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Drag HP vs mph

Discussion in 'Gen 4 Prius Technical Discussion' started by bwilson4web, Mar 18, 2017.

  1. bwilson4web

    bwilson4web BMW i3 and Model 3

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    This is a duplicate of a post in the Prius Prime forum earlier today so it may 'disappear'. If there were a way to make one post show up on two forums, this would be a good candidate because it includes the Gen-4 Prius. Just I wanted to share it with Gen-4 owners who might not be following the Plug-in forum. I am open to suggestions on how to share this with the community.

    Much to my surprise, the EPA test car data was updated in March: Data on Cars used for Testing Fuel Economy | Compliance and Fuel Economy Data for Vehicles and Engines | US EPA

    The March data includes some interesting car so I graphed their drag HP vs speed:
    [​IMG]
    • PRIUS Eco excellent, low drag - put the Prime one-way clutch in; replace the lead-acid battery with something lighter, and; add 2 kW for a deeper LiON battery capacity and it would put the Prime to shame.
    • IONIQ - excellent drag profile
    • Prius, Volt, Prime, and Camry Hybrid - all show good drag curves in effect lying on top of each other.
    Bob Wilson
     
    #1 bwilson4web, Mar 18, 2017
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  2. ATHiker

    ATHiker Senior Member

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    Not sure exactly what the graph is saying.

    Google tells me that HP loss due to aerodynamic drag is approximately:

    [ Cd * A * (V3) ] / 150,000

    Cd     = drag Coefficient
    A = frontal area in sq. ft
    V = Velocity (speed) in MPH

    Doesn't this suggest the Eco has a lower drag coefficient than the othe Prius Models (perhaps due to the lack of the rear window wiper)?

    0r do you think things like the clutch, battery and vehicle weight factor into the EPAs equation? If that is the case do you know thier formula?
     
    #2 ATHiker, Mar 18, 2017
    Last edited: Mar 18, 2017
  3. bwilson4web

    bwilson4web BMW i3 and Model 3

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    The EPA requires a set of roll-down tests that are reduced to three coefficients. There is a simple formula that converts them into the power required at any given speed:
    • Target Coef A (lbf)
    • Target Coef B (lbf/mph)
    • Target Coef C (lbf/mph**2)
    • v - mph
    • Drag_HP = (v*(Coef_A+(v*Coef_B)+(v*v*Coef_C)))/375
    The vehicle drag is the sum of aerodynamic, linear, and a static drag force which is why getting them from the USA "Test Car Database" is so useful. All of the forces must be accounted for and the roll-down coefficients have been very useful in predicting vehicle efficiency.
    Bob Wilson
     
    #3 bwilson4web, Mar 18, 2017
  4. ATHiker

    ATHiker Senior Member

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    Still confused.

    Does the kind of clutch and battery factor into what is showing on the graph?

    I would have thought the vertical axis on the graph would have been independent of all that -- in the same way it would need to be independent of different tires offered on the Eco.

    When I think of aerodynamic differences between the Eco and the other Prius Models, I just think of that rear wiper.

    If the vertical axis is to include more than aerodynamic drag, wouldn't the different tires on the Eco likely be the biggest difference between the different Prius Models.
     
    #4 ATHiker, Mar 18, 2017
    Last edited: Mar 18, 2017
  5. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

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    The Eco does not have different tires. It is lackiing the spare tire which makes it lighter.
    At one point I believe the Ecopia 422 Plus was rumored to be the Eco tire. That what my Liftback has. There are a variety of 15 inch tires used. Somebody here started compiling a list.
     
    #5 Prodigyplace, Mar 20, 2017
  6. bwilson4web

    bwilson4web BMW i3 and Model 3

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    I'm not too terribly worried about trying to map the three coefficients into specific drag forces. For example, I've seen the second term come in as a negative number. But that is a by-product of a methodology that uses roll-down metrics to derive the coefficients. It is a math trick that generates a quadratic equation 'curve fit' to the data.

    If you've used the trend line function in excel with a set of data, you may have noticed it can create parameters for up to a six degree polynomial. Within the data limits, a usable formula can be generated that fits the existing data points and give useful, intermediate values. The problem is understanding the data range limitations.

    Bob Wilson
     
    #6 bwilson4web, Mar 20, 2017
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  7. RCO

    RCO Senior Member

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    Please forgive my meagre understanding of aerodynamics, but some of the things discussed, eg weight, tyres etc. I suspect they affect friction drag and have no effect on the aerodynamics.
     
    #7 RCO, Mar 20, 2017
  8. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

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    Try dragging a 1 kg weight vs a 10 kg weight. All else being the same, the force (HP) needed to get to the same speed is more.
     
    #8 Prodigyplace, Mar 20, 2017
  9. RCO

    RCO Senior Member

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    Like I said that is frictional drag and inertia, not aerodynamic resistance.;)
     
    #9 RCO, Mar 20, 2017
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  10. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

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    (looks at thread title)
     
    #10 Prodigyplace, Mar 20, 2017
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  11. ATHiker

    ATHiker Senior Member

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    This is my take away:

    Someone measured something and then wrote down 3 numbers
    1. electric dynamometer target coefficient a
    2. electric dynamometer target coefficient b
    3. electric dynamometer target coefficient c
    These numbers get put into an equation showing that two very different vehicles perform almost exactly alike (regarding drag, anyway), but two very similar vehicles do not. The variance is assumed to be significant, and a function of the car's design.

    I think I am missing something fundamental.

    Google is not my friend tonight, so I am going out to Pep Boys to get an electric dynamometer gauge. If they don't have one, I am going to abandon this thread and go back to the "is black or white is a better color" thread.

    To paraphrase Dirty Harry, I man has to know his limits .
     
    #11 ATHiker, Mar 20, 2017
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