Regen while coasting -- why?

Discussion in 'Gen 2 Prius Fuel Economy' started by Sailfish11, Aug 16, 2007.

  1. Sailfish11

    Sailfish11 New Member

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    It seems like a waste to me. We use precious energy to build up speed; why give it away to coast? If we want to regen we can lightly press on the brakes. I know we can 'feather', but wouldn't it be easier to have the Prius coast in neutral (neither using nor generating energy)?
     
  2. icarus

    icarus Senior Member

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    You need to learn to pay attention to the arrows, and your right foot. You can "coast" without regen by keeping the arrows neutral (neither charging nor discharging.) You regen only when you take your foot far enough off to start the arrows flowing back into the battery. If you watch the arrows you will get the feel of it.

    You regen when you really need to slow down or stop, but not when you are cruising. Look at warp/stealth in these boards.

    Icarus
     
  3. jarrett_gorin

    jarrett_gorin New Member

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    I am new at Prius driving technique (I just got mine on Sunday), but I also have a question about this. Are you all saying that you should keep your foot slightly on the gas pedal when you coast?

    Also, I wanted to find out under what circumstances people use the "B" setting. I understood that you use it to regen when coasting down hills etc. and that it will also help you from building up excessive speed. I have also been using it occasionally as the first stage of stopping when I exit the freeway, since it provides a nice gradual slowing and appears to regenerate electricity doing that. Is this a good idea or not?

    Would appreciate input on this.
     
  4. qbee42

    qbee42 My other car is a boat

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    <div class='quotetop'>QUOTE(Sailfish11 @ Aug 16 2007, 11:53 AM) [snapback]496924[/snapback]</div>
    The regen during coasting is done to simulate engine drag on a normal car. Every effort was made to make the Prius drive and control in a fashion understandable to drivers. It's the same reason the Prius creeps forward when you take your foot off the brake. There is absolutely no reason for this other than that's how it works with a normal automatic transmission, so the Prius control system makes it work that way.

    Tom
     
  5. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(icarus @ Aug 16 2007, 11:04 AM) [snapback]496935[/snapback]</div>
    The original poster is already aware of this. His question isn't how to glide. . . .

    His question is: Why is the car designed to to regen when I take my foot all the way off?

    It seems like a better design for hypermilers would be to design the car to "glide" when you completely remove your foot from the accelerator and not to "regen" until you ever so slightly press the brake.

    Of course the car wasn't designed for hypermilers (it just seems that way), it was designed for average drivers.

    Perhaps average drivers expect a car to slow down a bit from engine drag when they remove their foot from the pedal?
     
  6. Washington1788

    Washington1788 One of the "Deniers"

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    <div class='quotetop'>QUOTE(JG in SB @ Aug 16 2007, 11:21 AM) [snapback]496945[/snapback]</div>
    B is used primarily for "engine" braking. For instance, if you're coming out of the mountains and all you're doing is coasting, once you build up an all green charge for your battery, you can switch to the B mode so your engine essentially helps slow you down rather than relying on your brakes -- it takes some of the pressure off them. As far as I know, that's really the only situation where you'd use B.

    I'm sure someone will correct me if I've missed something. :)
     
  7. tekn0wledg

    tekn0wledg New Member

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    That is correct.

    You can feel the drag of letting off the gas pedal in a normal car quite significantly. I believe the regen here was made to feel the same way.

    Most people let off the accelerator at driving speeds to avoid having to use the brakes. A good example would be when someone switches into your lane abruptly. It seems stupid to use the brakes if you can just let off gas for a few seconds to gain a reasonable distance from them.

    The autoregen at this point is probably used to simulate this type of behavior.
     
  8. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(JG in SB @ Aug 16 2007, 11:21 AM) [snapback]496945[/snapback]</div>
    Yes, if you are trying to maximize your fuel efficiency (mpg) and glide as far as possible while losing as little energy as possible.

    <div class='quotetop'>QUOTE(JG in SB @ Aug 16 2007, 11:21 AM) [snapback]496945[/snapback]</div>
    Most of the experts around here will tell you to use "B" when heading down a steep hill for a long distance. ( Like a mountain for instance ). In general it is reccomended to use "B" to keep from wearing down (or overheating) the brake pads when the battery is as charged as the car will allow, or when regen won't slow the car down enough.

    It is not intended to be used for charging the battery. When exiting the expressway, using the brake pedal will also "provide a nice gradual slowing and regenerate electricity"
     
  9. Godiva

    Godiva AmeriKan Citizen

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    <div class='quotetop'>QUOTE(Danny Hamilton @ Aug 16 2007, 11:25 AM) [snapback]496949[/snapback]</div>

    Why not?
     
  10. icarus

    icarus Senior Member

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    <div class='quotetop'>QUOTE(JG in SB @ Aug 16 2007, 09:21 AM) [snapback]496945[/snapback]</div>
    The trick is to use only as much decleration as needed. If you coasting just to maintain speed, or in anticipation of a traffic situation you want to coast without regen. If you need to slow down as for a light, then taking your foot off will start regen and slow you down faster. Using the "B" setting will not add to the regen, and in fact will reduce it. You should only use "B" for coming down a long hill or mountain grade where you would need the brakes, just like shifting down in a regular car. When you get off the freeway, anticipate your deceleration time and allow the car to come to a nice, gentle slow down with as little use of the brakes as possible. (I don't know, but I think the regen comes not from the friction brakes, but from the electric motor charging instead. I think the friction brakes do nothing more than conventional brakes. The advertisment about "regen braking" is (I think) a misnomer,,,correct me if I am wrong).

    Get used to the arrows and drive accordingly,

    Icarus
     
  11. geodosch

    geodosch Member

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    <div class='quotetop'>QUOTE(icarus @ Aug 16 2007, 01:52 PM) [snapback]497031[/snapback]</div>
    The term 'regen braking' is not a misnomer. Braking refers to the act of slowing or stopping the vehicle, which is not necessarily by means of a mechanical friction brake. The friction brakes are conventional brakes (in the sense of the type of brakes on most cars.) The regen braking is another type of braking on the Prius, which uses the drag on the EM which occurs when you turn them into generators.
     
  12. Sailfish11

    Sailfish11 New Member

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    <div class='quotetop'>QUOTE(tekn0wledg @ Aug 16 2007, 12:30 PM) [snapback]496952[/snapback]</div>
    I guess I can accept the argument that Toyota wanted to simulate a 'normal' car (same goes for the gentle forward creep when, from a stop, you take your foot of the brake while in drive). But coasting in a normal automatic will slow the car down only to a point -- it usually levels out at around 20 mph (depending on the car). The Prius' system continues to slow you down no matter how slow you're going. So at slow speeds it's not simulating a normal car at all.

    Feathering works, but involves diverting at least some extra attention inside the car, instead of out on the road.

    In the end, I guess I'm just venting....
     
  13. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(Sailfish11 @ Aug 16 2007, 01:14 PM) [snapback]497062[/snapback]</div>
    The Prius will also only slow the car down to a point -- it usually levels out at around 7 mph.
     
  14. icarus

    icarus Senior Member

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    <div class='quotetop'>QUOTE(GeoDosch @ Aug 16 2007, 11:03 AM) [snapback]497051[/snapback]</div>

    I understand the concept of regenerative braking.years. All Diesel/Electric locomotives have been using it for years. The difference is that the energy generated by turning the traction motors into generators is send to giant heating elements and is tossed off as waste heat. (Look at the top of a loco somtime). My point (question) was that when you apply the friction brakes you are in reality wasting energy. If you could coast to a (almost) stop without hitting the brakes at all you would be wasting the least amount of energy. Of course it is always better to maintain momentum rather than slowing down.

    Icarus
     
  15. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(icarus @ Aug 16 2007, 02:15 PM) [snapback]497132[/snapback]</div>
    I agree, when possible, it is better to minimize loss of momentum than to use regen or friction brakes. If you need to slow down, as a general rule the more gradually you can do so, the better. If you can "glide" without any regen down to your desired speed, you will have maximized the efficiency of the reduction in speed.

    However, it isn't clear from your post that depending on how hard you press the brake pedal, and how fast you are going the Prius chooses some combination of friction brakes and using the generator to reduce your speed. In the Prius the electricity generated while braking is used to charge the battery a bit, so that electricity can be reused in the future to re-accelerate the car.

    You'll never get as much back out as you put in, but it is certainly better than just generating heat and dissipating it into the atmosphere.
     
  16. geodosch

    geodosch Member

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    <div class='quotetop'>QUOTE(icarus @ Aug 16 2007, 03:15 PM) [snapback]497132[/snapback]</div>
    Stepping on the brake pedal on the Prius does not automatically mean you are engaging the friction brakes; it depends upon how hard you press the pedal. When you step on the brake, you're telling the Prius you want to slow down, & how hard you press lets it know how quickly you want to slow. It will decide if it needs friction brakes, or if regen braking is enough. Once you are below 7 mph you will definitely be using friction brakes, since regen does not work below that speed. Otherwise, the only way to tell when the friction brakes are engaged would be to rig an indicator, (or else listen carefully for any sounds they might make.)
     
  17. icarus

    icarus Senior Member

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    <div class='quotetop'>QUOTE(Danny Hamilton @ Aug 16 2007, 12:39 PM) [snapback]497158[/snapback]</div>

    I understand intuitivly that when the electric motor is being driving by the wheels, the turn into generators and therefore recharge the battery. The capacity to regenerate is limited (simply) by the speed of the turning. Question is, is the car "smart enough" to add extra regen when you first push the brake pedal, or as I suspect the regen is already taking place because you are off the throttle. As I said before, I suspect that all pushing the pedal does is add in the friction brake with no added regen.

    Icarus
     
  18. bwilson4web

    bwilson4web BMW i3 and Model 3

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    <div class='quotetop'>QUOTE(icarus @ Aug 16 2007, 03:25 PM) [snapback]497199[/snapback]</div>
    It modulates the regen current in response to the brake pedal. Only when you get beyond the regen range does the mechanical brake smoothly come in.

    <div class='quotetop'>QUOTE(Sailfish11 @ Aug 16 2007, 10:53 AM) [snapback]496924[/snapback]</div>
    Yes.

    Bob Wilson
     
  19. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(icarus @ Aug 16 2007, 03:25 PM) [snapback]497199[/snapback]</div>
    You suspect incorrectly. Pushing the pedal increases the regen. Pushing it harder increases the regen more. At some point it can't increase the regen anymore, so it engages the friction brakes along with the regen.

    Note that I've seen several discussions that claim that in a "panic stop" situation, the Prius ceases regen and uses friction brakes exclusively. I haven't bothered to investigate this yet, so I can't say if it is true or a misunderstanding of what is actually happening.
     
  20. justifyd

    justifyd New Member

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    <div class='quotetop'>QUOTE(qbee42 @ Aug 16 2007, 12:24 PM) [snapback]496947[/snapback]</div>

    Actually, both exist for useful reasons.

    The regenerative braking while coasting makes it a bit easier to pace traffic. Without it, you'd encounter even more situations where you'd have to manually engage the brake to keep from running into the back of the (conventional) vehicle in front of you.

    The creeping action is another convenience. Without it, stop-and-go traffic would give your right foot a bigger workout. As is, you don't need to switch pedals when crawling along.

    - Justifyd
     
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