Valve trains still seem to rely primarily on thicker oils for protection, moly reduces friction but it doesn't necessarily reduce wear, ZDDP is still the best 'common' anti-wear additive, OEMs trade off wear vs low viscosity with low friction additives, bearing wear correlated with visocisty, etc.., all the things that we learned in kindergarten :^)
The
http://lnengineering.com/sae/ link has some nice briefs.
http://sciencelinks.jp/j-east/article/200114/000020011401A0391179.php
Friction Analysis between Cam and Shin in Engine Valve Train System.
Abstract;A friction measurement apparatus has been newly developed to analyze the frictional behavior between a pair of a cam and a shim in a direct acting valve train system. This apparatus has enabled the real time measurement of frictional force during the rotation of a cam shaft by placing a tri-axial transducer between the shim and lifter. Frictional behavior between the cam and shim has been evaluated using commercial engine oil. Most of the contact regions, that is, the ramp, shoulder and nose part, exhibited frictional coefficients from 0.1 to 0.15. These regions are judged to be under the boundary lubricating condition due to the theoretically confirmed low oil film thickness. Only the contact area of the flank part showed a low frictional coefficient of around 0.05, due to the thick oil film under the elastohydrodynamic lubricating condition. The influence of an organo-Mo-type friction modifier(FM) added into the engine oil has been investigated. It was determined that the organo-Mo-type FM effectively decreased the friction between the cam and shim by means of reducing the friction in the boundary lubricating region. (author abst.)
http://www.lnengineering.com/sae/Design%...007-01-4143.doc
SAE Technical Paper Series. Design Considerations in Formulating Gasoline Engine Lubricants for Improving Engine Fuel Economy and Wear Resistance Part 1 Base Oils and Additives. 2007-01-4143
19. Adequate performance for a lubricant in the VIB sequence is most important to most OEMs as this is a good indicator of fuel economy, after which sequences to evaluate the wear performance are then considered.
20. These new lubricants must balance the fuel economy improvement FEI% and good wear resistance.
23. As speed increases, the coefficient of friction decreases, with Group IV oils having the lowest and Group II oils the highest but has little effect of the FEI %.
34. The poor performance of MoDTP and MoDTC-1 has to do with the interaction of ZDDP which leads to a decrease in tribofilm hardness even though these tribofilms were the thickest and fastest to form.
36. The base oil has no significant relationship on wear other than a slight improvement with oils with more methyl groups, which help to contribute carbon to the anti-wear tribofilm with a significantly lower wear coefficient.
37. But base oil does have an effect on hydrodynamic friction which viscosity and film thickness which is good for wear, bad for FEI.
SAE Technical Paper Series. Engine Oil and Bearing Wear. 810330.
1. There is a dependence on oil viscosity on bearing wear.
2. Bearing wear can be divided into two categories: catastrophic and non-catastrophic (normal wear and tear).
3. “The best correlation was found between connecting rod bearing wear and pentane insolubles.”
4. The bearing distress tests measured not in bearing weight loss but in bearing demerits were precise enough to allow researchers to differentiate between a 5W-20 and 10W-40 oil, showing sensitivity towards viscosity of oil used.
5. Average connecting rod bearing wear demerits for the 5W-20 was 133+/- 37 versus 55 +/- 4 for the 10W-40. TABLE 2.
6. Bearing distress in tests with Newtonian oils (mono-grade) were lowest with oils with higher viscosity at 100C and 150C.
7. Increase in oil viscosity showed a linear decrease in bearing wear demerits with data collected precise enough to determine a zero wear intercept of 13.7 cP at 150C. FIGURE 2.
8. Bearing wear is directly proportional to the HTHS viscosity of oils.
9. Bearing distress test wear was higher with multi-grade oils than their mono-grade base oils, so the additive packages used must have some effect on an oils performance.
10. The detergent type used was the largest contributor to performance of oils in the bearing distress tests.
11. TAN or total acid number of oils tested showed poor or no correlation to connecting rod bearing wear from corrosion.
12. As average sludge merit decreases there is an equal response in decreased bearing wear.
13. Under severe conditions the detergent inhibitor package can contribute to bearing wear.
14. Pentane insolubles were highest in 100% ester (Group V) oils in first drains.
15. Low ash-less dispersant oils showed highest average total connecting rod bearing weight loss.
http://physicsworld.com/cws/article/print/5020
But by combining models for the plain bearings, the piston [censored] and the valve train, researchers have found that they can model the lubrication conditions in a complete internal combustion engine. Since the bearings and piston rings are lubricated predominantly in the hydrodynamic regime, a lubricant with a lower viscosity should lead to a thinner oil film and thus lower friction. However, the valve train operates in the mixed-boundary lubrication regime, which means that lower friction can only be obtained with thicker lubricants. Engine-friction models enable us to study the trade-off between viscosity and friction, and therefore select the optimum lubricant to improve a vehicle's fuel economy