Gearbox accommodates optional electric drive

February 21, 2017 // By Christoph Hammerschmidt
Together, automotive supplier ZF Friedrichshafen and car manufacturer Porsche have developed an eight-speed dual-clutch transmission. The special thing about this gearbox: It can accommodate a hybrid module as an option, accommodating an electric motor without affecting the installation dimensions.

One major development goal was to make electric power an option for this transmission: Thanks to a new gear set design, it was possible to integrate a 100 kW hybrid module without increasing the length of the transmission, compared to the current 7DT sports transmission. Another goal was an integrated all-wheel distribution option, driving the front axle in the most fuel-efficient manner possible using a hang-on clutch. Power loss in the basic transmission is reduced by up to 28 percent thanks to several factors, including an optimized electronic transmission control unit (TCU).


ZF has customized an already compact hybrid module for the optional variant of the new 8-speed dual clutch transmission. It includes a torsional damper, a separating clutch with actuators as well as the electric motor, and it can be integrated directly into the clutch bell housing. This does not alter the dimensions of the overall transmission but has considerable impact on its properties: With 100 kW peak output, 55 kW continuous power and 400 Nm torque, a vehicle can accelerate up to 140 km/h on pure electric power alone. With this technology, all other hybrid functions are also possible– from recuperation to boost mode.

The TCU and the transmission software for the 8DT were developed by ZF independently of the Porsche assignment. The TCU is not integrated, but mounted externally. This prevents the electronics from limiting the transmission sump temperature to a maximum of 150°C.

The TCU allows the 8DT interact at high efficiency with other vehicle systems, optimizing fuel consumption in conjunction. When operating in ECO mode, the control unit lowers the transmission pressure when driving conditions are suitable. This reduces power consumption without impairing comfort or performance. In stop-start mode, the combustion engine can be switched off from a speed of 10 km/h, while the vehicle is rolling to a stop. Furthermore, coasting with the engine off is possible even at high speeds.