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Mechanical degradation of polymeric additives in lubricants has been a topic of extensive study since complex formulations were introduced to reduce the temperature dependence of viscosity. Many devices, which are able to shear polymers, have been tested for their abilities to simulate the degradation observed in engines and other lubricated systems. Conclusions drawn from these studies are often ambiguous as they depend on the test protocol and method of data analysis. In this work, a simple expression based on probabilistic arguments is used to describe kinematic viscosity data from a variety of degradation simulators. This expression provides a method of comparing extent and rate of degradation for different simulators.

Two commercial base stocks are evaluated using a controlled stress rheometer, a cold cranking simulator and a scanning Brookfield viscometer. These oils form wax structures below 0°C that significantly influence viscosity. These structures break down with time under shear. This degradation process depends on the magnitude and duration of applied stress. A constant stress causes a continuous decrease in viscosity even after an hour of shear. Viscosity approaches a limiting value if the imposed shear stress increases at a constant rate. Repetitions of this test lead to a lower viscosity that is relatively insensitive to shear rate over the range of applied stress. In all cases, the wax structure does not readily reform after degradation for oils held at the test temperature. These results are used to suggest the domain of current procedures and their relation to higher temperature measurements.

Polymer additives in commercial engine oils are mechanically degraded in flows involving large stresses. The oil stability is usually characterized by an asymptotic value called the fully-sheared viscosity. Published data from field and laboratory engine tests show that the kinematic viscosity of degraded oil does not approach such an asymptote. This viscosity reaches a minimum due to the compensating effects of mechanical degradation and oxidation. Kinematic viscosity data from various tests are correlated here using a logarithmic function. This correlation is used to compare the kinetics of degradation processes in different tests. These comparisons suggests that multi-grade oils degrade at a consistent rate in different engine tests. Simulation devices using a diesel fuel injector do not give results comparable to field test data. A milder technique for simulating mechanical degradation of engine oils is suggested to improve correlation with field data.

Fullerenes are spheroidal carbon clusters typically produced by arcing graphite rods in an inert gas such as helium. This process generates structures including C60, C70 and trace amounts of larger clusters. At ambient conditions, these clusters dissolve in a clear 100N bright stock at mass fractions up to 700 ppm. These solutions have a magenta color which is typical of solutions involving aromatic solvents like toluene. Solution density decreases with increasing mass fraction suggesting that fullerenes act like voids in the mineral oil. The low shear rate viscosity exhibits an unusually strong dependence on the volume fraction of fullerenes in solution. This dependence implies an effective diameter that is at least 50% greater than the molecular diameter. These solutions also exhibit abrupt shear thinning from their low rate value to the base stock viscosity. All of these results suggest that aromatic hydrocarbons adsorb onto the fullerene surface.

Monitoring pressure drop in an oil line has been proposed as a technique for assessing lubricant condition in an engine. This technique is evaluated using a commercial single cylinder, gasoline engine. The monitoring system consists of a differential pressure transducer and a thermocouple attached to a section of the oil line. Engine tests using single and multi-grade oils are carried out at a constant temperature (100 ± 5°C) and load (7 ± 1 N·m) over a thirty-six hour period. The pressure drop for single grade oil shows an average increase of 10% over the test period. This trend is similar for multi-grade oils over the first 20 hours. A larger rate of increase over the last 16 hours results in an average change of 50-70% by the end of the test. The corresponding change in kinematic viscosity is typically half of that for pressure drop. Differences between oils are associated with compositional changes in multi-grade formulations.

The pressure drop in turbulent pipe flow is suggested as a technique for monitoring the condition of oils during use in an engine. Several new and used oils are tested in a flow loop designed to simulate operating temperatures in an internal combustion engine. The used oils are obtained from four and six cylinder fuel injected gasoline engines typically used in city driving conditions. Changes in viscosity and composition are measured directly using a viscometer and gel permeation chromatography equipment. The proposed monitoring technique provides a sensitive measure of these changes.