September 8th, 2009

I’ve said (as have many others) that the Toyota Prius, (new) Honda Insight, and Chevy Volt all look similiar, at least insofar as the overall shape of the vehicles.  (My personal opinion is that that Volt is much better looking than the other two, with the new Prius coming in second, but this isn’t really related to shape.)  The reason for this is that they are all efficient vehicles, so one of their design goals was a low drag coefficient.  An article in this month’s Automotive Engineering International (Aerodynamics Soar) speaks to this, mentioning “complaints that cars like the Honda Insight and Chevrolet Volt, which balance similar missions of efficiency and cabin space, are derivative of Toyota’s Prius, when actually they are all recognitions of the fact that similar goals will produce similar designs.”

A recent video on the Chevy Voltage website talks a bit about the work that went into optimizing the aerodynamics of the Chevy Volt.  One remarkable data-point is that aerodynamic work on the Volt increased the all-electric range by 7 MILES from the original prototype!  Aerodynamic efficiency makes the Volt a PHEV-40, rather than a PHEV-33! To my knowledge, GM still has not announced what the Cd for the Volt is, except to say it’s the lowest of any GM vehicle since the EV1 (which had a Cd of 0.195).  For comparison, the Prius has a Cd of 0.25, and the new Ford Taurus (a modern vehicle for which aero is important, though not as high a priority as it is for hybrids) scores a 0.32.

The drag coefficient (Cd) is directly proportional to the amount of power needed to overcome the force of the air pushing against a vehicle in order to maintain a steady speed.  This power is also directly proportional to the frontal area of the vehicle, the density of the air, as well as the cube of the vehicle’s velocity.  So, fuel economy can be increased by improving the aerodynamics, reducing the size of the vehicle, driving in less-dense air … or, to an even greater degree, slowing down.  The third option seems difficult, and the fourth a bit boring…

If you still think that aerodynamics don’t matter, check out the extreme, where Cd is optimized at the expense of downforce, causing this Mercedes at Le Mans, and this Porsche at Road Atlanta, to become airborn!

Mercedes CLR at Le Mans, 1999

Mercedes CLR at Le Mans, 1999

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