Do Cars Really Need to Look Like UFOs to Save Gas?
With the world focused on fuel economy, advanced car design seems to be converging around one point in space. And I do mean, literally, a point in space — a point sometimes seen flying over Roswell, or crashing in the Bermuda triangle.
Regardless of what you think about this type of design, it begs the question: do cars need to look like alien spacecraft to get decent fuel economy?
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Recently, when I posted about the newly announced Honda Insight’s uncanny resemblance to the Toyota Prius, I sparked a rather *spirited* debate with fellow blogger and former Gas 2.0 writer, Ben Jones (ecomodder) regarding aerodynamics and why all high mileage cars are starting to look the same.
His point was that if we want good fuel economy, we shouldn’t be upset when all cars start to take on the same shape. Basically, we shouldn’t whine about cars looking more and more like flying saucers if we want to save gas.
But I wasn’t convinced that in order to get good fuel economy all cars will have to look like the Prius. I remembered reading about a Mercedes concept car based on the shape of a Box Fish that looked nothing like the Prius and had an unexpectedly better aerodynamic profile. Also, the Aptera Typ-1, which looks more like a mutant sperm than the Prius, has much better aerodynamics too.
So, you’re probably saying, “Yes, both the Typ-1 and the Mercedes Box Fish Concept have better aerodynamics than the Prius, but they’re so strange looking that they’ll only appeal to a very small portion of the population.”
And you’d be right. Which brings us back to the question of the Prius shape, but now framed in a different manner. Perhaps the answer is that the Prius shape is the most aerodynamic design you can have and still appeal to a large enough market to make a profit (and an environmental difference).
But is the Prius design really a good compromise? Does it do itself justice by trying to accomplish so much in one vehicle?
To get to the root of this, I decided to seek out an expert opinion and contacted MIT professor, Mark Drela. Dr. Drela is a professor of fluid dynamics in MIT’s Department of Aeronautics and Astronautics and has been an adviser to MIT’s Vehicle Design Summit regarding vehicle aerodynamics. In a response via email, Dr. Drela had this to say:
“All common cars, including the Prius, are aerodynamic bricks. The lowest drag shape that you can wrap around a passenger cabin resembles the front half of a sailplane fuselage. The Aptera comes pretty close, assuming they designed it properly.”
What he’s saying is that the Aptera Typ-1 probably represents the ultimate achievable aerodynamic shape for an enclosed vehicle with tires. He goes on to say:
“We know how to make extremely low drag vehicles (e.g. the Aptera), but the main problem is making them acceptable to consumers and making them ’safe,’ however that is defined. I see it mainly as a marketing and lawyering problem, not an R&D problem.”
So there you go. The Prius is not the most aerodynamic shape, it’s the most aerodynamic shape that’s also visually acceptable and safe enough to meet standards. But does aerodynamics even matter that much? I mean, some cars today are getting excellent mileage and don’t look like the Prius or the Aptera.
The issue comes down to the fact that achieving good fuel economy on the highway is an entirely different beast than achieving it in the city. Trying to make one vehicle that excels at both creates a vehicle that doesn’t do well at either. As Dr. Drela says:
“At steady highway speeds, low air drag is most important. If you’re braking frequently, then low mass is most important. A hybrid squeezes the two very different missions into one vehicle, and you sort of get the worst of both worlds. It doesn’t do either mission as well as a targeted vehicle, and is more complex to boot.”
Instead Drela suggests that a better solution would be to have two targeted-mission cars:
- An extremely light plug-in electric car for city driving
- An extremely low drag internal combustion car for highway driving
He points out that, obviously, owning two cars is more expensive up front, but that the lifetime economics and energy use of this arrangement might prove that it is the best solution.
Purchasing two vehicles up front might be out of reach for most people, but, if Dr. Drela is right, perhaps we should be looking at ways to make this easier for everybody to do? Government incentives?
Certainly the car companies would be happy to sell you two cars at once — perhaps they could work up a discount on the second car purchased if purchased at the same time? Does this seem like a fantasy world that sounds great on paper but would never fly in reality? What do you think?
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Image Credits: Toyota, Honda, Aptera
I think the better question is, why do we expect spacecraft to be aerodynamic? Any alien race capable of building interstellar spacecraft probably has better methods of getting stuff into orbit than rockets- maybe a space elevator. The point is, they’d probably assemble large spacecraft in orbit, so they didn’t have to lift it all from the ground at once, or build it on a low-gravity body so it would be easier to launch. In space, aerodynamics wouldn’t be a concern, and non-aerodynamic shapes would make much more functional sense.
Personally, I like the really aerodynamic cars.
Scott,
I think you’re underestimating the importance of low mass.
Even for the rather optimistic Cr = 0.005, I get the following gasoline consumption needed just to overcome rolling resistance:
Aptera: 0.16gal/100mi
Prius : 0.30gal/100mi
For the easily achievable Cr = 0.010, these will double:
Aptera: 0.32gal/100mi
Prius : 0.60gal/100mi
I also get the following gas consumption numbers needed to overcome air resistance at 40 mph, which might be representative of city driving:
Aptera: 0.16gal/100mi
Prius : 0.52gal/100mi
So in the city driving case, rolling resistance (which scales with mass) is comparable to air drag. And after you add the losses of brakes with inevitably imperfect regeneration, mass gets even more important.
So the bottom line is that even at the “fast” city driving speed of 40 mph, low mass is somewhat more important than low air drag for fuel economy. At 30 mph, low mass clearly dominates. This is my main rationale for an electric-only city car. The less extra hybrid stuff you have in the car (IC engine, gas tank, generator, etc.) the better.
At 70 mph highway speeds, the air drag of course dominates, and the picture changes substantially.
BTW, Anyone know the achievable brake regeneration efficiency on a Prius? The systems I’ve seen on Solar cars about 10 years ago looked good on paper, but were measured to be no better than 30% in practice. So they were almost not worth having, since they complicated the power electronics. I’m curious how much better the current systems are.
Dr. Drela.
First off, props for the nice work on Decavitator, among other things. My own efforts stem towards tilting trikes.
Without the R&D data from Toyota, it might be worthwhile to look at comparable mileage figures for vehicles that are not regeneration capable.
Honda Civic 2009 26 mpg city /34 mpg hwy
Toyota Yaris 2008 29/36
Toyota Prius 2009 48/45
For the cars without regeneration you’ll notice roughly 33% increase in mileage going from city to highway. For the Prius the figures are almost the same, with city mileage actually better at
So it appears that Toyota regeneration is adding close to 35% to the city mileage. And that may not be enough of a difference from your previous experience to account for specific driving techniques and traffic environments.
:)ensen
Mark,
You make several interesting points. I was not aware that rolling resistance was very near comparable to air resistance below about 40mph. I checked a couple of other model cars and that was true for them as well. The cubed relationship between power and speed for air resistance is probably responsible. Even so, I am not sure I would necessarily make the same conclusion that you do regarding the importance of weight versus aerodynamics.
Given any particular vehicle class or application is there room to reduce the weight of the vehicle by a factor of 2? That seems very ambitious. It would require making a four-passenger car the size of the Prius that weighs the same as the Aptera. On the other hand given that most modern vehicles have Cd values in the .25 - .4 range as compared to the Aptera’s .11, the Aptera stands as an existence proof of the fact that there is plenty of room to reduce drag by well over a factor of 2 with roughly the same mileage benefit that halving the weight represents for city drivers doing at least 35mph and far more benefit for anything much over 45mph. Given the cubed ratio it should be noted that even a small percentage of drive time at high speeds will tip the balance in favor of aerodynamics. Is not the class of vehicles restricted to usage below 40mph rather limited? That said by all means do both.
The figure that sticks in my mind for Prius regenerative breaking is 30% recovery, sorry no reference. However, I think that might be misleading in that the system can do far more if driven correctly. For instance my commute includes a 600 foot ridge between home and work. The Prius allows me to recapture some of the potential energy of the hill climb. It is especially helpful during a gradual 10+ mile down hill on both sides where I can typically maintain all electric drive over all of that distance. As a result I have averaged 1.97gal/100mile over the last 12,000 miles driven (about 50% of that commuting). I am unaware of a non-hybrid available in the US in 2002 when I bought mine that could come within even 75% of that performance. Thus I take issue with your characterization of the hybrid as the worst of both worlds.
I think the conclusion is inescapable that indeed cars of the near future will trend more and more to looking like UFO’s.
I have long thought that one possible part of the solution to our global crisis would be to make more use-specific vehicles. Prof. Drela makes a good point. The problem is though, avoiding the added insurance costs. You can’t physically drive two cars at once (unless you are towing… I guess) so you should be able to insure both vehicles at a lower rate. It will take new laws to force insurance companies to lower rates for people who own multiple cars for various uses.
>The cubed relationship between power and speed for air >resistance is probably responsible.
Air-drag fuel consumption per mile scales as the square of the speed, not the cube. It’s still a strong effect.
>Given any particular vehicle class or application is there
>room to reduce the weight of the vehicle by a factor of 2?
>That seems very ambitious. It would require making a
>four-passenger car the size of the Prius that weighs the
>same as the Aptera.
Yes, it’s a challenge. Here’s how I think about the problem:
Let’s say we take the money that’s sunk into all the extra “hybrid stuff” on the Prius, and use it instead to reduce weight. For example, one might replace most of the structural steel with carbon fiber. How would _that_ car compare to a stock Prius in fuel economy? That’s certainly a very interesting question. I don’t know the answer since I haven’t done the numbers, but I wouldn’t bet against the “light and simple” solution. In the aero business, light and simple usually wins.
There are also other considerations in such a tradeoff. Simplicity has reliability and maintenance side benefits.
>On the other hand given that most modern vehicles have Cd
>values in the .25 - .4 range as compared to the Aptera’s .11,
Side comment:
Cd is really misleading when comparing different cars. If the Aptera’s Cd had the same Cd as the Prius, the Aptera would still have less drag, because its frontal area is smaller, or at least it looks that way.
Rather than quoting Cd, everyone should be quoting “drag area”, or “CDA”, in ft^2 or m^2. That’s really what counts when calculating air drag, and it’s the only rational way to compare the air drag of two very different vehicles.
When I was advising the MIT Solar Car team on drag reduction, I pushed them to think in terms of CDA, and not Cd. One of the team leaders Goro Tamai wrote a good book on solar car aerodynamics (The Leading Edge) where he discussed this. The solar car community has now made the transition from Cd to CDA, but the rest of the auto industry hasn’t.
<There are also other considerations in such a tradeoff. Simplicity has reliability and maintenance side benefits.
I agree that at first blush one would think that simpler should be more reliable, but again my experience is exactly backwards in the case of my Prius. Maybe the way components are stressed over the life of the car makes a big difference to reliability and the hybrid has a more favorable stress profile that more than compensates for the increased complexity?
Driving in this morning I noted that I have recently turned 150,000 miles on my Prius. I mention this because it relates to the efficiency of regenerative breaking and reliability. I am still running with the original break pads (3X - 5X longer than normal, cost savings of $300-$1000 just for breaks)! At this point I expect them to last the life of the car. They are lasting so long presumably because regenerative breaking is actually doing a significant amount of the breaking work and thus is greatly lowering break wear.
For what it is worth, my Prius has the lowest maintenance costs of any car I have ever owned, by at least a factor of 4X. I have literally spent more on tire replacements than all other forms of maintenance combined. I can only guess why, but I assume it is because the hybrid electric drive is actually extremely reliable (very few moving parts) and that it has greatly reduced wear and tear on the IC drive line … not to mention the breaking system.
I seen 21% increase in my truck in better gas mileage and can help the envirnment alot burn 99.99% clean and you can possibly get up to 25% increase in mileage and after 3 oil changes many reported you can go 2-3 times longer if not satisfied you can return unused prtion and get full refund http://www.getmorempg.info
Mark Drela said “but were measured to be no better than 30% in practice. So they were almost not worth having”
Mark, could the same not be said for any internal combustion engine. After all only 30%, typically less of the fuel goes towards moving the car.
–DingBat