Evaluation of the Potential Use of Thermal Conductivity and Breakdown Voltage to Determine the Degree of Lubricating Oil Dilution with Diesel Oil

The dilution of lubricating oil with diesel oil (DO) is a significant operational problem in piston combustion engines, as it degrades lubrication conditions and may accelerate the wear of interacting components. This study aimed to evaluate the usefulness of selected thermophysical and electrical properties of lubricating oil for determining the degree of its dilution with diesel oil. The tests were conducted on mixtures of SAE 30 or SAE 40 lubricating oil with diesel oil over a concentration range of 0–100% m/m of the latter material. Changes in thermal conductivity, thermal effusivity, and breakdown voltage were examined as a function of the mixture’s fuel content. The thermal conductivity and effusivity of the tested oils were measured using the MTPS (Modified Transient Plane Source) transient method, while the breakdown voltage of the tested oils was measured at mains frequency using an apparatus in which the oil sample was exposed to an increasing electric field by gradually increasing the alternating voltage at a constant frequency until electrical breakdown occurred. An increase in the proportion of diesel oil caused a systematic linear decrease in thermal conductivity and thermal effusivity. A decreasing trend was also observed for breakdown voltage; however, this parameter exhibited significantly greater variation in results. The results indicate that thermal conductivity and thermal effusivity are more useful for assessing the degree of dilution of lubricating oil with DO than breakdown voltage.