TRIBOLOGY
Modern tools for tribological optimisation of EV transmissions: From basic tribology to digital twins
Boris Zhmud, Tribonex AB, and Michele Merelli, Particleworks Europe, EnginSoft
Today’s automotive engineering relies heavily on the use of computer-aided engineering (CAE) tools. To improve the efficiency of increasingly complex product development processes, a significant effort has been put in development of versatile CAE systems that allow dynamic simulation of engineered appliances – e.g. an engine or a transmission - by integration of computer aided design (CAD) and computational fluid dynamics (CFD) modules, laying a foundation for simulation-based design (SBD). Fundamentally, this approach implies coupling between the underlying fluid dynamics, solid mechanics, and mass and heat transfer equations to obtain a self-consistent solution describing the complex system dynamics. The introduction of meshless CFD methods in the 1980s-1990s, paralleled by increasing computer power, has revolutionised this field.
A new term was born: digital twin. A digital twin is basically a sufficiently accurate computer model of the actual appliance under development that allows virtual testing of its functionality (hardware prototyping). Despite a significant progress in development of digital twins, current models used in automotive engineering still fail to achieve sufficient predictive power and largely ignore numerous tribological effects that may have a large impact on the appliance function and longevity. For instance, a CFD model that may sufficiently accurately describe the oil flow in an EV reduction gearbox is unlikely to differentiate between, say, API GL-5 gear oil and ATF of similar viscosity grade despite enormous differences between these
two products. Macro-level models also have limited capacity in sensing effects of surface finish.
At the other end of the spectrum, we have micro-level models aiming to describe the tribology of lubricated contacts. These models use the Reynolds equation, which is a simplification of the Navier-Stokes equation of fluid dynamics that neglects inertial and gravity effects.
Such a simplification is valid because thin film lubricant flow remains laminar even at high sliding velocities. The corresponding lubrication regime in which the mating surfaces separated by a thin lubricant film experience elastic deformation under load is called elastohydrodynamic (EHD) lubrication. More general thermal elastohydrodynamic (TEHD) models further factor in heat transfer phenomena. The temperature distribution in the lubricant film is described using the heat transfer equation including sources due to shearing and compression of the lubricant. Heat transfer between the solids and the lubricant is realised using heat flux continuity conditions. In some models, energy dissipation due to plastic deformation of solids is also taken into account.
Elasticity of solids in the contact and the dependence of lubricant viscosity on pressure have significant effects on EHD lubrication and are ultimately related
Continued on page 14 LUBE MAGAZINE NO.181 JUNE 2024 13
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