The first automatic transmissions were two-speed automatics, and these were quickly replaced by three-speed and then four-speed automatics. Each successive change improved fuel economy and drivability, and so the trend continued on up through the numbers to the point where most current automatics generally have eight, nine or even 11 speeds.
A higher gear count reduces the size of the steps between adjacent gears, and that has several benefits. Fuel economy improves because the engine is more likely to be operating at its most efficient rpm. Likewise, more gears reduces the likelihood of back-and-forth dithering on grades because smaller steps keep the engine closer to its sweet spot in terms of power and torque. The presence of smaller steps also reduces the need for a slushy torque converter, a generally inefficient, power-robbing device that smooths what would otherwise be jarring shift-shock at each gear change.
A CVT maximizes the potential of this concept. And of course the degree to which a CVT lives up to the potential described above is only as good as the software that controls it. The main drawback that remains is one of perception and emotion, because a CVT changes how an engine sounds and a powertrain responds under acceleration.
However, many have found the lack of traditional gear changes to be disagreeably odd, especially aggressive drivers who are more likely to run the engine hard.
The car can sound like it's perpetually stuck in a low gear. This is why many newer CVTs artificially create stepped gear changes — usually in a "Sport" driving mode, but sometimes in normal operation. Now place the same gearset at the pedals, but flip it around. The latter car, on the other hand, would fly at 80 mph down the highway, but it would have almost no acceleration when starting out and wouldn't be able to climb hills.
So the transmission uses a range of gears -- from low to high -- to make more effective use of the engine's torque as driving conditions change. The gears can be engaged manually or automatically. In a traditional automatic transmission, the gears are literally gears -- interlocking, toothed wheels that help transmit and modify rotary motion and torque.
A combination of planetary gears creates all of the different gear ratios that the transmission can produce, typically four forward gears and one reverse gear. When this type of transmission cycles through its gears, the driver can feel jolts as each gear is engaged.
Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts. If you're wondering why the word "gear" still appears in the explanation of a CVT, remember that, broadly speaking, a gear refers to a ratio of engine shaft speed to driveshaft speed.
Although CVTs change this ratio without using a set of planetary gears, they are still described as having low and high "gears" for the sake of convention. Peer into a planetary automatic transmission, and you'll see a complex world of gears, brakes, clutches and governing devices. By comparison, a continuously variable transmission is a study in simplicity. Most CVTs only have three basic components:. CVTs also have various microprocessors and sensors, but the three components described above are the key elements that enable the technology to work.
The variable-diameter pulleys are the heart of a CVT. Each pulley is made of two degree cones facing each other.
A belt rides in the groove between the two cones. V-belts are preferred if the belt is made of rubber. V-belts get their name from the fact that the belts bear a V-shaped cross section, which increases the frictional grip of the belt. When the two cones of the pulley are far apart when the diameter increases , the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the cones are close together when the diameter decreases , the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger.
CVTs may use hydraulic pressure, centrifugal force or spring tension to create the force necessary to adjust the pulley halves.
Variable-diameter pulleys must always come in pairs. One of the pulleys, known as the drive pulley or driving pulley , is connected to the crankshaft of the engine. The driving pulley is also called the input pulley because it's where the energy from the engine enters the transmission.
The second pulley is called the driven pulley because the first pulley is turning it. As an output pulley , the driven pulley transfers energy to the driveshaft.
When one pulley increases its radius, the other decreases its radius to keep the belt tight. As the two pulleys change their radii relative to one another, they create an infinite number of gear ratios -- from low to high and everything in between. The simplicity and stepless nature of CVTs make them an ideal transmission for a variety of machines and devices, not just cars.
CVTs have been used for years in power tools and drill presses. They've also been used in a variety of vehicles, including tractors, snowmobiles and motor scooters.
In all of these applications, the transmissions have relied on high-density rubber belts, which can slip and stretch, thereby reducing their efficiency. The introduction of new materials makes CVTs even more reliable and efficient. One of the most important advances has been the design and development of metal belts to connect the pulleys.
These flexible belts are composed of several typically nine or 12 thin bands of steel that hold together high-strength, bow-tie-shaped pieces of metal. Type keyword s to search. Today's Top Stories. Future Cars Worth Waiting For: — Karl Tapales Getty Images.
Advantages of CVT Depending on the vehicle and the driver, a CVT can provide a smoother driving experience when comparing it to a traditional automatic.
A flexible belt connects the two pulleys The width of the pulleys changes depending on how much power the vehicle needs. What Is Launch Gear? Disadvantages of CVT While the lack of upshifting and downshifting is considered an advantage of the CVT, some drivers miss the sensation of the vehicle moving through the shift points.
There are limits on the engines that can work with a CVT in terms of power and size. They don't last as long as a conventional transmission. CVTs are harder to work on. Even basic maintenance often needs to be done by a trained mechanic.
Advantages of Automatic Transmission According to Mister Transmission , while there are some advantages to having a CVT, traditional automatic transmissions also offer some benefits. They have a more connected feeling between the engine and the speed of the vehicle.
The driver has a better sense of control with actual upshifting and downshifting. Automatic transmissions are less expensive to maintain. They offer a more engaging driving experience. Some of these include: They are less fuel-efficient than a CVT. Automatics produce more emissions. Car Tech. Share this on Twitter Share this on Facebook Email.
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