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Mountain or lowlands?

Discussion in 'Roadster: Performance' started by Dragon, Feb 5, 2011.

  1. Dragon

    Dragon Lightning Green Fairytale

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    We all know that the regenerative breaking on the Tesla Roadster works very well. If you're going down a mountain street, you will recuperate a lot of energy.

    Now a question came to my mind.
    Is it possible, that driving on mountain streets will give you more range?

    To simplify I list two options:
    a) Going 100 miles with ~50 mph on a flattish street
    b) Going 50 miles with ~50 mph up a steep mountain and 50 miles back down

    Were will more range be left? Option a or b? What do you think / are your experiences?
     
  2. ChadS

    ChadS Petroleum is for sissies

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    Regen is a lot better than braking; but it can never be as good as not using energy in the first place. You will always get less out of regen than you put in to getting up a hill or up to speed. So option 'a' is definitely better.
     
  3. Eberhard

    Eberhard #421 Model S #S32

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    driving on mountain road are more efficient because first: you are riding at lower speed, second: air draft is lower because at higher level, the air is thinner
     
  4. vfx

    vfx Well-Known Member

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    Electric propulsion is not affected as carburetor fitted engines are by mountain air changes.
     
  5. TEG

    TEG TMC Moderator

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    Even fuel injected cars can also be affected. The air is less dense at altitude so 'gassers' have to flow more of it to make the same power. Your intake manifold can become a 'bottleneck'.

    On the other hand, turbochargers tend to help a lot there as they just spin a little faster to make up the difference in required flow.
     
  6. Todd Burch

    Todd Burch Electron Pilot

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    #6 Todd Burch, Feb 6, 2011
    Last edited: Feb 6, 2011
    If you assume the car is going at a constant speed (50 mph) up and down the hills and on the flat route, then the only difference between the two routes is the changing air resistance (mountain route) and the use of regen (mountain route).

    Air density at the peak of Mt. Everest is about 50% of the sea level density, and since air resistance is a linear function of density, air resistance is about 50% of the sea level value on the peak of Mt. Everest.

    The Roadster's regen is approximately 80% efficient (if I recall correctly from the Tesla engineering blog).

    So on the flat route (assume sea level), you've got a constant drag force (call this 100% drag) and no regen.

    On the mountain route (assume starting and ending at sea level), air density (and therefore drag) starts to decrease from its 100% (sea level) value to some smaller amount at the peak, then increase again on the other side on the way down when regen kicks in.

    Long story short, the answer depends on how high the mountain is and what altitude you start at...you'd have to work out the math, which I don't have the time for right now (Super Bowl's on...).

    Anyway, it's a nonlinear problem requiring differential equations and I'm pretty sure it can't be answered without more information.

    Without doing the math, it's probably better to take the flat route as ChadS says unless it's a VERY high mountain route :). As a guess, your altitude change would probably have to be 15,000-20,000 ft (starting from sea level) to be better off taking the mountain route.

    Clear as mud?
     
  7. roblab

    roblab Active Member

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    As everyone knows, anecdotal evidence is more convincing than scientific. So for what it's worth:

    I drove a RAV4EV for several years. I live on a hill. Driving up the hill took roughly the same as miles on the flat, PLUS another 10% for climbing the hill. Driving down the same hill, it took the same as miles on the flat, but you got back about 5% of regen. As the man says, it's never enough to make up for the climb.

    So whenever we went anywhere, we would just discount the ride down the hill, go the distance, and figure on another 10 miles of charge to be used up coming home. That always worked, and we always made it home.
     
  8. vfx

    vfx Well-Known Member

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    I can confirm similar numbers for our Roadster commute.
     
  9. Dragon

    Dragon Lightning Green Fairytale

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    I was thinking that maybe, since the Roadster has a highly efficient electric motor, you won't need much more energy in going upwards. A less heavy battery pack would sure work. And if you can use the regen almost all the time going downwards...
    Maybe there are some few constellations, which it's really better going on the mountain. On a mountain with a form like these?
    442px-RechtwinkligesDreieck.svg.png
     
  10. TEG

    TEG TMC Moderator

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    Just guessing, but I think that average speed is one of the biggest factors. Hills tend to slow you down on average which can make up for other losses.

    [​IMG]

    Original question suggested 50mph either way, but in practice you often are going slower over hills compared to flats.
     
  11. Dragon

    Dragon Lightning Green Fairytale

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    #11 Dragon, Feb 7, 2011
    Last edited: Feb 7, 2011
    Your mountain looks a lot nicer than mine, but the way up is too steep. :tongue:

    It should be possible to extend the range a lot by going up slow (20-30 mph) and then going down as fast as possible using only regen to brake.
     
  12. vfx

    vfx Well-Known Member

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    On my local elevations I can set the cruise control and go the exact same speed up and down the same hill.
     
  13. bolosky

    bolosky Member

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    #13 bolosky, Feb 7, 2011
    Last edited: Feb 7, 2011
    While the drive train is about 80% efficient, you pay that penalty both on the way out and on the way back in, so you're only ~64% efficient on the round trip. So, you'd need quite a bit of reduction of drag due to lower air density to make up for this.

    Or...you can use my other idea. At the top of the mountain, fill your trunk with 500kg of rocks. Extra mass means extra gravitational potential energy to extract, means extra regen. Increasing mass by 40% should more than make up for the efficiency losses in the powertrain, and then the lower drag is all gravy. Of course, if you do that enough you wind up flattening the mountain... :)
     
  14. vfx

    vfx Well-Known Member

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    HA!

    Just make sure you take just the gold down the mountain. Less of it needed and you can buy more electricity with it. :)
     
  15. Nik

    Nik Dreaming no more :-(

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    This will be a Model S thread rather than a Roadster one then? ;-)
     
  16. AndrewBissell

    AndrewBissell Member

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    Put a water tank in the trunk. Fill at the top from a reservoir you allow to fill from rainfall. Dump water at bottom. Now you have turned your electric vehicle into an unusual hydroelectric generator.
     
  17. Brian H

    Brian H Banned

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    #17 Brian H, Feb 15, 2011
    Last edited: Feb 15, 2011
    Vewy kwever! Not quite Flux Capacitor level, but good! :tongue:

    This all reminds me of the old story about a prim spinster in one of the early cars, Model T or the like, being pulled over for speeding at 35 mph in a 20 mph zone on the town's mainstreet. The cop was astonished at her reckless misbehavior, and asked her what she was thinking. She said, "Oh, officer, I'm very low on gas, so I was rushing to the station at the end of the street before I ran out!"
    :biggrin: :confused: :wink:
     
  18. zack

    zack Member

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    Only drive downhill. 8^D
     

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