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Regenerative Braking Details?

Discussion in 'Prime Technical Discussion' started by Insirt, Jan 16, 2017.

  1. Insirt

    Insirt Junior Member

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    So a little technical question about regenerative braking on the Prime or any vehicle that has it:

    I know the basic idea: when you let off the accelerator the front wheels turn the motor-generators, regenerating power to use later, but my question is how, if at all, is that regenerative braking throttled? The regenerative braking occurs at (almost?) all speeds and at (maybe?) all amounts of pressure put on the braking pedal.

    Only when braking hard (and probably at a complete stop to hold the car in place) do I know the disk brakes are used for sure, but is the how can the regeneration generate different amounts of braking force across multiple speeds? The MG's are geared to the wheels at a fix gear ratio, so the amount of braking should be constant when you come off the gas: I see this when there is a little regen when coasting. But how is that braking increased when pressing on the pedal if the gear ratio is fixed?

    I imagine it is done one of a few ways:
    1. Regenerative braking is actually quite strong and always on, but some power is added to the MG to allow you to coast further like a normal car
    2. Like the above, but first one MG is engaged than the other for more regeneration and braking force rather than both at once
    3. The gear ratio when coasting/braking to the MG's is not fixed but somehow varied, allowing for varied regenerative braking
    4. Or something else I haven't thought of or found online...
    A bit of a long first post, but as an engineer I'm curious. Depending on how it is accomplished, the power train could be made more efficient or perhaps I underestimate Toyota.
     
  2. Lee Jay

    Lee Jay Senior Member

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    The power electronics hooked to the two motor generators can entirely control the current to/from the motors, and the torque is proportional to the current. This is how you can change your acceleration rate with the throttle pedal in Ev mode too. Same thing.
     
  3. Insirt

    Insirt Junior Member

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    Thanks, so essentially like my #1 method? In that case the power train may benefit from a change such as a regular CVT geared to the MG's so that the MG's could provide braking force at a wide range at all speeds. This would prevent the need for current input, potentially increasing efficiency, regeneration, and extending the life of the battery.

    Certainly it would add mechanical complexity which Toyota is hesitant to do...
     
  4. Lee Jay

    Lee Jay Senior Member

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    No.

    Power electronics can make the power flow into or out of the motors at any rate at any time within capacity limits. If you let off the pedal, it does some amount of power out (regenerative braking). If you push the pedal down slightly, it'll just decrease the power out, eventually crossing zero and becoming power in as you increase throttle pedal position. If you instead press on the brake pedal, it just increases the power flow out of the motors thus increasing braking. This can all be done at almost any speed and at any level. At very low speeds, it doesn't work because the motor voltages are too low, but everywhere else, you can basically get any torque you want, positive or negative, out of either motor at any time just by controlling the way the switching works inside the power electronics.
     
  5. mr88cet

    mr88cet Senior Member

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    There is also a mostly-but-not-entirely-traditional hydraulic friction-braking system connected to brake pedal. If you depress the brake pedal only a little, you will get more regenerative braking than friction braking (assuming that the battery isn't already full). As you depress it more, you get more of both, but regenerative starts to loose oomph as the car slows down.

    How much regenerative braking to engage, when you release the gas only without hitting the brake at all, is an interesting engineering-design question. On the Prius, they try to mimic the amount of drag you get from a typical automatic transmission. Teslas have a mode switch to do that, or what they call "normal" mode, which engages *a lot* more regeneration as you release the accelerator.

    I personally really like Tesla's "normal" mode, and I wish Toyota would provide that option too. It improves regeneration, and lets you do a lot more driving with your foot on just the accelerator pedal only. The Chevy Bolt (pure EV) and Volt (series (mostly) hybrid), co-opt the gearshift's "low-gear" setting for that purpose: it's not actually any lower-gear, but when you let off the accelerator, it decelerates regeneratively as if it were. Again, I wish Toyota had a setting for that.

    In the field of DC-to-DC electronics, used also in power supplies such as for computers, there are what are called "boost" and "buck" converters. They do in DC essentially what a transformer does in AC - they trade-off between voltage and current. As your car slows down, the voltage from the MGs decreases, and as the battery recharges, it's voltage increases. However, they can "boost" the voltage from the generators above that of the battery and thus still pump current into the battery. At some point -- at a few MPH -- the voltage from the MGs goes so low that it becomes pointless to even try, and it's all friction braking from there.

    You can literally hear the regeneration happening on your Prius: it sounds a little like on a manual transmission when you downshift; you hear a whirring sound of gradually-lowering pitch.
     
    #5 mr88cet, Jan 19, 2017
    Last edited: Jan 19, 2017
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  6. Lee Jay

    Lee Jay Senior Member

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    You get no friction braking as long as regenerative can generate the requested braking force.
    They do - B mode.
     
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  7. Trollbait

    Trollbait It's a D&D thing

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    B mode is a one speed bike compared to the multi-speed ones of what some of the others offer in terms of regenerative brake control. As seen here, it also leads to confusion since it works differently from the B mode on the hybrids.
     
  8. Lee Jay

    Lee Jay Senior Member

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    The brake pedal is the proportional version.
     
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  9. Trollbait

    Trollbait It's a D&D thing

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    And the volt has pedal shifters for incremental adjustment of regen in addition to the brake pedal and L(B) gear.

    Tesla uses the brake pedal just for the friction brakes. Acceleration and regen is through the prissy pedal, and regne is high enough to require the brake lights to operate. The same is true of other BEVs.
     
  10. fuzzy1

    fuzzy1 Senior Member

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    It is controlled in pretty much the same way power is controlled when in electric thrust mode. The computers decide how much voltage and current to apply to (or draw from) the motor-generators. Voltage and current determine power. Power and speed and some mechanical equations determine thrusting (or braking) force.
     
  11. Trollbait

    Trollbait It's a D&D thing

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    This has a clear explanation of how regen braking works.
    BMW i3 versus Tesla Model S 70D
    There is also a bit about how more people preferred one pedal driving.

    Aside, magnetic braking in locomotives is called dynamic braking. Without a battery, the electric generated by baking is passed through a bank of resistors to convert it to heat.
     
  12. mr88cet

    mr88cet Senior Member

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    That's interesting: At least on my 2009 (gen-2), every time I've put the shifter to B, it immediately turns on the engine.
     
    #12 mr88cet, Jan 19, 2017
    Last edited by a moderator: Jan 19, 2017
  13. Lee Jay

    Lee Jay Senior Member

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    That's because it doesn't have the Prime's large battery to absorb the power. That's one thing that's supposed to be different about the Prime - B just means braking, not necessarily engine braking.
     
  14. mr88cet

    mr88cet Senior Member

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    I'm "all in" for that P.Prime feature!

    Does B on the shifter affect anything else (on the P.Prime), or does it just increase regenerative braking?
     
  15. Lee Jay

    Lee Jay Senior Member

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    Supposedly, it just regenerates. However, that's dependent on other things like battery state of charge and battery temperature. It can use engine braking if needed.
     
  16. heiwa

    heiwa Active Member

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    I just had 5k service done on our Prime. I am not an engineer like many of you. My layman observations on 11 inch screen (in a sub menu of hybrid system monitor) and MID (hybrid indicator with eco score screen) have shown no active ICE engagement but more aggressive regeneration when going down hill on B shift position. Battery dose "seem" to accumulate charges faster than in D position when the battery is not full.

    Posted via the PriusChat mobile app.
     
  17. Arthur McBeth

    Arthur McBeth Junior Member

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    When you hit the brakes on a Prius Prime, does it do the same function as putting it in "B" mode?
     
  18. Tideland Prius

    Tideland Prius Moderator of the North
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    Yes but stronger (well, strength depends on brake pressure).
     
  19. ChapmanF

    ChapmanF Senior Member

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    I don't have a Prime, so what I'll say here might be more specific to an ordinary Prius.

    The computer in charge of power in and out of the MGs has been programmed to act in ways that will feel sort of familiar to drivers of other cars. The B mode is an interesting case, because they programmed it to feel like downshifting in a conventional car to slow it. So B mode is programmed so you feel more aggressive slowing than in D mode.

    At the same time, the designers knew that a lot of the times you want to downshift like that will be on long downhills where you want to control speed. On a long enough downhill, you'll eventually run out of battery capacity to fill, so if you have chosen to shift to B, that's a good hint to the car to kind of limit the rate of charge into the battery, so the battery can stay cooler and it will probably still be fully charged anyway by the time you reach the bottom of the hill.

    So the B mode is trying to meet two sort of weirdly contradictory demands: 1. slow more aggressively for the feel of it (that is, harvest kinetic energy faster), but, 2. don't dump the harvested energy into the battery as fast.

    It does that with an interesting balancing act, where it does harvest more energy, but also resorts earlier to spinning the engine with some of it, so the energy has another place to go besides the battery. If you have your foot on the brake, it may also increase the amount of friction braking in the mix, so some of the energy is dissipated that way too.

    There's nothing all-or-nothing about any of this; the computer has all of those tricks up its sleeve and can use them in different proportions, and it can change its mind and pick different proportions a thousand times in the time it takes me to type this period. ⇠ (that period there)

    In a Prime, the same basic ideas apply, but it can base its strategy on having a much bigger battery to dump energy in.
     
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  20. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    Curiously, the sophisticated Tesla has a very different approach to braking: you are supposed to slow down by letting off of the "gas" pedal, essentially down to a full stop, and that will use regeneration. It is called "single pedal" driving. But if you ever actually use the bake pedal, that is 100% friction brakes. Whereas in the Prime, you don't have to decide between letting off the gas pedal and hitting the brake pedal; you just let off the gas pedal and hit the brakes, and the system will decide the best way to slow the car.
     
    #20 CharlesH, Mar 7, 2020
    Last edited: Mar 7, 2020
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