TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker and becoming a Supporting Member. For more info: Support TMC

Why isn't regen integrated with the brake pedal?

Discussion in 'Model S: Driving Dynamics' started by dennis, Jan 24, 2013.

  1. Clint Eastwood

    Joined:
    May 18, 2013
    Messages:
    114
    Location:
    South Florida
    That's how I see it, but what actually slows the car? Because the car is slowed more than the friction of an ICE car that coasts longer but eventually will stop on level ground. If its not the brake pads, some power must be applied to reduce the speed of the car to that degree.
     
  2. PopSmith

    PopSmith Saving for a Model 3

    Joined:
    Jan 22, 2010
    Messages:
    609
    Location:
    Utah
    #122 PopSmith, May 30, 2013
    Last edited: May 30, 2013
    Being in motion means the car has kinetic energy. Brake pads use friction to "convert" the kinetic energy to waste heat. By comparison, an electric motor uses this kinetic energy to generate electricity which charges the battery pack.

    In both situations the kinetic energy is being used (which results in the car slowing down), but with an ICE it essentially becomes wasted energy in the form of heat. With an EV that energy is used to charge the batteries, which means it is used in a much more efficient manner.
     
  3. Clint Eastwood

    Joined:
    May 18, 2013
    Messages:
    114
    Location:
    South Florida
    See, that's the part I understand, but what if not friction of brake pads is actually slowing the car down?
     
  4. swaltner

    swaltner Active Member

    Joined:
    Oct 13, 2012
    Messages:
    1,215
    Location:
    Kansas, USA
    Physics, particularly conservation of energy, which states that energy can neither be created nor destroyed.

    As mentioned earlier, the car has a certain amount of kinetic energy (E = Mass * Velocity * Velocity). You are generating electricity (energy) inside the motor, which is being driven by the wheels. This electricity (energy) has to come from somewhere, which happens to be the kinetic energy of the car. Since you are reducing the kinetic energy of the car and the mass of the car can't change, the energy comes by slowing the car down.

    The same thing would happen by a windmill generating electricity, just on a much larger scale than what is happening on the car. The air moving across the windmill face generating 1 MW of electricity would cause the air to slow down. The very same principle is being applied in the car to slow it down.
     
  5. scaesare

    scaesare Well-Known Member

    Joined:
    Mar 14, 2013
    Messages:
    7,407
    Location:
    NoVA
    The responses ot your comment are all correct.

    But to be more specific, it's magnetism. When the system is in regen mode, there's a strong magnetic field induced in the motor/generator. The rotor (which is being turned by the drivetrain when decelerating) is being forced thru this magnetic field, which resists it. As the rotor is forced through the magnetic field, in generates the electricity which is funneled back to the battery pack.

    It's the same principle as what causes the motor to turn when electricity is applied to it in "forward" mode. If consuming 60 KWh of power will supply "N" amount of acceleration force, then reversing the system to build a magnetic field that uses the kinetic energy generate 60KWh of energy will also supply the same "N" amount of deceleration force.
     
  6. Clint Eastwood

    Joined:
    May 18, 2013
    Messages:
    114
    Location:
    South Florida
    That makes more sense to me. Thanks for taking the time to refresh my memory of physics classes from a time ago.
     
  7. scaesare

    scaesare Well-Known Member

    Joined:
    Mar 14, 2013
    Messages:
    7,407
    Location:
    NoVA
    Sure thing.

    I love the net effect... "one pedal driving" is very cool... and it's even more fun to watch your WHPM counters start dropping :)
     
  8. GeekGirls

    GeekGirls Kid in Candy Store

    Joined:
    Dec 30, 2012
    Messages:
    312
    Location:
    Seattle
    Its even more fun when your average since last charge goes negative! I have one regular trip where that's the case, having managed numbers as low as -24Wh/mi.
     
  9. strider

    strider Active Member

    Joined:
    Oct 20, 2010
    Messages:
    3,311
    Location:
    NE Oklahoma
    Just think about using your arm to turn a crank generator radio, flashlight, etc. You are taking energy from your muscles and spinning a motor (motor and generator are the same thing, just depends on the direction of energy flow) to generate electricity. In the model S, instead of your arm spinning they're using the spinning wheels.
     
  10. Clint Eastwood

    Joined:
    May 18, 2013
    Messages:
    114
    Location:
    South Florida
    I like that. Those crank generators sow down fast when you stop cranking. And that's where they release energy in the form of electricity. Though I still don't understand technically what brakes the car....
     
  11. brianman

    brianman Burrito Founder

    Joined:
    Nov 10, 2011
    Messages:
    17,191
    I'm not hand cranking my Model S no matter what anyone says.
     
  12. strider

    strider Active Member

    Joined:
    Oct 20, 2010
    Messages:
    3,311
    Location:
    NE Oklahoma
    The (magnetic) resistance of the generator to being spun is what slows down the car. When you're hand-cranking a generator you are having to constantly put in energy to keep it spinning. In the Model S the energy being put in is the speed of the moving car. As energy is used to spin the generator the car slows down.

    Wait, I have another tack. Take the above crank generator and connect it via a clutch to a flywheel. Disconnect the clutch and spin the flywheel. Once the flywheel is spinning engage the clutch so the generator will spin. The flywheel will slow down. Now imagine the flywheel is sitting on the ground and the whole contraption could roll like your car.
     
  13. eledille

    eledille TMS 85 owner :)

    Joined:
    Jan 13, 2010
    Messages:
    535
    Location:
    Oslo, Norway
    #133 eledille, Jun 1, 2013
    Last edited: Jun 2, 2013
    Mr. Eastwood, I have hands-on experiment for you :)

    Buy a neodymium magnet (you know, the super powerful kind that you can barely loosen without pliers). Find a piece of aluminium sheet, or even better, copper. No, wait, get one of each. Place the sheet of metal on a table, put the magnet on top. It will not stick, as aluminium (and copper) is not magnetic. Place a coin next to the magnet.

    Now quickly lift one end of the metal sheet up, so that the sheet is at a 45 degree angle to the table. The coin will immediately slide onto the table, while the magnet will slide down much more slowly.

    What happened was that the very powerful magnetic field through the magnet penetrated the electrically very conductive metal sheet, and when the magnet moves, its magnetic field will also move. When a magnetic field moves in the presence of an electrical conductor, electric current will be created in that conductor. This effect is called electromagnetic induction. This electric current will create a magnetic field of its own, which opposes the field that created the current in the first place, and thereby slow the fall of the magnet.

    Essentially the same thing is going on inside a regenerating motor. But the motor is more refined, and is able to capture the electric current so it can be used to charge the battery, instead of going to waste as heat.

    Next you can test different metals - the more conductive the metal, the slower the magnet will fall. Copper will slow the magnet more than aluminium, aluminium will slow it more than lead, and a sheet of plastic or wood will not slow it whatsoever. If the metal sheet was a superconductor, the magnet would not have moved at all.
     
  14. Clint Eastwood

    Joined:
    May 18, 2013
    Messages:
    114
    Location:
    South Florida
    20 years ago in college physics this may have made sense to me. Unfortunately I just read blah blah blah blah blah. Just kidding. But it amazes me the level of knowledge I no longer possess that at one time I did.
     
  15. brianman

    brianman Burrito Founder

    Joined:
    Nov 10, 2011
    Messages:
    17,191
    Another for the nitpicker's corner:
    The thread title is technically incorrect, as regen is integrated in both pedals in the Model S. Though differently between the two pedals.
     
  16. mknox

    mknox Well-Known Member

    Joined:
    Aug 7, 2012
    Messages:
    10,068
    Location:
    Toronto, ON
    Well, I learned something! I though the brakes were pure hydraulic and that all the re-gen was controlled from the accelerator.
     
  17. brianman

    brianman Burrito Founder

    Joined:
    Nov 10, 2011
    Messages:
    17,191
    When you press the brake pedal you're getting friction braking as well as maximum regen. At least that's my understanding and user experience.
     
  18. joshuaeven

    joshuaeven Member

    Joined:
    Nov 22, 2012
    Messages:
    246
    Location:
    Portland, OR
    You only get max regen because you took your foot off the accelerator.
     
  19. dennis

    dennis P85D

    Joined:
    Jul 26, 2012
    Messages:
    1,941
    Location:
    Silicon Valley CA
    Correct. Try coasting down a steep hill. Check your regen level (most likely less than 60kw). Now press on the brake. Regen does not increase.

    That observation is why I started this thread.
     
  20. Cosmacelf

    Cosmacelf Well-Known Member

    Joined:
    Mar 6, 2013
    Messages:
    5,934
    Location:
    San Diego
    It is possible that the amount of regen is limited by the car's speed. ie. at lower speeds, it is possible that the actual regen mechanism cannot grab a full 60 kW of power from the wheels. I'm just speculating here, but it is possible there is a technical hardware limitation, rather than just a software limitation.
     

Share This Page

  • About Us

    Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.
  • Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


    SUPPORT TMC