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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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    Those two statements are wrong: engine works in "cut-off" regime, no fuel injected. Therefore a high depression takes place in the cylinder so that very thin oil may make its way into the chamber....
     
  2. Doug McC

    Doug McC Active Member

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    Basic physics: energy conversion and thermodynamics
    EDIT: I just realized I missed read your post: if the ice needs to be warmed up (based on what the ECU is seeing) it will do so, and if it was running once you were done descending the hill it was just coincidence and (probably) had nothing to do with B Mode.
     
    #62 Doug McC, Feb 17, 2024
    Last edited: Feb 17, 2024
  3. Doug McC

    Doug McC Active Member

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    Perhaps it might be helpful to watch it again to refresh your memory.
     
  4. Lantaral

    Lantaral Junior Member

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    Yes, he says it doesn't use fuel, but it does spin up the engine. This requires power.

    There is only one other source of this power, and that is the traction battery. So while it's not fuel inefficient, B mode is power inefficient once it has filled the battery and is required to further slow the vehicle with engine braking.

    Also explains why on an especially long downhill, the system would need turn itself off, as the battery is eventually exhausted.
     
  5. Lantaral

    Lantaral Junior Member

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    Perhaps you should actually read a post before replying?

    Rewatched the video and corrected my post above before you posted your snarky reply.
     
  6. ChapmanF

    ChapmanF Senior Member

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    You have left out gravity. As a 3500 pound object hurtles down a hill, quite a lot of power is available from that source. Easily enough to twirl the engine at a few thousand RPM, which is what happens.

    On the contrary: the battery doesn't end up exhausted on a long-enough downhill, it ends up full—of the energy recovered from gravity. And when the battery is full, regen braking that charges the battery has to stop.

    The engine-twirl braking can continue for as long as there is downhill to twirl it.
     
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  7. Doug McC

    Doug McC Active Member

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    Either I watched a different video (even though it’s the same one), or TOTALLY misunderstood nearly everything he said, or perhaps you are mistaken.
     
  8. Doug McC

    Doug McC Active Member

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    At least you and I seem to have watched the same video. I was beginning to think I had lost my mind (of course, that IS STILL a possibility ;)).
     
  9. KMO

    KMO Senior Member

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    Um... You may want to think this through a bit harder. You're not drawing down the battery. The battery is full in that scenario, and the engine is providing a place to dump energy generated from rolling downhill. The power is coming from gravitational potential.
     
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  10. sylvaing

    sylvaing Active Member

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    The way I read it, MG1 is wasting power by turning an unfueled engine while MG2 is pushing power to the battery but since MG1 is wasting more power to than MG2 is giving, it's a net negative.
     
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  11. fuzzy1

    fuzzy1 Senior Member

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    ...NOT!

    In my experience in this mode, MG1 consumes only what MG2 gives it, not any more. ICE RPM goes according to what MG1 is driving it. But RPM is also capped by an ECU, so when it can't safely go any faster, MG2 output is capped accordingly. When this happens, the car starts creeping up to higher road speed, requiring the driver to apply friction brakes.

    Make that a net zero.
     
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  12. ChapmanF

    ChapmanF Senior Member

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    MG1 and MG2 share power on a common buss inside the inverter, so a simple clear way to say it is the wheels in this case are turning MG2 and generating electrical power, and in pure engine braking that power is going straight to MG1 to twirl the engine. (The rest of the power from the wheels is going mechanically through the transmission, and also contributing to the engine twirling.)

    The cable between the inverter and the traction battery only ever carries the net difference (+ or −) between what MG1, MG2, the air conditioner, and the 12 volt converter are doing. That difference can be into the battery, out of the battery, or nothing.

    The HV control ECU gets to decide how much power to send to or accept from MG1 and MG2, so it can make that net difference work out however it wants. When it wants to charge the battery, it chooses one imbalance between MG1 and MG2, and when it wants assistance from the battery, it chooses the other imbalance. A lot of the time, it doesn't want to either charge or use the battery very much, so it just arranges for one MG to be producing the same amount of electrical power that is going to the other one, plus the 12-volt system and A/C.

    Edit: jinx
     
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  13. vvillovv

    vvillovv Senior Member

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    There is a lot more room for regen with a Prime in EV mode. But, regen - generally - is dependent on a host of other conditions like ambient temp, battery temp, speed, SOC. Any others? The ECU's collect the current conditions / sensor data and than decide how much regen can be added to the battery for those conditions.
    As one example; if the battery temps are too cold regen to the battery is limited and the engine will spin / fire.
    If someone want to prove the Gen 4 and Gen 5 Primes always use engine pumping without fuel, I'm all ears.
    Put please keep in mind that regen with a warm larger battery functions way different than regen to cold battery and I believe it also is quite a bit different with a larger battery that allows more than a mile or so in EV mode.

    If it's possible to explain the three levels of regen / B mode in the Gen 5 Prime, please I'm very interested in having it explained or linked to a page in the doc. Thanks for bring that up ;)
     
    #73 vvillovv, Feb 17, 2024
    Last edited: Feb 17, 2024
  14. ChapmanF

    ChapmanF Senior Member

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    During engine braking, the engine is needed as a power sink, not as a power source. Injecting fuel would make it less of a power sink, so the engineers would need to have some other pretty good reason to entertain doing that.
     
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  15. vvillovv

    vvillovv Senior Member

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    Might a large heat sink like a cold battery be one of those possible conditions?
     
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  16. Doug McC

    Doug McC Active Member

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    I could have been much more explicit in pointing out your error and THAT would have been “snarky”. Simply, and politely suggesting you were in error is not being “snarky”, nice try. No body is on a “witch hunt” here and narcissistic attempts at insults don’t change the fact you were WRONG! Sorry.
    EDIT: Just rethought my response, so a correction: you are right: I now realize I am being a fool. Those who are familiar with my posts will most likely understand what I am saying.
     
  17. ChapmanF

    ChapmanF Senior Member

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    In braking or a descent, your situation is you already have more power than you want being thrown at you by momentum/gravity, and your mission is to get rid of it somehow. You don't need to make more.

    If you have a cold battery that you want to warm up by putting some small amount of power through it, why wouldn't you just use some of the excess power you're already trying to think of what to do with?
     
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  18. vvillovv

    vvillovv Senior Member

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    You're writing about 1 scenario, and one set of conditions. Might there be some other conditions that don't fall into your wasting energy by running the engine scenario that might change the behavior called for?
     
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  19. sylvaing

    sylvaing Active Member

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    Thinking about it, at the 10 minutes mark in the video, he does mention that the system becomes inefficient because you can't generate more than what you give. However, I would venture that this would be true without any potential energy (ie, level plane), because as soon as you're on an incline (and that's the main reason for the B mode from what I understand), you have all that potential energy that you also have to shed.
     
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  20. Doug McC

    Doug McC Active Member

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    Good point and well put.