Simulated and measured characteristics for churning oil flow with a coating surface over a wide range of temperatures

Gearbox efficiency is essential for new energy vehicles. Most of the energy in a gearbox is consumed by a load-independent factor, e.g., churning loss, which, however, has not been deeply addressed. In this study, load-independent losses are inhibited by oleophobic treatment of the surfaces of a transmission component. A polytetrafluoroethylene (PTFE) coating was prepared via electroless chemical plating. The contact angle (CA) and surface morphology of the coated surfaces were tested to elucidate the loss inhibition mechanism at the oleophobic interface. A visualization test rig for measuring the churning oil flow was built with the function of controlling the temperature in the range of −30 to 80 °C. The moving particle semi-implicit (MPS) method was used to further investigate the mechanism of oleophobic surface regulation. The oil velocity and pressure distributions, slip characteristics, and flow field around the coated surface were investigated. An analytical flow model was established to determine the flow structure and quantify its relationship with oil properties and surface characteristics. The results showed that the CA increased from 7.8° to 31.2° when the surface was coated with PTFE. The average reduction in the churning torque test values ranged from 20% to 36% across a wide temperature range of −30 to 80 °C, with a maximum of 50.7% at 40 °C. The simulated slip lengths ranged from 2.9 to 16.0 μm at different rotational velocities. The coating reduced the oil velocity and pressure, as well as the viscous shear and differential pressure resistance, on the surface of the rotating component. This study thus provides scientific support for improving the efficiency of gearboxes used in complex engineering.