Shift to B

Discussion in 'Gen 3 Prius Technical Discussion' started by kknguyen1168, Jan 10, 2013.

  1. kknguyen1168

    kknguyen1168 Member

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    hi all

    would you guys explain to me what exactly what happen when you shift to B ?

    V/r

    Khoi Nguyen
     
  2. bwilson4web

    bwilson4web BMW i3 and Model 3

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    In "B", MG2 begins working as a generator and part of the energy is sent to MG1 to force the engine to turn and sink part of the braking energy. This slows the vehicle without requiring the mechanical brakes or over-charging the traction battery.

    Measurements with OBD scanners have shown this can reduce the 'forced charge' heating from descending a large, steep hill. Often it means descending at the speed limit without having to use the brake.

    Bob Wilson
     
  3. tedjohnson

    tedjohnson Member

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    So Energy wise, where does the lost energy go? Is it heat from engine compression that goes out the radiator? Is the energy that would have gone into the battery without using B unrecovered??
     
  4. GasperG

    GasperG Senior Member

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    ICE is doing the braking just like you would use lover gear in a normal car. ICE is not burning fuel but it's pumping air like a compressor.

    Even using B mode ICE will work at very low rpm and providing very little braking if the battery is not fully charged, when the battery is full the ICE will noticeably spin in high rpm when you are doing a big descent.

    Even if you are in D mode and your battery is full, the computer will engage B like mode meaning ICE will spin and assist in braking.
     
  5. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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  6. tedjohnson

    tedjohnson Member

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    so basically that energy of compressing the air in the cylinders goes off as wasted heat and out the radiator. That's what I thought. Its lost. The best you can do is to recover as much as you can in the battery, thru regen, before the long downhill, so you do not have to use B . Not always possible tho......
     
  7. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Yes, B is a safety strategy, not an energy strategy.
     
  8. zhutton

    zhutton Junior Member

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    I've just had a lovely conversation with a police officer over the use of the B button engine braking..... 15 minutes later I drove off with a large grin and a cop with a sour face after proving he was wrong after not realising that people do use engine braking (which we are shown on the driving test) and he was too close behind me to notice this..... You write me a ticket, I'll have the book and write one back for you, good day officer!
     
  9. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I may be too provincial, but I can't imagine there would be many places in England where B mode would make any sense. The highest town is Flash at 1518 feet, you only want B mode if you have a drop over 600 feet.

    Staffordshire - Google Maps
     
  10. zhutton

    zhutton Junior Member

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    It's useful going through Cannock chase, some long hills that can let the car run away, b keeps tabs on this, it's not just for hills
     
  11. fuzzy1

    fuzzy1 Senior Member

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    More details please? For what offense did he intend to write you a ticket?
     
  12. xs650

    xs650 Senior Member

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    I have started using the cruise control for moderate descents. It's a nice alternative to using the brakes and using B. Is there a serious risk of battery overheating with that?
     
  13. Ragin Cajun

    Ragin Cajun Junior Member

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    It would think because the cop was behind him and saw he was slowing down without brake lights being on?

    I drove down Mount Lemmon using mostly B. At times it seemed the ICE was overrevving but the Scangauge never showed it above 4k rpm. It does sound funny though.
     
  14. bwilson4web

    bwilson4web BMW i3 and Model 3

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    I didn't test cruise control in descents.

    Bob Wilson
     
  15. zhutton

    zhutton Junior Member

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    apparently 'dangerous driving' until he realised he couldn't do anything....
     
  16. Jumpjet

    Jumpjet Member

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    Does B work the same in the PiP with the only exception being that it's able to store a lot more energy?
     
  17. David Beale

    David Beale Senior Member

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    You won't "overheat" the battery doing whatever. The cars systems will take care of themselves. Sometimes we tend to overthink things a bit!

    Keep in mind the Prius has very low drag, both aerodynamic and mechanical, and will "take off" down even the shorter hills. "B" can be useful even -out- of the mountains! Driving in "B" will lower your mileage a bit. Use appropriately.

    Also keep in mind GII cruise didn't have "braking" and those cars WILL take off down hills! GIII uses electric only I believe. On "real" hills you still have to pay attention! Such as highway 99 descending from Duffy lake, with parts at 18% grade! You have to drive!!! Carefully!!! ;)
     
  18. Sergio-PL

    Sergio-PL Member

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    It depends on what kind of moutain is ahead of you. If you are sure that it will overfill the battery it's better to switch to B. Engine will spin at very low RPM as long as full 8 bars (where the last one is veeery capable) go to the battery. After that it will do whatever is possible to keep slowing down - thus spinning ICE at very high RPM wasting all excesive energy in heat.

    Additionally whenever it will be possible to get rid energy from battery it will do it too. With 8 bars you can drive (below 74 kmh) with full HSI hybrid zone without ICE starting. With 7 bars (near 8) ICE will start in 75% of hybrid zone. Then it will go back to normal.
    If there is a long, flat road after the mountain and low speed limit (35 mph or so) you can drive in EV mode for few miles without ICE. Almost Plug-In Prius experience :)

    Using B mode in mountains instead of keeping brakes in CHG zone is also a safety feature. While in D mode with leg on the brake pedal you never know when computer decides that it needs discs (battery overheat protection, excessive energy, bump on the road, slippery road). And then you may run out of brakes without even knowing something wrong happened (not every Prius owner may notice electric/mechanic brake change).

    So my suggestion is - short downhill, especially when you see the end - keep in D mode, nothing wrong will happen.
    Long, few miles downhill with rapid curves - switch to B for your (and others) safety. 5% of energy loss is nothing compared to life.
     
  19. David Beale

    David Beale Senior Member

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    Uh, well you will never "run out of brakes". If you stomp on the brakes they will work just fine no matter what!

    In the mountains I run in "D" until I crest a mountain pass. Then I switch to "B" for the descent.
    I use cruise on "normal" mountain roads such as highway #1. Note that cruise doesn't work when in "B". On highway 99 not so much. It's play time!!
    I've only had one person pass me on the Duffy Lake section, a cop in his personal vehicle. He had to be patient as places I could pull over to let him by are few and far between. ;) Funny part is on that section of road the speed limit is hard to exceed due to the 10-20 km/hr corners and up to 18% grades. My kind of road!
     
  20. Sergio-PL

    Sergio-PL Member

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    You will... as soon as they overheat. It may cause to problems. Non recoverable spots on discs which don't generate friction and boil DOT4 in hydraulic system. Second one will cause lower braking pressure. First one permanently destroy pads and discs making them unusable.

    Of course it matters whether you are alone or with full allowed load. In the first case you may never notice it as long as you don't visit the race track. In second - it may be very easy to do.

    So if you stomp on brakes - car will do whatever it can to stop but can't overcome associated physics :)
     
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