Comment: Why oil-cooled motors are key to next-gen EV propulsion

Just like internal combustion engines, electric motors generate a considerable amount of heat during operation, even if in a much lower quantity. In electric vehicle motors, electromagnetic losses result in the generation and dissipation of energy in the form of heat. These losses are primarily caused by resistance encountered by the flow of electric current through conductive components and variable magnetic fluxes through magnetic components in the motor.

To improve the efficiency, longevity and performance of EV motors, it is essential to reduce these losses. As such, we are examining the optimal solution for oil-cooled motors that enables delivery of the same power output as larger units, but in a smaller, lighter, more-affordable package.

To avoid causing material degradation or demagnetisation, EV motors should be kept at less than 180° Centigrade, allowing the various components to operate at their optimal temperatures. With the highest temperature appearing in the winding copper core shrouded by an insulation layer which leads to poor heat dissipation, innovative cooling directs the fluid exactly where it’s required.

When looking at approaches to direct motor cooling, there are three methods that we’re currently researching and developing, all of which provide enhanced thermal management to improve efficiency and performance.

The first method is manifold dripping cooling, which precisely and uniformly directs cooling fluid on to the motor winding heads. This method requires less cooling fluid – rather than ‘bathing’ the rotor – and therefore removes parasitic losses due to drag, however does require a higher-pressure oil pump.

The second method - shaft centrifugal cooling - offers the same cooling capacity as other solutions, with significantly reduced oil flow rate. Cooling using the rotor-shaft, the centrifugal force sprays oil to the head windings, which has some limitations at lower speeds, but does provide uniform distribution at higher speeds.

And lastly, specific oil channels through rotor and stator, which would allow the fluid to improve the heat dissipation close to the source of the heat.

Using an electrically driven pump to move the oil allows control for maximum efficiency. The pump can be a stand-alone unit or, depending on application, shared with the vehicle’s gearbox.

Reducing losses through advanced thermal management technologies such as these is vital to the rollout and uptake of next generation electric vehicles. Although under continuous development, internally cooled electric motors are already hitting the market, starting in large EVs, before being democratised through mid-sized EVs and, eventually, all EVs in the medium term.

As a development partner to most global OEMs, our ambition is to drive forward these developments in oil-cooled motors, providing reductions in both weight and cost, while offering improvements in efficiency and performance.

The project has been part-funded by Basque country local grants with development taking place at our GKN Driveline Zumaia plant, where GKN Automotive’s eMotor development is concentrated. As we continue to test and refine the direct oil cooling feature, we are able to bring to market the next generation of eMotors which are even more efficient and robust than before.

Aitor Tovar is Chief Engineer eMotor Development at GKN Automotive