Motors in the wheels take EVs further
AT the Paris World Exhibition in 1900 Ferdinand Porsche, the eponymous founder of the German sports-car company, unveiled an electric vehicle (EV) with a radical design. Rather than a single motor located in its body, the car he displayed had them incorporated into its wheels.
Porsche’s big idea caused a sensation, but never took off. Combustion engines are too big and complex to fit inside wheels and electric motors, which are simpler, fell out of fashion. In-wheel motors (IWMs) mostly remained the preserve of electric bicycles and some motorbikes. But with EVs newly resurgent, carmakers are starting to take an interest in the potential benefits that IWMs can bring. Some new EVs could start using them soon.
Most IWMs work in a similar way. Fully contained within either two or all four wheels, they deliver torque (a twisting force) directly to each wheel, avoiding the energy losses associated with the gears and transmission links used by centrally mounted motors. This also allows each wheel to be independently controlled to suit different road conditions, which improves stability.
In addition to being simpler and potentially cheaper to install than conventional, centrally mounted engines, their energy efficiency allows an EV to travel a greater distance on a single charge. The space freed up within the body also allows for more spacious interiors, as well as lighter and more aerodynamic bodies.
Protean Electric, a company based in Farnham, south-west of London, produces an IWM made up of two concentric rings. One is an electromagnet embedded in a static component, known as a stator. A larger ring, called a rotor, contains permanent magnets and rotates around the stator. When an alternating electric current is induced in the stator it creates a rotating magnetic field that causes the rotor to spin in sync. Both rings—together with the associated electronics and a standard friction brake —form a unit small enough to fit inside a standard EV wheel. Protean’s motors are already being installed in light commercial vehicles converted to run on electricity by Protean’s parent, the BEDEO Group. ConMet, an American company, also fits them to the wheels of lorries where they work in reverse as generators, powering the vehicles’ refrigeration units.
One issue with IWMs is that they can be more vulnerable to the elements, as well as shocks and vibrations from the road surface. Andrew Whitehead, Protean’s boss, says his firm’s IWMs have been subjected to extensive testing in hostile conditions and should last the lifetime of a typical car, considered to be 15 years or 300,000km, and do so without any maintenance. He hopes to conclude an agreement to supply a mainstream manufacturer soon.
Another issue often raised with IWMs is that they can increase a vehicle’s “unsprung mass”, which includes everything—such as wheels, tyres, brakes and axles—not resting on the suspension. A high unsprung mass can interfere with the way a vehicle handles. As Mr Whitehead sees it, however, the absolute value of unsprung mass matters less than the ratio of sprung to unsprung mass. Slightly heavier wheels in a sufficiently heavy car, in other words, can be made imperceptible to most drivers with a few tweaks to the suspension.
Donut Lab, which makes IWMs for its parent, Verge Motorcycles, a Finnish company, claims to be able to reduce unsprung mass with its lightweight “Donut Motor”. Fitted inside the inner rim of the rear wheel on motorcycles, it looks like a wheel with a big hole instead of a hub (pictured). The company is developing IWMs for cars and trucks and reckons some could drive supercars and electric helicopters.
DeepDrive, a Munich-based company that makes IWMs for BMW and others, has a design that uses two rotors, one that revolves outside the stator and another inside, to increase the motor’s efficiency. The company reckons that, compared with a traditional EV set-up, their IWM could give an EV some 30% more range in typical driving conditions. One way or another, it seems Ferdinand Porsche’s motorised rubber is finally about to hit the road. ■
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