1 INTRODUCTION
Use of a Continuously Variable Transmission (CVT) in
a car can lead to lower fuel consumption. Because engine
revolutions can be chosen independently of vehicle
speed, efficient operating points of the internal combustion
engine can be reached that are unreachable with a
manual transmission or a stepped automatic transmission.
Unfortunately, because the energy consumption
of the CVT itself is higher than that of a manual transmission,
this efficiency gain is partly lost.
The pulleys of a pushbelt or chain type CVT are actuated
axially to adjust transmission ratio and to apply a
belt clamping force. In conventional CVTs this is done
using hydraulics. The CVT transmission ratio is dependent
on the ratio of primary and secondary clamping
force, and can be controlled by the pressure in the cylinders.
Hydraulic losses are however substantial. The oil pump
is directly coupled to the crankshaft of the internal combustion
engine. To realize high shifting speeds at low
engine speed, for instance during an emergency stop,
a large pump volume is required. However, when the
ratio is held constant at higher engine speeds, for instance
while cruising on the highway, an unnecessarily
large oil flow from high pressure to sump leads to large
energy losses.
By using a double planetary gear system and a screw
mechanism at both shafts, as shown in Figure 1, it is
possible to actuate the CVT electro-mechanically with
two electro motors at the fixed world [9]. By rotating
the primary motor Mp, a relative rotation between the