66 http://www.racecar-engineering.com SEPTEMBER 2019TECHNOLOGY – TRANSMISSIONS
I
f you were to ask anyone which
technologies have progressed the most
throughout the first five seasons of
Formula E, they would probably reply ‘motor
efficiency and battery capacity’, and they would
be correct. But this enhanced capability of the
motors, together with the increased freedom
of the regulations, has also driven significant
developments within the transmission, and
today’s single-speed Formula E gearboxes are
now achieving incredible efficiencies and are a
world away from the regulated 5-speed unit that
was first introduced in season one.
‘It’s interesting to analyse the transmission
because most of the time in Formula E we talk
about the progress on the electrical side,’ says
Thomas Chevaucher, technical director at DS
Techeetah, which won the 2018/19 drivers’ and
teams’ championships. ‘But the transmission
has evolved a lot throughout the history of
Formula E and it is difficult to imagine the
steps of development that have been made
so far. The main reason for this is because the
improvements in electric technology have
allowed the transmission to progress. At the
level we are now at in Formula E, what you^
gain or lose in the transmission is more than
what you can gain or lose on the electric side,
so the transmission is an important part of the
overall powertrain performance.’’Box of tricks
A transmission effectively transmits the input
torque from the engine or motor to the wheels.
It achieves this through several gear ratios which
provide the precise amount of torque to the
wheels to allow the vehicle to accelerate whilst
remaining within the most efficient RPM range
of the engine or motor.
‘We know the location of the motor axis
and the location of the differential axis and the
gearbox essentially joins these two together,’
says Steve Blevins, project manager at FE
gearbox supplier Ricardo. ‘It’s just a question of
joining those two points with the reduction ratio
we want, in the most efficient manner possible.’
For a conventional internal combustion
engine the most efficient rev range is termed
the power band and this is where the engine
speed produces the right amount of usable
torque for the engine to perform within its
optimum condition. For electric cars, this rev
range relates to the torque band, which is where
the motor operates at its highest efficiency.
‘The DNA of Formula E is to improve
efficiency,’ says Chevaucher. ‘This is because
we have a limited amount of energy from the
battery at the beginning of the race, and that is
not enough to complete the race at full pace. So,‘We have to lose as little energy as possible through the transmission
to maximise the transfer of torque from the motor to the wheels’
Figure 1: The torque curve for a typical internal combustion engine showing the familiar peaked shapewe have to make the best use of the energy
that has been made available to us by the FIA.
Every single percentage of efficiency or kilowatt
of energy that you save, allows you to go
quicker. In an electric car the main areas where
you lose energy is in the power electronics,
motor and transmission, which is why the
transmission is such a crucial thing to optimise
in Formula E. There is also a secondary effect. If
you have a loss, you have to use more energy
from the battery to achieve the same speed. So,
poor efficiency has a double disadvantage.’Efficiency drive
It is this drive for efficiency that has been the
focus of powertrain development since the
regulations opened up in season two. This has
led to an influx of investment, resource and
effort to try and minimise any losses within
the driveline, maximising performance and
efficiency. ‘To give an idea of the progress that
has been made on the global powertrain, the
efficiency of the overall powertrain in season
one was approximately 90 to 92 per cent, which
was already impressive,’ says Chevaucher. ‘For
season five, I would say we are now above
95 per cent, while the efficiency of the global
transmission is close to 100 per cent.’
In terms of motors, the high spec examples
we have today are capable of achieving peak
efficiencies of up to 98.5 per cent, which
has consequently unlocked significant
developments in gearbox technology. The
challenge right now is to try and operate
the motor within this peak efficiency band
throughout the range of motor speeds, and
this is the job of the transmission.‘We have to try and lose as little energy
as possible through the transmission and so
maximise the transfer of torque from the motor
to the wheels,’ Blevins says. ‘Therefore, we
need to achieve high gearbox efficiencies.
What we are also aiming to do is match the
range of gearbox efficiencies to where the
motor is most efficient. If we can match the
gearbox efficiency at that point where the
manufacturers are running their motors most
efficiently then that maximises the overall
performance of the driveline.’
As a result of this relentless drive for
efficiency, Formula E transmissions have evolved
from 5-speed boxes in season one to single-
speed boxes in season five. To understand
why it’s worth taking a look at a conventional
IC engine. Figure 1 is the torque curve for a
typical IC-engined racecar, which, as would be
expected, has the traditional peaked shape.
The question is, how many gear ratios will allow
the transmission to maximise the amount of
tractive effort applied to the track surface?Thrust curves
To analyse the above we need to take a look at
the thrust curves. These essentially show the
actual amount of tractive effort available at
the tyre contact patch throughout the speed
range of the vehicle. Figure 2 illustrates the
typical thrust curves for a 5-speed gearbox for
the IC-engined racecar. The blue line illustrates
the maximum speed limit of the racecar
dictated by aerodynamic drag.
‘The actual power the car is able to put
down onto the ground relates to the number
of ratios within the gearbox,’ Blevins says.