EXPLANATION
The low temperature, low shear viscometric behavior of an engine oil, whether new, used, or sooted, 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. Two forms of flow problems have been identified: 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.
The temperature scanning technique employed in this test method was designed to determine the susceptibility of the engine oil to the 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. In this way, both viscometric and gelation response are obtained in one test. This test also yields information on parameters called 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 slowly cooled. The second parameter is the temperature at which the gelation index occurs.
A shear rate of approximately 0.2 per sec is produced at shear stresses below 200 Pa. Apparent viscosity is measured continuously as the sample is cooled at a rate of 3 ° C per hour over the range of -5 to -40°C. Applicability of this test method to petroleum products other than engine oils has not been determined.
TEST SUMMARY
Used and sooted engine oils are analyzed using a special rotational viscometer with analog or digital output to a computer program. A specially made glass stator/metal rotor cell is attached to the viscometer and subjected to a programmed temperature change for both calibration and sample analysis. Following calibration of the rotor/stator set, an approximately 20 mL test sample of a lubricating test oil is poured into the stator and preheated for 1.5 to 2.0 h in an oven or water bath. Shortly after completing the preheating step, the room temperature rotor is put into the stator containing the heated oil and coupled to a torque sensing viscometer head using an adapter to automatically center the rotor in the stator during test. A programmable low temperature bath is used to cool the cell at a specified rate of 3 ° C/h from -5 ° C to the temperature at which the maximum torque recordable is exceeded when using a speed of 0.3 r/min for the rotor. After the desired information has been collected, the computer program generates the desired viscometric and rheological values from the recorded data.
TEST PRECISION
Since there is no accepted reference material suitable for determining the bias of this test method, no statement on bias is being made.