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"B" mode...what for?

Discussion in 'Gen 5 Prius Technical Discussion' started by peternumber2, Feb 14, 2024.

  1. peternumber2

    peternumber2 Junior Member

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    Hi all,
    Prius is the first toyota hybrid I own.

    Can anybody tell me the difference between regenerating enabling the "B2 mode and pressing the brake?

    thank you
     
  2. RRxing

    RRxing Senior Member

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  3. Trollbait

    Trollbait It's a D&D thing

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    For a Prime/PHV with a mostly discharged battery, there isn't much difference. You can get the extra regen braking of B by pressing the brake.

    With the hybrid or PHEV with a full battery, B will engage more engine/compression braking. It's like using a lower gear with a traditional transmission.
     
  4. Mendel Leisk

    Mendel Leisk Luddite

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    You can mostly ignore B. With a regular Prius its main usefulness is to reduce charging, substitute “engine braking”, on extremely protracted downhills, where there’s a risk of the hybrid battery getting so fully charged the car will revert to friction braking only, and overtax them.

    On the plug-ins B gear behaviour is maybe a little different?
     
  5. peternumber2

    peternumber2 Junior Member

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    ok, i suspected that. The only advantage of the B mode is that I'm sure it's not going to waste any energy by engaging the "real" brakes, right?
    Ok, so I expect to witness the engine revving when battery is full, right? Or does it have the possibility to lock the valves in position just like big trucks do downhill?
     
  6. ChapmanF

    ChapmanF Senior Member

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    No special valve business; the engine will just rev (up to a maximum safe RPM, never higher) as the battery approaches full charge. If the downhill is steep enough that even that maximum safe RPM can't use enough energy to hold your speed, then the car will still pick up speed, and you'll have to make some use of the brake pedal to control it.

    That all happens whether you're in D or B, but B shifts the balance so more engine twirling is used earlier, so the battery is charged less aggressively. B also offers more resistance than D does with the foot off the go pedal, so it feels like selecting a lower gear in a conventional car.

    On the plug-ins the behavior is maybe a little different?
     
  7. vvillovv

    vvillovv Senior Member

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    Shifting to B will produce a static amount of regen. If all sensor conditions permit that static amount of regen to the traction pack the Prius will slow down at a predictable deceleration rate on a flat road.
    If all conditions from the sensor thresholds are not met, the engine typically turns on due to a sensor threshold being exceeded for that amount of static regen B mode produces. ie: traction pack temps being to low or too high are the easiest to notice.

    Pedal braking regen is fully controlled by the ECU's. So the mix of regen to friction braking is dynamic so with pedal braking "engine on" scenarios are typically caused by other sensors readings beyond the thresholds of EV driving.

    B mode produces a static amount of regen.
    Pedal braking produces a dynamic mix of regen and friction brakes controlled exclusively by the ECU's.
     
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  8. ChapmanF

    ChapmanF Senior Member

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    It may come down to just what you mean by 'static'. The car is programmed to feel much like a conventional A/T car, where as your foot comes off the go pedal you get less go, then a brief sort of 'glide' point, then a bit of deceleration, that is maximum when your foot's completely off the pedal. In the conventional car, that deceleration is present both in D and L, but stronger in L. Likewise, in the Prius, it's present both in D and B, but stronger in B.

    The car provides that deceleration by regen, engine twirling, or a mix of the two, but it won't add friction braking into the mix on its own.* Once you go on the brake pedal, all three of those can be in the mix.

    The car can mix regen with engine twirling whether you are in D or B, but in B it diverts more power to engine twirling earlier, to charge the battery less aggressively. (In a plug-in, that diversion to engine twirling doesn't have to happen so early,)

    You may notice the deceleration rate change if battery conditions reach the point where only engine-twirling remains. The twirling just can't match the maximum deceleration rate available with regen.

    * at least not in any generation I know this about, absent some other feature like radar cruise.
     
  9. peternumber2

    peternumber2 Junior Member

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    What do you mean by "static" ? Constant power? Constant deceleration?

    In my prius phev, when battery was too cold, going downhill in B mode resulted in very little regen and very little decelaration, but engine did not kick in, so I had to break.
     
  10. peternumber2

    peternumber2 Junior Member

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    I would have said the opposite: more regen in B...
     
  11. Trollbait

    Trollbait It's a D&D thing

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    B is suppose to be like down shifting with a traditional transmission, but only about one gear step down. It will seem like less braking than what is possible in other cars.

    The Prime will do this by upping the regen amount. The bigger battery allows this. If the battery can't take that charge, the system should spin up the engine. That was the case with gen4 when the SOC was near full. Perhaps the system only looks at the SOC and not possible charge rate on deciding to use the engine or not. Maybe Toyota changed some things, like avoiding using the engine in B while in EV mode.
     
  12. peternumber2

    peternumber2 Junior Member

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    Quite likely
     
  13. Trollbait

    Trollbait It's a D&D thing

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    What I remember of my gen2, B couldn't slow down the car as much as others with multiple gear selections. Unfortunately, Toyota stuck with the Prius shifter instead of using a traditional one with the gen5 for those that use downshifting for speed control. The Camry hybrid with the shifter from the ICE model has 6 degrees of B.
     
  14. ChapmanF

    ChapmanF Senior Member

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    That's a difference between us. :)

    In general, when you compare the same amount of deceleration in D or in B, and look at how that amount of deceleration is achieved by mixing regeneration into the battery and engine twirling, the point of B is to move the balance earlier away from regen into the battery and more toward engine twirling. The purpose is to treat the battery less aggressively on long descents.

    Of course, a separate effect of B mode is to increase the amount of deceleration you feel as you back off the pedal. That is a separate effect independent of changing the balance between regen and engine twirling. Therefore, to "compare the same amount of deceleration in D or in B" doesn't mean the same go pedal position for both.

    So, the differences between D and B are twofold:

    1. Change the amount of deceleration felt at a given pedal position: D less, B more. The purpose of (1) is to make the car feel like a familiar A/T with a low gear.

    2. Change how that deceleration is achieved (the regen:engine balance): D more:less, B less:more. The purpose of (2) is to extend the life of the battery.

    (2) is quickly noticeable in a non-plugin. In a plugin, because the battery is larger, the car doesn't need to back off toward engine twirling so proactively.
     
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  15. AndersOne

    AndersOne Member

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    That could actually happen BUT makes the car even more inefficient compared to D mode besides the usage of motorbrake.
    I feel you meant it differently though...
     
  16. peternumber2

    peternumber2 Junior Member

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    Ok, it's plausible that, in order to lower the stress on the battery, the b mode privileges the ice braking.

    As a matter of fact, in my phev, the electronics chose to shut down the cold ice and reduce the regen of the cold battery.

    Maybe a too specific situation to take general conclusions...
     
  17. vvillovv

    vvillovv Senior Member

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    static - fixed - non-changing - the same amount - not dynamic - programmed amount that is always the same

    B has more behaviors it exhibits and be confused by in the Prime than it does in the regular Prius because of the Primes EV range, as if B in the regular Prius isn't confusing enough to most owners, especially owners who don't know how Prius produces regen, yet, probably due to the term 'regen braking'.
     
    #17 vvillovv, Feb 14, 2024
    Last edited: Feb 14, 2024
  18. ChapmanF

    ChapmanF Senior Member

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    I've got a dictionary too, but it doesn't say much about all of the factors (starting with go pedal position) that figure into the deceleration rate of a Prius, whether in D or B mode.
     
    #18 ChapmanF, Feb 14, 2024
    Last edited: Feb 14, 2024
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  19. vvillovv

    vvillovv Senior Member

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    your right, there are a lot of variables that I've forgotten about.
    At slower speeds and on flat surfaces B seems to behave in a fairly predictable way.
    Add hwy speeds and downhills, it's hard enough to see how regen is behaving after just letting up on the Go Pedal, much less all the different things that might happen when shifting to B, than try to define B behavior in an easy to understand couple of sentences.
    I tried to make some sense of how B works, but as you say, it's more complex than my simple examples of what I've seen happening,
     
  20. fuzzy1

    fuzzy1 Senior Member

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    The RAV4 Prime appears a bit different still, having removed the B label from its shifter entirely. It is labeled D, S (Sport), S+, and S-. My unit also has paddle shifters, missing from lower trims/options.

    The RAV4 Prime Owners Manual describes regular D (no number), and levels D1-D6 (paddle shift units only?) and S1-S6 (all units) for various levels of up/downshifting. My only experience so far is paddle shifting down the D series while descending the neighboring downhill in EV, and hearing the ICE spin up when reaching D2. I should learn a bit more tomorrow when returning downhill from a planned ski day.

    Even coming from a Gen3 Prius, this Owners Manual has far far too much new stuff for my senior citizen brain to absorb in just a week. Or possibly ever.