The fundamentals of driveline efficiency explained

Uwe Paksa, Vice President of Engineering for CV joint systems at GKN Automotive, explains how the CV Joint System influences driveline efficiency.

Driveline efficiency can be hard to understand. The simulation tools engineers use to calculate fuel consumption are complex but they focus mainly on total vehicle mass and so fail to explain the real relationships at work.

That’s why automakers sourcing drivelines usually specify targets for the weight, NVH and cost, but not for efficiency. The conventional view of driveline efficiency works back from the wheels to the engine, seeing each stage as a loss-maker.

If you minimise the losses as the power moves through the reduction gears, differentials, driveshafts and bearings, more power reaches the road. But how much potential is there to improve the CO2 ratings and the real world fuel economy?

Understand the angles

The biggest factor in driveline efficiency is actually the driveshaft installation angle, usually pre-determined by the vehicle architecture. Some automakers design the passenger cell around the engine with driveshafts that run at angles of just 1° or 2°, which helps transmit the power efficiently during normal driving.

That’s obviously not possible for every vehicle. Some automakers design the propulsion system around an optimised passenger cell instead. And SUVs and light commercial vehicles with high payloads often have high installation angles to improve ground clearance or suspension movement.

The result can be driveline installation angles of 5°, 8° and even 10°. For these platforms, losses at the joint may cost the car more than 1% of its total propulsion power.

More power, more freedom

More advanced driveline system design can make a real difference. GKN’s more advanced driveshaft systems can reduce the losses from ~1% by around 0.5%. That gives the automaker options: either use the additional design freedom or deliver 0.5% more power to the wheels.

The fuel savings for a highly optimised drivetrain depend mainly on the vehicle propulsion system and the duty cycle. The biggest factor is often the internal combustion engine’s parasitic losses – the power required just to keep the motor running.

The impact of drivetrain efficiency improvements otherwise depends mainly on the overall power demand at the wheels. The higher the power demand, the bigger the efficiency benefits of driveline optimisation are.

Electrification rewards efficiency

In driving cycles such as the NEDC and WLTP where the average power demand range is relatively small, a 1% driveline efficiency improvement can improve the CO2 score by around 0.5-0.6%. In high-speed, high-performance driving, the benefit is closer to 1%.

For hybrid and electric vehicles, because the losses from the engine are lower or non-existent, the impact of driveline improvements is greater. And, as energy is regenerated, the benefits of a more efficient sideshaft are doubled. Studies indicate that each percentage of loss reduction can improve the vehicle’s overall energy consumption by up to 1.5%. Improvements to the driveline system’s efficiency can help meet future emission targets.

Getting the driveline concept right early requires detailed expertise in the complete system integration, however. As the world’s number one supplier of eDrive, all-wheel drive and conventional drivelines, engineers at leading automakers worldwide now regularly consult GKN Automotive on driveline efficiency.