Low friction tribofilm formation and distribution on an engine cylinder tested with MoDTC-containing low viscosity engine lubricants

The piston-liner tribological contact has a key role in controlling friction caused energy losses in a passenger car engine. This study analyses the MoDTC-containing low viscosity lubricant performance and the resulting low-friction MoS2 tribofilm distribution on the liner surface. The engine liners were tested in a floating liner measurement system, where fully-formulated lubricants with different MoDTC concentrations were utilised to assess the impact of friction modifiers on the friction performance. Tested engine liners were dismantled from the measurement system, and MoS2 tribofilm formation on the liner was quantitatively characterised by Raman Microscopy mapping. The MoS2 tribofilm chemical composition and lamellar structure were further investigated by Transmission Electron Microscopy (TEM), High-Angle Annular Dark-Field Scanning (HAADF) and Energy Dispersive X-ray (STEM-EDX) Microscopy. The findings presented in this paper confirm that MoS2 tribofilm spatial distribution plays a pivotal role in reducing friction in piston-liner contact. 60% friction torque reduction was achieved at BDC area with 0.7% MoDTC concentration in the fully formulated lubricant. Moreover, MoS2 presence in the middle of the liner indicates MoDTC effectiveness throughout the whole engine cycle. Therefore, contributing to the overall energy savings in passenger cars.