Audi CVT

The core of the Audi Multitronic is a variator working with quite conventional conical pulleys of variable effective diameter, but in which the usual steel push-belt has been substituted for a specific link-chain of 37 mm width.

This chain offers a higher torque capacity for a similar packaging space, it has a better efficiency and it allows a smaller scrolling radius of the tracks. It consists of 1025 individual links and 75 pairs of pins made of high-strength steel. Either 13 or 14 links are lined up next to each other. The torque is transferred by the contact between the pulleys flanks and the ends of the chain's pins. The wear of contact surfaces is said to be only 0.1 to 0.2 mm in 300 000 km.

The input torque is detected by a torque sensor which directly and instantaneously adapts the necessary hydraulic clamping pressure, up to 60 bars. Thus the chain clamping force needed for an absence of contact slip is never set to an excessive level.

The hydraulic system is particularly smart. It consists of two independent circuits, including a low pressure one for cooling and another high pressure one for clamping and ratio adjustments. Moreover, pistons of great surface ensure the pressure of the pulleys flanks against the chain whereas the management of the ratio is ensured by pistons of a smaller surface. By this so called "dual piston" principle, the pressure which clamps and tightens the chain is separated of that which controls the ratio. Consequently only small volumes of oil are necessary to alter the ratio because the volume of oil of the large clamping cylinders is simply transferred from one to the other. A faster response to the instructions of the control system is thus obtained and the power needed by the hydraulic pump is reduced.