Minimum drag

The slipperier the better – that’s certainly the idea with vehicle design, where the less resistance through the air to the vehicle’s mass, the more efficient its passage. Designing a vehicle to slip through the air with the minimum resistance is an art form, and a vital part of designing vehicles for speed and efficiency.

With electric cars, the less resistance to that forward motion, the less power is expended in ‘fighting against the air’ and as a result, the vehicle has greater charge to travel further. 

On long journeys out on the highway for example, the drag constitutes the key driving resistance – far more important than the rolling resistance and inertia. The energy is lost which the car needs to overcome this resistance. That is why good aerodynamics are so important. In urban traffic, however, other factors come into play. Here an electric car can recover a large part of the used energy when braking, thereby reducing the importance of its mass.  

That is exactly what Audi’s engineers were looking to fine tune with hours and hours in the company’s wild tunnel facility. Here the Audi e-tron prototype was put through over 1000 hours of testing on the test rig at the Wind Tunnel Centre in Ingolstadt, facing down 300km/h winds generated by the five metre low-noise rotor – a 2.6 megawatt fan used by Audi enters to optimise drag and noise under the most extreme conditions.

The result of 0.28 is extraordinary for an SUV, and not only gives the Audi e-tron prototype the top result in the SUV segment but plays a decisive part in its everyday range of more than 400 kilometres in the WLTP cycle.

To achieve the drag coefficient of 0.28, the Audi engineers developed a wide range of aerodynamics measures in all body areas. Some of these technical solutions are evident at first glance, while others fulfill their purpose hidden away from sight. Thanks to these solutions, the drag coefficient for the Audi e-tron prototype is almost 0.07 less than for a comparable, conventionally powered vehicle.

Perhaps the most exciting and obvious of the devices used to give the e-tron prototype its ‘slippery’ exterior are the optional virtual exterior mirrors, which will make their world premiere in the volume-production version of the Audi e-tron prototype.

They are much narrower than the standard mirrors and reduce the vehicle’s width by 15 centimetres. Their specially designed shape not only reduces drag, but also cuts win noise ever further. Each of their flat supports integrates a small camera, with the captured images appearing on OLED displays in the transition between the instrument panel and door. The virtual exterior mirrors can be adapted for various driving situations, with three views available in the MMI system – for highway driving, turning and parking.

Standard adaptive air suspension – a pneumatic suspension with adjustable damping – also plays an important part, lowering the body by up to 26mm at speeds above 120km/h.

Out of sight, the underbody of the all-electric SUV is also fully enclosed with the front and rear area are fully panelled for improved air flow.

The attention to detail is extraordinary. An aluminium plate under the passenger cell protects the high-voltage battery against damage from below – its bolting points fashioned with bowl-shaped indentations, similar to the dimples on a golf ball, to improve air flow and efficiency even further. 

Every conceivable way of reducing drag and improving movement through the air had been used.

The controllable cool-air inlet – a frame behind the Singleframe with two electrically operated louvers – also helps lower drag. When closed, the air in this area flows with virtually no swirl, but as soon as the drivetrain components need cooling or the air conditioning condenser requires ventilation, first the top and then both louvres open. When the hydraulic wheel brakes are subject to high loads, the controllable cool-air inlet opens and releases two ducts which channel the cooling air into the front wheel arches to the brakes.

Side air inlets channel the airstream so that it flows past the outside of the standard aerodynamically optimised 19-inch wheels which sport flatter design than conventional wheels. Even the 255/55 tyres boast ultralow rolling resistance, the lettering on their sidewalls negative instead of raised to ensure even the slightest impediment to smooth runnings is removed. The series production e-tron prototype will make its eagerly anticipated entry onto the world stage later this year.