Electric engineering challenges: aerodynamics

Ahead of the official launch of Formula E’s new racing car, Current E will be delving into the engineering challenges presented by series.

First up: aerodynamics. The early sketches and CGI renderings of the Formula E racing car hint that reducing drag might be more important than creating downforce to the designers. Downforce is pressure exerted on the car by the air moving over it, which pushes it into the ground and makes it stick to the road. It’s the opposite of what happens with an aeroplane’s wing.

While a racing car benefits from downforce by becoming more stable around corners, and therefore faster through the twisty stuff, more downforce means more induced aero drag.

“More drag requires greater power to achieve similar speeds to a more efficient, slippery shape,” explains professional racing driver turned analyst, designer and engineer, Cliff Rassweiler in a recent technical article. Rassweiler has been pioneering innovative hardware and software for electric car racing for two decades. He still competes, in an all-electric Subaru Impreza, and continues to advance battery-powered racing technologies at testing and development firm Pro EV.

“In all-electric cars, more power means more batteries; more batteries mean more weight. More weight in a racing car is rarely a good idea.”

Formula E races will be held on street circuits, which will call for lower top speeds and better cornering ability. So the Formula E car needs enough downforce to remain on the road, but not so much that the car wastes energy trying to push through the drag and its own weight. It’s a delicate balancing act. And unlike Formula 1 cars which get lighter as they burn off their fuel, a Formula E car will carry the entire weight of the batteries for the whole race.

Wings

“What we see on the Dallara-designed Formula E car are single plane wings, rather than the multi-plane wing we’re used to in Formula 1,” says Rassweiler. “That suggests an approach targeting less downforce. The front wing looks pretty skinny – I wouldn’t expect much downforce from that. Quite what the winglets are for on the sides I don’t know: they might be there to provide greater impact protection given that there aren’t side pods to protect the driver.”

Also notable are the significant fairings front and rear, which will smooth airflow over the fat exposed tyres. Fairings behind each wheel would be very useful but would likely interfere with other bodywork and possibly make the car too long. The bargeboards remind us of cars of yesteryear: anyone remember Gumdrop?

Cooling

The air intakes are significantly smaller than you’d expect on a fossil-fuelled car. That’s fine, because while an internal combustion engine loses around 70% of the potential energy of its fuel as heat and so needs significant cooling, in electric motors, than inefficiency is reduced to around 10%, cutting heat output and therefore cooling requirements. Smaller air intakes reduce drag too.

Rassweiler again: “I’d guess that the car will use liquid cooling and normal radiators, probably with separate systems for the motors and batteries since they can have very different heat ranges. The intakes will change size when the car gets to testing: batteries, motors, electronics and chassis are all being designed by different partners in the consortium.”

Rear-wheel drive

From the pictures, it looks like the batteries will be positioned to the rear of the car. It also looks like traditional rear-wheel drive: if motors were being mounted up front to create all-wheel drive, they’d have to be as low as possible, which would require the nose to be flat rather than raised (as it is currently).

“Of course, wheel mounted motors are possible, but I’d think designers would prefer the drag of exposed half shafts and centrally mounted motors rather than having the motors as unsprung mass,” says Rassweiler.  “While I’m expecting a single motor and multi-gear transmission based on initial reports, I’d use a single gear transmission for its superior efficiency. If the design changes to accommodate separate motors, one driving each wheel, we won’t be able to tell from the drawing. “

Overall, the car has a high-drag shape similar to that of a Formula 1 machine. Is that the best approach? While it helps bring instant familiarity to the legions of F1 fans worldwide, we’d rather see something more efficient and truly innovative, like the designs released by Bluebird or the Nissan ZEOD. Perhaps that will come with the second season.

 

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