Sunday, September 17, 2006

Evolving Automobiles

The task of designing cars with ever-improving gas mileage has become a staggering challenge burdened by the conflicting simultaneous requirements of powerful acceleration and a strong and safe energy-dissipating structure. More power and safer structures require heavier engines and structural members which reduce mileage. Necessary leg and headroom for passengers and storage volume requirements conflict with the need for streamlined body styles that would reduce drag on the car (and so improve gas mileage).

So what is a poor mechanical engineer to do when faced with seemingly impossible constraints?
Well, the designers at Mercedes-Benz turned to a solution fine tuned over millions of years worth of prototypes; the female Boxfish, or Ostracion meleagris.

A boxfish

Over the milenia, this little aquatic denizen has evolved in an environment where hydrodynamic drag is one of the most significant factors influencing its survival. As the Mercedes engineers noted,
" has a great deal in common with cars in many respects. It needs to conserve its strength and move with the least possible consumption of energy, which requires powerful muscles and a streamlined shape. It must withstand high pressures and protect its body during collisions, which requires a rigid outer skin. And it needs to move in confined spaces in its search for food, which requires good maneuverability."
The Mercedes engineers constructed a CAD model to characterize the streamlined nature of its body and its Coefficient of Drag, Cd.

An aerodynamic calculation of the boxfish model
"Despite its angular structure, the boxfish has almost as good streamlining qualities as the water drop shape which specialists consider to be the standard for the ideal aerodynamic form. When exposed to an open flow, this streamlined shape has a Cd value of 0.04."
Their subsequent 1:4 scale wind tunnel model came very close with a Cd = 0.06.

Wind tunnel model of a boxfish

Using this model as a starting point, the engineers began slowly modifying the design to accommodate expanded passenger and trunk compartments while maintaining the angular outside contours. The final Cd = 0.95.

Wind tunnel model of a car with the contours of a boxfish

The final fully-functional and drivable concept car with tires and all maintains a Cd value of 0.19, one of the best ratings in the size category, and achieves a stunning 70 miles per gallon fuel economy standard with a top speed of 190 km/hr.

Mercedes-Benz bionic concept car

Compared with the current commercial streamline champion, Toyota's Prius hybrid, which has a Cd = 0.26, the little fish delivered an almost 30% reduction in drag!

Viva la Evolucion!

More details can be found at the concept car homepage, and this document "Gone Fishin'" from their web site. (All images credited to DaimlerCrysler)

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