Low Kinematic high hths?

[l322spouse]

Hello all, I've always thought about this but I do not think any forum post discuss this exact thing. I see how a lot of modern oils shear a little for improved mpg and still acceptable wear protection, and often have a little lower hths, coupled with a high viscosity index for their SAE viscosity, than you would expect from monograde of the rated sae visocisty, but my question is, what would happen if someone used an oil that was the opposite of the norm? Say, instead of a 5w-30 multigrade, an SAE 20 monograde, that had the same hths because of its shear resistance but lower kinematic viscosity? Are there particular parts that benefit from the thicker film without being such high shear, such as timing chains? A direct comparison would be off of the shelf PCMO 5w-30 vs 5w-20 redline, for what I mean

 

[3783-KC1]

Don't know too much as I'm new here, but I'll take a crack. In theory the 5w20 redline would be better off the fact that thicker oils increase temperatures at lubrication points (bearings, rings, etc) vs thinner oils. Also, there would be greater pumping losses with the thicker fluid in my opinion which would cause the engine to work a tiny bit harder. But there might be other things such as add-packs and overall engine design that would turn this theory on its head.

 

[twX]

Comparing two oils with the same HTHS, the oil that is more shear stable will have a higher viscosity at any shear rate higher than 10^6 1/s (the shear rate used for the HTHS test), and will be have a lower viscosity at any shear rate below 10^6.

Here's a chart that shows this. Three of the oils have the same HTHS, but the low-VI 10W-20 has a higher viscosity than the 0W-30 or 10W-30 at very high shear rates. It's even thicker than the high-VI 0W-40 at these shear rates, even though the 0W-40 has a much higher HTHS.

Shear rates at the pistons and cams are typically a lot higher than 10^6, so the viscosity at these higher shear rates is relevant to the protection of these components. The rings and bores tend to wear the most in the area near TDC, where shear rates are around 10^7 or even higher.

Fuel economy is best correlated to the viscosity of oil at the piston through its mid-stroke range, where shear rates are 10^6 to 10^7, since a large percentage of the friction in an engine is in this region.

Further complicating all of this is the fact that the shear thinning behavior depends on temperature. When oil is colder than 150 C, the oil will be fully shear-thinned at lower shear rates than the chart above suggests.

 

[l322spouse]

Comparing two oils with the same HTHS, the oil that is more shear stable will have a higher viscosity at any shear rate higher than 10^6 1/s (the shear rate used for the HTHS test), and will be have a lower viscosity at any shear rate below 10^6.

Here's a chart that shows this. Three of the oils have the same HTHS, but the low-VI 10W-20 has a higher viscosity than the 0W-30 or 10W-30 at very high shear rates. It's even thicker than the high-VI 0W-40 at these shear rates, even though the 0W-40 has a much higher HTHS.

View attachment 192737

Shear rates at the pistons and cams are typically a lot higher than 10^6, so the viscosity at these higher shear rates is relevant to the protection of these components. The rings and bores tend to wear the most in the area near TDC, where shear rates are around 10^7 or even higher.

Fuel economy is best correlated to the viscosity of oil at the piston through its mid-stroke range, where shear rates are 10^6 to 10^7, since a large percentage of the friction in an engine is in this region.

Further complicating all of this is the fact that the shear thinning behavior depends on temperature. When oil is colder than 150 C, the oil will be fully shear-thinned at lower shear rates than the chart above suggests.

View attachment 192741

Very interesting information about a range of things. Is there a particular load/rpm range you'd say those shear rate estimates pertain to?

 

[l322spouse]

Don't know too much as I'm new here, but I'll take a crack. In theory the 5w20 redline would be better off the fact that thicker oils increase temperatures at lubrication points (bearings, rings, etc) vs thinner oils. Also, there would be greater pumping losses with the thicker fluid in my opinion which would cause the engine to work a tiny bit harder. But there might be other things such as add-packs and overall engine design that would turn this theory on its head.

That's what I'm thinking, but that is also what leads me to ask why the major manufactures don't approach it like that either.

 

[twX]

Very interesting information about a range of things. Is there a particular load/rpm range you'd say those shear rate estimates pertain to?

The shear rate ranges I have marked on the chart are from the study. It modelled the engine from 750-5,000 rpm in medium load conditions. Most of the difference between the max and min shear rates are due to how the shear rate changes with crank angle. RPM doesn't have a very large effect on the shear rates.

 

[Fredxt]

Comparing two oils with the same HTHS, the oil that is more shear stable will have a higher viscosity at any shear rate higher than 10^6 1/s (the shear rate used for the HTHS test), and will be have a lower viscosity at any shear rate below 10^6.

Here's a chart that shows this. Three of the oils have the same HTHS, but the low-VI 10W-20 has a higher viscosity than the 0W-30 or 10W-30 at very high shear rates. It's even thicker than the high-VI 0W-40 at these shear rates, even though the 0W-40 has a much higher HTHS.

View attachment 192737

Shear rates at the pistons and cams are typically a lot higher than 10^6, so the viscosity at these higher shear rates is relevant to the protection of these components. The rings and bores tend to wear the most in the area near TDC, where shear rates are around 10^7 or even higher.

Fuel economy is best correlated to the viscosity of oil at the piston through its mid-stroke range, where shear rates are 10^6 to 10^7, since a large percentage of the friction in an engine is in this region.

Further complicating all of this is the fact that the shear thinning behavior depends on temperature. When oil is colder than 150 C, the oil will be fully shear-thinned at lower shear rates than the chart above suggests.

View attachment 192741

Thanks for the charts! Quick question, my cars oil temp never gets above 200F, does that mean that because oils doesn't get to 150C a low HTHS viscosity actually doesn't cause more wear?

 

[mitsuman47]

Thanks for the charts! Quick question, my cars oil temp never gets above 200F, does that mean that because oils doesn't get to 150C a low HTHS viscosity actually doesn't cause more wear?

Not necessarily. The oil temp is usually taken between the pump and where it makes it into the rest of the engine, which is where the temp will rise.

 

[twX]

Thanks for the charts! Quick question, my cars oil temp never gets above 200F, does that mean that because oils doesn't get to 150C a low HTHS viscosity actually doesn't cause more wear?

All of the studies I'm aware of that show thinner grades resulting in more engine wear are done with sump temperatures of 130 to 150 C, and they're always done at wide open throttle. These are the conditions where an oil can become too thin and increase wear. The piston rings and bores are most sensitive. If your sump temperatures are always
I'd be cautious about using too thin a grade even if sump oil temperatures are never very high, since the temperature of the oil at the piston rings near top dead center is more closely related to the cylinder liner and piston ring temperatures than it is to the sump temperature. This is because the oil film is thin here and the piston dwells near TDC for enough time for the oil film to approach the temperature of the liner/ring, regardless of the temperature the oil was in the sump. Now, splash lubrication from colder sump oil will keep the piston and liner cooler than if the sump oil was hot, but it's maybe not that much of a difference.

This figure is from a test that shows increased wear with oils thinner than 0W-20 when the oil is 130 C in the sump, but no increased wear when the oil is at 90C.

 

[l322spouse]

all of the points in this thread really still leave me wondering why oils like Mobil 1 racing oil is made that way, their 0w-50 seems like it could have just been a 0w-40 with less VII's. Is it possible it is just to comfort those thinking they need thicker? Is that perhaps the goal of the OEM's as well?

 

[l322spouse]

Comparing two oils with the same HTHS, the oil that is more shear stable will have a higher viscosity at any shear rate higher than 10^6 1/s (the shear rate used for the HTHS test), and will be have a lower viscosity at any shear rate below 10^6.

Here's a chart that shows this. Three of the oils have the same HTHS, but the low-VI 10W-20 has a higher viscosity than the 0W-30 or 10W-30 at very high shear rates. It's even thicker than the high-VI 0W-40 at these shear rates, even though the 0W-40 has a much higher HTHS.

View attachment 192737

Shear rates at the pistons and cams are typically a lot higher than 10^6, so the viscosity at these higher shear rates is relevant to the protection of these components. The rings and bores tend to wear the most in the area near TDC, where shear rates are around 10^7 or even higher.

Fuel economy is best correlated to the viscosity of oil at the piston through its mid-stroke range, where shear rates are 10^6 to 10^7, since a large percentage of the friction in an engine is in this region.

Further complicating all of this is the fact that the shear thinning behavior depends on temperature. When oil is colder than 150 C, the oil will be fully shear-thinned at lower shear rates than the chart above suggests.

View attachment 192741

Do you know what the shear rate of the timing chain would be? Could this be a place of lower shear rates?

 

[twX]

Do you know what the shear rate of the timing chain would be? Could this be a place of lower shear rates?

No idea. I also don't know whether oil viscosity is even relevant to timing chain wear.

Most research on timing chain wear focuses on the effects of soot in diesel or GDI engines. API SP and Dexos1 Gen3 have timing chain wear tests that address the timing chain wear that is caused by soot in GDI engines.

 

[CR94]

No idea. I also don't know whether oil viscosity is even relevant to timing chain wear. ...

We've seen BitOG posts claiming chain wear has been shown to correlate with viscosity of the base oil.