Why does the engine wind out when in B mode?

Discussion in 'Gen 2 Prius Technical Discussion' started by jerlands, May 15, 2019.

  1. jerlands

    jerlands Member

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    Going downhill, my engine winds out terribly when the battery is fully charged in B mode. I'm assuming this is because MG1 (which is the only thing driving (starting) and driven by the engine) is being fed current is such a way that it rotates in opposition to the direction of the engine?
     
  2. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    When you put it in B mode for hill descent, the computer assumes that there is more downhill road than space in the battery. It does so to prevent you from getting into a dangerous loss-of-brakes scenario once the battery fills up. So with B mode the engine is spun up and used as a torque sink. This is roughly the same as traditional engine braking in other cars.

    It can be a bit noisy but it is safe and effective.
     
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  3. jerlands

    jerlands Member

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    And this is accomplished via MG1 being used as a motor in opposition to engine rotation?
     
  4. fuzzy1

    fuzzy1 Senior Member

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    When the battery is full, then B mode is effectively the same as downshifting in a non-hybrid, winding up the engine's RPM. MG1 is used to spin it up, but the engine does the actual compression braking and makes the load jet engine / vacuuming / banshee noise.
     
  5. jerlands

    jerlands Member

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    What I'm trying to get straight in my head... MG1 is both a motor and a generator... the only thing connected to the engine... it acts a generator to charge the battery, as a motor to start the engine... but I'm confused how it is used in regenerative braking...
     
  6. jerlands

    jerlands Member

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    Yes but isn't this accomplished by opposing the direction of engine rotation with MG1.. that seems the way it's done to me?
     
  7. fuzzy1

    fuzzy1 Senior Member

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    Both MG1 and MG2 are mechanically connected to the engine, through the PSD which is effectively an open differential.

    Both MG1 and MG2 can be either motor or generator, depending on the need at the moment. The ICE does the compression braking.
     
  8. jerlands

    jerlands Member

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    Thank you... now I have PSD in the picture :)
     
  9. Prodigyplace

    Prodigyplace Senior Member

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    The engine is connected to the transaxle too.

    MG1 starts the engine & charges the battery through the engine excess power. On the prime it can also drive wheels through a one way clutch.

    MG2 drives the wheels & charges the battery through regenerative braking.
     
  10. Zeppo Shanski

    Zeppo Shanski Member

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    I never really understood why there was a "B" mode.
     
  11. M. Oiseau

    M. Oiseau 6sigma this

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  12. Prodigyplace

    Prodigyplace Senior Member

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    Prevent overcharging the traction battery when coming down Pikes Peak. :D
     
  13. srellim234

    srellim234 Senior Member

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    It also helps you from being put in the penalty box by a Ranger for overheated brakes too. Waiting for brakes to cool can really add some time to your descent.

    Driving Down Pikes Peak Mountain - Pikes Peak Colorado Springs
     
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  14. fuzzy1

    fuzzy1 Senior Member

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    As Jimbo frequently points out, federal regulations require some form of downshift available for hill descents. On all cars, not just hybrids. "B" mode serves that function.

    Flatlanders don't need it, but those of us driving in the mountains do need it.
    My spouse wanted to stop at the gift shop there. A minute later, an F-250 with Florida plates was ordered into the penalty box for a 30 minute cooling off period, stinking up the whole area with the aroma of burnt brakes.
     
  15. MickyMatter

    MickyMatter Active Member

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    You can find a German description of the different modes here:
    PSD – Prius Wiki
    (With Google translator or something similar you should understand it.)
    Look for 7.2 for braking in B-mode in general and there for the link to the B-mode particularly, too.
    There's a nice explanation, I think.

    Edit: maybe this link with the translation will do: https://translate.googleusercontent.com/translate_c?depth=1&hl=de&nv=1&rurl=translate.google.com&sl=auto&sp=nmt4&tl=en&u=https://www.priuswiki.de/index.php%3Ftitle%3DPSD&xid=17259,15700021,15700186,15700190,15700253,15700256,15700259&usg=ALkJrhjDCWAW7Ij7qgSjnDtt6d-mtwwKgg

    SM-G950F ?
     
  16. ChapmanF

    ChapmanF Senior Member

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    Engine braking (whether in a Prius or any other car) is done by the wheels spinning the transaxle and the transaxle spinning the engine. Everything is spinning in its normal direction, and the only thing different is what's spinning what.

    The engine is burning minimal fuel, or no fuel at all in modern cars, so it sure isn't spinning itself, and it takes plenty of work to externally spin an engine that's not under its own power, and that's what slows the car down.

    One thing that's different in the Prius is it's all under computer control. In a different car, you can downshift to get some engine braking, and you can overrev the engine if your speed is too high or your gear is too low or you're not paying attention.

    The Prius HV computer is coordinating all of this, and it knows the completely safe RPM range for the engine, and it will use as much of that range as it needs to, but never beyond it. Even if you think it sounds like it "winds out terribly", there's nothing terrible about it; it's well inside the engine's operating range and fully controlled at all times.
     
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  17. jerlands

    jerlands Member

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    What I experience in B mode going downhill and the battery fully charged is extreme force on the engine. It sounds like its reaching 7,000 rpm rapidly from like 3,000. I understand that during regenerative braking in B mode both MG1 and MG2 act as generators slowing the vehicle down and outputting electricity to the battery but when it's full it sends that excess current to MG1 and acts as an engine brake, putting an opposing force to the engine force and direction. I'm changing the battery in this thing and am going to test regenerative braking afterward to see if I get the same type of effect. I'm thinking the battery doesn't take current as well as a new battery so the excess developed won't be so great?
     
  18. jerlands

    jerlands Member

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    So were you outside your vehicle looking for brake pads smoking :)
     
  19. jerlands

    jerlands Member

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    Thanks... I found a video that was adequate enough to explain it to me...
     
  20. fuzzy1

    fuzzy1 Senior Member

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    We didn't have to go look for them. The heavy stench announced itself to everyone in the area.
     
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