LuK: an abundance of transmission options.

Considering that one in every four cars that rolls off assembly lines around the world is fitted with one of its clutches, very little is known about LuK, part of the Schaeffler Group, Germany's largest family owned business. However, this subsidiary has a portfolio of interesting products that include the dual mass flywheel, the twin clutch gearbox, the belt-driven starter generator and a continuously variable transmission (CVT). Additionally, it is the worldwide leader in tractor clutches. Transmission technology has developed into an innovative driving force in the automotive industry with both autoshift and twin-clutch gearboxes providing alternatives to conventional automated transmissions. For LuK, these gearbox variants comprise its XSG family that embraces Electronic Clutch Management (ECM), which dispenses with the clutch pedal, the Auto Shift Gearbox (ASG), where the actual gear shifting is automated, and the Uninterrupted Shift Gearbox (USG), where a partial filling of the torque interruption during a gearshift is achieved with an additional clutch. It also includes the Parallel Shift Gearbox (PSG), which belongs to the twin-clutch gearbox group, and the Electrical Shift Gearbox (ESG), in which a starter-generator is coupled in parallel to one of the two input shafts.

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It is the twin-clutch PSG that excites Dr. Peter Gutzmer, LuK president and CEO, who had spent 17 years with Porsche working in various positions in engine and vehicle development. Known as DSG--Direct Shift Gearbox--in Volkswagen-Audi parlance, it combines the advantages of a conventional six-speed manual-shift gearbox with the qualities possessed by a modern automatic transmission. The driver enjoys immense agility and driving pleasure with smooth, dynamic acceleration without any interruption to the power flow. The technical basis of the DSG, developed in the VW-Audi case by BorgWarner, is a double clutch. It consists of two wet plate-type clutches with hydraulically regulated contact pressure. One of the two clutches engages the odd-numbered and the other the even-numbered gears. This principle enables gearshifts to be made without interrupting the power flow and keeps the shift times extremely short. While the first clutch is transmitting the power, the second clutch is ready to engage the next gear, which is pre-selected. When the driver makes the gearshift, the first clutch is released and the second engages, so that the gear shift takes place in a fraction of a second. The driver can operate the DSG manually or allow changes to take place automatically. In the automatic mode there is a choice between the well-balanced, comfortable standard shift settings and a program with greater sports emphasis. Manual shifts are made either at the gear lever or at shift paddles behind the steering wheel. "I believe very strongly that within the next five years we will see this kind of transmission being more widely offered, especially in Europe," says Gutzmer. "It has several advantages with the fuel economy being for me more or less the hidden one. It has greater agility and driving comfort at the same time and this is what you really can tell. The customer also feels like a Formula One driver!"

Building on the PSG concept is ESG, in which a starter-generator is linked to one of the gearbox shafts. Advantages include functions such as start/stop, energy recuperation by regenerative braking and electric powered driving, all with a compact design. Furthermore, with the combustion engine switched off, the air conditioning system can be operated using the electric motor. The ESG can result in reduced fuel consumption over 20% compared to a manual gearbox. "Looking to the future we see hybrid solutions," says Gutzmer. "To reduce fuel consumption and emissions even further, the combustion engine will only be used when needed. We will see start/stop, and electrically supported driving, but the applications might be different in different parts of the world."

An important part of the argument in favor of the automation of manual transmissions is the improvement in fuel consumption, says Gutzmer. If the manual transmission is taken as a basis, automatic transmissions suffer from greater fuel consumption at the same shift point selection due to hydraulic power loss. Automatic transmissions and automated gearboxes can use the choice of more favorable operating points in the engine map for shift point selection to their advantage, resulting in reduced fuel consumption in legally defined cycles. Moreover, this reduction in fuel consumption through automatic shift point selection is also realized in practice, since the average driver using a manual transmission generally avoids driving at economic low engine speeds.

While the single clutch automated manual transmission has come in for some criticism for its lack of finesse and smoothness, Gutzmer believes that it still has a future. But he thinks they are only likely to come under threat once the economies of scale are applied to the double-clutch gearboxes and their prices start to fall. "I expect the double-clutch gearbox has the potential to come down the model range by 2010 to 2012. It is in the 1.6- to 3.0-liter segment where we feel the dual clutch approach is very appropriate."

Another solution that LuK has been actively involved in is the continuously variable transmission (CVT), particularly the MULTITRONIC transmission from Audi that features major components produced by LuK. In 1999 it became the first system supplier to introduce CVT component for torque ranges of 300 Nm and above, and put them into production. Together with high-performance engines, these components have redefined the concepts of dynamics and comfort with excellent fuel consumption figures. However, the surprising thing is that the concept has not been taken up by other manufacturers despite the torque capacity now being increased to around 400 Nm. The reason, says Gutzmer, is that the larger diesel engines are now exceeding this figure and the public at large still does not readily accept it. As evidence of this he points out that Audi had to introduce artificial shift points "which is pointless on a CVT but that is what the customer wants". As for handling greater loads, he says, "We would like to go higher than 400 Nm, but we do not have a solution to get the cost disadvantage out of that. We talk about split arrangements but we have not yet come to a cost-effective solution. However, we are working with others on higher torque ranges at the pre-development stage. If we succeed there might be a chance in 2012."

By William Kimberley, Editor

Automotive Engineer, London, [e-mail]