What Exactly does the Added Clutch Do?

Discussion in 'Prime Technical Discussion' started by mr88cet, Jan 18, 2017.

  1. mr88cet

    mr88cet Senior Member

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    As you all probably know, Toyota says that they added clutch to the drive system on the P.Prime, to allow both MGs to drive in EV mode.

    What two things specifically does that clutch make a mechanical connection between?

    Conceptually, I can see two ways that the power-split device could be clutched to allow both MGs to drive the wheels:
    • The ring gear could be connected to the sun gear, so that the entire planetary-gear complex is frozen together into a single unit, and both MGs turn in the same direction at the same speed.
    • The engine and thus the planetary-gear carrier is frozen still, and the two MGs turn in opposite directions, the sun gear turning considerably faster than the ring gear.
    Since it can go up to 84MPH in EV, I'm guessing it's the former. If it were the latter, the sun gear would have to turn mighty fast! If it is the former, it's kinda unfortunate that the ICE keeps cranking over in EV mode, especially if the oil hasn't had a chance to warm up. Sure would be great if the engine were completely motionless!

    Any of you know how it works?
     
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  2. bhtooefr

    bhtooefr Senior Member

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  3. MikeDee

    MikeDee Senior Member

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    Seems like the term Sprag clutch is a misnomer; if the Prius uses clutches like that shown in the article. Sprag clutches use bearings; not ratcheting pawls like a bicycle freewheel.
     
  4. mr88cet

    mr88cet Senior Member

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    So it looks like it's not as much a clutch as a ratchet.

    I was kinda hoping that the engine would for-sure not rotate (neither forward nor backward) in order to reduce wear and drag. No guarantee that it won't turn forward, it appears.
     
  5. bhtooefr

    bhtooefr Senior Member

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    However, what'll happen is that MG1 will apply torque to try to accelerate itself to higher negative RPMs, which would turn the engine in reverse. The one-way clutch prevents that, so MG1's torque instead spins the ring gear faster.

    Play with Toyota Prius - Power Split Device some... it doesn't show torques, only RPMs, but realize that MG1 increasing torque to increase vehicle speed would be in the direction of more negative RPMs.
     
  6. Lee Jay

    Lee Jay Senior Member

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    It's a sprag clutch.

    "A sprag clutch is a one-way freewheel clutch."
     
  7. Boomer 2298

    Boomer 2298 New Member

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    It’s a sprag clutch used in industrial power transmission for decades. Highly reliable.
     
  8. ChapmanF

    ChapmanF Senior Member

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    Can't believe we went four years without clearing that up.
     
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  9. PiPLosAngeles

    PiPLosAngeles Active Member

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    The press release says it's for 2.5L engines, which means it's not for the Prime?
     
  10. Tideland Prius

    Tideland Prius Moderator of the North
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    Not sure but there is a sprag clutch on the Prime to allow it to use both MG1 and MG2 for forward propulsion in EV Drive Mode.
     
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