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Old 05-06-2009, 03:44 PM   #1 (permalink)
Ernie Rogers
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Great forum, asking for help

Well, I just got around to looking in this forum today. You guys are doing great work, and thanks for assembling the drag table.

Today, I am embarking on a great task--beginning of a "white paper" on the potential for greater efficiency in cars. This is for the Sierra Club. I see the matter as logically divided into four or five parts--

1. Aerodynamics
2. Rolling resistance
3. Power plant efficiency
4. Other losses such as accessories, bearings, lubricants, etc.
5. Future prospects outside the paradigm such as pavement
types and modes of car use.

All suggestions are welcome. Citations to similar work would be nice. But, I have a specific request at the moment. There are a few outstanding examples of very low drag in "regular" cars

Aptera...................... 0.15
GM EV1.................... 0.19 (cone mod-- 0.15)
GM Precept 2000....... 0.176 (as I remember).

Those are the "easy" ones. More spectacular numbers have been achieved by SAE Supermileage competitors, close to Cd = 0.10.

I have worked on concepts in the neighborhood of Cd = 0.02, possibly the limit of what can be achieved at very high Reynolds numbers.

My question for today: The lowest documented Cd I have is 0.15. I am looking for published numbers that are lower than that. I know I should do some looking for myself, but I thought it would be nice to check in here first. The next place I would check is the U. of British Columbia Supermileage team, and a few other schools.

Thanks for help and suggestions.

Ernie Rogers

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Old 05-06-2009, 04:23 PM   #2 (permalink)
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Are you limiting your search to a particular vehicle class (ie. production possible passenger vehicles)? Or is it wide open?
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Old 05-06-2009, 04:58 PM   #3 (permalink)
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Hi Ernie,

Over sixty years after the P-51, I've yet to see extensive laminar flow on commercial aircraft, let alone production cars. So, you might want to start a separate study on what might be achieved, after we get the rest of the shapes right.
Perhaps the biggest area for improvement in North America is reducing wasted capacity. As I recall, it is hard to see around in traffic in your neighborhood from a small car, because of all the big, empty trucks. There are huge problems of perceived safety and status. However, we can remind folks that "John Wayne hauled his manhood around with only 1 HP."
Anyway, assuming that people will achieve enlightened self-interest regarding vehicles, we could encourage driver's co-ops to let people drive tiny cars and still have a big one available on occasion.
Personally, I'm sketching a cabinscooter that can take on a single adult passenger, bobsled-style, or the usual kids and groceries load. Occasional overloads would go on fold-down platforms beside the single rear wheel, on on a trailer. The driver's seat would recline, to allow sleeping over instead of some return trips.
See What is a Hypercar® Vehicle? for the hypercar project, too.
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Old 05-06-2009, 06:05 PM   #4 (permalink)
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The VW 1L and Daihatsu UFE III are both in the .15-.16 range.

MIT Solar Car: 0.11Cd.
0.11Cd means the MIT Solar Car can do 90 - News - PlanetTesla

Sometimes you find very low Cd reported for aircraft and fish, but they're not comparable to automotive Cd, since they use wetted area rather than frontal area for the calculation.
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Old 05-06-2009, 06:38 PM   #5 (permalink)
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Ernie, I don't know if you've already seen the white papers sticky, but you'll find a great deal of info in there. It's a collection of the best freely available papers I've read.


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Old 05-06-2009, 06:42 PM   #6 (permalink)
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Adjust the nut behind the wheel
Nissan Versa Aeromods
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Old 05-06-2009, 06:53 PM   #7 (permalink)
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low Cds

Ernie,I'm not sure what "category" of vehicle you're interested in.Subaru and GM have developed windtunnel models with Cd0.088 at critical Reynolds Number.Honda took GM's Sunraycer and did the mods Aerovironment knew about,but did not do,to push Cd0.12 down to Cd0.10 with their Dream-2 solar racer.--------- The twin-boom Tarf race car is rated at Cd0.10There is a university-sponsored mileage marathon car claiming around Cd 0.089 or something like that,but this is below critical Reynolds Number and is of dubious value to us,unless we're willing to drive at or below 20 mph.------- Many Bonneville streamliners have forms in Cd0.11-0.12 range( Goldenrod,Pumpkin seed,Ken Lyon's BMW 'liner).------ Ford's PROBE-V concept car was "producible",with Cd0.137.----------- The old-school theoretical limit for a vehicle on the road,in ground-effect,and above critical Reynolds Number is Cd0.10 (like Dream-2).The mileage-marathoner Cd claims may be a bit disingenuous do to their remarkably low velocities .These numbers need to be taken in a proper context.All these cars are "passive".Perhaps "active" aerodynamic cars will evolve with dynamic morphing abilities which can push the envelope a bit.We'll have to see.That's all I have off the top of my head.Good luck with the project,it sounds interesting.If you have access to "Enviromentality" Sierra Club of Dallas' cable-access television series,there are a couple of episodes I did back in the early 90s on aero.Around May/June 1991.
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Old 05-06-2009, 08:15 PM   #8 (permalink)
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That's really cool Ernie. I'm actually making a very simple "how-to" presentation to the local Sierra club chapter this month on how to use less fuel. I don't really have anything to contribute to the thread because I'm not that technically edjuhmakate'd, but I really respect this work.
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Old 05-06-2009, 11:56 PM   #9 (permalink)
Ernie Rogers
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Thanks, guys,

As usual, you are impressive. Those are exactly the information I was looking for.

You've made that step of my job much easier.

Now, I'll move over to the general efficiency forum and ask the same kind of question about tires and such.

Ernie Rogers
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Old 05-07-2009, 12:38 AM   #10 (permalink)
Ernie Rogers
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Looking for lowest friction at highway speed

Originally Posted by MetroMPG View Post
Are you limiting your search to a particular vehicle class (ie. production possible passenger vehicles)? Or is it wide open?
Hi, Metro,

Wide open.

Also, any low numbers you know of for rolling resistance, and high numbers for plug to wheels, etc.

Ernie Rogers

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