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Development of optical microscopy techniques enables evaluation of the dispersancy quality of new engine lubricating oils and permits prediction of sludge deposition severity before and during engine operation. By observing MIL-L-2104B engine oil samples under a microscope, size of the sludge agglomerates was related to sludge deposition potential and correlated to engine sludge ratings. Thus the prediction of engine sludging at any time during an LTD test is possible without the benefit of engine inspection. Sludge agglomeration temperature was found to be the basic criteria for determining dispersant quality of an oil. Using this principle and the microscope, a method was developed for screening new dispersant oils. Correlation of analytical and fleet ratings are also presented.

Under low temperature engine operation, sludge appears as insolubles in the lubricating oil or as deposits on engine parts. Its formation is initiated by liquid oxidation products, inorganic salts, and polymerized organic compounds that pass the piston ring zone. The liquid oxidation products undergo further chemical reaction in the crankcase oil medium, forming solid “sludge binders.” These “binders” are the essential ingredients that the oil must contain before the organic solids (carbonyls, sulfur, and nitrogen derivatives and polymerized hydrocarbons), inorganic salts, wear particles, and soot can be deposited as sludge.