Racecar Engineering – September 2019

68 http://www.racecar-engineering.com SEPTEMBER 2019

Drivewasprovidedbya singlemotorthat

was mounted longitudinally within a cast

aluminium bellhousing and the inverter was

mounted above the battery, with both the

motor and inverter supplied by McLaren

Applied Technologies. Torque from the motor

was transferred to the wheels via a Hewland

5-speed gearbox complete with pneumatic

paddleshifting. Under race conditions the

maximum available power from the motors was

limited to 150kW, as per the regulations.

‘You can see [in Figure 5] that at around

100km/h, the overlap of the curves show that

you could effectively either be in first, second

or third gear and yet still put the same amount

of tractive effort down to the ground,’ says

Blevins. ‘So, you’re carrying around parts in the

transmission that you don’t necessarily need.’

Second gear

For season two the battery remained fixed,

but the regulations opened up to allow

development of the powertrain parts

downstream of the battery such as the motor,

the gearbox, differentials and casings. This

saw the introduction of a wide variety of

transmission concepts, with some of the teams

shifting to 3- and 4-speed gearboxes, with the

motor either longitudinally or transversely

mounted, while other teams stuck with the

original season one hardware.

Only a few new powertrain regulations

were introduced for season three, the rest of

the rules stayed the same. This allowed a year of

progression for Formula E, rather than one

of redesign, which enabled teams to evolve

their season two concepts. Although by this

season all of the teams had moved away from

the original 5-speed gearbox.

‘We actually switched to a single ratio

gearbox in season two, but this was mainly

for simplification as there was not much time

available for us to develop a more complex unit,’

Chevaucher says. ‘Everyone knows that at that

point [season two] our car was good, but heavy,

so for seasons three and four we moved back to

a multiple ratio gearbox to work on optimising

the performance of the motor.’

The next major step came in season four

when the regulations increased the available

power from the motors to 180kW. This

effectively extended the power curve towards

the right, as shown in Figure 6. At this point

the majority of teams switched to a 2-speed

gearbox to reduce the tractive effort overlap

between the ratios when compared to Figure 5.

However, there is still an overlap, in this

configuration at around 150km/h you can

either be in first or second gear, giving the

driver a degree of flexibility as to when to

TECHNOLOGY – TRANSMISSIONS

‘This concept requires a large reduction ratio, so we have had to split

the reduction up so that it is a two-stage single-speed gearbox’

Figure 7: Motor thrust curves for season five. Power is up and single ratio gearboxes are now used by all the teams

Figure 6: Motor thrust curves for season four. At this point the majority of the FE teams had switched to a 2-speed gearbox

Figure 5: Thrust curves for the season one 5-speed gearbox. At around 100km/h you could use first, second or third gear