iMiEV CdA?

• What is the iMiEV CdA? I can't find it. http://energyandourfuture.org/node/5912

Cd:

• iMiEV 0.35 (Thanks to JoeS)
  Leaf 0.29
  Tesla .34
  Gen1 Honda Insight 0.26 (Thanks to JoeS)

CdA:

• Leaf 7.02 sq ft
  Volt 6.51 sq ft,

with both lagging the Drag coefficient King 2011 Prius 5.84 sq ft.

Cd is a coefficient that describes the aero efficiency of the shape. Cd is a dimensionless quantity that describes the "goodness" of shapes. Lower is better.

CdA is Cd times the frontal Area. If it has twice the frontal area, it takes twice the force to push it through the air.

• Drag force: Fd = 1/2 rho x v^2 x Cd x A

where:
Fd is the force of drag,
rho is the density of the fluid (air= 1.293 kg/m3 at 0 °C and 1 atmosphere)
v is the speed of the object relative to the fluid,( v^2 means v squared)
Cd is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car)
A is the reference area,

The specs in the iMiEV embossed booklet show Cd=0.35. I plan on taking a head-on photograph with a yardstick in order to attempt a cross-section area calculation. Having just completed my first long highway trip, the iMiEV's drag (and thus its highway energy consumption) is significant compared to the effortless coasting by my Gen1 Honda Insight with its small cross-section and Cd of 0.26. When will manufacturers wise up and produce super-aerodynamic vehicles, as that's the single best way of extending EV range instead of piling on expensive batteries? :evil: Aptera, RIP.

Couldn't we get the CdA by doing careful coast-down tests?

The force on the car from mechanical friction (rolling resistance) can be assumed constant for the moderate speeds the i-MiEV can obtain. Call this force Ff, then the total drag on the car, Ft, is:

Ft = Fd + Ff = 1/2 rho x v^2 x Cd x A + Ff

And, since for given atmospheric conditions rho is a constant that we can calculate, we can simplify the above equation to:

Ft = C x v^2 + Ff

Where C is the constant:

C = 1/2 rho x CdA.

Using velocity vs. time data from coast-down tests we can calculate the cars acceleration "a" at a number of velocities:

a = dv / dt (change in velocity divided by change in time)

And of course we can then get Ft by:

Ft = M x a

Now, even without knowing Ff, can't we find C and Ff in the above equation (Ft = C X v^2 + Ff), by using curve fitting?

JoeS said:

When will manufacturers wise up and produce super-aerodynamic vehicles, as that's the single best way of extending EV range instead of piling on expensive batteries? :evil: Aptera, RIP.

Click to expand...

I think they have wised up. Have you noticed how highly raked the windshield is on the i-MiEV, allowing for a smooth transition with both the hood and the roof?

But, as with almost all engineering solutions, there are compromises that can't be avoided.

Any highly aerodynamic shape will need to taper down toward the rear (like your Honda Insight). For a given length, this seriously reduces the room in the rear of the car. I much prefer having the room in the rear.

This is not to say there isn't a place for a more aerodynamic car with less room (again like your Insight). But I don't think you should blame the manufacturer. If there is someone to blame, I think it falls squarely on the rest of us consumers that like more room inside the car. In fact, I think your Insight demonstrates this. Honda put out a small aerodynamic car. If it had sold really well, I think that not only would Honda have continued in that direction, but that other manufacturers would have jumped on the band-wagon as well. I think Honda made later Insights larger because of market pressure, not because they weren't "wise" enough.

Quixotix, you are quite right about consumer demand driving the manufacturers, and I'm very pleased with our iMiEV spacious aft packaging. My own personal choices have always strayed from the mainstream. The Gen1 Honda Insight was an experiment and to this day I believe it is the most efficient gasoline-powered production car ever sold, anywhere (please correct me if I'm wrong). Nice companion to our present most-efficient electric iMiEV which, if one believes the EPA equivalency rating, is also the most efficient production car ever sold (to date). It was a very conscious decision to purchase the iMiEV for local utility driving and keep the Insights for our longer trips.

The sad thing about the Gen1 Insight is that it was a highly-specialized car which, if I understand correctly, cost Honda more to build on their small NSX production line than what they sold it for. It was simply never marketed, but gave Honda the bragging rights for most efficient vehicle for seven years. Visiting Honda dealers enquiring about the Gen1 Insight in the middle of the last decade was an exercise in futility: the sales people knew almost nothing about it, would tell you that it was only available on special order, and would desperately try to talk you out of it and immediately steer you to their Civic hybrid.

Back to aerodynamics and rolling friction: a coast-down test, starting from a significant speed such as 65mph, comparing the Insight and iMiEV would be interesting. All it takes is time, no wind, and a deserted highway...

Quixotix said:

JoeS said:

... Any highly aerodynamic shape will need to taper down toward the rear
(like your Honda Insight). For a given length, this seriously reduces the
room in the rear of the car. I much prefer having the room in the rear...

Click to expand...

Click to expand...

Anytime I read a discussion of aerodynamics, especially when connected
to EVs, I have to wonder what the numbers would be for the still-a-concept-
after-all-these-years iMIEV Sport -- the blue one:





Cool little video that suggests the i would have better CdA when driven backwards!

rokeby said:

iMIEV Sport ... Cool little video that suggests the i would have better CdA when driven backwards!

Click to expand...

iMiEV CdA TESTED: Forward vs. Backward
http://www.youtube.com/watch?v=eo7Ip12eA5k
The above video shows that the Takara Tomy Tomica 117 Mitsubishi I-MiEV forward drag is the same as its backward drag.

The iMIEV Sport shape looks like it has very low drag.

Thanks to rokeby for posting Craig Vetter's really Cool little video.

FiddlerJohn said:

Thanks to rokeby for posting Craig Vetter's really Cool little video.

Click to expand...

You're welcome...
And thank you for the iMiEV video.

As much as I like both videos, I think that they are both fatally flawed as
far as being a valid demonstrations. I can think of three of three objections
without serious effort:

1. The diameter of the tube is far too small. There is a near certainty that flow
and pressure waves from the model are reacting with the walls of the tube.
I suspect to minimize such effects the tube would need to be ~6" in diameter.

2. The model car is moving backwards and it's wheels are rotating backwards.
In full size wind tunnels tests are conducted with one of three conditions:
a. car stationary, wheels not rotating.
b. car stationary, wheels rotating on rollers
c. car held stationary over a moving ground plane (think conveyor belt), wheels
rotating.
I've never read of a full size test with the wheels going backwards. The
backwards spinning wheels are going to be throwing/pulling air up in front
of the wheel... quite simply, not right.

3. You can scale down the car and the wind tunnel, but you can't scale down
the air molecules. What might be a relatively large radius curve on a full size car
is a tight radius on the model. Granted, F1 designers do use models, but IIRC they
are 1:4 scale, not 1:64. You can't scale air viscosity or momentum for the models.

So, they're fun little videos, but they leave a lot to be desired as far as rigor goes.

I wouldn't bet the farm on the i having similar Cd (drag) going both forward and
backwards. And other than as a theoretical discussion, it isn't meaningful...
unless you can find a way to reverse the i's body on its chassis!

Besides a flat tray or sculpted sheet under the motor bay, the biggest practical aero improvement I'm pondering would be a partial Kammback. Picture a clear plastic 'eyebrow' over the rear window that starts at the roofline and sides of the rear hatch, and slopes down at 12-14 degrees. Where to cut it off is the main decision, the beltline is probably a practical limit, but an equilateral triangle would probably look better. A smaller one that protrudes no higher than the upper edge of a raised rear hatch would also avoid potential clearance problems. Many small cars now have a rear appendage like this, often called a rear visor or duckbill, though those usually just follow the flat roofline...

For those that routinely drive their i at speeds over 50 mph, here is an inexpensive
device that improves both stability and range.

http://airtab.com/gallery/autos.html

Like the example pictured above, I installed Airtabs (6) on the roof of my 2008 Smart and the change in stability at highway speeds was remarkable. This was especially true in crosswinds. The Airtabs also reduced drag and I consistently exceeded the EPA highway MPG by a wide margin. The rear of the i is similar to the Smart so I would anticipate the same type of improvement.

Airtabs come in white and black so you have a 50% chance of matching the color of your i. They also can be easily painted to match the silver and red cars. They are simple to install and can be removed without damage to paint or body. By just installing on the roof the Airtabs blend in better while still having the desired effect.

Hi Archie,

Can you post a picture of your Smart car with the Airtabs mounted? It sounds like something that might be good for those of us with a highway commute, as I notice that the i-MiEV really gets buffeted by the wind. Unfortunately, my anti-virus blocks that website, so I can't view the gallery.

Thanks,

Jenn

Archie

I wish I had known about these earlier. I am trading in my Smart for the i. After looking at the pictures I am a little confused about the location for attaching the air tabs. Some show on the roof and some are on both sides at the rear. Could you explain where they would go on the i.
Thanks

Dan

Oh Boy, AirTabs border on an article of faith. They've been discussed at length on EcoModders, and been classified as a "Unicorn" http://ecomodder.com/forum/showthread.php/do-air-tabs-work-2209-3.html
However, if they don't improve range but do increase stability, I'm interested in giving 'em a whirl.

I mounted six Airtabs just on the roof of my Smart. I could have mounted four more, for a total of ten, but was satisfied with the results just using six. Also by only mounting in the center of the roof I minimized the visibility of the Airtabs. Being a black Smart with black Airtabs they didn't jump out when looking at the car like some of the other examples on the Airtab web site.

Did they provide more stability for the Smart at highway speeds? Absolutely. No more grab the wheel with both hands when a semi passes or is just in front of the car. No more darting from side to side in a crosswind. Did they make the Smart handle like a Corvette? No, but it did make it handle like a regular compact sedan. Big improvement. Airtabs have the biggest effect on flat ended vehicles, like semi trailers, vans, Smarts and the i.

As for increasing range, don't expect any big increases, but it will help. Expect somewhere in the 1-3% range. Don't expect any increase in range at speeds below 45-50 mph. The faster you drive, the more the effect. I've owned a first gen Insight for 12 years and that car is loaded with improvements that give .5% here, 1% there, 2% over there, etc. The end result was the first car rated at 70mpg highway. I live where it routinely gets below zero and snows a lot during the winter months which drastictly reduces gas mileage. I still have averaged 62.5mpg over 12 years of use.

Airtabs are inexpensive and are attached using adhesive. This means you can remove them easily if not satisfied with the results. Hopefully someone with an i that drives at highway speeds will give them a try and report back with the results.

Archie,

Can you say whether the airtabs helped much with accumulation of dirt on the back of your smart? I live on a dirt road. The back of my subaru outback is always completely covered with a thick layer of fine dust from our road. I'm not looking forward to that on my sparkling white i. The dust storm is also rather rude to the neighbors hiking and horseback riding. I'd get the airtabs just for that.

Barb

Barb:

I don't think the Airtabs will reduce the dirt you kick up driving on the dirt road, both because of low speed and because the tires are what is mostly kicking up the dust. If once you get on a paved road you get up to speeds above 50 mph it could lessen the dirt that accumulated on the back, but it will not eliminate it. I still got dirt and road spray on the back of my Smart, just that the quantity was significantly less.

There is a product you may want to try which might help. The product is RejeX and here is the web site: http://www.rejex.com
I have used this product for about 6 years and it does what it says it will do. It can be used on glass also and this along with the Airtabs could keep the back end of your Subaru and the i cleaner. Hope this was helpful.

If I get a new job that requires freeway commuting, I will definitely try them. It looks like I will need to mount them on the roof-proper, not on the top of the hatch, because they might not clear when opening the hatch. Archie, does that sound right to you? Also, do you think there would be any advantage to also mounting some on the sides at the back of the car?

Thanks,

Jenn

With the background that Mitsubishi had with vortex generators on the Evo Lancer, one must wonder why Mitsu didn't incorporate them into the iMiEV.

http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf

Anyway, this topic is pretty nicely picked-apart in this series of four articles ...

http://www.autospeed.com/A_3058/cms/article.html

http://www.autospeed.com/A_3059/cms/article.html

http://www.autospeed.com/A_3059/cms/article.html

http://autospeed.com/cms/title_Blowing-the-Vortex-Part-4/A_3061/article.html

When testing (unscientifically) on a Gen1 Honda Insight they saw a 5%-7% increase in fuel consumption. But, is there any relationship between this and the squared-off iMiEV aft end?

BTW, the new Toyota Prius C has tiny vortex generators molded into it taillight lens assembly. Interesting...

http://ecomodder.com/forum/showthre...nerators-trailing-edge-side-toyota-20835.html

In any case, it still might be fun to try either these or other brands (anything physically smaller?) to see if they have any effect - like maybe just keeping the back of the car less dirty! :roll:

jennrod12 said:

It looks like I will need to mount them on the roof-proper, not on the top of the hatch, because they might not clear when opening the hatch. Archie, does that sound right to you?

Jenn

Click to expand...

Jenn, I just purchased some of these off the web and will be mounting them in a few days hopefully. I am planning on mounting them on the roof proper before the gap between the roof and hatch, might actually help to make it more aerodynamic there as the gap is wide enough for some drag. Noticed in photos that in the UK the Miev's seem to have an extended hatch cover http://all-mitsubishi.info/wp-content/uploads/Mitsubishi/i-MiEV/146289085/Mitsubishi__i-MiEV_White.JPG
wonder why US version didn't get this?

Cool, let me know how it goes! How many did you get? Hopefully you can post a picture, too.

I think we didn't get the extended hatch cover because our car is wider. I'd really like that hatch cover.

Jenn