The Power-Split architectures for Automatic Transmissions allow
the mainstream of power to pass through a high efficiency planetary
gears arrangement. The variator, which controls the overall gear
ratio of the transmission but usually exhibits a lower efficiency,
only sees a fraction of the power coming from the ICE.
RENAULT carried out a systematic study of the possible
Power-Split AT architectures in order to select the most flexible
and efficient variator. Their conclusion is that an electric
variator based upon two coupled electric machines, one working as a
generator and the other working as a motor was probably one the of
the best compromise considering all the existing technologies.
This architecture is optimised by the addition of a dual modes
structure, i.e. a device enabling two different gear ratios at the
variator output. This structure reduces dramatically the power that
passes through the variator and thus the size of the electric
machines required.
One mode is dedicated to the vehicle low speed range and the
other to the higher speed range. The mode change is made at a given
speed ratio. We developed and implemented a set of mechanical and
control system rules that make the mode change transparent and
seamless to the driver (no shocks, no vibrations, quick
response).
The key advantage of such a Power-Split automatic transmission
is its flexibility. Depending on the energy storage capacity fitted
in between the two machines, this transmission can work as a simple
Automatic Transmission (almost no energy storage), a mild hybrid
transmission (limited energy storage enabling stop & start and
brake & boost features) or a full-hybrid transmission
(high-energy storage enabling ZEV autonomy and Off Vehicle
Charging).
RENAULT is now running a comprehensive test programme on
prototype vehicles on both Automatic Transmission and Mild-Hybrid
versions of this infinitely variable transmission.