Quote:
Originally Posted by aerohead
Yet another thought-experiment from my tortured mind!
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I considered the difference Cd makes with respect to power,mpg,etc.,based on an 'ideal' low-drag body and current production cars.
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I calculated the difference in road-load aerodynamic horsepower for a car of Cd 0.30,and the same car at Cd 0.10 ( what was once considered the theoretical low-drag limit for passenger cars ),at 100 km/h ( 62 mph ) steady velocity.
Drag coefficient is the only variable for the aerodynamic road load power formula:
HP= V/550 [ 1/2 x rho x Cd x Af x V-squared ]
V = ft/sec
rho = 0.00238 slugs ( my apology for US units!)
Af = square-feet
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As it turns out after the math,the relationship is very simple.Cd 0.3 is 300% higher than 0.1 and so is the horsepower requirement.
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What this means is that the Cd 0.10 sees the same load at 89.4 mph ( 144 km/h ) as the Cd 0.30 car sees at 60 ( 100 km/h ).
-------------------------------------------------------------------------- A perverted way to look at this,is that the Cd 0.30 car has a 27.4 mpg 'headwind' penalty 'designed' into it's shape,compared with Cd 0.10 baseline.
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Over a statistical 12-year service life,the life-cycle cost to aerodynamic drag would constitute a 300% loss on investment for every highway mile traveled over the 12-year period,with :
* 300 % more fuel unnecessarily consumed( based on constant BSFC)
* 300 % more fuel cost
* 300 % more tailpipe emissions
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It's truly a great, get poor quick scheme!
And as a component of the world's invisible economy,a certain golden goose for those who sell the things that make things go.
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I no longer understand what constitutes a 'Sputnik-Moment' in the world of capitalist economics.
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Phil, nice post. I know you prize accuracy especially with data so I thought you might want to correct something. Comparing Cd 0.3 to 0.1 is a three fold change but not 300% difference. It could be either 200% increase or 67% decrease depending on baseline (direction of comparison). It doesn't change your main point. A two fold change would still back it solidly.
