B Mode, Battery Charge and Charge Rate

Discussion in 'Gen 2 Prius Technical Discussion' started by jerlands, Oct 24, 2020.

  1. jerlands

    jerlands Member

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    I've read "B Mode Made Clearer" http://techno-fandom.org/~hobbit/cars/b-mode.html but it doesn't seem to answer my question.

    I'm curious how the Prius spends extra energy when the battery is fully charged. MG2, by itself, can generate 100 amps of current... the full charging rate of the battery pack. When in B Mode, I had thought MG1 acted as a generator but this isn't making sense to me now. if the battery is fully charged and you're traveling a long descent, the only thing helping slow down the vehicle is the resistance supplied by the generating force of the MG sets, the compressive force of the ICE and the hydraulic brakes. it would seem that in B Mode MG1 is not acting as a generator but power is being supplied to it in a fashion that it would oppose both MG2 and the ICE?...
     
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  2. fuzzy1

    fuzzy1 Senior Member

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    MG2 is acting as a generator. The power is being pushed into the ICE for compression (or vacuum, depending on viewpoint) braking. MG1 is controlled however necessary to get the desired power flow from MG2 to the ICE, and sets the ICE RPM.
     
  3. jerlands

    jerlands Member

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    ok... so what exactly is pushing power into the ICE? you cannot speed up the ICE without an increase in speed... the only thing I see the system doing is pushing power into the MG1 (so that it opposes MG2 and ICE.) ..
     
  4. jerrymildred

    jerrymildred Senior Member

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    The car's inertia is the input force. The whole idea of B mode in a non-plug-in-Prius is to use the ICE to bleed off the car's kinetic energy rather than using the battery to absorb it. The wheels spin the transaxle shaft and the car's computer tells the inverter how much current to feed in which direction to the MGs to make them spin the ICE and use it as an air pump to dump energy from the car and it's contents so it will slow down. (or accelerate less if it's a really steep hill) So, a little energy comes out of the battery but only enough to get the MGs to do what they need to do to route the car's inertia to the ICE.
     
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  5. fuzzy1

    fuzzy1 Senior Member

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    I should have thought through slightly more before my previous reply ...
    Both other shafts are spinning and forcing the ICE shaft to spin too. The MG2/wheel shaft will spin a rate fixed by the ground speed. In most cases, MG1 will then be acting as a motor, spinning at whatever RPM and torque are necessary to force the ICE to spin at the desired braking RPM. MG1 should be getting all its power from MG2.

    ... or spin MG1 at whatever rate is necessary, keeping in mind that it is also a function of MG2 RPM.

    MG1 is not opposing MG2, but instead working in concert with it.

    I'm thinking that nothing is coming out of the battery at this point, it and all other overhead power needs are coming from MG2.
     
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  6. jerlands

    jerlands Member

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    Still... doesn't make sense to me... if B Mode simply used the compressive resistance of the ICE then slowing down in D Mode would be the same... even with the air intake open... if you increase the speed or the energy going into the ICE you will increase the speed of the vehicle. The only way to slow this thing down is by using MG1 as a Brake... sending power to it so that creates a magnetic field that opposes the rotation of both MG2 and the ICE...
     
  7. jerlands

    jerlands Member

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    If you're moving in the forward direction and applying a force to MG1 that opposes the direction of MG2 and ICE that makes sense...
    I don't think that is true... all the time... MG1 is basically the functioning alternator.. if you let your foot off the accelerator then MG2 becomes the generator because it is so much larger than MG1.. in fact MG2 can by itself exceed the 100 amp charging rate of the battery... so what does the Prius do then, when the battery is fully charged?... it can't drop the load on MG2 because it is needed to maintain the resistance to slow the vehicle down...
    I'm pretty certain now that MG1 is being used to oppose both MG2 and the ICE when in B Mode...
     
    #7 jerlands, Oct 24, 2020
    Last edited: Oct 24, 2020
  8. TMR-JWAP

    TMR-JWAP Senior Member

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    if you increase the speed or the energy going into the ICE you will increase the speed of the vehicle.

    The speed of the ICE changes the speed of the car when the ICE is the prime mover and the wheels are the load. During "B" braking, the wheels are the prime mover and the ICE is a load. (not a source of energy).
     
    #8 TMR-JWAP, Oct 24, 2020
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  9. jerlands

    jerlands Member

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    The inertia of the vehicle is the prime mover, not just the ICE. The ICE can help slow the vehicle just like any engine can using compressive resistance but if that were the only force resisting gravity the vehicle would continue to speed up. In D Mode the vehicle will continue to speed up... in B Mode the vehicle will slow down... so... what is causing the vehicle to slow down in B Mode?... I'm pretty sure it isn't just the added air to the compression cycle...
     
    #9 jerlands, Oct 24, 2020
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  10. dolj

    dolj Senior Member

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    Don't forget that when the battery is completely full, the VVT is used to reconfigure ICE so it becomes an inefficient super compressor to use as a load to dump any excess energy.
     
  11. fuzzy1

    fuzzy1 Senior Member

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    While conceptually the same, the degree is very different. See my past posts describing ICE RPM and speed control effectiveness when switching back and forth on the same hill descent.
    On that hill, D produced about 2000 RPM and my car kept speeding up. B produced about 4600 RPM and was able to hold my car back to a steady speed.
    That sounds like magnetic eddy current braking. But that would seriously overheat MG1, if it were even built for that sort of operation. But that isn't what Toyota's engineers have ever described to us.

    Please revisit that thought. The very early PriusChat folks were invited to talk to Toyota's Prius engineers during new product introductions. You are trying to contradict what those Toyota engineers said.
     
    #11 fuzzy1, Oct 24, 2020
    Last edited: Oct 25, 2020
  12. jerlands

    jerlands Member

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    the only way I see the engine speeding up while in starvation mode is to be driven by MG1?
     
  13. fuzzy1

    fuzzy1 Senior Member

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    Yes, it is being driven in part by MG1, which in turn is being driven by MG2.

    But don't forget the other part driving the ICE: a direct mechanical path (via gears) from the drive wheels, also driving MG2.
     
    #13 fuzzy1, Oct 25, 2020
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  14. TMR-JWAP

    TMR-JWAP Senior Member

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    The inertia of the vehicle is the prime mover, not just the ICE.
    ???????????? I'm pretty sure I stated:
    The speed of the ICE changes the speed of the car when the ICE is the prime mover and the wheels are the load. During "B" braking, the wheels are the prime mover and the ICE is a load. (not a source of energy).

    sooooo….. Wheels aren't going to be much of a prime mover if the car isn't rolling and decelerating.

    The ICE can help slow the vehicle just like any engine can using compressive resistance but if that were the only force resisting gravity the vehicle would continue to speed up.

    In normal D mode, the compressive resistance via the ice is minimized by the hybrid system to maintain momentum of the car.
    In B mode, it is not minimized, but is controlled in multiple ways to increase energy use (waste) to attempt to reduce speed.
    What makes you think the car will always slow down in B? A steep enough grade will still make the car speed up without use of friction brakes. B mode still has limits.
     
  15. jerlands

    jerlands Member

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    MG2 and the ICE are not mechanically connected.. only through the planetary... when in neutral and coasting down a hill there is no mechanical disconnect... but the engine doesn't experience any load...

    so... how exactly is it controlled other than via MG1 and the air intake valve?...
     
    #15 jerlands, Oct 25, 2020
    Last edited by a moderator: Oct 29, 2020
  16. fuzzy1

    fuzzy1 Senior Member

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    That planetary gearset is a mechanical connection. A substantial amount of power flows directly through that mechanical path, while another portion flows through the electrical path.

    The HSD transaxle operation always involves both the mechanical and the electrical path. Break either path, and it ceases to transmit any power.
    Whenever the engine is spinning but fuel is cut off, the engine is a load. The degree of load is a function of variable valve setting, intake manifold vacuum, and RPM. Heavy engine braking involves strong vacuum and high RPM.
     
  17. jerlands

    jerlands Member

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    the planetary gears can rotate freely in the ring gear without moving the ICE...
    none of that seems to explain what happens when the drive is in N Mode... the vehicle is coasting yet no load present on the engine? so... what happened to the mechanical path?...
     
  18. TMR-JWAP

    TMR-JWAP Senior Member

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    I am not a Toyota engineer. I don't know every fine detail about how the system works, as many things about the hybrid system are not available to the typical joe. In the big scheme of things, I don't spend my days worrying about things that are out of my control. The details of the hybrid system that you're asking about fall into that category. How much I know about "B mode", has no effect on 99.99% of the troubleshooting and repair that I (or anyone else) performs. Any document I could read is also available to you.
     
  19. jerlands

    jerlands Member

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    Prius (2nd Gen) P112 Hybrid Transaxle (eCVT) Operation
     
  20. TMR-JWAP

    TMR-JWAP Senior Member

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    There are plenty of good videos available on the net showing how the system works. They're made by people and facilities that teach the subject. Feel free to use them. They're good info and probably better than anything you're going to get on a text forum.
     
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