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Old 04-27-2010, 06:18 PM   #1 (permalink)
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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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Old 04-27-2010, 09:54 PM   #2 (permalink)
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Old 04-27-2010, 10:11 PM   #3 (permalink)
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1) Over 6+ years and close to 90K miles, my Insight has averaged 71.4 mpg, in what are not usually the best conditions for getting good fuel economy.

2) Internal combustion engines are very inefficient. Let's be generous, and say the Insight's engine averages 25% efficiency.

3) Likewise, assume an electric drive is only 75% efficient. This makes the math easy, just multiply by 3.

So with a pure electric Insight, I might expect 3 * 71.4 = 214.2 mpg equivalent.
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Old 04-27-2010, 10:45 PM   #4 (permalink)
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Inadequate batteries continue to stymie straight electric cars.
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Old 04-27-2010, 10:49 PM   #5 (permalink)
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The Insight's engine is an incredibly efficient one, and it mostly operates in the 30-40% efficiency range. The Insight's electric motor + inverter operates around 85% efficiency, but after losses in your charger and battery, figure maybe 75%.

100 highway mpg does not seem like a stretch to me for an ecomodded Insight driven at normal speeds. However, I've done the math on further gains, and I can confidently say no car with 20ft² of frontal area will achieve 200mpg at 55mph any time soon.

I feel you could "easily" exceed 200mpg in a streamlined feet-forwards motorcycle or trike with an Insight engine. Apparently, you can do it even with a Honda motorcycle engine: 214 mpg with DIY aerodynamic fairings on a Honda 125cc motorbike | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com
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Old 04-28-2010, 03:14 AM   #6 (permalink)
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Quote:
Originally Posted by jamesqf View Post
2) Internal combustion engines are very inefficient. Let's be generous, and say the Insight's engine averages 25% efficiency.

3) Likewise, assume an electric drive is only 75% efficient. This makes the math easy, just multiply by 3.
This makes the math easy, just add 25% and 75%.
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Old 04-28-2010, 09:33 AM   #7 (permalink)
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Aerohead, is it possible that they have also reduced rolling resistance and powertrain losses?
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Old 04-28-2010, 03:53 PM   #8 (permalink)
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Darn it. Here I was under the impression that we could just dream our way to 200 mpg.
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Old 04-28-2010, 05:43 PM   #9 (permalink)
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75%

Quote:
Originally Posted by jamesqf View Post
1) Over 6+ years and close to 90K miles, my Insight has averaged 71.4 mpg, in what are not usually the best conditions for getting good fuel economy.

2) Internal combustion engines are very inefficient. Let's be generous, and say the Insight's engine averages 25% efficiency.

3) Likewise, assume an electric drive is only 75% efficient. This makes the math easy, just multiply by 3.

So with a pure electric Insight, I might expect 3 * 71.4 = 214.2 mpg equivalent.
james,somewhere,in another thread we did a workup of the total energy balance of a plug-in electric car,based on coal-fired generation and found it no better,and I think,actually less than an I.C. car.
The good news,was that the electricity could be provided from renewables,however,the overall efficiency isn't any better,it's just cleaner.
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Old 04-28-2010, 05:54 PM   #10 (permalink)
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Hypercar FAQ

I think the folks at the Rocky Mountain Institute may be a little optimistic with their modeling. But here it is straight from the Rocky Mountain Institute.

Excerpts from RMI.org - The HyperCar FAQ


Modeling by RMI suggests that full-sized Hypercar® vehicles should be able to get 90 miles per U.S. gallon of gasoline or equivalent (2.6 L/100 km) in the near term and 200 mpg (1.2 L/100 km) in the long term.


How could Hypercar® vehicles attain such dramatically improved fuel economy?
A car's fuel economy can be improved by reducing any or all of the following:
· aerodynamic drag
· rolling resistance (due mainly to tires)
· drivesystem inefficiencies (the drivesystem includes the engine and all mechanical components
connecting the engine to the wheels, such as the transmission and differential)
· energy lost during braking
· accessory loads (lights, audio system, climate control, instrumentation, etc.)
Minimizing these losses piecemeal is good, but redesigning the entire car for maximum overall
efficiency—taking an integrated, "whole-system" approach—is much better.


Ultralight Construction
... All told, it should be possible to make Hypercar®
vehicles that are 50-65 percent lighter than conventional cars of the same size.


Aerodynamics and Rolling Resistance
Today's cars are already fairly sleek, but aerodynamic drag can be further cut by 40-50 percent or more through cab-forward design, a smooth underbody, a tapered rear end, minimized body seams, and aerodynamically designed air intakes, suspension, and wheel wells.

FWIW,
Tim

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