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Coasting in 'B' Mode

Discussion in 'Gen 2 Prius Main Forum' started by Marvinh, Jun 4, 2008.

  1. Marvinh

    Marvinh Marvin

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    I know there are many posts about 'B' mode, but I have a similar question :rolleyes:. So please don't flame me :)!

    I just drove the Shendandoah National Park in Virginia. Lots of hills and I utilized the 'B' mode going down hills when necessary. It worked great! And the power going up the hills was excellent as well! Plenty of power to spare!

    I found when coasting down the hills in 'B' mode a slightly different sound was generated. Like a smooth hum. When lightly pressing the accelerator to apply power the hum would instantly stop and the MFD would show power supplied either from battery or ICE or both.

    So here's the question: What created the humming sound? Was it the electric motor or the power inverter running to slow the car? And is it correct to coast downhill in 'B' mode? I know some people say to just press the brake to get the same effect. But I preferred to use 'B'.

    Thanks! Marvin
     
    #1 Marvinh, Jun 4, 2008
  2. qbee42

    qbee42 My other car is a boat

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    No, the hum comes from the ICE spinning to waste energy. B mode tells the car to use the engine for braking. When you coast with your foot off of the accelerator, the Prius applies a small amount of braking to simulate engine drag. In the case of B mode, the braking is done by spinning the engine (ICE) in a high drag configuration. When you press the accelerator, the Prius stops simulating engine drag. In the case of B mode, it stops spinning the ICE in drag mode, so the humming sound stops.

    Tom
     
    #2 qbee42, Jun 4, 2008
  3. qbee42

    qbee42 My other car is a boat

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    Why do you prefer to use B mode? Pressing the brake will slow you just as well as B mode, but will recapture more of the braking energy. B mode only makes sense if the hill is too long and steep for the battery to absorb all of the recaptured energy. Obviously there is no way to know this from the top of a strange hill, but a little common sense will get it right most of the time. If you look out the window and see mountains, B mode is a good idea. Otherwise you probably don't need it.

    Tom
     
    #3 qbee42, Jun 4, 2008
  4. bac

    bac Active Member

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    Really, there is NO reason to get on the gas in "B" mode. Use the engine brake only on steep downhills while coasting or braking. :)

    ... Brad
     
    #4 bac, Jun 4, 2008
  5. richard schumacher

    richard schumacher shortbus driver

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    An excellent point which suggests an easy rule-of-thumb: if you find yourself using the gas pedal you should not be in "B".
     
    #5 richard schumacher, Jun 4, 2008
  6. SparrowHawk60

    SparrowHawk60 Happy to be green!

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    B mode is like a "jake brake" on a big rig. Lots of un-needed noise for little result...
     
    #6 SparrowHawk60, Jun 4, 2008
  7. Marvinh

    Marvinh Marvin

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    I find 'B' mode to very efficient. It seems like an easier way to keep control on a downhill then riding the brake. And I used the brake as well on some steeper sections.

    Thanks for your replies! Marvin
     
    #7 Marvinh, Jun 4, 2008
  8. morpheusx

    morpheusx Professor Chaos

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    using the B mode is similar to if you have ever driven a stick shift and down shifting to slow the vehicle down coming off the interstate or to a stop light.
     
    #8 morpheusx, Jun 4, 2008
  9. qbee42

    qbee42 My other car is a boat

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    When you say you find it very efficient, do you mean efficient from the driver's standpoint? I can see where it would save all that wear and tear on your shoe leather by avoiding the brake pedal. It certainly isn't efficient from an energy standpoint.

    Tom
     
    #9 qbee42, Jun 4, 2008
  10. Marvinh

    Marvinh Marvin

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    Yes. Efficient from a driver's standpoint. And not wearing down the brake pads. ;).

    Marvin
     
    #10 Marvinh, Jun 4, 2008
  11. qbee42

    qbee42 My other car is a boat

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    The brake pads are not an issue here. They won't wear any faster without B mode, unless we are talking about mountain roads. Since you need the accelerator on part of the downhill, that probably isn't the case.

    Tom
     
    #11 qbee42, Jun 4, 2008
  12. hobbit

    hobbit Senior Member

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    You can read my account, including lots of Prius energy management
    specifics, of driving the same route. I tend to alternate between
    D and B depending on slope and how much downhill I think might be
    left before I get to start re-using the energy I've collected.
    .
    _H*
     
    #12 hobbit, Jun 4, 2008
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