Silent Running

ZF is reducing fuel consumption with technology that temporarily allows a car to sail as quietly as a sailing boat. What exactly is behind the concept?

If you have two new cars and one of them can suddenly freewheel and the other is even able to switch off the engine completely, then it can be said that everything is literally in the green range – assuming that both vehicles are equipped with ZF’s fuel-saving sailing function. This separates the engine from the rest of the driveline in suitable situations and automatically engages the clutch. ZF has now also made this possible for automated manual transmission (AMT). The clutch-by-wire system had already established the basis for sailing on vehicles with manual transmission. “And also ZF’s dual clutch systems and automatic transmissions are ready to sail,” says Jörg Buhl, who oversees the design of control systems for manual and automatic car transmissions at ZF. “All in all, this probably makes us the only partner that supplies a sailing concept for every car segment and virtually all commonly used transmission types.”

ZF’s clutch-by-wire system actuates the clutch electronically and, if necessary, without driver interaction.

A ZF test vehicle recently demonstrated the potential of the AMT sailing function: It consumed up to 8.5 percent less fuel than an identical car without this feature – recorded in normal road traffic conditions with the engine running at idle speed while sailing. An assumed actual consumption rate of 5.9 liters per 100 kilometers could therefore drop to 5.4 liters in future. “In view of the ever stricter emissions standards being imposed around the world, sailing could soon play a key role,” says Buhl. The EU, for instance, has set a fleet fuel economy target for manufacturers of just 95 grams of CO2 per kilometer by 2021, which is equivalent to around 4.1 liters of petrol and 3.6 liters of diesel per 100 kilometers.

Clever clutch actuation

The sailing function makes the most of situations in which the car also has sufficient momentum to freewheel forward. Such situations include country roads with a slight downward gradient or longer stretches of straight road approaching city limit signs. Intelligent clutch actuation systems from ZF then temporarily disengage the combustion engine so that it doesn’t suppress this kinetic energy. These can be electronically controlled and activated by wire, providing the basic prerequisite for the sailing function.

The disengaged engine can then idle while consuming a minimal amount of fuel. Ideally, however, it stops completely for a short period of time, because this is the only way in which zero emissions can really be achieved. “It is ultimately the vehicle manufacturers who decide how this concept is integrated into a car,” says Buhl. “Meanwhile, we have further developed the sailing function, turning it into a sailing manager that can react even better to each driving situation.” Modular software elements make it possible, for example, to connect the feature to a navigation system or ZF’s adaptive cruise control (ACC) and also enhance it by adding a traffic flow or rolling resistance detection system.

Systematically exploiting the advantage

Besides the sailing function itself, ZF also manages the electric peripheral equipment, including the EPS and integrated brake control (IBC). As such, every individual component within the system architecture ultimately interconnects in concert. This also proves advantageous at the end of the sailing phase when the restarted engine re-engages without jolting the moving car. In the case of automated manual transmissions, ZF has been working since the 1990s on continuously improving clutch actuators and their control systems. This wealth of experience benefits both customers and car drivers when it comes to sailing.

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