Raindrop are Round(-ish)

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Alternative Transportation Fuels

Tesla Motors' recent press blitz about their roadster renewed my interest in alternative transportation fuels. Their white paper about well-to-wheel efficiency was particularly interesting, however, my inner skeptic wanted more! It turns out that diesel cars are not far behind.

Using Tesla Motors' formulae, the production four passenger Audi A2 1.2 TDi with a fuel efficiency of 2.99 ^liters/_km (or 79mpg) has a well-to-wheel efficiency of 0.84 ^km/_MJ! That closely approaches the Tesla Roadster's 0.89 ^km/_MJ when it's fueled from the USA's dino-powered electric grid. Diesel fuel's excellent well-to-station efficiency (91%) makes it stiff competition for electricity generated from fossil fuels (41%).

Tesla Motors, however, can easily crank up their efficiency with improved aerodynamics. If they snugged their power plant into a sleeker body, it's efficiency would increase a lot!

Why? Here are some numbers. Lacking specifics from Tesla Motors on drag (C_d) and C_d * frontal area (C_dA), I’ll just assume they're close to the very similarly shaped Lotus Elise S2: C_d=0.408, C_dA(m²) = 0.653 or C_dA(ft²) =7.00. This means the Roadster has aerodynamic drag somewhere between a Chevy Lumina and a BMW 325i.

For comparison, the best production cars today have a C_d ~ 0.25 (Prius, Audi A2 1.2 TDi, Lexus LS430 etc..). The GM EV1 had a C_d = 0.195 and C_dA(m²)=0.366 or C_dA(ft²)=3.95.

Update June 2008: Tesla Motors has issued additional information and they have improved on the Elise's Cd; the Tesla Roadster is reported to have a C_d = 0.3 and a C_dA(m²) = 0.49