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Front air dam on car for mileage?

  1. Aug 19, 2005 #1
    do front air dams help to increase gas mileage a bit?
     
  2. jcsd
  3. Aug 19, 2005 #2

    NateTG

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    They can, depending on the car since they affect the aerodynamics.
     
  4. Aug 20, 2005 #3
    I don't know how much the air dam effects mileage (though I suspect they do), but I can say that they effect handling. In the past, I have had several Corvairs, and (despite a certain lawyer, who doesn't drive) they are very quick responding, great handling cars - - - as long as the air dam is in place. Without it though, they get squirrely at speed in cross-winds.

    KM

    PS. Though rear-engine cars are great in the hands of those who understand the feel and intricacies of driving, I wouldn't put one in the hands of a driving 'neophyte'. Oversteer will allow you to do great things handling-wise with a car, but for the uninitiated, it will also allow the driver to get into trouble. That's why front-wheel drive cars are so popular with manufacturers.
     
  5. Aug 20, 2005 #4
    Air resistance is proportional to velocity squared. Therefore, if you drive at speeds of about over 100miles/hour, then aerodynamics takes on an effect in a cars preformance and gas mileage, but at lower speeds, the effect is neglegable.

    Regards,

    Nenad
     
  6. Aug 20, 2005 #5

    Stingray

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    Where did 100 mph come from? Drag actually does have a significant impact on fuel consumption at regular highway speeds (~70 mph). Total drag might consume up to ~20 hp at that speed, which is significant.
     
  7. Aug 20, 2005 #6

    Hurkyl

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    I hope nobody's suggesting breaking any speed limits, especially by significant amounts... :mad:
     
  8. Aug 20, 2005 #7

    Clausius2

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    I don't think so. In an upper limit, the power lost in aerodyn drag can be estimated as:

    [tex]W=0.5\rho C_x S U^3=0.5\cdot 1.2 Kg/m^3 \cdot 0.35\cdot 1.5 m^2 \cdot (28 m/s)^3 \sim 7 kW \sim 9 HP[/tex]

    at 100Km/h, in a car with frontal area of 1.5 m2, standard air conditions, and aerodynamic drag coefficient 0.35 (medium).

    Actually, the vehicle will loose a 8% or less of useful energy wasted in aerodynamic friction. To say the truth, automobile manufacturers don't worry too much about the aerodynamics of low powered cars. The external shape of this kind of cars are dominated by how it looks more than how it aerodynamically works.
     
  9. Aug 20, 2005 #8

    Stingray

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    Clausius,
    While I agree with your calculation, I postulated 70 mph=110 kph, which is actually fairly low for most American highways (despite being above most speed limits). Also, cars popular here tend to be a bit bigger than 1.5 m^2. 20 hp is not unreasonable for a sedan if the parameters are changed slightly.

    Your claim about 8% losses is completely incorrect, though. At your estimate of 9 hp lost to drag, that would imply that it requires 110 hp to go 100 kph, which it clearly does not. Manufacturers do worry a lot about aerodynamics. Even the horribly underpowered Prius and Insight have highly optimized shapes. Cars haven't been designed on looks (and packaging) alone for more than 20 years now.

    Edit: If you pulled the 8% statement from some website, you might be misunderstanding the wording. As I remember, that's about right for the fraction of aerodynamic drag that arises due to skin friction rather than pressure effects.
     
    Last edited: Aug 20, 2005
  10. Aug 20, 2005 #9

    Clausius2

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    Well, if you wanna change the parameters, I will change the whole formula, because it is completely innacurate. The [tex]C_x[/tex] factor is a rough simplification of the car's aerodynamic. It is based on averaged experimental results, on how it is measured the coefficient. Also, it is false that the aerodynamic drag is proportional to the square of the speed.

    Well, this is your personal opinion. I don't think so. I don't think that Ford thought too much in aerodynamics when designing the Ford Focus. On the other hand, i do think that Ferrari thinks of aerodynamics indeed. But the purposes of both cars are different.

    Also, i didn't understand what you meant in your edit.
     
  11. Aug 21, 2005 #10

    Stingray

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    Really? I thought it was reasonably accurate (~10%) over 50 mph or so. In any case, my only point was that a large fraction of the power needed to maintain highway speeds is lost to drag.

    If you look at the drag coefficients of even the lowliest economy cars over the past thirty years, you'll see a steady downward trend. I don't believe that's accidental.

    In the particular case of the Honda Insight - a car whose sole existence is to optimize fuel economy - the engineers spent a great deal of effort getting a 0.25 drag coefficient. As far as I know, that's the lowest of any production car.

    Also, the Ford Focus has a better drag coefficient than any Ferrari (but they're more concerned with lift than drag). It's actually as good as older high-end cars that are known to have had extensive aerodynamic development (e.g. MB E-class).

    Drag can be split up into two parts. One is roughly independent of the object's length (in the direction of the flow), while the other is not. This latter effect is directly due to viscosity, and as I remember, it is ~10% of the total drag in most cases.
     
  12. Aug 21, 2005 #11

    Clausius2

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    How can you prove that?



    Stop writting about aerodynamics. This statement proves you have NO IDEA of what are you talking about. I am not going to say anything more, because you have disqualified yourself in this stuff.
     
  13. Aug 21, 2005 #12

    Stingray

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    Manufacturers usually give their cars' specifications. Play with google a bit, and you'll get that the Focus has a drag coefficient of 0.32. The 360, F430, 550, and 575M Coupes are 0.33 (convertibles are higher). The Enzo is 0.36. You can look up older models if you want, but I doubt you'll find anything lower. I can give specific links if you want.

    You don't have to act so rude. I may be wrong about the 10% factor for cars, but skin friction is certainly a real effect. If I said something else incorrect, I'd actually like to know.
     
    Last edited: Aug 21, 2005
  14. Aug 22, 2005 #13

    Cliff_J

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    Clausius2 - normally you have very informative posts but I do believe you have a mistake or poorly phrased statement above. If 9HP is the aero drag estimate and this is 8% of the overall friction then the overall losses would be 112.5HP at 100KpH, a very high number. Did you happen to intend to say 80% of the overall friction, for a total of 11.25HP at 100KpH, a more reasonalbe sounding figure?
     
  15. Aug 22, 2005 #14

    Clausius2

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    No I didn't mean anything of what you said. I meant an 8% of the TOTAL available engine power is lost in aerodynamic drag at 100km/h. I could have said an 8.1%, an 8.5%, a 10%, but never a 20%, a 30% or so, which I think they do are very high numbers.
     
  16. Aug 22, 2005 #15

    FredGarvin

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    They really should be stating at what Reynolds number those Cd's are calculated at. Without some kind of reference, the number is not quite meaningless, but leaves a lot open to question. Chances are it's not an apples-to-apples comparison
     
  17. Aug 22, 2005 #16

    Stingray

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    That's true. Comparing them too strictly isn't quite fair. Still the Reynolds number for all interesting speeds is extremely high (at least a few times [tex]10^6[/tex]). If you look at typical graphs of drag coefficients versus Reynolds number, there's often a severe dip around [tex]5 \times 10^{5}[/tex], and then the coefficient increases again and becomes nearly constant for a long time. Of course that dip moves around depending on the various things (e.g. surface roughness), but I recall it occuring at very low speeds for most cars.

    The dip might still be important near 100 kph (I imagine this would be a nice goal for the engineers to place it where people drive a lot), but I'm going to assume that manufacturers measure things at higher speeds where the drag coefficient is approximately constant. As evidence, calculated and measured top speeds (when its drag-limited) usually come close, which wouldn't happen if only the lowest possible drag coefficient was given.
     
  18. Aug 22, 2005 #17

    Stingray

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    Huh? Common cars range between 100-400 hp in their optimal gear, and much less in a realistic gear. Also, fuel economy (the original point of this thread) is terrible at full throttle. I don't understand your point.
     
  19. Aug 22, 2005 #18
    Stingray,

    When C2 referred to total available power, he probably meant gross vs net. "Full throttle" would not make any sense in the context.

    So, 8% of total power = 8% before the inefficiency of the transmission, inefficiancy of the tires, etc., are factored in.
     
  20. Aug 24, 2005 #19
    If the cars drag coeffecient is high would that also lead to a high downforce?
     
  21. Aug 24, 2005 #20

    Stingray

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    Not necessarily, but optimizing downforce usually has a negative effect on drag. In F1 cars, for example, drag coefficients near 1 are not uncommon. This is done in order to acheive lift coefficients around -3.

    There are, however, very few street cars that generate downforce. Almost all actually lift at high speed. A few modern Ferraris and other exotics are the exceptions (still, their lift to drag ratio is very small).

    Eliminating lift is generally much more important than reducing drag for high-powered sports cars, so this design goal is probably the reason for their mediocre drag coefficients.

    Aerodynamics also affects engine and brake cooling, and these systems must be very efficient on exotic sports cars. Again, getting this right is probably more important than lowering drag.
     
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