Automatically Assessing Low-Temperature Low-Shear Viscometric Response of Engine Lubricants with Decreasing Temperature

Abstract

Successful low-temperature operation of engines depends on the lubricant being able to flow to the oil pump. ASTM D5133, Standard Test Method for Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils Using a Temperature-Scanning Technique, is one of two tests the industry uses to assess whether the engine’s oil pump will be able to lift the lubricant from the sump at low temperatures. This test method requires several distinct steps to accomplish the required thermal conditioning. Accomplishing these steps in a consistent manner is necessary to minimize the variation in test results. Currently, ASTM D5133 has a repeatability of 16 % and a reproducibility of 29 %. This study demonstrates that automating the thermal conditioning steps yields better precision. Based on the samples tested, the automated procedure reduced the variability in test results by approximately 50 %. The data obtained show that the automated method provides a clearer distinction between lubricants whose flow properties were above the specification limit while having little impact on those at or below the specification limits. As a part of this study, the impact of small changes in shear rate was evaluated using the automatic procedure. Poor flow of lubricants at low temperatures can be due to the presence of wax crystals. ASTM D5133 uses a cylindrical rotor with a narrow gap between it and the stator, thus limiting crystal size. A portion of this study evaluates the impact of replacing the cylindrical rotor with a vane rotor. These measurements indicated that a vane rotor yields a stronger response for those samples with a test value greater than the specification limit but not those at or less than the limit.