CVT Principles
Unlike conventional planetary automatic transmissions that provide a limited number of gear ratios, usually three, four or five, the CVT, as its name suggests, continuously varies the gear ratio.
A low gear (low ratio) makes it easier to pull away from a rest position, the drive pulley being relatively small, while the driven pulley is large by comparison. The drive belt is used to transmit power and torque.
The CVT uses a primary pulley and a secondary pulley. Both pulleys have one fixed half and one mobile half, controlled by hydraulic pressure. The position of the drive belt on the pulleys will determine the ratio. If the mobile half of the pulley is close to its opposite half then the drive belt is forced to travel around the outer circumference. When the pulley is open wide then this circumference is reduced. The primary and secondary pulley mobile halves are diagonally opposed so when the drive belt diameter is reduced on the primary pulley, it increases on the secondary pulley. See Fig 1 .
To pull away, a low ratio is required. To provide this, the primary pulley is open, allowing the drive belt to sit down into the pulley and forcing it to run around the outer part of the closed secondary pulley.
As vehicle speed increases, a higher gear ratio is required. To do this, the primary pulley gradually moves towards its fixed partner, increasing the pulley circumference. At the same time the secondary pulley is forced apart reducing pulley diameter, therefore creating a higher gear ratio. See Fig 2 .
If acceleration continues to take place, further up-shifts may be made until the drive pulley diameter is as large as possible and the driven pulley diameter is as small as possible. Therefore, for every revolution of the drive pulley the driven pulley revolves several times.
This degree of change can be controlled to ensure that the most suitable ratio is provided. An overdrive ratio is obtained when the primary pulley is fully closed and the secondary pulley is fully open. The secondary pulley is now forced to rotate approximately two and a half times for every turn of the primary pulley.