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Click & Clack on B-mode...pretty close.

Discussion in 'Gen 2 Prius Main Forum' started by efusco, Jan 7, 2009.

  1. efusco

    efusco Moderator Emeritus
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    http://www.mercurynews.com/weekenddrive/ci_11355768?nclick_check=1

    Q: Dear Tom and Ray: We recently bought a 2008 Prius, and just love it, love it, love it. But we're having a bit of a discussion about how to use the B on the gearshift. I say it's like a downshift, and you should use it only temporarily, while going down a steep hill or coming to a stop. You use it to save the brakes, then return to D. My husband thinks it's fine to drive most of the time in the B mode, since it regenerates electricity and recharges the battery. Who's correct? And if B is the gear to use all the time, why didn't Toyota just call it D and forget the separate braking option? Thanks for your help!
    Gayle

    TOM: Gayle, you're 100 percent right, and your husband, unfortunately, has his head up his brake line.

    RAY: When you shift into the B mode, you increase the drag created by the regenerative portion of the brakes. And while that extra friction does produce some electricity to help recharge the battery, the B mode's primary purpose is to simulate downshifting, or engine braking, when you're going down a long, steep hill.

    TOM: The B mode's secondary purpose is to give guys something to fiddle with while they're driving. And after that comes creating more electricity to recharge the battery.

    RAY: In any car, hybrid or not, when you go down a long, steep hill, you run the risk of overusing your brakes. That makes the brakes overheat, which leads to brake fade (when the brakes become less effective). Even worse, if the brakes overheat badly enough, the brake fluid can boil, and cause the brakes to fail completely. So it's always recommended that you downshift in those circumstances, rather than ride the brakes, to keep the car at a safe, reasonable speed.

    TOM: But since the Prius has an unusual propulsion system and transmission, they've essentially faked a downshift mode for just that purpose.

    RAY: So unless you're driving under conditions where you specifically need the equivalent of engine braking, just leave it in D, Gayle. But try not to rub it in too much. Remember, husbands have extremely fragile egos.
     
    #1 efusco, Jan 7, 2009
  2. JimboK

    JimboK One owner, low mileage

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    Someone reading that might be led to believe that somehow one can magically create energy using B and, therefore, improve fuel economy with it. It would have been nice if they had explicitly dispelled that notion.
     
    #2 JimboK, Jan 7, 2009
  3. patsparks

    patsparks An Aussie perspective

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    You're right Jim. At least they knew something about it even if they messed up the science a bit. After all is said and done the advice was right, use it only on long descents but how easy would it have been to say it forced the wheels to spin the engine just like downshifting, which dumps energy through the engine. Light braking would achieve more regen.
     
    #3 patsparks, Jan 7, 2009
  4. qbee42

    qbee42 My other car is a boat

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    Agreed. Their recommendation was correct, even though their information was flawed.

    Tom
     
    #4 qbee42, Jan 7, 2009
  5. bwilson4web

    bwilson4web BMW i3 and Model 3

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    There is a subtle, battery heating effect they are not aware of that recommends using "B" when descending large hills at high speed. Using "B" does spin the engine a little more to reduce the charging load on the battery and subsequent heating. However, it is a very subtle effect, ~1C for a 500 ft. hill, and not something to 'set your hair on fire' about.

    My comment is more along the lines of 'polishing the musket balls' and I agree with their recommendation.

    Bob Wilson
     
    #5 bwilson4web, Jan 7, 2009
  6. patsparks

    patsparks An Aussie perspective

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    Hey Bob, I haven't got a scan gauge yet but when I come down my local hill if the battery has 1 bar to "full" I use B to slow the rate of charge but if I'm a little late and the battery goes up one more bar the engine spins very fast, so I can hear it spinning up and then spins faster again if I brake lightly.
     
    #6 patsparks, Jan 7, 2009
  7. qbee42

    qbee42 My other car is a boat

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    Correct. When you press on the brake, you are telling the control system to slow down. The control system in turn spins the engine faster to burn off more energy. I find it amusing when you do the opposite on a long downhill: press the accelerator and hear the engine slow down.

    Tom
     
    #7 qbee42, Jan 7, 2009
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