Soot-related oil thickening problems have been reported over the years by multiple OEMs in Europe, Japan, and in the U.S.A (1,2,3). The earliest problems, from the late 1970s, were often attributed to adverse changes in operation [lower engine speeds, heavy loads and low air/fuel ratio, or severe operation such as stop-and-go service (3)] which led to a high soot generation rate. In the late 1980s, the emission legislation became more stringent and soot-related oil thickening concerns resurfaced. It appeared that even engines that produced a relatively low level of soot in the exhaust gas showed a high level of soot contamination in the lubricant (4). For the oil and oil additive industry, the Mack T-7 engine test offered a useful tool to evaluate the ability of oils to disperse soot, but it has been noted that the industry remains without a test based on a European engine to adequately evaluate an oil's ability to disperse diesel engine soot.
Data from our recent study indicate that a useful test using a European engine has been developed to measure soot-related oil thickening, diesel engine sludge performance, and diesel engine oil gelation. Engine test results were also obtained on field-tested oils and correlation between the engine and the field test has been established. Coupling the engine test data with oil seal compatibility data from the VW 3344 laboratory bench test, a trade-off among the ashless dispersants that provide effective diesel engine oil dispersion performance and those that provide effective VW 3344 performance was observed.
*Numbers in parentheses indicate references listed at the end of the paper