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Old 04-17-2010, 07:27 AM   #11 (permalink)
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i think what frank is saying is that aerodynamic drag is not the only factor influencing fuel consumption, there's rolling resistance, vehicle weight, the gear ratios. also aerodynamic drag increases with speed, so vehicles driven at slower speeds will see less benefit from aero improvements that vehicles driving a lot at highway speeds.

thus mathematically a 3% reductions in aero drag equals not a 3% improvement in FE.

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Old 04-17-2010, 08:22 AM   #12 (permalink)
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True, but a 2-4% improvement in FE is HUGE. Find two or three of those that your competitors miss, and you've got a car that people will think of as more efficient than the rest.

Note also that a typical car burns 7000 gal during its lifetime. Each 1% improvement is 70 gal, or $150-300 worth of fuel.

I figure a dual camera replacement mirror system might cost $200 over the life of the car, while saving $1200 in gas and another $1200 worth of gas-fueled wars, resource depletion, and climate change.
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Old 04-17-2010, 09:46 AM   #13 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
3 to 6% drag reduction does not equal or lead to 3-6 more mpgs or even 3-6% more mpg.
From preliminary coast down numbers earlier this year, a 3 to 6% drag decrease would equate to a 2 to 4% decrease in fuel consumption.

At 55 mph, 66% of the overall drag was associated with air drag...

Some math...

(3 * 0.66)% - (6 * 0.66)% mpg decrease

Every little bit helps, Jim.
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Old 04-17-2010, 12:02 PM   #14 (permalink)
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Hi All,

Its commonly said that Aero Drag reduction in % will improve highway mileage by half that %. For example, improve aerodyamic drag by 20 % will give a 10 % reduction in highway fuel consumption.

On the blown mirrors idea, the issue is air friction. A scoop system has air friction too. Probably more then just letting the air flow over the top of the car. To get it less, the tubing that routes the air to the mirrors, would need to very large. For example, for dust collector systems, the difference between 4 and 6 inch diameter is quite significant to how easily the low pressure air will flow down the tubing.

The way one might get an adavantage is route air that is already coming through, to the mirrors. And the exhaust it uniformly around the whole circumfrence of the rear of the mirror. Exhausting only partially around the circumference will result in lift or sideways thrust, which will generate more drag.

So, now, how does one get a 6 inch hole from the engine compartment to the backside of the mirror?
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Old 04-17-2010, 01:18 PM   #15 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
Are you suggesting we add extra mirrors, since the result would be "only" a 2-4% increase in fuel consumption?
Nope, not suggesting that.

Just sayin', I don't want complicated, expensive, relatively unreliable systems to replace simple, cheap, proven ones for a fractional mpg gain (ex. 33 mpg x 1.02 = 33.66. And as I noted somewhere here on EM re: sidewinds I'm not so sure these mirror drag effects could be proven at all anyway).

http://ecomodder.com/forum/showthrea...s-11580-2.html



I suspect I drive through x-wind yaw more severe than depicted most of the time. How much "frontal" area and drag are these mirrors adding? I bet the downwind mirror adds 0 to drag; while the upwind one may add zero to frontal area but may add some form drag...
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Old 04-17-2010, 01:54 PM   #16 (permalink)
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gain

Quote:
Originally Posted by Frank Lee View Post
All that complexity, cost, and expense for a potential .00000001 mpg gain? I think not.
Frank,I hear what you're saying.
I'm seeing more and more side mirrors with built-in turn indicators,or electric mirrors.My CRX was pre-wired for accessories I never had in the car.'guess my thought is,that auto makers are okay with the wiring issue.
The electronic speedometers are a cost saver over gear-driven,cable-operated units.They save weight and can't break as in my T-100.
I would suppose that the camera system could save weight,couldn't be broken off the car,could easily be demisted/de-fogged/wiped,and I suspect at a cost 'savings' over conventional mirrors.
Electronics costs, do not necessarily represent their actual 'cost.' And 'economies of scale' with mass-production should push cost through the floor.
As far as drag reduction,I'm probably thinking more of 350-class pickups with mirrors of the frontal area of Sam Whittingham's Varna racing cycle.
And with the speeds people drive in Texas,the cameras might compensate for the added resistance.
Just some churnings.
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Old 04-17-2010, 02:01 PM   #17 (permalink)
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blown mirror

I looked at Hermann Schlichting's Boundary Later Theory.He addressed this issue,and from the tone of his writing,this is not a project you would attempt without a wind tunnel.
If the air jet is not perfectly sized,sited,and flow calibrated,it will increase drag due to induced vorticity.
If I had to rate the project on a "Buy or Bust" basis,I'd have to rate it a "bust."
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Old 04-17-2010, 10:36 PM   #18 (permalink)
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how 'bout mirrors that flip out when you need them. i already have 3 mirrors inside the rabbit.
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Old 04-18-2010, 09:42 AM   #19 (permalink)
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If the housing around the mirror fits tight enough to reduce frontal area, then there would be no room for air jets and such.

The trailing edge of the housing should be closing in shape to reduce the wake behind the mirror.

I do favor the idea of a flip-out mirror however. I think Metro was the first to mention this, in the postings that I have seen.

Jim.
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Old 04-19-2010, 09:07 AM   #20 (permalink)
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Frank - thanks for that picture, and the thought it provokes.

But then again, we do have several empirical tests showing an improvement. I'd guess in a crosswind the effect would be less, but still nonzero.

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Last edited by PaleMelanesian; 04-19-2010 at 09:22 AM..
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