ZF Shifts EV Transmission Efficiency up a Gear

ZF looks at the math and decides for many applications, 2-speed EV transmissions would be sensible.

August 22, 2019

Stuart Birch

ZF R&D is driving towards 2-speed EVs, and called its 2-speed concept transmission system (above) “a paradigm shift for electromobility” during a recent test ride in a development vehicle at the Lausitzring track, south of Berlin. (ZF)

ZF Project Engineer Volker Vogel checked that the Lausitzring test track ahead was clear and announced: “Now from stationary on full throttle.” From the rear of the prototype van came the almost instant and familiar whine of a very busy electric motor as the speed built. Then, the unexpected: A slight change in volume at 70 kph (44 mph) as the transmission shifted from first to second gear. “It is only at prototype stage.” Implicit in Vogel’s words was the need to make allowances. But that was not necessary; as shifters go, this was a real smoothie.

The EV transmission debate for passenger cars has so far – mainly for reasons of cost, complexity and weight – been overwhelmingly on the side of single speed. But ZF reckons it is time to look again at the math and has decided that for many applications two ratios would be sensible. It seems a natural thing to do from the manufacturer of a wide range of automotive components, because the Z of ZF stands for Zahnradfabrik: gear factory.

We had an early experience of the transmission (top) at Lausitzring between Berlin and Dresden, Germany. There, Stephan Demmerer, Head of ZF Advanced Engineering e-Mobility, plainly regarded the new transmission as a “paradigm shift for electromobility”; pun intended. A totally new design, the thinking behind its worth is its contribution to an optimal combined efficiency, at present claimed to be 4.7% better than that of a comparable single-speed.

This could be honed if the “sailing mode” is linked via the car’s CAN communication to a GPS/camera system able to anticipate downgrades, corners, traffic light changes and other predictive scenarios in both urban or motorway situations. It also has scalability, embracing heavier and faster vehicles, and better tackles towing, explained Demmerer. The new transmission also has an automatic hill-start capability when both clutches are closed. The compact 2-speed unit incorporates both shift and power electronics.

Efficiency sweet spot

In the prototype, the 2-speed is claimed to deliver more than 200km/h (125mph) compared to a single speed’s 160km/h (100mph) from a series production 140kWh motor. The real aim is to keep in the motor’s efficiency sweet spot to reduce motor rotation speed. “With longer range we can use a smaller battery, which reduces cost and weight,” said Demmerer. “The 2-speed transmission can be accommodated in a package similar to that of a single speed. So we are developing a modular and scalable transmission and e-motor set – great combinations!”

Although a smaller battery can be used depending on an EVs category (high performance, off-road capable, urban), to gain an overall financial offset (manufacturing; end-user benefits), ZF has yet to clarify potential overall 2-speed unit costs including its power electronics. The key for better efficiency is in a specific e-motor, dedicated for working together with the 2-speed transmission, said Demmerer: “An even better efficiency at the sweet spot of the efficiency map in combination with the 2-speed gearbox leads to an efficient system layout.”

A new e-motor is also being developed with a maximum power rating of 140kW. The present situation of OEMs having to choose between high initial torque or high top speed, may be resolved by 2-speed use. The 2-speed gearbox has been designed to cope with motors delivering outputs up to 250kW.