Correlation of the Dielectric Constant and Conductivity of Marine Motor Oils

The study of marine lubricants using modern methods for quality control and diagnostics of engine conditions is an important task. The solution to this task determines the technical condition of ships, their performance, and accident-free operation in maritime transport. In this study, the properties of several marine motor oils were studied using broadband dielectric spectroscopy. The complex dielectric constant, conductivity, and loss tangent of base oil, Shell Rimula 15W40 oil fresh and used for 250 and 500 h were studied. The influence of the test voltage frequency and temperature on the selected electrical parameters was determined. In used oils, the DC conductivity decreases slightly, which is apparently due to the depletion of additives. The increase in the conductivity of all studied oils with increasing current frequency is caused by the displacement current. It is shown that dielectric losses increase with frequency as long as the polarization has time to follow the change in the field. A shift in the frequency at which the maximum loss occurs with a change in temperature was discovered. The activation energies of polarization and conductivity were determined. The thermal activation energies obtained from the conductivity and dielectric constant are not significantly different. For the first time, the distributions of the dielectric constant relaxation times for the studied lubricating oils have been obtained. The dielectric relaxation time distribution function depends on the composition of additives in the oil, wear particles, contaminants, and the chemical degradation of the oil during operation. For the first time, correlations have been established between conductivity and dielectric constant at different temperatures and frequencies. These correlations can be used to select frequency ranges that provide stable parameter values and maximum diagnostic ability.