Driveline technology is hot these days, with Tier 1s rolling out new axle-disconnect, electric axle and lightweight innovations in rapid cadence. In the vanguard of this trend is GKN Driveline, whose Twinster dual-clutch masterpiece gave the 2016 Ford Focus RS and Range Rover Evoque dynamic torque vectoring functions across the entire speed range. That was followed by eTwinster, bringing e- axle technologies pioneered in the Volvo XC90 T8 and other hybrids.
Now comes eTwinsterX, which leverages those learnings and integrates a compact 2-speed transmission into a unit aimed at broad vehicle applications—from city cars to plug-in SUVs, high-performance hybrids and EVs, and the upcoming wave of electrified light trucks in North America. The system is engineered for front-, rear- and AWD configurations and is claimed to be the first e-axle that is fully off-road capable. It can incorporate electric motors from 80 to 120 kW (107 to 161 hp) and peak torque to 147 lb·ft (200 N·m).
Aimed to launch in the 2020 timeframe, the eTwinsterX enables the electrified vehicle’s propulsion system to deliver “what every OEM likes—efficiency, performance and a combination of the two,” noted Ray Koczera, Senior VP of Engineering. “You always want the electric motor running in its ‘sweet spot’ and the best way to expand that operating range is with two speeds.
GKN engineers prefer the two-speed addition for good launch performance and pure electric mode in its lower ratio, with the second, longer ratio used to support higher speed operation. “It gears the electric motor down improving efficiency by up to 10% and better performance, as you have more torque to launch the car and support to V-max,” explained Theo Glassman, VP of Advanced Engineering. He said eTwinsterX can also produce a negative torque effect to slow the vehicle, and provide a limited-slip differential function.
Automotive Engineering’s European Editor Stuart Birch was among the first media to sample eTwinsterX in prototype form last year during winter testing and came away impressed; see http://articles.sae.org/14630/.
Multi-speed transmissions provide benefits in the European drive cycle where “you’re running very low torque, so you have a 300-kW e-machine running at 30 kW or less,” Glassman said. For hybrids, the front axle needs to have an automatic or dual-clutch transmission “in order to harmonize the front and rear power sources for maximum effect.”
Achieving imperceptible shift interruption with 2-speed gearboxes has been an ongoing challenge for EV engineers, Glassman conceded. The eTwinsterX employs a new concept of power shift—it doesn’t have dual clutches or a torque converter. Rather, it uses a single friction clutch in combination with a free-wheel system that can be locked.
“In first gear it is locked and the clutch is open with the planetary gearset providing the first gear ratio,” Glassman explained. “Then you release the over-running clutch, lock the second gear shift clutch and change from first to second gear without any interruption. Basically the planetary gearset is inter-locked, and works like a conventional helical geartrain with the same efficiency as a two-stage helical gear system.
“That’s why it offers good performance in first and great efficiency in second gear,” he said. The unit delivers 2,581 lb·ft (3,500 N·m) to launch the vehicle and operates at up to 155 mph (250 Km/h).