ASTM D4684 for yield stress and apparent viscosity of engine oils
ASTM D4684 standard test method for determination of yield stress and apparent viscosity of engine oils at low temperature
3. Summary of Test Method
3.1 An engine oil sample is held at 80°C and then cooled at a programmed cooling rate to a final test temperature. A low torque is applied to the rotor shaft to measure the yield stress. A higher torque is then applied to determine the apparent viscosity of the sample.
4. Significance and Use
4.1 When an engine oil is cooled, the rate and duration of cooling can affect its yield stress and viscosity. In this laboratory test, an engine oil is slowly cooled through a temperature range where wax crystallization is known to occur, followed by relatively rapid cooling to the final test temperature. These laboratory test results have predicted as failures the known engine oils that have failed in the field due to the lack of oil pumpability. These documented field failing oils have all consisted of oils normally tested at -25°C. These field failures are believed to be the result of the oil forming a gel structure that results in excessive yield stress or viscosity of the engine oil, or both.
4.2 Cooling Profiles:
4.2.1 For oils to be tested at -20°C or colder, Table X1.1 applies. The cooling profile described in Table X1.1 is based on the viscosity properties of the ASTM Pumpability Reference Oils (PRO). This series of oils includes oils with normal low-temperature flow properties and oils that have been associated with low-temperature pumpability problems (1-5). Significance for the -35 and -40°C temperature profiles is based on the data collected from the "Cold Starting and Pumpability Studies in Modern Engines" conducted by ASTM (6,7).
4.2.2 For oils to be tested at -15 or -10°C, Table X1.2 applies. No significance has been determined for this temperature profile because of the absence of appropriate reference oils. Similarly, precision of the test method using this profile for the -10°C test temperature is unknown. The temperature profile of Table X1.2 is derived from the one in Table X1.1 and has been moved up in temperature, relative to Table X1.1, in consideration of the expected higher cloud points of the viscous oils tested at -15 and -10°C.
