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

Gearbox efficiency is essential for the range of new energy vehicles. Most of energy in gearbox is consumed on the 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 to the surfaces of a transmission component. A polytetrafluoroethylene (PTFE) coating is prepared by electroless chemical plating. The contact angle (CA) and surface morphology of the coated surfaces are tested to elucidate the loss inhibition mechanism on the oleophobic interface. A visualized test rig for measuring the churning oil flow is built with the function of temperature control at a range of -30℃-80℃. Moving Particle Semi-implicit (MPS) method is used to further investigate the mechanism of oleophobic surface regulation. The oil velocity and pressure distributions, the slip characteristics and flow field around the coated surface are investigated. An analytical flow model is established to demonstrate the flow structure and to quantify the relationship with oil properties and surface characteristics. The results showed that the CA increased from 7.8° to 31.2° when the surface is coated by PTFE. The average reduction of the churning torque test values ranged from 20% to 36% across a wide temperature range of -30℃ to 80℃, with a maximum of 50.7% at 40℃. Simulated slip lengths ranged from 2.9 μm 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 the efficiency improvement of gearboxes used in complex engineering.