I came across this article while I was lurking in my "Lubricants" folder. Appoligies if this has been previousely posted.
What I was really reading about was the relationship with pistons, liners and rings with lubrication and friction. It was interesting to note that at mid-stroke the rings and piston are at hydrodynamic lubrication and at TDC and BDC they were at, either, mixed or boundry or both. Oil friction was highest at mid stroke with least wear. ZDDP and Moly reduced friction at TDC and BDC, with Moly being slightly better, but wear increased with TDC being worse. HTHS and shear played a big part in the mid stroke area along with oil film strength. They ran tests with 30w, 20w-50 and 10w. The least wear, and greatest friction, was the 20w-50. The auther stated a concern with the use of thin oils in this area but also stated engine designers are expirementing and using various metal coatings on pistons and rings to mitigate the use of lighter oils and reduce friction for economy and enviromental issues.
Here is the article:
Understanding HTHS
When oils started being graded, when VI was starting to be used, the only oils that were in use was oil that came out of the ground, and was refined.
These oils displayed "Newtonian behavior", didn't matter how hard you worked them, they were the same viscosity all the way through.
By "worked", I mean shear...for example, two plates (or a bearing surface) traveling at 1m per second, and 1mm apart would have a shear rate of 1000/s.
Then we developed the technologies to change the behaviors of the oil with viscosity index improver's, and by observation, the new grades did not reflect their grade in their wear behavior...so they started looking at why.
What was found that at very high shear rates, the viscosity modifier additives started to "flatten" out and the apparent viscosity if the oil dropped...in the above plate example, 1mm separation, the rate is like 1km/second. the shear rate in this case is 1,000,000/s
Given that the shear rate is the surface speed/the gap, there's places (like the loaded side of the bearing) where the oil film thickness (gap) is so small that the loaded part of the bearing is in this high shear regime.
That's the "HS" part of the equation.
Re the 150C, that's the sort of temperature that is seen in the big end bearings, even IF the sump is tooling along at 100C.
Most of the heat that the oil deals with is related to the work that the engine does on it through shearing (note them mechanical "force", not breaking it up)...bearings have a big temperature rise. Pistons and rings do too, but the oil there is on the cylinder walls and the heat goes straight to the coolant.
So High temperature - think big ends. High Shear - think big ends.
That's why it came into being.
What I was really reading about was the relationship with pistons, liners and rings with lubrication and friction. It was interesting to note that at mid-stroke the rings and piston are at hydrodynamic lubrication and at TDC and BDC they were at, either, mixed or boundry or both. Oil friction was highest at mid stroke with least wear. ZDDP and Moly reduced friction at TDC and BDC, with Moly being slightly better, but wear increased with TDC being worse. HTHS and shear played a big part in the mid stroke area along with oil film strength. They ran tests with 30w, 20w-50 and 10w. The least wear, and greatest friction, was the 20w-50. The auther stated a concern with the use of thin oils in this area but also stated engine designers are expirementing and using various metal coatings on pistons and rings to mitigate the use of lighter oils and reduce friction for economy and enviromental issues.
Here is the article:
Understanding HTHS
When oils started being graded, when VI was starting to be used, the only oils that were in use was oil that came out of the ground, and was refined.
These oils displayed "Newtonian behavior", didn't matter how hard you worked them, they were the same viscosity all the way through.
By "worked", I mean shear...for example, two plates (or a bearing surface) traveling at 1m per second, and 1mm apart would have a shear rate of 1000/s.
Then we developed the technologies to change the behaviors of the oil with viscosity index improver's, and by observation, the new grades did not reflect their grade in their wear behavior...so they started looking at why.
What was found that at very high shear rates, the viscosity modifier additives started to "flatten" out and the apparent viscosity if the oil dropped...in the above plate example, 1mm separation, the rate is like 1km/second. the shear rate in this case is 1,000,000/s
Given that the shear rate is the surface speed/the gap, there's places (like the loaded side of the bearing) where the oil film thickness (gap) is so small that the loaded part of the bearing is in this high shear regime.
That's the "HS" part of the equation.
Re the 150C, that's the sort of temperature that is seen in the big end bearings, even IF the sump is tooling along at 100C.
Most of the heat that the oil deals with is related to the work that the engine does on it through shearing (note them mechanical "force", not breaking it up)...bearings have a big temperature rise. Pistons and rings do too, but the oil there is on the cylinder walls and the heat goes straight to the coolant.
So High temperature - think big ends. High Shear - think big ends.
That's why it came into being.
