What is a CVT? and How does it work?
Features, Advantages, and disadvantages
Newer vehicles are often fitted with a continuously variable transmission, which ensures a smooth driving experience.CVT provides an infinite number of gear ratios, making gear change operation an unnoticeable experience. In this article, we will take a look at the inner workings of CVT, including the reverse gear.
The basic parts of a CVT are demonstrated ( See Image ). Two conical pulleys and a steel belt make up a basic CVT. As you can see ( in the image ) the input pulley turns the belt, and the belt turns the output pulley. The gap between the cones determines the effective diameter of the pulleys. For this configuration, it is clear that the output speed will be less than the input speed. As the gap decreases, the effective diameter of the pulley increases. This simple mechanism forms the basic operation of CVT.
Now, just see what happens to the arrangement when the right side cone of the input is moved to the right, and the left side cone of the output is also moved to the right. You will see ( in the image ) that the gaps are simply the reverse of the previous case. This means the effective diameter of the pulleys is also the reverse, and hence, we will get an overdrive. This simple mechanism provides countless gear ratios for the CVT According to the driving condition, a transmission control unit will adjust the width between these cones. And as a result, unlike any other kind of transmission, the speed change will be smooth and unnoticeable. You might be wondering About this Question.
Q: How a reverse gear is even possible with this mechanism?
A: In order to achieve these forward and reverse gear ratios, an interesting planetary gear set arrangement is used in CVT. First, let’s take a look ( see image ) at this gear mechanism, and then we will move to the reverse gear. The input shaft is connected to the sun gear. The carrier of the planetary set is fitted with the input pulley. This means that the output of the planetary set is drawn from the carrier. A two-row planet gear set is fitted with the carrier. If the ring gear also turned at the input speed, the carrier will rotate at the input speed as well. This is possible just by connecting the sun gear with the ring gear. A clutch pack is used for this purpose. If the clutch pack is well-pressed, the frictional force between the plates will lock the sun gear to the ring gear. This is how the forward gear is achieved.
The whole mechanism moves as a single unit here. Now, let’s keep the ring gear stationery with a second clutch pack. The result is quite different. The stationary ring gear will force the output carrier to turn in the reverse direction, so does the input pulley. So, to achieve the reverse, just release the first clutch and engage the second clutch. It is interesting to note that unlike any other type of transmission, CVT offers a variety of gear ratios for the reverse gear also, even though it is not programmed to do so. Despite its very interesting features, there are certain disadvantages to CVT. The belt is the weakest part of this mechanism. It really limits the transmission’s torque carrying capacity, even though the belt technology has been continuously improved. To learn more about the inner workings of CVTs, and the latest advancements, please check Thomas Schenke’s CVT Post. Also, make sure not to miss any of his other posts.