ZF launches clutch-by-wire for passenger cars

A ZF innovation has brought fuel-saving coasting function to manual transmissions for the first time and gives vehicle manufacturers new scope for designing much more efficient passenger car drivelines with manual transmissions.  

With Clutch-by-Wire (CBW), the technology company has launched an electromechanical actuator system that, for the first time, controls the clutch independently of the driver's left foot and without a mechanical connection to the pedal. This adds functions to manual systems that were previously reserved for automatic or automated transmissions: above all fuel-minimising coasting.

Compared to systems that change gears fully automatically, the market share of manual transmissions will continue to contract slightly. However, absolute values and current forecasts also show that the overall sales curve for manual transmissions will continue to point upward for a long time. This applies above all to vehicle segments from the medium class down, and especially to emerging markets. This and steadily tougher emission and consumption regulations worldwide make it imperative to exploit efficiency potential in conventional drivelines as well.

ZF says it has also identified useful potential in the clutch system – and already realised a current development project named Clutch-by-Wire. The idea is that the clutch is actuated electronically by an integrated electric motor. "Fuel consumption and therefore CO2 emissions drop by up to 10 percent," explains Jörg Buhl, Head of Actuator System Design/Engineering at ZF Friedrichshafen AG. "This figure results solely from automatic disengagement and engine switch-off in appropriate driving situations, known as the coasting function. Our Clutch-by Wire system allows vehicle manufacturers to achieve these effects in manual as well as automated transmissions."

No more stalling
The option of controlling the clutch by control unit and actuator alone with no mechanical connection to the pedal also opens up a raft of new comfort and safety benefits for manual transmissions; this includes start-up without operating the clutch pedal, as well as an anti-stall function that kicks in for instance if the clutch pedal is released too soon or during emergency braking. The anti-stall feature then fully or partly opens the clutch before the engine speed drops below a critical level. Added to this is the creep function where controlled slipping makes maneuvering and driving in traffic jams easier.

At the same time, because the pedal is independent of the driveline, the design possibilities for different models are almost limitless. For instance, the pedal could be made easy to press even if it comes with a sporty characteristic curve. A sensor detects actuating speeds and paths, and the electronic and mechanical systems translate these into the clutch action the driver wants. Once set, the pedal feedback remains constant throughout the vehicle's entire service life. Even clutch wear doesn't affect it because the new CBW actuator automatically balances this out. In this context, it is crucial that, although car drivers benefit from lower fuel consumption, new functions, and an ultra-precise coupling feel, they still control the vehicle in the familiar way. Vehicle manufacturers on the other hand can continue to install the same volume production transmissions as before.

A compact revolution
The CBW actuator comes with a powerful, brushless DC motor that actuates the clutch in place of a cable-controlled or hydraulic system. The integrated ZF CCU (Clutch Control Unit) controls the system depending on the pedal parameters and the function requirements in each case.

When designing both actuators and software, the developers benefited from ZF’s  expertise in automated manual transmissions  and also managed to combine high effectiveness with low electricity consumption in the CBW actuator as well as uniting the system components in a compact design that saves installation space. As a result, the CBW from ZF can be integrated into almost all common manual-control drive concepts relatively easily and cost efficiently.