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Optimal braking

Discussion in 'Gen 2 Prius Main Forum' started by mdarby, May 16, 2006.

  1. mdarby

    mdarby Junior Member

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    Is there a consensus on how much gentle braking can be applied to use only regenerative braking versus friction braking?

    Has this been quantified? For example, at 60 mph can one brake to reduce speed by 5 mph / second and thus stop in 12 seconds? Can one feel when the friction brakes engage, or hear the difference somehow?

    Thanks!
     
    #1 mdarby, May 16, 2006
  2. hobbit

    hobbit Senior Member

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    Not really, not without instrumentation. Above 7 mph or so it's a
    smooth continuum based on how hard you're braking [limit 100A charge
    current into the battery], and how fast you applied the pedal [panic
    stop, i.e. fast pedal swing, uses friction brakes much earlier in
    the game]. Below 7 or so mph it's all physical since the large
    motor isn't spinning fast enough, and you generally *can* feel that
    transition especially if the rotors have grown a little rust.
    .
    If you're handy with electronics or want to spring for some of the
    add-on products that tell you some of this information, the info is
    definitely kicking around the board and elsewhere. Holler if you
    want specific pointers, or search away...
    .
    _H*
     
    #2 hobbit, May 16, 2006
  3. mdarby

    mdarby Junior Member

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    _H*,

    Thanks for the response. For the example given, would braking from 60-0 in 12 seconds, with brakes gradually applied (i.e. no "panic") exceed the 100A charge limit, in your estimation? If so, how slowly would one have to brake not to "waste" the car's kinetic energy?

    Michael

    <div class='quotetop'>QUOTE(hobbit @ May 16 2006, 10:16 AM) [snapback]256250[/snapback]</div>
     
    #3 mdarby, May 16, 2006
  4. Sufferin' Prius Envy

    Sufferin' Prius Envy Platinum Member

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    Two things I wish Toyota would have done with the MFD.

    1) Have the hubs on the wheel icons turn red when mechanical brakes are engaged.
    2) Have the gas engine icon turn red while gas is being burnt, and yellow while the engine is spinning but not burning gas for protecting the other HSD components from over-RPM damage.
     
    #4 Sufferin' Prius Envy, May 16, 2006
  5. efusco

    efusco Moderator Emeritus
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    This is a complicated question.
    It's been shown that braking at a rate of about 60amps is the most efficient in regenerating. That was slowing from 50-10mph in 20 seconds (I think, someone correct me if that's wrong)...it's moderate to light/moderate amount of pressure on the brakes.

    The problem is that when you make the amount of regen the goal then you lose part of the equasion...distance.

    The example I always use is this. You're traveling at 50mph and can maximize regeneration by slowing over 20 seconds...let's say that happens over a distance of 1/2 mile.

    So, the place you want to stop is 1 mile away.
    You can "inefficiently" coast to that stop and regenerate less energy, but not burn any more fuel since you'll be coasting.
    OR, you can drive 1/2 mile then brake at 60amps and maximize your regenerated energy but have to burn gas to get to your point where you begin braking lest you stop 1/2 mile before the stop sign.

    IMO (and I have no hard numbers to justify it), the net energy gain is going to be better for the coasting 1 mile stop than the 60amp stop over 1/2 mile.

    <div class='quotetop'>QUOTE(Sufferin' Prius Envy @ May 16 2006, 12:04 PM) [snapback]256331[/snapback]</div>
    While the whole yellow red think would, no doubt, result in as many or more confused questions than the battery colors do, I agree with you. One thing I still want is some means of telling when the friction brakes are engaged.
     
    #5 efusco, May 16, 2006
  6. mdarby

    mdarby Junior Member

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    So Evan, do you agree with the following earlier post about the technique of shifting to "B" mode at 20 mph or less?

    =======================

    The Ideal would, of course, be to only coast and never brake.

    If the choice is between slight braking all the way to the light, versus coasting then 'hard' braking, then the slight braking wins.

    Remember, all braking below about 8 MPH is mechanical. Regeneration cuts out at that point.

    Above 8 MPH, slight braking regenerates, but 'hard' braking can also engage the mechanical brakes, which is wasteful.

    Also, remember that below 20 MPH you can use the "B" mode to get maximum regenerative braking without touching the pedal.

    That's one of the tricks the Japanese hyper-milers use in city traffic. Since it's very hard to tell exactly how hard you can push the pedal without crossing over to mechanical braking, if you use B to brake initially coming to a light you are guaranteed maximum regeneration.

    However, above 20 MPH B mode will spin the ICE for resistance, which is just as wasteful as burning brake pads smile.gif

    <div class='quotetop'>QUOTE(efusco @ May 16 2006, 01:22 PM) [snapback]256368[/snapback]</div>
     
    #6 mdarby, May 16, 2006
  7. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(mdarby @ May 16 2006, 04:04 PM) [snapback]256495[/snapback]</div>
    No, I don't.
    First of all, if the ICE is still spinning (which it not infrequently is even below 20mph), B-mode will force it to continue to spin pretty much draining as much juice as you're regenerating....or more.
    Second, B-mode does NOT "guarantee maximum regeneration". In fact it peaks out at about 20-24 amps. I can regenerate nearly 100 amps, under certain conditions, even when slower than 20mph.

    Now, once you get to the point that you're coasting I'll sometimes use B-mode b/c it'll grab an amp or two where with just Drive the ECU will actually use a little juice to force the car to roll forward slightly. Net gain maybe 2-3 amps for about 2 seconds...pointless, but it makes me happy.
     
    #7 efusco, May 16, 2006
  8. ken1784

    ken1784 SuperMID designer

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    #8 ken1784, May 17, 2006
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