Electric engine cooling and lubrication

 

Electric engine is a key point of next-gen powertrain. It is important to well design its cooling and lubrification to improve efficiency and durability.

Hopefully CFD is a great tool to help in this task!

Thanks to Valentin Bonnifet, Field Application Engineer by Nextflow Software our partner, that created news to present its activity.

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Oil jet under aerodynamic load

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Global warming limitation has become the mankind’s major challenge for the coming years. Among the large lifestyle and technology changes that need to be undertaken, electric vehicles (EVs) coupled with de-carbonated electricity production represent a foreseeable future for personal transportation. Since the beginning of this decade, the world has come to the verge of a new age of private transportation with the advent of a new generation of EVs. Although the first road vehicle to reach 100km/h in 1899 was the electric-powered car named “la jamais contente”, thermal vehicles have been prevailing during the following century. Some years ago, the electric technology was still limited to concept-car prototypes, public transportation and some few restricted range delivery vehicles. Indeed, this previous generation of EV suffered from its relatively low autonomy range, compared to conventional thermal vehicles. However, the recent Lithium battery improvements changed the game and is boosting EVs as alternatives to thermal vehicles for a daily usage.

Battery is not the only area for improvement: electric engine (e-motor) also needs a careful attention. Indeed, e-motors must not only be powerful, efficient, lightweight, quiet, robust and affordable, but also able to deliver high torque for a wide speed range to ensure driving comfort. Anyone who believes that an e-motor is as simple as a rotor shaft surrounded by copper wires spooled around stator cores underestimates the extent of physical phenomena that occur in such a system. Its design requires a large panel of numerical tools, able to cover mechanical, electro-magnetic, acoustic, thermal and internal aerodynamic analysis. In such system, physics interconnections are strong such as coupling between mechanical resonance, electro-magnetic waves and air acoustic propagation, but also coupling between thermal dissipation, Joule effect and rotor induced aerodynamic flow. Those multi-physics interconnections make e-motors study challenging.

Even though e-motors efficiency (about 90%) remains better than thermal engine (about 40%), an important amount of loss, converted into thermal power, must be dissipated. The direct air-cooling approach is not compact enough for modern EV requirements, and liquid-based technology is needed. The rotor, which is warmed by induced Foucault current, is not efficiently cooled down by legacy liquid cooling systems and its so-called “water jacket” around the engine casing. To cross the air gap that thermally insulates the rotor from the stator, what an easier way than directly supplying the rotor with coolant! Of course, water cannot be used as coolant, since it conducts electricity. However, oil would be a perfect candidate as it is a non-conductive material, and also addresses both cooling and lubricating needs.

Two main oil injection technologies are present on the market. Centrifugal oil injection through the drilled shaft are suitable to e-motors embedded in wheel. Nozzles fastened to the e-motor side covers are often dedicated to e-motors fastened to car frame. While the first injection technology uses the centrifugal effect to widely spray the internal engine parts at any rotating speed, the second allows a more accurate focus on warmest parts. The present article focuses on the latter technology: injection nozzles fastened to e-motor side covers.

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Computational fluid dynamics (CFD) analysis of e-motor cooling/lubrication systems may provide relevant information to help engineers enhance the engine robustness under mechanical and thermal loads. Whatever the chosen CFD strategy, some assumptions must be made on the multi-physics coupling. Such simplification process makes the digital twin more affordable for iterative design process purposes. Due to the high rotating speed (nominal range from 14,000 to 18,000 rpm) and the complex geometry of rotor coil, the chaotic shape of the jet and the atomization into droplets after impact can be observed on the figure above. This chaotic behavior prevents simulation engineers from making use of geometry symmetries for problem reduction.

To perform oil injection study and simulate its multi-phase behavior, conventional Finite Volume (FV) multi phase solvers must use Volume Of Fluid (VOF) or Level-Set techniques. However, such approach is computationally intensive, since both mesh refinement and mesh overset techniques are needed around the oil free-surface and the rotating bodies respectively. In contrast, Lagrangian approaches can intrinsically capture complex free surfaces dynamics. Moreover, the use of a particle-based solver would save the user from doing a tricky and fastidious volume meshing operation. Computations in this article have been performed using the SPH-flow solver, based on the Lagrangian Smooth Particle Hydrodynamics (SPH) method. The video below shows the time-depend field of particles in the entire fluid domain. Note that droplet velocity magnitude can reach 150 m/s downstream rotor impact!

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Multi phase solvers allow to compute the mutual interactions between the oil jets and drops and the rotor-induced airflow. Due to the high rotating speed of EV motors, the airflow velocity can exceed 100 m/s, and its influence on one-millimeter-large oil droplets must not be neglected. However, although influence of air on oil is obvious, the other way around is not. Consequently, a unidirectional weak coupling can be performed: a standalone, oil-free, airflow is computed first. Then, its results can be imposed as external input into an oil flow computation. An example of such an airflow is presented above where streamlines are colored by velocity magnitude. Such coupling strategy has been made possible thanks to the ability of the SPH method to deal with monophasic free-surface flows.

In an upcoming article, the focus will be put on another electric engine cooling and lubrication technology: the oil injection through shaft, which benefits from the centrifugal acceleration to spray the rotor and stator coils.

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