What's the difference between...

Discussion in 'Knowledge Base Articles Discussion' started by Kaos1, Mar 18, 2006.

  1. darelldd

    darelldd Prius is our Gas Guzzler

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    I'll have to chime in here... but not with Prius experience, unfortunately. I have lots of EV experience here, and I can tell you with confidence that using some regen when going down hill can be MORE efficient than letting it coast the whole way. Why, you ask? Air resistance. If you use regen to hold you back to a reasonable speed (say 50mph or lower) your air resistance is a small fraction of what it would be at say 90mph. At 90, you're tossing a HUGE amount of your potential energy into the wind. At 50, you're throwing a lot away in the regen conversion process, but not nearly as much as you throw to the wind at 90. I've tested this time and time again, and regen comes out ahead every time. Of course there are less extreme cases where it is not so clear-cut... and the EVs I've used are more efficient at regen.... so as usual - YMMV!

    The easy way that I've confirmed I'm right is in the roll-out at the bottom of the hill. If I let it coast the whole way down the hill, I roll out one mile. If I regen down 75% of the hill, and let it coast the last 25%, my roll-out is one mile still (I achieve the same terminal velocity in that last 25%). So all the regen I stuffed in during the first 75% of the hill is "free energy." The only difference, is that I didn't get down the hill as fast, but when I got to the same point at the end of the hill, I had more energy left than I would have if I'd coasted the whole way.
     
  2. mssmith95

    mssmith95 Michael

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    Interesting,

    Will have to test that. However, it is never a perfect situation where I have no vehicles in front of me, and I can use the same lane, and go into the hill with the exact same speed.

    Darelldd...the only question I have it that in order to maintain traffic speed I have to use some throttle...not just coast in regen....so am I using more energy having to use the throttle?
     
  3. darelldd

    darelldd Prius is our Gas Guzzler

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    Not really sure I understand the question... you'll certainly be using more energy if you use throttle instead of regen or coasting! I'm sure I'm missing the point of your question.

    As for my "test" I am able to use the same start speed, and have nobody in front of me. I drive on this same hill several times per week, and I've been doing it for many years. If I coast from a start speed of 50mph, I can get up to about 75mph at the bottom. If I maintain 50mph with regen, I end up ahead of the game.
     
  4. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    the car will go farther in neutral using less gas than driving in stealth given the same amount of energy requirement. in neutral, there is less drag on the system then in drive. to overcome even the smallest amount of drag, you need power from something. in neutral, gravity can be enough if there is a slope. if in drive, you must use energy ultimately derived from gas.

    so if on a sufficient downhill, ya, do neutral. but keep in mind, the Prius is one of most free-wheelingst (is that a word??...cwerdna... please dont yell at me!!) vehicles there is. it coasts very well so going too fast may be a concern. but in any case, it can delay the time that you need to accelerate.
     
  5. storm petrol

    storm petrol Junior Member

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    Hello, darelldd,

    I agree with your thinking regarding energy losses related to air resistance vs using regen at lower coasting speeds where friction with the air is a smaller percentage of the total energy lost due to all frictional forces. Some fraction of the difference can then be stored as electrical energy in the battery, which could then attenuate the rollout. An interesting thought experiment would involve a theoretical hill of such length that the car reaches its terminal velocity, where the frictional force of air resistance has balanced the conversion of potential to kinetic energy and there is no further acceleration from drifting. At this point it is obvious that if one were able to hold the car at an approximately equal but somewhat lower speed while charging the battery, it would be a win-win situation. This strategy will work well for certain circumstances, as it does in the example of your hill with the one mile roll-out.

    There are assumptions here that are not applicable to all circumstances, however. One assumption that is fundemental is that the battery will be capable of accomodating all of the charge that would result. Another is that the rate of conversion to electrical storage will remain constant as one uses regen to reduce velocity to a moderate pace. On a hill several miles long, where a top drifting speed of 90 mph is achievable, the Prius battery is incapable of accomodating most of the charge. I believe that as the charge level in the battery rises, the efficiency of the energy conservation realized by MG1 declines. Eventually the vehicle must invoke energy wasting strategies to prevent damage to the battery. Braking becomes mechanical. Clearly, under these circumstances, neutral drifting is once again the clear choice for maximum efficiency of transport. Close inspection of the scenario in the infrared frequencies would reveal a contrail of low frequency radiation resulting from the friction of air and car at high velocity, the clear sign of wasted potential.

    Which raises another question that I wonder about. To what extent does a clean, slippery coating of wax or Zaino reduce this frictional loss?

    It is very significant in boats.

    What do you think, darelldd?

    storm petrol
     
  6. mssmith95

    mssmith95 Michael

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    Or, even better...drafting behind an eighteen wheeler (not recommended for the faint at heart!).

    I just love the feeling of shifting into Neutral and feeling the car "release" from its constraints!

    These cars really are too much fun!
     
  7. darelldd

    darelldd Prius is our Gas Guzzler

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    Dave -

    It is quite sad that you'll never be able to drive an EV1. I thought that I'd been in aerodynamic cars before... but oh man! Coasting down hill with 0.19 Cd in a dense little car with LRR tires is something truly magical. There are hills where most other cars slow down in neutral, while the EV1 would gain 20 mph! A lead-acid rocket-ship! The Prius, by comparison feels somewhat like driving a barn door. :)
     
  8. darelldd

    darelldd Prius is our Gas Guzzler

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    Yeah, this is one of the reasons I stuck the caveat in there regarding my EVs. I could regen down hill for many, many, MANY miles before having to worry about running out of battery capacity to fill! The ONLY time that's a concern is if you live at the top of a hill, and begin your drive with a full charge (people in these situations typically just terminate their charge early, and top up on the way down the hill). After you've driven for a few miles, you simply never have to worry about regen capacity. There's always more than you can fill. Always.

    It has always bugged me that while I'm charging UP a hill, I see that my ICE is not only working for traction power, but it is also needlessly charging my battery as well... so when I start down the other side of the hill, there's nowhere to put the regen charge! Obviously a case where I wish I could do more of the thinking for the car.

    This is one of those fun things I've played with. If you go too fast, you bleed off to much PE to the wind. If you go too slow, you get almost no regen happening. The sweet spot for my Rav4EV is 35-45mph - most bang for you regeneration buck.

    Well, I joke about a fresh wax job in my Rav4EV FAQ regarding max range tips... but the difference would be insignificant at best. Boats deal in a medium that is far more dense than air, so the effect is magnified by orders of magnitude.
     
  9. darelldd

    darelldd Prius is our Gas Guzzler

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    I'm quite curious as to how or why this would be???

    Neutral doesn't disengage anything, we all agree. This is not a "clutched" system. Stealth (or "no arrows" as I like to call it to distinguish from any of the other stealths mentioned) seems to do exactly what Neutral would do as far as energy consumption. I can't see where any more drag would be introduced into the system in D (and no arrows) than in N. ???
     
  10. KMO

    KMO Member

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    I believe Neutral totally "disconnects" the two MGs. If you're feathering in D, presumably you're not going to be continuously applying exactly the right pressure to have zero torque, so there will be small current fluctuations in and out, causing small losses.
     
  11. Ray Moore

    Ray Moore Active Member

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    Ah, a topic that is near and dear to my heart. Ya'll have covered it fairly well, so I will just add that my many 60+ MPG tanks are due in large part to the technique of coasting in neutral. I limit the speed to the 60s. I have done this for 46,000 miles without mechanical issue. I have never had a safety issue occur with this method.

    I would like to see a can-view owner report the speed of MG1 at 65 mph in neutral.
     
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