Use of "N" for coasting

Discussion in 'Gen 3 Prius Technical Discussion' started by tedjohnson, Aug 8, 2010.

  1. qbee42

    qbee42 My other car is a boat

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    I believe this is a legal issue. Neutral implies that power has been removed from the drive train. In the case of the Prius, the gearing is always engaged, so Neutral is simulated by removing the field excitation from both MG1 and MG2. This eliminates any chance of actually applying power while in N, but it also leaves us with the potential for over-speed and the inability to charge the HV battery while in N.

    Toyota took the safe route from a legal standpoint, but not the most elegant engineering solution.

    Tom
     
  2. bwilson4web

    bwilson4web BMW i3 and Model 3

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    It might be interesting to compare and contrast:

    • fuel consumed during ICE running "N" descent
    • kWh consumed by MG1 during ICE running "D" feathered descent
    My rule of thumb is:
    1 kWh electrical ~= 3 kWh gasoline equivalent energy
    It certainly would be interesting to see fuel flow as a function of speed in an "N" descent along with ICE rpm. I'll see if I can put something together on this.

    Bob Wilson
     
  3. GWhizzer

    GWhizzer not so Senior Member

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    You may be right here. Although why not at least provide a warning so the operator could take corrective action? I mean, it gives me a highly visible warning bell and screen message when it takes me out of EV mode, and I honestly couldn't care less about that.

    It seems to me is that the best solution is to simply leave it in D and let the Prius manage it by freewheeling the ICE as necessary. Particularly if it is true that it would be in fuel cut-off under such coasting circumstances and not burning any gas - all upside and no down...
     
  4. qbee42

    qbee42 My other car is a boat

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    Certainly there could be a warning. On the other hand, the manual clearly states to never shift into N while moving. Likewise it is illegal to coast in N in nearly all jurisdictions. Toyota may have thought that was good enough. It's a very rare situation that could ever result in over-speeding MG1.

    Tom
     
  5. GWhizzer

    GWhizzer not so Senior Member

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    Ya, you're probably right. Anyway, I've already concluded that leaving it in D is the best approach. By trying to use N, I'm just placing the management burden on me and if I don't do it right I could be either overspinning MG1 or freewheeling/running the ICE unnecessarily. Besides, Ken has indicated that coasting in N may actually use more fuel than coasting in D.
     
  6. tedjohnson

    tedjohnson Member

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    Hey guys try it. I find using both approaches on the same hill many times, that coasting in N takes you further and faster than coasting in D with the regen compensated by slight throttle feathering . I don't have a scan gauge to record it but my seat of the pants experience leads me to think it might be so. I could be wrong but maybe someone in a well instrumented 2010 could verify. Thanks.
     
  7. Arthur

    Arthur Member

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    What about the NHW20 (2004-2008 models)?

    If I were to start out in EV mode (at about 30 mph), and then shift into neutral, how could I avoid any potential risk to MG1?

    I have a Hymotion system, and my "hybrid threshold" seems to be between 40 and 41 mph, if I understand that term correctly.

    Under certain circumstances, that is the speed at which the RPM gauge on my ScanGauge starts showing that the ICE is spinning (assuming that the ICE was not running before that). So, I assume that that is my effective "hybrid threshold."

    With the Hymotion system, the car behaves a little differently if can manage to start your trip without letting the ICE start. Once you get into EV mode, the ICE will stay off (as long as certain conditions continue to be met). In this "mode," I can coast down my hill in neutral and the RPM remains at zero, even if I go between 45 and 50 mph.

    However, I have no idea what MG1 is doing under these conditions, or whether passing 41 mph puts it under any kind of risk.

    If you have any ideas on this, I'd love to hear them.

    Thanks.

    Arthur
     
  8. GWhizzer

    GWhizzer not so Senior Member

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    But how much fuel was burned each way? Even if it did coast farther, Ken has indicated it would burn more in N than D above 50 mph so there might be little/no/negative gain. Unfortunately I don't have the instrumentation to test this. Maybe Bob will be able to in the near future. It might be an interesting experiment (and it also might be negligible) in the long run...
     
  9. qbee42

    qbee42 My other car is a boat

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    Only by keeping your coasting speed to a reasonable value.

    Tom
     
  10. Mike4puy

    Mike4puy Junior Member

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    I'm a new boy but why on earth do you need to coast in a Prius, aren't those MPG's I'm seeing enough without coasting. When coasting you DO NOT HAVE FULL CONTROL of the vehicle, not a good idea from a safety stand point.:(
     
  11. tedjohnson

    tedjohnson Member

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    It's free mpg, no the Prius was a come down MPG wise for me. It is the best now but back in 2000 the Insight was the best by about 10 mpg better than the Prius. After 10 years and 262,000 miles I sideways graded ( not up nor down) . What is out of control? The brakes work the same , the steering works the same.....you just have no engine braking, which you do not have on the Prius anyway, just a little regen, when you take your foot off the gas. My Saab 60's version freewheeled by pulling a lever on the dash, My Ford Aerostar van freewheeled. it is a common way to get good gas mileage.
     
  12. qbee42

    qbee42 My other car is a boat

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    Generally your comment is correct. Neutral in the Prius is a different matter, since there is no real neutral. While I don't find any need to coast in N, with the Prius it's not really a safety risk.

    Tom
     
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  13. ken1784

    ken1784 SuperMID designer

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    There are a lot of variables to do experiment it, such as speed, terrain and etc.
    I think such experiment is just wasting time to find little/no/negative gain.
    When you see over-speed in N, you have to apply brakes. Then, you are wasting energy to make heat on brakes.

    My recommendation is staying in D at highway speed.
    When you see over-speed going downhill in D, Prius regenerates electricity to apply brakes.
    If a downhill is not so steep, just lift off the accelerator in D.
    If it is slight downhill or flat, you can still drive in warp-stealth mode using no fuel.

    I believe driving in D at highway speed is the best solution for saving energy, safety and legality.

    [email protected]
     
  14. ThePriusMan.com

    ThePriusMan.com Waiting for my Prius

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    I coast in my 4Runner all the time to get better MPG, Especially on the way to the beach, where there are long hills, Why do I not have control over my car during those 15 mins of coasting?
     
  15. qbee42

    qbee42 My other car is a boat

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    You probably do have control. The laws against coasting in neutral stem from transmissions that were hard to reengage. It was difficult to get the old manual transmissions back into gear once your speed built up, and older brakes were not up to stopping a speeding car without the help of engine braking. In those days, coasting could lead to an out of control runaway.

    Tom
     
  16. donee

    donee New Member

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    Most states have anti-downhill coasting laws. Its legal to coast on level and uphill terrain.

    But, the question is, what does your state law define as coasting? Many state laws require that the gearing be switched to be in a coast. That never happens in a Prius. When yoiu put a Prius in N, all the gears are still meshed the same ways as they always are. If your state law require the gearing to be disengaged to be a coast, a Prius N coast, is not legal coasting.

    Shifting to N at highways speeds in a Prius is playing with fire, however. Keep it under 42 (2nd Gen) and 45 (3rd Gen) when shifting to N. The car will readily warp stealth anyway, on any prolonged down hill run with sufficient slope to keep the car rolling along at the same speed, without shifting to N above those speeds, and below about 64 mph.
     
  17. xs650

    xs650 Senior Member

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    A Ford Model A 1928-1931) would be hard to get back into gear with a dead engine. A 1932 or later Ford with synchromesh isn't hard to get back into gear. I believe most of the competition got synchromesh in a similar time frame.

    I suspect the laws predate synchromesh.
     
  18. tedjohnson

    tedjohnson Member

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    This has gotten really confusing. The question is simple - If I drop into N at 50 mph and then coast a lot faster downhill, dropping back into drive when I get back down to 50 , will it hurt the PSD or its components. Some have said that it would be fine, some have said it was dangerous, some argued that it would not give significant MPG savings . My experience is that it indeed will give better mpg than leaving it in D and feathering the gas and of course it is a lot easier to do. I am getting a solid 60 mpg my first 11 tanks of gas. I have a lot of hills I drive up and down and many are long and sloped just right for a 5 minute coast which really kicks up the mpg. My only question is - will this in any way harm my Prius? Thanks all. Ted
     
  19. bwilson4web

    bwilson4web BMW i3 and Model 3

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    As long as you are over 46 mph or the ICE is running, no problem. If in doubt, shift briefly into "D" and then back into "N".

    Bob Wilson
     
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  20. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    "The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone synchronizers were called "Porsche-type" for many years after this. In the early 1950s only the second-third shift was synchromesh in most cars, requiring only a single synchro and a simple linkage; drivers' manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again. With continuing sophistication of mechanical development, however, full synchromesh transmissions with three speeds, then four speeds, and then five speeds, became universal by the 1980s"

    Albins off Road Gears :: Synchromesh and Dog Engagement Gearsets... What are the differences? :: Transaxle :: Transfer Gears :: Gear Design :: Performance Gears :: Gearsets :: Dog Gears :: Performance Transaxle :: Sequential Transaxle

    So yes, many laws were written before the 1980s.
     
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