Explained: ZF two-speed EV gearbox

By Daniel Gardner

German transmission giant cuts new teeth on two EV ratios

With the exception of a handful of high-end high-performance models, all electric vehicles use a simple single-speed transmission to send power from the motor to the wheels.

The solution is cost-effective, and the incredible torque characteristics of an electric motor ensure swift acceleration even with one gear. But now an innovation by transmission giant ZF is about to take EVs up a gear – literally.

Its new universal drive unit, offering a turn-key drivetrain for manufacturers looking to develop a mainstream EV, incorporates a 140kW electric motor and, crucially, a game-changing two-speed gearbox.

At low speeds and in stop-start traffic, an electric motor is at its most efficient, offering regenerative braking to recoup power under deceleration and without power-sapping drag to fight. But at cruising speed, the rapidly spinning motor puts a high demand on the battery, compounded by aerodynamic and tyre resistance.

However, when vehicles fitted with ZF’s new unit pass 70km/h, the gearbox shifts up into its taller cruising ratio, lowering the motor’s RPM for greater efficiency and optimised torque.

Until now, manufacturers have had to choose between low-speed torque or high-speed efficiency and lock that decision into the single gear ratio, but this potentially revolutionary twin-ratio solution is offering something closer to the best of both worlds.

High hopes Down Under

Touring this great land of ours under EV power is a tall order, but multi-speed ’boxes could be part of the answer. Lower gears would provide maximum towing torque for trailers and caravans, while higher gears would impart long legs for vehicles on the interstate run, reducing charge consumption.

Ratio to the finish

As with manual and automatic gearboxes, expect to see EV transmissions with more ratios. ZF is up to nine in its automatics for ICE, and more cogs in an EV gearbox will bring similar benefits – more potent acceleration and torque, increased motor life, and longer cruising range.

Rival gear

Global auto supplier GKN is already manufacturing a two-speed transmission it calls the eAxle, which is, in part, responsible for the dazzling performance of the Porsche 918 Spyder and BMW i8. But with the potential to drive a greater number of mass-market cars, the unit cost of ZF’s transmission will plummet.

Source: WHICHCAR

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