Racecar Engineering – September 2019

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

‘Controlling howoil moves around inside the gear-case is important; it

obviously provides lubrication, but it also helps with gearbox cooling’

TECHNOLOGY – TRANSMISSIONS

shift gear. Unfortunately, the motor

performance wasn’t quite there to allow

accelerating from zero to maximum speed

without a gear shift being required resulting

in a single torque-interrupt event.

‘The biggest step so far was between

seasons four and five,’ says Chevaucher.

‘Developing a Formula E gearbox is a very

long process because we’re not optimising

a known architecture like you are for other

categories of racing. We are building new

architectures which means there are lots of

possible routes of development that are not

usually possible in conventional IC engines.’

Fifth gear

Season five saw the introduction of the Gen2

car as well as an increased battery capacity,

allowing the race to be completed with a single

car (before this they were changed mid-race).

The amount of available power and speed from

the motors was also increased, power to 200kW

in normal race configuration, and as a result, the

power curve extended even further to the right.

This led to most teams having to effectively start

their transmission design from scratch, and all

converged on a single-speed transmission.

‘The improved torque and speed range

of the motor means that you can now cover

0-200km/h in one gear,’ says Blevins. ‘During

the first season of Formula E, you had to carry

five gears around to achieve that performance,

but now we are able to achieve a better

performance envelope with just one ratio. That’s

essentially why everyone has now converged

towards a single-speed driveline. However, this

concept requires a large reduction ratio which

is too much to do on a single gear pair, so we

have had to split the reduction up so that it is a

two-stage single-speed gearbox.’

With all teams now converged on to the

same single ratio concept, large performance

gains are difficult to make. Therefore, the teams

and their suppliers are now having to chase

tiny percentages to improve efficiency in an

attempt to beat the competition. So, which

areas are Formula E transmission suppliers

focussing on now? Well, to determine the areas

of potential improvement, you first need to

identify where the losses are.

‘One area of loss comes from the gear mesh

itself,’ Blevins says. ‘Theoretically there is a

perfect gear alignment which can be achieved

by using the ideal shape of a gear tooth which

is a true involute. Gears should roll together,

rather than slide and when the shape of the

gear teeth migrates away from that true involute

you start to induce sliding. This sliding effect can

also be caused by shaft deflection, as the gear

centres effectively increase under load. Under

Optimising the meshing of two gears depends on everything from tooth geometry to shaft deflections and case design

this condition the tip of the tooth can also begin

to interfere with the mating gear creating a

further transmission loss. We can predict this

and refine the micro geometry of the gear by

adding tip relief. But that assumes a degree of

deflection, which will change with the level of

torque transmitted. Using our experience, and

a duty cycle supplied from our customer, we

can calculate what average torque the gears

are transmitting, allowing us to optimise the

amount of tip relief required.’

Oily drag

There are also losses associated with the

gearbox oil. If the gear assembly is dipped

into a sump then the gear teeth have to be

dragged through oil as the gear rotates.

Therefore, the gearbox layout, gear geometry,

case design and oil management are all

optimised to minimise this drag. With the

mandated multi-speed gearboxes in season

one these features were previously not as

important, but the competitiveness of current

Formula E is forcing suppliers to focus on

developing these finer details.

Temperature is another enemy of efficiency

and is the second function of the oil. Typically,

Formula E transmissions run cooler than, say, a

GT gearbox simply because the fewer number

of ratios means there are fewer gears meshing

together. Also, relative to GT, Formula E boxes

are usually transmitting less power. These

lower transmission temperatures mean that

fewer and smaller oil coolers are required,

reducing weight and aiding packaging.

‘We take oil management very seriously,’

Blevins says. ‘Controlling how the oil moves

around the inside of the gear-case is very

important; it obviously provides lubrication,

but it also helps with gearbox cooling. We

therefore aim to introduce oil only where it’s

needed; trying to keep it away from rotating

components elsewhere to minimise any

churning losses. We have also done a lot of