Apart from enabling continuous ratio change under load,
the Continuously Variable Transmission (CVT) offers,
between limits, the ability to choose engine rotational
speed independently of vehicle speed. Here lies a
potential efficiency benefit, because the engine can
operate more fuel efficiently. There are unfortunately
considerable power losses within the CVT itself, causing
many CVT equipped cars to be less fuel-efficient than
cars with a manual transmission. The internal losses are
caused for a substantial part by the CVT’s hydraulic
actuation.
An electromechanical CVT pulley actuation system was
designed to overcome the hydraulic power loss and
hence improve CVT efficiency. Spindles are driven from
the fixed world through epicyclic gearings by electric
motors that are placed outside the transmission housing
in a cool environment. A mechanical link between the
adjustment mechanisms on the two shafts provides
energy exchange, thus lowering shifting power demand
and actuator size. Because of this, only one electric
motor actuates pulley movement on both shafts for ratio
change. A second actuator keeps the belt tensioned and
controls belt clamping force. A high degree of
independency between ratio- and clamping force
actuation can be established.
A prototype was designed as a modification on a
commercially available belt type CVT, which is currently
in the realization phase. After trials on a test rig, the
transmission will be built in to a vehicle to further
evaluate the new concept.