More About Coastdown Testing
Tuesday, August 31, 2010 at 11:34AMThe engineering department at Edison2 greatly appreciates the informed comments and gentle prodding we get on the Blog page. We have and will continue to do our best to put actual information up here. We hope everyone will realize that we also have our jobs to do so sometimes it might be a while to answer even really good questions.
We didn’t have stopwatch on it but it seemed to take only about 5 minutes before Kevin pointed out that the C term in our last blog post didn’t make a lot of sense. He’s absolutely right, it doesn’t, and the reason for that is also the reason that the car industry needs coastdown testing and does not just rely on wind tunnel numbers.
The SAE standard that defines coastdown testing is J2263. It’s available for download from multiple places online but because it’s copyright material you have to have to pay for it. It’s an interesting read if you’re into this kind of thing (and a great cure for insomnia if you’re not) and it delves into some of the complexities that surround this. For example, how do you account for rotating weight? A wheel not only has linear inertia because it’s travelling but it also has rotating inertia because it rotates. J2263 discusses this at length and in detail.
Whether or not stuff like rotating inertia is significant depends on the numbers you’re trying to find. If all you really want to know is how much drag there is at any given speed, the coastdown method is great. Plot speed against acceleration (and, since mass is constant, acceleration gives you force) from the coastdown and you have the drag profile. It happens that car drag profiles very reliably fit the A + Bx + Cx^2 three term general form.
Here’s a parallel: if you want to know downforce, do you take a bunch of pressure taps and attempt to integrate the pressure contours you generate over the car’s planform area? You could but it’s kind of messy. Far better to measure the downforce directly because that intrinsically integrates the air pressure over the whole body.
So what happens is, the coastdown people do a curve fit on the speed/drag plot and that gives the A, B and C numbers and all the complexities are handled right there. While the C number, being squared, sort of corresponds to aero, it doesn’t necessarily do so exactly. It’s just like saying we don’t care very much what our pressure distributions are because we know accurately how much downforce we have.
That said, we at Edison2 like to be sure of our ground as much as the next person so we asked the coastdown engineers to dive into the sea of numbers and calculate our Cd given our 1.702 m^2 (18.3 ft^2) frontal area. These guys are good at this and proved it when the number they delivered, 0.157, agrees within 2% the number we saw in the wind tunnel.
It’s an interesting factoid that the Very Light Car rolled about 8100ft (over 1½ miles) while coasting down from about 71 to 10 mph (the standard is actually from 115 to 15 km/h). If we were to assume linear speed decay, the force to decelerate would be about 20.4lb. This matches the ABC coastdown numbers at a little over 40mph, right in the middle of the speed range. Overall, we’re happy that we’re dealing with facts.
And if you really want to see how good the VLC is, take your own road car up to a bit over 70 on a level road, knock it into neutral and see how far it goes. Even though it weighs some multiple of the VLC, bet it’s less than 1.5 miles. Please use common sense and do this only where it’s safe and legal.
Edison2’s engineering department goes to a lot of trouble to model performance, which is why we were able to make some very good primary decisions about our cars’ layout and characteristics. But we’ve also learned a hard lesson over the years: there comes a point where you just have to go out and try it. When what you observe in reality doesn’t line up with the model, the reasons why are worth study.
Reader Comments (58)
Jim,
I'm still not able to visualize how a tall vehicle with narrow track (such as the miastrada concept) can handle well around corners. I'll try to read the patents more carefully...but I remain very skeptical.
The only way I can visualize this really working is to have the upside-down-U car cross-section (similar to Neil's photo, but with outrigger rails which trail from each front wheel to the corresponding rear wheel on that side)....and THEN come up with a tilting mechanism that makes it ride like a motorcycle.
This is esstentially what the K-Way Motis (another XPrize competitor) does.
see: www.k-waymotus.com/KWayDefault.aspx
and www.youtube.com/user/TTWVehicles
Although low-slung ballast would help....I don't think it is enough. It actually would not be that hard to hydraulically make the left and right struts actuate to accomplish the tilting. Today's electronic controls would have no issue creating "coordinated turns", although emergency maneuvers could be a challenge.
In effect, you'd be riding a very tall, very strange 4-wheel motorcycle. However, your low-drag aspirations may be able to be realized (at least partly...since the wheel outriggers and struts probably will negate most or all of the advantage.
Kevin
Kevin,
The K-way Motus looks awesome. It reminds me of a vehicle called the Carver which is another three wheeled articulated vehicle. It actually made it on to "Top Gear" some years ago.
So, will Edison2 get their $5M on Sept 16?
John C. Briggs
John C Briggs - I predict / hope yes. Edison2 has been kind-of quiet as of late...
Kevin
Jim Bullis,
We've already had a Manhattan project to redistribute water across the American west. Please check out the outstandingly well-researched and thorough book "Cadillac Desert" by Marc Reisner. You will learn why your idea has not-- and will not-- gain any traction.
Me personally, I have two big things against being high up off of the ground.
Number one, history.
Now if you look at all of the cars with the absolute best aerodynamic efficiency, the Nuna II being at the top, all of those vehicles are very low to the ground. The Nuna II even exceeding Edison 2's VLC gets a 0.07 Cd and that car is as low to the ground as it's possible to be.
Number two, stability and fun. Now think about this, a golf cart and a go kart can accomplish about the same straight-line speed, however, which one is more stable and much more fun to corner? The go-kart. There is one reason that sports cars are so low, not just for better CG, but also because it feels faster (the sports car's high priced selling point), and in part that is from being lower to the ground. The graphical detail of the roads gives the driver a zoomed in look as it is flying by beneath him/her. The reason many people complain about the ride quality is the same reason sports car enthusiasts love the road feel, the responsiveness, the handling agility, etc. If I'm going to buy a car I want to FEEL safe when I take hard corners, and I want it to be fun. That's one thing that I, personally would not want to give up most in a car, is cornering, and cornering feel. Personally, I wouldn't even care if the Tango could out corner all of the other competitors, I wouldn't feel safe driving it, and I would rather have a low slung sportier car. So that's my main reasons against high up off of the ground cars, all for efficiency. Even if it is more efficient, I would rather sacrifice that efficiency, but given the cars in history with the lowest Cd, all being very low to the ground and super energy efficient, I won't have to worry.
Oliver and Team - wishing you the best of luck this week. I'm confident you guys will win, and you deserve it.
When you start shopping for a powerplant for the "non electric" version of the VLC, consider Ecomotors for a biodiesel engine. These guys have some serious financial backing, and although I'm not sure that I buy all their efficiency projections, it is clear that the engine will be lightweight and the "base" 2-cylinder model model is about the right size (75HP, 95lb weight). see: www.businessweek.com/magazine/content/10_32/b4190036463034.htm
Also, as I've said before - please consider adding "dual" struts out to the wheel pods, to visually merge them into a more appealing body, less "go cart like", like the GM concept car "Autonomy" I mentioned previously. I know this will add a little drag - but you'll have more customers !
Kevin
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What is you aerodynamic engineer opinion about covering car in dimples like on golf ball ?
Like they did on MythBusters and aerodinamic resistance decreased 10 %
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-------Josh Trutt,
Ah yes, the first comments out of the gate say 'it can't be done'. That is to be expected. Even the evidence is heaped on showing all the terrible things that have happened, mostly irrelevant but ugly stuff, and blunders that don't have to happen.
But maybe before the final pronouncement, we might take note that there are some different problems floating around now that were not on the table over the last 100 years when water history was written.
Thanks for the pointer to the 'Cadillac Desert' by Mark Reisner which clearly chronicles the water history of the West. It does appear to revel in the failures, which were significant, but it also points out the wildly successful projects as well. Yup, some of the dams failed, but the reason why the Pacific Northwest enjoys cheap hydro more than the rest of us is that the Grand Coulee dam is so enormously productive. And the California central valley was transformed into one of the most productdive regions in the world by the California aquaduct started in 1963. Then of course is the water grabbing history of Los Angeles, though if you live there you do not really want to turn off the flow from the Owens Valley project.
Of course there are sacrifices for any big project, and they should be carefully considered.
But now, getting to the main point, have you noticed that the whole globe is about to become a disaster zone? Or so some would have it. Under this circumstance, we might convince the smelt defenders to yield, since it ends up being one huge environmental disaster versus a relatively minor one.
I accept the general point that CO2 has to be put under control, but it is not a panic situation, and it can be fixed with real engineering solutions. But these have to recognize the economic realities and resource realities that are fundamentally part of our developed world, yes, that is a result of the industrial revolution which has run on cheap energy for a couple hundred years.
As to the story of Western US water problems, which easily can be made to look terrible compared to the simple but massive successes that can be measured in quantitative terms, that must be a part of the data base that would be used in designing the North American system.
Yup, Great Lakes, watch out. But nope, we should not drain 'you', just use this idle mass of fresh water for the general well being of us all, in a way that keeps everyone going along. A real plan would include a system of aquaducts and storage reservoirs, pumping with electricity production, with pumped storage to keep energy for pumping costs down.
A well thought out plan might even steer clear of Hudsons Bay, though that might get a little less water from north flowing rivers if and only if the salinity balance can be reasonably maintained.
My assertion is however, that if those who are serious about fixing global warming would like to really solve the problem, rather than grandstand against the very industrial forces that put us in the developed world that we enjoy, here is something to think about.
But I guess I can not count on Josh Trutt to provide traction for this.
sar315,
Dimples might help but shape is probably a lot more important for cars. We are not even close to getting the shape right for most cars, so the dimples will probably have to wait a while.
Kevin,
Whether the low slung ballast is 'enough' depends on the numbers, but conceptually, just realize that there is no hard limit on how wide the wheel trains can be spread.
Whether the aerodynamics of wheels, struts, etc. negate the advantages also depend on the numbers, but again realize that the wheels and ballast in between form columns that are faired in to make single aerodynamic entities, and these are much lower in cross sectional area than the main body. So things start to make sense here.
For struts, think of the kind of supports that are used in wind tunnels, and look at the rules for shaping struts to cut down their Cd. Nothing is perfect here, but again, one needs to stop thinking of conventional wheel wells and low slung bluff bodies that we all have known and loved for all these years.
There is a new world out there.
Kevin,
I looked at that Business Week article on engines which of course is interesting, but a little hard to get excited about, given the lack of any real facts except some rah rah about Gates and Khosla getting interested.
Bill Gates recently recommended the '-- Without Hot Air' book by Prof. Dr. David MacKay who decrees that the Second Law of Thermodynamics is to be ignored in his analysis of electric vehicles because it is 'time honored' to thus ignore physics, though he also says he knows better. (see page 27). (MacKay is Chief Advisor to UK DOE and Climate.) Since Gates gets his physics, I guess, from MacKay, perhaps we should not imagine that his success in selling software indicates probable expertise in energy.
Probably not Jim. I'm not a physics expert, or an engineer so I will have to trust Edison 2 on what the best real world design should be. As far as betting on more customers from said "syle", GM's "Autonomy" has no evidence to back that design up on sales since it itself never went on sale, so I'm sorry Kevin, but I have to disagree with you on that, and one other thing. Most of Edison 2's customers will be engineers, and in the engineer and scientific world, aesthetics are much less of concern for the true raw purpose and efficiency. Plus I really like the go-kart design, I think it's boldly stating; this car really is a true game changer.
Wait, do you think that Edison2 will build this car? Have they said they will build the car? I assume this is a dead-end technology.
John C. Briggs,
I think Oliver Kutner said they are building advanced models and would also like to sell the concepts.
The Xprize requires a plan to build and sell cars, whether or not it is real is another matter.
Jim,
Thanks for the input.
Sorry to be skeptical, but I feel like this is the end-of-the-line for this project. Who wants to spend big bucks to put this into production only to find out that it will not sell.
Reminds me of the last exercise, the Partnership for Next Generation Vehicles
http://en.wikipedia.org/wiki/Partnership_for_a_New_Generation_of_Vehicles
That produced 80 MPG prototypes by real car companies. It went nowhere.
Hope I am wrong and these vehicles become available for sale.
Thanks
John C. Briggs
To Jim - my only comment is that yes indeed there is a relatively "hard limit" to how wide the track can be, as dictated by existing traffic lane width and parking spaces. In other words - it better not be wider than a Hummer, and even that would be pushing it.
To all:
Dimples can help reduce drag of poorly streamlined shapes (such as round balls) by keeping the boundary layer energetic and therefore keeping flow attached further downstream and subsequently reducing the size and force of the negative pressure wake. However, for very well streamlined shapes (such as the VLC), dimples would not likely help, and might even hurt. The flow already stays attached the whole way, and leaves a very small wake.
I believe I heard Oliver state that his intent was to find a manufacturer to make the VLC, and let the Edison2 team be "advisors" on how to implement the important efficiency details. They have a few patents, such as their unique "in the wheel" front suspension (not sure how valuable this really is...but it indeed might be). He also said that in order to "follow the money" available for such ventures, it pretty much needs to be an electric vehicle.
Lastly, I guess style in unimportant to some - but to most people it is pretty important. Also, style is in the eye of the beholder, but I don't think anyone but extreme nerds would prefer a go-cart-like look vs. an organic curvy sportscar look. For the XPrize, it made sense that Edison2 use the single struts with optimized aero drag. For a real product - aesthetics matter if you want to be mainstream. My point was that it wouldn't take too many changes and those changes would not hurt aerodynamics that much - to make the VLC body more sexy. If you look at some of the Edison2 videos, you'll see renderings on the workshop wall of car concepts that certainly show that these guys know HOW to make a great looking car if they choose to.
Does anyone remember what day the XP results will be announced ?
Kevin
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Did you think about selling Edison2 as a kit car ?
Kit car doesn't need to meet most government regulations, thus you could make it rather cheap.