Significance and Use
5.1 Significance of Low-Temperature, Low Shear Rate, Engine Oil Rheology—The low-temperature, low-shear viscometric behavior of an engine oil determines whether the oil will flow to the sump inlet screen, then to the oil pump, then to the sites in the engine requiring lubrication in sufficient quantity to prevent engine damage immediately or ultimately after cold temperature starting.
5.1.1 Two forms of flow problems have been identified,4 flow-limited and air-binding behavior. The first form of flow restriction, flow-limited behavior, is associated with the oil's viscosity; the second, air-binding behavior, is associated with gelation.
5.2 Significance of the Test Method—The temperature-scanning technique employed by this test method was designed to determine the susceptibility of the engine oil to flow-limited and air-binding response to slow cooling conditions by providing continuous information on the rheological condition of the oil over the temperature range of use.4 ,5,7 In this way, both viscometric and gelation response are obtained in one test.
5.3 Gelation Index and Gelation Index Temperature—This test method has been further developed to yield parameters called the Gelation Index and Gelation Index temperature. The first parameter is a measure of the maximum rate of torque increase caused by the rheological response of the oil as the oil is cooled slowly. The second parameter is the temperature at which the Gelation Index occurs.
Scope
1.1 This test method covers the measurement of the apparent viscosity of engine oil at low temperatures.
1.2 A shear rate of approximately 0.2 s-1 is produced at shear stresses below 100 Pa. Apparent viscosity is measured continuously as the sample is cooled at a rate of 1°C/h over the range −5 to −40°C, or to the temperature at which the viscosity exceeds 40 000 mPa·s (cP).
1.3 The measurements resulting from this test method are viscosity, the maximum rate of viscosity increase (Gelation Index), and the temperature at which the Gelation Index occurs.
1.4 Applicability to petroleum products other than engine oils has not been determined in preparing this test method.
