In the complex and demanding world of production-vehicle aerodynamics, breaking through the 0.25 Cd (coefficient of drag) level might be compared to the challenge once faced by aircraft designers struggling to make the supersonic breakthrough.
But whereas Chuck Yeager rocketed past Mach 1.0 in 1947, with several jet fighter types following suit soon after, the down-to-earth “0.25” barrier has taken rather longer to breach convincingly. Since 1995, the club of production vehicles that have attained 0.25 Cd or better has been small indeed. They include the GM EV-1 (0.195 Cd), the 1999 Honda Insight, 2001 Audi A2, and 2010 Toyota Prius (all 0.25 Cd), and the 2013 Tesla Model S (0.24 Cd).
But now Mercedes-Benz has achieved a 0.22 Cd (Cd x A, 0.49 m²) with the base version of its new CLA four-door, five-seat coupe. The 1.6-L CLA180 BlueEfficiency Edition incorporates narrower tires (width of 195 mm/7.7 in) on 15-in wheels with aerodynamic covers, and lower ride height. Mercedes rather boldly claims it as the most aerodynamic (production) car in the world, immediately after Volkswagen declared much the same for its extraordinarily advanced, unconventional hybrid XL1 (see www.sae.org/mags/aei/11874). The difference, however, is that Mercedes’ claim concerns a car that is on sale and will be built in relatively high volume, while the XL1 has yet to be given a price tag and will be built initially at two per week.
Both cars, though, are fine design and aerodynamic engineering achievements. In fact, Mercedes’ BlueEfficiency version of the B-Class is already claiming a 0.24 Cd, but the leap to 0.22 is a signal advance. For a regular version of the CLA, the figure is still a very impressive 0.23 Cd (Cd x A, 0.51 m2). Until only a few years ago, the general feeling among the automotive design and engineering fraternity was that below 0.25 Cd, vehicles were likely to look “odd” and probably lack multi-role capability. But an amalgam of the application of advanced CFD and extraordinary attention to the finer points of design detail herald a new dynamic.
Does this mean still lower figures from practical, conventional vehicles? Norbert Fecker, who headed the aerodynamics program for the CLA, said: “We have to try. It is all about working in details—lots and lots of details!”
But it is likely to take a huge effort. Fecker says the program to drive down the CLA’s Cd figure was more intense than for any previous Mercedes, with engineers and aerodynamicists developing the car very closely with designers.
“All details of the car’s design were considered,” he said. “To get the Cd value down we looked at saving thousandths—0.001, 0.002, or 0.003—which combined to make a big difference.”
It was not just the application of one component or solution but continuous refinements such as a slightly higher trunk lid, a change of offset (from the concept car) between rear deck and shoulders.
Designer Mark Fetherston said of the teamwork between designers, aerodynamicists, and engineers: “We worked together on every single exterior aspect of the car. We did a quarter-scale clay and then a full scale that went straight into the wind tunnel. Work there also included changes to the rear spoiler and the position of the wheels in their arches. In terms of form, taking it to the wind tunnel was not a difficult job, and we did not lose our original design.”
Fetherston made the point that some aerodynamic tweaks that work for one design may not for another. “It is about individual consideration, and then integration into the whole design,” he said.
Although the CLA is the latest version of the new-generation hatchback A-Class and has the same platform and powertrains, it shares no body panels with its sibling, with the exception of its panoramic roof. The angle between the hood and the low-shouldered A-pillars also is the same.
Mercedes’ aeroacoustic wind tunnel at Sindelfingen, Stuttgart, has a 265-km/h (165-mph) capability both with regard to airflow (it has a 9-m/29.5-ft fan) and to its multi-belt system for a car’s road wheels to simulate real-world, on-road conditions.
Design refinements proven in the tunnel included adjustable radiator shutters and rear lamp lens fins. The raised trunk lid creates a partial Kamm-type tail. A diffuser is fitted below the rear bumper, and the car’s underbody is enclosed, including the middle section of the rear axle. The muffler also received aerodynamic attention.
The A- and B-Class vehicles already use serrated spoilers on front and rear wheelarches to reduce linear turbulence, deflect airflow from the wheels, and stabilize shear layer. The CLA gets something similar but with further detailed refinements.
Fecker explained that a coupe-type bodyshell provides an optimum shape for a practical car because it allows the rear quarters to taper more markedly. A little extra length (30 cm/11.8 in) over the A-Class hatch also helped.
The ecological result of sharply focused aerodynamic work is demonstrated clearly, said Fecker, with an improvement of Cd by a factor of 0.04 cutting fuel consumption of a car cruising on an autobahn at 130 km/h (81 mph) by 0.5 L/100 km, which equates to approximately 13 g/km of CO2.
“Engineers would need to find a weight saving of 35 kg in the chassis structure to manage a similar drop in CO2 emissions,” according to Mercedes.
The production CLA follows on the reveal of the Concept Style Coupe at the Beijing Motor Show in 2012, and also has strong design cue links to the larger, highly individualistic CLS four-door coupe, unveiled 10 years ago before entering production in 2004. The second-generation CLS was launched as a MY2011 model.
The CLA is being offered with a range of diesel and gasoline engines; the high-performance CLA 45 AMG is the range topper. Both front-wheel drive and all-wheel drive 4MATIC will be available, the AMG having it as standard.
The CLA is available with two suspension setups—comfort and sport—the latter allowing the car to sit 20 mm (0.8 in) lower at the front and 15 mm (0.6 in) lower at the rear.
The new car is being built at Mercedes’ new facility at Kecskemét, Hungary. Production started in January.