Shifting up

Single-speed transmissions have proved the norm for today’s electric vehicles, but a greater number of ratios could bring performance and efficiency gains

The prevailing wisdom when developing battery-electric vehicles at present is to use a single-speed transmission. In theory it offers enough of a balance between efficiency and performance for most driving situations, and it is also cost-effective to implement.

But, as the performance of electric cars increases and the number of vehicle segments the powertrain is adapted for expands, that wisdom will change.

GKN has been at the forefront of many electric vehicle projects, perhaps most notably developing systems for BMW’s i brand, more specifically the i3 and i8. Ramon Kuczera, GKN’s product technology vice-president, says: “Currently almost every eAxle on the market is a single-speed, the exception being the BMW i8 for which we do a two-speed.”

On the i8 the rear wheels are driven by a gasoline engine via a six-speed automatic transmission, while the front wheels receive their power from the electric motor via a two-stage automatic transmission. Combined maximum output is 266kW and combined peak torque is 570Nm in the all-wheel drive plug-in hybrid sports car.

The i8 might be something of an exception both in terms of performance and technology – the battery-electric Nissan Leaf, for example, uses a single-speed reduction gear with a final drive ratio of 8.1938 – but GKN sees that changing in the future.

“We see an evolution in the number of speeds, but then also in terms of integration,” says Kuczera. “The next generation is trying to make a commonised housing, put everything into a tighter package rather than it just being everything bolted together, minimising high-voltage cabling and integration into the vehicle.” 

GKN had to take a fresh look at the design of its two-speed system for the i8 application. To make it narrower, engineers located the shift mechanism on the input shaft instead of the intermediate shaft in the middle of the transmission, resulting in a package that weighs 27kg and measures less than 325mm by 562mm by 313mm.

How far the number of ratios can climb is difficult to foresee – the increase in traditional transmission systems for combustion engine vehicles has been significant. You don’t have to look that far back in history to when six-speeds were seen as the optimum, but now firms are developing 10-speed systems.

tags: December 2015 GKN Transmissions