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Does your electric motor regenerate electricity while rolling in neutral?

Discussion in 'Gen 3 Prius Technical Discussion' started by Priipriii, Aug 14, 2022.

  1. Priipriii

    Priipriii Member

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    I am trying to flat tow a non-working prius. I know by manual and after reading tons of threads on here, you are NOT supposed to flat tow a prius due to electric motor connected to the wheels at all times, even in neutral. But with that logic, even driving down a hill while in neutral is the same damage as towing the car. I do not see how it can be bad for the car UNLESS, the transmission is constantly charging electricity while its in neutral.

    So I would like to know, if I have it in neutral, will I need to have a load to discharge the created electricity from the electric motor, or does it not generate any electricity in neutral to begin with? Does anyone actually know what is being done inside the car's transmission while its being flat towed?
     
  2. Tombukt2

    Tombukt2 Senior Member

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    Are you towing the car in the ready mode? I wouldn't think if the car's not ready voltage isn't moving or it's going nowhere I would think your electrical system has to be switched on already. But that's just me I don't even know how to flat tow a Prius. So you're talking about I guess you would have to make it ready to get it in neutral and for it to stay there. I don't know I think there's a way to put it out of gear without having to do all that but weird
     
  3. ChapmanF

    ChapmanF Senior Member

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    The two motor-generators in the transmission are made with permanent magnets, so they make voltage any time they move. There is no way to make that not happen.

    The driving mode "neutral" simply keeps all of the switching transistors turned off, so the motors are open-circuited. Current is not flowing, but the full open-circuit voltage being generated, depending on their RPM, is appearing at their terminals and across the gated-off transistors.

    The RPM of MG2 is always directly proportional to road speed, regardless of driving mode.

    If you're towing down the street in "neutral", most likely the engine is not spinning, and that will mean MG1 is doing fairly high RPMs backward. Depending on how fast you tow, that might put fairly high voltages across the transistors, and even high physical stresses on MG1's rotor itself. Early Prius generations were programmed to limit engine-off operation to below around 42 mph, if I remember right. The limit may have increased some in the later generations, but I don't have the details at my fingertips.

    I generally avoid rolling down a hill, or any significant distance, in "neutral". I'll do stops in "neutral" on residential streets sometimes, as a way to clean rust off brake rotors.

    I think that if you shift to "neutral" from a speed above the limit, where the engine is already running to keep MG1's speed down, the engine keeps running, so MG1 stays within safe limits.

    If you shift into neutral at a lower speed, where the engine isn't running, there could be a question of what happens if you are rolling downhill and pick up speed. In the Gen 2 New Car Features manual, on page TH-46 under "Shut Down Control", it explains that the ECU can veto your selection of "neutral" in three special circumstances, and picking up speed beyond the safe range is one of them. So it can then decide "neutral" wasn't quite what you meant, at least long enough to crank up the engine and keep the speeds under control.

    That might be a bit of software they added for Gen 2, so I might be more leery of letting that happen in a Gen 1. I assume Gen 3 is also smart enough to do that.

    If the reason you're towing is because of any problem that would keep the engine from spinning, that safeguard won't be available ....

    You don't have to be in READY to shift into "neutral"; it's also available in IG ON.

    But probably the "you must not have meant neutral, I'll spin the engine for you" safeguard is not available except in READY. (None of the high-voltage circuitry is active except in READY.)
     
    #3 ChapmanF, Aug 15, 2022
    Last edited: Aug 15, 2022
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  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    While Toyota is required by law in the US to offer an N, it is almost never a good idea to use it.

    If you feel the brakes are rusting, shifting to N before braking at highway speeds may clean them.

    Pushing your Prius around a parking lot can be done in N.
     
  5. ChapmanF

    ChapmanF Senior Member

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    Shifting to N before braking at perfectly tame residential speeds, coming up to an ordinary stop sign, cleans rust off my brakes just fine. I prefer that to doing the same at highway speeds.

    Doesn't require making any big production. I generally aim for, if there are passengers with me, they don't notice me doing anything special, if they weren't watching my hand on the shifter.
     
  6. Tombukt2

    Tombukt2 Senior Member

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    We have the country out here in North Carolina we can whip down backcountry roads 90 miles an hour and shift to neutral and break all we want rarely is there anyone around. I need to round up a set of rear discs for my Gen 2
     
  7. bisco

    bisco cookie crumbler

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    what is the minimum speed you need to tow it at?
     
  8. ChapmanF

    ChapmanF Senior Member

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    If you're whipping down the backcountry road at 90 miles an hour when you shift to "neutral", the engine is already spinning, which is what makes that ok.
     
  9. Priipriii

    Priipriii Member

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    So based on all you said, a working prius can be flat towed in neutral, but its engine will be turned on while doing so. But a non-working prius that does not even start, cannot be placed in neutral. Thus the wheels will be locked to the transmission/engine?
     
  10. ChapmanF

    ChapmanF Senior Member

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    I don't think it's quite that simple.

    You can shift to "neutral" from IG-ON (two power button presses without brake) or from READY (one power button push with brake).

    So you can get to "neutral" easily in a Prius that won't start. Only if the Prius actually won't even go IG-ON is that a problem.

    But from "neutral" in IG-ON, the car won't be able to spin the engine to keep MG1 speeds safe if you tow too fast.

    From "neutral" in READY, it probably can, as long as the engine is spinnable and the high-voltage systems aren't messed up. But that's getting further and further from what you'd normally think of as towing a car in neutral.

    Toyota recommends just not flat-towing the car, and that's probably good advice, given the complexity involved.
     
  11. fuzzy1

    fuzzy1 Senior Member

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    If I remember right, OP's Gen3 (model year 2013) has this limit set at 46 mph, just a small increment higher than previous generations. Gen 4 (2016+ Liftbacks) bumped this threshold speed up much more.
    The 'transmission' / engine / MG1-2 are always mechanically 'locked' to the wheels, even in neutral. There is no mechanical disconnect. The risk is from MG1 spinning too fast. If the engine is operable, the car's computers can keep MG1's RPM down to a safe speed. When the engine is not operable, that protective action is not possible.
     
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  12. douglasjre

    douglasjre Senior Member

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    No. And don't do it
     
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  13. bisco

    bisco cookie crumbler

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    get those roller skates for the front wheels
     
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  14. Tombukt2

    Tombukt2 Senior Member

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    Yes I have a set here I took out of the box and looked at to use to push the cars around the garage floor where I used to live and they've sad ever since and they make some larger ones that have bigger wheels that the tow drivers use I can probably borrow a set of those.