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How Oil Cooling Can Make EVs Even More Efficient

How Oil Cooling Can Make EVs Even More Efficient photo
How Oil Cooling Can Make EVs Even More Efficient photo

It doesn’t matter where you stand on the EV adoption scale—when you get down to it, electric cars are machines, and machines are cool. Up until now, I'd never given much thought to electric motor cooling; after all, EVs' motors operate at over 90% efficiency. How much energy could they be throwing away? Then I stumbled onto an opinion piece by Aitor Tovar, chief engineer of eMotor development at GKN, a company with millions of drive units currently on the road in hybrids and fully electric cars. Quick summary: An oil-cooled motor is more expensive upfront, but offers greater benefits to the whole vehicle.

My first reaction: “You’re telling me a guy in the business of selling motors wants customers to switch to more expensive motors?” Insert shocked YouTuber face. But it turns out Tovar isn't the only one saying as much, and it's an educated opinion worth considering. EVs are complicated, and we have to take a deeper look and weigh the advantages against the costs. And bonus: descending this rabbit hole gives us the opportunity to nerd out on electric motors. If you don't like electric cars, pretend we’re talking about a drill.

2025 Porsche Taycan Turbo GT.
2025 Porsche Taycan Turbo GT.

A Quick Refresher on Electric Motors

Wrapping a wire around an iron nail and then connecting the two ends of that wire to a battery creates an electromagnet. It’s a science fair classic that beats the pants off Billy Sedgwick’s papier mâché volcano. Electric motors—AC, DC, brushed, brushless, synchronous or not—all use electromagnetism to turn electrical energy into mechanical energy. Motors use metal windings as the electromagnet. The current switches directions to change the polarity of the magnets, to then attract and repel the rotor to create torque. As electricity flows through the motor, resistance in materials cause them to heat up as they vibrate faster on a microscopic level. There's also some heat generated in the permanent magnets. It isn't much, but it all adds up.

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Like internal combustion engines, electric motors are designed to operate in a certain temperature range. There may be significant temperature variation in the different components, but the windings' temperature should be between 100°C and 150°C (212°F–302°F). As temperature increases, so does resistance in the windings. Thermal expansion can further decrease efficiency by modifying tolerances inside the motor, causing even higher temps and eventually permanent damage.

Nothing Explodes, So How Hard Can Cooling Be?

Internal combustion engines are horribly inefficient. Part of that is because they send so much energy out through the tailpipe. However, internal combustion engines also run coolant through their blocks and heads to carry away energy, which is then passed through a radiator before entering the environment. Cooling itself is a sign of inefficiency.

The very best internal combustion engines struggle to achieve 50% efficiency at peak torque; most gasoline engines are in the 20% range. Your average electric motor is more than 90% efficient. Well-optimized motors running at peak output can even approach 98% efficiency, and that’s not counting EVs' ability to use the same motors to recapture energy through braking. Sure, they sound like cordless power tools when running, but anytime I can give the Second Law of Thermodynamics the finger, I get excited.

Sometimes, though, trading a marginal amount of motor efficiency for the sake of the entire system's efficiency is a worthy tradeoff. A smaller motor may have to work a little harder and create more heat compared to a larger one, but there's a benefit in saving weight. Plus, a larger motor running at a low power output isn't as efficient as a smaller one operating in its sweet spot. It’s all a balance.