Why does the engine wind out when in B mode?

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

  1. Skibob

    Skibob Senior Member

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    Don’t know what to say. If your concerned you could sell the Prius and buy a regular car I guess.
     
  2. jerlands

    jerlands Member

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    I'm just discussing this to learn more...
     
  3. jerlands

    jerlands Member

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    Like how do I release the electrical connector to the trunk light :)
     
  4. jerlands

    jerlands Member

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    some of my interest, since I had the episode where I thought it ran away, was to see what was what. But for it to run away MG1 would have to power MG2. I've learnt there is an electrical path between the two but I don't know if it's possible with electrical interlocks and such.
     
  5. Skibob

    Skibob Senior Member

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    Could you have experienced brake fade? When was the last time the brake fluid was flushed?
     
  6. ChapmanF

    ChapmanF Senior Member

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    MG1 is involved in everything the transmission does. Any time MG1 is not "involved" (supplying torque one direction or the other), it freewheels and the transmission is effectively in neutral, communicating zero torque between engine and wheels. ("Zero" but for the drag of some bearings, anyway.)

    The Prius transmission is an almost artistically elegant, deceptively simple mechanical/electrical contraption that is very rewarding to study and figure out. It has a surprisingly long history: you can recognize an astonishing lot of it over a century ago in the 1908 patent by John Godfrey Parry Thomas, only he had no way to imagine silicon computers fast enough to monitor and control such a thing, so it had to stay an interesting idea until Toyota could fill in that part of the picture.

    It can provide one of the most satisfying "Aha!" moments available anywhere, when the understanding starts to fall into place. But that can't really be rushed quite the way a lot of new owners want to, by trying to think simplistically about what any single part (like MG1) is "doing" to any single other part at some particular time. Without all the parts acting together, the transmission isn't there.

    I'd suggest that the "Aha!" moment comes faster if you step back and think of it as algebra, involving the torque relationship between the three players (fixed, never changing), the speed and power relationships between the players (not fixed, continuously variable), and the voltages and currents flowing between the two MGs, which are three-phase AC—that is, waveforms—such that not just volts and amps matter, but frequency and phase, controlled by the microsecond timing of the solid-state switch flips inside the inverter ... which doesn't even look like it's part of the transmission really, but is where the crucial magic happens.

    Surprisingly, the battery is one of the least important parts to keep in mind for any of that. It really serves as a slosh tank for brief moments when either more power is being generated than used or vice versa, only for as long as the computer needs to adjust the other components to be in balance again. Other than EV mode or in a PiP or Prime, that's about all it's needed for, though it's also handy for starting the engine.

    I'd venture that if you spend a while just driving the car and getting used to it, while steadily sipping at information you can find on how the transmission works (and on the basic topics that go into that), and wait a bit longer before trying to impose your own assumptions on what "would have to" happen in one operating mode or another, you'll get the "aha!" when you get it, and the wait will be worth it.
     
    #46 ChapmanF, May 16, 2019
    Last edited: May 16, 2019
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  7. Skibob

    Skibob Senior Member

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    That’s a good description. I think of the battery as more of a capacitor when the car is in ready, but slosh tank is probably a good way to explain it. Saying capacitor would just confuse some people.
     
  8. jerlands

    jerlands Member

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    I started thinking along the lines of regenerative braking quitting for some reason and if I was going down hill at the time (I think I know where I was :) the sudden release of resistance might have felt like powered acceleration?
     
  9. bisco

    bisco cookie crumbler

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    that's usually around 6 mph, especially when you hit a bump
     
  10. Skibob

    Skibob Senior Member

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    I’m pretty sure if your going 65 down a hill and the battery is 100% charged the brakes will kick in. Think about it? It’s the only path left. Other than engine braking.
     
  11. Skibob

    Skibob Senior Member

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    Get the brake fluid flushed.
     
  12. jerlands

    jerlands Member

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    I'm confused about this then because I understand excess energy get routed to MG1. no mention of it activating brakes or any other type of intelligent action if the car speeds up excessively while going downhill.
     
  13. ChapmanF

    ChapmanF Senior Member

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    Some details have changed over the generations; I've known a Gen 1 and a Gen 3 well, haven't spent enough time in a Gen 2 to be sure which one it's more like, but I think in these details it's more like Gen 3.

    In Gen 1:

    If you press the brake pedal, it will do regenerative braking to the extent there's battery capacity to use, and apply the friction brakes for the rest.

    If you shift to B and stay off the brake pedal, it will do somewhat more aggressive regenerative braking, but send less of the harvested power to the battery, and use the rest to spin the engine. It will not apply the friction brakes unless you use the pedal.

    The Gen 1 cruise control only really plays the go pedal. You can use it in D or B. In D it will tend to pick up speed downhill, but in B it will generally do a nice job holding speed both up and down hills. Downhill it will do some battery charging and some engine spinning. If the downgrade and speed represent more power than the combined limits of available battery capacity and engine braking at the maximum allowed rpm, the car will pick up speed. Again, it will not apply friction brakes unless you use the brake pedal.

    In Gen 3:

    Using the brake pedal, you get regenerative braking, with friction picking up the rest. If the battery is near full, you can get some engine braking, even if you are not in B.

    If you shift to B and stay off the brake pedal, the behavior is as for Gen 1.

    In Gen 3, you cannot use B together with the cruise control. Cruise only works in D. However, the Gen 3 cruise control in D will control speed both uphill and down, so there's no need to use B with it as there was in Gen 1. Going downhill, it will use regenerative braking, splitting the harvested power between the battery and engine spinning as capacity permits. Again, once the speed and downgrade amount to more power than the battery and engine spin can consume, the car will simply pick up speed. It will not bring in the friction brakes on its own, but only when you use the brake pedal.

    Again, I think the Gen 2 behavior is like the Gen 3, but somebody with a Gen 2 can confirm if I remember that right.

    Also again, it doesn't help understanding to insist on saying things like "routed to MG1" during engine braking. All the parts of the transmission act together. Of the torque applied to the engine during engine braking, MG1 is supplying only 28%.
     
  14. fuzzy1

    fuzzy1 Senior Member

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    ???

    Are you confusing threads? Without a foot on the brake pedal, the brakes are not applied.
     
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  15. jerlands

    jerlands Member

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    Sorry, I must be the only one hearing these terms used like current dump or current sink for MG1. In B Mode that 28% might be true as far as regenerative braking but when the battery is full (or approaching full) current gets fed to MG1 (or dumped) and this is the cause of the high revs I complain about :)
     
  16. Skibob

    Skibob Senior Member

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    Just making a point. The OP has 2 threads regarding his car and going downhill.
    Honestly I give up with this guy.
    Best of luck.
     
  17. jerlands

    jerlands Member

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    stop fartin' on the way out...
     
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  18. ChapmanF

    ChapmanF Senior Member

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    Internet forums are full of things posted by lots of different people, some of whom understand what they're posting about, and the posts don't come with labels. Case in point: your thread here is now a permanent part of PriusChat, and other new folks will be coming here and getting odd ideas about how the transmission works, which will end up being hard to talk them out of so they can learn.

    It would not be impossible for you to learn this stuff if you wanted. The 28% / 72% torque split is one of the few things about the transmission that is absolutely fixed at all times (it comes straight from the number of teeth of the sun gear, planet gears, and ring gear in the PSD). Those gears never change, never shift, never unmesh.

    That split means that no matter what the torque is at the engine, whether algebraically positive (engine sourcing power) or negative (engine being used as a sink), 28% of that is accounted via the sun gear (= MG1) and 72% via the ring gear (= final drive and MG2, or the MG2 reducer in Gen 3 and later). No more, no less.

    Now, that fixed split is a split of torque. Power is a product of torque and rpm, and the rpms at all three components, and therefore the power ratios, are variable; that's what the transmission is for, after all. But if you take a little time learning how that happens, the behavior of the car will make more sense to you.
     
  19. jerlands

    jerlands Member

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    I still don't have the PSD envisioned because it sounds more and more to me like a sum of vectors adding up. There is no fixed point in that device, just three things acting together producing an end result.
     
  20. jerlands

    jerlands Member

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    All those gear relations are important for things like how many times MG1 will turn in relation to ICE rotation for x effect but when your forces acting on the ring (MG2 motorized or MG2 generized) it has a relation with both the the state of sum forces acting on the sun gear (Mg1) and the planetary (engine.) The drive is chained from the ring so the ring is the focus of the net forces derived from the PSD. Ring Gear = Planetary +/- Sun
     
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