Originally Posted By: Shannow
OK...your point is...Designers know their stuff, and pick a point in the "safe" hydrodynamic range. Designers, knowing your stuff add a safety margin.
Good, we agree on that.
Originally Posted By: Shannow
So you can install (and advise others) to install a thinner oil.
Mostly based on Shell's study at
http://priuschat.com/attachments/fuel-efficient-motor-oil-technical-article-pdf.11772/ which I'll quote below.
Originally Posted By: Shannow
Problem is, it's not your reading, or analysis of the Streibeck curve (whether it's logarithmic, in pressure units or other), you are simply guessing that it's somewhere, and guessing that changing the oil viscosity moves it somewhere else, and guessing that place is OK.
You can easily put in 8 cs vs. 10 cs (going from a 20 to a 30 oil) into the ZN/P relationship and shift the curve in
http://www.stle.org/resources/lubelearn/lubrication/ .
And yes, any design engineer would place it around 0.4 (maybe upwards of 1). Put 8 cs in instead of 10 cs (again, 20 vs. 30), worst case moves the 0.4 to 0.3 --->
Not much movement on the Stribeck, obviously. The point.
Originally Posted By: Shannow
Except that means that they LOSE their designer installed safety margin,...
The discussion was, in going from a 30 to a 20 weight, and we see that only moves the Stribeck a small amount. Emphasis on SMALL amount. Very little margin lost.
Originally Posted By: FetchFar
..... As for the left part of the curve, thats in a mixed-boundary lubrication all the way to near-zero, so we almost totally depend on the anti-wear additives to save bearings from destruction there, nothing more can be done, like on start-up in crank/pin bearings, or on cam lobes full time, timing chain pin/links, etc....
Originally Posted By: Shannow
With the increases in fuel economy requirements, all of the manufacturers (Honda, Ford, etc.) are telling us that they are operating in boundary mode more and more often, and that the new standards require new and better friction modifiers to provide adequate protection under those conditions...was discussed before here
http://www.infineum.com/sitecollectiondocuments/notebooks/gf5/ResearchReport.pdf
Page 78 or thereabouts.
Thats a pseudo-ZN/P, for the entire engine, for discussion purposes. The ZN/P for the crank journal bearings SPECIFICALLY remain hydrodynamic for running conditions, a very important point to remember. Its well known that the valvetrain, timing chain, rings, operate in mixed-boundary conditions, always have and always will.
Originally Posted By: Shannow
So if you "guess" that the designers know their job......
They do know their job, not a guess. The same people I observed in Mechanical Engineering school & the SAE who did this specific task are sharp people, no doubt.
End of discussion for me. I'll just let the "Improved Fuel Efficiency by Lubricant Design : A Review by R.I. Taylor & R.C. Coy, Shell Research & Technology Centre, Thornton, United Kingdom paper take it from here. Address all questions to Shell from now on please. LOL...
"In our laboratory, it has been observed that in a modern gasoline engine, well designed
automotive bearings can be lubricated with oils as thin as 2.3 mPa.s (and a 20 weight oil has 2.6 to 2.9 mPas) without any observable wear on either con-rod or main bearings.
The assumption that lower viscosity lubricants automatically give rise to thinner oil films in key lubricated contacts in a gasoline engine is also open to question, particularly in the case of piston rings. Laser Induced Fluorescence measurements have found that, in a Nissan gasoline engine, the mid-stroke top ring oil film thickness was greater for an SAE-5W/20 lubricant than it was for an SAE-15W/40 lubricant. "