The effect of combustion on MoDTC friction reduction capabilities determined by engine tests

Molybdenum Dithiocarbamate (MoDTC) is an excellent friction modifier used in oil formulations for internal combustion energies. In the search of minimising transition metals contents in lubricating oils, extensive work has been carried out in benchtop tribometers to produce optimised formulations. However, tribometers present major limitations mimicking the extreme environment present within the engine cylinders and assessing the effect of this environment on the tribochemistry of this additive. This work presents a combined analysis using single-cylinder engine tests and surface analysis techniques: Raman and XPS. Raman microscopy maps the distribution of MoS₂ and XPS identifies all the molybdenum species formed by tribochemistry. This approach enabled assessment of the effect of the combustion on MoDTC by comparing the results of experiments executed under fired and motored conditions. Areas where the MoS₂ solid lubricant and other molybdenum species formed were identified on the liner and linked to the corresponding friction force attained during the engine test cycle. As expected, the effective MoS₂ formation on returning points on the liner delivered efficient friction reduction. Noticeable differences in the tribochemistry of MoDTC and distribution of molybdenum species were found between fired and non-fired tests. In motored tests, MoS₂ was generated in both returning points, whereas in fired tests MoS₂ was generated only at the bottom, with oxidised molybdenum species present close to the top dead centre. Tribochemistry mechanisms under fired conditions are proposed. Understanding these mechanisms constitutes an invaluable tool to transfer the results obtained with bench tribometers to real engine operating conditions.