Pulsing brakes at the bottom of a long steep hill

Discussion in 'Prius v Technical Discussion' started by MrZap, Feb 22, 2021.

  1. MrZap

    MrZap New Member

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    We regularly drive our 2012 v up to the ski hill. On the way down, we experience brake pulsing when nearing the last part of the hill. Is it possible that the battery is becoming overcharged and then causing problems with the regenerative braking system as it tries to add more charge to the full battery and it is rejected?

    I ask this because I can definitely delay the point on our trip down the hill at which the pulsing starts by keeping the battery charge as low as possible. I do this by coasting downhill to higher speeds then braking hard to reject more energy as heat through the brakes as opposed to lighter more constant braking that would capture more to the battery. I also drive any flatter sections using EV as much as possible. By doing this, the battery reaches full further down the hill and the pulsing starts later in the trip down.

    It's only a matter of 10 seconds or so of driving on the flat and the pulsing will be gone. Once pulsing is observed, I have tried coasting down more to see if perhaps it was related to heat and heat dissipation in the brakes but this has no effect.

    This is just my observations and there could be some kind of coincidence going on. Side note: I don't like to use the B mode as the battery fills up too quickly and once the battery is full the engine revs really high and I'm worried that this is not good for the life of the engine. Also, this does not seem to help with the pulsing brakes.

    Anyone comments appreciated.
     
  2. fuzzy1

    fuzzy1 Senior Member

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    Are you using B mode for the descent?

    The battery won't overcharge, the computers make sure of that.

    Never coast down steep or long hills, keep it in gear. When the battery is full, some energy is diverted to spinning the engine faster, as normal engine compression braking. D mode does only a limited amount of this. B mode starts doing engine braking immediately and can go full bore. (It will get loud, but it won't overspeed or be damaged.)
    While this does delay battery filling, it does so by heating up the brakes. This is the wrong way to go down long or steep hills, because it can overheat the brakes, causing failure. Or warp them, causing brake pulsing.

    Use B mode to scrub most of your downhill energy, then the brakes to handle any excess. This will prevent them from overheating.
     
    #2 fuzzy1, Feb 22, 2021
    Last edited: Feb 22, 2021
  3. MrZap

    MrZap New Member

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    Just to clarify, I'm coasting in D mode. I'll see if B mode makes a difference next trip.
     
  4. jzchen

    jzchen Senior Member

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    The only normal operating pulsing I understand happens when ABS kicks in, as in wheel slip. (It sounds like that as you mention allowing the car to accelerate and then hitting the brakes)...

    moto g(7) power ?
     
  5. fuzzy1

    fuzzy1 Senior Member

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    Is this to Cypress Mountain? If so, @Mendel Leisk is quite familiar with that hill, and D mode doesn't sound appropriate from what I remember of his descriptions.
     
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  6. Mendel Leisk

    Mendel Leisk Senior Member

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    More familiar with Seymour, but for either yes,,that’s a time to use B. When it flattens out enough that I need to use gas I’ll shift back to D, but mostly it’s unrelenting steep, and I keep it in B. B reduces regen btw.

    We don’t go up those hills regularly, in winter I can’t take the cold, and don’t ski, so it’s just to sightsee. And I believe it’s tough on the car.
     
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  7. fuzzy1

    fuzzy1 Senior Member

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    I haven't actually been to Seymour, Cypress, or Grouse, so am not keeping them straight. A visit to each would greatly help my mental confusion among them.
     
  8. MrZap

    MrZap New Member

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    Close. Mount Washington over on the island.
    From my experience, abs is much more rough feeling. More on/off than what we see so I don't think it's that.
    Maybe what I'm seeing is the battery has reached a full state of charge and the computer is now applying the friction brakes and perhaps there is a problem with the friction brakes. However, I thought that I would be engaging the friction brakes just by pressing quite firmly on the brake pedal and braking hard. But this doesn't seem to add up either as when I brake purposefully hard earlier in the trip down the hill, there is no pulsing.

    I assume the computer decides for any given brake pedal position how much regen and how much friction to use based on charge, temperature, etc. Once the calculation is made, how does the car actually do that? And is it recalculated and modified throughout a braking cycle? Lots of questions. There must be a point where no matter what, the friction brake is applied. I can't imagine this is a fully brake by wire system.
     
  9. Mendel Leisk

    Mendel Leisk Senior Member

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    I’ve never seen regen quit, even coming down Seymour. It does get really full, seems to run EV even past the median (as displayed on Hybrid System Indicator), like it’s trying to get charge down more aggressively, once we’re down, driving on the flat.
     
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  10. jerrymildred

    jerrymildred Senior Member

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    This is exactly the situation B mode is intended for. The revving won't hurt the engine. It's just acting as an air pump to slow the car. You have a good idea in keeping it in EV on the flat at the top to make room in the battery, but once you start down that steep slope, just put it in B and let the car do its thing while you modulate the speed with the brake or accelerator. Don't forget to put it back in D at the bottom. ;)
     
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  11. rjparker

    rjparker Senior Member

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    I would verify the rotors are not warped.
     
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  12. Mendel Leisk

    Mendel Leisk Senior Member

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    You can DIY the check. Dial indicator with magnetic base (check for trueness), and micrometer (check thickness) are what’s needed. Both items about $50. I can post repair manual info in a bit, if there’s interest.
     
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  13. sam spade 2

    sam spade 2 Senior Member

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    USE B. That is what it is MADE for.
    OR set the cruise for a reasonable speed and let it try to maintain it.
    The result will be about the same.

    The pulsating might indicate that your rotors have been overheated in the past and they are a bit warped.
    Might.

    More likely it is as you suspect and the system is switching from regen to friction and back.
    BUT......if the engine never speeds up much, you are using too much friction brake.
     
  14. Mendel Leisk

    Mendel Leisk Senior Member

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    Repair Manual excerpt attached with brake rotor specs and inspection instruction. Note, this is for regular 3rd gen Prius, but maybe "close enough for government work". I think the spec's would be very close, or the same. I believe rear brakes are a little different on the v, have an integrated drum brake (for parking brake). Sadly there doesn't seem to be a PDF version of the v (or c) RM.

    FWIW: I think way too many rotors are needlessly replaced, and/or remachined. Mechanics can't be bothered to check them, and customers are impressed by shiny/new.
     

    Attached Files:

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  15. fuzzy1

    fuzzy1 Senior Member

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    I have skied there, both downhill and nordic, but that was a very long time ago.
    No, the computer will never apply the friction brakes. Only the driver can do that.
    Are they warping from excessive heat part way down, then at least partially un-warping when cooling down later?

    Put the car in B mode at the very beginning of the descent, before much or any battery charge has accumulated. The presence of brake problems down the hill is a very strong indicator that D mode is the wrong mode. N is even worse.

    Mt. Washington's base elevation is 3558 feet. Courtenay is at sea level. That elevation change is more than enough to get very hot brakes.
    Cruise control is not a good idea on windy twisty mountain roads, or on winter roads. And the results won't be exactly the same, B mode will spin up engine braking earlier, well before the battery is full. Cruise control will tend to fill up the battery first, before engaging substantial engine braking.

    If the road isn't straight and bare and dry, then stick with B mode.
     
    #15 fuzzy1, Feb 23, 2021
    Last edited: Feb 23, 2021
  16. ChapmanF

    ChapmanF Senior Member

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    I sometimes start out on windy twisty roads by setting the cruise control at its minimum allowed speed (25 mph), and then using my right foot to drive the car. It will eagerly slow the car when backing off the pedal, sort of like B mode. It does use the battery harder than B mode, so I ought to remember to just shift to B when a long descent is coming up, but I don't always.
     
  17. MrZap

    MrZap New Member

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    Does anyone have a link for a good run down of how braking works for these cars? Is friction braking just delayed in the pedal stroke and the first bit of stroke is for regen braking only or are they in tandem? Is most typical daily driving (without any big hills) done with regen braking only? I have heard it said that the last bit of a stop is accomplished with friction brakes only. If the computer will not apply friction brakes, does the regen brake back off as a Prius comes to a stop and if so, why don't I notice the reduced braking? Is this just a gradual and natural regen fade as the wheel rpms drop or does the computer back the regen off?

    I'm pretty sure it is something to do with charge. Last time, my better half drove down. She doesn't use the same battery discharge strategy and the pulsing started much earlier for her. But maybe whe was riding the brakes more. On a long hill like this, I don't ride the brakes for long periods. I typically use the brakes a little firmer than needed and get off the brakes as much as possible to give them a chance to cool down. Perhaps this is not a valid method as I have developed that technique without outside advice. I should check into this. Once at the bottom, I have tried the brakes shortly after and there is no pulsing.

    Sunrise parking lot to parking lot at the bottom where the road finally levels out: 1100 m - 140 m = 960 m = 3148 ft over a distance of 14.2 km = 8.9 mi but the steepest stretch is at the bottom with a bit of flat in the middle.
     
    #17 MrZap, Feb 23, 2021
    Last edited: Feb 23, 2021
  18. ChapmanF

    ChapmanF Senior Member

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    It's a balance between regen and friction, calculated by computer, constantly recalculated during your use of the brakes, and a more complicated dance than most people guess.

    This illustration is from the 2001 New Car Features manual, introducing the first generation of Prius:

    [​IMG]

    Starting with Gen 2, they were able to eliminate the little sliver of hydraulic running down the lightest-braking edge of the graph:

    [​IMG]

    That illustration comes from the 2nd gen NCF manual.

    If you are looking for a deeper level of detail than "it is computed moment to moment, it depends on a lot of things, charge is one of them, regen is limited both at higher and lower speeds and most prominent in between, and the system will (try to) seamlessly blend between regen and friction at the transitions", then the best place to look is in the NCF manuals themselves.
     
  19. fuzzy1

    fuzzy1 Senior Member

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    Chapman has provided a lot of core detail. I'll try to fill in a bit more around the sides.
    This depends a lot on the driver and style. Regeneration is limited by the maximum battery charging rate, which is sometimes reduced for temperature or other reasons. It is easy to exceed this limit, and many people's natural braking style easily and commonly goes higher. For many people, it requires some training or practice to stay under the limit when practical, which also requires starting braking sooner.

    A regular Prius has a smaller battery than any plug-in car, thus a smaller regeneration braking limit too. A Prime can do a bit better, a Tesla can do a lot better.

    It doesn't reduce total braking, just transitions to friction-only to make up for the loss of regeneration. Though the transition is not always seamless, sometimes I feel it.
    This descent vastly exceeds the battery charge capacity, so a large amount of energy must to thrown away somewhere. Assuming the distance is too short and steep for air drag and wheel rolling resistance to take much, then the only remaining choices are brake heating, and engine compression braking.

    You have been mostly avoiding engine compression, so that leaves mostly brake heating. Over short time periods, it doesn't matter much if one rides the brake gently or pulses the brakes harder with gaps in between, the total heat is the same. Aside for stopping at pullouts to give the brakes time too cool, the only practical alternative is send some of that energy into the engine instead. Wind it up to high RPM as a vacuum pump or air compressor, heating up some air getting pumped out through the exhaust system. It won't be anywhere near as warm as regular fuel-burning engine exhaust.
     
  20. MrZap

    MrZap New Member

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    I'm trying to rationalize these statements and the graphs above.
    Referring to the two top graphs:
    • When drivers demand is constant, I assume this also means a constant pedal position. Maybe it is more accurately pedal force but assuming no fade, this would be the same thing.
    • The constant demand is then satisfied with a combination of the friction and regen brakes.
    • From this, I assume that friction brake is the white area above the grey regen area and below the demand line.
    • Since the graph shows regen changing over time, it follows that the friction brake must be controlled by the computer to change with time inversely to the regen brake otherwise the constant braking demand will not be met?
    The only other option I see is that the friction brake is constant and directly proportional to pedal position/force and regen is added on top of this but then your total braking force changes over time with changes in speed, temp, charge etc . I'm guessing this is not the case but it it is, does our human brain somehow accommodate? Sorry if this seems a bit in the weeds or if I'm missing a simple concept but it is an interesting topic and vital to an understanding of how they work.

    Also, can someone tell me what generation a 2012 v is equivalent to?
     
    #20 MrZap, Feb 23, 2021
    Last edited: Feb 23, 2021
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