The Coefficient of Drag Story

The tale of the coefficient of drag is almost a century old. The recent past however is a good starting point.

The year 2000 was a remarkable year for the automotive ingenuity and yet at the same time it was the worst year for corporate irresponsibility. It was the best of times it was the worst of times.  The year saw the release of GM Precept, Ford Prodigy and Chrysler ESX-3. All extremely fuel efficient cars and all broke the 80 mpg target set by the cooperative research program “Partnership for a New Generation of Vehicles (PNGV)”.  And yet these PNGV prototypes made by the “big three” (GM, Ford, Chrysler), were never rolled out, despite being designed for mass production. The sorry excuse given at the time is even more baffling today than it was then.  It was said that USA has no interest in high fuel economy cars. With this statement the plugged was pulled on project.

The 2000 GM Precept

GM Precept: A car ahead of its time

The partnership between the big three was dissolved in 2001 by the Bush administration at the behest of auto-makers themselves.   However, the remarkable designs that the OEMs were able to achieve at the time still hold within them golden nuggets of engineering information. For auto-makers around the world, the design of PNGV prototypes are both aspirational and educational. Within the design concept, there were novel ways in which the big three  reduced the Coefficient of drag (CD).

Traditionally cars have been streamlined by making the surface as smooth as possible and making the shape as close to a slashed tear drop as possible. It should be noted that a tear drop shape has the lowest possible drag coefficient (0.04) but is impractical for a car because of low volume to surface area ratio. To reduce the drag on a spacious family size car is a challenge. The PNGV prototypes went the extra miles in streamlining their cars. They removed the wing mirrors and replaced them with cameras to provide the rear view.  The door handles were also removed.  The protrusions underneath the car platform were concealed with a metal plate providing a level surface.  An air duct that ran through the body of the car not only reduced the stagnation area upfront but also filled up the wake region with the air passing through the duct. The exhaust pipe was also positioned so to pack the wake region with exhaust fumes thus reducing the pressure difference between the front and back of the car.  Lastly the cars,  in particular GM Precept adopted a gradual tapering shape at the back-end,  reducing the adverse pressure gradient and in turn delaying the separation of Boundary Layer. It is because of these design changes that the GM Precept was able to achieve an astonishingly low coefficient of drag of 0.16. Even to this day,  this value of CD remains  one of the lowest in the industry ( for a Sedan).

Coefficient of Drag is the measure of aero- dynamism  or degree of streamlining of  a body. A low Coefficient of Drag indicates a profile that will move through a fluid with small amount of resisting force. The force of drag on auto-mobile is directly proportional to the fuel consumption. At highway speeds (70 mph and above), more than 80 percent of the engines output is consumed in overcoming the drag force alone.

It should be noted that most modern sedan cars rarely achieve a coefficient of drag less than 0.24 . This makes the GM Precept almost 30% more efficient than the most aerodynamic car in the market.  By observing the values of CD for the cars that are popular among green enthusiasts, one can truly appreciate the engineering excellence of Precept. The Tesla Model S has a CD of 0.24.  The Chevy Volt on the same index sits at 0.28  while the Prius was measured at 0.3. Cars marketed as urban vehicles, such as the modern crossovers or off-road vehicles have extremely poor coefficient of drag. For example Nissan Qashqai has a CD of 0.34 and Land Rover Evoque has 0.34. Similarly vans also fall in the category of bluff bodies (non-streamlined).  Ford Transit has a CD of 0.43. This is understandable as the mentioned cars (Crossover, Off-road and Loading vehicles) are not designed to be driven on highways for most part of their service. However, when they are driven at highway speeds, they can consume almost twice compared to the GM Precept.

Having noted that it is still a struggle for engineers to achieve anything below 0.24, it makes all the more remarkable that a car from 1930’s was able to achieve an exceptionally low value of  0.21.  The Tatra T77 was a Czechoslovakian car designed and developed by Hans Ledwinka and Paul Jaray.  It was launched in 1934. The vehicle was one of the first of its kind to transition from boxed or “plate” type vehicles (like Ford Model T) to a more rounded configuration. The distinguishing feature of the car was the vertical fin at the rear of the body.  The fin restricted any laterals movement in the air , stabilizing the flow and in turn aiding a delayed separation of air boundary layer.

Tatra T77 low coefficient of drag

The Tatra T77 (Picture Courtesy Oliver Kurmis)

Starting from 1930’s many streamlined features of cars- like rear wheels aerodynamic covers-  continued till the next four decades.  In the post  Oil crisis era, when the insecurity of fossil fuel was arrested, designers started fidgeting with car designs again. Only this time Coefficient of drag wasn’t a consideration at all. The cars lost their curves and instead became edgy. In aerodynamic terms car started becoming bluff again.  A very prominent example of this was the Delorean DMC 12 as featured in the block buster “Back to the Future” .  The drag coefficient for the seemingly sporty car was reported 0.33. For many sports cars during the 80’s,  the main design objective was reducing the frontal area rather than the coefficient of drag.

Fortunately, common sense prevailed and cars returned to their curvy form by the end of the 80’s.  The contrast between the shapes of Toyota MR2  MK-1 (CD 0.32) and MK-2 (CD 0.31) encapsulates this transition very succinctly (as can be seen in the picture).

Evolution of MR2

Reverting back to the curvy shapes from the boxy shapes of the 80’s

Electric vehicles initially were not concerned with CD as their limited range and speed meant priority was given to room and reduced weight.  For this reason many initial EV models had a higher CD. As the battery performance improved and as more EVs with highway speed capability appeared, the shapes also became aerodynamic.

Going back to the year 2000, one has to wonder if the “Big Three” would have continued on the path set by PNGV, how would that have shaped the auto industry today. It would surely be US cars being sold around the world rather than foreign cars being sold in the US.

Useful Resources

List of Coefficient of Drag of Vehicles

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