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"B" mode...what for?

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

  1. ChapmanF

    ChapmanF Senior Member

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    The word 'tolerate' there is kind of a giveaway that the cold temps maybe aren't anything the passengers actually want.

    So the basic choices still boil down to: capturing some of the energy into the battery, using some of it for something you actually want (like, the amount of cooling that's genuinely comfortable), and trading the rest away for things you don't really want (whether that's excess cooling, excess heating with the heat pump, or twirling the engine).

    As long as what you're trading it away for isn't anything you really want, you're just getting rid of it one way or another.

    Kidding aside, you could combine that technique with the engine twirling to handle a somewhat steeper descent; the A/C can consume 2 or 3 kW if you get it to run flat out (I assume the heat pump is similar), and that could be noticeable on top of the ~ 13 kW you can use up twirling the engine.
     
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  2. vvillovv

    vvillovv Senior Member

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    I liked your customized alternative more when I first read it than I care to say after now, but I will anyways, after reading what others think of your inventiveness.
    When I got the 2014 Gen 3 Plug-in base model, I got it mainly to learn about the new to me plugin thing back than. I got hooked on seeing the 999.9 mpg dash readout and sharing my MIDs comms pics in the Top 20 MPGe thread and getting in as one of the last members to the Blizzard Brigade, I learned a lot from owners who'd had the Plug-in a lot longer than I did.
    One thing I'd do during summer EV driving was after running out of EV range with only a few tenths of a mile to get to a charger or home and knowing I'd have some downhill glide between where i was and my destination, I shut off the car. I leaned about creep home mode and when I could use it to make it back while using minimal ICE on for the trip. I also learned just how fast the PiPs 999.9 mpg would drop as soon as the ICE started to spin.
    I know there are a lot of chat members that don't hyper-mile and some that probably cringe when reading about some of the inventive ways of doing it.
    Thanks for adding your observations. I'm one of the members here that appreciates your inventiveness.
     
  3. fuzzy1

    fuzzy1 Senior Member

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    It sounds like you drive either shallower hills than me, or hills that allow much higher legal or safe speeds that produce far greater air drag.

    On my regular hills, while I do use full blast AC in season (or electric window defrost in other seasons) when descending long hills, as an additional energy bleed mechanism, in D mode it just isn't anywhere enough braking force to keep down to legal and safe speeds. B or CC mode engine twirling is required to 'save the brakes', and even then some friction braking is still occasionally required.
     
  4. peternumber2

    peternumber2 Junior Member

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    no it won't
     
  5. PriusV17

    PriusV17 Active Member

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    If max regen is 26k and max AC is 3.4k it's still not bad. The main reason I don't want engine revving is engine wear. If you can get enough brake control from AC you are sacrificing the brakes with the aid of the AC to save your engine from friction wear.

     
  6. ChapmanF

    ChapmanF Senior Member

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    The A/C only gets near 3kW when the compressor runs flat out (which might be hard to get it to do, even when you request max A/C, depending on the actual temp and humidity).

    And if you can get the compressor to run flat out, there's friction wear involved with that too. An A/C repair after a compressor failure might not be a lot cheaper than an engine.
     
  7. PriusV17

    PriusV17 Active Member

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    I don't know how durable an AC compressor is long term but people from LA traveling to Vegas in the dead heat of summer 110F will max their AC for 4 hours straight. If using it for 30-40 mins downhill helps let's hope it won't do any long term damage.
     
  8. fuzzy1

    fuzzy1 Senior Member

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    I didn't get anywhere near 26 kW from engine-twirling braking in D-mode in my Prius. Testing several times on a particular hill, in full-battery conditions where B-mode would spin the ICE to its maximum 4800-4900 RPM and almost keep road speed steady, switching to D-mode would drop RPM to low-mid 2000s (better details in some old thread here) and gravity-driven road speed increased much faster.

    I didn't have my ScanGauge gauges set to monitor actual regen or engine-twirling power, but seem to remember that others who did (Chap??) found significantly less than 26 kW.
    Others have pointed out that with no fuel being burned, cylinder pressures and engine wear are much lower than when climbing that same hill.
    I'm regularly traveling hills where AC is simply not enough. I use hard AC and full engine twirling, and still need some brake friction pad too.

    Cutting the engine twirling makes the brakes get much hotter. Having witnessed a pickup and a real truck suffer too-hot brakes, and reading of another member here who had to rebuild his overheated Prius brakes twice after relying on faulty claims of other posters, I'm not going to risk that.
     
    #108 fuzzy1, Feb 19, 2024
    Last edited: Feb 19, 2024
  9. KMO

    KMO Senior Member

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    I now feel compelled to ask what do you do when you have to go up a hill? Does the answer depend on your passengers?

    (Is this really a cunning technique to prevent people asking you for for a lift?)
     
  10. ChapmanF

    ChapmanF Senior Member

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    How many hours will they put on their engine in that trip?

    Yeah, I was seeing about 13, linked upthread.
     
  11. Tom Redinger

    Tom Redinger New Member

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    You may have found the answer by now BUT: "B" mode is basically like shifting from "3" to "2" on an only stick shift car. The gear ratio is changed such that you will slow down going downhill without breaking. (I assume, but don't "know" that it also puts more regenerative energy into charging the battery just as using the brake does.) On minor hills it limits the braking needed to keep speed down.
     
  12. fuzzy1

    fuzzy1 Senior Member

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    In general in non-plug-ins, this similarity to downshifting (on automatics too, not just stick shifts) happens only under deceleration. Under acceleration, and propulsion in conditions requiring the ICE, it is essentially a "no-op".

    Well, except for locking out cruise control. Maybe some corner cases too. And plug-ins have more modes.
     
  13. Trollbait

    Trollbait It's a D&D thing

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    The shift in gear ratios that could happen with B may mean more spirited acceleration. At least, that is how it sounds like what happens with the Camry hybrid.
     
  14. GoodOldBob

    GoodOldBob Junior Member

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    I noticed on my Gen. 4 2020 Prius L Eco that in B mode, the cruise control refuses to engage. However, switching back into D mode returns the cruise control functions.
     
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  15. ChapmanF

    ChapmanF Senior Member

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    Yeah, that change happened after the gen 1 Prius, where you could still use cruise in B mode.

    That helped in gen 1, because the cruise algorithm in gen 1 was just copy/pasted from other cars, and it was just like a machine pushing the go pedal for you. It wasn't very good at holding your set speed downhill, but using it in B made it better at that.

    In the later gens, you aren't allowed to use cruise in B mode, but it's ok, because the cruise control on its own knows how to hold your speed downhill, using the same approaches B mode uses (and it can use them more strongly than B mode does).
     
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  16. fuzzy1

    fuzzy1 Senior Member

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    If so, that would be a new feature since my Gen3, and I don't remember it being mentioned for Gen4.
     
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  17. fuzzy1

    fuzzy1 Senior Member

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    ... as declared in the Owners Manual, top of page 334, in the first version OM of that model year (through February), or page 333 in the second version OM (from March).

    Toyota Manuals and Warranties | Toyota Owners
     
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  18. Trollbait

    Trollbait It's a D&D thing

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    The Prius manuals mention B only applying engine braking or stronger off pedal braking.
    The Camry hybrid has the ICE shifter, and its manual states, "By selecting shift ranges using S mode, you can control accelerating force
    and engine braking force." The ICE manual mentions gear ranges with S mode, so it isn't a simple cut and paste between the two.

    The question is if these similar hybrid systems have different shifter software based on the installed shifter. B was included to give the driver an option analogous to downshifting for controlling descent speed. Downshifting is also used to supply more power to the drive wheels by operating the engine at a higher rpm. Is B a full range downshift emulator, and Toyota never mentions it because it is presumed Prius drivers would only be using it for deceleration? Or did they add for hybrids with access to a multi 'gear' B? Also possible the increased acceleration force isn't something noteworthy considering the braking force from B.
     
  19. fuzzy1

    fuzzy1 Senior Member

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    At least on my Gen3 Prii, after using B for its intended downhill purpose, I inadvertently left it in B for extended periods on subsequent flats and uphills more than enough times to see how it behaved. For steady cruising and for uphills, it was no different than D whatsoever. Without looking at the gear selection indicator, the only reminders that I wasn't in regular D, were enhanced deceleration when gliding to stops, and failure of the ICE to shut down during such glides or when actually stopped.

    Its usual engine control laws, trying to stick to a set torque-vs-rpm load line on the BSFC chart occasionally copied here, don't really need any D vs B downshifting effect on the cruising and acceleration side.

    My new RAV4 Prime lacks any B, but does have generic D, plus D1...D6 and S1...S6. I haven't yet meaningfully tried its numbered gears, but am suspecting that the Ss are more like your Camry hybrid, and the numbered Ds like a Prius B expanded to multiple steps.
     
  20. ChapmanF

    ChapmanF Senior Member

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    ... which is something useful in a tranny that has only fixed choices of ratio.

    With the Prius CVT, running the engine at the best RPM for the amount of power you want, and matching that RPM exactly to the wheel speed, is just what the car normally does all the time.
     
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