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Design-headwind
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 3X 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 mph '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 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 ------------------------------------------------------------------------- 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. |
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. |
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 |
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. |
...but, it's certainly not a "get poor" scheme for the petroleum companies!
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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 ,, http://www.sciencemuseum.org.uk/imag...5/10314748.jpg |
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. |
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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] :( |
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regen
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Until we 'cure' gridlock and un-synchronized traffic signals,it's a fabulous way to get a little back.:thumbup: |
mph was what I meant,sorry!
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It should have read mph,instead of mpg.Sorry! With respect to the 'unsightly' issue,it could all come down to that. The 1929 'pregnant' Buick and 1934 Chrysler/DeSoto/Imperial Airflows are mentioned as examples of styling before their time. If they don't sell,they'll disappear from the market. Education could be a factor although I'm not holding my breath on that one.You'll notice Energy Secretary,Steven Chu's deafening silence on the issue. The science is there.The votes are out on whether or not consumers are ready. Hollywood and Madison Ave. could probably invert market tastes within 90-days with an efficiency corollary to Smoky and the Bandit,The Fall Guy,and HUMMER-toting 'Governator' Arnold Schwartzenegger . Re-defining an Americans right of passage into adulthood and quelling fear of self-emasculation will play big roles if things are to ever change. |
linear?
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If you're at Cd 0.40 and you streamline down to Cd 0.25,then, 0.4/0.25 = 0.625,or,only 62.5% of your original drag remains,a 37.5 % drag reduction. The horsepower necessary to overcome aero drag to maintain the same velocity would be reduced by 37.5 % also,freeing up some power for hill climbing,passing,etc.. If your engines BSFC map didn't move around do to the change ( a better bet today with EFI engines ) ,given rolling resistance as a constant,you'd pick up around 18.75 % better mpg at this speed.More at higher velocity. |
personal tailwind
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Unless that deer finally strikes back,your literally on Easy Street.:thumbup: |
accuracy
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I ran calcs for a Miata-sized car of 17.7 square-ft Af,90.93 ft-sec,at Cd 0.30 and 0.10. Road load @ 0.30 was 8.637 hp Road load @ 0.10 was 2.879 hp Delta-HP = 5.757 hp 8.637/2.879= 3 ( this is where I'm getting the 300% ( 2.879 X 3 = 8.637 ) -------------------------------------------------------------------------- On the Cd part, 0.30/0.10 = 3 ( 300 % ) -------------------------------------------------------------------------- Since Cd was the only variable it was looking like the exercise could be reduced to just comparing Cds,and on a percentage basis.Is this valid? |
Percent - evil?
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Cd1 = 0.3 Cd2 = 0.1 Cd1 is 300% *of* Cd2. [multiplicative] Cd1 is 200% *larger* than Cd2. [additive] Same for horsepower, and it doesn't change your bottom line as I said before. (Call me a nit picker but an error of 100% seems hard to dismiss. I thought it was kind of humorous. I hoped you did too. If not, sorry.) I wonder what my Cd is riding motorcycle... |
hard to dismiss
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I think I've been stuck in 'multiplicative' mode.Once a value is in the calculator I just carry it through.Lazy huh? Is '3X' a more accurate representation of the Cd comparison? Before we get too far into this it would be nice to have clarity. -------------------------------------------------------------------------- With respect to your bike,Hoerner posted Cd 0.90 for un-faired MCs.Ouch!!!!!!!!!!!!!!!!!! |
Sorry for interjecting but yes, 3X or 200 percent increase... Everyone at school looked at me like I was crazy trying to make this point... We took the smallest number/the greatest number X 100(%) and called it a percent increase. I tried to say the greatest number and smallest number was the same in one trial (hey it could have been) so we had a 100% increase with the same number! Just got glazed over stares.
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Thanks for the bike Cd data point. While 0.9 is high, frontal area is so low I still enjoy 90+ mpg without any aero mods whatsoever. And it is a bike after all. Knees in the breeze is a big part of it. Rock on! |
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