Auto-Future: Active Aerodynamics
Life’s a drag. If you’re an automobile, the faster you go, the more of a drag it becomes. As early as the 1920’s, engineers realized that a car’s shape was no less important than an airplane’s; it determined the the automobile’s aerodynamic efficiency, which has a major impact on its fuel efficiency. (Cars may not have been born from jets, but the same rules apply.) For mass motoring, decades of cheap gas made automotive aerodynamics more of an optional art class than a required science. Now, with government regulators demanding maximum fuel efficiency, aerodynamics are back in play, headed for the mainstream. Active aerodynamics are taking center stage.
Active aerodynamics uses moving surfaces or parts to change the aerodynamic behaviour of a vehicle. The most obvious example: the retractable spoilers found in a number of sports cars (e.g. Audi TT). Another recent, less famous but equally important application: BMW cars equipped with the Efficient Dynamics package. Air flaps at the front of the vehicle regulate air flow to the engine. If the ECU determines that engine cooling is not needed, the flaps close to reduce drag.
Other manufacturers are sure to copy Bimmer’s lead. At the same time, various carmakers are developing active aerodynamics that reduce drag at higher speeds. Saab has shown a proposal (don’t they always?) to improve a hatchback’s drag by using an extending rear parcel. Although the feature is an aesthetic nightmare, the principle is sound.
This little black deflector on a Citroen C4 Picasso may also become an active surface. Reports indicate that the feature could be part of a rear light module including the mechanism for surface actuation. At the same time, automakers are experimenting with downforce– aerodynamic aids that “press” the car towards the pavement to increase road holding. High end automobile manufacturers have spent considerable effort modifying the underside of their vehicles to improve handling (e.g. the Ferrari F430). Actively manipulating these surfaces may yield some incremental benefits for overall fuel efficiency.
But incremental aerodynamic and, thus, fuel efficiency gains are all the rage. There’s no question that the quest for better high speed mileage will lead to more vehicles with active surfaces.
More by David Moreira
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Well, let's do the maths. Let's say you improve your car from a very respectable 0.30 to a best in the world 0.26, based on a frontal area of 3 square metres. You will save roughly 3 hp at 75 mph. Does that sound like it would lead to a "major impact on fuel efficiency" ? No.
Porschespeed: Being an owner of an '87 928, I can speak directly about this. It wasn't so much they were a PITA to keep working, it's that typically the fix was simple when they did break, but not really worth the money. The claimed improvement in aero was to bring the car's cD down to .34 from, I believe .38. But nobody could ever really provide any proof that having the operational flaps would improve fuel economy. Plus, if they failed in the closed position, you really increased the odds of destroying the engine - cooling was not one of the highlights of the 928. So most people disabled them in the open position, and Porsche eliminated them in '91 after a spate of customer cars came back with closed flaps and ruined engine blocks. Mine? The flap motor is dead, and one of the wires going back to the cooling ECU is shorted. So it's permanently open. It seems recently more cars have these, typically some type of rear spoiler, and I think they have improved reliability. But I suspect a more effective item would be a full undertray. The 928 S4 and later had a partial undertray as well. It *was* a PITA to take off an on, and added, at least for me, an hour to any work under the engine. So it's off now, and I haven't noticed a change in fuel economy. Some say it's mainly do direct cooling air to the hydraulic motor mounts, another item that tended to crap out. But I think a well designed full undertray could accomdate routine maintenance without adding significantly to billable work hours, really improve fuel economy, and not be susceptible to failure like active aerodynamics.