What's the difference between...

Discussion in 'Knowledge Base Articles Discussion' started by Kaos1, Mar 18, 2006.

  1. Kaos1

    Kaos1 Junior Member

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    Is there any difference, mechanically, between coasting with the transmission in neutral and driving in drive with no arrows between the battery/MG1/ICE/wheels?

    Kaos1
     
  2. Tideland Prius

    Tideland Prius Moderator of the North
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    The difference is that you're in gear and have control over acceleration if the situation arises with coasting. In neutral, you're on your own.
     
  3. jeneric

    jeneric New Member

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    I wonder this as well, and have decided not to use N for that purpose for now. In N, I don't think the ICE can charge the battery at all, and you may lose excess energy if it has to run or something. Kinda like, when you're parked, it warns you not to leave the car in N.
     
  4. jeneric

    jeneric New Member

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    How long does it take you to shift into drive? About the same time as it would take someone to let out the clutch or shift to a lower gear for more accelleration, I suppose. If you're in N, you probably have your hand on the...um...ever since I've heard it called "clitoris" vs. stick, it's kinda stuck with me, and are ready to shift when you need the power.

    Today, I had the bright idea to let my 2 year old use the garage door opener. She wasn't holding the button long enough, so I ended up having to open it and telling her she wasn't pushing it long enough. So after it opened I booted up the car and started backing out, with a glance I saw the garage door was coming back down. I've never had to shift back into drive so fast in my life. This is right after getting a lifetime warrantee on the garage door, good for life or until someone runs into it. Anyway, made it back in in time without hitting the car in front of us either.
     
  5. darelldd

    darelldd Prius is our Gas Guzzler

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    Uhhh, yeah. But the comparison by the OP was between DRIVING in N or DRIVING with no arrows. In neither situation will you be charging anything. I don't advocate driving in N... but mechanically it sure seems like it would be the same as driving with no arrows (you ever try to hold to no arrows over a bumpy road? Tough game!)

    If I had my way (have I ever?) I'd have made the default "zero throttle" mode to be full-on coast (no regen). We could then add a button to create the regen of "off throttle drag." That's how the production EVs were (most of them) and it is the way to go IMO. The brakes still work... the throttle still works, but if you want to coast, all you need to do is remove your foot from the throttle pedal.
     
  6. jeneric

    jeneric New Member

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    Right, but I assume it's because he wants to know, like I, if instead of having to feather the gas pedal to achieve gliding, he can just shift to N. I'm saying that when you feather the gas pedal, if the ICE needs to run or something, the energy will start going to the battery. If you're in N, you may miss out on that energy.
     
  7. darelldd

    darelldd Prius is our Gas Guzzler

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    I gotcha. Its just that you aren't likely to get into "no arrows" if the battery needs charging anyway. Anyway... "mechanically" I'll bet they're the same, or same enough. And no, you won't get charging while in N (apparently). I know I've read this in the manual before... but the ICE will NEVER come on when in N? Or you just won't get regen? If only the latter, then there really should be no difference between "no arrows" and N while driving.

    If N doesn't allow the ICE to come on, then that could be used as a poor-man's EV mode. Or maybe the ICE will come on, but still won't allow charging? Now I'm getting myself all confused. :)
     
  8. tochatihu

    tochatihu Senior Member

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    As I understand it, MG1 cannot start the ICE in neutral, because there is no electricity flow to/from either MG. Perhaps the ICE can be started anyway (add fuel and spark), if it is spinning at an appropriate speed?

    But Kaos1, don't think of it as a conventional neutral, because MG2 is always connected to the drive wheels. The rpms of ICE and MG1 are as defined by the nomograph (have you seen the nomograph?). Prius neutral is not a condition where the wheels are disconnected from the other drivetrain components.
     
  9. Kaos1

    Kaos1 Junior Member

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    I can verify that the ICE will run in neutral. Ever try coasting in neutral from a cold start when the ICE is still warming up? Somewhere between 25 and 30MPH, you can get the magic 99.9mpg. That is because the ICE is still running and consuming fuel.

    The reason that I ask is that there are times that it is very difficult to feather the throttle to achieve the perfect glide. When the engine is cold, bumpy roads, or when traveling at higher rates of speed, popping the transmission into N is the easy way to go. I am not concerned about storing electrons during these times, just rolling as efficiently as possible. As far as any need to accelerate, D is engaged with a thought.

    The only concern that I have is there any undue strain on the system? I don't think that there is. The way that the planetary gears work, unlike a traditional tranny, all the gears are meshed all the time, I think.


    Kaos1
     
  10. Kaos1

    Kaos1 Junior Member

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    tochatihu,

    Can you post the nomograph, or a link?

    Kaos1
     
  11. richard schumacher

    richard schumacher shortbus driver

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    Mechanically? No. All drive system components (ICE, both motor/generators, and drive wheels) are always mechanically engaged with each other through the planetary gearset and drive chain. The shift lever is an input to the computers which decide when and how to let the engine and the motors operate. What I'm not certain of is whether, when in N, the ICE will start as needed to prevent max RPMs being exceeded by MG1. If the car is OFF and in N, exceeding about 45 MPH will damage MG1; possibly the same thing can happen while driving and coasting in N downhill. I'm not about to experiment with it!

    For tonnes of detail see http://www.ecrostech.com/prius/original/PriusFrames.htm
    Follow "Understanding Your Prius" -> "The Power Split Device" to get to a version of the nomograph.
     
  12. Kaos1

    Kaos1 Junior Member

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    So, if one is going downhill on the highway at speed, say 65 or so, and releases the throttle (green arrows to the battery) then depresses the pedal to the point where all arrows are absent, this will damage MG1? I have done this, in fact there are times while using cruise control that the car will do this on it's own. Is this any different than using N?

    Kaos1

     
  13. Tideland Prius

    Tideland Prius Moderator of the North
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    Longer than just pressing the accelerator since your foot's already slightly depressing it to glide.
     
  14. mssmith95

    mssmith95 Michael

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    I have been experimenting with this Neutral technique for the last 2 tanks of gas, and have found it very useful!

    I have a long downhill strech in my last 10 miles of my 35 mile commute home. If I fully let off of the throttle, I can not maintain enough forward momentum to coast down the entire strech without giving it some throttle. If I shift to neutral, I can easily maintain freeway speeds, and still can shift quickly to drive if needed to slow down.

    Those last two tanks, using this neutral technique throughout my daily driving, instead of messing with that "no arrows" G-spot, which is sometimes impossible to maintain, seems to have increased my MPG by 2-3.

    The one thing I have noticed, that I can not verify, is that once I shift to Neutral, I seem to lose any potential energy that has not already shown on the MFD. Since I assume the MFD only refreshes the battery level on the screen every few minutes, I do not know if I am really losing potential energy or if I am crazy.

    To better explain. I can go down the same long downhill strech, with my foot off of the throttle and never shift into neutral. By the bottom, I usually have an almost full battery level. However, if I shift into neutral just before the bottom (maybe a few hundred foot difference), and before the battery level has refreshed, I do never get the full battery on the MFD! I have tried this time and time again, with the same result...so my conclusion is that somehow I am losing that built up energy by shifting into neutral. I would think that once the energy is transferred real time to the batteries, but this seems to show otherwise.
     
  15. jeneric

    jeneric New Member

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    Is the last few hundred feet a lot of braking? I think in Neutral, you might not get energy from regen braking.
     
  16. jeneric

    jeneric New Member

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    It looks to me like the MFD refreshes battery level pretty much in real time.
     
  17. priusham

    priusham New Member

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    Neutral disengages the electric generator/motor function so the batteries do not charge, there is no "drag" from the electric generator/motor, you'll gain speed gliding downhill.

    The batteries will still drain as all the car's electronics are still drawing several amps of current.

    I do this all the time in safe situations, it's warned on PriusChat that "driving" in neutral is illegal in some places.
     
  18. priusham

    priusham New Member

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  19. storm petrol

    storm petrol Junior Member

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    Re:mssmith's questions about potential energy.

    The amount of available potential energy in your vehicle at the top of the hill reflects the investment of the chemical energy in the fuel you burned to get there. There have been significant losses due to friction in the engine during conversion of this chemical to mechanical energy. There are frictional losses via air resistance, tire to road interface, etc. There is a huge loss of energy as waste heat flowing out of the block, down the exhaust and out into the atmosphere.

    In any case, as you sit in your car at the top of the hill, you have an enormous amount of potential energy in the mass of your car and yourself which you can think of as being the equivalent of the energy it took to get you up there.

    Now you get to choose how you will release/use that energy.

    If you put the car into neutral and let it go, that potential energy will be converted into the kinetic energy of motion most efficiently. If you do not touch the brakes, encounter another uphill or run into something, you will get the maximum benefit out of that conversion. You would be able to drift until all of that potential energy had either moved you down the road or disappated as waste heat due to friction in the transmission, in the running gear, friction against the road, flexing of the tire side walls, etc.. It is these frictional losses that ultimately slow you to a stop. (Unless you encounter another uphill, in which case some of the remaining kinetic energy of motion will begin to be converted back into potential energy again.

    In neutral, the Prius is not converting any of that energy into electrical energy to be stored in the battery.

    In this scenario, if you put it in drive and drift down the hill, the green arrows actually represent a quite significant loss of potential energy that is being converted into electricity instead of forward motion. Unfortunately, this conversion results in a loss of more than half of the energy it absorbs. This is clearly a waste of energy compared to drifting in neutral. If you will have to stop at the end of the hill anyway, not a big deal. You have saved some significant percentage of that stop as electrical energy in the battery. However, if you can just accelerate by drifting downhill in neutral and then allow the car to naturally slow due to frictional forces (other than physical brakes or regen) down to the cruising speed you wish to maintain and then put it back into drive and begin to burn fuel again, you will have enjoyed the maximum benefit from your energy investment.

    Every time you see green arrows, whether from braking or drifting in drive, you are suffering a loss of kinetic energy. Less of a loss then a non-hybrid car, but a loss. Obviously, this is highly desirable compared to, say, colliding with the rear end of the car in front of you. In this case that energy is very poorly invested in increasing the disorganization (entropy) of the two vehicles and their occupants and producing a great deal of waste heat.

    The ultimate trick for high mileage is to always avoid using the brakes or generating electrical energy while drifting as much as is practical and safe.

    In my own case, I have drifted in neutral at speeds as great as 90 mph. Obviously, I do not recommend that you try this at home. It is dangerous for a number of reasons. However, I do not have any sense that the car has been adversely affected.

    And it has helped me to acheive some very significant mileage gains.

    storm petrol
     
  20. mssmith95

    mssmith95 Michael

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    Storm petrol,

    Thanks for that response!

    That is exactly what I figured...and why I know that using Neutral has definately improved my MPG numbers. As others have stated, it is really hard to get no arrows on the MFD...it has to be a perfect senario. Shifting to and from Neutral works great for me! I just always make sure to be in D while braking to minimize the strain on the brakes. If I already have a full battery, I always use Neutral to glide, and then just use little bursts in D to keep my momentum going. As long as it is not adversely effecting the gears...why not?!

    Thanks for the help
     
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