Development of Mini-Rotary Viscometer Measurement Techniques for Highly Sooted Diesel Engine Oils 2002-01-2795

In 1999 the ASTM Low Temperature Rheology of Used Engine Oils or LOTRUO task force was formed within Subcommittee D02.07 to address potential method and measurement issues for low temperature rheological determinations of used engine oils. A primary focus of this task force was to assist the heavy duty engine oil classification panel which was developing the new PC-9 category, to include a used oil pumpability specification from one of the new multicylinder EGR diesel engine tests. With anticipated soot loadings of 5-10% in some of these used oils, there was concern that standard ASTM test methods developed for fresh oil pumpability might not be suitable for these sooted oils. The task force conducted some preliminary work on a used Mack T8E test sample of approximately 5% soot loading. These data indicated that variation in preheating conditions could have significant influence on the low temperature properties measured by standard procedures. Additional rheometric studies showed that quiescent heating was probably building structure in the oil due to soot agglomerization.

To address these concerns, a modified MRV procedure was developed in which the sample is heated externally, then agitated to remove any soot-agglomerized structure, and then cooled via the ‘TP-1’ cooling portion of the standard D4684 procedure. At about this time, additional end-of-test drain oils became available to the group from Mack T10, Cummins M11-EGR and Mack T8/T8E engine testing during the PC-9 precision matrix. These samples ranged in soot content from 5 to 10% and showed a good range of viscometric response. An interlaboratory round robin study was initiated in late 2000 on nine samples. Both the standard D4684 method and the ‘external preheat’ MRV were run at -20°C and -25°C on the samples. Analysis of the data indicated close agreement in measured viscosities for the same oil measured by either method, although the yield stress values were generally lower in the case of the external preheat. Precision statements at -20 and -25°C for viscosity and yield stress have been developed for both methods with the used oils, and a section dealing with sooted diesel engine oils has been added to revise D4684; the external preheat has been drafted into a new proposed ASTM D02 standard test method.

Follow-up work at one of the LOTRUO test labs on many of the end-of-test samples from the PC-9 precision matrix showed an excellent correlation between viscosity as measured by D4684 and by the external preheat. The latter has been adopted as the preferred method for used oil analysis of 75 hr Mack T10 samples as specified in the PC-9 (CI-4) category