GKN has developed the first two-speed eAxle; the technology has entered production on the BMW i8 plug-in hybrid sports car, (earlier post) in which it provides power to the front wheels from a 96 kW, 250 N·m (184 lb-ft) electric motor. (A 170 kW/231 hp three-cylinder combustion engine in the BMW i8 drives the rear wheels.)
Production electric and axle-split hybrid vehicles have used single-speed transmissions; however, GKN said, using a fixed ratio limits electric drives in terms of either acceleration or top speed as they must disconnect at certain speeds to prevent the motor overspinning. GKN said its two-speed eAxle is the first in series production to enable the electric drive to boost the vehicle’s performance across its entire speed range.
|GKN eAxle. Click to enlarge.|
Giving the electric motor an additional gear ratio improves acceleration and pure electric range, benefiting both driving dynamics and CO2 emissions. GKN’s two-speed eAxle also enables the motor and all associated systems to be downsized, reducing mass and further increasing efficiency.
Being the first Tier One to deliver a two-speed eAxle demonstrates GKN’s leadership in electric drive technologies and hybrid driveline systems. Two-speed eAxles will help manufacturers enhance hybrid and electric drivetrains and support the trend to downsize eMotors to reduce weight and cost. With the technology proven in a high-performance vehicle with high levels of refinement, significant savings in CO2 are possible.—Theodor Gassmann, GKN Driveline’s Vice President Product Technology eDrive Systems
GKN Driveline itself currently offers two other different electric axle solutions: eAxles for M-4WD (a lightweight gear box with active controlled clutch for electric motor assisted AWD solution) and eAxles for Hybrid AWD (an electric drive axle unit for axle split hybrid systems incorporating a proprietary disconnect clutch technology, which facilitates on-demand all-wheel-drive).
The new two-speed eAxle technology enables automakers to produce “axle-split hybrids”. A conventional or hybridized engine provides the primary power to either the front or rear wheels, with the other axle driven by an eAxle module. The technology responds intelligently to deliver an instant high-torque all-wheel drive experience, a useful pure electric range or a refined, efficient parallel hybrid mode. The architecture offers such a strong combination of efficiency, traction, dynamics and packaging.
Designing the two-speed eAxle for a high-performance sports car application placed tight packaging requirements on GKN’s development team—to achieve high torque density in a package that weighs just 27 kg (60 lbs) and measures less than 325mm by 562mm by 313mm.
|Two views of the front axle electric motor and eAxle in the i8. Click to enlarge.|
To make the system narrower, the shift mechanism sits on the input shaft instead of the intermediate shaft in the middle of the transmission. Even with the input shaft spinning at up to 11,400 rpm, GKN’s software control of the eMotor and the synchro-actuation achieves a smooth gearshift in all conditions.
Gear shifts are completely transparent to the driver. The smooth electromechanically actuated shift between the synchronized ratios has enabled our eAxle to set new benchmarks for comfort and NVH. The eAxle combines superior efficiency of 97% with high gear shift comfort and low gear noise. And yet the shift mechanism is a proven technology with components already in series production, providing a robust and cost-effective solution.
By drawing on all the expertise and knowledge within GKN’s global engineering network, we were able to take the eAxle from concept to production in just 24 months. Going forward, pairing electric powertrains with multi-speed transmissions is enabling GKN to supply downsized eDrive modules for future hybrid and electric vehicles. The technology also enables more radical future eMotor downsizing strategies.—Theodor Gassmann
GKN said that its technologies help drive 50% of the world’s vehicles. GKN has invested more than £50 million (US$80 million) in the development of key technologies to support customers hybridizing and electrifying global platforms.