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regenerative deceleration

Discussion in 'Gen 4 Prius Fuel Economy' started by alexgrigori, Oct 31, 2016.

  1. alexgrigori

    alexgrigori Active Member

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    <<acceleration is a privilege, but deceleration is the obligation>>

    In this forum acceleration has been discussed many times. IMO, from the fuel economy point of view, brakeage is even more important. During a speedup, MG2 can assist ICE (or vice versa) and kinetic energy typically not wasted (except small loss due to friction). OTOH, the ability to recuperate the kinetic energy is limited by MG2 itself (or, most likely, by AC to DC inverter - TBC). All energy does not fit this this envelop is wasted in mechanical brakes.
    I've read that some hybrid cars have a designated option to slow down car exclusively by electrical generator while a mechanical brakes can be used only for emergency.
    Also I've heard that regen brakeage has been improved for gen 4. So, if road condition permits, are there any ways to insure 100% regen deceleration up to full stop? Are there any limitations car's computer can inform about, like battery/inverter too hot, etc.?
     
    #1 alexgrigori, Oct 31, 2016
    Last edited: Oct 31, 2016
  2. bisco

    bisco cookie crumbler

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    i worry that giving people more than one option to brake, will cause accidents. the human brain isn't always that quick. i'm happy with keeping it brake pedal only, and improving the efficiency with each generation.
     
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  3. alanclarkeau

    alanclarkeau Senior Member

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    I don't think it's just emergency braking - even "normal" braking, say on a downhill slope EXIT ramp from 100 to 60km/hr, you will, in most situations brake harder than just using REGEN braking, thus using some mechanical brake.

    Another consideration - if you drove for 2 yrs using just REGEN and then had a Panic Stop - the rust build up on your Disks would rip the brake pads to shreds in one stop.
     
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  4. fuzzy1

    fuzzy1 Senior Member

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    That would require a very high power electrical system and battery, such as that of a Tesla.

    For the Gen3 Prius, the regeneration limit was set by the battery at about 27kW. For comparison, emergency braking at highway speed can exceed 500kW.
     
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  5. alexgrigori

    alexgrigori Active Member

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    This is from a recent Bolt review:
    The Bolt’s default drive mode is designed for drivers making the transition from gasoline-powered vehicles with minimal regenerative braking when the driver lifts off the accelerator pedal. Those who know and enjoy the novelty of one-pedal driving can shift the gear selector into its low mode for up to 0.3 g of regenerative braking when they aren’t touching either pedal. A small paddle behind the left steering-wheel spoke increases the amount of regenerative braking whenever it’s held, in either drive or low mode, and it can be used to bring the car to a complete stop.
    2017 Chevrolet Bolt EV Test &#8211; Review &#8211; Car and Driver

    IMO this is nice to have option we may miss in our Prii
     
    #5 alexgrigori, Oct 31, 2016
    Last edited: Oct 31, 2016
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  6. Coast Cruiser

    Coast Cruiser Senior Member

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    I want nice simple brakes. Press the pedal, stop the car.

    If the Prius gets any more complicated, I'm going back to my 1995 Camry! :LOL:
     
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  7. ChapmanF

    ChapmanF Senior Member

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    For purposes of testing for brake drag, I've been known to make completely-brakeless stops by using B mode down to about 6 MPH, then shifting to reverse until the car just comes to zero (at which point it's ok to stab the brakes, and won't heat the rotors or mess up the test). It works but it's fiddly; I certainly wouldn't do it in normal driving. (I scope out the place my test will end up in advance.)

    One of the good things about the Prius brakes working exactly the way they do is that if anything surprising should happen with traction while you are slowing regeneratively, the car can switch directly to 4-wheel hydraulic ABS so quickly you almost don't notice (though that almost has been the genesis of countless PriusChat threads...).

    -Chap
     
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  8. Coast Cruiser

    Coast Cruiser Senior Member

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    I have yet to hear, feel, or see any "regenerative" braking. What am I missing?

    But I did notice a weird thing involving my brakes when I was driving around in PWR mode for 5 months. Many times, the brakes would feel very mushy, weak, and I had to push the pedal further down to slow the car. Now that I switched back to ECO mode, the brakes feel good and strong.

    Just another Prius Oddity...
     
  9. alanclarkeau

    alanclarkeau Senior Member

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    Whenever the Hybrid System Indicator is down below the ZERO level, in the CHG area, it's doing regenerative braking and charging the battery.
    upload_2016-11-1_14-55-20.png

    Alternatively, if you have the Energy Monitor displayed you'll see power coming from wheels to battery - that's regenerative braking.
    upload_2016-11-1_14-57-8.png
     

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  10. Coast Cruiser

    Coast Cruiser Senior Member

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    Thanks. Understood. Why don't they just call it "battery charging." It's not really braking as we know it; it's not something I can feel in the brake pedal... or is it?

    I still get grabby brakes often. And I've had mushy brakes. Will they ever feel normal??? This Prius sure is a strange car. :)
     
  11. alanclarkeau

    alanclarkeau Senior Member

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    Not initially - on level ground, you could be driving at 110km/hr and take your foot off the throttle and it would slow down - you would see the Monitor or Indicator showing charge - it would be slowing down quicker than - say a Petrol Corolla - in neutral. My understanding - if you brake with the pedal enough for the Hybrid System Indicator "CHG" zone to be almost full (blue):
    upload_2016-11-1_15-33-37.png -
    it is only using regenerative braking. Should the window be COMPLETELY blue, you could be then using some mechanical brakes.

    NORMAL - I don't think there is such a thing. My Fiesta and Focus had similar brake feel, my Dad's Camry needed considerably more force to brake and didn't feel as progressive. But he thought his was NORMAL - I didn't - partly what you get used to. I remember the first Mercedes ('60s) I drove, the accelerator needed to be planted so deep to get acceleration if felt quite odd - I far preferred my Corolla and VW.

    STRANGE - yeah maybe - I've often bought cars a little left-field - and PRIUS is a little different.
     
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  12. Coast Cruiser

    Coast Cruiser Senior Member

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    I find it to be strange, weird, odd... but in a fun kind of way. I never know what to expect from day to day. :D

    But the most odd thing is the grabby brakes. Just a light, easy, gentle push on the brake pedal..... and then GRAB!!! :eek: I better not have coffee in my hand.
     
  13. bisco

    bisco cookie crumbler

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    probably first year new generation blues. in time, toyota will come out with a recall or tsb. until then, dealers will have no clue.
     
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  14. RCO

    RCO Senior Member

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    Bisco's up early this morning.,.:cry:
     
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  15. alexgrigori

    alexgrigori Active Member

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    I'd like to understand what is going on during the regen. Please correct me if my description is incorrect.
    MG2 is 650 VAC 3 phase motor-generator. As a motor it can produce about 60kW, but I could not find its generating capacity.
    Anyway, 3 phase 650 VAC from MG2 must be converted to 1 phase 650 VDC and I am not sure it can be done with inverter which propels the car. Finally, 650 VDC should by converted to 220-240 VDC to charge the battery. And I do not know if propelling booster can be reversed or separate DC-DC converter is needed.
     
  16. RCO

    RCO Senior Member

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    Afaik it would need a rectifier to convert AC to DC
     
  17. RCO

    RCO Senior Member

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    It needs a rectifier to convert AC to DC. The inverter converts DC to AC and I don't think either can function in reverse.
     
  18. alexgrigori

    alexgrigori Active Member

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    If I get you right, the regeneration requires a completely independent electrical circuit with designated rectifier and DC-DC step-down converter.
    BTW, I did not recall any info about such circuit, all descriptions only mentioned booster and inventer used for the car driving.
     
  19. ChapmanF

    ChapmanF Senior Member

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    No ... no, it doesn't. The conversion from DC to AC (and back) is all done by large transistors in switching mode in a bridge configuration (how do you rectify AC with a bridge of switches? Turn these two on for half the waveform, turn those two on for the other half ... how do you invert DC to AC with a bridge of switches? Pretty much the same way), the voltage adaptation between the 200-ish volt battery and the 650-ish volt motors is done by bucking/boosting using inductance and capacitance at the switched pulse edges. The computer driving the precise timing of the switch transitions has complete control of the process, forward and reverse.

    This is one of the most studied, discussed, and understood topics in all Priusland ... in addition to the Toyota manuals and course materials (all on techinfo, though you need $15 to play, best starting place is New Car Features Manual), there are lots of technical discussion threads here on PriusChat going back years, all there for the searching.

    -Chap
     
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  20. fuzzy1

    fuzzy1 Senior Member

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    The battery is the weakest link of the regeneration path, so the higher 60kW rating of MG2 doesn't help. The full 60kW rating is used only during acceleration when both the ICE and battery are providing power at the same time.

    All the high voltage motor-drive / inverter / rectifier stuff is built to run power both ways, as both directions are needed at various different times during normal propulsion operations. Given this, no additional hardware is needed for regenerative braking, just more software.

    Unlike the MG driver hardware, the high-voltage to 12V inverter is one-way only.