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Better MPG: B Mode or Braking?

Discussion in 'Prius v Main Forum' started by alanvw, Dec 10, 2011.

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  1. alanvw

    alanvw New Member

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    To get the best MPG, is it better to use B mode then brake at the last minute or just use the brake more?

    Both seem to trigger regen but simple braking seems to get the car into EV mode faster
  2. Gurple42

    Gurple42 New Member

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    Only use B mode for long downhill "engine braking". That's it's only purpose.
  3. Chazz8

    Chazz8 Gadget Lover

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    The Prius will use B mode (engine brake) all by itself when it is appropriate. It does not use regen in B mode, it simply spins the engine parts to slow down the vehicle. Your Prius will automatically switch to B mode if you go down a big hill and you have filled you battery up with brake regen power.

    To maximize MPGs, use the break early and try to time it so the traffic light turns green before you get there and you still have momentum because you didn't have to come to a complete stop.
  4. qbee42

    qbee42 My other car is a boat

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    The purpose of B mode is to deliberately waste energy. For better mileage, you want to recapture energy, not throw it away. Don't use B mode for braking unless your battery will run out of room, which only happens on long downhills (essentially, only in mountain driving).

    Tom
  5. J5A

    J5A Active Member

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    When I occasionally use B Mode the charge bar does activate. Still lost energy, Tom?

    But, I only use it so it doesn't appear I am braking in certain situations; people behind me seem to get pissed if I brake on a downhill with no one in front of me.
  6. DragonflyDM

    DragonflyDM Junior Member

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    Yes..I thought the B mode was to charge the battery by not disengaging the engine. That isn’t the case?
  7. tonyrenier

    tonyrenier I grew up, but it's still red!

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    When I first bought my Prius 2 1/2 years ago I experimented with everything and asked lots of questions on this very knowledgeable forum. Playing with B Mode was one of many attempts to improve my numbers, it just put extra drag on the car, that's all. Maybe some day when I get back to the Rockies.......
    I now have settled on ECO Mode, anticipating traffic lights, a Scangauge II, inflating tires to 40 PSI, grill blocking and overall sensible driving. At just over 30K miles I have a 55.1 mpg (calculated) total fuel consumption, how could I ever complain.
  8. qbee42

    qbee42 My other car is a boat

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    The 'B' in "B Mode" stands for Brake, as in engine braking. It simulates down-shifting in a normal car, but only for the braking part (no effect for hill climbing or acceleration).

    To understand why B mode wastes energy we need to look at how Prius normally brake. Unlike a normal car, the Prius has two ways to brake: 1) Friction braking, and 2) Regenerative braking.

    Friction braking is the standard form of braking that we all know, using disk or drum brakes with pads or shoes. Regenerative braking, on the other hand, uses electrical generators to convert the car's kinetic energy into electrical energy. In other words, your Prius slows itself by making electricity with motor-generator 2 (MG2).

    The Prius does as much braking regeneratively as possible. This is because it is easier on the brakes and recovers valuable energy. There is a limit, though, and that is battery capacity. Once the HV battery gets charged up there is no place to put the electricity made by regenerative braking. Engine braking comes to the rescue. With engine braking, the second MG, MG1, is used as a motor to spin the engine. The engine is not burning fuel in this mode, but instead acts as a load for MG1. The engine is a big air pump, and it takes energy to pump air. That's what it does with engine braking. MG2 slows the car by making electricity, and MG1 dumps the electricity by spinning the engine which pumps air.

    Your Prius will automatically switch to engine braking when the battery is near the high charge limit, so you don't have to do anything special to use it.

    So where does B Mode fit in? B Mode tells your Prius to start using engine braking right now, and not wait for the high battery limit. By using B Mode you tell your Prius to waste energy.

    When would wasting energy be useful? It is useful for saving the friction brakes on a long downhill. Say you are descending a long mountain grade. You are riding the brakes, but hey, no problem, your Prius has regenerative brakes. So far the friction brakes aren't even warm. Since this is a long downhill, your battery eventually fills up. Your Prius automatically engages engine braking, but engine braking alone is not enough to slow your car, so the friction brakes automatically engage. Everything is under control, but you are now wearing away your friction brakes. Wouldn't it be better to minimise the use of friction brakes?

    B Mode allows you to minimise the use of friction braking when you know in advance that your battery will hit the high limit. This is an important concept, so read that last sentence again. If you know in advance that plain regenerative braking will run out of space, you manually engage B Mode and start throwing away some of the energy. By doing this from the start of the decent, the battery charges more slowly and the use of friction braking is forestalled.

    Tom
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  9. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    B Mode is to provide Engine Braking when going down long hills. You would use it like L mode in a 'normal' Automatic Transmission. (for almost all of us, that is never) Someone at Toyota was too honest to label it L as it does not have lower gearing.

    If you know you will descend more than 600 vertical feet, engaging B can save your friction brakes by slowing you via engine braking before your HV Battery fills.

    If, not engine braking will only happen after your HV Battery fills, so you will use more friction brakes, and they will get hotter. Cool brakes are safe brakes.

    B Mode does charge the battery, but not as efficiently as just driving your Prius normally. (If you did not buy a Prius for gas mileage, go for it!)

    An example of when to use B Mode:
    http://www.oregon.gov/ODOT/MCT/docs/EmigrantHill.pdf
  10. ken1784

    ken1784 SuperMID designer

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    I wonder why you guys did not talk about your speed range using B mode.

    Following speed numbers are based on my experience on JP Gen3 Prius, so those speed numbers may be different on NA Prius v.
    So, please check the speed with ScanGauge or other equipment on your Prius v.

    Above 43km/h (27mph), B mode causes engine braking which is wasting energy as people wrote.
    Between 43km/h to 10km/h (6mph), B mode causes more regeneration than D, which is good for fuel economy.
    Below 10km/h, there is no more regeneration, then the friction brake takes care everything.

    Ken@Japan
  11. efusco

    efusco Troll Slayer Staff Member

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    Ken, it's only good for fuel economy if you intend to slow at that exact rate that is induced by B-mode. And you can simulate B-mode rate of regeneration by using the brake pedal. Better, you can exactly control your rate of deceleration/regeneration with the brake pedal which is better for fuel economy than B-mode.

    Again, the situations where B-mode is advantageous are very rare if they exist at all.
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  12. ken1784

    ken1784 SuperMID designer

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    I mean...
    B mode is "B mode plus foot brake"
    D mode is "D mode plus foot brake"

    Ken@Japan
  13. robertg97

    robertg97 2ndOwner.PriusII.2005

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    I get the impression from a couple of above comments that you think you have to have your foot on the brake to get battery regeneration; however, on my 2005 Prius, regeneration starts as soon as I take my foot off the gas pedal. Thus, the farther one drives in a coasting mode, the more energy recuperates into the battery (either of the braking methods wastes energy, but 'B' is the worst) . I only slow down this way when there is no one close behind me though. When it is easy to do, I get over, to let the gas guzzling, fast drivers pass me. When you can coast, in place of braking, your mpg goes up quickly. I am averaging around 60 MPG in mixed highway and stop and go driving. Also, once I tried driving using coasting in place of braking exclusively, when I was on the road all alone, and got around 80 mpg over several hundred miles. Its too bad not everyone drives this way and I feel pressured to give into their stop at the last minute ways.
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