The effect of temperature, pressure and shear rate on the viscosity of engine oils.

摘要

The wide variation of temperature, pressure and shear rate in lubrication systems can significantly affect oil viscosity and the load bearing capacity of lubricant films. Mechanical and chemical degradation of the oil also influence viscosity through changes in the molecular weight distribution. These issues have motivated the present investigation.; Expressions are developed to describe viscosity and kinematic viscosity data over a wide range of pressures and temperatures. These correlations describe viscosity data for several mineral and synthetic oils to within 5% average deviation at pressures to 350 MPa (50 ksi) and temperatures to 373 K (100{dollar}spcirc{dollar}C). Coefficients in these expressions scale with choice of reference state. The pressure and temperature dependence of shear thinning fluids can also be described by these correlations. A capillary-type instrument is developed to obtain viscosity data for pressures to 70 MPa (10 ksi) and temperatures to 373 K (100{dollar}spcirc{dollar}C). This instrument is used to obtain viscosity data for a variety of mineral oils and binary mixtures. An expression to predict mixture viscosities over the mentioned range of pressure and temperature is presented.; A series of correlations is proposed to predict viscosity data for hydrocarbons and mineral oils in terms of the molecular weight, viscosity and density in a reference state. These expressions can predict data for several fluids to within experimental error over ranges of temperature and pressure typical of hydrodynamic lubrication.; A method of monitoring the effectiveness of lubricants during use is proposed. Several used and new oils are tested in a flow loop designed to simulate operating conditions in an internal combustion engine. The proposed technique provides a sensitive measure of changes in viscosity and composition of oils.