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Old 07-13-2006, 04:19 AM   #1
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Long Range EV Aero Design

I was thinking to myself about crosswinds the other day, how to design a bodyshell that would maximize range. With a high enough Cd, it might even be able to be driven by lead acid batteries.

I figure that there has to be a market for something that:
a) Can go 300 highway miles with lead acid batteries.
b) Can do it in a strong crosswind.
c) Can protect from weather.
d) Can carry perhaps up to 4-5 people.

The shell should be built with plastic panels to minimize cost.

This doesn't leave me with many choices. Naturally, if we are to minimize drag, the airfoil or a corruption of it for the sake of convenience is the only real way to go.

Since we are talking about highway miles, I assume 60mph as a base speed.

Now, our vehicle design will depend on the crosswinds we expect.


Here is the average wind speed for the USA. To convert m/s to kph, multiply by 3.6.

According to the Beaufort scale:
Near Gale: 62
Gale: 75
Strong Gale: 87

Now, we are going at 100kph, or 60mph.

air speed = (highway speed^2 + high wind speed^2)^0.5
theta = arctan (high wind speed/vehicle speed)

So, different theta will be:
Near Gale: 31 degrees
Gale: 37 degrees
Strong Gale: 41 degrees

And air speed will be:
Near gale: 80kph
Gale: <80kph...

Conclusions from that are that if we take maximum crosswinds into effect, we don't need to take into account as much theta as if we just looked at theta alone. That is because the largest theta comes from a slight tailwind, which has a partial effect of slowing the true airspeed and hence the drag on the aircraft.

The driver of the vehicle could also be advised to SPEED UP to reduce drag in an exceptionally high wind. Counterintuitive, but it means that the car is facing more straight on and is so better equipped to take the drag.

However, we WILL need to take into account the crosswind. That means that at some maximum angle facing the front of the car, the car needs to be producing as little drag as if it were facing headwind.

The only way to do that is to imagine a maximum rear width of the car. Now, start drawing an airfoil section towards the front, such that it is drawn parallel to the maximum expected angle that the wind will be facing the car at highway speed.

Basically, at any angle between these two maximum angles, the wind should see nothing but airfoil (corrupted or otherwise). And when I say airfoil, I mean a neutral airfoil, not a wing. If anything, the rear should be sloped slightly upwards to add some downforce for stability.

The wheels should be faired as well.

This shape seemed so logical to me, I went out and bought some plasticene right away and molded it, since I don't know how to do the same thing on computer.

The resulting plan view of the car looks like one of those ghosts from pacman.




front


rear


rear side


side


top - (pacman)


side

Note how small the rear area is. If necessary, the car can be extended some more so that there is no turbulent rear area, no "stall" region, however, this will not be necessary IMO.

I show how small the rear vacuum area of my prototype is (dark brown shading) with the equivalent section of an opel calibra sized rear end of roughly the same volume (dark blue). We know that the best case Opel Calibra with wheel skirts and undertray is approximately Cd 1.7, of which most will be derived from the rear vacuum area. Thus we can estimate that if we have half that area, then the drag of my prototype will be approximately 0.75. AND, that drag will be irrespective of whether there is a crosswind below galeforce. If the Opel Calibra is in a crosswind, note that the rear area where there is a vacuum baloons out to the largest purple area, because it includes both the back AND the side.



Since this shape will get a Cd of approximately 0.75, which is less than a third of most cars, it stands to reason that the same battery load will take it three times as far. Which puts it certainly within the ballpark of 300miles with regular lead acid batteries.

Sure, it looks whacky by conventional standards (kind of like one of those Nasa experimental rocketships), but I see no reason why it couldn't:
a) carry 5 passengers
b) be electric powered
c) go 300 miles with conventional lead acid batteries
d) be immune to most crosswinds.
e) be stable at speed and in all manner of wind
f) handle reasonably well - all batteries will be placed between the wheels below the floor.
g) be built reasonably cheaply - needs a chassis, wheels, electric motor, lead acid batteries, windscreens, lights, plastic shell and some sort of spaceframe to protect/fit the batteries.
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Old 07-13-2006, 04:52 AM   #2
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One problem. Obtaining a Cd under .11 is theoretically impossible for a road car that is within the domain of turbulent flow. I understand airfoils and vehicles using laminar flow can go a lot lower, but they are very limited in practicality and utility, and even the smallest imperfection will trip the flow and destroy the drag reduction.

With golf cart batteries or cheap off the shelf deep cycle AGMs, getting long range is theoretically possible. I elaborate on that in the following topic:

http://www.gassavers.org/showthread.php?t=1298

Forgive me if anything in that post turns out incorrect. I have triple checked though and part of it was from an EVDL post I made weeks ago.

Anyway, 300 miles range would indeed be possible for a ~4,000 pound luxury size car with a .12-.13 drag coefficient and 22 square foot frontal area. But given the Ford Probe V's astonishingly low .137, that doesn't seem feasible for a car people are willing to accept.

Although yiou could achieve that goal with a quirky-looking design, a design that I myself would be rather fond of being that I like things that are 'dfiferent', you must also realize the practicality of what will sell.

The GM Precept achieved a .16 Cd, Dodge Intrepid ESX2 a .19 Cd.

So a .16-.18 Cd for a midsize or luxury size car is definately achievable, while remaining very paletable to the 'normals'. And if you read the post I refer to in the link above, 200-250 miles range is well within reach even then. That should certainly be enough.

A car that has 200-250 miles range and looks normal is probably far more sellable to a car that looks quirky buy gets 50 miles more range.

An electric musclecar with attention paid to aerodynamics for a .16-.18 Cd, professional finish, 200-250 miles range at 65 mph, is designed to comfortably seat 5 adults at least 7' tall and 220 pounds, uses lead acid batteries, has DC drive, does 0-60 mph in 6 seconds, tops at least 150 mph(could probably do much more with such good aero, but might be limited for stability reasons), is probably feasible for $25-30k assuming the parts like Zilla controllers and PFC chargers remain hand-built. Much cheaper with mass production of parts.

The battery pack with proper management and around 30 miles of typical daily use would last in theory well over 70,000 miles, but in practice such packs have lasted 40-50,000 miles(ie. Brian Matheny's Chevrolet S10's pack of Trojan T125s, costing ~$2k and lasting 45,000 miles before it only had 80% of rated capacity). Even with maintenance free AGMs to achieve high performance, such a car would be much cheaper to run than most gas cars so long as gas stays above $1.50/gallon.

I'll add more later on when I get the time, but I'm about to go drinking and smoking ganja with some friends for a few days.

I'll be back later, and I will leave to you some cost feasability and energy studies relating to producing EVs and/or their batteries.

Studies on EVs and power plant emissions(EVs are by far cleaner than gas cars): http://www.sherryboschert.com/Emissions%5B8%5D.pdf

Energy and Pollution in making EV batteries: http://www.osti.gov/bridge/servlets/...ble/201715.pdf

EV Production cost study(Outdated, parts/batteries are cheaper now, doesn't use good aero): http://www.ipd.anl.gov/anlpubs/2000/05/36138.pdf


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Old 07-13-2006, 05:31 AM   #3
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toecutter, I think the first step is to get a reliable estimate of how low a drag my shape will produce. Then figure out how many batteries it will require to get it to go 300 miles. I appreciate all the links on the batteries, I will go look at some now.
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Old 07-13-2006, 05:39 AM   #4
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Quote:
I think the first step is to get a reliable estimate of how low a drag my shape will produce.
That will be your most difficult part.

Without a wind tunnel, estimates are just that.

I estimate I will be able to get my Triumph GT6 to have a Cd around .25-.28 with extensive aero mods, but in reality, it may not get that low.

Due to turbulence and the fact that cars are kept on the ground, even airfoils like those solar cars raced by colleges are slightly above that elusive .11 and just can't go lower.

I'm no aeronautical engineer or expert, but my own guess is that you will not be able to get below .13 or so for a practical road going car.

But please do try. If you prove my guess wrong, I will be very pleased.

Even if you get .11 Cd and 18 square foot frontal area, you're still going to need ~80 Wh/mile for a passenger car at 60 mph highway speeds. Packing enough lead in then becomes the problem, and you get back to the problem of weight...

For that type of car, 150-200 mile range seems very doable on about 800-1,000 pounds of lead acid battery. I don't know about 300 miles.
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Old 07-13-2006, 06:37 AM   #5
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Quote:
Originally Posted by The Toecutter
Even if you get .11 Cd and 18 square foot frontal area, you're still going to need ~80 Wh/mile for a passenger car at 60 mph highway speeds. Packing enough lead in then becomes the problem, and you get back to the problem of weight...
Hmmm. Do you know why you can't get past 0.11 for a car? What is the issue? What are the constraints?
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Old 07-13-2006, 06:56 AM   #6
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I don't know the finer details, but I believe it has to do with the turbulence caused between the car and the road, an issue aircraft do not have to deal with. The top of the car does not have to deal with this, while the bottom, no matter how smooth, does.

An expert like Phil Knox could elaborate way more than I.

This may be slightly mitigated by a full wheel faring(barring the contact point with the ground) and having the wheels housed well below the car itself. This would require an axel-less design, perhaps suited to three wheels, and the ride height of the car itself might wind up being a few feet from the ground(with just the wheels and their housing touching it).

There are cars that have cracked that .11 barrier, but they are far from conventional or practical. A single person car is one thing. Getting a real family car to do that, sized large enough to fit a family(even if it may be very unconventional in appearance), will be a real challenge. I don't know if it can be done.
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Old 07-13-2006, 07:23 AM   #7
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Quote:
Originally Posted by The Toecutter
There are cars that have cracked that .11 barrier, but they are far from conventional or practical. A single person car is one thing. Getting a real family car to do that, sized large enough to fit a family(even if it may be very unconventional in appearance), will be a real challenge. I don't know if it can be done.
Yes, you certainly have that right.
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