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"Switching" from Regenerative to Friction Braking a Myth?

Discussion in 'Gen 2 Prius Technical Discussion' started by ilusnforc, Jul 7, 2010.

  1. ilusnforc

    ilusnforc Member

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    I have found other threads on Regenerative VS Friction braking, but all of them seem to indicate that in an unexplained way the Prius is capable of 100% regen while pressing the brake pedal and then magically switching to friction braking. Maybe I'm taking it too literally, but that's how I'm interpreting what I keep seeing in many threads:

    I first had a 2000 Honda Insight several years ago before M-IMA. The brakes in the Insight are the same as any other ordinary car, they are hydraulic and when you press the pedal it applies friction brakes. The only difference is that when the brake lights are activated (by the brake light switch under the brake pedal) the computer would apply 100% full regen. To eliminate any unwanted friction from braking to maximize regen potential, people were tapping a momentary switch into the brake light wiring under the brake pedal so the brake lights could be activated by a push button. There was no "switching" from regen to friction, you press the brake and friction is applied in the same way any other car does but because the brake lights activated it triggers the computer to apply regen.

    It was very simply, you're cruising down the road, good SOC on the battery and everything is normal. You take your foot off the accelerator and you see 3 regen bars appear and it will stay like that until the car is not moving fast enough to produce any regen. If you either press the brakes or push a button that activates the brake lights then all regen bars light up and as the car decelerates the regen bars gradually disappear one by one until it is deactivated and you continue on friction braking only (if the brake pedal is physically being pressed).

    Although I haven't seen anything that implies the same for the Prius, I still believe they work much in the same way. I can believe that the Prius may have a more advanced or complex way of controlling variable regen depending on how hard you press the brakes, but still I believe the brake pedal mechanically still functions exactly the same as it does with any other ordinary car and simply the fact that it is being pressed causes the computer to apply more regenerative braking in combination with friction braking. I just can't see that pushing the brake pedal would cause 100% regenerative braking until you reach a certain point on the brake pedal that would then magically mix in some friction braking and/or once you reach 7 mph it deactivates regen and finally "switches" to friction braking without changing the brake pedal position. I just believe the way it is commonly worded to be inaccurate. I think this change that is felt at 7 mph is simply the load of the electric motor/generators being removed and the feeling of friction only continuing. Not a 100% change from regen to friction.

    Not trying to rant or anything, just tring to get some clarification. Does anyone have any documentation to support either theory or debunk the "myth" of mysteriously "switching" from regenerative to friction braking? Just trying to obtain and share a better (and more correct) mechanical understanding of how it works. Thanks!
     
    #1 ilusnforc, Jul 7, 2010
  2. Rokeby

    Rokeby Member

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    #2 Rokeby, Jul 7, 2010
  3. efusco

    efusco Moderator Emeritus
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    In the Gen I Prius there was always some mild friction braking even during regen.
    In the Gen II and Gen III there is no friction braking unless you are below 7mph, if the system detects a loss of braking (bumps, sudden rapid application, etc.).
     
    #3 efusco, Jul 7, 2010
  4. Patrick Wong

    Patrick Wong DIY Enthusiast

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    I've attached a couple of excerpts from the 2004 New Car Features manual that may be of interest.

    Within the brakings file, find the chart on page CH-37. Note that with very gentle braking force, the chart shows 100% regen braking initially. However it would be incorrect to say that all braking under all circumstances at speeds above x mph is always regen. For example if you do a panic stop, the braking system will switch to friction.

    Also see the chart on page CH-36 which compares 2G to Classic and shows that 2G enjoys a greater proportion of regen braking. This chart also shows some period of time where 2G regen braking can be 100%.

    Although it is true that the Prius friction brakes are of conventional design when viewing the brake parts located at the four wheels, it would not be correct to say that the friction brakes are "just like" a conventional vehicle. Note that under normal circumstances there is no direct hydraulic link between the brake master cylinder and the brake lines leading to the four wheels. Instead, the brake pressure accumulator pump provides hydraulic pressure for the brake system (this is the buzzy-sounding pump that you will hear operating as you repeatedly pump the brake pedal.) See the braki1 file for more details.
     

    Attached Files:

    #4 Patrick Wong, Jul 7, 2010
  5. qbee42

    qbee42 My other car is a boat

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    Adding to this, you will also get some friction braking if more braking is required than what is available from pure regeneration and engine braking. Likewise a panic stop will go directly to friction braking.

    Tom
     
    #5 qbee42, Jul 7, 2010
  6. Mike Dimmick

    Mike Dimmick Active Member

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    Yes, it's more accurate to say that braking is a blend of regenerative and friction braking, with regen braking preferred where possible. Friction brakes are used for sharp braking as they respond faster than regen. Regen requires that the tyres are gripping, if they slip the car drops into friction-only.

    It's my view that the 7mph cutover to friction braking is not that it's not possible to recover any more energy from regeneration, but that it's the point where the HV ECU switches to creeping. Creeping requires the main motor MG2 to be driven as a motor, rather than be used as a generator to brake. Suddenly instead of taking torque from the wheels to make power, it's applying torque to the wheels to drive, and the friction brakes have to work harder against that to slow you down.
     
    #6 Mike Dimmick, Jul 8, 2010
  7. ilusnforc

    ilusnforc Member

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    Does anyone have a thermal camera? :D I'd be curious to see video footage from a thermal camera of the front rotors under different braking conditions to see at what point are the friction brakes actually being applied.
     
    #7 ilusnforc, Jul 8, 2010
  8. Oddest_raindrop

    Oddest_raindrop New Member

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    thermal camera not needed... heavy stopping you will use friction breaking. If you break lightly you can use just regen down to around 7 mph. You can feel the swithc with light breaking. You can also feel the friction break being used when pressing hard. You can also use a scangauge to monitor the battery terminal current and tell when it quits charging. If you use mostly regen you can hear the friction break when you use them.
     
    #8 Oddest_raindrop, Jul 8, 2010
  9. N8JC

    N8JC New Member

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    It is a fact that the generation III does not mechanically apply friction to break the car until you reach the 7 - 9 MPH threshold unless the driver applies emergency braking. I personally have confirmed this. Every once in a while after a rainy spell my brake rotors develop a very thin layer of oxidation which usually wears off after a couple braking cycles. The reason why I mention this is because if I am braking from a moderate speed it is VERY obvious when the friction braking kicks in because you hear the noise caused by the oxidation on the rotor.
     
    #9 N8JC, Jul 8, 2010
  10. qbee42

    qbee42 My other car is a boat

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    You don't need a camera; you can directly measure the signal applied to the friction brakes. One of our technical members (hobbit) has done exactly that. The comments posted on this thread are consistent with hobbit's measurements.

    Tom
     
    #10 qbee42, Jul 8, 2010
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