Between dual clutch transmissions (DCT), continuously variable transmissions (CVT), and nine-speed automatics, the world of transmissions is confusing. So we broke down each technology, how it works, and what it does well.
It wasn’t that long ago that car buyers had a pretty simple choice, when it came to transmissions: manual or automatic. Today, that simple selection has turned into a confusing morass of acronyms and newfangled technologies.
The advent of dual clutch (DCT), semi-automatic, and continuously variable transmissions (CVT) is ostensibly a good thing. But the downside is that buyers are now beset with a host of questions: Is a DCT better than a CVT? Do I need nine speeds, eight, or just seven?
These and other questions are important ones to ask. Whether you’ll get a useful answer from a dealer is another issue altogether. Never fear. We are here to sort the wheat from the chaff and explain what all those technologies actually do, and which is the best choice for you.
Manual transmissions are disappearing from the scene, but they are still worth discussing. Manuals are the original form of transmission technology and a good place to start when understanding how a transmission is supposed to work.
Transmissions exist not only to help transfer power from the engine to the wheels, but also to allow the engine to operate over a broader rev range. Just like the gears on a bike, the various gears in an automotive transmission help the engine move the car under different loads from a flat road to under difficult conditions such as an uphill start.
Manual transmissions tend to be light and well suited to transferring power compared to other designs.
To this extent, a manual transmission is pretty straightforward. The trick, as anyone who has learned to drive stick can attest, is changing gears. When the engine is sending power through the transmission, changing gears can only be accomplished with a great deal of grinding metal. To solve this, manual transmissions use a clutch.
Clutches may be complicated to engineer, but in principal they are actually quite straightforward. In essence, a clutch is two or more discs that are pressed together with springs. One disc is connected to the engine, the other to the transmission. When the two are engaged they are locked together with friction. The driver can change the state of the clutch with a pedal, allowing the transmission to match speed with the engine as the two discs are pressed together.
The advantages of this simplicity are profound. Manual transmissions tend to be light and well suited to transferring power compared to other designs. This has made them the traditional favorite in performance designs. However, many factors threaten the future of manual transmissions from shift speed, weight, and human frailty.
Manual transmissions require the driver to get the most out of the transmission, rather than a pre-programmed shift pattern controlled by an ECU. These days, that is a tough ask, as computers can typically do much better. This is why modern automatics tend to have better EPA fuel economy ratings as well as 0-to-60 times than their manual counterparts. That being said, most manual drivers — including myself — will smugly tell you that they are having way more fun.
Manual transmission takeaways:
- Fun to use
- Requires acquired still to operate
The standard automatic is still by far the most common type of transmission found in new cars in the U.S. However, a lot has changed from the brainless and sluggish three-speed automatics of the 1980s.
Automatic transmissions still use gearing that is fairly similar to that found in a manual transmission. How they change gear is very different, though. Rather than a clutch, automatic transmissions use a device called a torque converter, or sometimes a fluid coupling.
Torque converters are a tricky piece of engineering that is challenging to both explain and understand. However, the principal is somewhat simple.
Rather than using a physical connection, a torque converter uses hydraulic fluid — typically oil — to form a connection. In practice, a torque converter is usually a series of three different turbines. One connects to the engine, one to the transmission, and one joining the other two.
The engine spins the first turbine and creates a flow of fluid to the turbine connected to the transmission. This turbine then can rotate with the flow, transferring power. The central turbine acts to redirect the flow back from the transmission turbine towards the engine. This system allows power to be transferred. It also allows the transmission to engage and disengage without causing the sort of damaging friction that would be present in a physical connection.
The disadvantage of this system is that it is inherently somewhat inefficient. The hydraulic connection is not as good as transferring power as a physical connection and there is also a small parasitic loss to keep the hydraulic system pressurized.
However, there are also some important advantages conveyed by modern automatic transmissions. Because engineers don’t need to worry about overwhelming drivers, they can use more gears. Most significantly, when a computer controls shifting, it can be done at the perfect moment to maximize fuel economy.
Automatic transmission takeaways:
- Requires one foot to operate
- Allows for precise computer control
- Can offer as many gears as the engineers want
- Inherently less efficient
Continuously variable transmission (CVT)
CVTs, have been around for a while, but have recently grown in popularity thanks to their promise of light weight, low cost, and good fuel economy. But be warned, while all of those things may be true, it also comes with some real penalties.
Unlike all the other types of transmissions, a CVT doesn’t use gears per se. Instead, a CVT can range through an almost limitless set of gear ratios, thanks to a unique and rather odd design. While engineering solutions vary, the best way to envision a CVT is by picturing the arrangement of gears on a bike.
On a bike, the gears are arranged in a cone and the drive chain jumps between them to change the ratio. On a CVT, there are two smooth cones arranged in an hourglass formation. The chain sits between the two cones. Moving the cones in or out, thus moving the chain to a wider or narrower space on the cone, changes the gear ratio. The effect of this system is to create an infinitely variable set of gears.
The advantage of this arrangement is that — at least, in theory — it allows the engine to remain at its most efficient rpm level at all times. CVTs are also very small and light, which makes them ideal for compact cars.
However, there are a few problems with the CVT. First, CVTs are not quite as good at transferring power from the engine to the wheels as traditional, geared transmissions. This means that engines will lose a little of their potential power. More significantly, CVTs frequently offer an unpleasant driving experience.
The most obvious problem with CVTs is the “rubber-band” effect. Under hard acceleration a CVT cannot keep up, making the engine rev extremely high with little result before snapping back down to a lower RPM. Even high-quality CVTs are frequently slow to respond to rapid throttle increases. This makes them less than ideal for most performance applications.
Continuously Variable transmission takeaways:
- Offer good fuel economy
- Disappointing driving dynamics
Semi-automatic and dual-clutch transmissions (DCT)
The two newest innovations in transmission technology, semi-automatic and dual-clutch, are actually based on the good, old-fashioned manual.
Semi-automatic transmissions, which were pioneered by high-performance automakers like Ferrari, are in essence electronically or hydraulically controlled manuals.
The advantage of semi-automatic transmissions is that they retain the lightness and efficiency of manual transmissions.
Like a manual transmission, semi-automatics use a physical clutch. Rather than being directly controlled by the driver, they’re operated by an automatic system. Depending on the particular transmission, the driver may either retain manual control of shifting or put the transmission in full automatic mode.
The advantage of semi-automatic transmissions is that they retain the lightness and efficiency of manual transmissions. The issues most users face with semi-automatics are their clunkiness and slow shift times, which is most often a complaint of modern Aston Martin drivers.
But those interested in getting these benefits without having to operate a clutch pedal should be even more excited about another innovation: dual-clutch transmissions.
DCTs, originally pioneered by the Volkswagen Group, are clearly the product of evil German magic. As the name suggests, they use two separate clutches. Typically, the clutches are arranged concentrically with one clutch for even numbered gears and one for odd.
The advantage of this arrangement is that the transmission can have the next gear already engaged by the time the previous one is released. In effect, a dual-clutch box is almost two separate transmissions in one.
While the additional clutch means added weight, it also translates to literally instantaneous shifts. Also, because there is no interruption in power, shifts can be extremely smooth, resulting in a more refined driving experience. The principal reason DCTs have not been more widely adopted is their relatively high cost.
Still, from a technological standpoint, these transmissions offer the best compromise between efficiency and performance.
Dual-clutch transmission takeaways:
- Incredibly fast-shifting
- Refined power delivery
- Expensive and complex
Different automakers commit to transmission technologies for any number of reasons: cost, branding, familiarity, priorities, or simple dumb obstinacy.
In fact, because of the different advantages and disadvantages, it is hard to say that any one technology is best. So when shopping for a car, transmission type is important, but it is more important to consider what that the buyer’s priorities are than choosing the “best” transmission type.
For example, I love driving a manual and will happily put up with a tried leg, worse fuel economy, and reduced acceleration if it means getting to be more directly involved in the act of driving. Whatever you choose, if you are having fun when you are in the driver’s seat, whether that means you feel connected, disconnected, or are getting the best possible efficiency, you made the right decision.