Deposit formation in gasoline engines: Part 2. Dispersant effects on sequence VE deposits

The effects of dispersant concentration, basicity, and molecular weight on ASTM Sequence VE engine sludge and varnish control are reported. The lubricants used an American Petroleum Institute (API) SG/CD-derived performance package with differing amounts and types of experimental dispersants. After determining the effect of dispersant type and concentration on deposit formation, a second set of engine tests was run on an API SF formulation to verify the observations. Viscosity increase and insolubles content are shown to be poor predictors of sludge performance. Rather, particle growth and eventual particle size determine Sequence VE engine sludge severity. In turn, dispersant level and dispersant chemistry control particle size growth. For the systems studied, insufficient dispersant level, dispersant basicity, or dispersant molecular weight contribute to excessive particle growth and engine sludge formation.