Pretty much all mainstream electric cars integrate their electric motors with single-speed, often 10:1-ish gear reductions, mostly because it’s cheap and easy, and it works. But now, the German transmission company that changed the industry with the now-ubiquitous eight-speed automatic is showing off an electric drive unit with multiple speeds. Two, to be exact.
In its press release, ZF refers to its new contraption as a “2-speed electric drive.” Like a Tesla drive unit, it integrates an electric motor with power electronics and a gear reduction all in a single unit, except instead of a single-speed reduction as is common among EV manufacturers (EV supercar maker Rimac does offer multiple speeds, and so do some Formula E cars), there are two ratios.
ZF claims the new unit, can provide “up to” a 5 percent increase in range when compared to a single-speed unit, though the transmission manufacturer doesn’t go into detail on that figure, though it does stress the “integrated” nature of the setup as key to improvements energy conversion efficiency.
Perhaps more importantly, the two-stage shift element should allow for an automaker to better trade off acceleration and top speed. From ZF’s press release:
The 2-speed concept offers benefits for OEMs who are pursuing performance. “Until now, with electric motors, vehicle manufacturers have had to choose between high initial torque and a high top speed”, explains Hellwig. “We are now resolving this conflict and the new drive will be compatible for performance and heavier vehicles – for example for passenger cars towing a trailer.”
ZF mentions both a 140 kW as well as a 250 kW unit, saying the modular nature of the two-speed e-drive allows it to be “fine-tuned and scaled up.” As for the shift strategy, ZF has some pretty unconventional ideas. From its press release:
Shifts take place at 70 km/h. By connecting to the vehicle’s CAN communication it is also possible – if the customer so wishes – to devise other shift strategies, possibly linked to digital map material and GPS. For example, the vehicle could identify from the GPS route programming how far it is to the next charging station, enabling it to respond predictively by switching into Eco-mode. More effective shifts would also be possible in accounting for topography on the interstate, and on inter-city journeys. The software in the drive can also be updated thanks to the network link to Cloud services via over-the-air updates.
It’s an interesting setup, and I’m curious to see if any mainstream automakers will think it’s worth the cost, since single-speed units seem to do just fine, as electric motors have ample low-end torque, high enough revs, and good enough efficiency across the power band to make do with just one gear ratio. You can learn more about why most EVs use a single gear in Engineering Explained’s YouTube video below:
I’ll be reaching out to ZF engineers to see if I can learn more about the benefits of this new two-speed drive.