Continuously Variable Transmission (CVT)

Continuously Variable Transmission (CVT) is a type of automatic transmission that can change through an indefinite number of gear ratios possible between the maximum and minimum values of gear ratio.

A typical mechanical transmission such as a 4 speed gear box has a fixed number of gear ratios. For example,

• 1st gear ratio- 3.4:1
• 2nd gear ratio- 2.5:1
• 3rd gear ratio- 1.7:1
• Final gear ratio- 1:1

In the case of a CVT, it can seamlessly vary gear ratios between the maximum gear ratio (3.4:1) and the minimum gear ratio (1:1). This ensures maximum fuel efficiency.

Design:

CVT transmission doesn't employ the conventional planetary gear system used in automatic transmissions to vary the gear ratio. The most commonly used system for CVT is the pulley system that allows indefinite number of variations in gear ratios between highest and lowest values.

A pulley based CVT uses three main components:

• Electronically controlled and oil pressure supported cones
• The cones form a pair of pulleys- one of them is the 'input driving' pulley and the other is the 'output driven' pulley.
• High strength alloy steel belt running through the pulleys.

The pulleys can vary its diameter when the cones move apart or towards each other. The cones have an angle of 20° and they face each other. Belts form the main link between these pulleys. Rubber belts in the shape of 'V' pass through the gap between the cones. Nowadays, more efficient flexible metal belts are used to handle more torque.

Working:

The engine power is transferred from the input driving pulley to the output driven pulley via the belt. By varying the gap between the cones in the pulleys, an infinite number of gear ratios can be achieved.

The cones can be hydraulically operated or spring operated to change the distance between them. When the cones of a pulley are far apart, the radius of the belt revolving around the pulley becomes smaller. Conversely, if the gap between the cones reduces, the radius of the belt revolving around the pulley increases.

In order to keep the belt tight, if the radius of the belt over one pulley is small, then the radius over the other pulley has to be larger. This relative variation in radii of the belts over the two pulleys helps in varying the gear ratios.

High Gear:

If the radius of the belt over the driven pulley is smaller than its radius over the driving pulley, then the CVT operates at a smaller gear ratio (say 1:1), thereby increasing the vehicle speed.

Low Gear:

Conversely, if the radius over the driven pulley is higher than the driving pulley, the gear ratio is increased (say 3.4:1) and the vehicle speed decreases.