MGU: Twin axial units / GKN EVO

Transmission: Single gear / Hewland

In short: Twin motors provide good torque but their weight and layout make the car difficult to drive

NextEV TCR and DS Virgin Racing have both gone for two motors driving through a single gear. The Chinese team has been supported by German engineering company Rational Motion and Chinese/American transmission specialist Omnigear, with a powertrain that eschews radial flux motors in favour of axial flux motors.

The motors appear to be supplied by EVO, which has been bought by GKN. Rather than cylindrical-shaped radial flux motors familiar from first generation Formula E powertrains, the axial flux motor is large in diameter and short, somewhat like a pancake. It spins more slowly but with greater torque than a comparative radial flux motor.

While season two regulations permit two motors, they aren’t allowed drive the wheels independently. That’s led NextEV to couple the motors together, needing only a single reduction gear ahead of the differential. Moreover, being a low speed motor (peaking around 8,000rpm*; radial units elsewhere on the grid spin to around 20,000rpm), the reduction gearing ratio is lower, which improves efficiency.

The inverter needs to be different to single-motor powertrains, too. The McLaren component has been switched for a large, flat unit. Still mounted above the battery, the NextEV inverter is formed of three parts: a side unit takes the DC supply from the battery and is then connected to two inverters stacked on top of each other, one for each motor.

This type of multiple axial motor configuration was used in Drayson’s record-breaking electric car, in which Yasa motors were mated to Rinehart Motion Systems inverters (which look very similar to those being used in the NextEV car).

Without a full gearbox to act as a structural element, a skeletal tubular spaceframe has been built around the motors, to provide support for the differential and rear suspension. This structure isn’t particularly lightweight – nor are the axial motors, which, for all their simplicity and torque, are heavy items. These weight penalties have pushed the car’s already rear-biased weight distribution further towards the back axle, which has made the car pretty tricky to drive. For evidence, just look at Nelson Piquet’s tail-happy, pendulum-like cornering performances thus far in the second season.

We’re told by industry insiders that the team specified the motors to cover anticipated regulation changes next season, when power was due to be substantially increased. That effectively means the motors have far more capacity than can be used under the regulations. Although the motors have decent weight to torque characteristics, they therefore pose something of a weight handicap compared to powertrains designed specifically for this season.

The handling problems are thought to centre around the chassis design, which was reported to have been twisting during preseason testing at Donington Park, causing problems for the powertrain components.

For Oliver Turvey, a DNF in Malaysia came courtesy of the throttle pedal sticking open. Under the “drive-by-wire” system, pedal inputs are sent via electronic signals to the ECU. The aluminium throttle pedal rotates in a floor-mounted clevis; the pivoting also moves the rotary throttle pedal sensor. The fault could lie in either the electronic sensor or the mechanical part of the pedal. A similar problem affected Piquet in China; Berthon and Felix da Costa have both had throttle issues at Aguri over the first and second seasons. If they are all related, it is clearly an issue that needs to be resolved, although NextEV’s problem may lie in how the team’s software is interpreting and responding to sensor signals.

NextEV is reported to be exploring a single motor configuration in place of dual motors for season three, given that permitted peak power is unlikely to change, as well as getting serious about carbon fibre. Whether that will be enough to convince Piquet to stay, given that he is unlikely to be able to mount much of a championship title defence in this season’s car, remains to be seen.

Scarbs is technical editor at Current E. Follow him: @ScarbsF1

*Article updated 01.02.15 from previously quoted “3,500rpm”