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Wind speed meter?

Discussion in 'Technical' started by ratsbew, Mar 3, 2012.

  1. ratsbew

    ratsbew Member

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    Since aerodynamic drag is the largest force acting against a car at highway speeds, why don't electric cars have an anemometer to tell what the speed is through the air? This would certainly increase the accuracy of range calculations.

    A 20mph tailwind would give many extra miles while a 20mph headwind will take away a ton of miles.

    A simple wind speed turbine in the grille would probably be enough for the calculations.
     
  2. Yggdrasill

    Yggdrasill Active Member

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    I doubt it would improve estimates significantly. If you're driving at a constant speed in the same direction, the estimates based on the energy consumed over the last X seconds would be more than enough to get a good estimate. If you're driving at widely varying speeds and/or directions, the wind speed meter would be useless.
     
  3. dhrivnak

    dhrivnak Active Member

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    I do think this could help. With my Roadster I frequently drive 110 miles to Knoxville along the same route and average a consistent 235 W/Mile. But one day, a day my wife joined me, there was a strong headwind and rain and I average 285 W/Mile a 20% increase. With the cruise control I can keep the speed constant and there is little traffic along the route so I am convinced it is the weather that made such a difference. We were really sweating making back home. There are no charging stations along the route.
     
  4. cinergi

    cinergi Active Member

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    In fact it doesn't provide any more information than what's already available -- the power being consumed at a given speed. If you have a tail wind, it'll be less -- which the computer can see and will factor into its range calculations.
     
  5. Yggdrasill

    Yggdrasill Active Member

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    One thing the car could take into account is the weather. If the car could access a weather database, it could be able to assess the driving conditions and adjust the estimates accordingly. This might be most effective when setting a specific destination on the GPS, because then the car would know how much wind it would receive from which direction and at what speed. It could know the approximate temperature of the road along the entire route. It could know where the road would be wet and where it would be dry. And so on.
     
  6. AnOutsider

    AnOutsider S532 # XS27

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    I mean no offense to your wife, but could it also have been the extra weight in the car?
     
  7. bonnie

    bonnie Oil is for sissies.

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    Sooooo many ways to have worded that better ...
     
  8. AnOutsider

    AnOutsider S532 # XS27

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    Lots of retyping before I posted, but yeah....
     
  9. Doug_G

    Doug_G Lead Moderator

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    Not likely at highway speeds. You might get slightly more rolling resistance, but the mere 90 lbs :wink: will be insignificant compared to the 2700 lb vehicle not to mention the much heavier driver (right?). The weight might matter a little bit for acceleration, but at a constant speed it won't matter at all.

    At high speeds the aerodynamic drag becomes a really big factor, so I could see how a headwind could significantly affect your range. Even a big crosswind.

    Also I've done road trips with my wife in the car, and the spreadsheet still matched our performance very well (slightly conservative in fact).

    Next time you see the Wh/mile that high, slow down a little. Your return trip will be a lot less tense.
     
  10. vfx

    vfx Well-Known Member

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    Please teach us the way....
     
  11. bonnie

    bonnie Oil is for sissies.

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    Doug_G did it well, above. Or perhaps the original message could have said 'the addition of a passenger' instead of 'no offense, but ...'

    You have to bring flowers home a lot, don't you?
     
  12. AnOutsider

    AnOutsider S532 # XS27

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    Nope, the great thing about my wife is I can be direct (even when it comes off as being an a$$) and she's fine with it :)

    ..not that I was trying to be one in that post mind you, it could have been a male passenger and I would've posited the same. Thanks for the clarification though Doug, that makes sense.
     
  13. vfx

    vfx Well-Known Member

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    Never. I tell her I love her every day for putting a down payment on a Tesla.
     
  14. bonnie

    bonnie Oil is for sissies.

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    Ahhhh ... smart man.
     
  15. Robert.Boston

    Robert.Boston Model S VIN P01536

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    The truly smart range calculator will interact with the route set on the GPS to figure out the terrain looking forward and predict the SOC at your declared destination. If the answer is negative, the car will suggest strategies, including slowing down and intermediate charging points.
     
  16. PV4EV

    PV4EV Member

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    You make an interesting point here !


    Above about 60mph, the dominant energy drain on the car will be due to aerodynamic drag. Rolling resistance is usually a constant irrelevant of road speed. A small percentage change in the weight of entire vehicle will have little effect on the energy required to push the car along at a steady speed.

    However, you mentioned that there was a strong head wind and rain. A 20 mph headwind will be like driving 15-20mph faster and will increase energy drain.

    Rain on the road will effect rolling resistance and reduce the friction/grip of the tyres on the road, BUT there will be a significant amount of energy needed to constantly displace the water on the road as well as all the water going around the treads and being flung off behind the tyre and up into the inner wheel arch.

    If there is half an inch of water on the road each tyre will be constantly displacing that weight of water multiplied by the contact patch and vehicle speed. This adds up to considerable amount of energy. I don’t have the equations to hand, but from what I remember seeing in studies on this, rain on the road can increase energy usage by 15 to 20% or more, dependant on speed.

    So loosely speaking, head wind and rain combined could dramatically increase your energy drain by 30-40%


    I've also seen debates on other weather aspects, such as the marginal effect of absorbing all the energy of large rain droplets hitting the car at speed multiplied by its frontal area, and so on.
     
  17. ElSupreme

    ElSupreme Model S 03182

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    From someone who has raced bicycles headwind and rain make a HUGE difference. I just don't know how hard it is to 'measure' these phenomena and adjust rain for it. I mean a dead head wind is a lot different than a slight crosswind. A lot of time the crosswind seems worse. I happen to think it is way to many variables, and stupidly complex fluid flow equations (some of which are NOT SOLVABLE) to get any real calculations out. Maybe a warning notification on the dash "Wind/Rain predicted on route range may be affected." that you can turn on or off.
     
  18. dhrivnak

    dhrivnak Active Member

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    Knoxville, 110 miles away has many EV events I try and support. On that day I was going to Earth Fair to show the car and the event coordinators went out of their way to ensure I could charge on 110V when I arrived and during the event. But having to drive 220 miles is a bit of a stretch for my Roadster under the best of conditions. We checked the radar and new we would have some rain but we also had a BAD headwind of a good 25mph. While I drive a modest 54mph with the headwind I found my watts/mile was a good 20% higher than before. I really could not slow down as then I would be late to the event. So when I arrived I had used 31 kwh well above my previous two trips of 25.5 kwh. And since I had to get back home that night another 110 miles I was sweating it as I knew the lights would further diminish range and temps in the 40's meant a little heat would be nice especially after sitting in the cool wind all day.

    I had a long run with extension cords during the event so rather than charge at 120V I was charging at 105V so I was not charging very fast. We did make it home but with the car saying the battery was so low it could not calculate the remaining range. So the trip and event did little to endear my wife to the car. And I have to agree at 150lbs I have a hard time seeing where that extra weight made much of a difference. And as luck would have it, the winds died down by the time we had to go home so I could not benefit from a tail wind.
     
  19. NigelM

    NigelM Recovering Member

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    Ask any golfer and they will tell you that a strong headwind will significantly reduce distance. Oddly, a tailwind never helps quite as much?
     
  20. Robert.Boston

    Robert.Boston Model S VIN P01536

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    The asymmetry is consistent with physics. Suppose you're driving at 50mph (car or golf ball). With a 20mph wind at your face, the effective speed is 70mph; with the wind at your back, it's 30mph. Since drag forces increase approximately with the square of the speed, that's a lot bigger penalty into the wind than you'll get from the tailwind.
     

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