Maybe due to the AF/AW additives used, since some take a lot of heat and pressure to activate fully.

It's because ZDDP film changes form at higher temperatures

 

[ZeeOSix]

It's because ZDDP film changes form at higher temperatures

Yep, I know. He was talking about HPL oil: "HPL's Euro/Supercar oils don't see peak CoF until 305°F."

Not sure if they use a lot of ZDDP in their Euro/Supercar oil formulation, or some other additives.