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Old 04-27-2010, 06:18 PM   #1 (permalink)
aerohead
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lunar streamlining yields 108-mpg hwy

Coming back from outer space I had been contemplating Amory Lovins' 200+ mpg 'Hypercar'.
For many a moon I've tried to wrap my mind around the physics of such a car and how you could really pull it off.
So to make big mental leaps I thought I could simplify the aerodynamic part of the equation simply by eliminating it.
By inserting a 'Lunar' atmosphere into the Road-Load equation it would be quick and dirty way to look WAY forward.
Here are the assumptions ( there are always assumptions!) :
A 'typical' 1000-kg passenger car of 'typical' frontal area,and 'typical' Cd which returns 28-mpg HWY,has a Road-Load of 28-horsepower at 70-mph, constant BSFC of 0.459 pounds gasoline-per-horsepower-hour,and 'typical' all-season steel radial tires at 'typical' inflation pressures.

When the 'atmosphere' is removed,all there remains is the Rolling-Resistance and powertrain loss,which we're going to ignore.

When the aerodynamic drag is removed,the Road-Load is reduced to something on the order of 8.4-Bhp required to impel the car down the road at 70-mph.

By taking the Road-Load for one hour,at 8.4-horsepower,and 0.459 Pounds per horsepower-hour BSFC you end up with less than seven-tenths of a gallon required for the one hour of driving,or,70-miles.
This works out at 108-mpg HWY.A bit short of 200-mpg.
If you could get the BSFC of a stationary 2-stroke,compound-turbo diesel,at around 0.36-pounds/hour you still come up short.

Now if anyone has seen the Cd 0.16 "Hypercar" at Rocky Mountain Institute,Old Snowmass,Colorado,even with its light composite carbon-fiber body,200+ still seems like a 'reach.'

GM's all-composite Ultralite,at 1,400-pounds, Cd 0.19,and turbo-diesel could only manage 100-mpg,and it never went over 60-mph.

I'm a fan of RMI and I think they have done,and continue to do good work,but I have difficulty with "Hypercar's" numbers.
I realize that electrics and hybrids get 'considerations' from the EPA for CAFE purposes but 200+ mpg seems like a stretch.
I like to think that they are not,in the parlance of Louis Black,"just makin' the ---- up."

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