by The_Toecutter » Tue 28 Feb 2006, 22:12:52
$this->bbcode_second_pass_quote('', 'I') am reluctant to believe most of the fuel gains come from reduction of drag. I would have a feeling, depending on the possible headwind and the average speed of the vehicle drag could significantly reduce fuel consumption. However, when the speed is say 50KM/h (forget the headwind for this example) does the difference between .21 and .26 drag co-efficient make that much difference in fuel consumption?
One way to get a decent estimate on the difference in fuel consumed between two cars with identical engines without going into a very deep automotive technical analysis is to calculate the horsepower needed to move the car a certain speed. This is what will mostly determine the constant speed fuel consumption. The equations used aren't 100% accurate, the drag equations don't account for outside wind speeds, and the rolling force equation is an estimate that is roughly within 3% margin of error up to about 70 mph, as rolling resistance does vary with speed in reality.
Lets say we have the Toyota Prius, with its .26 coefficient of drag and 21.9 square foot frontal area. It comes equipped with size 185/65R15 tires with about a .006 coefficient of rolling resistance. Its curb weight is about 2,900 pounds.
Now that is all the data needed to reasonably estimate horsepower needed to go a certain speed. We will compare the car with a lower drag version of itself having a .20 drag coefficient with all other parameters the same, same frontal area, same weight, same tires, at the speed you describe, 50 km/h, or about 30 mph. This is also about the speed where aerodynamic drag force balances out with rolling drag force in most cars, generally between 30 mph and 40 mph. Also, it will be assumed 15% stray losses(accessories, ect.). Assume both cars are run on flat ground.
Rolling resistance:
Fr = Cr * W = .006 * 2,900 = 17.4 lbs.
The above figure can be used for both Prii since both are assumed same weight and tires and such.
Cr = coefficient rolling resistance, no units
W = vehicle curb weight in lbs.
Fr = rolling force in lbs.
Aero Drag:
Fd = Rho * Cd * A * V^2
For .26 drag coefficient Fd = .002558 * .26 * 21.9 * 30^2 = 13.1 lbs.
For .20 drag coefficient Fd = .002558 * .20 * 21.9 * 30^2 = 10.1 lbs.
Rho = constant to adjust for normal air pressure, convert from Metric to American, and to account for the ½ normally present in the formula Fd = ½ * Rho * Cd * A * V^2
Cd = coefficient drag
A = vehicle frontal area in square feet
V = velocity in mph
Fd = aerodynamic drag force in lbs.
Total Drag = (Fd + Fr) / S
For .26 drag coefficient Total Drag = (13.1 + 17.4) / .85 = 35.9 lbs.
For .20 drag coefficient Total Drag = (10.1 + 17.4) / .85 = 32.4 lbs.
Fd = aerodynamic drag force in lbs.
Fr = rolling force in lbs.
S = 100 percent minus 15 percent stray losses and written as decimal, so expressed as .85
Total Drag = Total drag force in pounds
HP = Total Drag * V / k
For .26 drag coefficient HP = 35.9 * 30 / 375 = 2.87 horsepower
For .20 drag coefficient HP = 32.4 * 30 / 375 = 2.59 horsepower
HP = engine horsepower required to cruise at speed
V = velocity in mph
k = 375, a constant to adjust for V in mph, drag force in lbs., and HP in horsepower
You get a difference of 9.75%.
So even at 30 mph, cutting the Prius aero drag coefficient from .20 to .26 will yield ~10% difference in power needed to cruise at that speed. This would equate to about 10% better fuel economy at that speed.
If you adjust the speed to 65 mph, the difference is far more drastic. The .26 drag coefficient Prius then requires 16.10 horsepower to cruise at 65, while the .20 drag coefficient Prius requires 13.20 horsepower to cruise at 65 mph.
This is a difference of 18% in required power! Going from .26 to .20 would result in about 18% better highway mileage.
So basically, cutting the coefficient of drag from .26 to .20 on a car might give it an overall decrease in fuel consumption of about 13-15%.
Of course, you did choose a relatively aerodynamic car to start with, lowering the disparity a bit.
Most cars on the road have a drag coefficient of about .35, frontal area about 24 square feet, 3,200 pounds weight, and tires with a Cr about .010. If you go through the same formulas above, you'd find that bringing the drag coefficient to .20 from .35 while keeping everything else the same would yield a decrease in power consumption of 16% at 30 mph, and 38% at 65 mph, and thus decrease fuel consumption by roughly the same amount. ~40% more highway fuel economy, or about 30% more combined fuel economy.
This is without any LRR tires, no weight reduction of any kind, without putting a diesel powerplant under the hood, without reducing engine size o horsepower, without shrinking the car to reduce frontal area, without synthetic oils, without CVT transmissions. Just pure aerodynamics and nothing else.
Add in all those other things combined, and you'll see about ½ the benefits of all the aerodynamic improvements at highway speeds.
So how does the thought of a gas chugging V8 musclecar achieving 35 mpg fit you? Or gas-powered 160 horsepower 4-cylinder and 6-cylinder mid size cars getting 50 mpg without hybrid drives sound? Or a 180 horsepower biodiesel mid size car getting 70 mpg sound?
That Loremo has a 12.8 square foot frontal area, and .20 coefficient of drag. This is a Cd*A of 2.56. Those things multiplied for the Loremo are about 1/3 the Cd*A of a typical car. THAT is where it got most of its fuel economy. Compare a typical car, with a Cd*A of about 8.4, or a 24 foot square area and .35 drag coefficient.
$this->bbcode_second_pass_quote('', 'I') guess one of the reasons a low drag co-efficient wasn't successful is because people don't want to have to lie down in their cars to drive them.
Myth. Low drag coefficient doesn't mean low to the ground or small. It means streamlined.You could make it smaller and thus reduce frontal area, but get a Cd of about .20, and that's not necessary.
Here are pictures of some of the cars I mentioned above, all of them taller than most sedans on the road, all of them mid to full size cars capable of seating at least 4 people:
You know why it wasn't successful? The major automakers never offered a car with low drag coefficient in the U.S., and only recently started offeribng cars with drag coefficients in the .25-.30 region. Why? A few reasons. Buckminster Fuller, who built the Dymaxion car was told outright by auto execs that they refused to produce his car on grounds that there would be less advancements that they could sell out in the future would they produce his car immediately. Notice how cars have gotten very slowly more aerodynamic as oil got more rare? The industry wants to ration out advancements as slow as possible to maximize profits on each advancement made. Thus today, we're finally seeing .25 Cd Honda Insights that match the 1933 Dymaxion and .26 Cd Toyota Prii that match the 1921 Rumpler in drag coefficient. Further, less drag = less horsepower for a given speed needed = less engine wear = less money for the Detroit(and Europe, and Japan) pricks. Today we are just now getting cars that catch up to those that smaller industries have built over 70 years ago. This is insulting to my intelligence when I hear automakers say they are doing everything to maximize fuel economy or cut drag in their present auto fleet, when many of them before have made feasible concept cars years back with literally half the drag coefficient of their current models! Our automakers cannot even match in their commercial models the body design efficiency of cars 70 years old and only top of the line models are beginning to touch the high .20 region today, with most of today's cars being around the .35 region!
The Tatra T77a was wildly successful I might add. It was one of the fastest production cars of its day, one of the only cars of its model year able to reach over 100 mph. The British secret service in Germany particularly loved them as getaway cars that also got over 25 mpg, and these were huge 5,000 pound tanks of a car! WWII killed the company.
The Alfa Romeo BAT shows that 50s style road sharks could also be efficient. Most of the cars America made in the 50s had horrible aerodynamics, but that classic style baby boomers swoon over can be somewhat duplicated in a low drag design.
That Ford Probe V concept? The same 112 horsepower common rail diesel used in Europe's Opels if shoved into that a commercial version of thing built fully of composites and with LRR tires and CVT might be able to return 120-130 mpg, 0-60 mph in about 8-9 seconds, and top speed around 170 mph. That is what an optimized for efficiency midsize or full size car with normal performance of cars today might look like today with the very best of all components and design cherry picked. Or, you could take the 20 horsepower Loremo engine, and maybe get 200 mpg, compared to the Loremo's 157 mpg. The drag reductions would make all the difference.
$this->bbcode_second_pass_quote('', 'I') think with my layman knowledge these things attribute to better fuel economy in order of importance :-