Although Electric Cars Don't Need Petroleum, Their Motors Still Require Oil

After more than a century and a half of development and improvements, the internal combustion engine’s thermal efficiency is still a joke compared to electric motors. That’s because of the more complex design of the ICE, and the high temperatures, pressure, and friction it must endure. Nevertheless, electric motors’ thermal management is not as simple as you might think. And oil is a big part of the equation.
Oil cooling makes electric motors more efficient 6 photos
Photo: BMW / Image by jcomp on Freepik
Forms of motor cooling for electric cars in 2022Oil-cooling helps increase powerOil-cooling helps designing more compact electric motorsInverters can be cooled by oilMost electric motors wil be oil-cooled by 2033
When people talk about electric cars, most of the debate is about batteries and their shorter range compared to conventional cars. Generally, battery improvements are related to their thermal management.

Active cooling with water-glycol-type coolants increases batteries’ performance, durability, and fast-charging capabilities. By comparison, air-cooled batteries – which means they don’t really have a thermal management system – are much more prone to critics.

On the other hand, a thermal management system makes the cost of a battery go higher and requires regular maintenance. This is an uncomfortable truth for proponents of EVs. And there’s more. The electric motor and power electronics also require some thermal management.

EVs' motors are efficient, but also demanding

While it’s true an electric motor has no moving parts that get in contact with one another – thus no losses because of the heat and no intensive wear or degradation – it’s also true an electric motor gets hot and it should be cooled down.

Oil\-cooling helps increase power
Photo: GM
Usually, electric motors and inverters’ optimal operating temperature is around 60°C (140°F). The cooper windings in the stator generate the electric fields used to drive the rotor. Consequently, a high amount of heat is generated, and it needs a cooling circuit.

You might think it’s not as complicated as an ICE cooling circuit, but it is kind of tricky. For instance, water-glycol-based cooling systems are very limited because of their electrical conductivity. This means the risk of an electrical short circuit is serious if it gets in direct contact with electrical components.

That’s why the most common way to cool an electric motor is to use a 'water jacket' around the outside of the stator. But this simple solution is almost of no use for high-power electric motors, which require better cooling on the inside for improving their efficiency as much as possible.

What’s better than water glycol?

It turns out that water-glycol coolants can be replaced by oils. As a matter of fact, engineers used it for decades in industrial electric motors, while in a conventional car, the oil in the transmission is used both for lubricating and cooling the moving parts.

Inside an electric motor, direct contact with oil poses no short circuit risk and it removes the heat from internal parts more effectively, especially from a high-speed moving rotor. The oil also lubricates the rotor’s bearings, while stator windings’ dilatation and wear caused by the heat is greatly minimized.

Oil cooling can successfully replace the 'water jacket' and this can lead to smaller and more power-dense electric motors. Oil-cooling system can remove double or triple the heat compared to a water-glycol cooling system and even ten times more than an air-cooling system.

Oil\-cooling helps designing more compact electric motors
Photo: Bosch
In simple words, using oil for cooling can allow a serious increase in the electric motor’s power, without the need to radically change the design. However, other issues arise. For instance, the heated oil must come out of the motor and be cooled before re-entering the motor.

As such, oil-cooling thermal management adds a lot to the complexity of modern and powerful electric cars. This also means an increase in the price, which is still a burden for EVs’ competitiveness, despite the performance benefits outweighing the complexity of oil cooling.

Hail the oil cooling systems

In recent years, many electric motors design were changed to benefit from oil cooling. Research company IDTechEx reveals that in the first half of 2022 oil cooling for electric motors was adopted by 50% of the new electric cars sold worldwide.

In the meantime, only one-third of EVs were using the water-glycol coolant solution, while electric motors cooled by air had a small 13% share market. In the next ten years, forecasts favor the oil-cooling solution market share increase, while air-cooled electric motors are expected to almost be phased out from cars.

Forms of motor cooling for electric cars in 2022
Photo: IDTechEx
The market evolved quickly, and engineers created integrated thermal management systems for both battery and electric motor, which are more efficient overall by combining oil cooling and water-glycol cooling. But things will get even better.

The industry is working hard to find the best way to integrate inverters into electric motors. This way, the drive system would be more compact and more efficient, and also suitable for a larger number of applications.

Nowadays, inverters are mostly cooled by water-glycol coolants. In the near future, integrating inverters into electric motors would provide the benefits of direct oil cooling, further enhancing EVs’ performances and range.

An unexpected opportunity for Big Oil

These forecasts are particularly useful for the oil industry. The mass electrification of road transport and heavy investments in hydrogen and near-zero-emissions fuels for ships, airplanes, trains, and heavy trucks and machinery will lead to a massive drop in petroleum fuel demand.

Oil companies are already changing their priorities to favor the petrochemical sector, but the oil business is nothing to ignore. According to predictions, the EV market in all transport areas is expected to generate $2.6 trillion (€2.4 trillion) by 2042.

Most electric motors wil be oil\-cooled by 2033
Photo: GM
As oil-cooled electric motors will most probably account for more than three-quarters of electric vehicles, the demand for oil coolants is expected to be a very profitable business. Oil coolants are either mineral oils or synthetic oils, which are sourced from crude oil.

Of course, the industry will still have to apply best practices for recycling these oils or for near-zero-emissions sourcing of raw materials, and also for carbon-free technologies for manufacturing it. For now, I’m sure this might be another reason for EV antagonists to attack electromobility – just like the battery manufacturing-related pollution debate.

There are still challenges along the way, but at least using oils in electric motors’ thermal management system is much more common sense than burning them in combustion engines. And that really is part of the evolution to a more sustainable transportation system.
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About the author: Oraan Marc
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After graduating college with an automotive degree, Oraan went for a journalism career. 15 years went by and another switch turned him from a petrolhead into an electrohead, so watch his profile for insight into green tech, EVs of all kinds and alternative propulsion systems.
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