Can a high HTHS ever be detrimental to engine protection?

[camrydriver111]

- Increase engine load, which may increase wear of rings, liners, bearings during engine warm up

According to some BITOGers, thick oil will warm up faster, so thicker is beneficial during warm up.

 

[Fa24wrx]

A higher-HTHS oil will only result in less wear when the oil is very hot.

When the oil is not very hot, thicker grades of oil will:
- Increase fuel consumption
- Reduce oil flow and delay oil pressure rise on cold starts
- Increase maximum cold oil pressure
- Increase likelihood of oil filter bypassing
- Increase load on battery and starter
- Increase engine load, which may increase wear of rings, liners, bearings during engine warm up
- Higher air and fuel flow may result in more dust contamination, filter loading, and injector wear

Thinner grades provide very small, but guaranteed benefits most of the time. The tradeoff is a higher risk of excessive wear in extreme conditions.

Here's a figure from a study showing bearing wear vs HTHS, with the engine at WOT with 150°C oil sump temperature. As viscosity is reduced, wear decreases, until it starts to increase exponentially. A 0W-20 would have little safety margin in this engine at 150°C (2.6 cP). With oil temperatures of 100°C, there would be plenty of safety margin since the viscosity would be >5 cP.

View attachment 210796

So this 0w20 in this motor showed to be fine until 150c? I get oil temps of up to 230f in the summers and use 0w20 but probably switching to 5w30 Valvoline ep with an hths of 3.2

 

[ChrisD46]

HTHS essentially is the rating of a lube's ability to retain vis in extreme heat.
How in the world would a lube with high HTHS value be "detrimental to engine protection"?
As opposed to what as an alternative; a lube with low HTHS which degrades quickly?

I also disagree with your poorly defined statement that "a higher HTHS gives better wear protection". That's a very broad paint brush with which to paint. To be more specific, lubes with a high HTHS value likely will protect LONGER (extended OCIs), but in shorter OCIs they are not shown to do much different at all. Much of this would depend upon the specific application and use factors.

Wear protection I believe is different than viscosity HTHS . How oils compare in a 4 ball or other friction test may be better ?

 

[dumkid]

A higher-HTHS oil will only result in less wear when the oil is very hot.

When the oil is not very hot, thicker grades of oil will:
- Increase fuel consumption
- Reduce oil flow and delay oil pressure rise on cold starts
- Increase maximum cold oil pressure
- Increase likelihood of oil filter bypassing
- Increase load on battery and starter
- Increase engine load, which may increase wear of rings, liners, bearings during engine warm up
- Higher air and fuel flow may result in more dust contamination, filter loading, and injector wear

Thinner grades provide very small, but guaranteed benefits most of the time. The tradeoff is a higher risk of excessive wear in extreme conditions.

Here's a figure from a study showing bearing wear vs HTHS, with the engine at WOT with 150°C oil sump temperature. As viscosity is reduced, wear decreases, until it starts to increase exponentially. A 0W-20 would have little safety margin in this engine at 150°C (2.6 cP). With oil temperatures of 100°C, there would be plenty of safety margin since the viscosity would be >5 cP.

View attachment 210796

Isn't this why they make multi viscosity oils like 0w40. Good protection and good cold flow

 

[I<3oil]

Isn't this why they make multi viscosity oils like 0w40. Good protection and good cold flow

Correct

 

[twX]

Isn't this why they make multi viscosity oils like 0w40. Good protection and good cold flow

Yes, but while a 0W-40 may have similar viscosity to a 0W-20 at the CCS test temperature of -35°C, it will be thicker at every temperature warmer than this, so basically all the time for most engines. For instance, its KV40 will be almost double that of a 0W-20.

 

[ZeeOSix]

I also disagree with your poorly defined statement that "a higher HTHS gives better wear protection". That's a very broad paint brush with which to paint. To be more specific, lubes with a high HTHS value likely will protect LONGER (extended OCIs), but in shorter OCIs they are not shown to do much different at all. Much of this would depend upon the specific application and use factors.

Engine wear protection is from two things: 1) Viscosity (more HTHS means more MOFT), and 2) The AF/AW additives (tribofilm, aka film strength).

HTHS viscosity is the main protection factor, and the tribofilm/film strength is the secondary protection factor when the MOFT goes near or to zero. Higher HTHS will always give more MOFT and therefore more wear protection when all other factors are held constant, like the AF/AW package being the same between viscosities.

The protection from viscosity could decrease (depending on exact formulation) with how long the oil has been ran because as the VIIs shear down badly with miles, that will also reduce the HTHS viscosity. The amount and type of VIIs can have an effect on the HTHS viscosity with miles. Run a no-VII oil like what HPL makes, or a straight weight if you want some HTHS stability with use mileage.

 

[ZeeOSix]

If you're having to increase viscosity to obtain that HTHS then you have added heat.

Not enough heat to really be concerned about. When a thinner oil is used, the MOFT is smaller which actually increases the shear rate of the oil film inside journal bearings. That higher shear rate also increases the temperature, so over-all there really isn't much of a difference going on with heat rise in the bearings.

 

[ZeeOSix]

There's no reasonable way to conceive that a quality HTHS lube is going to harm an engine in terms of wear in the inferred application.

What is bad for engines is using a "W" rating that is wrong for the cold temperature start-ups, and a HTHS that is too low for the application. Using more HTHS viscosity that is "needed" never hurts anything mechanically.

 

[ZeeOSix]

Good post, I've been wondering the same thing, I found this link on another forum:

Low HTHS oils | News - GNV Oil Gruop

ООО «ДЖИ ЭН ВИ ОЙЛ ГРУПП», является официальным дистрибьютором корпорации «Кувейт Петролиум Интернейшнл Лубрикантс». И имеет эксклюзивное право на продажу продукции Q8 Oils на территории России, и в ряде стран СНГ.

q8oils-russia.com

It shows wear rates decreasing up 3.5 and then cut off at 3.5 It would be good to see chart after the right.

The wear curves are probably just about flat-lined above 3.5 cP HTHS. That flat line wear is most likely due mainly to the components in boundary layer lubrication. The wear protection in those components is relying a lot on the film strength of the oil.

 

[ZeeOSix]

It can if the hydrodynamic friction from the higher dynamic viscosity is great enough to cause excessive bearing heat leading to fatigue of the soft metals. It takes a lot to make this happen though and unlikely with a daily commuter. It would be more likely in something like NASCAR with high rpm and tight bearing clearance. They see a ~75°F temp rise across the bearings with 0W-16 oil with oil temp at bearing exit around Daytona and Talladega at 350-360°F.

At a constant RPM, the smaller the MOFT due to less viscosity, the higher the shear rate and the higher the temperature rise. And of course the RPM ads to the shear rate. A MOFT that is half as thick will have twice the shear rate. Tight bearings will get smoked easier than looser (to a point) bearings.

 

[ZeeOSix]

Here's a figure from a study showing bearing wear vs HTHS, with the engine at WOT with 150°C oil sump temperature. As viscosity is reduced, wear decreases, until it starts to increase exponentially. A 0W-20 would have little safety margin in this engine at 150°C (2.6 cP). With oil temperatures of 100°C, there would be plenty of safety margin since the viscosity would be >5 cP.

What oils were used in that test to achieve the different test HTHS viscosities ... and were they multi-viscosity oils and all have the same AF/AW formulation? What are the symbols on the curve representing?

Happen to have a link to the study? I'd like to see the details of that testing.

With the sump at 150C, the oil inside the journal bearings will be even higher than that under those conditions.

 

[twX]

What oils were used in that test to achieve the different test HTHS viscosities ... and were they multi-viscosity oils and all have the same AF/AW formulation? What are the symbols on the curve representing?

Happen to have a link to the study? I'd like to see the details of that testing.

With the sump at 150C, the oil inside the journal bearings will be even higher than that under those conditions.

The dark circles are monogrades. The lighter shapes are multigrades that use different types of VII. The same additive package was used for all oils except for one of the multigrades.

Effect of Oil Rheology on Journal Bearing Performance: Part 4 - Bearing Durability and Oil Film Thickness 892154

 

[Brian123]

A higher-HTHS oil will only result in less wear when the oil is very hot.

When you say the oil is very hot, what temperature are you talking about? Something like a Temperature of 210 or 220 Degrees Fahrenheit. I think different areas inside an engine will have different oil temperatures.

 

[twX]

When you say the oil is very hot, what temperature are you talking about? Something like a Temperature of 210 or 220 Degrees Fahrenheit.

It will be different for every engine, but generally much hotter than 220°F. For the engine in the study I posted, bearing wear only started increasing with a sump temperature of 150°C (300°F), with oil thinner than an xW-20. In another study, piston ring wear doubled with oil grades thinner than xW-20 at a sump temperature of 130°C, but cam and bearing wear did not increase at all at this temperature.

There are some scenarios not taken into account in these studies. Problems with an engine's oiling system, like a clogged oil filter or oil passages, or excessive oil aeration, will reduce oil pressure and might lead to catastrophic bearing wear even if oil temperatures aren't so extreme.

A thicker grade of oil just might be enough to save the bearings or at least prolong the inevitable in those scenarios. So, it's important to have some safety margin, but with oil temperatures <110°C there should generally be a healthy safety margin with the recommended grade.

I think different areas inside an engine will have different oil temperatures.

The oil at certain engine components will be hotter than the sump temperature, but this is taken into account in these studies. Sump temperature is just used as a reference because it's easier to measure.

 

[ZeeOSix]

The dark circles are monogrades. The lighter shapes are multigrades that use different types of VII. The same additive package was used for all oils except for one of the multigrades.

Effect of Oil Rheology on Journal Bearing Performance: Part 4 - Bearing Durability and Oil Film Thickness 892154

That's an interesting study, and a lot to digest - thanks for the link. Pretty wild test setup for it being done back in 1989. It points out that VIIs have an effect during high shear rates, and the ties into the many past discussions here about how the dynamic viscosity when sheared beyond the standard HTHS measurement rate of 10^-6/sec that the dynamic viscosity becomes as low a possible - termed HTFS (High Temp/Full Shear).

Lots of the data in that study does show that higher HTHS viscosity does result in more MOFT, such as in Tables 2 & 3, and Figure 8. Figure 8 basically shows a general overall linear relationship that higher HTHS viscosity gives more MOFT. Figure 6 doesn't seem to really explain why the higher MOFT would result in slightly more bearing wear, but I think there is something else going on there that can't be seen. They mention bearing cavitation earlier in the study (they assumed it wasn't happening), and it could be possible that the slight wear difference with the higher HTHS oils was due to bearing erosion wear due to any cavitation that could be going on. They also mention possible "reduction in oil flow" with the higher HTHS oils, but I highly doubt that due to the PD oil pump and the viscosity the oils were under test. Journal bearings will "self pump" oil volume based on all kinds of factors, and there will also be some added flow on top of that due to the pressurized oil supply from the pump. They also point out that the Figures 6a and 6c both showing a similar behavior doesn't seem to make sense ("fortuitous" ... happened by chance/accident as shown below). Figure 6 info does seems a bit nebulous for why the testing showed that relationship - no solid explanation by the authors, just theories.

The bottom line in this study, and pretty much every other one looking at engine wear with respect to HTHS and MOFT is don't let the oil film thickness between moving parts get too close to the "danger zone" with respect to the engine's use conditions. Increased HTHS and MOFT headroom is a simple way to ensure that.

 

[ZeeOSix]

It will be different for every engine, but generally much hotter than 220°F. For the engine in the study I posted, bearing wear only started increasing with a sump temperature of 150°C (300°F), with oil thinner than an xW-20. In another study, piston ring wear doubled with oil grades thinner than xW-20 at a sump temperature of 130°C, but cam and bearing wear did not increase at all at this temperature.

In most of these studies, the rings seem to one engine component that takes a beating, and the wear rate of rings vs HTHS seems to be a pretty strong correlation - probably even more so if you looked at ring wear vs the real time HTFS viscosity going on in the ring pack. Oil temps in the ring pack can be brutal. It would be interesting to see if piston oil squirters on some of the high performance engines helps reduce ring wear by keeping the ring pack a bit cooler. Components in full hydrodynamic lubrication like journal bearings are not as sensitive to the viscosity until the MOFT goes near or to zero and surfaces start really rubbing together, then only the film strength of the oil due to the AF/AW tribofilm and the sacrificial top layer of the bearing kicks-in to mitigate wear and damage.

 

[twX]

It points out that VIIs have an effect during high shear rates, and the ties into the many past discussions here about how the dynamic viscosity when sheared beyond the standard HTHS measurement rate of 10^-6/sec that the dynamic viscosity becomes as low a possible - termed HTFS (High Temp/Full Shear).

They mention that maximum shear rates in bearings an order of magnitude higher than the HTHS measurement, or around 10^7/sec, but I've seen other studies estimate maximum shear rates in bearings at only 2 x 10^6, a lot closer to HTHS. I've also seen data showing a very strong correlation between HTHS and oil pressure, with both high- and low-VI oils, so I think HTHS is still a good approximation for bearings in most engines. Shear rates at the piston rings and cams are higher.

They also mention possible "reduction in oil flow" with the higher HTHS oils, but I highly doubt that due to the PD oil pump and the viscosity the oils were under test.

Some engines will have oil pressure regulated by a pressure relief valve even when the oil is hot. This seems to be fairly common with American designs, and could explain the comment about lower oil flow.

The other study I mentioned is the Japanese Toyota study, and while the data is noisier, there's no obvious reduction in bearing wear with thinner oils. The oil pump on this engine would not be in pressure relief, so flow would only be reduced very slightly with the thicker oils.

 

[twX]

In most of these studies, the rings seem to one engine component that takes a beating, and the wear rate of rings vs HTHS seems to be a pretty strong correlation - probably even more so if you looked at ring wear vs the real time HTFS viscosity going on in the ring pack. Oil temps in the ring pack can be brutal. It would be interesting to see if piston oil squirters on some of the high performance engines helps reduce ring wear by keeping the ring pack a bit cooler.

Yes, I think the rings are the most sensitive to thin oil because of the high temperature of the oil film at the top ring. I've read that the temperature of this oil film tends to rise to a temperature somewhere between that of the piston ring and that of the liner, as heat transfers from the piston to the liner through the oil film. Keeping the pistons cooler with oil squirters should help maintain higher MOFT.

I don't see MOFT at the piston rings as a big concern, since the wear only increases modestly and only near WOT, and this wear will be dwarfed by cold-start piston ring wear in most applications. Bearings are the bigger concern due to how quickly they can fail when oil pressure gets critically low.

 

[ZeeOSix]

They mention that maximum shear rates in bearings an order of magnitude higher than the HTHS measurement, or around 10^7/sec, but I've seen other studies estimate maximum shear rates in bearings at only 2 x 10^6, a lot closer to HTHS.

The shear rate inside the journal bearing is due to the relative speed between the bearing and journal surfaces, and the oil film thickness, so as the MOFT goes lower the shear rate goes up, which also heats up the oil more from shearing. With all other factors constant, a MOFT half as thick will have a shear rate twice as high. Of course bearing size and speed due to engine RPM are also factors. A smaller diameter bearing will have a higher shear rate at the same RPM compared to a larger diameter bearing. For that study to say the bearing shear rate was well above the standard 10^-6/sec at 3000 RPM it might be from pretty small diameter bearings.

I've also seen data showing a very strong correlation between HTHS and oil pressure, with both high- and low-VI oils, so I think HTHS is still a good approximation for bearings in most engines. Shear rates at the piston rings and cams are higher.

The graph below shows an almost perfect linear relationship between oil pressure and the HTHS viscosity. This is data someone on this board had, and I plotted it for visual clarity. Engine oil pressure is primarily a function of all the journal bearings clearance because they have the highest resistance to flow in the system. The oil pressure vs KV data isn't as linear because not all oils depending on their VII formulation will show the same percentage change in viscosity under HTHS conditions.

X-axis is oil pressure, and Y-axis is KV100 and HTHS viscosity.

Some engines will have oil pressure regulated by a pressure relief valve even when the oil is hot. This seems to be fairly common with American designs, and could explain the comment about lower oil flow.

The other study I mentioned is the Japanese Toyota study, and while the data is noisier, there's no obvious reduction in bearing wear with thinner oils. The oil pump on this engine would not be in pressure relief, so flow would only be reduced very slightly with the thicker oils.

Yeah, not many engines would be in pump relief with fully hot oil ... maybe some of those crazy pumped Subarus, lol. Even is an oil pump is in pressure relief with hot oil, there's still a high volume of flow going on. Journal bearing wear rate doesn't seem to vary much until the MOFT is not longer large enough to ensure full hydrodymaic lubrication. Running an oil that puts the MOFT closer to zero makes it more likey to cause more wear depending on the use conditions that could cause inadequate MOFT (ie, don't run 0W-20 on the race track unless you can keep the oil temps way down). More MOFT adds more headroom and therefore less chance of added wear.