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cruise control

Discussion in 'Technical' started by Kipernicus, Dec 21, 2009.

  1. Kipernicus

    Kipernicus Model S Res#P1440

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    Location:
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    First there was cruise control, constant speed
    Then there was radar adaptive CC, constant speed except when car in front is slower

    What about some sort of constant power output cruise control, where on a flat stretch of road you set the speed, but if the road starts going uphill the car can slow down (to some preset minimum) in order to maintain the original power output? There's no need to have the motor and battery work harder to maintain 75mph going up a big hill since most drivers slow down anyways.

    Is this possible? Good idea? Maybe I should post this in the suggested features section.
     
  2. shark2k

    shark2k Member

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    Except that you are supposed to maintain speed while going up hills (unless the speed limit changes of course).

    -Shark2k
     
  3. johnr

    johnr Member

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    #3 johnr, Dec 21, 2009
    Last edited: Dec 21, 2009
    I think a moderate reduction in speed (say, 5 mph or so) would be reasonable and safe. A load-adaptive cruise control such as this would be possible - just dilute the existing power requested vs speed achieved feedback loop. (Did that even make sense? Well maybe you can figure out what I meant, LOL)
     
  4. Tdave

    Tdave Member

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    What do you gain? Sure going slower reduces drag and saves some energy. But that's also true on level ground. So why not just go 5 mph slower all the time and save more energy? I don't understand the reasoning behind this idea.

    The fact that my car's cruise control can exactly maintain the same speed regardless of terrain, especially downhill through use of regen, is an advantage I'm enjoying.
     
  5. johnr

    johnr Member

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    Good point - I suppose when going at highway speeds the wind resistance typically far outweighs the resistance from gravity on hills.
     
  6. bolosky

    bolosky Member

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    Not necessarily. By my calculation driving a Roadster up a 10% grade at 60 mph is over 30 kW just to go up the hill, while the overall loss on the level at that speed is roughly 15 kW, so you'd more-or-less triple your power.

    The reason that slowing down doesn't help is that you use the same amount of energy to get to the top of the hill no matter how fast you go. If you go slower, you draw less power for a longer amount of time, for the same overall energy. (Here I'm talking about the energy to overcome gravity, not the drag and other losses, some of which depend on speed.) The formula for gravitational energy is mass times height times g (the gravitational acceleration). It doesn't depend on speed.

    On the other hand, you get all of that energy back when you go back down the hill, so it's pretty much a wash.

    I think that a constant speed cruise control is the right thing. You really wouldn't like what constant power would do, you'd speed up and slow down a lot.
     
  7. shark2k

    shark2k Member

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    I know electric motors are a lot more efficient than ICEs, that said I'll ask my question. Isn't the bold not necessarily true because when you are going faster wouldn't the motor generate more heat waste (thus using more electricity)? I'm thinking if you go slower it might take longer but you create less heat waste (wasted electricity) so you could end up using less juice. Correct me if I'm wrong, as I was a little confused while typing that out and I could very well be wrong.

    -Shark2k
     
  8. bolosky

    bolosky Member

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    There will be some additional inefficiency operating at higher power due to going up the hill faster, but it's probably pretty small. To see where the energy goes, check out the most recent (i.e., 1 year old) engineering blog post on teslamotors.com. Look down at the third graph (the one with many lines). The only one of these lines that depend on power (as opposed to speed) is the green "drivetrain" one, and some of that (the gearbox and some of the motor losses, for example) still depends on speed.

    If we take my hypothetical 10% hill at 60mph, we'll increase from 15kW to roughly 45 kW, but maintain speed. Looking at the graph above, 45kW on level corresponds to about 95 mph. Looking back at our green line, at 60 mph it's at about 80 Wh/mile and at 95 it's at 125 Wh/mile. So you can bound the extra loss by the difference: 30 Wh/mile. I'd bet that the bulk of the loss in the green line is in the spinning, speed dependent components rather than the electrical, power dependent components. Bottom line, we're talking 10-15 Wh/mile extra inefficiency due to the higher power.

    Slowing down from 60 mph to 25 mph (roughly what you'd get at 15 kW power on that hill) would save the better part of 100 Wh/mile overall, but mostly it's just because of lower drag at lower speed.
     

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