Most electric cars like the Tesla and Nissan LEAF— heck, even the 250 MPH Koenigsegg Regera!– use a one-speed transmission, or single reduction gear, that is linked to the electric motor. It’s a simple, durable solution that takes full advantage of the fact that electric motors generate peak torque at 0 RPM. Around town in stop-and-go traffic, it works great, and it was the ideal solution for electric cars that were built around commuting and shorter driving distances. Notice how I said, “was”? That’s because today’s electric cars offer ranges in the hundreds of miles and, with an infrastructure of fast-charging stations that’s just getting bigger every day, will soon be good enough to serve as solid road trip companions. At highway speeds, another gear is probably what you’re gonna want, and that’s exactly what ZF has built.
In practice, the transmission shifts at just under 45 MPH (70 km/h), allowing for the shorter gear to be used in normal driving and reserving taller gear for stuff like accelerating onto on-ramps and passing. 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.”
If all that sounds familiar, that’s because you’ve heard it before. Back in the early days of Tesla Motors, Elon Musk launched his original Tesla Roadster with a 2-speed transmission for exactly those reasons. It wasn’t until they “upgraded” the car to version 2.5 that the single speed transmission with the hand-wound stator ended up being the norm for those guys.
According to ZF, electric cars that are fitted with its 2-speed “e-drive” transmission aren’t just quicker– they also show a 5% increase in driving range. “For electric vehicles in everyday use, it is important to obtain as much range as possible from each battery charge”, states Bert Hellwig, Head of System House at ZF’s E-Mobility division. “Every percent of improvement in energy conversion efficiency translates into two percent more range.” To increase the performance rating of the new electric axle drive system, ZF leveraged its expertise in systems to develop a new electric motor with a maximum power rating of 140 kW paired with a two-stage shift element. “Bringing together our know-how in relation to electric motors, gearboxes, and power electronics ensures that we achieve the best possible range from each battery charge.”
In any event, the modular design of the e-drive transmission also allows it to be easily fitted to a range of electric motors, with ZF mentioning 180 and 350 HP options having been tested. ZF also added that the compact design of the two-speed system would make it suitable (if not ideal) for passenger car use where interior space can be limited. On a related note, Kreisel Electric, together with Sala Drive, unveiled an automated two-speed transmission specifically designed for use in electric power trains last year, which can handle up to 805 HP (600 kW) and an input torque of up to 900 NM.
Paul Tan wrote that ZF explained that vehicle efficiency can be further improved by linking the 2-speed transmission to the vehicle’s CAN Bus and using a myriad of sensors in order to devise alternative shift strategies. For instance, an onboard computer module could analyze the GPS route programmed into the navigation system by the driver to identify upcoming terrain, and respond in a proactive/predicative manner by switching gears to maximize range and power harvesting.
All of that sounds pretty good, doesn’t it? More range, more performance, and probably more dollars added to the price of the car, too– would a 5% increase in range combined with zippier performance be worth the extra ducats to you, as a consumer? Let us– and the car companies!– know in the comments section at the bottom of the page.