How much MPG improvement do you get with regenerative braking?

Discussion in 'Gen 2 Prius Fuel Economy' started by burritos, Jul 11, 2007.

  1. burritos

    burritos Senior Member

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    One of the reasons I really like the toyota hybrid technology is that it attempts to recapture the energy lost with braking. Philosophically, I feel better when the energy that normally is wasted as heat when braking is redirected to propel me at a later time. This in turn of course contributes to less gas use. Does anyone know how much this really helps though? Is this just a gag selling point? Am I feeling better for a .2 mpg improvement(not worth it) or a 2 mpg improvement(statistically relevant)?
     
  2. Tideland Prius

    Tideland Prius Moderator of the North
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    <div class='quotetop'>QUOTE(burritos @ Jul 11 2007, 09:21 AM) [snapback]476865[/snapback]</div>
    Keep in mind, even if the amount of fuel saved is not significant, you are saving on brake pad wear so that's another way to save the environment if that's your concern. You also minimise the amount of brake dust that is produced and so forth.


    I think it is worth it. This means I can chug around the top of the mountain in EV mode and know that I can recharge it on the way down. So think about it - no emissions at the top of the mountain, no emissions going down the mountain. That's pretty good I think.
     
  3. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(burritos @ Jul 11 2007, 11:21 AM) [snapback]476865[/snapback]</div>
    Your question doesn't have an answer. It really depends on your personal driving profile.

    If you:
    Live next to an expressway entrance ramp.
    Work 75 miles away next to another expressway ramp.
    Commute during non-rush hour traffic when you never need to slow down until you reach your destination.
    Exclusively use your Prius as your work commuting car and nothing else.
    Wait until the last second to slow down and then brake hard.

    You'll gain pretty close to 0 mpg because of the regenerative braking.

    If you:
    Always drive less than 40mph.
    Frequently need to slow down from 40 mph but rarely less than 10 mph.
    Coast (not glide) or brake lightly for every slow down.
    Accelerate at a rate that uses maximum current from the battery.

    You'll probably see a huge gain in MPG because of regenerative braking.

    There are a lot of variables that have to be taken into consideration before you can figure out how much the regen is helping you personally.
     
  4. fan-atic

    fan-atic New Member

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    <div class='quotetop'>QUOTE(Tideland Prius @ Jul 11 2007, 11:49 AM) [snapback]476894[/snapback]</div>
    I think Tideland got it right: it saves on the brakes and does save some energy.

    As for the real numbers, I can make some guesses based on a few discrete points of information: 1) the Energy display shows regeneration as little green cars, each worth 50 watt-hours. I don't see many cars usually. 2) The Traction battery holds 1200 watt-hours full capacity I think. (Could be as high as 1600 however). 3) A gallon of gasoline has 36600 watt-hours of energy. In a normal car only 20% of that is used. In a Prius probably 35% is used (12,800 watt-hours). So if you see 10 little green cars in 50 miles and you get 50mpg, that would mean 4% is due to regeneration.

    A lot of the automotive media seem to think that ALL the energy for the battery comes from the braking. They need to be educated. Most of it comes from MG1, the starter motor running as a generator.
     
  5. burritos

    burritos Senior Member

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    <div class='quotetop'>QUOTE(Danny Hamilton @ Jul 11 2007, 12:12 PM) [snapback]476919[/snapback]</div>
    Define huge please.

    If I take the car and slowly accelerate the car to 40 mph at the cost of X gallons of gas, how much energy will I get back if I:

    1. Abruptly stop(not slam on the brakes but brake as hard as I can still using regenerative braking).
    2. Brake slowly.
    3. Coast to a stop.
     
  6. Pinto Girl

    Pinto Girl New Member

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    <div class='quotetop'>QUOTE(burritos @ Jul 11 2007, 02:16 PM) [snapback]477033[/snapback]</div>
    I don't know the answers, but I will say that you'll cover more distance if you coast to a stop than if you don't; doesn't regen capture 30-something percent of the energy thrown away during deceleration...?

    I agree, I too think there's something just philosophically 'right' about at least attempting to recapture this energy.

    Every time I commute on CalTrain, for example, it galls me how their locomotive accelerates hundreds of tons of engine and cars (oh, and the weight of the people, too)...then, a minute later, throws it all away as the train comes to a stop to pick up more commuters.

    I think they even pour energy *into* the traction motors, so they can help to slow things down more quickly...

    ??????
     
  7. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(burritos @ Jul 11 2007, 02:16 PM) [snapback]477033[/snapback]</div>
    Can someone with a CANView chime in here please?

    If you get the car up to 40 mph with no cargo, and only yourself in it and then coast down to 10 mph on a flat in windless conditions, what sort of electric generation do you see? I assume it will change over time as you slow down, so for a rough estimate, how about noting the instantaneous current every time your speed drops 5 mpg.

    Same with a consistent gentle brake in identical conditions? (Does CANView have an indication as to how far the brake pedal is pressed?)

    I'm not sure how to define a "hard but not panic brake" in a consistent way, but if you want to offer some numbers here as well that'd be great.
     
  8. gge5

    gge5 New Member

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    <div class='quotetop'>QUOTE(Danny Hamilton @ Jul 11 2007, 12:51 PM) [snapback]477053[/snapback]</div>
    This link should answer your questions:
    http://vassfamily.net/ToyotaPrius/CAN/brindex.html

    The most efficient brake pedal position read by CANView is at 17 out of a maximum of 127 (13% depressed).


    <div class='quotetop'>QUOTE(ken1784 @ Mar 11 2007, 10:40 PM) [snapback]404066[/snapback]</div>
     
  9. Earthling

    Earthling New Member

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    Toyota engineers might be able to estimate how much regenerative braking gives you in terms of mpg. I can't.

    I don't think the Prius could get anywhere near 50 mpg without it.

    A minor point perhaps: every time a conventional vehicle stops, it releases heat into the atomosphere from the brakes. The Prius regenerative braking isn't 100 efficient, but a sizeable part of this energy goes into the hybrid battery instead of directly into the atomosphere in the form of waste heat.

    Harry
     
  10. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(Alan D @ Jul 11 2007, 03:31 PM) [snapback]477079[/snapback]</div>
    Ok, so if I'm interpreting the spreadsheet correctly, about 4,905 joules of energy are recovered with optimal breaking.

    I guess the next question is, how many joules of energy are required to accelerate a Prius from 5mph to 40mph.

    Anyone want to do the math on that one? Maybe I'll try later tonight if nobody posts an answer by then.
     
  11. qbee42

    qbee42 My other car is a boat

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    Our best estimates are that about 40% of the braking power is actually delivered back to the wheels as driving force. This is only a rough estimate, but probably in the ball park, considering losses making the electricity, storing it in the battery, and turning it back into rotational energy.

    Tom
     
  12. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(Earthling @ Jul 11 2007, 04:05 PM) [snapback]477096[/snapback]</div>
    Nah, all the energy eventually makes its way to the atmosphere as heat.

    Sure the battery acts as a temporary holding tank for some of the energy, but it all gets used eventually, and the end result of all of it is heat. The Prius just manages to get more miles out of each joule of energy in the gasoline before it becomes heat, but it releases the same amount of heat per gallon as any other gasoline powered vehicle.

    As for the 50mpg, that has a lot more to do with the small ICE and the atkinson cycle. Heck, the hypermilers are getting around 100mpg by barely using regen braking at all. Everytime you recover energy with the brakes you waste some. You are far better off (from a fuel efficency standpoint) not using the brakes at all, regen or otherwise. I average just short of 55 mpg and the vast majority of my driving is expressway at 55 to 65 mph without using the brakes.
     
  13. gge5

    gge5 New Member

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    <div class='quotetop'>QUOTE(Danny Hamilton @ Jul 11 2007, 02:08 PM) [snapback]477098[/snapback]</div>

    from http://vassfamily.net/ToyotaPrius/CAN/eveffindex.html:

    """
    I switched to EV mode and accelerated from stop to 50km/h than back to until regen was observed.
    This graph shows, that regeneration is around 50% :
    ~54% observing currents and voltages.
    43%~57% observing SOC.
    """

    This, however, is only accurate if you've accelerated in EV mode which has a more efficient transfer of energy from storage to wheels than the combustion engine. According to http://www.fueleconomy.gov/feg/atv.shtml a typical automotive gasoline engine (ignoring draws from accessories and idling) is 37.6% efficient in converting the gas into usable kinetic energy. Tesla claims electricity to kinetic energy efficiency around 85%. I'll let you make a guess as to where in the spectrum the Prius's more efficient ICE and electric motors fall.

    I found http://www.teslamotors.com/display_data/te...ation_final.swf to be a very good presentation from Tesla Motors.
     
  14. ystasino

    ystasino Active Member

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    <div class='quotetop'>QUOTE(burritos @ Jul 11 2007, 12:21 PM) [snapback]476865[/snapback]</div>
    Well in my commute, regenerative braking regularly helps charging the battery. I think that in the city regenerative braking does have a significant effect to charge the battery and help with the P&G.

    On a good commute back and forth I get about 68mpg with significant battery charging (at least 3 bars each way) coming from the progressive breaking and the coasting with the teal arrows condition.

    We are certainly talking an improvement much higher than 2 or even 5mpg. For my commute. I'm surprised this is even a question.
     
  15. Earthling

    Earthling New Member

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    <div class='quotetop'>QUOTE(Danny Hamilton @ Jul 11 2007, 05:13 PM) [snapback]477104[/snapback]</div>
    Well, yeah, but...400 miles/50 mpg = 8 gallons of gasoline worth of heat <--Prius

    400 miles/12 mpg = 33.3 gallons of gasoline worth of heat <--F-250


    Harry
     
  16. gge5

    gge5 New Member

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    From the Tesla Motor "Touch" Blog:

    """
    The energy conversion efficiencies from chemical to electrical (battery), DC current to AC current (inverter), electrical to mechanical (motor), and torque to force (transmission and wheels) are all quite high and work just as efficiently returning energy into the battery. The bigger problem is aerodynamic losses and higher speeds and rolling friction of the tires. These both act to slow the car, but the energy dissipated cannot be recovered. We must also remember that, even though the battery-to-wheel conversion efficiency is pretty good (up to 80% or so), the energy makes a full circle back into the battery and it gets converted twice for a net efficiency of at most 80% * 80% = 64%.
    """

    It's interesting that in their presentation they claim 85%, and here they claim "up to 80% or so," but that's marketing for you.

    This brings up an interesting point: I'm sure someone knows off the top of their head, but does the Prius use the same electric motor for both acceleration and regeneration? If that were the case, then you would want to accelerate at the same rate as where the regenerator is most efficient (you would want to accelerate to 40MPH in 22.5 seconds).
     
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