Using B

Discussion in 'Gen 3 Prius Technical Discussion' started by DaYooper, Jun 14, 2011.

  1. DaYooper

    DaYooper Member

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    So I just read on wikipedia that using B to reduce speed doesn't recover energy? And that the use of B may promote excessive gear wear? Is this true? I have become so accustomed to using B like down-shifting because according to the HSI I am creating more charge. I'm going to have to look at the manual.
     
  2. nerfer

    nerfer A young senior member

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    Yes, if you do a search on PriusChat, you'll see that B is used for engine braking (like for long downhills so you don't burn up the friction brakes on the wheel). I believe this means the electric motor/generators can't be used to recharge the battery (I'm not real sure how the Gen III differs on this from the Gen II, esp. regarding the display).

    You're better off just taking the foot off the accelerator entirely, lightly braking if needed. I would bet you'd see an increase in your mpg. For sure, you betcha, ya.
     
  3. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    B Mode adds engine braking to regen. So more energy is wasted, than pure regen, as engine braking does not charge any battery. B Mode is useful when descending long inclines with more than a 600 foot drop.

    You certainly MAY use B mode any time, it is just less fuel efficient. Both the Motor/Generators and the Internal Combustion Engine are on the same gearset, I do not see how mach more gear wear would occur using the ICE over MG1, MG2 may have marginally less wear.
     
  4. Kentwang

    Kentwang Junior Member

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    I was told by a local Toyota sell man to use B when you try to slow down or getting off the freeway, insteal of the brake. So may be he was wrong.
     
  5. dtuite

    dtuite Silverback

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    As I understand it, once the battery state of charge reaches 80%, (SoC is only allowed to vary between 20% and 80% to maximize battery life.) then the car spins the ICE without injecting fuel (B mode) anyway. So if you started in D at the top of a long steep hill, you'd be in B (effectively) anyway by the time you reached the bottom. (B is for what Nerfer said.) I'm not sure what "gear" would be involved. (But I'm sure somebody will straighten me out. On average, I'm only 80% right on my posts.)
     
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  6. Teakwood

    Teakwood Member

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    As has been said above, "B" adds to regen. :)
    I have never heard that it promotes excessive wear; that concept sounds like pure brain-fart.:rolleyes:
     
  7. phoenixgreg

    phoenixgreg Senior member

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    I don't think B braking gets you any advantage in city driving. I ride with a friend who does this a lot just because he likes "down-shifting". In fact, a disadvantage is that you are slowing down rapidly with no brake lights coming on so you stand the chance of being rear-ended too (I pointed this out to my friend).

    As others have said, B is good for long rather steep descents, but if you use your cruise control, it will effectively give you "B" style braking to keep you near the setting. I've seen the SOC continue to charge when this happens to a point where I believe 80%+ has been reached (all bars in SOC lit). I'm thinking the system has a way to "bleed off" more than 80% somehow as the descent continues.
     
  8. adamace1

    adamace1 Senior Member

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    B mode dosen't make sense to me. if i go down a long hill and fill the battery the car will use the ICE as a brake on it's own without me switching modes.

    If you useng B mode all i think your doing is turning energy that could have been put into the battery into Engine wear and tear and reducing your mpg.
     
  9. DaYooper

    DaYooper Member

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    So B mode its not using Mg1 to generate power to recharge the batterry? Is that correct?
     
  10. dancergene

    dancergene New Member

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    according to my scanguage when i use B the amps to the traction battery increase a lot. i live in the hills so i get to experiment quite a bit with the brakes and B.
     
  11. richard schumacher

    richard schumacher shortbus driver

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    No, "B" doesn't damage anything. Yes, "B" recovers some energy; no, not as much as just plain old braking in "D" because the battery often can't take it that fast, and you end up spinning the engine instead. "B" is to keep the brakes from boiling. Unless you live on Pike's Peak or the like if you use "B" in everyday driving you're just wasting gas. Try to find something else for your hands to do while driving :_>
     
  12. fuzzy1

    fuzzy1 Senior Member

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    MG2 is generating electric power. Some of that recharges the battery, if it is not full (8 bars full, not the wider range set aside for aging and other factors). The rest spins the engine as a brake.
    Some hills are too steep for D mode, causing the car to roll too fast, so B mode provides stronger braking force.

    Some hills are too steep for B mode, requiring friction brakes.

    Some hills are too steep and long for sustained friction braking, requiring brake cooling stops. See 'Pikes Peak'.

    While both modes will end up using the same engine braking method, B applies more of it than does D. While I'm using D for probably 2-3% slopes, the 1000-5000 foot descents on the 5-7% slopes start with B mode.

    The difference between this and a nonhybrid is that downshifting with a traditional large engine provides much stronger braking than can the Prius' tiny engine. None of them fire the brake lights.
    Unless the battery is nearly filled, this is more like light pedal braking without the brake light, because CC doesn't spin up the engine as a brake. Only when the battery nears full does it act like B in spinning the engine.
     
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  13. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Yes but not the way you mean it.

    If you do not touch any pedals in D, then MG2 simulates 'engine braking using regen.

    If you hit the Brake Pedal in D, MG2 does much more Regen, excess energy is scrubbed off by the Friction brakes.

    If you use B, MG2 does more regen and the ICE is used as an air pump to squander energy, even before you apply the brake. Excess braking still uses Friction Braking. On a long downhill your friction brakes are unlikely to overheat, they are getting all the help they can. (This is very similar to how L feels in a 'normal' Automatic transmission, which you would also use on long downhills. http://www.seattlepi.com/default/article/Car-Talk-When-to-shift-an-automatic-Answer-s-1275519.php)

    Once the Battery Computer decides the HV battery is 'full' then all you have left is the ICE as an air pump and friction brakes. I would want to minimize the time spent in this mode.

    Engine noise is very counter intuitive in air pump mode. Loud equal slowing, quiet equal going.

    http://www.techno-fandom.org/~hobbit/cars/b-mode.html
     
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  14. DaYooper

    DaYooper Member

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    You guys are awesome. Such wonderful knowledge here on PC!
     
  15. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Some purist at Toyota did not label it L, as the gearing is not any lower than D. He sure added confusion by not calling it L. (I believe it is labeled L in the Volt, I do not expect it to use lower gearing there either)
     
  16. qbee42

    qbee42 My other car is a boat

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    He was wrong, as usual.

    It does add extra wear, as MG1 spins the ICE to pump air, but that is really a technicality. From a practical standpoint it makes no difference.

    It makes sense if you drive in the mountains, where engine braking alone is insufficient. In that case the Prius must also ride the friction brakes, which is something you want to minimize. Manually engaging early engine braking will minimize friction braking in this situation.

    That is correct. MG2 is used as a generator. Part of the power generated by MG2 goes to the battery and part goes to MG1, which is used as a motor to spin the ICE. Once the battery hits the high limit, all of the power goes to MG1 to spin the ICE.

    Yes. This is because B mode applies braking unless you press on the accelerator pedal. Pressing the brake pedal without B mode will do the same thing, except more amps will make it to the battery. Unless your hills are mountains, or you just like experimenting at the expense of mileage, you should stop using B mode and just step on the brake.

    Tom
     
  17. nerfer

    nerfer A young senior member

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    I'm willing to bet you're also slowing down faster in B mode, and that's why the amps increase.
     
  18. qbee42

    qbee42 My other car is a boat

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    This is exactly the point. Adding a bungee cord to the brake pedal would do the same thing.

    Tom
     
  19. Kb2lzh

    Kb2lzh Junior Member

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    I to drive mountain roads 3 to 4 times a week and have used cruse to keep me at the desired speed. This seems to work very well, and air braking only takes place when the battery is full toward the bottom of the hill for me. Enjoying my 2011 Prius.
     
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