GKN, the world’s leading supplier of automotive driveline components and systems, has produced the industry’s first two-speed eAxle. The technology has entered production on the BMW i8 plug-in hybrid sports car.
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 its associated systems to be downsized, reducing mass and further increasing efficiency.
eAxle technology enables automakers to produce “axle-split hybrids”. A conventional or hybridised 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. No other hybrid architecture offers such a strong combination of efficiency, traction, dynamics and packaging.
Until now electric and axle-split hybrid vehicles have used single-speed transmissions. 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’s two-speed eAxle is the first to enable the electric drive to boost the vehicle’s performance across its entire speed range.
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 27kg and measures less than 325mm by 562mm by 313mm, some remarkable design decisions were made.
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 11,400rpm, GKN’s precise software control of the eMotor and the synchro-actuation achieves a smooth gearshift in all conditions.
“Gear shifts are completely transparent to the driver,” said Gassmann. “The smooth electromechanically actuated shift between the synchronised 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,” said Gassmann. “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.”