The two-way full oil cooling technology was exposed at Xiaomi's automotive technology conference last year. This is a novel solution for motor oil cooling with higher heat dissipation efficiency. Now, let's take a brief look at liquid cooling and some innovative oil cooling solutions, and introduce bidirectional full oil cooling technology.
Liquid cooling and heat dissipation are the mainstream choice for high-power motors
In order to improve the performance of electric vehicles, electric motors are moving towards higher power, and heat dissipation is one of the current challenges. When the temperature inside the motor rises to a certain level, the lubrication effect of the bearing and the insulation function of the internal components will be destroyed. The higher the power of the motor, the more powerful the cooling solution is required. Therefore, improving heat dissipation efficiency has become an indispensable part of motor research and development.
Commonly used heat dissipation methods are air cooling and liquid cooling. Among them, air cooling is suitable for low-power motors due to its cost advantage. Liquid cooling, on the other hand, has a stronger heat dissipation capacity and is the mainstream choice for high-power motors.
Liquid cooling is further divided into two types of heat dissipation: indirect liquid cooling (water cooling) and direct liquid cooling (oil cooling).
Indirect liquid cooling arranges the cooling water channel in the stator windings of the motor, and cools and dissipates heat through the cooling water circulation. Water is electrically conductive, so it cannot be in direct contact with the motor, and the heat dissipation is relatively poor, so it cannot be applied to ultra-high-power motors.
Direct liquid cooling is subdivided into two forms: spray liquid cooling and immersion liquid cooling. Both use non-conductive and non-magnetic oil as the medium, so they are also called oil cooling. The former sprays cooling oil on the stator and rotor inside the motor, while the latter uses oil ducts to feed cooling oil into the stator and rotor. Both technical routes allow the cooling oil to be in direct contact with the components, and the heat dissipation effect is strong. In addition, direct liquid cooling can also make the motor structure more compact, catering to the trend of electric drive integration.
Innovative oil cooling solution
At present, some oil-cooled motors are innovative in terms of oil circuit design. For example, BorgWarner's centripetal oil cooling, Renault CMF's rotor direct injection, Schaeffler's in-tank oil cooling, Mahle's "immersive" cooling, Hasco Electric's grid waterfall oil cooling, and Xiaomi Auto's two-way full oil cooling.
Bi-directional full oil cooling technology
At the end of last year, the high-speed Xiaomi super motor V8s was exposed at the Xiaomi automotive technology conference, and the two-way full oil cooling technology was also announced. The highlights of the technology are the dual-circulation oil circuit, the "stepped" dislocation design of the lamination group, and the S-shaped three-dimensional channel of the rotor.
Immersion liquid cooling generally uses a single oil inlet and outlet for cooling. As the temperature of the cooling oil gradually increases, the heat dissipation becomes worse and worse, resulting in uneven temperature distribution throughout the motor.
The bidirectional oil cooling technology improves this situation by innovating the oil circuit design and changing the placement of the lamination group. Two oil inlets and oil outlets are arranged on both sides of the stator core of the motor, which are arranged alternately. The cooling oil flows from both sides into the cooling channel inside the stator core at the same time, resulting in a more uniform temperature distribution. The stator core is composed of laminated groups stacked on top of each other. Each adjacent stack is not aligned, but slightly misaligned, allowing the cooling oil to reach more area as it flows through it, enhancing heat exchange. For the rotor part, Xiaomi Auto has designed an S-shaped oil circuit, showing a spiral three-dimensional effect. For the same volume, this form of heat dissipation area is larger. Officially, after applying this series of designs to the motor, the temperature can be reduced by up to 30 degrees, and the comprehensive heat dissipation efficiency can be increased by more than 50%.
epilogue
Innovative oil cooling usually improves the design of the oil circuit to improve the heat dissipation efficiency. However, the current novel design is still relatively few, because the complex oil system design and high process level requirements are still difficult for the industry to challenge. It is believed that as more and more high-power motors are introduced, the progress of oil cooling technology will be significantly accelerated.