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Automatics are complicated devices, too complex to get into detail here. However, before we can explain the new style ones, let us state some basic facts about traditional automatics, which have been around since the 1940s.
The conventional automatic tranny relies on hydraulic pressure, supplied by the ATF (automatic transmission fluid). The gear ratios are provided by planetary gear sets (picture our Solar System as a set of interlocking gears), and a fluid coupling called a torque converter. Up until recently, a typical automatic had 2, 3, or 4 forward gears, or speeds. Recently, in the interest of better fuel economy, the industry has introduced 6, 7, 8, and even 9-speed gearboxes.
It is this desire to improve mileage which has led to the development and use of these new-fangled automatics. The concepts are not new, but their widespread application is. Let's look at them in more detail, starting with the "DCT".
DCT = DUAL CLUTCH TRANSMISSION
Mechanically, the DCT has more in common with a conventional manual transmission. Like a stick shift tranny, the gears are on two shafts (NOT planetary). But the DCT uses two clutches, not just one. The first clutch works the "even number" gears (let's say 2, 4, and 6), while the second clutch works gears 1, 3, and 5.
The clutches are operated via computer (so the driver still only has two pedals). By alternating which clutch will apply the next upshift, the upcoming gear can be pre-chosen, and shifts can happen more quickly than any human could manage. (Fun fact: every new Ferrari sold in the U.S. today uses a DCT automatic transmission.)
DCTs can show marked improvement in fuel economy due to many efficiencies, including rapid shifting, direct torque delivery, and lack of hydraulic "slipping" losses endemic in traditional automatics. There is a long list of manufacturers today using DCTs, including Volkswagen, BMW, Ford, and Honda.
Some drivers, not accustomed to the different driving characteristics, have complained of noisy and harsh shifts. Overall, the industry continues to adopt the DCT to more models, and consumers are certainly pleased with the MPG numbers, which now beat the figures achieved by manual transmissions.
CVT = CONTINUOUSLY VARIABLE TRANSMISSION
Unlike ANY of the transmissions covered above, a CVT does not have "gears". Instead, a belt or chain runs on two pulleys, a drive pulley and a driven pulley. These pulleys are shaped like spindles, and can vary their diameters. As the chain rides on the wider or narrower part of the pulley, the "gear ratio" changes.
The object is for the CVT, with its infinitely-variable ratios, to always keep the engine in its most efficient range. This maximizes fuel economy, as the engine is never turning "too low" or "too high". To the driver, the perception may be of an engine speed that never changes, which can be disconcerting. However, there is the complete elimination of any "pause" in between shifts. So the seamless changes and fuel mileage improvement tend to quell any concerns.
Subaru began installing CVTs in the late 1980s. They have also been embraced by Nissan, Honda, Toyota, Audi, and Mini, among others.
You may be driving a car with a DCT or CVT and may not even realize it! Many vehicle manufacturers have installed them in thousands of cars and trucks. Consumers are certainly enjoying the mileage gains, smoother shifts, and greater power delivery. Still, conventional automatics continue to be used, especially in heavier and more powerful vehicles. As the auto industry chases better fuel economy, and as drivers get accustomed to their function, expect to see the installation of DCTs and CVTs continue to proliferate.