Continuously Variable Transmission Mechanics and Benefits

The Evolution of Continuously Variable Transmissions

The Continuously Variable Transmission (CVT) is widely used in mopeds. It also began being used in cars in the 1960s, though it has not had much success until recently. In theory, CVTs are ideal as they vary the speed ratio continuously; thus, we can describe it as an automatic transmission with an infinite number of gear ratios.

This feature allows the vehicle to move along the curve of maximum power, which is impossible with automatic or manual gearboxes that produce steps or jumps between different speeds.

How the CVT Pulley System Works

A CVT is a system featuring two pulleys whose effective inner diameters are variable. The transmission between the two pulleys is achieved via a belt made of metal links. By varying the diameter of the pulleys, the system progressively varies the gear ratio.

As the belt is an inextensible element, the opening of one pulley involves reducing the diameter of the other. Even so, you obtain an infinite number of developments, achieving a continuous variation of the drive. Hence, this system is also referred to as stepless transmission.

Pulley Layout and Diameter Adjustment

The layout consists of two pulleys. If the face of the movable drive pulley that transmits the engine torque moves closer, the effective diameter of the pulley becomes larger. The belt, having an almost constant length, travels on smaller effective diameters when a pulley opens, resulting in a lower gear ratio.

Hydraulic Control and Pressure Management

The change in pulley width and effective diameter is managed using a hydraulic control system that dispenses oil at the adequate pressure at all times. The hydraulic control continuously monitors several parameters:

• Throttle position
• Operating conditions
• Vehicle speed
• Engine speed
• Gear ratio

This oil pressure also serves to lubricate the entire assembly and maintain the drive belt tension by applying the correct pressure on the driven pulley.

Modern Advancements in CVT Technology

Currently, the system transmits compressive forces, pushing the link that precedes it, rather than relying on traction like a conventional belt. Therefore, belt tension is a critical component in the correct functioning of this continuous change system. The tension depends on both the torque to be transmitted and the transmission ratio.

At the beginning, this technology was used in small-displacement cars because the chains could only resist the stresses caused by low-torque engines. Today, more resistant chains and belts have been developed to support the high torque values of high-powered cars.