Why electric vehicles must have different driveline targets

Current driveline technologies have been defined and optimised for conventional powertrains. That doesn’t make them the best solution for electric vehicles, says research by GKN Automotive.

Electrification is advancing faster than many realise. Automakers come to GKN Automotive when driveline innovation is needed – and the current level of interest is unprecedented. There has never been so much simultaneous change in drivelines across the entire automotive industry.

A few years ago, EVs were a small niche that had little in common with mainstream vehicles. The second wave of EVs electrified conventional platforms, sharing the same front-wheel drive platforms and using the prop shaft tunnel to package the battery.

Will front-wheel drive continue its dominance? Looking at the next wave of EVs, it’s a big question. New skateboard architectures are mainly rear-wheel drive, but some are adding an eDrive to the front to produce an all-wheel drive.

Going electric changes hardware requirements

Data acquisition, benchmarking, and extensive testing of hybrid and electric cars is now giving us a clearer picture of the different duty cycles and control strategies. One thing is clear: the requirements are not the same as for conventional powertrains.

These eDrive configurations have transverse electric propulsion systems to keep the car’s centre of gravity close to the middle axis. As a result, some applications require significantly shorter side shafts.

If these vehicles have the same degree of wheel suspension movement, then the drive shaft needs bigger articulation angles and that may change some basic requirements for the constant velocity joints. If these are SUVs or crossovers with larger eDrives, the driveline will also need more ground clearance, which would also have an impact on the installation angle.

Torque oscillations under control

Combustion engines steadily load up the driveline, but EVs just accelerate. The split-second launch is part of the appeal. With combustion engines, misuse of the clutch can shock the driveline with half shaft torque levels that are up to three times higher than usual.

Surprisingly, hard-accelerating electric cars do not have this high shock factor. Oscillation control algorithms in the eMotor software can minimise such overloading. By controlling the torque signal sent to wheel, it can reduce wheel spin or any extreme vibration in the torque signal.

This control can be improved further by adapting the drive shaft performance. GKN Automotive’s research indicates that during hard accelerations on split-mu surfaces, stiffer drive shafts can make software control of oscillations much easier.

The company expects demand to increase for high-performance monobloc tubular shafts, which can be 30-50% stiffer. And GKN is working on rear-wheel drive shaft concepts with double the stiffness.