Automotive Engineer

The 1970s fuel crisis was a wake-up call to the industry in two respects – that fossil fuels are a finite source of energy and the vehicles consuming them were not efficient enough. Aero-dynamic development became a priority. The highly streamlined models that resulted set new drag-coefficient benchmarks.

When Audi launched the 100 sedan in 1982, it was so proud of the Cd of 0.30 that the figure was etched into the glass. That was bettered two years later by the W124-series Mercedes E-Class sedan, but its 0.29 Cd was nowhere to be seen in the glazing or in the PR material.

When the current E-Class was launched, Daimler highlighted its 0.25 Cd, and that, at 0.24, the coupe version was the most aerodynamic production car in the world.

Another benchmark has been set with the latest B-Class compact. Despite the mini-MPV body – far from the ideal shape for drag – the car achieves 0.26 Cd. Dr Teddy Woll, Daimler’s head of aerodynamics, acknowledges that many consumers were unaware of the progress the OEM has been making because it was not talked about much until now, but has a pragmatic view of what his engineers achieved.

“The currency of aerodynamics is drag counts, which is 1/1,000th of a Cd, so from 0.29 to 0.25 is 40 drag counts in 25 years of sedan development,” he says. “Now we’ve gone from 0.30 in the outgoing B-Class to 0.26 which is a leapfrog in seven years. And we have an optimum eco package with a lot of measures which then reaches 0.24 – a phenomenal drag factor for such a relatively boxy car.”

The designers and engineers worked together closely, right from the beginning, and with common aims – it wasn’t always like this. In former times, Woll says, the stylists and his aerodynamics people came together in the wind tunnel and fought over solutions. Now the engineers use a development tool to calculate the first proportional styling model, and use it to discuss problems and possible solutions with the design team. “We show them where we have losses, and what might be done,” says Woll. “They see this, understand, and start thinking with us. Some of them like achieving very low drag figures – it wasn’t always so important to them.”

Sedans have lower form drag because the trunk helps the airflow to remain attached to the vehicle’s surface for longer, resulting in smaller, less turbulent wakes. The B-Class does not have this advantage. So achieving an extremely low drag coefficient meant optimising every last detail.