Thanks for the excellent post, toecutter!
It seems like you have been as obsessed about this as I have been the last few years. Long have I looked at the eco-speedster and thought "Why don't they produce something, anything like it for the public?"
BTW Here are pictures of the Wraith interceptor (Cd around 0.20 according to toecutter):
Note: sideskirts, front air dam (which I'm not a big fan of, I think undertrays are way better and more practical, if not from a maintenance POV, at least from a speed hump POV), barely vestigial 3-box design (note no difference in angle between windscreen and hood, as in 99% of cars), and my favorite, the boattail that both
slopes up from the floor and slopes down from the roof. Booyah! Also note that it reaches its largest frontal area approximately 1/3 back, like a fast swimming animal such as a mackerel, shark or killer whale. No rear wheel fairing either, which goes to show how great a shape it is already!
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Originally Posted by The Toecutter
It is a custom called the AeroCarbon built onto a Lotus Elise chassis with a Honda engine.
It has a drag coefficient of .22. Nothing spectacular. I think it's quite ugly, especially for having a Cd as high as .22, when other cars that look much better have attained .16-.18.
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Uhuh. It looks quite similar to the UFE-III, only not as good. I wish I had received the $2 million dollars of pork that went their way.
Again, note the aerodynamic cues taken from marine animals, such as the complete lack of a nod towards conventional 3-box design.
Note the way the rear sweeps up from the bottom as well. Not surprisingly, the Cd is 0.16.
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Fuel economy and performance can coexist nicely. If fuel efficiency is to be marketed to the public, it has to be fast. The truth is, a more efficient engine with lower horsepower numbers is not going to affect fuel economy more than ~20-25%. Compare the EPA mileage data on cars of a certain model with their different choices of powerplants. The V8 Mustang, for instance, gets near the same fuel economy as the V6. The V6 Corolla gets about the same fuel economy as the L4.
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Yes, that's obvious if we look at the above graph, taken from this
page.
Let's have a look at some equations and sample figures just to get a feel for it. Take a toyota camry like car. Cd = 0.3 (camry actually has 0.28 as of 2007, but we are making it more similar to an average family car), frontal area = 26 square feet, or 2.41 square metres. Kerb Weight 3200lbs, or 1454kg. Max Power = 121kW (158hp).
Acceleration is shown roughly by 0.0832kW/kg.
If the car is cruising at 70mph, then what is the load on the engine?
We know that at cruising speed, drag dominates. So if we can compute the drag, we can basically compute the load on the engine. Again, if we use this
site, we find that the equation for power loss due to drag is:
Substituting into the equation (air density is 1.2kg/m^3 at sea level),
P = 2.41 * 0.3 * 31^3 * 1.2 /2
= 12.9kW
(Edit: Please note that you can double-check my calculations
HERE. Enjoy, have a play around, it's great!)
So, a typical highway load for a standard passenger car is around 13kW. Acceleration requires the maximum load to be 121kW, or such that the average load at highway speed is roughly 10% of rated load.
If we look at the above graph and see where the efficiency would typically be for the average engine, it's at roughly 20% out of a total of 28%. Hence the maximum drop in fuel consumption by shifting to a gutless, unroadworthy engine is 32%, and a more realistic drop is 20% by shifting to a more anaemic engine where the cruising load is only 30% of rated load.
Of course, in so doing our max horsepower has dropped by 70%, which will not inspire the public.
Drop the weight to half and do nothing to the Cd, we can halve the engine size and have the same power to weight ratio, giving the same usable acceleration, lower top-end speed (which most drivers don't see anyway), and a 20% reduction in fuel usage at 70mph.
Drop the Cd by half down to 0.15. This means that the power needed for cruising at 70mph jumps down to 6.5kW (won't actually be quite so good because of diminishing returns due to other factors like rolling resistance, but close enough). Now to produce 6.5kW, the engine is 15% efficient. That means that it's consuming 43kW of fuel energy from gasoline (6.5/.15). By contrast, the regular camry is consuming 65kW of gasoline (13/.20).
This means that halving drag gives us a 34% reduction in fuel consumption, whereas downsizing the engine gives us a realistic 20% reduction in fuel consumption, and a MAXIMUM POSSIBLE reduction of 32%.
Of course, this analysis depends on that curve being correct. Supposedly it was supplied by the Clevelend RTA, whoever they are. I suspect that is at an rpm/throttle point that gives maximum efficiency for a given load. Note that a typical passenger car like the Camry will NOT be operating in this manner because the motoring public (or the journalists) like to have plenty of acceleration in 5th, which means that there could in fact be larger gains made IF THE SAME GEARING IS KEPT (which it probably won't be).
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The real truth is, it's mostly drag that effect's fuel economy.
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Yes. If we can reduce the drag from that camry down to 1.0, then we can halve the fuel consumption. (The limit is dropping it to zero, and then we are limited by whatever the engine consumes just to keep itself ticking over.)
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It upsets me when designers try to tackle increasing fuel economy through mostly engine downsizing, when that is where there are some of the least significant gains to be made. The real gains to be made are the low hanging fruit that is aerodynamics: you expend far less effort, sacrifice far less quality, for much greater fuel economy.
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Exactly.
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That Ford Probe V? If someone somehow managed to shoehorn a 350+ horsepower V8 into it, I bet it would still get ~45 mpg if driven conservatively!
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That's a good guess.
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If you've ever seen the movie "The Wraith", that concept Dodge M4S turbo interceptor that made its appearance in that movie had roughly 440 horsepower. Guess what fuel economy it got? THIRTY-ONE mpg. Why? Its drag coefficient was around .20. Here we are, with our car fleet not even approaching that, still struggling with inline four cylinders and V6s of around 170 horses.
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Yes. One thing to note about turbos is that they are designed for fuel economy - they permit a smaller engine to have a higher maximum horsepower. I'm trying to remember what the downside is - besides turbo lag, when the turbos spool up it would be less efficient than an NA engine.
But yes, your point remains that it is due to ridiculous drag coefficients on passenger cars that the state of things are where they are today. Drag coefficient is really the low hanging fruit here, waiting to be plucked. I suspect that it's due to the unintuitiveness of Cd, people don't take the time to understand the principles, to study nature, and to refine. They'd rather take a 3-box and tweak it by making it "look streamlined".
You might be able to get weight down to about 800kg if you really tried, which will enable almost a halving of engine size for the same performance. This will almost halve city fuel consumption, and as stated before it should have maybe a 20-30% effect (max) on highway FE.
All three in combination are best - lowering Cd, lowering kerb weight, lowering engine size, enabling a tall gear for cruising.
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Imagine making an ultra aerodynamic car. Could have performance out the *** and high horsepower, yet still get double the combined fuel economy of cars available today. All this, with no magical new technology needed.
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Yes. This is the highly frustrating aspect of the focus on the hybrid magic bullet. Instead of saying "we will reward very high combined mpg cars with special lanes and subsidies", they have said "we will reward not particularly amazing cars with fancy, expensive technology special lanes and subsidies", constraining the design process unecessarily.
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But when consumers demand fuel economy, Detroit punishes them with anemic powerplants they don't want and dramatically downsized cars that they don't want. Aerodynamics are everything. Literally.
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Exactly. Why does econo="econobox"? Why not "econocar"? I suppose that's what comes with having bean counters run the company, and not having engineers who can do finance and marketing run the company.
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