

All-wheel-drive vehicles are  nothing new. Neither are electric cars. Now combine and tweak the technologies: When every wheel is powered by its own electric motor, that’s attention-getting, and it makes the $450,000 gull-wing Mercedes-Benz SLS AMG Electric Drive a harbinger of the future. Imagine if you could do away with the traditional driveshaft that dictates where the powerplant goes and imagine if you could speed up or slow down each wheel. Safety, performance, and space efficiency would all improve. So would bragging rights at the gym, so long as you can explain it to non-techie colleagues.

This mega-sports car produces 740 hp in electric form. By comparison, the gasoline-powered SLS AMG GT ($200,000) produces 538 hp. Wilbur and Orville made do with 12 hp at Kitty Hawk but then they could only travel a few hundred feet and didn’t have Bluetooth. With the electrified gullwing Benz, you can travel 155 miles, or you can reach 155 mph top speed. Pick one. That’s the capacity of the 1,208-pound, liquid cooled battery pack that runs down the centerline of the SLS: 12 modules of 72 lithium-ion cells, 864 cells total.

Mercedes Benz SLS AMG Electric Drive; (BR 197); Paris 2012

Torque vectoring goes to the next level


Acura, Audi, and BMW use forms of mechanical torque vectoring that add weight, complexity, and cost to take handling to the next level. Torque is power (more important than horsepower) and vectoring means the power is sent to the wheels that need it most. The electrified Mercedes with its AMG Torque Dynamics system provides selective all-wheel-drive and each wheel can be powered or slowed independently, all with electronics and electric motors, and the torque vectoring works accelerating or decelerating. For $450K, it is adding some cost, too. The battery pack and electronics was developed by the Mercedes-Benz AMG subsidiary that also created the Formula 1 kinetic energy recovery system (KERS).

When a car goes around a corner, the outer wheel travel more distance than the inner wheels. In most cars, the car’s differential allows that to happen in proportion to how far each wheel needs to travel, but mechanical differentials are stupid and think spinning one tire on ice or snow is actually the car taking a very tight corner. Torque vectoring shifts power to the wheel that needs it in combustion-engine cars using either a complex, heavy pack of clutches in the drivetrain along with electronics, or electronic torque vectoring  does a credible job with electronics and selective braking of the wheels. The goal is to promote stability in rain and snow or on loose gravel. It also lets you drive faster than you could otherwise by overpowering the outer wheels, which helps track you through a corner faster if lap times matter, or more safely if you’re a normal driver who went into the corner a little too fast. If you’re going around a corner that arcs to the left, slightly overpowering the right rear wheel points the front of the car a bit to the left. This works so well that torque vectoring is not allowed in many forms of car racing on the grounds it takes too much control away from the driver.


How it works, how the AMG drives

Mercedes Benz SLS AMG Electric Drive; (BR 197); Paris 2012Mercedes places each of the four motors near a wheel but not in the wheel hub itself. That’s because anything that’s part of the wheel-tire-brake assembly is unsprung weight and 50 pounds of added unsprung weight is harder on the car’s handling that when it’s sprung weight attached to the car itself. A central controller constantly directs varying amounts of power to each wheel.

Since it’s an electric, there’s also regenerative braking. In the torque vectoring example above where the right rear wheel is overpowered to help you through a left-hand turn, the Mercedes microprocessors could also direct the left front wheel to do a bit of regenerative braking to further help the car’s sure tracking through the corner. In the future as GPS gets more precise, the car would know how sharp the curve is and how long it lasts before the road straightens. Overlay the current weather conditions and then AMG Torque Dynamics would always be able to stay inside the margin of error. Or you could just drive slower, if that’s possible in this car.

I drove a gasoline-engine AMG SLS recently and if it’s slower than the SLS Electric Drive … it doesn’t matter. This is everything you could wish for in a sports car. Visually it’s like nothing else on the road, a combination of modern and classic design. Gullwing doors have announced “Mercedes” since the 1950s and you could imagine Clark Gable and Sophia Loren emerging as the doors lifted up (both owned gullwings) or Selena Gomez today. About the only knock on the SLS is that the center stack could use a swift boot into the 21st Century; the LCD should not be smaller than a Samsung Galaxy.

Front-drive Lexus + hybrid-electric system = all-wheel-drive

2013_Lexus_RX_450h_002Some makers such as Lexus turn a front-drive vehicle into a hybrid all-wheel-drive without adding a rear driveshaft by powering  the rear wheels through electric power. On the best-selling Lexus RX SUV, it’s an electric cable that snakes back to the rear axle, where an electric motor powers the rear wheels, but only when called for. Most of the time it’s front drive with short shafts powering the front wheels. But in sloppy weather, or when maximum power is required, the rear motor kicks in the and the RX is driven at all four corners.

The idea of decoupling the powerplant from a rigid driveshaft isn’t limited to cars. It has been proposed for naval vessels where the diesels or turbines would generate electricity to power electric motors that turn the ship’s propellers. And it’s how diesel railroad locomotives work: 4,000-hp engine turns generator, generator powers electric motors.

SLS feels expensive? Don’t worry; they won’t sell you one anyhow

You can buy the gas-engine SLS gullwing in the US, but for now the 2014 Electric Drive SLS won’t be offered. With its carbon-fiber body attached to an aluminum space for rigidity and light weight, the Mercedes-Benz AMG SLS Electric Drive will sell for €416,500 ($543,000), or about $450,000 outside of the European VAT. It will come in six colors. The most prominent are Chromatic Blue (as seen above) and a matte yellow called Electricbeam.

Now read: Ferrari’s new ‘mild hybrid’ LaFerrari supercar produces 963 hp