How Oil Cooling Can Make EVs Even More Efficient
Water and motors aren’t a good combination, so manufacturers have to be more creative in cooling electric powertrains.
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.
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.
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.
The kind of oil cooling Tovar is talking about for electric motors is similar to what we see on high-performance internal combustion engines. The oil can be circulated inside the motor through journals, and sprayed on components where journals aren’t practical. If you’ve ever seen an internal combustion engine with oil squirters aimed at the bottoms of the pistons, those are there for cooling and not lubrication. Yes, you can fill the bottom of the motor with oil for cooling, but just like an engine, splashing moving parts through oil creates parasitic drag. That’s one of the reasons high-performance engines use dry-sump systems. The research indicates that the most effective way to cool a motor is through a combination of pumping oil through the motor shaft and using journals to carry oil to the outside of the rotor using centrifugal force, combined with oil squirters on components like the outside of the commutator or even the ends of the windings.
You Want to Put What in My Motor?
Automotive glycol-based coolant is very effective, but a poor choice for electrical devices because it's too conductive to be used inside a motor. Some motors do use traditional automotive coolant, but only in water jackets around the outside, which isn’t that effective for cooling the entire motor.
Whatever you put inside a motor can’t be nonconductive, because it will build up a static charge as electrons accumulate on the surface of the fluid. Any coolant used also needs to have good lubricity since that is a secondary purpose. Oh yeah—EV makers would love for this to be a lifetime fill, so it needs to have enough surfactant content to clean the system. It can’t contain oxides either. Did I mention that the same cooling loop will likely be used in the gearbox, whether it's single or multi-speed? It's a tall order for one fluid to check all those boxes, but it turns out that automatic transmission fluid (ATF) is pretty much ideal.
Please give me a few sentences to address the storm I can already see brewing in the comments. To be fair, I know it’s going to come from both ends of the spectrum. On one side, Obi_Wan_GREENobi will be furious I’m supporting the use of fossil-fuel-derived fluids in EVs. Meanwhile, Darth_V8er is going to stroke his lightsaber as he teases the tree-huggin hypocrites still needing oil for their EVs. I’ll be the center of the light and dark, Father@Motoris: Any reasonable person knows humanity is not getting away from dead dinosaur juice anytime soon. But, we should be making a concerted effort to limit how much of it we depend on. In this case, we're talking about a few liters of ATF for a hundred thousand or so miles in a motor before being drained and recycled, compared to a gallon of gasoline to travel 30 miles that’ll never be recovered. May the force be with you.
Yay, We Optimized Motor Cooling. So What?
Just like gasoline-powered cars, EVs are fitted with motors that rarely use their full potential. Even accelerating onto a highway will only deploy half the available power; if we’re talking about something like a Tesla Model S Plaid or Porsche Taycan Turbo S, the owner typically won't tap into more than 30% of available power. But, manufacturers use larger motors so those massive performance figures can be maintained without overheating, just in case a customer actually takes their electric supercar to a track day. If we optimize the motor’s cooling, though, we can downsize it. That not only saves weight and space, but also makes the motor more energy efficient at normal power outputs.
Right now, oil-cooled motors are being utilized on the highest-performance EVs. Manufacturers are hesitant to spend the money on lower performance (read: lower priced) EVs and hybrids, although smaller cars would likely benefit the most. As more cars adopt hybrid powertrains, maximizing system efficiency, keeping prices down, and not adding to a vehicle’s maintenance schedule is going to be a priority. Returning to the original opinion piece that started this whole thing, if manufacturers can use smaller motors and, in the case of hybrids, an existing cooling fluid loop in the car, it balances out the cost of adding cooling equipment inside the motor.
Say whatever you like about EVs, but they’ve brought a revolution of interest in vehicle optimization. As someone who has been in and around race cars most of my adult life, that’s interesting to me. If you enjoyed nerding out on this, wait until we get to the aerodynamic interface of wheels and tire sidewalls.
To go way more in-depth on temperature versus efficiency, this paper from Victoria University, Melbourne Australia has some great info. And if you want to build your own simple electric motor with stuff you probably have lying around the house, check out this video.
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