The classical automatic transmission is no longer unrivalled. Sequential transmissions conquer the market. Indeed, they did not yet accomplish the full shift comfort, however, excel the automatic transmissions in terms of efficiency. With the help of the twin clutches displayed above, they manage a shift speed which makes them interesting also and especially for sporty vehicles. And they are almost as compact as traditional friction clutches.

Because engaging takes place automatically, the friction surge should be small, e.g., while driving in reverse, to protect the driver against slight acceleration movements and thus uncontrollable behavior. Modern engines develop such a high torque already for the initial drive that the clutch has to fulfill particular requirements. Direct shift gearboxes are intended for almost the whole market range, from economically to sportily, lowering the production costs due to large numbers.There are clutches with completely dry friction surfaces (see picture 2) and others running in an oil bath (see picture 1). These are called wet clutches. They feature a larger friction surface with the same maximum torque requirements. The picture on top is interesting, because the internal clutch with the lower diameter has the bigger number of lamellae. Up to now multi disc clutches were almost exclusively used for two-wheeled vehicles and in combination with an automatic transmission, now they also spread out to the remaining four wheeled vehicles.

While however classical wet clutches operate in two-wheel vehicles mostly manually and perform their task constantly in oil, causing a drag loss, they co-operate in the car with an automatic transmission system with torque converter, very seldom already fully automatic. They are activated by a control unit, either hydraulically (see picture 1) or electrically (see picture 2).

Wet clutches are preferred at the moment, because the oil cycle leads away the developing heat faster. Besides, new oil is supplied only as long as the actual engagement lasts. Driving at a constant speed without shifting gears there is no oil to avoid drag losses. Thus, this design may compete in the main operating range with the dry clutch. Of course, the linings are not the same, but the reduced friction of the wet linings is sufficient once the clutch is engaged.Since the clutch is automated, the possibilities have increased. Thus it is possible to use, as with construction machines, the hydraulic or electric energy either for opening or for closing. If the designer predicts that the clutch will mostly be running in its biggest gear, it is for example possible to design the clutches in such a way that one is open without additional energy and the other is closed. Generally, the designer presets both clutches equally. Interesting is the possibility to operate both clutches at the same time via one hydraulic or electric lever element. 03/07

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