Engine friction reduction trends

JAG

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The main points should be familiar to those of us who read oil papers, but I’m posting it because it’s a good paper. It’s from 2016. The statements below are not all in the paper. Some come from other sources. Thinner oils push the lubricant regimes to the left on the Stribeck curve. Yep, it’s still true...darn physics. Friction modifiers help reduce mechanical friction when asperity contact occurs. Fuel economy benefits of thin oils are especially realized (compared to thicker oils) when friction modifiers are added because of the increased asperity contact caused by thin oils. Near the top and bottom dead center piston positions are the problematic regions due to the low relative speeds there causing oil film thickness collapse. The piston rocking back and forth there makes it even worse for the film separating the piston skirts from cylinder walls. There, the shear rates are very low, so low-shear rate kinematic viscosity is the driver rather than the high shear rate like is used in the HTHS test. The paper: https://link.springer.com/content/pdf/10.1007%2Fs40544-016-0107-9.pdf
 
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Originally Posted By: Marco620
Interesting... Very interesting. So.. Keep using redline 0w20 in my car, right??
Maybe, maybe not... If this is an overriding or controlling issue:
Quote:
Near the top and bottom dead center piston positions are the problematic regions due to the low relative speeds there causing oil film thickness collapse. The piston rocking back and forth there makes it even worse for the film separating the piston skirts from cylinder walls. There, the shear rates are very low, so low-shear rate kinematic viscosity is the driver rather than the high shear rate like is used in the HTHS test.
then you'd want a fluid with a higher kinematic viscosity, but not really a higher dynamic (HTHS) viscosity. Or, in other words, kind of the opposite of what you get from Redline's fluids: A reasonably '[censored]' 10W30 rather than a Redline-type 0W20. ...but that's only _if_ that one quoted principle is overriding or controlling in terms of overall wear that leads to engine failure or replacement.
 
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Originally Posted By: Onetor
What a paper to digest. I think I need to read it twice. I'm a newbie,,,,
Yeah. They throw in a little of everything---plus more than a few typos. Yes, I made it all the way through.
 
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Originally Posted By: JAG
Near the top and bottom dead center piston positions are the problematic regions due to the low relative speeds there causing oil film thickness collapse. The piston rocking back and forth there makes it even worse for the film separating the piston skirts from cylinder walls. There, the shear rates are very low, so low-shear rate kinematic viscosity is the driver rather than the high shear rate like is used in the HTHS test.
These graphs were posted in another thread awhile ago - don't recall which thread or by who. Shows that ring wear is a function of oil temp, oil HTHS rating and engine RPM. Still shows that higher HTSH will reduce ring wear in certain operating conditions. Also, it's pretty typical that higher viscosity oils will also have higher HTHS ratings. Another reason to use thicker oil IMO is to reduce ring wear based on these graphs ... especially if you're pushing the engine hard (load and RPM), and elevating the oil temperatures above what you'd see in normal street driving.
 
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Originally Posted by ZeeOSix
These graphs were posted in another thread awhile ago - don't recall which thread or by who. Shows that ring wear is a function of oil temp, oil HTHS rating and engine RPM. Still shows that higher HTSH will reduce ring wear in certain operating conditions. Also, it's pretty typical that higher viscosity oils will also have higher HTHS ratings. Another reason to use thicker oil IMO is to reduce ring wear based on these graphs ... especially if you're pushing the engine hard (load and RPM), and elevating the oil temperatures above what you'd see in normal street driving. [Linked Image] [Linked Image]
I'm not seeing the same conclusion from those graphs, especially the 2nd one that seems to show that an HTHS of 2.6 has the minimum average wear over the entire rpm range. That HTHS would be typical for a "thinner" oil (XW20) wouldn't it? (Also, they really need another data point in there around 2.9.) Maybe I should change my preference for using "thicker" oil (picked up from reading too much stuff on this site LOL). It would be useful to have a link or citation for the graphs.
 
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^^ Agree with neo3. There are also odd inconsistencies in speed vs. wear in those plots. What is "h·piece" in the vertical axis? (I assume something related to run time or number of revolutions.)
 
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Originally Posted by neo3
I'm not seeing the same conclusion from those graphs, especially the 2nd one that seems to show that an HTHS of 2.6 has the minimum average wear over the entire rpm range. That HTHS would be typical for a "thinner" oil (XW20) wouldn't it? (Also, they really need another data point in there around 2.9.) Maybe I should change my preference for using "thicker" oil (picked up from reading too much stuff on this site LOL). It would be useful to have a link or citation for the graphs.
The 2nd graph is showing that increased wear can occur for oils that are less than 2.6 HTHS when oil temperature gets elevated some. But it also says if you are using a 20 wt you really have no headroom, but are right at the verge of having more engine wear is the oil temps start to rise above "normal". Look how the wear rates all increase when you go a hair below 2.6. Personally, I don't want to always be operating on the edge of the wear cliff when pushing the engine hard. HTHS of 2.6 is probably fine for tootling around like grandma on the streets. grin2 Notice the wear is basically flat lined for the 2000 RPM case, regardless of the HTHS.
 
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Yes, those generalities do seem to be in there, but there also appears to be a very large amount of variation (scatter, really) such that those graphs are not adequate to give me a visual representation I can have confidence in.
 
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^^ Yes. It's interesting that 2.6 mPa·s appears to protect better than 3.1 at most speeds---unless that's merely an artifact of the scatter.
 
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