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TechnologiesElectric Mobility

Efficiency Turbo for Electric Mobility

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With its EVmore drive concept, ZF demonstrates how new ideas for optimized components in the electric driveline can make the overall system much more efficient and sustainable.
Frank Thoma,
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Frank Thoma has been corporate editor at ZF since 2011. With a degree in journalism, he has been planning, writing and editing articles for all of the company’s internal, external media.
The situation at ZF was a bit like an opening scene in the action movie series "Mission: Impossible". Hollywood star Tom Cruise, aka Ethan Hunt, normally gets details of his tricky missions in a variety of unusual ways. "Find the vehicle with the most efficient electric driveline at present and make it substantially better," is Dr. Otmar Scharrer's concise instructions to a team of engineers. Scharrer is Head of Research & Development E-Mobility at ZF. The task is not easy since the electric drives that leave the factory are already about three times more efficient than the internal combustion engine.

Recently unveiled: ZF EVmore

Recently unveiled: ZF EVmore

"Mission accomplished" is the feedback from the team. This success is visible in the form of the ZF EVmore, an all-electric drive concept that is tailored to maximum efficiency. And it is no exaggeration as, after all, an efficiency of 96.3 percent from the battery to the wheels sets the new benchmark for close-to-production technology. Of course, the 96.3 percent relates to the best operating point rather than to the average. This was made possible thanks to a variety measures on the motor, transmission and electronics. "With the EVmore, we have reduced drive losses by 30 percent compared to the volume production vehicle we chose," reports Dr. Stephan Demmerer, Head of Advanced Development Electric Drives at ZF's Electrified Powertrain Technology Division.
Here are some of the details of how the engineers managed to design the driveline so that the end result is a close-to-production ultimate efficiency car.

Two electric drives for maximum variability

Two electric drives for maximum variability

The EVmore uses two electric drives: One on the front and one on the rear axle. They deliver a combined system output of around 350 kW. ZF's (decouplable) four-wheel drive eConnect is the result of the interaction of these drives. In everyday situations during normal driving in traffic, a permanently excited synchronous motor (PSM) at the front, which is tailored to maximum efficiency and forms part of the ZF portfolio, delivers the propulsion. To this end, the engineers used thinner sheet metal in the rotor and stator, for example. The rear-axle drive also features state-of-the-art technology with an asynchronous motor (ASM). Whenever more power is required at short notice – when setting off on a hill or when overtaking – the rear-wheel drive automatically switches on and off again within milliseconds.
At the heart of the driveline is the efficiency-optimized 800-volt motor on the front axle, which uses virtually no heavy rare earth elements.

Green credentials: Almost no rare earth elements

Green credentials: Almost no rare earth elements

The focus with the driveline design was, however, not only on performance and efficiency. It was also on sustainability. A permanently excited synchronous motor (PSM), the kind the EVmore uses on the front axle, normally uses permanent magnets with heavy rare earth elements. These elements ensure the magnets are thermally stable. "Thanks to improved cooling of our motor, we can almost completely dispense with the use of heavy rare earth elements. Mining these elements is not environmentally friendly and is restricted to a handful of places around the world, which creates a certain dependency," explains Dr. Demmerer. In addition, software throttles the motor so it never operates at the performance limit. This is also beneficial for temperature buildup in the electric motor.

Lighter thanks to clever material and function reductions

Lighter thanks to clever material and function reductions

The transmission on the front axle also ensures greater efficiency thanks to less weight and reduced friction. Where an auxiliary shaft after the input shaft helps to reduce the engine speed in the usual two-stage transmission, ZF's new single-stage transmission dispenses with the intermediate shaft. This transmission type has only been used in Formula E so far. Fewer moving parts mean less tooth engagement, less friction in bearings and less weight. On average, losses generated in the transmission are around a third less. "At a speed of 100 km/h, losses can even be reduced by 50 percent. To achieve this, we have also reduced the number of rotating parts from nine to six," says Dr. Christian Guerlich from the development team.
Taking the lead from motor racing transmissions: Less weight and the elimination of the intermediate shaft increase the efficiency of the single-stage transmission of the EVmore.

For the perfect driveline, the associated control software is at least as important as the electric motor and transmission. Engineers therefore developed special drive software for the EVmore. It automatically controls the motors on the front and rear axles, depending on the driving situation, delivering either maximum efficiency or maximum performance. The holistic optimization approach for electric motors, transmissions and inverters with software increases the range of the EVmore by around five percent compared to the original vehicle. Incidentally, the decision for the 800-volt technology in the EVmore is also beneficial when charging the battery pack with up to 350 kW as it adds fast-charging capability to the vehicle. This in turn cuts waiting times at the charging station substantially. It takes under half an hour to charge the battery from 20 to 80 percent of its capacity. This also demonstrates how much "more" the EVmore offers compared to current volume production technology in electric vehicles.
"With the EVmore, we have reduced drive losses by 30 percent compared to the volume production vehicle we chose."
Dr. Stephan Demmerer, Electrified Powertrain Technology Division at ZF

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