This year at the North American International Auto Show (NAIAS) in Detroit, Schaeffler showcased the eDifferential. The system is an active electric differential that combines an electric drive with the option of controlling the power in each wheel individually. This facilitates torque vectoring (distribution of torque between the right and left wheels), which is beneficial for driving dynamics, safety, and comfort. “With a small additional motor we can actually split the work completely to one wheel or the other, or even reverse the torque on one wheel compared to the other. So if you’re doing handling maneuvers, cornering, or in a safety situation, we can help to drive the car around the corner rather than just having the front wheels steer it around the corner,” says Hemphill. eAxle The company also has its eye on the fully electric future. Like many others in the game, Schaeffler’s eMobility Systems Division is looking to drive down costs and increase range. One of the group’s projects is the eAxle, a modular solution for electric transaxles. To electrify drivetrains, it’s important to find the right balance between effort and benefit. Criteria such as vehicle class and powertrain size (in terms of power and torque) are key elements in achieving appropriate electrification. So Schaeffler is developing several versions of its eAxle, including single- and two-speed options, with or without torque vectoring. With the addition of a two-speed transmission, an EV can get a much higher startup torque by having a lower gear ratio, and at the same time a higher top speed for the vehicle because of the other gear
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Image courtesy of Schaeffler Group USA Inc.
the tech
Schaeffler‘s eDifferential An active electric differential
available. “What you can really do is downsize the electric motor and power electronics, and get the same vehicle performance so everything gets cheaper.” A good example of the savings is seen between Schaeffler’s firstgeneration single-speed transmission and its second-generation two-speed transmission. Hemphill boasts that “we made a substantial reduction in motor size and got even better start-up torque and top speed out of it. We went from 105 kW down to 60 kW on the motor size. At the same time we raised the axle torque from
1,200 to 2,000 Newton-meters, and raised the top speed capability from under 150 to 250 km/h.” Theoretically the power electronics should shrink linearly in terms of the cost with the motor’s power rating, a 40 to 45 percent savings. The sweet spots An electric motor has “islands of efficiency” with regard to torque generation and the speed of the rotor. When the rpm of the motor is fixed to the vehicle’s speed, the case with a single-speed transmission, you slip in and out of the most efficient zone (90
Engineering Notes Torque vectoring A normal vehicle has a differential that allows the outside wheel to turn faster than the inside, but torque cannot be guided. Some vehicles use traction control, which applies one brake to steer more torque to one wheel. In true torque vectoring, torque can be actively guided to one wheel or the other, and the wheel speed can be changed. This means the outside wheel can be driven faster than the inside wheel and push the car around the corner.