Motor-Oil Characteristics and Performance at Low Temperatures 280010
RESULTS of an experimental study of the viscosity characteristics of motor oils at low temperatures and their influence upon cranking torque and circulation within the engine are presented by the authors.
At temperatures in the neighborhood of 0 deg. fahr., even oils of asphaltic origin appear to possess some plastic characteristics, while those of the mixed and paraffin-base types deviate widely from the generally accepted laws of viscous flow. Oils of these latter classes have apparent viscosities which tend to increase with decreasing shearing-stress and to become somewhat greater than might be expected from a study of their characteristics at normal temperatures. However, as resistance to cranking the engine is due mainly to oil in thin films on the cylinder walls, the relatively small temperature-viscosity coefficient of the wax-bearing oils gives them a marked advantage over those of asphaltic origin, an advantage which becomes greater as the temperature is lowered.
Circulation tests in an engine equipped with a comparatively small-mesh screen over the pump intake indicated that circulation was not obtained until the oil in the sump attained its “pour-point” temperature. In general, the work indicates that a low-temperature viscosity coefficient is highly desirable to minimize cranking effort, and that free circulation requires an oil the effective viscosity of which does not increase too rapidly at very low shearing-stresses.
The discussion includes statements that many of the experiments described in the paper have been duplicated by other experimenters and that the results confirmed those obtained by Mr. Wilkin. Another discusser comments upon the results of experiments he made in the course of an investigation conducted at the Bureau of Standards relating to the problem of the lubrication of an aviation engine during the starting and warming-up periods. The need for a cold-test oil is emphasized, and a device for measuring oil-film shear-resistance is described. Tests using electrical means of measuring breakaway torque are cited, a process for dewaxing paraffin-base oils is outlined, and comparative results of tests of an asphaltic-base and of a paraffin-base oil are shown in charts.