how to use "b" for driving.

Discussion in 'Gen 2 Prius Fuel Economy' started by m1j1tc0rn, Feb 17, 2004.

  1. m1j1tc0rn

    m1j1tc0rn New Member

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    hi all,
    i can't find any detailed analysis anywhere on when and how to use "b" in maximizing my efficiency.

    in short, here's my deal:
    i commute down a windy mtn road every morning for about 12 mi. not stop and go, 40mph the whole way. previously, i had to sit on the brakes or constantly downshift.
    will "b" help me here or should i just leave it in normal "d" mode?

    thanks for your help.
    -r

    (04.sliver.9.leather)
     
  2. efusco

    efusco Moderator Emeritus
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    If you rarely need to get on the gas on that drive then B mode is probaly ideal in the situation you describe.
     
  3. m1j1tc0rn

    m1j1tc0rn New Member

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    thanks, i'll try this next week with it and following w/o.
    -r
     
  4. rockluvr

    rockluvr New Member

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    Please post your results of using "B" and not using "B".

    Saturday I drove on a windy road in the foothills of Colorado (285 from Morrison to Evergreen and back for those that know the area) and I expected to use "B" for the first time. I found that I didn't need it. Because of the sharp turns I was actually going the speed limit (~30 mph, 20 mph on some turns). I was in stealth most of the time (on the way back down, that is) and didn't have to brake much. I was almost disappointed that I didn't have to go to "B". Guess that will have to wait until I go on a longer trip into the mountains.
     
  5. m1j1tc0rn

    m1j1tc0rn New Member

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    not to be thick, but i would think your situation would have been ideal. i'm not planning on braking much... just coasting. is that not what "b" is all about?
    thanks again.
    -r
     
  6. rockluvr

    rockluvr New Member

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    "B" is like downshifting to slow the car down when going down steep inclines in order to save the brakes. Since the car must use the engine to do this, it will use some gas. For coasting, the car is in pure electric mode if it is on at all (other than recharging the battery) and therefore is a more energy efficient mode. I have had my car since Dec 24 and have not had an occasion to use the "B" mode yet. I'm certain that if I take
    I-70 west from Denver into the mountains that I will have to use "B" mode to slow down the car on those extremely steep grades. It actually surprised me that I DIDN'T have to use it on the way down from Evergreen.
     
  7. satchm0

    satchm0 New Member

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    From what I understand, when you're in "b" mode and you're coasting (foot off of the gas pedal), you are braking - engine braking.
     
  8. jsorger

    jsorger New Member

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    I have found that when using B for extended periods (2-5 mile steep grades), occasionally the engine will rev very loudly (as if to object).

    I also find it interesting that if you ride B to a complete stop the engine will not shut off. You must put it in D or P for that to happen.
     
  9. m1j1tc0rn

    m1j1tc0rn New Member

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    i think the revving is actually a fan cooling the battery when it has hit over 100% "juice." perhaps i'm wrong. i have concluded that b regens the battery quickly and saves me brake-wear, but doesn't increase mpg.

    i use it only on long declines (where i live that is bear creek rd/hwy 17) to save my brakes.

    -r
     
  10. rockluvr

    rockluvr New Member

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    Out of curiousity, what does the Energy Monitor for current mpg state when you are going down those steep declines in "B" mode?

    For instance, in "D" mode, it would recharge the battery to full green and state 99.9 mpg.
     
  11. HTMLSpinnr

    HTMLSpinnr Moderator
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    Let me add my 2 cents on this:

    Wayne has done some regen studies on B mode vs. light braking (which doesn't engage the brakes - much) and has shown that B mode is the least efficent mode.

    The sound you are hearing is the engine revving - it's compression braking. Do not be alarmed, the engine can never overrev as the ECU will not allow it.

    Honestly, you'd be best off lightly using the brakes. The only exception to this is if your battery is completely full, and you're no longer regeneratng - in which case B mode will help you stay off the brakes a bit.

    You'll know the battery is completely full if the engine is revving w/o being in B mode (though not as high) and your foot is off the accelerator, and the energy monitor shows all green bars to the top of the battery.

    hope this helps,
    -Rick
     
  12. davidbw

    davidbw Member

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    Another crazy question on braking while going downhill.

    If you set the cruise control at a slow speed, but fast enough to engage the cruise control, and then go down a hill. Is the Cruise control smart enough to use regenerative braking to keep the car at the set speed? Would this be more efficient that "B" mode or using the brakes?
     
  13. HTMLSpinnr

    HTMLSpinnr Moderator
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    I've felt to some degree it will do it, however it's not like B mode, and probably isn't much more than what regular off-pedal regen would be. Someone with an amp meter like Wayne's setup would need to find out. My gut feeling is that there isn't much difference, and this can be verified with the ButtDyno going down a hill, then canceling cruise control - if you speed up, it had some effect - if the rate of decel/accel remains the same, it doesn't.

    One great thing about the Classic is that I could drop to B mode while keeping the cruise on, and cruise would control the amount of engine braking, maintaining my rate of speed. With the 04, it's pretty much a manual deal if the grade is fairly steep (as far as highway grades are concerned).
     
  14. Wolfman

    Wolfman New Member

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    I've found the "B" mode to be completely unnecessary. I've had my Prius over Raton pass on the CO/NM line. The grade is a 6 to 7 percent grade. The car absolutely held its set speed on the cruise control, even going downhill. The only time you should use "B" is if you are going over a particularly steep grade, and need the extra assistance in holding the cars speed in check. There is NO economy enhancement in "B" mode.
     
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