High Pressure Volumetric Properties and Viscosity of Base Oils Used in Automotive Lubricants and Their Modeling

Abstract

Temperature and pressure effects on density and viscosity are reported for six base oils consisting of four mineral oils and two synthetic oils composed of poly(α-olefins). Using a variable-volume view cell, density data were collected at 298, 323, 348, 373, and 398 K from 10 to 40 MPa. The data were then fit to the Sanchez–Lacombe equation of state and used to determine isothermal compressibility, isobaric thermal expansion coefficient, and internal pressure. Compressibility and internal pressure were found to vary based on composition, specifically with cycloalkane content. Viscosity data were collected as a function of temperature, pressure, and rotational speed using a custom-built high pressure rotational viscometer. Data were collected at 298, 323, 348, and 373 K from 10 to 40 MPa at rotational speeds of 300–800 rpm. A free-volume model was used to model the viscosity and relate the viscous effects to density.