The San Diego Alternative Energy Meetup Group Message Board Cars & Transportation › Auto's wind drag coefficient?

Auto's wind drag coefficient?

A former member
Post #: 294
Remember back in the 70s when the auto makers would proudly display their auto's wind drag coefficient? And companies like BMW would proudly take out center two page advertisements that they would have no part of destroying their auto's looks for mere efficiency? When are the auto manufacturers going to start displaying this number again? We need it!
A former member
Post #: 5
We need it!

But what are WE going to do with it? In fact, if we really must have it, there are lots of data sources of a simple Cd (see, for example, Wikipedia). While we're at it, why don't we demand that the auto manufacturers also display frontal area, rolling drag, tire friction, engine efficiency, etc., etc.

One of my points is, automobile efficiency is a function of a multitude of coefficients that depend on a multitude of variables (including, importantly, how we drive; windows open, jackrabbiting, speed, etc.).

Hey, I'm all in favor of decreasing form drag (take off the roof rack when not in use, don't buy the spoiler,roll up the windows, etc.)! But the main way that most of us, not buying a new car every year, can increase the efficiency of our car(s) is consolidate trips and minimize trip mileage and, for heaven's sake, slow down (velocity squared, remember)!
A former member
Post #: 298
We need it!

But what are WE going to do with it? In fact, if we really must have it, there are lots of data sources of a simple Cd (see, for example, Wikipedia). While we're at it, why don't we demand that the auto manufacturers also display frontal area, rolling drag, tire friction, engine efficiency, etc., etc.

One of my points is, automobile efficiency is a function of a multitude of coefficients that depend on a multitude of variables (including, importantly, how we drive; windows open, jackrabbiting, speed, etc.).

Hey, I'm all in favor of decreasing form drag (take off the roof rack when not in use, don't buy the spoiler,roll up the windows, etc.)! But the main way that most of us, not buying a new car every year, can increase the efficiency of our car(s) is consolidate trips and minimize trip mileage and, for heaven's sake, slow down (velocity squared, remember)!

You've raised a few intersting issues.
0) cd
1) frontal area
2) rolling drag
3) tire friction
4) engine efficiency
5) how we drive
6) windows open
7) jackrabbiting (which i had never heard before but is self explanatory.)
8) speed

Thank you.

Now help me out here. I see the first 3, cd, frontal area, rolling drag, and tire friction, as possibly being combined for a total along with being listed separately. Imagine the tested car being pulled by a rope attached attached to a scale so the energy requirements to overcome drag at various speeds can be graphed. At all speeds the vehicle is capable of driving at. Do you think this graph would become something we could all appreciate?

Engine efficiency is difficult to judge on hybrids. Both power sources need to be listed separately using standards accepted across the board. Standards that mean something rather than hokey pokey numbers their lobbyists pushed through the politicians.

Windows open, how we drive, jackrabbiting, speed are all sort of the same. And a few displays taking into account speed, time, elevation grade, wind, acceleration, and energy being used. With a computer taking this into account the waste could be displayed. I know I would then like to tell it I don't give a expletive about my driving speed. I'd like no speed limits in fact. So being able to tell the computer to ignore that speed parameter and instead concentrate on helping me save energy while driving 80+.

So to summarize. We need graphs showing overall drag. Standardized details for hybrid power plants. And new style GPSs that take into account wind, elevation, acceleration, and energy useage to give the user useful feedback on saving money.
A former member
Post #: 6
Well, since this discussion belongs to Dave and is titled Auto's Wind Drag Coefficient?. and he has told us that he would like to save money while driving at 80+, clearly Cd is very important to him! But we know that the wind drag is also proportional to frontal area (how do we measure it?), air density (do most of your driving at high altitude) and the square of the relative speed of vehicle and air (always drive downwind).

But, in addition to annual depreciation, insurance, taxes, garaging expenses (?), maintenance, repairs, etc., one has to buy all of those tankfuls (or batteryfuls) of gasoline, diesel, cng, electricity, etc. each year. Note that the former expenses tend to be proportional to the initial cost of the vehicle, but change (initially decrease) with the age of the vehicle. The cost of the fuel per year is the product of the average price of an equivalent gallon, miles driven per year, and divided by equivalent mpg. So, to reduce the cost per year, look for the low-cost vendor (but don't drive too far or run out of gas/diesel/cng/electricity finding it), consolidate/reduce trips, and (at last, we get to the issue) increase equivalent mpg! See, we didn't mention Cd once in this paragraph; only equivalent mpg.

So, as we knew all along, mpg is really what we want to know about and to maximize. How do we learn about mpg? We look at the EPA sticker (or look up mpg for the vehicle in question on the internet) and first try to understand what it means. We find that the city and highway mpg numbers are measured by complicated, EPA-prescribed test trip loops with factors applied (the prescription changed in 2008, to the detriment of the Prius, among others). But the EPA test drivers do not drive at 80+, and nobody else drives exactly like the EPA tests either. Still, let's give EPA credit for giving the customer their best shot at a simple two-number measure of a complicated quantity, mpg.

Now, if we want numbers for mpg under more specific driving conditions (c. f., 80+) we have to do some research or measurement. First, we need to note that, as previously mentioned, mpg depends on too many variables to usefully display. The most important variable is vehicle speed (assuming no wind, flat highway, windows rolled up, etc. etc.). We can find (in Wikipedia, again) some plots of old data on mpg vs. speed for a few vehicles. These are interesting; they show, of course, differences among the vehicles, but all are characterized by a maximum of mpg at speeds in the range of ~45-60. Poorer mpg at low speeds, and, as we expect, decreasing mpg at higher speeds as speed increases. It shouldn't be too much to expect every vehicle model to be provided with this figure of mpg vs. speed.

Some vehicles are equipped with instruments that display "instantaneous" mpg. One should be able to generate data on mpg vs speed using them (make sure you have a good test track). They clearly do some computing and should be able to display some weighted averages of value to the curious driver. Since I don't know how they work or if they are standardized among different manufacturers, I don't know how accurate they are or what their response times are. In any event, the interested driver should be able to learn efficient driving techniques by watching the dial (but don't get too distracted).

Bottom line. Cd is interesting, but not by itself. MPG, is informative, especially with a little more info on speed dependence. Don't know how GPS could help. Lots of ways to save money on transportation. Some by using good sense, some with lifestyle changes.
A former member
Post #: 299
Have you ever thought autos should be sold as commodities? Like computers for instance. Imagine the body, interior, suspension, and power supply all sold separately. Where you could concentrate on upgrading the exterior for something more aerodynamic each year...
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