Design-headwind

[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 3X 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 mph '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 3X loss on investment for every highway mile traveled over the 12-year period,with :
* 3X more fuel unnecessarily consumed( based on constant BSFC)
* 3X more fuel cost
* 3X 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.

[NeilBlanchard]

Aerodynamic drag and rolling drag are total losses, and this makes them the most important things to reduce.

Weight, on the other hand, provides higher kinetic energy and so by coasting (or using regenerative braking) some energy can be regained. So, lowering weight helps in some situations, but some of the kinetic energy can be regained and so it is not a total loss.

[Sularus]

a little off topic, but:
I no longer understand what constitutes a 'Sputnik-Moment' in the world of capitalist economics.
- aerohead

Too bad it isn't capitalist economics anymore. It has turned into Socialist economics.


Anyway, weight is reducible, but at usually high cost. Aerodynamic drag, thanks to people like the wonderful people here, is rectified for not too much cost other than time and some elbow grease.

Just remember Neil, higher weight may lead to longer coasts, but the amount of energy required to move it is the problem. It would be more beneficial to lower the weight to reduce the required energy needed to accelerate and maintain speed at the same time of lowering aerodynamic drag. I know it is fairly unreasonable in already production vehicles being modified by shadetree mechanics in their garages, but it is a goal nonetheless.

J

[Kodak]

Nice job punching all those numbers. What formula did you use to calculate the 27.4mpg headwind comparison?

The numbers make me wonder how automakers haven't employed some of these technologies. I've seen the wheel covers, but that's really it. I suppose the simple answer is that some aero changes might seem unsightly - or at least pose a major marketing risk.

[gone-ot]

...but, it's certainly not a "get poor" scheme for the petroleum companies!

[MGB=MPG]

its better than it was.

is that a linear calculation/formula ?

whats the figures for a .4 and the next goal a .25

my car looks slick and is only rated at .4 ; .3 +/- seems to be current industry standard ..


approach to theoretical perfection is asymptotic . effort to achieve each incremental gain increases at an increasing rate ,,

[basjoos]

In addition to reducing fuel costs and tailpipe emissions, reducing drag also reduces the load on the engine and transmission, potentially extending its service life, an additional cost savings.

I've always said that driving my low Cd car is like driving in my own personal tailwind or that I am drafting myself.

[KamperBob]

Originally Posted by aerohead

Yet another thought-experiment from my tortured mind!
----------------------------------------------------------------------------
I considered the difference Cd makes with respect to power,mpg,etc.,based on an 'ideal' low-drag body and current production cars.
----------------------------------------------------------------------------
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
--------------------------------------------------------------------------
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.
--------------------------------------------------------------------------
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.
--------------------------------------------------------------------------
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
-------------------------------------------------------------------------
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.
--------------------------------------------------------------------------
I no longer understand what constitutes a 'Sputnik-Moment' in the world of capitalist economics.

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.

[redyaris]

Originally Posted by basjoos

In addition to reducing fuel costs and tailpipe emissions, reducing drag also reduces the load on the engine and transmission, potentially extending its service life, an additional cost savings.

I've always said that driving my low Cd car is like driving in my own personal tailwind or that I am drafting myself.

The non fuel cost benifit of low drag is so often over looked. Althogh I can not quantify it precisly I do believe that I benifit more from the low maintainance/operational costs than I do from the fuel cost saved.
one thing I have noticed recently is that when driving at 92km/hr the "sand" [1/4" minus gravel] used on the roads for traction on snow and ice, that gets flung onto my cars windshield does not crack the glass. How I found this out is when doing a long FE test run at Posted speed limits of 110km/hr on sanded roads I got 2 'stars' marks on the windshield from road "sand" throne up by passing trucks. I have had lots of hits from the sand/gravel at 92km/hr [57mph] but no cracks until I was going 110km/hr [70mph]

[redyaris]

Originally Posted by Old Tele man

...but, it's certainly not a "get poor" scheme for the petroleum companies!

Caveat emptor