How more cost efficient driveshafts can cut battery costs for EVs

Different driveline efficiency equation means direct savings in electric vehicles.

Regeneration strategies in hybrid and electric vehicles can have an impact on the driveline requirements. There are two key questions. How much energy can be harvested from the street and how much of that can the driveline really return to the battery?

Electric drivelines don’t just transfer power from the differential to the wheel – they also transmit the negative torque that enables the eMotor to recharge the battery. How much and how often vehicles use this two-directional torque flow is important.

In standard drive cycles, around 30% of the energy transmitted to the wheel can return to the side shaft on its way back to the battery. But in city driving cycles or hilly country roads, there is a significant amount of additional loading. GKN Automotive researchers test vehicles have measured 50-60%.

Theory vs reality

In theory every positive torque has an equivalent negative torque that enables the eMotor to regenerate energy. But that’s not what happens in reality.

Regeneration depends on different parameters: the interaction with the mechanical brake system, the efficiency of the electric drive system and the inverter and battery infrastructure are all factors. Some brake blending is necessary for an effective and comfortable hand-over between mechanical brake and recuperation brake.

Regeneration is also a question of vehicle stability. With rear-wheel drive layouts over-braking the rear axle can make the vehicle unstable. And finally, there is the battery. Even if the eMotor could receive 100% of the brake energy, the battery simply could not absorb all of it.

Battery cost savings

For combustion engines, efficiency improvements mean CO2 reductions that prevent penalties. But making a powertrain 1% more efficient does not translate into a 1% reduction in fuel consumption. It is less – and the savings vary according to the driving cycle, the power demand and the drivetrain’s friction losses.

The model is different with electric drive systems. The side shaft not only transfers the power from the engine to the wheel; it also helps recharge the battery during brake energy regeneration.

When GKN Automotive designs a side shaft that is 1% more efficient, in some driving situations it can reduce energy consumption by much more than 1%.

And the pay-off is in battery cost savings, not CO2 penalties. More efficient drive shafts can mean a smaller battery. In some applications the latest CV joints can reduce side shaft losses by 0.5%. With a 60kWh battery pack costing in the region of 9,000 euros, the potential saving for the automakers is €45-50.