BTW, this same topic and paper and others was discussed at least a year ago.
Quote:
The results demonstrated that under mixed lubrication regimes, the friction performance of engine oil improves with oil aging. However, it can not be assumed that the fuel economy of vehicles would improve appreciably with continued aging. There are other component systems e.g., the piston ring/bore contact, bearings etc.. that operate primarily in the hydrodynamic regime where the viscosity plays a significant role. The viscosity increase associated with oil aging would lead to increased frictional loss at these contacts. Therefore, the net fuel economy gain or loss would depend on the balance of boundary friction decrease with aging and the hydrodynamic friction (due to viscosity increase) increase with increased viscosity due to aging. Also, a remarkable improvement in the wear performance with oil aging in valvetrain contact is encouraging. This could be an enabler for achieving higher oil drain intervals although various other factors need to be considered. However, it would be interesting to know how the wear characteristics of other engine components change with engine oil aging.
Yes, the visosity does increase which provides a slightly thicker hydrodynamic film.
With any study, there are a number of things going on. The wear study appeared to be for cam tappet shims only after run-in of the cam assembly.
I counted at least four different oils as well, so one has to pay close attention to the bookeeping.
One of the oils had a Mag/calcium sulfonate detergent package while another had a calcium packge only (R208). R208 also had had a supplemental AW additive in addition to reduced ZDDP.
Quote:
RO 168 and RO 207 are factory fill oils (MY 2001 and MY 2003, respectively) containing 0.10 wt% phosphorous while RO 206 and RO 208 were developed as prototype GF-4 oils containing 0.05 wt% phosphorous. The formulation of oils RO 168 and RO 207 are similar, with the difference in the detergent system; oil RO 168 contains Ca and Mg sulfonate detergents, while RO 207 contains only the Ca sulfonate detergent. oil RO 206 is similar to oil RO 207 but contains only half of the ZDDP oil RO 208 is similar to oil RO 206 but contains a supplemental antiwear additive.
In addition to an increased viscosity, one of the conclusions was that reduced wear vs. oil aging had to have another cause. The inference is that while ZDDP decomposes, it's the ZDDP decomposition products (phos, sulfur, zinc) that may provide better wear protection as the oil ages.
The thing we don't know is, what is the formulation of these oils. We have to assume these were GroupI/II oils I think.
Most oils contain a bit of ester for seal swell and a fatty acid ester as a friction modifier.
While a good study, one of the questions NOT asked was this: Are these esters polymerizing or transforming under engine conditions with more aging and are they thus providing a thicker tribofilm? Owing to the Chemistry of these components one would have to say yes. Also realize, ZDDP is a Phosphor, sulfur, zinc compounded as an ester as well.
One of the theories I had ( I may not have been the only one with this theory) about early wear was that, due to the greater detergency of the fresh oil, two things would happen: 1. The detergents would strip wear metals and the dispersant would then disperse these metals into the oil, so a short UOA test would most likely show a lot of wear metals not picked up by the low detergency used oil, 2. fresh detergents would also reduce the concentration of AW additives on metal surfaces, thereby allowing temporary wear until the AW additives could compete with the detergents. There is another study that used different detergents with various AW additives, but I can't recall all of the relevant conclusions.
My info is that this study was born out of another previous study to determine how low a concentration of zddp could be tolerated (apparently 200 ppm is the limit), and or how certain (proprietary) supplemental (notice he didn't identify this supplemental) additives affected wwear. It was seen that there was a trend that seemed to show that aging oil was producing lower wear. So the original study was extended (this study) to determine what was happening with the oil and its additive decomposition products.
Much surface analysis using microscopy was done in this study, but what was lacking was an adsorption analysis of the molecular products of the oil. I have to asume that if that data was released, it would show the exact base oil and additive compositions and the cat would have been let out of the bag, I.E., proprietary data.