ZF announced its new two-speed electric drive for passenger cars, which integrates an advanced electric motor with a shift element and power electronics. The improvement in energy conversion efficiency compared to previous e-drives extends the driving range for each battery charge. Its compact design makes the new drive system of interest for passenger cars in the compact class. The modular design of the unit can also be fine-tuned and scaled up for use in sports and performance vehicles.
“For electric vehicles in everyday use, it is important to obtain as much range as possible from each battery charge,” stated Bert Hellwig, head of System House at ZF’s E-Mobility division. “Every percent of improvement in energy conversion efficiency translates into 2% 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,” said Hellwig.
Vehicles with the new two-speed drive consume less energy, which in turn extends range by up to 5% when compared to a one-speed unit. Shifts take place at 70 km/h (43 mph). By connecting to the vehicle’s CAN communication, it is also possible—if the customer wishes—to devise other shift strategies, possibly linked to digital map material and GPS.
For example, the vehicle could identify from the GPS route programming how far it is to the next charging station, enabling it to respond predictively by switching into eco-mode. More effective shifts would also be possible to account for topography on the interstate and on inter-city journeys. The software in the drive can also be updated thanks to the network link to Cloud services via over-the-air updates.
For vehicle manufacturers, the new two-speed drive offers two options for using improved energy conversion efficiency. The OEM (original equipment manufacturer) could either go for an extended range while retaining the same size of accumulator, or can use a smaller accumulator.
Thanks to the design of this unit that optimizes installation space, the two-speed system is suitable for compact passenger cars where interior space is at a premium.
The two-speed concept offers benefits for OEMs who are also pursuing performance.
“Until now, with electric motors, vehicle manufacturers have had to choose between high initial torque and a high top speed,” explained 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.”
ZF’s modular approach combines the two-speed gearbox with even more powerful electric motors rated for up to 250 kW. This delivers enhanced acceleration and, potentially, faster top speeds. With its modular concept, the new drive can meet a variety of requirements.
In addition to the new e-drive, ZF has developed a new eight-speed transmission for rear-wheel- and all-wheel-drive vehicles with front-longitudinal drive configurations, for which Fiat Chrysler Automobiles has selected it as supplier. It is the second largest single order in ZF’s history; only three months ago, ZF signed a first contract for delivery of the latest generation 8HP to BMW, which then marked the largest single order in ZF’s history.
"We are pleased being nominated as global transmission supplier by FCA,” said ZF's CEO Wolf-Henning Scheider. “This is our second major order for the new 8HP, and it confirms our strategy to focus on plug-in hybrids as an every-day solution and to develop attractive products in these areas.”
The lead production facility for the new (and fourth) transmission generation, which will start series production in 2022, will be ZF’s plant in Saarbruecken. The company also plans to start production of the technology at further locations including the U.S. and China in the future.
The new eight-speed automatic transmission could be installed in almost all vehicle segments with a front-longitudinal drive configuration. A technical innovation of the upgraded transmission is the integration of an electric drive.
With this modular transmission concept, manufacturers will be able to easily change from one transmission variant to another, which gives them the flexibility to react to market requirements. It features a new modular construction system that allows for both 48-V mild hybrids and plug-in hybrids with electric power of up to 160 kW.
Traditionally, hybrid transmissions are built by taking an efficient automatic transmission and replacing the torque converter with an electric motor with a higher power density. ZF has designed this new transmission for hybridization from the start. The modular construction system enables mild, full, and plug-in hybrid drives to achieve top performances between 24 and 160 kW.
The power electronics are no longer designed as a separate unit, but instead are fully integrated into the transmission housing without increasing its outer dimensions. With a new, significantly smaller hydraulic control unit, ZF has created the required installation space for the electric and electronic components.
Although ZF estimates that at least 70% of all new vehicles in 2030 will still have an internal combustion engine, a plug-in hybrid drive could considerably lower the engine‘s CO2 emissions. This is contingent on electric range and electric power, both of which must allow for driving in everyday traffic with battery power only.
While ZF’s electric motor has a maximum power of 160 kW, continuous output is rated at 80 kW. The maximum torque is 450 N·m, which allows for swift passing in e-mode.
This does not require a significant increase of the packaging size since ZF relies on a new generation of internally developed electric motors and uses welded copper rods instead of coiled copper wire. This technology, known as the “hairpin technique,” allows the copper fill level to be significantly increased, which has a good impact on the power density.
In addition to plug-in hybrids with high voltages of around 300 V, ZF believes that mild hybrids will also play a big role in the coming decade. They have a voltage level of 48 V and allow for considerable CO2 savings by generating power through regenerative braking recovery that can later be used as drive power.
The 48-V drives can be installed in several driveline locations; on the crankshaft at the engine output and on the input shaft are particularly efficient. The new generation from ZF is suitable for both installation types. The electric motor can reach a maximum power of up to 25 kW and thus optimally support the internal combustion engine in virtually all operating parameters.
Electric motors must be controlled via power electronics, which both convert the direct current from the battery into the required alternating current and control the power and speed of the electric motor. Until now, these power electronics were housed in a separate, shoebox-sized box for all series hybrid transmissions. However, with the fourth generation, ZF integrated the complete power electronics into the transmission housing for the first time. This is a great advantage to automotive manufacturers since hybrid drive assembly is no longer considerably more complex than that of a conventional transmission. In addition, fewer high-voltage cables are needed in the vehicle, which enhances safety.
The engineers at ZF were faced with the challenge of fully integrating the power electronics into the transmission without increasing the outer dimensions. They achieved this through an ingenious cooling concept, as well as through several other developments. The power semiconductors, especially the IGBTs for the high-voltage model, produce a relatively high amount of waste heat. This is dissipated by connecting the power electronics to the refrigerant circuit of the vehicle‘s air conditioning system.
However, the most important development was the considerable decrease in the size of the hydraulic control unit, which triggers mechanical shifts in the transmission. The hydraulic control unit in the current generation of the eight-speed automatic transmission requires a volume of 3.1 L, but in the next generation, it will shrink to 1.8 L. This is made possible primarily by using direct shifting valves. In contrast to the electric pressure actuators that were previously used, these electromagnetic actuators no longer require additional pistons and bushings.
Additional components of the new eight-speed construction kit were designed for hybrid operation, for example with the oil circuit. Previously, two oil pumps were used, but in the future a single power-split pump will be used. When the internal combustion engine is shut down, it is driven by a small, directly attached electric motor.
The mechanics of the new transmission system also contribute to efficient hybrid operation. Although the body remains the same with four planetary gear sets and five shift elements, efficiency was further increased by fine-tuning the friction power. Thanks to this, CO2 emissions were lowered by 1 g/km during operation with the internal combustion engine. During electric operation, the range has increased accordingly.
ZF will begin manufacturing the new-generation 8-speed automatic transmission in Saarbrücken, Germany, in 2022. The market launch in China and the U.S. will follow shortly thereafter. ZF can thus make significant strides toward ensuring that hybrid drives gain quick acceptance while reducing vehicle CO2 emissions in market segments where electrification cannot be fully implemented immediately.