Plug-in Hybrid The best of both worlds

Plug-in hybrids are becoming increasingly popular because they combine the range of an internal combustion engine with the reduced emissions of an electric motor.

In hybrid vehicles, an internal combustion engine works alongside a locally emission-free electric motor. But in many full-hybrid vehicles, these two components work very unevenly. Existing full hybrids, for example, can only drive around three kilometers (two miles) on battery power, at speeds never exceeding 30 mph (50 km/h). Most of the time, these hybrid vehicles are powered by their internal combustion engines. The correspondingly small battery is only recharged while the car is driving – usually by recovering kinetic energy during braking maneuvers, a process known as “recuperation” or “regeneration”.

A plug improves performance

That’s all due to change in the near future: future hybrid models will rely on more powerful batteries that can also be recharged from external power sockets. ZF is supplementing this plug-in hybrid technology with a hybrid powertrain that enables the cars to operate just like all-electric vehicles: with a range of up to 50 kilometers (30 miles) per battery charge, they can meet most commuters’ daily travel needs using electric power alone. The electric drive is also more than capable of coping with brief sprints at speeds of up to 120 km/h (70 mph) on major roads or highways – the gasoline engine only fires up at higher speeds. The presence of an internal combustion engine also solves the range problem that still afflicts all-electric vehicles.

With the new plug-in hybrid system, ZF has made the next big step in developing the hybrid technology the company has been offering since 2008. “We’ve introduced so many innovations and new developments that we’re now offering automakers a de facto new all-in-one system that will enable them to electrically power a large part of their model range,” explains Dr. Ralf Kubalczyk, head of Hybrid Transmission Development at ZF, and also in charge of integrating the all-in-one system. The more powerful electric motor plays an important role in this new setup. ZF will soon start to produce 90 kW electric motors for the plug-in hybrid system.

Two concepts

ZF's Schweinfurt plant started producing DynaStart modules capable of producing 12 kW back in 2008.

Alexander Gehring, head of Electric Drives at ZF, and his team are working on two different concepts: first, on a hybrid module that can be incorporated into ZF’s own transmission systems. In this case, the electric motor, separating clutch and torsion damper have all been redesigned for the system – and installed in the transmission’s clutch bell housing. But the team is also developing modules for other transmissions, so that automakers’ development engineers don’t have to make any further changes to the transmissions they are already using. A few things have changed in the transmission itself, which is based on ZF’s successful 8-speed automatic transmission, the 8HP. Kubalczyk’s team of engineers have customized it to form an integrated plug-in system.

The third key component is the power electronics, which play a vital role in the plug-in hybrid system’s energy management by converting the battery power from DC voltage into the high AC voltage that the electric motor needs to drive the car. The power electronics also level out any voltage fluctuations that drivers would otherwise experience as unexpected and uncomfortable power fluctuations when accelerating. Not only does ZF offer customers an all-in one, single-sourced system. All the components in ZF’s modular hybrid toolkit are also available separately.

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