My 12 year old asks, "Does the Prius not coast as far as other cars?"

Discussion in 'Prius v Technical Discussion' started by mikenancy1, Dec 1, 2012.

  1. mikenancy1

    mikenancy1 2012 PiP (Base), '10 Highlander Hybrid, '05 Prius

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    I'm attempting to explain the regenerative braking system to him, so his question is based on my weak attempt to explain how the battery is recharged from the kinetic energy generated when you coast or put your foot on the brake. Any answers that make me look smarter than I am will be appreciated.
     
  2. walter Lee

    walter Lee Hypermiling Padawan

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    Link simple noun-verb-objects sentences like a pathway
    show two pathways
    one pathway goes from A to B
    the second pathway goes from B to A
    (A= Yin/white/on, B= Yang/black/off)

    Simplify the truth to make it easier to remember and understand- as long as its mainly/most oftenly true - your kids will forgive you later for any minor bending of the truth. :rolleyes::whistle::coffee:

    For example...

    Putting your foot on the brakes and off the accelerator(A)
    slows down the wheels
    so that the wheels are turning the electric motors
    which causes the electric motors to fill up the battery with energy (A).

    Putting your foot on the accelerator and off the brakes(B)
    speed up the wheels
    so that the electric motors are turning the wheels
    which causes the electric motors to empty out the battery of energy(B).

    FYI - to get the highest fuel efficiency from any vehicle - a Prius driver should to avoid braking if possible because regenerative braking only recovers about 33% to 50% all the energy needed to stop the car. It's more energy efficient to retain some if not most of a vehicle's current speed/momentum (via smart braking/slowing down but not completely stopping) than it is to use the regenerative braking system transfer all of a vehicle's mechanical energy (speed/momentum) into the battery as electrical energy. So it is not the top speed that makes a vehicle energy efficient but the fact that a vehicle doesn't have to slow down as much and stop that matters most. The highera vehicle's top speed the more likely and more often that vehicle will have to slow down while a vehicle at a slower speeds will not as often need to slow down and/or stop. Hence - on a flat road surface, vehicles are often more energy efficient when moving at lower constant speed than when moving at higher variable speeds( There are exceptions to this but that requires you introduce your child to the concept of gravity and potential gravitational energy). Every energy transition from electrical to mechanical and from mechanical to electrical involves some energy loss - this is high school physics concept (the Second Law of Thermodynamics) and if your kid understands this you might want to introduce him to the Mean Value Theorem and some calculus ...;):p:D:love:
     
  3. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    User expectation.

    Previous cars had transmissions with engine braking so the Prius (who's transmission does not have engine braking natively) adds it in software so drivers get the experience they are used to from previous cars. Hypermilers then have to learn how to 'glide' to bypass the software.

    This is similar to why we all type on Qwerty keyboards, even though that layout was designed to minimize the keys jamming on a typewriter with individual keys. Modern typewriters use a wheel or ball, so they can't jam, computers have no print head at all, but here we are still using Qwerty.* Many faster layouts exist, but it would take time to relearn them.
    Dvorak Simplified Keyboard - Wikipedia, the free encyclopedia
    Velotype - Wikipedia, the free encyclopedia

    Similar reasoning explains why the US still uses the English system of measurements even though metric is clearly superior. "We have always done it that way." is a hard nut to crack. This can be very obvious to a 12 year old for whom 'always' is a much shorter period of time.

    * One way to tell the age of a typist, is if they put two spaces after a period. We were taught that in the dark ages of monospaced fonts, and now we can't unlearn it. Since the rise of the Macintosh and on screen kerning, there is no need for two spaces after a period, but my fingers keep doing it.
     
  4. cwerdna

    cwerdna Senior Member

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    Do you have a Prius v wagon?
     
  5. JasonW

    JasonW Junior Member

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    My Prius v ( lowercase v ) will coast extremely well... however you have to just touch the gas to do it. Otherwise, it will just slightly regenerate, and slow the vehicle.

    If you want to explain regeneration simply, use the motor example...

    Some motors can work both ways. If you put electricity in, the motor turns. If you turn the motor yourself, it puts electricity out. So sometimes the batteries make the motors turn, sometimes you use the spinning wheels, to make electricity for the battery. The more electricity the motor makes, the more it makes the car slow down.

    -Jason
     
  6. RobH

    RobH Senior Member

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    On a downgrade that requires slight braking to maintain 65 mph, I see most other cars without their brake lights ON. So they're using gas to maintain their speed, while I'm in regen mode...:D
     
  7. telmo744

    telmo744 HSD fanatic

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    Not always. No gas pedal means no fuel injection in other cars, although their engine is air/friction-braking. So is Prius (at 65mph), but at a very low rpm and less air braking (valve timing allows less pump losses)...
     
  8. miscrms

    miscrms Plug Envious Member

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    If you put both cars in neutral, the Prius should coast farther than most cars. If there was no friction, both cars would coast forever (conservation of energy/momentum). In reality, friction causes energy to be dissipated as heat through rolling resistance (heats tires), air resistance (heats air) transmission losses (heats gears/oil), etc. Since Prius is very aerodynamic, has low rolling resistance tires, and less moving gears all these loses should be lower which equates to losing energy slower or coasting farther.

    When left "in gear" the Prius should coast a similar distance to other cars. In normal cars leaving the engine engaged while coasting increases friction / resistance which creates more heat, causing the vehicle to slow down faster. When leave the Prius in gear (really just a computer difference, there is no clutch in the Prius) it adds a similar amount of resistance by using one of the electric motors as a generator, which converts some of the energy that would have been wasted as heat into electricity and storing it in the battery.

    Or something like that ;)

    Rob
     
  9. MTL_hihy

    MTL_hihy Active Member

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    Kids often need to see how it works in very simple terms so something like this might be good to demonstrate how a Prius works.

    Motor Generator Kit
     
  10. Tim Jones

    Tim Jones Active Member

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    I have tried coasting (N) on my exit and the car goes much farther......
     
  11. Ronald Doles

    Ronald Doles Member

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    Buy a generator and light kit for his bicycle. He can see the difference between the generator engaged and not engaged. It takes extra energy to pedal the bike with the generator engaged and it slows the bike down if you stop pedaling.
    [​IMG]
     
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