EXPLANATION
When an engine oil is cooled, the rate and duration of cooling can affect its yield stress and viscosity. In this test method, used 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. As in other low temperature rheological tests such as Test Methods D3829, D4684, and D5133, a preheating condition is required that all residual waxes are solubilized in the oil prior to the cooldown (i.e., remove thermal memory). However, it is also known that highly sooted diesel engine oils can experience a soot agglomerization phenomenon when heated under quiescent conditions. The current method uses a separate preheat and agitation step to break up any soot agglomerization that may have occurred prior to cooldown. The viscosity of highly sooted diesel engine oils as measured in this test method have been correlated to pressurization times in a motored engine test.
This test method covers the measurement of the yield stress and viscosity of engine oils after cooling at controlled rates over a period of 43 or 45 hours to a final test temperature of -20 or -25° C. The viscosity measurements are made at a shear stress of 525 Pa over a shear rate of 0.4 to 15 s(-1). This test method is suitable for measurement of viscosities ranging from 400 mPa•s to >400,000 mPa•s, and is suitable for yield stress measurements of 7 Pa to >350 Pa. The test method is applicable for used diesel oils. Other used or unused engine oils or other petroleum products have not been tested by this procedure.
TEST SUMMARY
A used engine oil sample is heated at 80° C and then vigorously agitated. The sample is 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.
TEST PRECISION
Where X is the mean of the results.
Since there is no accepted reference material suitable for determining the bias for this test method, no statement on bias is being made.