Measuring the Effect of Oil Viscosity on Oil Film Thickness in Engine Journal Bearings

The minimum oil-film thickness in the front main bearing of a 3.8 L, V-6 engine was measured at 3 000 r/min, and 140 N·m using an electrical resistance technique. For a series of seven Newtonian, single-grade oils, film thickness correlated with oil viscosity measured either in a kinematic or in a high-shear-rate viscometer. For a series of fifteen polymer-containing, non-Newtonian, multigrade oils, however, no single measure of viscosity adequately correlated with film thickness for all of the oils. By eliminating four multigrade oils from the combined single and multigrade data sets, it was possible to correlate film thickness to the viscosity (of the remaining multigrade and Newtonian oils) measured at 150°C and 5 × 10⁵ s⁻¹, conditions which are believed to be representative of temperatures and shear rates in bearing oil films. Possible explanations for the lack of correlation with the entire set of twenty-two oils are discussed.