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.

High-Pressure Density, Viscosity, and Modeling of Mixtures of a Poly(α-olefin) Base Oil Lubricant with Polymeric Additives

“…Densities for PAO 4 across this same region were previously measured 19 and modeled with the S−L EOS. 22 Figure 2 shows the effect of temperature and pressure on the density for the mixtures containing approximately 0.7 mass % of each polymer additive in PAO 4. Figure 3 compares the effect of these polymeric additives on the base oil densities as a function of the addition level at 323 K. Tables 1 and 2 show the fitted S−L EOS parameters for these mixtures, along with RMSE, % AAD, and % .…”

“…The details of the viscometer have been previously described in an earlier publication. 22 The rotating shaft is magnetically coupled to a ThermoFisher Haake Viscotester 550 (±0.00015 N m) which is used as the torque transducer and the motor. Viscosity is determined from measured torque required to rotate the cylindrical shaft at a given rotational speed.…”

“…Viscosities for pure PAO 4 had been previously measured using the same instrument and have already been reported. 22 2.4. Modeling of Volumetric Properties with the Sanchez−Lacombe Equation of State and the Viscosity with Free Volume and Scaling Theories.…”

“…Density data were fit to the Sanchez−Lacombe equation of state (S−L EOS) as was employed following the procedures described in our earlier publication in connection with modeling the pure base oil PAO 4. 22 The basic equation is given by i…”

“…Base oils, including PAO 4, have been previously modeled using the free volume theory. 22 In its simplest form, the wellknown Doolittle equation 28 describes the viscosity in terms of free volume…”