Lubrication is one of the major challenges faced by manufacturers of rotating machinery, and especially transmission systems, drivetrains and other gearboxes in the automotive industry. Lubrication has a direct impact on efficiency (torque) and reliability of gearboxes as it limits friction and power loss, as well as ensures cooling and heat transfer.
The diversity of involved physics makes the design of a gearbox very complex. Physics in the realm of fluid dynamics include viscosity, surface tension, thermal effects and multi-fluid effects. In addition, experiments are barely viable as high-speed rotations (typically 5,000-6,000 rpm) make observing fluid flows very difficult, even on a testbench prototype with Plexiglas.
Note that extreme rotating speeds (beyond 15,000 rpm) makes this challenge even more difficult from all points of view, as in the case of electric motors or transmission systems in aircrafts and helicopters for instance.
Thus, engineering simulation appears as the best solution to design gearboxes. Design engineers can test and compare several design choices, optimize the oil flow and quantity in order to improve design efficiency and quality while reducing manufacturing cost (less oil means less carter volume/space, so less material and weight).
However, simulating the lubrication of a gearbox remains complex, almost out of reach to many traditional Computational Fluid Dynamics (CFD) software. Indeed, traditional CFD methods like Finite Volume (FV) rely on volume meshing and have many difficulties addressing moving mechanical parts (pinions and gears), complex fluid topologies (splashing, spraying, droplets coalescence) and complex physics at play. In contrast, the Smoothed-Particle Hydrodynamics (SPH) method offers remarkable possibilities because of its particle-based and Lagrangian nature: particles naturally follow the fluid flow induced by mechanical movements.
As the SPH method is clearly identified, gearbox designers need to find the optimal CFD software that implements this method and has a proven track record of both academic and industrial cases in production at major manufacturers. Gearbox simulations have to be tested and validated on various designs and use cases, with varying rotating speeds, temperature ranges, different oil viscosities and densities. Results obtained by simulations, torque and power loss in particular, have to be confirmed by experimental measures so manufacturers can gain enough confidence on predictive results delivered by future simulations on new gearbox designs.
As an illustrative case, Renault Sport experienced gearbox overheating with their Formula E racing car on the famous Monza track (measuring +2°C per lap), making finishing a race impossible as the gearbox would eventually break. By simulating the gearbox lubrication in the extreme acceleration conditions of the “Curva Parabolica” curve, engineers realized that the sloshing of oil induced by centrifuge forces was starving some pinions of oil, causing the heat rise. Thanks to new simulations, engineers could understand and characterize the problem, and adjust the design to ensure efficient lubrication.
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