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September 2008 Posts
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Maybe our EV's should be flying
Electric Airplane
quote comment
Sorry, I missed it on the podcast.
SMART
The SMART Cd is 0.29 which I find shockingly low. The link is above.
http://www.daughtersoftiresias.org/apterawiki/Main_Page
Fd is the force on the vehicle due to air resistance (drag) in Newtons Frr is the force on the vehicle due to rolling resistance in Newtons F is the total force on the vehicle in Newtons V is the vehicle's velocity in m/s a is the vehicle's acceleration in m/s^2 A is vehicle frontal area in m^2 M is vehicle mass including occupants in kg rho is the density of air which is 1.22 kg/m^3 at sea level g is the gravitational acceleration constant which is 9.81 m/s^2 Cd is the vehicle's drag coefficient we want to determine Crr is the vehicle's coefficient of rolling resistance we want to determine
Now for some formulas:
Fd = -Cd*A*0.5*rho*V^2 (formula for force due to air resistance or drag) Frr = -Crr*M*g (formula for force due to rolling resistance) F = Fd + Frr (total force is the sum of Fd and Frr) F = M*a (Newton's second law)The Cd is not that valuable of a number, since it is so dependent on frontal area. wikipedia listsCdA for several cars an civic being 7.04 while a hummer H2 being 26.5.IMHO, the best number would combine all the constants til you getkV^2, where k is the combination of all the constants at sea level. It is interesting to note every 1000 meters, or 3000 feet you will lose 10% in drag. Thus Denver gets about 17% better drag and fuel economy due to their elevation.
Bill,
I think you meant F = Fd + Frr + m a You used the same "F" twice in your post implying Fd + Frr = m a which is not the case. We could also add in gravity for an angled road something like mg sin(theta). But I think the important forces are the rolling resistance and the air resistance.
You are of course correct about the value of CdA being a better measure of performance. It is both Cd and Frontal Area the matter to fuel efficiency.
However, Cd is popular because it shows the smoothness of the car. It also does not change if the car is big or small. It is dimensionless which is nice as well, don't have to worry about feet or meters like you do with CdA.
LaterJohn C. Briggs
Fd+Frr+Fengine+Fgravity=m*A, gravity is very important as there are certain locations in CA I can coast for 10-20 miles. One being Whitney Portal to Lone Pine, you could turn your engine off. I copied the numbers from "Calculate your cars drag coefficient".
I am not sure you can scale a car and it would have the same Cd.
energy = Av^2+B would be a much simplier way than using the above formulas.
Bill, I was wrong, sort of.
Equations
The website assumes that you are coasting to a stop so F(engine) = 0 and
m a = Fd + Frr
So the equation is correct for that one special case of the engine being turned off.
I couldn't find any information about the scalability of Cd with car size. I assume it scales but I don't know.
The Wikipedia entry Drag shows an equation with no dependance on size. On the other hand, it does indicate that the Reynolds number may be a factor. Since Reynold's number depends on size, perhaps Cd is effected by size.
Interesting info John and Bill- of course, thanks to you two, I now have a few million less brain cells trying to figure out the math:-( But a good primer of design efficiency!
For Rick at the Oil-Capital-of-the-World- Myers motors url: http://www.myersmotors.com/ But I think you meant Miles Electric Vehicles, whose url is: http://www.milesev.com/# Oddly, I don't mind that "80's" look that bothers Bo- there is something to be said for a mainstream-looking car, especially when you are already trying to sell a new technology to hard-to-change consumers- after all, not everyone is as fore-sighted as we EVCast listeners;-) But what I am concerned with, is the quality of Chinese vehicles more than anything else with the Miles EV's- and besides- I have bought mostly foreign cars for years- it would be nice to buy a well-made, cutting-edge American EV instead;-)
Why the unexpected result of hurricane Gustav- prices are falling due to a lack of damage and the speculation that had pushed prices up, gas inventory rising, and more people driving less- this year was the first drop in mileage driven in the US in a long time.
Hey Rick! Thanks for the welcome mate!
(I'm a few days behind the ev casts - I'm playing catch-up). :)
