When drivers of future Mercedes AMG models press the accelerator pedal on their electric performance cars, they get extra zest from the batteries of something that sounds straight out of ‘Back to the Future’.
No, not flux capacitors, but axial flux motors.
Mercedes-Benz AG and Ferrari NV are switching to this type of electric motor to generate torque against the headrest. Axial flux motors are much smaller than the commonly used radial motors, but have a more powerful effect.
High-end engines like these will be critical for brands like AMG and Ferrari as they race to electrify the high-performance vehicles that bring prestige and huge profits. All EVs offer the feeling of instant acceleration, from Nissan’s Leaf to Tesla’s Model S Plaid. While in the combustion era faster times and higher top speeds were achieved with more engine cylinders, manufacturers will differentiate performance EVs by making the most of batteries with lighter and more efficient engines.
“The power-to-weight ratio is truly a record number and much better than conventional engines,” said Markus Schaefer, Mercedes’ chief technology officer, of the carmaker’s upcoming AMG electric vehicle platform. “It will take advantage of the small size of the engine.”
With each press of the accelerator, EV drivers push hundreds – and in some cases thousands – amps of electrical current to copper coils. When these coils are activated, they become electromagnets with attractive and repulsive forces. The magnetic force created by a stationary stator surrounding a rotating rotor produces the torque that spins the vehicle’s wheels.
In axial motors, instead of a rotor spinning inside a stator, disk-shaped rotors rotate next to a central stator. This causes the flow of current – the flux – to pass axially through the machine rather than radially from the center. Because the motor generates torque at a larger diameter, less material is needed. Yasa, an Oxford, England-based manufacturer of engines used in Ferrari’s SF90 and 296 GTB plug-in hybrids, uses just a few pounds of iron for its stators, reducing the machines’ mass by as much as 85%.
Yasa’s motors are the brainchild of Tim Woolmer, whose work on them was the focus of his PhD in electrical engineering at the University of Oxford. Within a few years of completing his PhD, Jaguar Land Rover was making plans to use Yasa’s engines in the C-X75, a hybrid-electric two-seater with enough horsepower to rival the Porsche 918 Spyder, McLaren P1 and Ferrari LaFerrari. While JLR eventually dropped the project due to financial constraints, Yasa’s engines found their way into the Koenigsegg Regera hybrid hypercar, followed by the Ferrari SF90.
In July last year, Mercedes announced that it had acquired Yasa for an undisclosed amount and would place its engines in AMG models that will hit the market from 2025.
“If you look at the history of the auto industry in general, the auto companies wanted to have the engine, their core technology, in-house,” Woolmer said in an interview. “The batteries, the motors, this is their core technology now. They recognize the importance of long-term differentiation in these spaces, so they need to get it in-house.”
The most important aspect of axial motors is their form factor potential, according to Malte Jaensch, professor of sustainable mobile powertrains at the TUM School of Engineering and Design in Munich. Their smaller size allows automakers to fit one engine to each wheel, which is not feasible with radial engines.
Putting a motor on every wheel — or at least one on every axle — could translate into hair-raising EV driving performance. The innovation enables torque vectoring that better controls how much power the motors send to each individual wheel for improved agility. Fast corners can help AMG and Ferrari drivers overcome the lost roar of their eight-, 10- or 12-cylinder engines.
Yasa’s motors could also completely eliminate the need for a drivetrain on the so-called skateboard under the center of an EV, Woolmer said. That would create more space for engineers to pack batteries, make more room for larger trunk spaces front and rear, or allow designers to experiment with new aerodynamic ideas.
The small size and light weight of axial motors are not only beneficial for high-performance cars. They also find a home in aerospace, which led to Yasa spun off its electric aviation division, Evolito, last year. The world’s fastest electric vehicle, Rolls-Royce Plc’s electric plane, called the Spirit of Innovation, uses three axial flux motors to drive the propeller. The plane can travel about 380 miles (612 kilometers) per hour, making it faster than the Spitfire fighter that was powered by a Rolls-Royce V12 engine.
“The most important thing is their efficiency,” said Matheu Parr, Spirit of Innovation project leader at Rolls-Royce. “This allows you to keep the weight of the aircraft low.”
Axial motors are not necessarily the death knell for radial motors, which deliver higher top speeds. This prompted Ferrari to use two radial engines on the front axle of the SF90, along with an axial engine on the rear axle. For the 296 GTB, handling was considered more important, so only a lighter axial motor was used between the engine and transmission.
“It’s just a matter of what kind of driving experience you want to design for your customers with a specific engine,” said Davide Ferrara, Ferrari’s electric motors manager. “Different voices make sweet notes.”
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