There is a school of people on BITOG who think that wear is solely determined by the high-temperature, high-shear viscosity measured at 150 Â°C and 1,000,000 1/second shear rate (HTHSV) and the larger the HTHSV, the less the wear on the critical engine parts is. Tribology (friction, lubrication, and wear) is very complicated and I don't like oversimplifying this very complex problem into a single parameter of the oil, namely HTHSV. Here is an old paper from the dawn of the HTHSV idea, where they measured the minimum oil-film thickness (MOFT) and wear while varying the HTHSV and oil and viscosity-index improver (VII) type and amount. It's a very systematic study. They don't do studies like that anymore. In conclusion, they determined that the HTHSV does not reliably predict the MOFT and wear. MOFT on the other hand directly correlates with wear and there is a minimum MOFT beyond which catastrophic wear occurs. There is also a minimum HTHSV beyond which catastrophic wear occurs but that depends on the VII type and content (but not on the additive package) and on the engine. Some of the conclusions:
- There is a minimum MOFT beyond which catastrophic wear occurs.
- HTHSV is deficient in determining the MOFT and wear. MOFT can vary by more than a factor of two for different oils with the same HTHSV.
- Increasing the viscosity beyond the required minimum actually increases the bearing wear.
- Catastrophic wear only happens in extreme driving conditions such as wide-open throttle (WOT).
- Only connecting-rod bearings (big-end bearings) experience catastrophic wear, not the main bearings, piston rings/cylinder liners, and the valvetrain, in extreme driving conditions such as WOT.
- VII type is crucial in determining the MOFT and bearing wear. (I also interpret that as the VII content being crucial.)
- KV100 is no worse than HTHSV150 in correlating with the MOFT. However, they are both deficient correlators with the MOFT and wear.
- The data in the paper is suggesting that monogrades usually result in a higher MOFT than multigrades. (My instinct also always tells me to use an oil with the least amount of VII as possible).
- Applicable shear rates in the bearings are 10,000,000 - 50,000,000 1/second, which is well beyond (10 to 50 times larger than) the 1,000,000 1/second the HTHSV is measured and reported at. HTHSV still decreases within this shear-rate range with the increasing shear rate, which makes HTHSV alone not sufficient to determine the MOFT and bearing wear. (I interpret this as the reason why the base-oil viscosity at 150 C plays a role in addition to HTHSV.)
- HTHSV is a deficient predictor of an oil's load-carrying capacity and there needs to be a new ASTM Sequence "MOFT test" to characterize a commercial oil's load-carrying capacity before it can be certified. (I find this the most striking! A such ASTM test has never been implemented in the 30 years since this paper was published!)
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