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Puzzle: 75 mph vs 80 mph

Discussion in 'Gen 3 Prius Fuel Economy' started by bwilson4web, Jun 28, 2009.

  1. bwilson4web

    bwilson4web BMW i3 and Model 3

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    This evening I drove these segments:

    1. 34 43' 04.09" N 86 38' 08.91" W - 34 37' 50.26" N 86 53' 14.42" W
      1. West: 14.3 miles, 45.7 MPG, 75 mph, 84F to 91F
      2. East: 12.3 miles, 45.6 MPG, 75 mph, 91F to 85F
    2. 34 37' 18.17" N 86 54' 01.07" W - 34 26' 55.87" N 86 54' 06.48"
      1. South: 11.4 miles, 47.0 MPG, 80 mph, 91F to 94F
      2. North: 10.1 miles, 48.4 MPG, 80 mph, 94F to 91F
    There was a wind blowing from NWN, ~15 mph, a cross-wind for the East-West segment. It was a tail and head wind for the North-South leg although I could not verify it from the road with a flag.

    Hypothesis:

    • 5F makes a significant impact on mileage
    • East-West road had higher rolling drag than North-South road
    • ZVW30 is subject to higher cross-wind drag than direct head-on
    It is a puzzle. But compared to the 39 MPG measured at 59F with the defroster on, this is much better.

    Bob Wilson
     
    #1 bwilson4web, Jun 28, 2009
  2. F8L

    F8L Protecting Habitat & AG Lands

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    I was going to go with wind first then road surface second. If the road was that much worse east to west you would have noted it. :)
     
    #2 F8L, Jun 28, 2009
  3. donee

    donee New Member

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    Hi Bob,

    Cross wind drag is greater than head on, due to an increase in apparent cross section area, combined with the shape the wind sees of the oblique vehicle has a poorer Cd. The change in drag due the reduction in in-line wind speed is small compared to these other effects, unless the apparent wind direction is well off the axis of the car. Which at high speeds requires a big difference from the actual wind direction and the direction of the car.
     
    #3 donee, Jun 28, 2009
  4. Celtic Blue

    Celtic Blue New Member

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    I agree with donee. I had a recent interstate trip in the GenII 2008 where I was getting ~52 mpg (don't remember the precise number) for ~350 miles @ 72 mph. Then I hit a 20-30 mph crosswind that was almost perfectly perpendicular. I held the same cruise speed and AC blower speed, but my mileage tanked to 46.6 mpg over the next 150 miles. (I had a fill about 10-15 miles before the winds picked up and was just returning to the previous 52 mpg range when the wind began to kick up and systematically degrade efficiency--we checked windspeeds at various points along the way via weather stations.) I had to slightly countersteer most of the time after the wind picked up as well, as evidenced by the position of the steering wheel, so rolling friction was also increased (as well as power steering consumption.)

    From an angle the car has a larger cross section, and a much worse Cd.

    p.s. On the other hand by far the best mileage my V8 Tundra has ever gotten was a full tank at about 80 mph in West Texas. There was probably a slight impact from the bed being loaded in a way that reduced eddies behind the cab, but it seemed to be more of an example of finding a sweetspot in the gearing and engine efficiency. It doesn't get very good mileage at a cruise speed 20 mph slower...which is disappointing.
     
    #4 Celtic Blue, Jun 29, 2009
  5. fuzzy1

    fuzzy1 Senior Member

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    I remember this being explicitly called out by the Palm simulator for Prius 2G fuel consumption, in some thread one of these sites (PC or CleabMPG).
     
    #5 fuzzy1, Jun 29, 2009
  6. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Feel free to double-check my work:

    Temperature effect

    density = 360.77819 * (t ** -1.00336)

    85F ~= 303K :: 1.16804614 (kg/m^3)
    95F ~= 308K :: 1.14902117 (kg/m^3) :: -1.6% lower density

    Velocity effect (v**2)

    75**2 :: 5625
    80**2 :: 6400 :: 13.7% higher drag

    Crosswind velocity effect

    15 x 75 :: 76.48 velocity (right angle 15 mph)

    75**2 :: 5625
    76.48 ** 2 :: 5850 :: 5% higher drag than no crosswind

    arccos(75/76.48) ~= 11 degrees, relative air velocity

    No single effect explains the results although we don't know the Cd with an 11 degree offset, relative wind. If the cross-wind is a significant effect, I expect wheel well covers might improve cross-wind performance. But there was nothing in the steering or handling to indicate an 11 degree offset was causing a problem.

    The weather map and temperature changes indicate a frontal boundary. It is possible that the North-South wind direction and velocity were different than the East-West runs. It is not uncommon at a frontal boundary to have a 180 degree change in wind velocity and this would explain the improved North run at 80 mph.

    I tried to fill in a test at 35 mph only to confirm there seems to be a different traction battery SOC target at lower speeds. Somewhere above 40-45 mph, the system wants to maintain a higher SOC, typically within 2 bars of full. At lower speeds, the system seems to draw down the traction battery to 2-3 bars. All of my 35 mph runs were worthless, 99 MPG, as I watched the traction battery SOC run down.

    Bob Wilson
     
    #6 bwilson4web, Jun 29, 2009
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