Originally Posted By: bwilson4web
Hi,
I'm trying to resolve some of your comments with the R&G measurements of Mobil 1:
Originally Posted By: Gokhan
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(1) HTHS viscosity
xW-20 oils have HTHS viscosity of 2.6.
xW-30 oils have HTHS viscosity of about 3.0 - 3.1 but 5W-30 oils (especially dino ones) usually permanently shear to much lower values.
. . .
The best friction modifier to date is the molybdenum-sulphide compounds. Boron compounds are also used. Since molybdenum disulphide is graphite-like and not soluble in oil, organic molybdenum-sulphide compounds are used.
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As far as moly friction modifiers is concerned, Toyota oil is said to have as high as a monstrous 500 - 1000 ppm level of moly.
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The following results are from R&G Labs of Tampa FL:
You mention "HTHS viscosity" but my lab reports "cTs @40C", "cTs @100C", and "viscosity index" (a pure number relationship between the two.) I'm confused about the units of "HTHS" which doesn't seem to have any dual-temperature range metrics but just one number.
As for molybdenum content, Mobil 1 appears to be at 100 ppm and you're suggesting 200 ppm is needed. What we don't know is the extent that boric CLS works or doesn't work in a moly environment. Certainly it was not mentioned in the vendor's documentation.
Now my understanding is the moly compounds are not chemically bonded to the surfaces but has to be available in the oil during operation or as part of an assembly grease. But I had not considered the hypothesis that moly or any other solid, lubricant additives might block formation of the boric acid layer. ... something to ponder.
Thanks,
Bob Wilson
The units for high-temperature, high-shear viscosity is cP, and it's measured at 150 C (high-temperature)
and 1,000,000 (10^6) 1/second (high-shear). High-shear means two engine parts sliding against each other with oil flowing between them and the shear rate is the relative speed of the parts divided by the distance between them.
Engine oil is a so-called non-Netwonian fluid, where the viscosity depends on the shear rate. This is primarily due to the presence of viscosity-index improvers in engine oil, which are made of very large molecules. Viscosity-index improvers may temporarily shear at high-shear rates, leading to a temporarily smaller viscosity, or they can permanently shear at high-shear rates, resulting in an oil with permanently smaller viscosity.
Originally, people thought that specification of the simple, kinematic viscosity (such as saying SAE 10W-40, which tells you the approximate kinematic viscosity vs. temperature) was enough. But in the 1970s, people realized that the kinematic viscosity had little use and that's when they discovered that the HTHS viscosity was what determined engine wear and fuel economy, not the kinematic viscosity.
In fact the first of the two most important findings was that the minimum oil-film thickness (MOFT) in a bearing -- the smallest spacing between the rotating concentric cylinders in the bearing (which dynamically moves as the bearing rotates) -- was given by the HTHS viscosity and, as a result, bearing wear and failure was directly determined by the HTHS viscosity. More the HTHS viscosity, more the MOFT and less likely the bearing failure. This was especially important in diesel engines, where high torque at low RPMs could further decrease the MOFT. (MOFT is directly proportional to HTHS viscosity and RPM and inversely proportional to torque.) Once the MOFT becomes zero, there is no oil film and there is direct metal - metal contact and you are at the mercy of antiwear, extreme-pressure, and friction-modifier coatings on the metals.
The second crucial finding was that the fuel economy was also directly determined by the HTHS viscosity. Less the HTHS viscosity, more the fuel economy. In fact the relationship holds so extremely well that you can simply tell from the HTHS viscosity alone what the fuel economy of an oil will be -- save for the effect of the friction modifiers.
So, at the end, HTHS viscosity has become the single, most important measure of motor oil and you can tell how much an oil has wear protection and fuel economy from the HTHS viscosity alone (save for the antiwear, extreme-pressure, and friction-modifier additives). Note that most modern gasoline engines don't require too high HTHS viscosity to protect against wear and you can benefit from fuel economy of small HTHS viscosities without causing significantly more wear than with larger HTHS viscosites in gentle driving conditions (larger HTHS viscosity would be good for hard driving).
Here is the preview of a great book on HTHS viscosity:
High-temperature, high-shear viscosity be James Spearot
Obviously the measurement of HTHS viscosity is fairly complicated and requires expensive equipment and for that reason used-oil-analysis labs don't report this single most crucial quantity for engine oil.
As far as the friction modifiers are concerned, they are similar to antiwear additives, the most famous antiwear additive being ZDDP. They do form thin coatings on the metal parts, just like the antiwear additives. ZDDP forms a thin coating of phosphorous and zinc and moly forms a thin coating of molybdenum and sulfur, along with other elements in the lattice. Note that moly is not only a friction modifier but an extreme-pressure additive and an antioxidant as well. ZDDP is an antiwear additive and an antioxidant. All antiwear additives, extreme-pressure additives, and friction modifiers used in oil are organic (oil-soluble) -- they are
not solid particles (except for ad hoc aftermarket products which contain inorganic [not oil-soluble] molecules in suspension). Activation happens either at high temperature or when the metal parts rub against each other (which also generates heat due to friction). After the activation due to rubbing and/or high temperature, micron-thin films of phosphorous and zinc or sulfur and molybdenum coat the metal and they serve as antiwear, and/or extreme-pressure (against scuffing), and/or friction-modifier additives. ZDDP alone increases friction but ZDDP and moly together decrease the friction (more than moly alone) and decrease wear (more than ZDDP alone). For this reason a precise balance of ZDDP and organic moly works best to form the strongest antiwear and extreme-pressure coating with the least friction coefficient.
Here is a good
presentation on moly.
I said 200 ppm Mo but chances are that even 50 - 100 ppm of Mo may be sufficient if trinuclear organic moly is used according to the presentation above. Mononuclear and dinuclear forms are not as affective and you would need more Mo ppm for these kinds. Also, any organic moly that doesn't contain sulfur is useless as an oil additive. Trinuclear moly is made and patented by Infineum/Exxon-Mobil (Infineum also partly owned by Shell).
