Coasting in neutral

Discussion in 'Gen 2 Prius Main Forum' started by yuanmeinert, Oct 15, 2007.

  1. yuanmeinert

    yuanmeinert New Member

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    I have read many of the comments about coasting downhill in neutral versus gliding by feathering the gas pedal to maintain a zero arrow state (stealth or warp stealth), but still do not have a clear understanding of the advantages and drawbacks to coasting downhill in neutral.

    For a long, straight downhill stretch on the interstate, perhaps several miles, it is certainly easier to shift to neutral than to maintain a zero arrow state by feathering the gas pedal. It would also appear that there would be less drag (and therefore higher speed) in neutral than with the transmission engaged. This higher speed would then result in coasting further up the next hill before needing to use power to achieve a particular speed.

    Ignoring factors such as exceeding speed limits, local laws about driving in neutral, and less control of the car while in neutral, what are the pros and cons of coasting downhill in neutral? I am particularly interested in some of the previously posted suggestions that coasting in neutral could harm an engine or transmission, especially at high speeds. Is there real evidence for this? I have learned from and really appreciate many of the detailed technical responses to previous posters on a variety of subjects and look forward to being further enlightened. Thanks.

    Larry
     
  2. richard schumacher

    richard schumacher shortbus driver

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    No evidence that I know of in the form of pictures of an exploded transmission, or the like, but it is plausible. The engine has to spin to keep one of the electric motors from over-reving, and it can't do that in N. It is also possible (though probably unlikely) to completely drain the battery, since no generating occurs in N.

    Keep in mind that the transmission is always engaged mechanically: there is no clutch to separate, and everything continues to spin.
     
  3. auricchio

    auricchio Member

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    Richard's point about over-revving is plausible, and therefore enough reason not to coast in neutral.

    Also, in some states coasting downhill in neutral is illegal.
     
  4. Doc Willie

    Doc Willie Shuttlecraft Commander

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    What about coasting in Neutral during Pulse & Glide maneuvers? There is no real danger of overspinning anything, and it is a hell of a lot easier than keeping your foot in that EXACT position.
     
  5. patsparks

    patsparks An Aussie perspective

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    What is the maximum speed you can glide without the ICE spinning over? Is it 42mph? I would think up to that speed it is safe but over that you are asking for trouble. Of course if you activate the brakes to control your speed you will only have friction brakes so engaging drive would be better if your speed went too high. It's a rare hill that is exactly right to coast all the way down at a constant speed, if you traverse one, good luck to you I reckon.
     
  6. a priori

    a priori Canonus Curiosus

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    Somehow, this answer seems comforting, but I'm wondering why there is any real difference in effect at that speed. I know the ICE starts spinning at 41/42 -- I can feel it -- but how does this mean anything in terms of safety when gliding in neutral? I don't do it that way. I glide in gear and just balance my foot on the pedal. This seemed awkward at first, but I've become quite accustomed to it.

    I don't have much choice, because Illinois requires that you have the car in gear. I also believe I have more control over my car when it is in gear. Further, I like the ability to add a bit of electric power or grab a bit of regen in order to adjust my speed. More than anything else, though, I like the idea of saving my brake pads by using regen braking. If this happens only when the car is in gear, then I will never glide in neutral.

    Still, I think the OP's question is a worthy one, and I'd like to hear more responses from the other knowledgeable ones on PC. Thanks, Richard, Rick & Pat for your answers.
     
  7. bestmapman

    bestmapman 04, 07 ,08, 09, 10 and 16 Pri

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    OK Lets get some points clear about N (Neutral)

    1) N is a condition where both electric motors due not engage under any circustance. What this means is that whatever the state of the ice is when you select N is where the ICE will stay. If the ICE is off then it will stay of in N. If the ICE is on it will stay on in N.

    2) N does not physically disconnect anything. It means literally that the electric motors freewheel. both MG1 and MG2 do not act as a motor or generator. They just spin as required.

    3) Below 42 MPH the ICE will go off when you glide. This is essentially the same state as N is. There is no advantage to going to N except you don't have to hold the pedal feathered.

    4) Above 42 MPH the ICE always runs. If you select N above 42 MPH then the ICE runs by burning fuel. If you feather the pedal and obtain warp stealth, then the engine spins but the engine does not use any fuel. If you go to N above 42 MPH the ICE uses fuel to idle.

    5) Warp N This is where you select N below 42 MPH with the ICE off and then go faster than 42 MPH. The ICE will not come on because the electric motors will not engage. This mode is not recommended because you can overspeed MG1.

    Link on how the HSD works
    http://prius.ecrostech.com/original/Unders...gOnAsIDrive.htm

    Article and simulation on MG1 vs ICE vs MG2 speeds
    http://eahart.com/prius/psd/

    Article on Pulse and Glide
    http://www.cleanmpg.com/forums/articles/t-...m-fe--1224.html

    Now with all this said, practice and see what works best for you.
     
  8. a priori

    a priori Canonus Curiosus

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    Great help and explanation. Thank you.

    Question: If in N neither electric motor (MG-1 or MG-2) engages, then how are we in danger of an overspeed with MG-1 when in Warp N? One of the first posts noted there was a danger like this. I don't doubt it. I'm just wondering if your statements are inconsistent. Truly, it is my understanding that is likely to be inconsistent and not your explanation. I'm just hoping to learn more about this car.
     
  9. bestmapman

    bestmapman 04, 07 ,08, 09, 10 and 16 Pri

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    <div class='quotetop'>QUOTE(a priori @ Oct 15 2007, 09:17 PM) [snapback]526107[/snapback]</div>
    Read this article
    http://prius.ecrostech.com/original/Unders...gOnAsIDrive.htm

    You need to change your thinking on MG1, MG2 and the ICE. When I first got the Prius, I too had difficulty understanding the PSD. Hear is how to it works. All three motors/engine are always connected to the wheels through the PSD. Nothing disconnects, connects, disengages or engages in the PSD. This sounds strange untill you think of a normal differential in a car. In a differential the back wheels turn independeltly. If one wheel loses traction, then the other wheel loses power, as the wheel that has lost traction just spins. The PSD is similiar to this although different.

    Power to the wheels: If the ICE is running, the only way it can get power to the wheels is if the ICE gets to "push" off of something. The ICE (in a sense) pushes off MG1. Think of the differential. If you can visualize the MG1 as the wheel that has lost traction, then this wheel has to get traction for the other wheel to spin. This is similiar to how MG1 reacts to the ICE. Not exactly but similiar. In order for the ICE to pass power to the wheels, MG1 must resist spinning to allow power to pass to the wheels. Not a perfect example but it helps me understand it.

    Back to your original question.

    If in N neither electric motor (MG-1 or MG-2) engages, then how are we in danger of an overspeed with MG-1 when in Warp N?

    Both motors all always "engageed". They are engaged in this sense. They are ALWAYS physically connected to each other through the PSD. What they do is vary there electrons in (power to the electirc motors) or electrons out (generaton from the electric motors), or no electrons flowing ( N neutral). If no electrons are flowing, the ICE is off and the car is moving, then MG1 must turn to compensate. In fact MG1 turns in either direction as needed. If the ICE is not running (Warp N) and car goes about 61-65 MPH(I am not exactly sure of the speed), then MG1 will overspeed because MG1 must turn to compensate for the ICE not turning.

    Hope this helps.
     
  10. a priori

    a priori Canonus Curiosus

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    Yes! Thank you for the further explanation. I think it was your use of "engage" that got me stuck. I had forgotten that there really is no clutch mechanism and that everything "connects" electronically. So it is the MG-1 that really acts as a "counter" here (a bad comparison, I suspect).

    With this explanation, it seems clear to me there is no reason to use neutral when traveling at any highway speed at all. I use it just to move the car around the driveway. Because I live at the top of a slight incline, I've used it to allow me to back out of the drive and coast down my street before turning the car "on" when I leave very early in the morning. Otherwise, it just doesn't get used.
     
  11. daniel

    daniel Cat Lovers Against the Bomb

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    There is never any reason to put the car in neutral unless you are being towed, or pulled through a car wash, which is a form of being towed.

    There is never any benefit to driving the car in neutral.

    However, in neutral THE TRACTION BATTERY WILL NOT CHARGE and this can be very bad, because if you allow it to discharge completely you are in for very expensive service.

    The Prius was designed to combine gas and electric drive systems. When you drive in neutral, aside from getting no power to the wheels, AND HAVING NO POWER IF YOU NEED TO MAKE AN EMERGENCY MANEUVER, you are denying the car the ability to operate was it was designed to do.

    Please do not ever drive your Prius in neutral. Use neutral only if you need to be towed or pulled or pushed.
     
  12. patsparks

    patsparks An Aussie perspective

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    Have a look at this
    http://eahart.com/prius/psd/
    Near the bottom of the page is a simulator of the HSD and power split device.
    See what happens if you raise the speed of MG2 to 2535 RPM speed at 43MPH while the ICE is at 0 RPM You will see that MG1 exceeds the maximum rated speed of 6500 RPM.

    Start the ICE and see that the speed of MG1 drops even at ICE idle speed of 880 RPM to 2993 RPM. With the ICE idling at 880 RPM the maximum speed is 65 MPH then the speed of the ICE will need to increase above idle.
    If you haven't already, go have a play with the simulator.
     
  13. PA Prius

    PA Prius Active Member

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    Larry, many opinions here as you can see. Mine is that I consistently use "N" -- when I'm not in the mood to Pulse and Glide, and especially to Warp N. I do put it back into D when approaching 62 mph.

    Has anyone heard of anyone doing damage by coasting in N above (or below) 62 mph?

    PA P
     
  14. bestmapman

    bestmapman 04, 07 ,08, 09, 10 and 16 Pri

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    <div class='quotetop'>QUOTE(efusco @ Aug 30 2006, 10:22 AM) [snapback]311561[/snapback]</div>
    I new I had seen it somewhere. 10,000 RPM limit for MG1. If you use that simulator, then it works out to be about 61 MPH. I think that simulator shows max of 6500 RPM thus the difference.
     
  15. Kidd

    Kidd New Member

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    Not going into Neutral when placed in "N"

    I have owned my Prius for just a week now, and aside form the obvious that it is the most incredibly fun car that you could ever drive, coming onto an exit ramp, I put the car into Neutral /"N" to see what would happen, going from the display on the dash, it did not go into neutral. It just stayed in Drive. I have an "08 Touring. So how do you get it into neutral or to coast out of gear when driving then?
     
  16. Doc Willie

    Doc Willie Shuttlecraft Commander

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    You may have to hold the shift knob in "N" for a 1/2 second or so for it to shift.
     
  17. qbee42

    qbee42 My other car is a boat

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    I agree with daniel, for all of the same reasons.

    Tom
     
  18. Matt Herring

    Matt Herring New Member

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    From the article you posted...

    "By spinning the planet carrier and the ICE at 1000 r.p.m., MG1 is protected at speeds of up to 65 m.p.h."

    Thanks...I'll keep an eye on it when coasting at highway speeds.
     
  19. donee

    donee New Member

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    Hi All,

    Just lifting the pedal , and initiating a warp-stealth ( yellow arrows only at highway speeds) can be done up to 64 ish (maybe 65) mph. In this mode the engine is controlled to be spining at 1100 RPM I believe, but its unfired (not combusting gasoline). Above that speed the engine will be fired and run faster. This is a similar situation when gliding, or EV'ing and you exceed 41 mph - the engine comes on.

    Its not practical (or legal in many places) to initiate a neutral glide on the interstate as these roads have a 45 mph minimun speed limit, and you need to be below 42 mph to get the car into neutral with the engine at 0 RPM.

    I do not know of any advantage of shifting into neutral at 42 to 65 mph in the Prius. The engine will most likely drop to 1100 rpm, and MG1 will spin up quite fast. I do not think you would save any energy over a warp-stealth.

    Neutral gliding below 42 mph saves significant scalable drain on the battery. This has been shown conclusively. The faster the car is going when it is in a DRIVE mode glide, the more battery current is being draw. Below about 15 mph, the extra current is negliable, and a neutral glide is not going to be advantagous.

    <GUESS MODE ON> The reason for the current, is probably something to do with the the windings losses in the MG1. The controller is trying to keep MG1 at a speed where torque can be instantlly applied to restart the engine. In order to do this a pulse width has to be set that results in a potential at the windings (averaged over a half cycle?) that that counters MG1 back emf (generator voltage, proportional to motor rpm). This probably does heat MG1, where at Neutral glide would result in it cooling (no current flow). With the car in neutral, MG1 free wheels, and presents a voltage at the inverter transistors (which are off). The inverter transistors can resist this voltage, as they are rated to handle all the voltages MG1 can generate, as they have to go open circuit midway in the pole cycle (pulse-width modulation of motor field coils), to control the motor in normal operation. <GUESS MODE OFF>
     
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  20. Simtronic

    Simtronic Member

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    This post will certainly change my driving as I have been in N a lot in my hilly area, I always put it back in D long before I needed to touch the brakes and did so when speed needed curtailing. I didn't realise I could overspeed or damage anything doing so.
     
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