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Shifting to "B" mode when stopping

Discussion in 'Gen 2 Prius Main Forum' started by mdarby, May 12, 2006.

  1. mdarby

    mdarby Junior Member

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    I try to coast to a stop whenever possible, but when a little extra stopping power is needed, is shifting to "B" mode a smart strategy? Is a Prius' current momentum converted to battery charging power with the same efficiency as if one coasted in "D" mode?

    [EDIT] I just saw a FAQ on this, which suggests to me that at lower speeds, "D" and "B" equivalently convert momentum to battery power, subject to an upper limit on the rate of charging. Since "B" is a more aggressive stop, it's more likely that regenerative braking in "B" will exceed that threshhold and thus "waste" potential regeneration. That is a potential downside with "B". But I am comparing braking to a full stop in "B" versus slowing in "D" and having to use friction brakes.

    So, if I am stopping from about 45 mph, and can do so completely with "B" regerantive braking, I assume that's more efficient than using "D" and not slowing completely thus forcing use of friction brakes.

    Thanks,
    Michael
     
    #1 mdarby, May 12, 2006
  2. finman

    finman Senior Member

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    My 2 cents worth: just drive and brake normally.

    Use B mode only for long downhills where the battery fills to max and friction brakes HAVE to be used. Engaging B mode then uses engine compression to slow u, thus limiting use of friction brakes. Any other situation and regen will be reduced, since engine braking does NOT recharge the battery. B)
     
    #2 finman, May 12, 2006
  3. Presto

    Presto Has his homepage set to PC

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    Stopping with the brakes is far more efficient (in terms of regen) than using the B mode. B mode is like 1 or L on other cars as it uses the ICE (with no fuel consumption) to assist in stopping. Don't worry about friction brakes kicking in. They won't engage unless you pratically stand on the brake pedal. For comparison, when using B mode to stop, it will maybe generate 20-30 amps while regular regen braking will see 70-90 amps of regen.
     
    #3 Presto, May 12, 2006
  4. qbee42

    qbee42 My other car is a boat

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    B mode throws away power by spinning the ICE in a high drag state. Don't use B unless you need to dump power, such as a long, steep downhill.

    Tom
     
    #4 qbee42, May 12, 2006
  5. mdarby

    mdarby Junior Member

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    Thanks to all, that's amazingly concise and clear.

    I look forward to getting better mileage!
     
    #5 mdarby, May 12, 2006
  6. jamarimutt

    jamarimutt New Member

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    You can shift to B in order to slow down when apporaching a light. No one has shown that it'll do any harm. If you shift to B, after you stop shift to N and then to D, and 8 out of 10 times the ICE will immediately shut down.

    I don't buy all the talk about the friction brakes not working except in emergency stops or below 8 mph. I belive that they work much more frequently, just by listening to the typical pad-rotor contact noise when braking.
     
    #6 jamarimutt, May 12, 2006
  7. seasalsa

    seasalsa Active Member

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    <div class='quotetop'>QUOTE(jamarimutt @ May 12 2006, 04:55 PM) [snapback]254427[/snapback]</div>
    How do you explain the cold rotors after miles of driving?

    Here is what Toyota has to say on the matter.
    "Paul Williamsen, curriculum development manager of Toyota Motor Sales, said the regenerative braking is the greatest contributor to the HSD’s fuel-efficiency gains. Williamsen, who manages technical training for Toyota dealers and technicians, said the new system exploits the regenerative braking so much better that the conventional brake pads and shoes may never come in contact with their discs and drums under normal driving conditions. "
     
    #7 seasalsa, May 12, 2006
  8. ken1784

    ken1784 SuperMID designer

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    <div class='quotetop'>QUOTE(mdarby @ May 13 2006, 06:33 AM) [snapback]254356[/snapback]</div>
    Just a moment, please. :)

    No one mentioned the speed when to use the B mode.
    Using B mode above 35km/h(22MPH), the ICE spins, but the ICE does not spin when you switch to B below 35km/h.
    Shifting to B below 35km/h is very common improving regenration technique among Japanese mileage enthusiast.

    Ken@Japan
     
    #8 ken1784, May 12, 2006
  9. qbee42

    qbee42 My other car is a boat

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    <div class='quotetop'>QUOTE(ken1784 @ May 12 2006, 11:02 PM) [snapback]254535[/snapback]</div>
    I recall reading that technique is a hold-over from the Gen I Prius, which was not nearly as good about regen braking as the current models. Correct me if I am wrong.

    Tom
     
    #9 qbee42, May 12, 2006
  10. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(ken1784 @ May 12 2006, 10:02 PM) [snapback]254535[/snapback]</div>


    If the ICE is still turning B-mode will make it continue to turn even below 20mph. It will force it to turn at any speed above 20 even if it was off.



    I question the utility of B-mode below 20 with the ICE not spinning. I don't think it does any better of a job of regeneration than careful application of the brakes (even in Japan!).



    Now, my dirty little secret.... I sometimes use B-mode as I'm approaching a stop. As one's speed drops below ~10mph with the foot off the accelerator a little bit of power (1-2 amps) will be drained to sustain the forward roll. If, OTOH, at about 10mph you drop into B-mode you'll instead see a tiny bit of regen!..Again, 1-2 amps only, and only until you hit 7mph where you'll see a drain again until you stop. This probably matters not at all...but I'm a nerd.
     
    #10 efusco, May 12, 2006
  11. Presto

    Presto Has his homepage set to PC

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    <div class='quotetop'>QUOTE(jamarimutt @ May 12 2006, 04:55 PM) [snapback]254427[/snapback]</div>
    There has been extensive testing done on this. I remember there was a page that had gobs of info on the Prius and it's HSD system. It was proven that that the gen-2 Prius didn't apply friction brakes until the brake pedal was almost pressed all the way down. The author was a hardcore Prius geek and his brake pads showed almost no wear after many miles on the odometer. Perhaps someone here would be kind enough to post a link to that site as I have lost it.


    EDIT: found the link.... here it is, folks: http://privatenrg.com/#100kBrakePads
     
    #11 Presto, May 13, 2006
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