AT, CVT, DCT: What are the Differences?
January 9, 2013
Automatic transmissions are widely used across many international markets. A range of technologies is available, with different solutions tending to find favor in different regions. But how do each of these work, how have their respective markets evolved, and which technology – if any – will dominate the global market of the future?
The traditional automatic transmission (AT) has been fitted to most passenger cars in North America for more than 60 years. The earliest ATs were three-speed, with four-speed units becoming common in the 1980s; since then, six-speed and eight-speed units have followed. ATs are based on a hydrodynamic torque converter used in conjunction with epicyclic (sun and planet) gear sets. The torque converter comprises three basic components: an impeller driven from the engine, a turbine linked to the transmission, and a stator which assists in the torque multiplication. The unit is oil-filled and transmits torque between the input and output shafts by means of the forces arising from the flowing oil, rather in the manner of a centrifugal pump and turbine. Inside the transmission, the path taken by the torque through the epicyclic gears is controlled by a series of bands and clutches to provide a number of fixed forward speeds and reverse. Compounding of epicyclic gear sets (several into one) has made it possible to increase the number of speeds within a reasonable package size.
The latest transmissions also have energy-saving functions built in, such as torque converter mechanical lockup and “neutral-idle” control, which disengages the engine from the gearbox components when the vehicle is stationary. Complex electronic control strategies of the clutches and brakes within the transmission also increase efficiency – as does the greater number of ratios, which allows the engine to be run closer to the most efficient part of its regime for longer periods of time. The current state-of-the-art in passenger car transmissions is eight speeds, and in 2013 ZF will release a new nine-speed automatic transmission for transverse front-drive applications.
Dual Clutch Transmissions (DCTs) have only emerged into the mainstream in the last 10 years, although the principle was first shown by Automotive Products in 1980 as the “Hot-Shift” gearbox. The DCT resembles, in effect, two concentrically arranged manual transmissions, each offering an alternative torque path and respectively accommodating sequential ratios and its own clutch unit. One path provides first, third, fifth and possibly seventh gears, while the other provides even-numbered ratios and reverse. Processor controlled pre-selection of the synchronizers for each successive gear is made in parallel with precise, real-time control of the switching of the torque path between the two clutches – and equally precise control over the torque delivery of the engine – as each gearshift takes place.
The potential for absolute refinement, along with excellent drivability and the lack of torque interruption, make DCTs an appealing option in high-performance applications, too. The reason for the considerable delay between DCT’s invention and its widespread adoption was largely due to the need for innovation in clutch and control technology, something which has occurred in the intervening years.
Now, however, the increasing need to cut fuel consumption and CO2 emissions has made the reduced losses, low weight and compact packaging delivered by the DCT an even more attractive option. VW introduced its DSG six-speed, dual wet clutch transmission for transverse applications in 2003 and developed a seven-speed dry clutch version in 2008, limited to 250 Nm (184 lb ft) torque. The absence of oil in the clutch assembly further reduces drag and, therefore, energy losses. While DCTs have been popular for European passenger car applications – from small cars to luxury vehicles such as Bugatti’s Veyron – there has been much discussion of the technology’s potential for future heavy truck and off-highway applications.
Another popular alternative to the AT is the Continuously Variable Transmission (CVT), first introduced by Dutch firm van Doorne in the 1950s when DAF fitted the Variomatic transmission to the 600. A van Doorne-type CVT comprises two split pulleys joined by a steel belt. The ratio is continuously changed as the halves of one pulley close up, increasing its diameter, while the other pulley opens, decreasing its effective diameter − and vice versa. The system has been regarded as relatively inefficient, especially at steady higher speeds, due to the energy expended in clamping the belt. Newer designs, however, have improved efficiency thanks to intelligent control of belt-clamping force. They perform especially well in conditions of fluctuating speed and are now less limited in the amount of torque they can handle. Jatco and its parent company Nissan are often seen as leaders in CVT technology.
The function of the CVT was traditionally based on rpm and, under acceleration, the engine tended to rev higher while the transmission adjusted its ratio to increase the vehicle speed. Many drivers, especially in Europe, disliked the fact that the sound of the engine did not match up with the speed of the vehicle. As a result, the control of Audi’s Multitronic CVT, launched in 2000, was torque-based. This allowed the engine speed to rise in parallel with the speed of the car. It was equipped with a belt design resembling a linked chain, capable of transmitting higher torque. The capacity of the latest Multitronic transmission is now 400 Nm (295 lb ft).
While ATs, DCTs, and CVTs each have their own unique strengths and weaknesses, they are all subject to active ongoing technical development and have extremely complementary geographical market segments where they are popular. It is unlikely, therefore, that any single automatic transmission technology will come to dominate the global market, even in the long-term future. In the absence of widespread electrification or some other form of shift away from liquid fuels and the internal combustion engine as a power source for passenger cars, the AT will likely predominate in the US and neighboring countries. European markets favor DCTs and Asia favors the CVT-based solutions. These are just preferences, however, and all three automatic transmission technologies are likely to be offered by different automakers on a global basis.
© 2013, The Lubrizol Corporation