A variator (CVT) is a continuously variable transmission that, within a certain range, allows you to achieve the most smooth change in the gear ratio. The main difference between the variator gearbox and its analogues is the rather efficient use of the internal combustion engine power.
CVT box (constantly changing transmission) allows the best way to change the torque from the internal combustion engine, transmitted to the wheels, considering engine loads, vehicle speed, etc. As a result, the variator allows for high fuel efficiency.
Distinctive features, device and principle of operation of the variator
To begin with, there are several types of CVT gearboxes. However, cars use a V-belt and toroidal variator, while a V-belt variator is much more common.
The device of a modern variator box includes the following elements:
- A special mechanism that is responsible for the transfer of torque from the engine to the variator and also disconnects the box and the internal combustion engine when put into neutral;
- The variator box itself (another name is “variator transmission”), as well as a separate mechanism that allows the car to move backwards;
- The electronic control system of the box with input and output sensors;
You can use a torque converter, a centrifugal automatic clutch, an electronically controlled electromagnetic clutch, and a multi-plate clutch to transmit torque from the engine. A torque converter (GDT) is often used for these purposes, which makes it possible to achieve smoothness and thereby increase the resource of a variator gearbox.
As for the gearbox itself, the V-belt variator includes one or two belt drives. This transmission is actually two pulleys that a V-belt connects. The pulley forms a pair of tapered discs, and the discs can move and expand. This way, it is possible to change the diameter of the pulley itself.
Hydraulic pressure, spring force and centrifugal force are used to move the cones. The tapered discs are angled 20 °. This allows the belt to slide over the surface of the pulley with minimal drag.
At the initial stage, such variations were equipped with a rubber belt, but the resource of the belt itself turned out to be small. Today, instead of rubber, a metal variator belt is installed. Such a belt is strong and flexible, less noise is generated during the gearbox’s operation, and the solution also lasts much longer than a rubber analogue.
The frictional force transmits rotation between the variator pulleys and the side surface of the belt. Also, some CVTs may use a steel chain. In this case, we are talking about a wedge-chain variator.
This type differs from the V-belt variator in that the end surface of the chain transmits the torque during its point contact with the tapered discs. At the same time, the tapered discs are made of durable steel, and the wedge-chain variator has the best efficiency.
We also note that the variator is not capable of providing the ability to move backwards. Various mechanisms are integrated into the box for a car with such a gearbox to have a reverse gear. Most often, it is a planetary gearbox, which is actively used in automatic transmissions .
As for the control of the variator, the system is electronic; it makes asynchronous changes in the diameter of the pulleys, taking into account the operating modes of the car’s engine. The clutch and planetary gearbox are also controlled.
The driver controls the gearbox using a selector, and the modes themselves resemble the modes of operation of a hydromechanical automatic transmission. Moreover, in the variator box, you can choose fixed gear ratios, which allows you to implement a manual gear shift function, like Tiptronic, on an automatic transmission.
We also add that the toroidal variator differs from the belt and chain analogues described above in that it has two coaxial shafts. Such shafts have a spherical or toroidal surface. Rollers are clamped between the shafts.
To change the gear ratio in such a variator, it is necessary to change the position of the rollers. In this case, the torque is transmitted using frictional forces between the working surfaces.
How a CVT works in a car
If we talk about the principle of operation of the variator box, the diameter of the variator pulley is constantly changing. The change implements the servo. When the car is just starting, the drive pulley of the variator has the smallest diameter; that is, the tapered discs are completely divorced.
The driven disc at this moment has the largest diameter when the tapered discs are compressed to the maximum. As the engine speed starts to rise, the diameter of the drive pulley becomes larger and the diameter of the driven pulley decreases, which allows a reduction in the gear ratio.
As you can see, the variator, by decreasing and increasing the diameters of the pulleys, allows the engine to operate at optimal mode to obtain maximum power and achieve better dynamics.
Pros and cons of the variator
First of all, such transmission is continuously variable. This means that while driving, there is no need to switch to the so-called “stages”, while there is a constant conversion of torque, there are no delays between switching from the stage (gear) to stage.
As a result, maximum comfort is achieved when driving a car with a variator. However, this solution is not without its drawbacks. First of all, installing such a transmission assumes certain restrictions on power and torque; that is, the variator is not installed on cars with a powerful engine. Another disadvantage is the complexity of the device of the variator transmission itself, low maintainability and high cost of the variator repair itself.« Back to Glossary Index