The degradation and analysis of lubricating oil in diesel engines
Investigations into lubricating oil degradation and engine wear were carried out during the course of this work. Lubricating oil degradation models were studied and verified. This study has resulted in the development of an integrated, miniaturised, automated oil analysis system for the determination of water content, Total Base Number and Total Insoluble Matter for new and used oils. The study on the engine wear and oil degradation was based on two air- cooled Petter AA1, single cylinder engines running on a 1% sulphur diesel fuel and a minimum military specification oil (OMD 75). The results obtained indicated that the combination of engine operating conditions, diesel fuel and lubricating oil used, resulted in the premature loss of oil consumption control due to severe bore polish which was caused by corrosive wear. Oil analysis methods were developed requiring minimal quantities of oil sample in order not to disturb the engine/lubricating oil system integrity. The oil analysis methods developed were automated into a single integrated instrument for the determination of water content, Total Base Number and Total Insoluble Matter for new and used oils using 1gram (or less) of oil sample and with analysis times in the region of three minutes for doing all three analysis. A detailed study into new and used formulated oil chemistry revealed that the measurement of Total Base Number as determined by the Institute of Petroleum standard test methods (IP 177 and IP 276) provided information that were overestimates of the true levels of basicity of the oil. A new technique, based on the same chemistry as the original (IP) method, was developed for the Total Base Number determination with improved end-point detection for not only new oils, but more importantly for used oils as well. Other oil analysis data obtained during this study revealed the need to modernise the existing Industry test methods with regard to chemistry, equipment and functionality, for example Total Acid Number determination. The data obtained from the analysis of the used oil was studied and the rates of change of each of the measured parameters were calculated, based on the Quasi-Continuous Stirred Tank Reactor model. The data obtained from this model was compared to . two previously proposed models which indicated that the QCSTR model is a better model for representing the degradation of lubricating oils in running engines. This work has shown the need for the trapping of oil from the top-ring zone to understand the nature of the degradation and interaction of this oil with that (bulk) oil in the sump in order to successfully model the degradation characteristics of the oil.