Annoy As Little Air As Possible
Friday, August 20, 2010 at 06:00PMTo say that the Very Light Car displays great aerodynamic efficiency is an understatement. This summer we spent a shift in the GM wind tunnel in Warren, Michigan, and we just flat-out nailed it. We recorded a 0.16 coefficient of drag: the best results ever seen in their wind tunnel, we were told.
No wonder. Our Chief of Aerodynamics is Barnaby Wainfan, the aerodynamics fellow at Northrup Grumman. The goal of aerodynamic efficiency is to punch as small a hole in the air as possible and try to close it seamlessly behind you, or as Barnaby says “annoy as little air as possible.”
Air separates as it flows around a car and inevitably creates a “dead zone” of low pressure behind, an inefficiency known as “base drag”. A way to help close the air is with a tail: either a horizontal “beaver tail”, seen on our X Prize competitor Aptera, or with the vertical “fish tail” we employ.
A big part of not annoying air is keeping it from entering the car except as necessary, such as for cooling. Contemporary cars often have oversize grills and usually exposed wheel wells: both allow air to enter and bang around inside the car. The car grabs this air and accelerates it to the speed the car is traveling and the energy for this comes from the engine.
With the Very Light Car we enclose the wheels outside the body in pods. This design allows a smaller frontal area, with each wheel pod also poking its own small hole in the air. We are deliberate about exactly where and how much air enters the car. We then put the air back outside where it will do the most good, reintroducing it into the “dead zone”. We also avoid exposing anything to the air that we do not have to, using cameras instead of mirrors and flush-mounting the door latches.
Drag coefficient (Cd) might be the headline number but to know actual drag you need to factor in this frontal area. When Cd and frontal area are multiplied together, the product, CdA, allows you to directly compare different cars. It turns out a Hummer H2 (which not only is huge but also has a terrible aerodynamic shape) has about 9 times the drag of the Very Light Car. Even a 2010 Prius, a car with a very good Cd (.25), has almost twice our drag.
At Edison2 we take great care and pride in making only definite and supportable claims about our car. Our drag numbers were obtained by the dedicated and competent engineers running one of the world’s top wind tunnels. We have since backed those numbers up with the best-ever coast down results measured by an equally impressive and credentialed facility.
The overall goal in aerodynamic efficiency is a car that moves through the air with minimal effort. The ultimate would be if the only drag was the friction of air against the car’s body. That perfection may be unattainable, but the Very Light Car comes close.
Reader Comments (39)
Kevin,
Your apology is noted and appreciated, and it is also noted that it was not a retraction of your machine gun attack. I suggest we both note that we have no idea of the experience of each other, and I point out that I could not begin to tell you about mine. But you might surmise that challenging authority is not something I shrink from doing, and I have never been much when it comes to cheering other people along without thinking about it. All we can really do is to work with substantive things that we say here.
Some of your points are interesting, so let's discuss. You are successful in commercializing products, as you say, so perhaps you can tell us something about your research in the
Stirling engine in that regard?
Your level of understanding of technology, significant as you say, is given some credence by your ability to perceive the bogosity of the MPGe comparisons by the Xprize, and Dr. David MacKay. Do you realize that you are up against our Argonne National Laboratory on this matter? Now is that being respectful? Chuckle. Did you enter into the discussions at the Xprize forum on this under a different name? You can see some of a two year tirade by me on the subject. I also engaged in time wasting combat on Wikipedia in writing the definition of MPGe. Were you there? Did you try to show the distortion by Dr. MacKay of the MPGe business in public forums, such as the Economist magazine? You can see my efforts in comments I made there.
So now let's try to get constructive. You said, "--many people suggest putting a small steam engine or peltier device to run on the exhaust heat of cars. Sounds good...but the temperature is low so the efficiency will be very low, and the total amount of useful work that can be extracted is extremely small (not cost effective). Similarly, the waste heat of a natural gas burning hybrid car + generator is too low to efficiently run a condensation-cycle air conditioner with enough BTU output to be useful."
This is important to my planning, and of course the Peltier stuff is one of those 'do a little - accomplish a little' kind of things. But then you confuse the two issues of temperature and quantity of heat in regard to waste heat running a diffusion cycle air conditioner. It is my understanding that engine exhaust temperature is plenty high for efficiency of these devices, and the quantity of heat, even of a 16 hp engine, is more than adequate. Yes indeed, using a typical car engine for these purposes is generally not meaningful because the amount of heat that is discharged is far in excess of the needs of a typical household. But here is the point!! I am not talking about a typical car engine, That is your assumption contrary to my actual premise. So the bizarity of my premise is that we would actually make cars that could go fast on 16 hp and still have a size of engine that could be integrated into the bigger energy system of household needs and electricity generation.
So Kevin, I would appreciate if you would step up to defend and support the points you have made. I would like to offer you advice in the same spirit you did for me, and suggest that when you answer, that you attempt to think beyond the usual level of a competent engineer, and actually try to think, yes, think outside the box of the way things actually are, and try to get into how to make things better.
But the bottom line is that a Cd of .16 is not the place to stop. You really ought to read the 1982 paper by Morelli to get a better idea of the possibilities. Yes, and there ar problems as well, some of which you correctly note.
But in skimming my patent applications you might not have noticed a lot of attention was given to making the wheel train system work in such a way that very good aerodynamics would be achieved for the lower vehicle parts. Then you might check out fluid dynamics fundamentals, and perhaps you would warm up on the Navier Stokes equations. If you do that nicely, then we can get togethere and be the first to solve Maxwell's equations.
Ok, I was kidding. I have not solved either the Navier Stokes equations or Maxwell's equations. I am limited as an engineer after all, and so have had to rely on computer approximations in addressing various issues in developing systems.
Have you folks seen the Mercedes development model of the Bionic/Boxfish that is reported to have a Cd of just 0.095?
http://img.photobucket.com/albums/v724/NeilBlanchard/CarBEN%20EV%20Concept/Bionic-Car-Body-lg.jpg
It would require articulated front wheel panels, that stayed flush at highway speeds and moved to make room for low speed sharp turns. I think a vertical hinge near the centerline of the wheel, so that only the part covering the part of the tire that extends out moves.
No cooling, but an EV would require very little cooling; and ventilation air intake for the passenger compartment could be right at or just below the stagnant point, and exhaust vent could be located on the back in the low pressure zone; like Edison2 an Aptera have done.
Do you think it could be kept below 0.15? How about 0.13? Or even below 0.11? This seems to be a very practical shape, that would make a comfortable and usable car, in my opinion. And it starts with just about the lowest Cd of any car I have heard of; save some streamliners, which obviously would be near impossible to use on the streets and highways.
Sincerely, Neil
John C. Briggs,
If coal is banned or heavily penalized on a national basis, and at the same time use of natural gas expands to fill the gap and to fill other uses as well, we will see a quick rise in the price of natural gas to parity with gasoline. The only thing keeping the price of natural gas as low as it is is the presence of abundant and cheap coal.
Yes, the coal is truly abundant at even the very low prices that now prevail. Not so for natural gas. The so-called reserves of natural gas have been rapidly disappearing, now that the price of that fuel is $4.50 instead of the $7.50 which seems to have been the 'economic condition' under which the abundant reserves appeared. (An analyst named Lipper stated this $7.50 as the price point for the estimates on Nightly Business Report a few months ago.) The EIA states shows the rigs devoted to natural gas have dropped to half the number that were in use a couple of years ago.
Also, your idea of the effect of the price of natural gas on electricity prices is incorrect. The price changes we saw during the Enron crisis were highly insulated from the actual natural gas price events, and if the disastrous levels had continued, we would have seen something a lot worse. Various things happend, including expanded use of coal in California, both internally and in buying electricity on the spot market from other sources, which would have largely been coal fired. Then the impact was delayed by contracts and pre-set rates that caused Calpine and PGandE to go bankrupt.
So I really do not see that we can change things in the power generation and use world without some serious problems, and yes, I think that we will all be quite unhappy when we realize that even talking about the changes could be a major reason the economy is not recovering. None of us will be selling cars if the economy does not get fixed soon.
Neil Blanchard,
The boxfish had a Cd of .06 in free flow conditions, and then when lowered to be near the ground the same body went up to .095, but it still did not even have wheels.
What you suggest is right about how to make it work with wheels, but even if perfectly executed, it would go up a little.
As I recall, Daimler Benz achieved .16 without doing much with fairings. That was not made entirely clear. I would guess they could get it down around .14, but that is only a guess.
But why not get brave and ask for the original .06?
Impossible things might turn out to be just things you are not familiar with.
Jim,
From a relatively creative guy, this is a startling statement
"So I really do not see that we can change things in the power generation and use world..."
That's it, we are screwed, nothing can possibly change? Well, then, I would suggest the same is true for cars. Nothing can possibly change in the fuel source or usage of cars, so you are wasting your time even trying.
John C. Briggs
Hi Jim,
As I understand it, this is the model that got Cd 0.06 and obviously, it has no wheels and cannot be used as a car.
http://www.ecofriend.org/images/mercedes-benz_bionic_concept_car2.jpg
This is the model that got the Cd of 0.095 and it has wheels, so I have to assume they tested it on the ground:
http://www.carbodydesign.com/concept-cars/2005-06-23-mercedes-bionic/3.jpg
The thing is, if you take the 0.06 shape and put it on a set of wheels, then the drag would probably go up to more than the 0.095. The wheels must touch the ground, and the two volumes will also interact. So you still have ground interaction AND you have the interaction between the two volumes.
Sincerely, Neil
John C. Briggs,
Thanks for catching that poor wording on my part about changing things in the power generation and use world. That is a writing error-- no it is an incomplete thinking error. Ugh.
I meant to be saying that to change things by taxing power generation we would be looking for serious trouble. I strongly favor changing the way we generate and use power; but I do not think actions should be taken that will significantly burden the economic system which can not handle it.
Neil,
Right you are!
JIm,
For any design to be commercially successful, it needs to be practical. Your "miastrada" idea seizes on one factiod (that the Cd of an airship body is much lower when raised vs. close to the ground) and then 100% runs with it without regard for other important details. Specifically:
1. The center of gravity vs. narrow track makes handling abyssmal, as is therefore safety. If you can't solve this problem - the idea is a non-starter. It might be possible to improve it by using batteries as ballast (like the Tango), down low, say between the front and rear wheels. This suggests a body form more like an upside-down U, with a clean "tunnel" down the middle. Get rid of all the other wires and struts, just have 2 strong slender vertical ones to two wheel "outrigger bars". Note that this is not unlike some of the "Ecomarathon" winning body forms.
2. The vertical cross-section of the outrigger bars can be cut in approx. half by getting creative with the wheels. Consider using small diameter wheels, and have several along a rising curve in front to simulate a larger wheel when going over a sharp bump - but 98% of the time, cars ride on flat ground. This would allow the "outrigger bars" to be half as high (but still need slender struts upward to tie to the body). This may be moot, however, if the wheels can be rather narrow - because then the width of the "wheel pods" and the struts become similar.
3. I think the most interesting thought experiments are to think about WHY the Cd drops when you raise the body - and try to leverage the appropriate physics in your favor - but WITHOUT raising the body to a ridiculous level. There are several aero treatments that I believe have not been sufficiently researched for cars: A) Control the flow with thin, transparent "walls" that extend forward from the body, held in place with slender streamlined struts. There are many variants possible - visualize a conventional car body shape with a clear tube surroundign the body at a distance, with the leading edge extending, say, 3 feet in front of the body. In this way, you can possibly control the airflow locally, without allowing far-field flow to be affected (as much).
B) Control of flow patterns using active boundary layer control - such as sucking or blowing with a fan system through slots or ports. This has merit...but the challenge is to improve the energy lost to drag to be MORE than the energy used for the blower system. Overall...I think passive systems will always be most practical.
Anyway, this is some of my thinking on the miastrade idea. Also, I see no benefit whatsoever for all of those wheels - stick with 4 (unless you need more to go over bumps, as mentioned above).
Although I honestly believe the miastrada concept is fully into the realm of "crackpot"....like many such ideas it has some threads of real basis. My feeling is "live and let live" and hope that creative ideas can lead to something better. Good Luck,
Kevin
Kevin,
I am not sure what makes something a factoid. But there is a powerful principle (80% drag reduction) about aerodynamic bodies that is represented by the airship form. Compare this to the bluff body that the automotive world usually accepts without serious question. And there is a second topic about the large degrading effect (40% drag increase) of a ground plane, and this is nearly impossible to fix without raising vehicles to ridiculous heights; unless, that is, the body is a cylinder of about four feet diameter. And this diameter is only possible if tandem seating is used. Now things start to change dramatically, since the four foot diameter body running at around 2 feet above the road is not an uncommon vehicle height. In fact, people are very fond of riding at that height, if you notice the marketability of vehicles of such height.
Yes, the Ecomarathon body forms are relevant. So is the Morelli work that showed how high a body needs to be above the ground to escape the ground plane degradation effect. The Ecomarathon cars and the solar cars as well are content to place the driver in a nearly prone position, so the body forms are extremely thin vertically, and these tend to come out more like wings than airships.
Your item (1): Absolutely right, except handling the problem can be accomplished with the 'six wheel system' which is the way the articulated vehicle works. You might find the combination of stabilizing and steering interesting, though it is admittedly complicated. I can tell you that a simple 'thinking tool' model that I ride down the driveway has shown this system to work remarkably well. But simplifying the whole thing seems reasonable, along the lines you point out, since the most important thing is to get the drag to be a lot lower. And yes, the ballast could be like that of the Tango, except that it is split into the two sides so that air flows under the vehicle about as well as it does over the top. And no, the Tango width is not something that has to be matched. Though it could be matched with the articulated six wheel system, that could be put off until a later time, and we could just spread the wheel trains as much as needed for stability.
Your item (2): Yes, there is much room for improving the wheel trains as aerodynamic entities. The pictures shown on the miastrada web site does not include any serious detail on these, as it is only a concept drawing. The struts can be made quite slender, so it seems likely that these will remain thinner in cross section than the wheels. It is important to interfere with the symmetrical airflow around the body of revolution in order to fully enable the airship performance. Remember, we are comparing body drag that is reduced from the best production cars by a factor of six. So yes, the struts and the wheel trains have to be given serious attention.
Your item (3): Future arrangements are interesting, but since it is possible to achieve an airship body drag coefficient of .04, why not start there? Of course it will look crackpot, even though it will be about six feet high; go look at a Yukon.
Sometimes I yearn to be popular. But it is a lot more fun to be a crackpot. 'Crackpot' sometimes accomplishes something. Popularity is for sheep.
Thanks for listening and for your reactions. Even crackpots need to pay attention to the market reaction.
Hello Jim,
By raising the "people part" of the chassis up high, you have not improved anything aerodynamically around the wheels, batteries, and the motors. So, what is gained by raising the "people part"?
As was pointed out (and implicit in my email to you) is the higher center of gravity of such an arrangement; all else being equal. The only "fix" for higher center of gravity is added ballast weight; like the Tango. Added weight to an already heavier vehicle because you cannot raise the "people part" up without adding some structure that would not be there otherwise.
The other practical problems with the situation is sight lines for the driver in front of the vehicle, difficulty is entry and exit, crash worthiness of two main masses separated by space, very complicated and power consuming hydraulics or electrical assist for the steering, software programming and CPU for drive-by-wire-everything.
I'm not saying that you should drop your initial idea, far from it! It is always good to be willing and able to "try out" something new and different. I am saying that you need to be willing to step back, reevaluate, and be realistic with yourself.
If there is anything we can learn from the X-Prize and Edison2 in particular, it is we need to apply the KISS method at all times, and look for the elegant, simple, most efficient way to move people. Have you seen this vehicle?
http://img.photobucket.com/albums/v724/NeilBlanchard/DSC04228.jpg
http://img.photobucket.com/albums/v724/NeilBlanchard/DSC04229.jpg
http://img.photobucket.com/albums/v724/NeilBlanchard/DSC04231.jpg
http://img.photobucket.com/albums/v724/NeilBlanchard/DSC04227.jpg
It gets the entire chassis up off the ground and is much simpler. It is good to have as many people thinking about this challenge as possible.
Sincerely, Neil
Neil,
Are these pictures showing you and your work?
Your four photobuckets illustrate the body form I am talking about, only I would make it larger to accommodate the larger humans more comfortably. The spacing off the ground meets my rule. My larger body would mean a greater ground clearance and a greater overall height, but still it could come out around 6 feet overall in height. I would also use pitch down to get similar viewing for the driver.
I would use much thinner connections to the wheels, so the body aero function would be better retained. The rear wheel and equipment looks bigger in the picture than the smoothly faired in versions would accommodate. Maybe that is just the angle effects. However, unless this is very narrow, there is a significant disturbance to air flow by this rear wheel and machinery, and it would degrade the performance of the large main body.
But there is no need for this, since the rear wheel support could be done with two rear wheels that follow the front wheels. So with two wheels in line on each side, and narrow struts holding the body up, you get my configuration. That is, this is my alternate configuration; which is shown in the patent applications, not the main picture which is the six wheeled articulated form.
Anyway, if there is a wheel train on each side, with heavy items low between the wheels, a better weight arrangement is achieved than that shown in your pictures. And then it is not necessary to have such a wide wheel base. It would probably need to be only a little wider than the Tango, and we have evidence that this can work quite well from their results.
Not my stuff -- in fact I have misplaced the name of this project. The text on the display board could be read, I suppose...
Neil
I just had a thought -- the Edison2 VLC should be added to the list of low drag cars on Wikipedia! It'll go on the concept car list for now, I think?
http://en.wikipedia.org/wiki/Automobile_drag_coefficient
I think all the listed Cd numbers are of the "old" variety, and maybe a couple of sentences on the new SAE standard could also be added?
Sincerely, Neil
I am currently studying aerodynamics as part of my engineering course and the ideas about this very light cars makes me curious. If this research and study would be successful as well how aerodynamic efficiency works, I guess more studies can be made out of it.
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drill edm
I had a question about the shape of the VLC body. I have studied enough aerodynamics and fluid flow to have a fairly calibrated eye for low drag shapes (not that complicated, but more so than one would guess). I fully understand why the streamlining of the body and wheel pods comes back to a nearly sharp edge. However, from my understanding of things - the shape of the LEADING edge is not so critical as long as there are no sharp edges. Therefore, my question to Barnaby (or anyone who can answer it) is why was the "pointy" nose chosen ? It seems that a more bulbous nose shape could have more storage space for the same drag. Was it for driving visibility ? Cool looks ? Or...is there a subtle aero principle at work that I just don't understand ?
thanks,
Kevin
Well now that the 4th gen iteration of the body has been seen, i lament the apparent loss of visibility in the newer more bulbous nose, though as you point out, it is likely noticeably lower in drag.