Regenerative vs. Friction Braking

Discussion in 'Gen 2 Prius Main Forum' started by mtsarpilot, Aug 9, 2006.

  1. mtsarpilot

    mtsarpilot Junior Member

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    So you are breaking and getting blue arrows showing regeneration going on. How do you know if all of the braking is coming from regernative or if some is coming from friction? If you are doing a slow deceleration to a stop, is it all regen, or are you still losing most of that kinetic energy to friction?
     
  2. seasalsa

    seasalsa Active Member

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    Normally friction braking only occurs at speeds below 8 MPH or in a panic situatiion.
     
  3. joelparks

    joelparks New Member

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    I think without additional instrumentation you can't (unless you've got a very sensitive seat-o-the-pants).

    I have a 2005 with Can-View and with it I can see amps going back into the battery.

    And I can see that when I go over little bumps approaching a stop line, there is a certain threshold where if the bump is significant enough, the regen kicks out and the amps go to a minimum, meaning some software (skid control ECU?) has decided we're about to have a loss of traction and we'd better use real brakes instead of that regen thing! And it seems to me the bump size theshold is set too low for this anyway. Just my observation.

    Anyway, if you don't have a Can-View, our guru Hobbit has developed a little circuit you can fabricate and hook up to your Prius (tapping into signals sent to the skid control ECU) with LEDs to indicate brake pressure on a per-wheel basis.

    I haven't tried it myself but _H* certainly has and if I could finish all my other projects I would do it.

    Joel
     
  4. Tideland Prius

    Tideland Prius Moderator of the North
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    The only way without a CAN-VIEW is... well actually there are two ways that I know and use.


    1) hold the brake pedal til you come to a stop. Around 8mph, you'll feel the car "give" and suddenly "lose" braking power. This is when it switches from regen to friction braking. If you feel it, that means you were regenerating all along. If you don't, then you were braking too hard

    2) This is the method I used to gauge how much to press the brake pedal before I found out about method one. Brake like you normally would. At 20mph, shift to B mode. The amount of drag you suddenly feel depends on how much you were regenerating before shifting. If you feel big difference and a lot of drag, you were braking too lightly. Try pressing harder next time. Keep repeating til you know how much to brake without relying on the B mode to tell you. Shifting to B at 20mph won't turn on the engine and will engage maximum regen.
     
  5. bruceha_2000

    bruceha_2000 Senior Member

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    <div class='quotetop'>QUOTE(mtsarpilot @ Aug 9 2006, 10:06 PM) [snapback]300562[/snapback]</div>
    Coastal Tech sells a little device with leds that show the relative amount of friction braking and has an engine run light. I have not used this product. Brake/Engine run link
     
  6. mtsarpilot

    mtsarpilot Junior Member

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    <div class='quotetop'>QUOTE(Tideland Prius @ Aug 9 2006, 11:23 PM) [snapback]300666[/snapback]</div>
    Yup, this method works well. There is a noticable transition at 8 MPH. Good call. Thanks.
     
  7. Scott_R

    Scott_R Member

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    <div class='quotetop'>QUOTE(bruceha_2000 @ Aug 10 2006, 04:28 PM) [snapback]301055[/snapback]</div>
    I've seen that, but it's not shipping yet, is it?
     
  8. MtBiker

    MtBiker Junior Member

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    One question of mine I haven't seen answered in the numerous regenerative braking posts I've reviewed concerns how the braking forces (regenerative or friction) are distributed to the individual wheels.

    What is the source of braking forces applied to the rear wheels in normal braking situations (ABS not activated)? Put another way, are regenerative braking forces (RBF) being applied to the front wheels only while friction braking is being used to slow the rear wheels?

    Assuming the electric motor is the source of the RBF and because the Prius is front wheel drive, it would appear the resultant RBF would be applied to the front wheels only. When the MFD shows blue arrows flowing from one set of wheels only (I'm assuming the fronts), it appears the other set (I'm assuming the rear) doesn't have any regenerative capability of their own and thus would not provide RBF to those wheels.

    I hope I haven't overlooked some simple explanation and am just broadcasting my ignorance.
     
  9. daniel

    daniel Cat Lovers Against the Bomb

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    <div class='quotetop'>QUOTE(MtBiker @ Nov 6 2006, 03:43 PM) [snapback]344755[/snapback]</div>
    You are correct that regen braking is applied to the front wheels only. But since the front wheels do most of the braking anyway (in a conventional car) due to physics, this is not a problem, and the car does not apply the rear friction brakes during normal regenerative braking.

    Below 8 mph or if you press hard enough on the brakes to indicate a panic stop, the friction brakes are applied. Also the ABS and VSC systems use the friction brakes.
     
  10. MtBiker

    MtBiker Junior Member

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    <div class='quotetop'>QUOTE(daniel @ Nov 6 2006, 06:20 PM) [snapback]344785[/snapback]</div>

    So, if I'm driving on a surface with reduced traction (wet/snow/ice) and apply the brakes normally, the ABS has to first detect front wheel traction loss in order for the rear friction brakes to be activated?
     
  11. Tideland Prius

    Tideland Prius Moderator of the North
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    <div class='quotetop'>QUOTE(MtBiker @ Nov 7 2006, 07:51 AM) [snapback]344975[/snapback]</div>
    It will apply any of the 4 brakes that it deems necessary to slow you down. If you're accelerating and traction control kicks in, it's probably braking the front wheels (since the Prius is FWD) and may or may not retard engine output. But if you're going around the corner and start to slide, the VSC kicks in and it will brake each individual wheel to control the slide and get you back on track
     
  12. daniel

    daniel Cat Lovers Against the Bomb

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    <div class='quotetop'>QUOTE(MtBiker @ Nov 7 2006, 07:51 AM) [snapback]344975[/snapback]</div>
    That is correct. But it will detect it in the tiniest fraction of an eyeblink. It is truely amazing.

    Actually, it would be more correct to say the computer detects it, and applies ABS or VSC as needed
     
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