The Bearing Oil Film Thickness of Single and Multi-Grade Oils—Part 1: Experimental Results in a 3.8L Engine

Bearing oil film thickness (BOFT) measurements were obtained in the front main bearing of a 3.8-liter engine. Engine speed, engine load, and oil temperature were varied to determine the effect of these parameters on BOFT. For single-grade engine oils, the minimum oil film thickness (MOFT) correlated with the Sommerfeld number (Speed × Viscosity/Load). In addition, it was also determined that the type of dispersant package used in a particular single-grade oil did not affect the oil's MOFT values.

For multigrade oils (SAE 5W-30, 10W-30, 10W-40, and 15W-40) MOFT values could not be related to the Sommerfeld number with a high degree of correlation. Although high-shear, high-temperature (HSHT) viscosity was found to be important, the viscosity index (VI) improver type was also a factor in determining MOFT. The contribution of VI improver to MOFT was found to be dependent on the SAE grade and VI improver type.