Fuel Economy Gains Getting Harder to Garner - Just Ask Mercedes-Benz

Fuel Economy Gains Getting Harder to Garner - Just Ask Mercedes-Benz

By John O'Dell December 14, 2009

Comfort Innovation for 2011 E-Class Convertible Costs .0025 Gallon Per Mile

AirCap, that shiny black "wing" at top of windshield, almost eliminates front-seat turbulence with wind speed at 75 mph in Mercedes-Benz wind tunnel. Tall passengers in rear get a bit more breeze. Trade-off for comfort is a very slight reduction in long-term fuel economy.

Fuel economy is a hard-won gain for most automakers. Improvements over the years in aerodynamics, lightweighting and powertrain efficiency have gobbled up the low-hanging fruit, leaving only the hardest-to-pick gains still on the tree.

And it is oh, so easy to pick up a little here by dint of a lot of fine tuning - only to lose it over there.

A case in point is the impact of a new comfort-boosting innovation on the 2011 Mercedes-Benz E-Class Cabriolet due out next year.

Thanks to a lot of attention to tiny details, the classy new four-seat convertible sports pretty impressive aero numbers: a drag coefficient of 0.28 with the soft top up - slightly better than the then world-leading mark turned in by the 1984 E-Class sedan and not too shabby when compared to the 2010 E-Class sedan's 0.24 coefficient.
But drop the top and - as is always the case - aerodynamics suffer.

In the 2011 E-Class Cabriolet's case, the Cd drops by half a tenth, to 0.33. That cuts fuel economy by about a tenth of a gallon per 100 kilometers (62 miles) when driving at a steady 62 miles an hour, according to Mercedes aerodynamics guru and wind-tunnel chief Teddy Woll.

What also suffers when the top comes down on a cabriolet is cold-weather comfort, so Mercedes went looking for a way to fix that so the convertible could be used opened up all year.

The result is an invention Mercedes calls the AirCap, a pop-up deflector that changes the flow of air over the open cabin of the car, greatly reducing interior noise and turbulence and enabling warm air from the heater to stay inside, with the passengers, instead of being sucked out by the wind.

But that the AirCap does more than reduce wind. It also reduces the cabriolet's coefficient of drag a bit more, by an additional half a tenth, for a final Cd - top down and AirCap deployed - of 0.38 versus the convertible's top-up reading of 0.28.

That - if I've done my math correctly - is sufficient to decrease fuel economy by as much as one gallon for every 400 miles at a steady 62 mph.   

The AirCap, in effect, negates aerodynamic gains Woll said were made made by aerodynamic improvements such as louvers behind the radiator grille that close when there's no need for extra cooling; small tabs on the bottom of the side mirrors to smooth out the turbulence the protruding mirrors cause; tiny spoiler lips molded into the taillight lenses to create a uniform spoiler edge across the entire rear of the car; and a fully paneled underbody.

But when the air is crisp, the leaves are turning and the sun is shining, I'd trade .0025 of a gallon of gas per mile for a warm and cozy top-down drive in the country without a tad of conscience.

John O'Dell, Senior Editor

John O'Dell:  is an AutoObserver Senior Editor. Follow @AutoObserver on Twitter.