Automotive Engineering - May 2021 - 6

TECHNOLOGY
REPORT

CVT is Bosch's answer to upgrade EV efficiency
Enhanced ratio coverage

Bosch CVT4EV packaged with traction motor and inverter.
As electric vehicles (EV) move from niche offerings to mainstream products, research and
development to improve all aspects of EV functionality and efficiency is accelerating. Bosch
recently revealed results of an effort to improve
one of the most crucial aspects of EV development - traction motor performance - by fusing
electric motors with a transmission technology
familiar to the internal-combustion world: continuously variable transmission (CVT).
Bosch revealed that coupling a lightly modified conventional pushbelt CVT it has dubbed
CVT4EV with a typical EV traction motor can
both improve performance and reduce energy
consumption compared with today's typical
setup of a traction motor integrated with a
single-speed transmission. Fitted to a
C-segment demonstration EV, Bosch said the
CVT4EV improves acceleration and top speed
and reduces torque demand on the traction
motor, making it more efficient in many driving situations.
Perhaps best of all, the company said using
the CVT4EV could allow an automaker to fit
essentially the same electric powertrain across
a range of vehicle sizes from the C segment to
F-segment large cars and SUVs - and even
G-segment light-commercial vehicles (LCVs).
It also can be used in dual-motor vehicle layouts that power both axles to impart all-wheel
drive (AWD).
6 May 2021

The CVT4EV's chief advantage is a wider ratio spread than even the 2-speed step transmissions available today for some higher-end
EVs. The CVT4EV also offers the ability to
consistently maintain the traction motor's
speed in a more-efficient range that's less
tied to road speed or load, said Gert-Jan van
Spijk, transmission director for Bosch
Transmission Technology.
" Fixed-ratio transmissions have to deal with
a compromise between the required wheel
torque to perform with high loads and gradients, against maximum-speed requirements, "
said van Spijk in an interview with Automotive
Engineering. " At the same time, fixed-ratio
powertrains have to deal with the efficiency of
the electric motor 'as it is,' following the load
and speed conditions. "
A 2-speed transmission offers the first stage
of improvement on this, he said, but with a limited ratio coverage - typically up to 2.0 per step
- and with disruptions in torque and speed, influencing acceleration, efficiency and comfort.
" If a larger ratio spread is required, typically
more steps have to be added. CVT is a more
logical pairing, as it has the ability to improve
the electric motor's efficiency while keeping
the 'fluent' power characteristic of an electric
machine, " van Spijk asserted.
Due to the wider flexibility of electric motors, the CVT4EV ratio spread needs only be
between 3-4 to one, he said, as opposed to a
ratio spread of 6 to 8 when coupled with IC
engines. This is one reason that CVT-equipped
EVs will not evince the sometimes harsh and
strained-sounding NVH characteristics that
have brought them criticism in ICE vehicles,
particularly higher-powered premium models,
van Spijk said.
" The flexibility of the electric motor puts different requirements to the control of the CVT,
meaning that shift speed is much lower than in
ICE applications, " he explained. " With this, the
inertia effect of rotating masses is hardly noticeable. It has become one of the bigger benefits of CVT compared to other multi-speed
solutions for EVs. "
Demonstration of a CVT4EV prototype to
Bosch specialists, customers and press resulted
in positive feedback on the driving behavior,
including remarks like " not distinguishable
from single-speed,'' he added. " Because

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Automotive Engineering - May 2021

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