CVT vs. Automatic Transmission
Are CVT and automatic transmissions really all that different?by Dim Angelov, on
In general, there are three types of transmissions – manual, automatic, and CVT. Each of them have their own pros and cons. The automatic and CVT gearboxes are similar in that they do not require driver input in order to change gears. However, where they differ is in the way they function internally. So what’s the difference? Let’s find out!
Before we dive into the technical details, it’s worth noting that both automatic and CVT gearboxes have a few sub-types, depending on their construction.
In the case of the automatic, we have AMT (Automated Manual Transmission), Planetary or Torque converter automatic transmission, and Double-clutch gearbox – DST, DSG, PDK, etc.
Planetary/Torque Converter Automatic Transmission
The planetary/torque converter automatic transmission is still the most wide-spread type of automatic gearbox. A torque converter normally has two rotating elements. There’s an impeller, which is mechanically driven by the prime mover – in this case, the internal combustion engine – and a turbine which is what turns the transmission’s input shaft. In order for the system to work, a stator is positioned in-between the impeller and turbine. Its job is to alter the flow of the hydraulic fluid, returning from the turbine to the impeller.
When the impeller rotates with the engine, transmission fluid is flung across the stator and rotates the turbine. The impeller, turbine, and stator, lock and unlock, with the help of a clutch-system. The system remains unlocked most of the time, but when the transmission shifts into overdrive, the stator locks, and creates a direct link between the engine and transmission, allowing for a 1-to-1 ratio. This helps improve fuel economy and reduce parasitic loss of power. When unlocked, the turbine and impeller can spin at different speeds, which is also why you can stop a car with an automatic transmission in gear without the engine stalling.
Automated Manual Transmission (AMT)
The AMT is the simplest and most in-expensive implementations of an automatic gearbox. It is essentially a manual gearbox which is operated by an electro-hydraulic system, which comprises of a dedicated electronic control module and а hydraulic system. The gearbox monitors parameters, such as accelerator position and speed in order to actuate gear changes, without additional input from the driver. This is similar to how a normal automatic transmission works, but the design is largely different.
Double clutch transmission
The DCT is the third sub-type of automatic. It is essentially two gearboxes in one. For this reason, they are often compared to two manual gearboxes, merged together. There are two separate input shafts, each with their own clutch. One of the gearboxes is responsible for the uneven gears – 1,3,5,7, and so on, while the other is responsible for the even ones – 2,4,6,8, etc. When one gear is engaged, the next one is already selected. When it’s time to upshift, the clutch immediately engages the next gear while releasing the current gear, making for almost instantaneous gear shifts – impressively quicker than a single clutch or a torque converter automatic. The design includes two clutch packs that are situated on the same axis as the flywheel. This means the outer clutch pack has a larger diameter than the inner one.
The concept for a dual-clutch gearbox actually came to be in 1939, courtesy of Adolphe Kegresse – a French engineer. Due to a lack of financial resources, however, the project was not finished. This brings us to the main drawbacks of the double-clutch gearbox – its complex and very expensive to manufacture.
Continuously variable transmission (CVT)
Although its application in motor vehicles is relatively recent, compared to traditional automatic, the concept of the CVT transmission is actually quite old. The first CVT dates back to 1490 and was designed by Leonardo DaVinci. It was officially patented much later, in 1886, by Daimler and Benz. Even as early as 1910, when Zenith brought out a motorcycle equipped with their own CVT, its efficiency became so apparent that it was banned from hill-climb racing.
As mentioned, both the automatic and CVT gearbox have the ability to shift without any driver input. However, this is where the similarities end. Unlike traditional automatics, the continuous variable transmission does not have a set of gears. The most common type of CVT works through a system of two pulleys. One pulley is connected to the engine and the other is connected to the wheels, via a flexible chain belt. There are cones attached to each pulley. The cones move, which increases or decreases the belt diameter, thus changing the gear ratios as needed. The width of the pulleys changes depending on how much power is needed. While one pulley becomes larger, the other becomes smaller. Because of this, a CVT gives you a near-infinite number of gear ratios to work with, while effectively eliminating the need to shift. For the same reason, the CVT is sometimes referred to as a gearless transmission. That said, in some models, you have the option to switch to manual mode, just like in many traditional automatics, in which case the CVT emulates a number of pre-set gear ratios and behaves similarly to a traditional automatic.
Another less common type is the turoidal CVT. Instead of an impeller and a turbine, it has two discs - an input disc and an output disc. There are rotating power rollers in between them, instead of pulleys. These rollers will pivot towards the outer rim of the output shaft, for low gear / high torque or towards the outer rim of the input shaft for high speed. The rollers themselves are moved by a hydraulic piston. A well-known fact is that continuous variable transmissions cannot handle high horsepower and torque, which is why they are used almost entirely in economy cars. They also have a history of being problematic.
Nissan CVT transmissions in particular have been used the longest since becoming mainstream. Through extensive research and innovation, Nissan recently introduced a new toroidal CVT transmission. What is remarkable about it is that it is the first CVT designed for high torque applications. Because of the rollers being made of very hard steel, they are able to sustain forces of up to 10 tons.
While the performance of traditional automatics is being improved by adding more gears, the development of CVT transmissions focuses on more compact packaging and less friction. An example is the Xtronic Nissan CVT transmission, which in its 3rd generation is more compact, provides a wider range of gear ratios, improves economy by 15 percent, and reduces friction by a whole 40 percent. The latter which reduces powertrain loss and therefore improves performance. Because a CVT transmission does not perform gear shifts, it needs to constantly keep the engine in the upper rev range, while the car is getting up to speed. Moreover, to some, it creates the same sensation as a slipping, worn, clutch.
Like traditional automatics, a CVT transmission requires fluid to work properly. The CVT transmission fluid has special friction modifiers, which allow the chain belt to maintain the necessary grip with the pulleys.
What is a CVT transmission?
A CVT (Continuously variable transmission) is a type of gearbox that does not have gears. Depending on the type, a CVT may have a system of pulleys and cones or a system of discs and rollers.
How does a CVT transmission work?
The CVT does not have pre-set gear ratios. Instead, it uses cones or rollers to adjust the gear ratios accordingly. Because it does not have conventional gears, the number of gear ratios available is virtually infinite.
What does CVT stand for?
CVT stands for Continuous variable transmission
Is a CVT an automatic transmission?
In a way, yes. Similar to traditional automatics, it does not require driver input in order to shift the gears. However, because it does not have a set of gears, like in any other type of transmission, it is often referred to as a gearless transmission.
How many grs does a CVT transmission have?
A CVT has no gears. Instead it has a system of belt-driven pulleys or rollers, which alter gear ratios.
What is a torque converter?
A torque converter is, essentially, a fluid coupling device. It facilitates the transfer of power output from the engine into the transmission. Torque converters suffer from a small parasitic loss of power due to the nature of fluid coupling, but when locked the engine and transmission are mechanically coupled – something that usually happens when you’re cruising in overdrive. A Torque converter also makes it possible to stop your vehicle without the need to shift into neutral to prevent the engine from stalling.