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An investigation of an engine field-failure found low-temperature incompatibility to be the root-cause of an engine pumping failure. This was established from an examination of the rheology of the new and used oils. It was later discovered that some SAE multigrade oils that contain higher-cloud-point basestocks are incompatible with other same-W-grade oils that contain VI improvers that have a propensity to interact with wax precursors. The latter oils, which failed the Scanning Brookfield test, but not the TP1 mini-rotary viscometer test, were found to be incompatible with a number of commercial multigrade oils at low-temperature.

Results from this study indicate that the reproducibility of the CEC Standard Test Method, CEC-L-14-A-88, a diesel injector mechanical shear stability test for engine lubricants, is not applicable to certain engine lubricants. The test is subject to a material bias which is caused by operational parameters-the allowance of a broad valve opening pressure range - and the use of a relatively insensitive calibration fluid. The magnitude of the bias depends on VI improver molecular weight and molecular weight distribution, as determined by gel permeation chromatography, otherwise known as size exclusion chromatography. The bias can be minimized or eliminated, however, by determining viscosity loss at a specified gauge pressure. It was thereby possible to improve reproducibility and obtain excellent correlations between diesel injector and both gasoline and diesel laboratory engine tests results.

A journal bearing rig has been used to determine the viscosity of ten multigrade oils under hydrodynamic lubrication conditions at a mean shear rate of 5 × 105 s-1 and a temperature of 150°C. These conditions closely approximate the shear rate and temperature likely to be encountered when an oil is subjected to severe service in engine bearings. The same rig has been used to study the effect of lubricant viscosity on wear under boundary lubrication conditions at 150°C. Bearing wear was determined by weighing the bearing shells. The results obtained under static load showed that for single-grade lubricants wear increased markedly with decreasing viscosity. However, for a number of polymer-containing oils including 10W30 and 10W40 multigrades, similar experiments showed that wear was not influenced by high-shear viscosity at least not in the region between 2.1 and 4.9 mPa.s.