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Why the downhill fuel use in B-mode?

Discussion in 'Gen 2 Prius Technical Discussion' started by tanglefoot, Jul 10, 2015.

  1. tanglefoot

    tanglefoot Whee!

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    I've always been a little curious why the ICE receives fuel when going downhill in B-mode. I've noticed that at times (especially at lower speeds) going downhill in B-mode that the instantaneous consumption bar will be in the 30-50 mpg range at times that I don't think the engine would be receiving fuel if the system were in D-mode.

    Just a curiousity/discussion topic.

    Thanks!
     
    #1 tanglefoot, Jul 10, 2015
  2. fuzzy1

    fuzzy1 Senior Member

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    Does this happen in situations where the ICE could quit spinning in D mode, and where you either don't really need engine compression drag, or the D-mode synthetic drag from regeneration is sufficient?

    If so, then B mode is preventing the ICE from shutting down, even if it needs to burn fuel to keep spinning. If D mode works, keeping speed down to the range you want, then don't use B mode.
     
    #2 fuzzy1, Jul 10, 2015
  3. wjtracy

    wjtracy Senior Member

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    ...believe I have accidentally driven in B mode the engine is working...I consider it the equivalent of downshifting in a regular car
     
    #3 wjtracy, Jul 10, 2015
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  4. ftl

    ftl Explicator

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    According to Prius guru Hobbit, who has made extensive technical studies of the functioning of the car, B mode uses fuel only below 17 MPH:

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

    "Fuel usage in "B" is somewhat mitigated by the fact that when decelerating above some nominal speed, somewhere around 17 mph, no fuel is sent to the engine and it just spins "dry". It's still wasting energy and slowing the car, but there's no reason to throw away gas along with that. This is sometimes called "fuel-starve" mode, and is also used in some conventional cars during high-speed coasting conditions."

    He wrote this about the Gen I, but I believe it's the same in later models.
     
    #4 ftl, Jul 10, 2015
  5. tanglefoot

    tanglefoot Whee!

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    Ahh--thanks for the resource. It does seem to burn fuel in B at times that the ICE would be stopped in D. I guess it makes sense to keep the engine turning when the car's at low speed, even though I suppose the MGs could take care of that without using fuel (MG1, are you busy?).

    I'll have to try the 1-pedal EV driving in B-mode between 10 and 19 mph. Wow, there's so much to learn about these things, isn't there?
     
    #5 tanglefoot, Jul 10, 2015
  6. Melchior

    Melchior Junior Member

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    Can you explain that please? What exactly do you mean and why would you use that method?
     
    #6 Melchior, Jul 15, 2015
  7. tanglefoot

    tanglefoot Whee!

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    In hobbit's document linked above, it indicates that (paraphrasing by memory here) B-mode does not spin the engine via electric motor to waste energy at speeds of approximately 10-19 mph, allowing efficient, electric driving in that range without using the brake pedal to slow down, for driving using only the accelerator pedal. For the most part, it sounds like more of an entertaining quirk than a practical application.
     
    #7 tanglefoot, Jul 15, 2015
  8. uart

    uart Senior Member

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    Yep. I sometimes use it when driving at slow speed down my driveway. Because "B" doesn't spin the engine at those low speeds then you get the extra regenerative braking without the extra losses. TBH though, it's nothing that couldn't be achieved by just staying in "D" mode and using light application of the brake pedal. :)
     
    #8 uart, Jul 16, 2015
  9. uart

    uart Senior Member

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    Re the (sometimes) use of a small amount of fuel in "B" mode. I would assume it is the same reason that the engine often receives a small amount of fuel when regenerating downhill in "D" mode (with no accelerator input). When you cross that magic 42 MPH (67 km/h) threshold it forces the engine to spin up, and at that instant there is a small amount of fuel injection. I've always assumed that this is to make the process as smooth as possible, to try to prevent any noticeable "jolt".

    My guess would be that the mysterious "B" mode fuel usage at certain lowish speeds would be a very similar phenomenon.
     
    #9 uart, Jul 16, 2015
    Last edited: Jul 17, 2015
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