March 16, 2009

Reducing Drag on Cars and Trucks by 15-18%

Researchers have achieved 15 to 18 percent reduction in drag by placing the actuators on the back surface of cars and trucks. Deploying this improved drag reduction on cars and trucks would be equal to the improved CAFE standards from 2008 to 2010. Being able to apply mileage improvements to the active fleet of 100 million older cars in the USA would have over ten times the impact of just improving the mileage of new cars.

The idea behind the active flow control (AFC)is to deploy actuators on the surface of these vehicles to modify the flow in a way that the overall resistance is reduced. Using computational fluid dynamics software, Agarwal has found that the actuators modify the flow, which results in drag reduction, which in turn reduces the fuel amount needed.

This is a simpler version of aeromodding a car to improve aerodynamics and reduce fuel usage.

Edmunds explains how drag effects fuel mileage

For a full-size truck, a change in drag coefficient of 0.01 is approximately equal to an improvement in fuel economy of 0.1 mpg on the combined city/highway driving cycle. The same drag coefficient reduction can improve a car's fuel economy by approximately 0.2 mpg.

At stop-and-go speeds, drag isn't a big deal, but the faster you go, the more it matters. At 70 mph, you've got four times the force working against your vehicle that you have at 35 mph.

An imaginary car has a curb weight of 3,527 pounds, a Cd of 0.30, a frontal area of 23.7 square feet and 9 pounds of rolling resistance for every 1,000 pounds of weight. If we put a gas-burning engine in this car, expect reasonable performance and drive it on a combined driving cycle, we can expect to get 23.8 mpg. Add 10 percent to the drag coefficient, we'll now get 23.3 mpg. Take 10 percent from the drag coefficient, we'll now get 24.3 mpg.

* 10% better drag is about 0.5 mpg for 23.8 mpg average US car
* 15% better drag is about 0.75 mpg
* 18% better drag is about 0.9 mpg

Improve aerodynamics by:
* Reducing the use of roof racks
* Rolling up your windows and turning on the air conditioner at higher speeds, typically above 35 mph
* Replacing a broken or missing front air dam
* Running narrower tires
* Choosing smoother wheels (ideally, flush discs like those on vehicles trying to set land speed records)

You can reduce your vehicle's aerodynamics by:
* Lifting it — "an inch of increased ride height degrades the coefficient of drag by about 10 drag counts [.01]," says Wegryn.
* Choosing more "open" wheel designs (although, for many owners, this advantage will be offset by the fact that "open" wheels promote better brake cooling)
* Installing a bug shield
* Adding a rear spoiler, in some cases

OTHER CLAIMED MILEAGE IMPROVER
Apnano claims an oil additive they make improves mileage by 5%.

Apnano site

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