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
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.