Prius Coefficient of Drag

As far as I know its tested in a wind tunnel.

 

You're confusing frontal area with Cd. Measure the frontal area by measuring the vehicle height and width and multiplying. Multiply that figure by the Cd. This is the rectangular plate (bluff shape) that will produce the same drag as the car. Don't remove the space under the car. The air there is considered moving with the car, so that space is considered part of the car.

You must take the maximum height and maximum width. Don't worry that there are curves, the Cd figure takes that into consideration.

I don't know of any computer program that can accurately calculate Cd, I believe it still has to be measured in a wind tunnel.

Cd is, in effect, the aerodynamic design reducing the effect of the frontal area. In the Prius, it is even helped by the "rear diffuser" on the rear underside (black plastic part curled down on each side).

 

Modern CFD codes are incredibly powerful but you still need to go full-scale in a wind tunnel to get real numbers. CFD is very good at giving the engineers an idea of what is better than something else but not necessarily profficient at telling you exactly what the numbers will be. Close, but not as good as a well-controlled experimental data.

The drag coefficient CD is backed out using the measured force, fluid velocity, fluid density, and the reference area. Here's a page worth looking at:
http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

 

I remember seeing somewhere that it was 0.26 and was 2nd lowest, with something like the Honda Insight (no longer in production) being the lowest. It's out there on the web somewhere.

Reminds me that I'm curious as to the drag coefficient of the Buggatti Veyron. Not that I'm thinking of getting one.

PS Excellent wikipeadia article, has a table with drag coefficients of vehicles:

http://en.wikipedia.org/wiki/Drag_coefficient

 

PS
Good article with a table of vehicle drag coefficients:

http://en.wikipedia.org/wiki/Drag_coefficient

 

<div class='quotetop'>QUOTE(sharkmeister @ Aug 1 2007, 09:16 AM) [snapback]488571[/snapback]</div>

He said Cl (coefficient of lift), not Cd.

Tom

 

I was reading in Wired yesterday that there is a car that is designed from a boxfish, no joke, and has a Cd of like .19. It looks like a freaking Element so I'm not sure how that works. I believe it's called the Wow. I could be wrong since they were talking about 6-7 cars that have yet to come out.

 

That boxfish thing was built by MB. Here's their explanation:

http://www.daimlerchrysler.com/dccom/0,,0-...0-0-0-0,00.html

And an article with some interesting images from the design study:

http://foodfordesign.blogspot.com/2007/06/...oncept-car.html

 

:car:Yes, the Boxfish was in a recent National Geographic - a short part of an article on man's mimicry of nature to develop new technologies.. But a few notes on Buckminster Fuller's Dymaxion (drag coefficient of .26)- in essence modeled after the fuselage of a DC3 plane - is very close to the ideal "teardrop" shape for aerodynamics, and has no real undercarriage, plus only a single centered "tail-wheel", so the shape can come to a more gradual, smooth taper at the rear end of the vehicle. Powered by a standard 1933 Ford drivetrain, it got 30 MPG, and had luxurious seating inside, comparable to a modern van. While never a production car, Fuller tried to interest the auto makers in producing it, but he was turned away.
A certain Mazda Miata (year and style escape me) has a drag coefficient of .20, and a frontal area of 16.5 sq.ft., but didn't get exceptional mileage for some other reason - probably because of the massive engine and drivetrain the auto maker put into it!
I think the Prius, and most other cars have a lot of room for improvement. The Miata has the shape of a flattened jellybean - very smooth.. I think that's the secret - shape the car like a fish, a diving bird, or a raindrop falling in air - flattened out to desired shape for the purpose.
Looking at the GM Ultralight, I think the car could be produced with something like the Audi's aluminum monoque chassis - instead of using expensive, exotic, and probably toxic composites, Audi's methods lighten the vehicle while adding strength and stiffness to the frame - it's not all that much added force to move along 2200 lbs. vs. 1400 lbs. at highway speeds, and would make such vehicles possible to afford!
The Prius does appear to have one weakness as far as drag goes - and this is true for almost all vehicles today - the aft end is chopped off fairly sharply. Maybe I'm just seeing it wrong, but that's the way it looks to me - so if this could be brought out more, and gently rounded so the air can flow along with less of a "stall wing" effect in the rear, it's worth the effort. I did the calc's and came up with the following estimates: to run a vehicle @ 60 MPH constant speed, what is the HP required? Corvette = 26 HP; Prius = 25 HP; Miata = 16 HP; Ultralight = 14 HP; Typical SUV = 53 HP. The Prius is one of the best without a doubt! But a body redesign would help in the mileage department, and moving the design more towards electric and away from engine power is definitely the way to go, especially now with the new lithium batteries (safe, fast recharge, and powerful!) coupled with some ultracapacitors. Double the aerodynamics, double the engine and battery capacity & efficiency, and 4X the mileage should (in the ideal) be possible!:fish2:

 

Yes, the Boxfish is fantastic! But the article doesn't clarify what the drag coefficient for the auto is - just that the Boxfish's shape is .095. Still, I'm sure it's impressive. But the Boxfish is pretty smoothly tapered in both the front and the rear - more than most cars. A cab-forward design could make the concept vehicle more attractive from the driver's perspective, but then there might be less collision protection. Either way, it's a heck of a lot better than the typical boxy vehicles on the road - makes one wonder if there's an oil supply problem or an applied technology nearsightedness problem..
PS - where do you put the eyes?:fish2:

 

25 HP for Prius @ 60 MPH is high. How did you get that estimate? I think it is between 15 - 20 HP.

 

If you were to round off the rear end of the Prius, as ChuckKtoo suggests, the Prius would have more drag, not less, unless you extended the length of the car by about fifteen feet. Streamlining as suggested by ChuckKtoo only works if the taper is gradual and extended over a long length. When the taper gets too abrupt the airflow begins to separate and develops large amounts of drag. If you are going to have separation, it's better to roll the turbulent air into a cone following the body where it acts as a virtual taper and reduces overall drag.

That's the whole point of the chopped off rear end. The design is called a Kammback after the German aerodynamicist Kamm. The truncated rear end and little "spoiler" on the lip force the airflow into a rolling ball behind the vehicle. This ball of air follows along and acts as an extension of the body, allowing the moving air to flow off the back of the car with less drag.

It's not a coincidence that so many cars use a chopped off rear end.

Tom

 

Hi Chuck...,

Composites need not be toxic or expensive. A spin-off of the Rocky Mountain Institute - Fiberforge, uses PEEK high temperature engineering thermoplastic (remeltable) and carbon fiber to make strong light parts that are recyclable and non-toxic and cheap (considering the lifecycle - no rust). The manufacturing process is actually cheaper than stamped steel up to about 10K parts per year. It is similar to vacum forming of thermoplastic sheet.

I also personally like the idea of putting more petroleum into solid objects, rather than CO2. And the lightweight structure will reduce operating petroleum consumption.

 

There is no such thing as a "static" coefficient of drag. By definition, the coefficient of drag, normally written as Cd, is the resistive force to forward motion created by parting the air, divided by the frontal area of the object, in this case the car. So it is a dynamic measure. To refer to it as static is meaningless.

Also it is normally determined experimentally. While it could be done using a scale model, it is preferably done at full-scale in a wind tunnel, where Reynolds Number effects are eliminated.

 

You may want to double check your calculations:

My 2003, NHW11 model has a higher coefficient of drag and a similar cross-sectional area. I'm fairly confident the power to match the drag at 60 mph requires closer to 17 hp. The predicted and measured MPG on a standard day are pretty close too.

Bob Wilson

 

Bob, you have a chart for everything. Have you charted your use of charts?

Tom

 

Aww now you've done it Tom. Asking Bob if he's charted his use of charts is like a circular reference in Xcel... It cannot be done. Bob's probably already errored out.