CVT transmissions have more problems

Converter, CVT, double clutch: advantages and disadvantages - Automatic is not just automatic

Munich / Friedrichshafen - Today's automatic transmissions no longer fit the cliché of the lame fuel guzzler. Modern transmissions are efficient and sporty and largely regardless of the principle on which they are based. There are essentially three systems:

  • the automatic converter
  • the dual clutch transmission
  • the continuously variable transmission (CVT)

Automatic converter: Offers a high level of shift comfort

At ZF in Friedrichshafen, a transmission is put through its paces on the test bench Source: ZF

With the classic automatic converter, a torque converter ensures a high level of comfort when starting and shifting. It is also suitable for high torques and therefore for powerful engines. But: "The automatic converter has disadvantages in terms of efficiency", says Karsten Stahl from the Research Center for Gears and Gear Construction (FZG) at the Technical University of Munich. Part of the engine power "seeps away" into the components.

An intelligent control system and highly efficient components can largely compensate for the disadvantages today. So achieved one modern automatic converter under certain circumstances a lower fuel consumption than a manual transmission. In terms of switching speed, modern systems can no longer be compared with old automatic converter systems.

And they have another advantage: "Torque converters still offer the highest starting and maneuvering comfort", says Jürgen Greiner, Head of Development for Passenger Car Transmission Systems at ZF. This is ensured by planetary gears, which, however, are limited in their speed stability and are therefore not suitable for high-revving sports engines.

Dual clutch transmission: especially for sporty engines

Dual clutch transmissions are now just as comfortable as the automatic converter. A - insignificant for most drivers - However, according to Stahl, the disadvantage of the technology is that gears can only be shifted sequentially, So no jumps over several courses are possible.

Therefore a dual clutch transmission can withstand higher speeds than an automatic converter. "This makes it particularly suitable for very sporty, high-revving engines," says ZF developer Greiner. Very dynamic gear changes without a noticeable interruption in tractive power also speak in favor of use in sports cars.

Infinitely variable transmission: that does not require any gears

This selector lever controls a BMW dual clutch transmission with seven gears Source: BMW

With a continuously variable transmission (CVT), as the name suggests, there is no fixed gear. The translation is continuously changing. "The efficiency is, however, in particular because of the

required oil pressure pump less favorable than with a modern automatic transmission "says Stahl. Here too, some of the power in the transmission is lost. In Europe, CVT transmissions are also unpopular because of their acoustic behavior. Engine RPM doesn't follow speed and acceleration, which many drivers feel strange.

However, the continuously variable ratio optimizes the consumption of the car, as the engine can be operated in the most efficient speed ranges much more frequently than with other concepts. Audi, Toyota, Nissan and Subaru are manufacturers who have CVT transmissions in their product range.

Which transmission the manufacturers use in which vehicles has partly technical reasons, but it is also a philosophical question. "The strengths and weaknesses of automatic torque converters and dual clutch transmissions are different and must therefore be assessed depending on the respective vehicle concept," says Edmund Bauchrowitz, Head of Advanced Development for Transmissions and Drivetrains BMW. The Munich company use both concepts. Installation space, weight, the desired level of comfort when starting up, efficiency and the desired number of gears all play a roleas Bauchrowitz says.

Dual clutch transmission: The S-tronic from Audi with seven gears for transversely installed front engines Source: Audi

According to Uwe Keller, Head of Powertrain Development at Mercedes-Benz, they offer modern automatic converter systems Benefits for cars with rear-wheel or four-wheel drive. Dual clutch transmissions, on the other hand, combine the shifting convenience of automatic converter transmissions with a compact design, which is helpful in front-wheel drive cars.

Individual gear setting

"The customer requirements, size and performance of the car as well as the desired performance and characteristics of the model play a decisive role in the selection of the transmission," says Michael Schöffmann, Head of Transmission Development at Audi. So would Double clutch and CVT transmissions, for example, are only used up to an engine torque of 400 Newton meters.

There should be a suitable automatic transmission for every type of car driver. And for those who are still skeptical, some manufacturers have a technical trick ready: With the control technology, today's generation of transmissions can be completely adjusted to the customer - regardless of whether it is comfortable or particularly sporty.


source: dpa