CVT – Continuously Variable Transmission

This type of power transmission (gear box) is used on some motorcycles,scooters, cars with engines up to 2 liter, and some front-wheel drive vehicles with a transaxle integrated into the transmission.
It is also widely used in industry, where there is a similarity between variable speed operating systems in machines as well as in cars. In cars, high-capacity rubber belts are used, which are built for use in industry.
This type is called “step less transmission” as it provides infinite reduction ratios between the engine and the final drive. The advantage of using this transmission is that it allows the engine to rotate at nearly a constant speed with varying load or operating requirements, at which speed is most fuel efficient. The system is also characterized by its small size and smooth operation compared to other power transmission systems.
In automotive applications, there are two sets of wet clutch discs that transmit engine torque to a planetary gear set and from there to a drive shaft to provide forward and reverse movement. The main parts of the system consist of a primary pulley, a steel transmission belt, an output pulley (secondary), and a hydraulic control unit to control all operations including changing the pulley diameters according to operating requirements, road load, and engine condition.
This power transmission system works by changing the operating diameters of the two pulleys as shown in the figure. The bobbin has a V-shaped track along which the steel belt runs. The varying range of reduction ratios is affected by changing the travel position between the two pulleys. The power transmission gear from the engine drives the primary pulley, which is connected by a steel belt to the output pulley.
Both pulleys have a fixed cone and a movable cone, with a hydraulic cylinder integrated with them. Activation of the hydraulic cylinders causes the cone to move axially and changes the distance between the fixed and moving cone of the spool. The greater the distance between the two cones, the lower the belt on the pulley and the smaller the operating diameter of the pulley, and vice versa.
When starting, the primary pulley is automatically (self) adjusted to the maximum distance between the two cones (minimum operating diameter). At the same time, the secondary pulley is set to the minimum distance between the two cones (maximum operating diameter), which corresponds to the highest reduction ratio (lower shift). The diameters of the pulleys change inversely as the engine speed increases, resulting in a continuous change in the reduction ratio. Since the two pulleys are shifted at the same time, the tension in the belt is permanent and continuous.

The figure shows how the continuously variable transmission works, as it is clear that the operating diameter of the pulleys determines the ratio of the output speed.

Changing the distance between the two cones, leading to a change in the operating diameters of the pulleys, is carried out automatically by means of hydraulic valves. These valves are controlled by input signals via various sensors such as the throttle position sensor, engine speed sensor, and torque requirement.
Performance comparison, this system is slower in response to engine torque, than a conventional automatic gearbox. But the manufacturers of the system claim that it is better in terms of acceleration than the traditional system after covering 40 km / h. It features quiet gear shifting with optimal gear selection as needed.
This system is smoother than automatic transmission, easier to manufacture and maintain, and achieves better fuel economy by avoiding losses in the torque converter, but the amount of torque transmitted is limited by the tension in the belt or track, and this is why this type is applied to vehicles with low power.
The system smoothly shifts gears to maintain speed at the engine’s maximum efficiency value. This improves fuel consumption and reduces exhaust emissions.
Scope of use: The system is used in small tractors that do home gardening, and most snowmobiles and motor scooters. There are some car companies that have introduced the system in their car, such as Nissan Murano, Honda, Audi, Mini Cooper. 
 There is a great development and improvement in terms of performance and torque transferred from the first models of the system, as well as the spread of its use in many cars.