Volume 4, Issue 10 (November 2007)
Viscometric Temperature Sensitivity of Engine Lubricants at Low Temperature and Moderately High Shear Conditions
Low-temperature viscosity of engine oils is a key indication of a lubricant’s capacity to provide wear protection during starting and subsequent operation of an engine. Some 40 years ago the Cold Cranking Simulator (CCS) was developed to measure the low-temperature viscosity of engine oil with viscometric conditions similar to a starting engine. The importance of cranking viscosity has grown since the test was originally developed and is now one of two low-temperature viscosity measurements that define the SAE grades for engine oils. Previous studies evaluating viscosities determined by the CCS have focused on test precision and correlation to engine starting performance. This study evaluates the effects small offsets from the indicated temperature have on the measured apparent viscosity of engine oils. Interest in this topic was driven by the observation that some engine oil formulations have abnormally high variation in viscosity. All the low-temperature viscosity measurements were made in the CCS in accord with ASTM D 5293-04. A small temperature offset was achieved by adjusting the CCS temperature probe calibration so that the indicated test operating temperature was offset from the true temperature. The viscosity data were collected using automatic thermoelectrically cooled CCS instruments. Comparative viscosity data were collected on instruments using cold methanol to control the sample temperature. The oil samples in this study consisted of base oils, a selection of commercial engine oils of the API performance category SL, recent ASTM Interlaboratory Crosscheck Program (ILCP), and from the Low Temperature Engine Performance (LTEP) study which was conducted in the 1990s. The LTEP oils are of an earlier performance category and thus have a different composition than either the API SL commercial engine oils or the ASTM ILCP program oils. Results of this study show that when the CCS stator and sample are warmer than the indicated temperature, the measured viscosities are higher than when the viscosity is measured at the correct (true) temperature. As would be expected, the opposite response is seen when the offset is in the opposite direction. This response to the temperature offset is opposite of what would be traditionally expected—lower temperatures typically result in higher measured viscosities. As seen in the study, this is a result of the way the instrument is calibrated and not a fluid anomaly. Some of the API SL oils exhibited more than a 5 % change in measured viscosity from a 0.5°C shift in temperature. Base oils and synthetic formulations only had a change of around 1 % due to a 0.5°C shift in temperature. The samples of API SG oils have temperature sensitivity lower than the API SL oils tested. This study compares data obtained on instruments using two different methods of controlling sample temperature. For the samples evaluated in this program, the data indicated no relative bias between the two methods (thermoelectric and cold methanol) of stator temperature control.