Taking EV drag coefficients to new lows to boost range

The next generation of electric vehicles is taking the art of reducing aerodynamic drag to a new, much lower level.

Not since the low-slung, streamlined General Motors EV1 from the late 1990s and the 2013 Volkswagen XL1 have production cars and light trucks slipped through the air with such efficiency.

For EVs, a low aerodynamic drag coefficient, or Cd, is crucial for lowering wind and road noise that would normally be masked by the piston engine. More importantly, a sleek, wind-cheating body enables a vehicle to go farther on a charge — no trivial matter for consumers considering their first EV.

The lower the aerodynamic drag, the less power required to propel the vehicle at speed.

"The pressure is on to save energy any way possible. It's critically important for range," said Paul Snyder, chair of Transportation Design at Detroit's College for Creative Studies, one of the nation's premier schools for automotive designers.

Three sedans are contending for most aerodynamically efficient EV.

The Mercedes-Benz EQS large luxury sedan has a drag coefficient of 0.200, making it the most aerodynamically efficient production Mercedes car yet.

The new Lucid Air sedan, which won Motor Trend's Car of the Year award in November, has a 0.200 Cd.

Tesla claims the new Model S Plaid, which can reach 60 mph in under two seconds, is the most aerodynamically efficient production car on the market, with a 0.208 drag coefficient.

The figures for these cars are not an apples-to-apples comparison, though, because different wind tunnels yield different results, test procedures vary, and the figure used to derive the drag coefficient rating is based in part on the car's frontal area, which can differ from vehicle to vehicle.

Still, the lower the drag coefficient, the less power it takes for the drivetrain to move the vehicle and the farther it can go on a charge.

Mercedes-Benz engineers calculated that reducing the Cd on an EV from 0.23 to 0.22, would yield a 2 percent increase in driving range at 56 mph, which adds about 9.3 miles of range to a charge.

Tesla says that a 10 percent improvement in aerodynamic efficiency yields a 5 to 8 percent increase in the range of one of its vehicles.

For Lucid's Air sedan, low aerodynamic drag played a key role in meeting one of CEO Peter Rawlinson's major goals: a 500-mile range between charges for one version of the car.

To help achieve that, the California startup hired Jean-Charles Monnet away from the Red Bull Formula One racing team to head Lucid's aero development.

He didn't try to incorporate aero tricks from F1 racing on the Air sedan. But his 11 years at Red Bull did influence how the car manages airflow under, over, through and around the vehicle. Monnet said he was extremely proud of the diffuser.

The diffuser attaches to a vehicle's underbelly and usually consists of vertical slats mounted to a flat panel that covers mechanical components, such as the axles and suspension parts.

According to a Car and Driver report on race car aerodynamics, the "diffuser acts as an expansion chamber to manage the air as it exits from underneath the car and reintegrates it with higher-pressure ambient air. Smoothing this transition reduces turbulence and drag in the car's wake and improves airflow under the car."

In a 2017 test of the Air's aerodynamics performance on the racetrack, the car reached 235 mph and the tires remained firmly planted on the track.

"The design of the diffuser started early on," Monnet said. "We only used computational fluid dynamic simulations. We went straight on the track at 235 mph. When you have that speed, aerodynamics is critical, and the diffuser performed as well as we expected."

Snyder, of the College for Creative Studies, believes the EV era will see aerodynamic drag coefficients for cars in the low 0.20s become normal, not exceptions, as the design language of cars changes to adapt to electric drivetrains. It'll drop for pickups and SUVs, too, he says, but that challenge is far more difficult because of their body styles.

As Ford was sculpting a smooth body for the Mustang Mach-E, the aerodynamic team was busy focusing on the details, from the wheels, to the active grille shutters to the underbody, says Debbie Hands, aerodynamics supervisor for the vehicle. Ford's hot-selling electric crossover has a drag coefficient of just 0.28.

"As we were working through the development process, we had the styling studio in the wind tunnel with us to try and tune every last little bit we could," Hands said. "In our work, we were really touching every part of the exterior."

EVs have aerodynamic advantages over internal combustion engine vehicles. Perhaps the biggest: the grille.

On vehicles such as the Mustang Mach-E, Air or Porsche Taycan, the grille is either closed or reduced in size to little more than a slit under the fascia since no radiator is needed to disperse engine heat. Air going through a grille to cool an engine is one of the biggest drags on a traditional car or light truck.

"The grille actually traps a lot of air. There are more efficient ways of cooling the battery through the undercarriage, so that's a big part of it," Snyder told Automotive News.

Look for exterior body parts, he says, to become hidden, very smooth or mounted flush with the body.

Automakers "are spending money in areas that really make a big difference," Snyder said. "Door handles and mirrors and electronic navigation systems are being covered and disguised and designed for slippery airflow."

At Lucid, Monnet may have one of the slipperiest cars on the road, but the quest to lower aerodynamic drag continues.

"With every vehicle, you try to understand how the airflow behaves around the vehicle. And what is important is the cooperation between the engineers and designers," he said. "We are all marching towards the same goal."