Search Results

A portion of the Institute of Materials database which contained data on more than 650 multigrade oils was searched to obtain an estimate of the percentage of oils that meet SAE J300 low-temperature pumpability requirements but exhibit properties which make the low-temperature properties suspect. The suspect oils studied either failed the Scanning Brookfield test (SBT) or produced significantly different viscosities in the one-day mini-rotary viscometer (MRV) test and the two-day MRV test, which uses the slow-cool, TP1 cooling cycle. Several of these commercial multigrade oils were obtained from the Institute of Materials (IOM). The oils were then blended with off-the-shelf commercial (CO-numbered) oils having the same SAE W-grade designation and evaluated in the two-day MRV test (TP1-MRV), which has been a reliable predictor of engine lubricant pumpability characteristics.

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.

The Mini-Rotary Viscometer (MRV) procedure, ASTM D3829, was designed to predict the pumpability characteristics of engine oils. The viscosity and yield stress measurements determined by this instrument correlate with the 16 hour cold-soak laboratory engine test results that were carried out in a joint ASTM/SAE program in the early 1970’s. However, several oils that had passed the ASTM D3829 procedure and the viscosity classification requirements in SAE J300 SEP80 caused pumpability field failures in engines during the winters of 1980 through 1983. These oils can be divided into two types of failures: those which failed after an overnight exposure to low temperature, and those that require several days exposure before problems developed. This paper describes the development of a series of temperature profiles for use with the MRV that clearly predicts the low temperature characteristics of these two groups of oils.

The effects of engine oil viscosity and chemical composition on connecting rod bearing wear have been studied using single cylinder CLR engine tests. Two procedures were followed. One procedure, developed by Automotive Research Laboratories, Inc. (ALI), was used to evaluate both single grade, Newtonian oils and multigrade, non-Newtonian oils. The results indicated that there was no relationship between the high temperature, high shear rate viscosity of multigrade oils and bearing demerits. The chemical composition of the engine lubricants appeared as a primary variable which overshadowed the viscometric considerations. A more severe test procedure was developed using the single cylinder CLR engine. The results from this procedure were found to correlate with those from the ALI test programs. Chemical composition was again found to have a dominant effect on bearing wear.

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.