Electric cars may only represent two percent of the overall automobiles sold within the United States last year, but that market share is only poised to grow. With Porsche, Mercedes-Benz, Jaguar, and Audi all tossing well-made hats into Tesla’s ring, the electric drivetrain is being evolved in a number of unique ways, based on the manufacturers’ philosophies for what the electric car should be.
The specific components and applications may vary, but the basic tenets of an electric chassis largely remain the same: the combustion engine is replaced by electric motors, powered by a controller that draws juice from a series of rechargeable batteries. That much you knew, right? But if we dig a little deeper, there’s some fascinating science happening beneath the floorboards of EVs. (Bear in mind, what comes next is oversimplified because we’re not engineers, nor do we wish to beleaguer you in technical jargon. (Also, a hat tip to this Donut Media video for helping our edification.)
Electricity itself cannot be stored in a battery, but electrical energy can be housed within the chemicals of the battery. Those two-node terminals are comprised of different metals, separated by an electrolyte which helps bring the chemicals from the nodes together in harmonious equilibrium, forming an electrical circuit and producing energy.
If you popped an EV onto a lift and started dismantling the battery pack from the chassis, you’d get down to individual battery cells. These are metal cases housing long spirals of two metal sheets separated by a thin perforated plastic sheet. The sheet that acts as the positive node is made of lithium cobalt oxide, while the negative node sheet is carbon. The whole setup is soaked in organic material, like ether, that functions as the electrolyte. While the cell is charging, lithium ions move through the ether and attach to the carbon. When the battery is discharging, they move back home.
So how does the car actually move? Magnetism. When you send an electrical current through a series of wrapped wires or other metal conduction bar, you generate a magnetic field. The forces from that field can actuate motion in an induction motor. There are two basic parts to this kind of motor: the rotor and the stator. The rotor houses the conduction bars and the stator provides alternating current (AC) to move the rotor. The speed of the rotor comes from the frequency of the AC. Mash the throttle on an EV and you’re increasing the AC. This system requires an inverter module, which sits near the battery and converts the direct current into AC to drive the motor. Because the rotor and the stator don’t physically connect—rotor bearings keep them separate—there’s less wear and tear than in a combustion engine.
Also, unlike combustion engines, electric motors can produce vicious amounts of torque immediately, and all the way up to 20,000 rpm. Due to this, there’s not a need for a transmission or torque converter. Further, a combustion engine uses pistons to create side-to-side motion that must be converted into forward movement, electric motors typically only produce rotational force, which is far easier to translate into forward motion. Lastly, the beauty of an induction motor is that when it’s not employed to move the wheels forth, it can harness the speed of the wheels to act as an alternator and recharge the battery system under freewheeling or braking.
This fundamental architecture can be invariably tweaked for a variety of applications. When Porsche was designing the newly-launched Taycan Turbo and Turbo S, the German marque didn’t want to veer far from its sports car heritage. Eschewing “green driving” attributes in favor of repeatable, blistering performance meant the wizards in Stuttgart gave it a massive 800-volt system, doubling the 400 volt systems found in other EVs, which helps increase performance and aids in diminishing charging times since it can handle higher power flowing into the battery pack. (The Taycan can charge at a peak energy transfer of 270 kW, meaning you can take the battery from five to 80 percent in about 23 minutes on a high-speed DC charger.)
Porsche also used innovative drive motors, one mounted on each axle, that have “hairpin” winding in the stator coils. These wires are rectangular instead of round and bent into a hairpin shape before being affixed to the stator core by laser beam welds. The design means more densely-packed wiring which affords more copper in the stator. That translates to more power and torque without enlarging the stator itself. While it doesn’t technically need one, the Taycan also includes a two-speed transmission on the rear axle, which realises quicker acceleration from a standstill in first gear, and a long ratio in second gear promises higher efficiency and higher power reserves, especially when traveling at breakneck speeds.
The Taycan uses permanently excited synchronous motors, which differ from the induction motors described above. That goofy name means it uses AC and DC current to keep the rotor speed and the speed of the stator equal, whereas in the induction motor, the rotor is always lagging behind the stator a bit. (For reference, Tesla uses induction motors.) The synchronous motors in the Taycan are more efficient than induction motors of the same output and voltage rating, and they also realise higher constant power over the entire speed range.
The sum of all of this is a 750 hp electric missile (in the Turbo S. The Turbo boasts 616 hp) and 775 ft lbs of torque that’ll launch the car from zero to 60 mph in 2.6 seconds. As we noted when reviewing the Taycan during a first-drive, it’s absurdly fast. But that’s precisely what Porsche sought during the engineering phase.
Mercedes-Benz launched its eSUV, the EQC, this year—the first salvo in a barrage of 10 EVs forthcoming from the tri-star marque over the next three years. Here, the focus was to make a smooth and silent car that still ticks the right Merc hallmarks of quality, safety and comfort. It required tranquility from the drivetrain to help customers who are making the jump from combustion to electric. Employed on each axle is an induction motor and, in concert, 402 hp and 561 ft lbs of torque are generated, which is more than ample to get the 5,300-pound ute hauling.
“Performance is very easy in an EV,” says Dr. Martin Hermsen, senior manager, E-Drive Development & System Integration, who was responsible for the EQC’s drivetrain engineering. “You need a very large battery to reach the requirement for the range, and with this large battery, you have enough power. The question becomes how you will use this power? We put the motor, inverter, and differential all in the same housing, one on each axle, but the front motor is more efficient while the rear system is more for power,” says Hermsen. The 405-volt EQC can adjust in real time to shift between the front and rear motors—or employ both—depending on how you’re driving. Both motors will kick in when you’re in sport mode or are really stepping on it. Relaxed drivers will only see the front motor engaged.
And it’s indeed a remarkably smooth ride when you’re not lead-footing it. We tested the EQC in a three-country jaunt from Copenhagen to Berlin and found it approachable and intuitive, especially when you’re trying to maximize the 220-mile range in eco mode. “When you buy an EV, your belief is that it should be a green driving vehicle. We wanted to deliver that,” says Hermsen. Steering wheel paddles typically reserved for shifting gears are repurposed in the EQC to toggle between regeneration modes. (The EQC does not feature a transmission at all.)
When you lift off the accelerator, the recuperation of energy starts. In D+ mode, the lightest setting, the recuperation feels like very mild engine braking while the wheels convert and feed energy back to the 80 kWh battery pack. In D-, lift off the accelerator and you’ll feel the EQC’s prow drop as it regains even more energy from the wheels. In D – -, the highest setting, the Mercedes slows so much, traditional combustion engine drivers may feel like something’s gone wrong, but the vehicle is trying to capture as much energy as possible.
In this mode, one-pedal driving is enabled, meaning you needn’t touch the brakes at all if you’re doing it right. “We use a lot of data, including traffic patterns and the car’s camera systems to help achieve the most intelligent and efficient driving possible,” says Hermsen. One-pedal driving may take a little getting used to for new EV drivers, but there’s a gamification aspect to the affair that makes it addicting. You’re constantly trying to see how much energy you can recoup and how long you can go without having to employ the brake pedal.
Mercedes-Benz pledges that its whole portfolio of vehicles will be electrified (fully or partially) by 2022, and the point of the EQC is to make the transition from combustion to pure electric as seamless as possible. For the customers who want the full green experience, it’s a wonderful one. For the folks who want to crank it into sport mode and rip, it’s equally happy to comply.
Audi also chose to launch its first EV in the form of an SUV, the e-tron, for similar reasons to Mercedes-Benz. It cited a need for new EV purchasers to feel comfortable with the vehicle, and the most acceptable body styling was a ute. “We kept the traditional SUV shape,” says Matt Mostafaei, Audi Connected Vehicle and e-tron product manager, “which hurts in terms of efficiency, but helps with the familiar feeling.”
Underpinning the e-tron is a 95 kWh battery pack—good for 396 volts—that feeds energy to dual induction motors, a smaller one driving the front axle, and a larger motor in the rear. Combined, they generate 402 hp and up to 490 ft lbs of torque (when using an electrical boost feature). Those induction motors were chosen because of the “advantage to decouple the axle to eliminate drag while freewheeling,” says Mostafaei. “The motors bolster our philosophy that the e-tron should feel as normal as possible to drive.”
That freewheeling means Audi wasn’t as concerned with one-pedal green driving. “Using the brake pedal is the most natural thing for a driver going electric,” says Mostafaei. So, Audi wisely reimagined the braking system. When you hit the brakes on the e-tron, a small fluid reservoir behind the firewall fills up, mimicking the feel of traditional hydraulic brakes. However, 90 percent of the time, the brake-by-wire system is actually engaging the motors to slow the wheels down. If the stopping event is under .03 gs, the car can recuperate energy without you feeling much of anything. (The e-tron does know when the friction brakes are required during harder stops, and they work perfectly in those scenarios.)
We’ve driven the e-tron several times and this clever setup works so well, you’ll never know when the motors are slowing you versus the actual brake rotors. Like the EQC, there are also regeneration paddles, which can also help bolster the recuperation. And the whole system can tack on an additional 30 percent to the e-tron’s range, which is about 200 miles.
Audi also rethought the shape of the cells within the battery pack. “We use pouch cells instead of the common cylindrical cells,” Mostafaei explains. “They’re cheaper, easier to source and lighter. We can monitor the temperature better, too. They also have more surface area so you can condition the battery better for faster charging.” The e-tron can get up to an 80 percent state of charge in about 27 minutes on a 150 kW DC fast charger. Mostafaei further adds that Audi is experimenting with the chemistry within the battery to help improve the life cycle of the cells.
The e-tron can also manipulate its ride height to help increase efficiency, dropping down to reduce drag or raise up if you need to plow through some snow or venture off-road, where it is surprisingly capable. “Had efficiency been our sole goal, we wouldn’t have added roof rails and heated seats and enlarged the cargo bay,” says Mostafaei. “But we want you to live with the e-tron as you would any other SUV.” Next for Audi is the e-tron GT, a four-door performance sedan that will semi-rival Porsche’s Taycan. Of the car due out next year, Mostafaei claims: “It’ll be a sporty car, but still will focus on luxury and comfort as well.”