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Soot was recovered from end-of-test (EOT) oils from several different Diesel engines. After resuspending the recovered soots in fresh high dispersancy engine oil, the infrared absorptivities of the soots were determined over a range of frequencies. On the basis of a statistical analysis of the data, recommended 1870 cm-1 absorptivities for soots from various engine tests are: GM 6.2 L, Mack T-8, Cummins L-10 HST, 59.0±0.5; GM 6.2 L, Mack T-8, Caterpillar 3176, 0.02% sulfur fuel, 58.2±0.4; Caterpillar 3176, 0.02% sulfur fuel and 0.2% sulfur fuel, 56.7±0.3; Caterpillar 3116, 53.3±0.4 and OM 602A, 47.8±0.2. In all cases, the units for the absorptivity are cm2/centigram. Over the range of 3800 cm-1 to 1870 cm-1, the dependence of absorptivity on frequency was linear for all the soots. Use of these absorptivities to determine soot concentrations in used engine oils is discussed. Of particular concern are the procedures required to obtain reproducible results.

“Oil thickening” due to soot has long been recognized as a potential problem in diesel engine lubrication. Soot-related viscosity increase in used crankcase oils is remarkable in that relatively small concentrations of soot produce large viscosity increases. Classical studies of suspensions of rigid nonagglomerating spheres in liquids suggest that the viscosity increase produced by 6% (by weight) soot in an oil should be less than 10%. In fact, the viscosity increase in such a mixture may be as much as several hundred percent. This discrepancy implies that the effective volume occupied by soot particles suspended in oil is very large. Past theoretical and experimental work on the rheology of particle suspensions provides a method of quantitatively determining the effective volumes (“voluminosities”) of soots in oil suspensions. Three different used oils were characterized by controlled shear rate rheometry.