Does downshifting increase fuel economy...

Discussion in 'Gen III 2010+ Prius Main Forum' started by 67tony, Jan 17, 2012.

  1. 67tony

    67tony Junior Member

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    ...save wear on brake pads, both, or neither?
     
  2. Tech_Guy

    Tech_Guy Class Clown

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    Downshift what? There is no transmission (other than the planetary power split device).

    see: http://eahart.com/prius/psd/

    Keith
     
  3. fuzzy1

    fuzzy1 Senior Member

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    Do you mean B mode? It can't improve fuel economy above what regenerative braking can accomplish, and in most cases cannot even match the regen brakes.

    On long mountain descents, B mode helps prevent brake overheating. It will also reduce brake pad wear on shorter descents that won't cause overheat but will exceed the regen capacity.

    On flat ground and short hills, B mode has little use. Stick to regen braking, which recaptures more energy and doesn't wear the pads.
     
  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers, Nadir of Wrongness

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    I am going to assume you mean on a manual transmission car. The Prius has no ability to downshift, that is why they called it B instead of L, the gearing is not Lower.

    Downshifting NEVER improves fuel economy.

    If you have a car from the 1960s or earlier, the brakes are so weak that downshifting may be needed to stop the car.

    By the 1970s, disc brakes begin the revolution where brakes 'just work' in any automotive application barring long 'steep' descents and racing. If your car was made in the 70 or later down shifting to stop is totally unneeded. Downshifting to descend steep hills is still a good idea.

    In snowy or very rainy weather in a RWD vehicle, downshifting will add drag to the back of the vehicle, which may keep it aimed forward. In a FWD car like the Prius, it is no help with stability.

    Downshifting NEVER improves fuel economy.
    http://en.wikipedia.org/wiki/Disc_brake#History
     
  5. 67tony

    67tony Junior Member

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    Sorry I wasn't clear. Yes, I have a 2010 Prius III, and I was referring to the "B" mode.
    Reading that it uses the engine to slow the car, I equated it to "downshifting".

    fuzzy, I think I understand regenerative braking, but am not seeing how it does not wear the pads.
     
  6. Much More Better

    Much More Better Active Member

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    Don't think you can outsmart the car :)
     
  7. 67tony

    67tony Junior Member

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    Not trying to...just wondering when, and how often, I should use "B".

    :)
     
  8. JimboPalmer

    JimboPalmer Tsar of all the Rushers, Nadir of Wrongness

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    Mount Arvon is 1,979 feet tall, so a road straight down it would need B mode. In general, however, Michigan is not going to offer many steep descents with over 600 foot of drop. It is possible you never need B mode until you visit the Rockies. 1200 miles is no more than 3 tanks.
     
  9. fuzzy1

    fuzzy1 Senior Member

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    Regen braking reduces or eliminates pressure on brake pads, instead getting the drag force from one of the motor-generators, specifically MG2. This reduces brake wear while the recovered energy is stored in the big battery. This works only for light to moderate braking, depending on speed, and only when the big battery not full. Exceed the regen limits, and the brake pads come back into play.

    A Search will find many threads about B mode (and other modes, because the search function can't use single letter search tags).

    How often do you descend 1000 feet or more on mountain roads of 4% grade or steeper? It becomes useful, though not necessary, at about half that. We have plenty of these roads out here, but I'm not familiar with Michigan.
     
  10. macman408

    macman408 Electron Guidance Counselor

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    An electric motor works by running a current through some coiled wires, creating a magnetic field, which pushes against some permanent magnets, spinning the motor. An electric generator is made of the same parts (and, in fact, one machine can be either a motor or a generator, depending on how it is operated, as the Prius's are). Instead though, something (the car's momentum) turns the motor, which moves the permanent magnets past the coiled wires, generating an electrical current. The power from this is then stored in the battery.

    So it's not "braking" by using the brake pads, it's more like engine braking. Unfortunately, however, your gasoline engine is not capable of sucking CO2 and H2O, compressing it, providing heat, and generating gasoline and oxygen. :D The electric motors do work as generators though, and recover a decent percentage of the energy that would otherwise be lost as heat in the brakes.
     
  11. macman408

    macman408 Electron Guidance Counselor

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    Isn't it at least somewhat possible (and I'm off-topic, talking only about non-Prius vehicles here), if downshifting means that the engine runs in fuel cut-off rather than the RPMs being slow enough that fuel is still injected?
     
  12. F8L

    F8L Protecting Habitat & AG Lands

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    I think the OP is hoping that using B mode will aid in filling the HV battery and by this route raising fuel Eco economy.

    The problem with this line of reasoning is that if you are using the brakes to slow down significantly fast enough to warrant using B mode then you have already wasted too much fuel trying to maintain a high rate of speed until just before a stop. It is more efficient to coast or glide to a stop which would the only require gentle braking to come to a stop.
     
  13. qbee42

    qbee42 My other car is a boat

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    Seeing that you live in Michigan, you will never need B mode unless you drive out of state. Save it for driving in the mountains.

    Tom
     
  14. 67tony

    67tony Junior Member

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    Great information, thanks for all the replies.
    Earlier I claimed I knew what regenerative braking is...it is now apparent I did not!
     
  15. qbee42

    qbee42 My other car is a boat

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    I suspect you do. What you might not have realized is that the Prius uses regenerative braking when you press the normal brake pedal. You don't have to be in any special mode to make it work.

    Tom
     
  16. ETC(SS)

    ETC(SS) The other One Percenter.....

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    Re: Does downshifting increase fuel economy...

    No.

    Unless you often drive through a really hilly area, or you're into slowing down your car so that LEO doesn't see you mashing on your brakes, I'd probably leave the B mode alone.
    For me, it's like the EV mode.

    Just a play-pretty.
    Not really worth using.

    OK...maybe the EV mode is more useful if you're trying to sneak up on a cat or something, but they've got pretty good hearing and the tires always give you away.
     
  17. SageBrush

    SageBrush Senior Member

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  18. SageBrush

    SageBrush Senior Member

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    Good advice given -- ignore 'B' mode. It is only good for long steep downhills where riding the brakes might lead them to overheat. I live in the tail of the Rockies and can only think of one place I have used the 'B' mode.

    As to what it is -- IIRC air inflow to the engine is restricted, causing the engine to draw off kinetic energy aka speed. If you are looking for an old world metaphor I think it is closer to the 'Jake' brake of trucks than downshifting in manual cars.
     
  19. ETC(SS)

    ETC(SS) The other One Percenter.....

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    The G3 has a glass jaw.
    Hit too many dogs with them.....and it'll cost you, unless they're like, Yorkie sized. :D