Taking an idea to success
The dry dual clutch
A technology that more than 15 million motorists are taking for granted today ultimately looks back on a near-100-year development history: the dual clutch transmission. Up until 1931, Morgan has been installing an early version of the two-speed dual clutch transmission with two drive chains to the rear wheel into its three-wheelers. And the first patents hark back to the Frenchman Adolphe Kégresse in 1939. In the 80s, Porsche experiments with the first “wet” and pretty rough Porsche dual clutch transmissions (PDK) in racing. Audi adapts the Porsche system for use in rally racing, but deploys it only in two trial runs. At the end of 2002, VW installs the first wet dual clutch transmission (DSG) in the Golf R32. In 2008, the dry dual clutch begins to combine efficiency and comfort.
The operating principle of the dual clutch sounds simple. It consists of two partially automated manual transmissions which, combined, enable fully automated gear changes without interrupting traction. One of the transmissions is responsible for the even and the other one for the odd gears. In the best case, the dual clutch delivers optimized performance, fuel economy and a gain in comfort. Whereas in wet systems torque is transferred via plates running in oil, dry dual clutches deliver traction via the friction linings of the clutch plates.
As an innovation driver, Schaeffler’s LuK brand opts for the development of a dual clutch at the end of the 20th century as well. Dr. Wolfgang Reik (68), a physicist and former LuK chief developer, recalls the beginnings of the development project: “The efficiency of the automatic torque converters was pretty poor back then. Especially in the part load range, their operation was less than perfect, so we wanted to combine the two worlds of automatic and manual transmissions – and so shifting comfort and efficiency.” A roughly outlined objective had been set, but how exactly was it to be achieved? A tricky question which Reik and his colleagues extensively discussed too: “Some of us wanted to embark on the dual clutch adventure while others tended to prefer a much simpler automated manual transmission. However, the interruption of traction with those transmissions is much harder to accept for more exclusive passenger cars.” The discussion drags on for months on end – not surprisingly, considering that the decision will set an important direction and involves millions of deutschmarks in capital expenditures. Ultimately, the “or” turns into an “and” – with both systems continuing to be pursued. In 1997, marking the first step in the field of automated manual transmissions, the world’s most compact automated clutch system goes into production in the Mercedes A-Class, followed three years later as a world first by the automated LuK manual easytronic transmission of the Opel Corsa.
Meanwhile the dual clutch advocates continue to struggle with another fundamental decision: Should a wet or a dry version be developed? Dr. Reik: “The ‘dry’ group used the argument of better efficiency as wet plates always involve minor drag torques. The proponents of the ‘wet’ solution believed that in the case of high engines torques dry dual clutches would not achieve the service life of the whole car. This was almost a religious war – after all, both sides used the right rationale to support their positions.” Development partner VW finally opts for the dry version, not least because Schaeffler engineers predict up to ten percent better fuel economy compared with the wet DSG of the design back in those days and more than six percent compared with a manual transmission.
Some of us wanted to embark on the dual clutch adventure while others tended to prefer a simpler solution
Dr. Wolfgang Reik,
then chief developer at LuK
Even though the operating principle of the new dry dual clutch is similar to that of its wet cousin, a number of hurdles have to be overcome before the efficient shifting aid is ready for the market. “For two years, we’d visit VW every week and sometimes go home with doubts in our minds due to the many issues,” Dr. Reik recalls. “As soon as we’d successfully resolved a concern two new ones would emerge, so it wasn’t uncommon for someone on our team to become discouraged. Fortunately, we weren’t all discouraged at the same time …” Then, in 2007, VW starts installing the dry dual clutch, initially in the Golf and Touran.
The meticulous and untiring development work done by Dr. Reik and his Schaeffler colleagues on the one side and VW on the other pays off. Motor journalists are full of praise for the new component. “ADAC Motorwelt,” notably the magazine with Europe’s largest circulation, writes: “The dry clutch now being utilized exhibits no abnormalities even when the engine is cold in winter, the shifting events are smooth and hardly perceptible. On the whole, the transmission in the Golf leaves an excellent impression. “Auto Bild” says: “The transmission changes gears as gently as an automatic in a luxury sedan. When starting from rest, the clutch engages gingerly and a creep function assists in maneuvering. The Golf practically always operates in the optimum engine speed range across the seven speeds.”
Today, Schaeffler produces both wet and dry dual clutches, with the dry ones accounting for the larger volumes. The dry solutions tend to be utilized more for cars up to 150 kilowatts and the wet ones for the powerful models up to 500 kilowatts. Dr. Reik regards the dual clutch system as an “invention by and large completed.” For his successors in the Schaeffler research departments, though, he already sees new challenges on the horizon, for instance, hybrid cars. Reik: “If you position the electric motor there on the transmission input, you need a third clutch in order to be able to shut off the IC engine and drive in fully electric mode.” So, following one and two clutches, there’ll be a triple in a manner of speaking.
The fully variable UniAir valve control
UniAir is another typical example of Schaeffler technologies drivers arguably never get to see but that save them money, provide comfort and benefit the environment. It’s the world’s first electrohydraulic system for fully variable control of the engine valves in gasoline and diesel engines. “So far, the system is exclusively utilized on the intake side of gasoline engines,” says Michael Haas (56), who is in charge of the variable valve train product line at Schaeffler. “However, there are no limits to its utilization in diesel engines or on the exhaust side.”
With UniAir the engine can always operate in the optimum efficiency range. Fuel economy with UniAir in combination with downsizing improves by up to ten percent. At the same time, power output can be raised by ten percent and torque in the lower engine speed range by up to 15 percent. In addition, vehicle occupants benefit from a more comfortable ride, especially in dense traffic. “UniAir is not only efficient in terms of fuel economy and emissions but enhances driving pleasure as well due to its fast response,” says Nicola Morelli (48), Director System Development UniAir, adding that, “UniAir responds faster to the driver’s demands, particularly at lower speeds.”
UniAir technology is invented in 1999 by Centro Ricerche Fiat (CRF). However, the Italians lack the know-how for development, industrialization and manufacturing that is necessary to put the system into production, so they turn to Schaeffler to obtain it. Together the automobile manufacturer and the automotive supplier by 2009 develop the product which Schaeffler has since been manufacturing and marketing as fully variable valve control thanks to an exclusive license. Initially sold to FCA (Fiat Chrysler Automobiles), the system dubbed as “MultiAir” operates in engines of Alfa Romeo, Fiat, Chrysler and Jeep. It’s the core element of the “TwinAir” two-cylinder family. “We’ve made the system salable,” Haas agrees, “because there’s a world of difference between inventing and industrializing.” This is due to the many components belonging to UniAir, as well as the precision in the microscopic range. “Manufacturing requires appropriate production processes that only Schaeffler masters,” says Haas. “We’re the hydraulics specialists with the requisite precision plus the capability of manufacturing the right components at the lowest possible costs.” The first UniAir engine in 2009 is the Fiat four-cylinder from the Alfa Romeo MiTo 1.4, a F.I.R.E. unit (Fully Integrated Robotized Engine) that has been completely assembled by robots.
There’s a world of difference between inventing and industrializing
Michael Haas,
Director Variable Valvetrain Product Line
A fully variable valve control system specifically developed for Jaguar Land Rover has recently been introduced in the Ingenium four-cylinder engine family, combined, for the first time, with two hydraulic camshaft adjusters. As a result, for instance, the engine can be shut off with low vibrations in start-stop mode and instantly restart. Fast control of the valves with cyclical precision is another advantage. It allows spontaneous torque development without impairing efficiency due to excessive retardation in ignition timing.
Now Schaeffler is working on making UniAir even more efficient. Conceivable for instance is a reduction of components, as well as the utilization of enhanced combustion strategies. “There’s still ample development potential,” says Haas. “We’re also thinking about an integration of fully variable UniAir valve control directly in the cylinder head …”
In conventional valve control, the cams of the rotating intake camshaft cause the valves to open and close. The air required for combustion is controlled by the position of a throttle valve. The disadvantage of this type of valve control is that timing is optimized for a specific load condition of the engine. Even VCT variable cam adjustment cannot reflect all the continually changing driving conditions, especially not at the frequently required pace. As a result, the engine does not run efficiently in all ranges. By contrast, Schaeffler’s fully variable UniAir valve control always optimally adjusts to changing power requirements.
