Coasting in neutral

[yuanmeinert]

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

[richard schumacher]

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.

[auricchio]

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.

[Doc Willie]

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.

[patsparks]

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.

[a priori]

<div class='quotetop'>QUOTE(patsparks @ Oct 15 2007, 06:51 PM) [snapback]526081[/snapback]</div>

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.
[/b]

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.

[bestmapman]

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.

[a priori]

<div class='quotetop'>QUOTE(bestmapman @ Oct 15 2007, 08:07 PM) [snapback]526099[/snapback]</div>

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.

. . .
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.
[/b]

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.

[bestmapman]

<div class='quotetop'>QUOTE(a priori @ Oct 15 2007, 09:17 PM) [snapback]526107[/snapback]</div>

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?
[/b]

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.

[a priori]

<div class='quotetop'>QUOTE(bestmapman @ Oct 15 2007, 09:11 PM) [snapback]526135[/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.

Hope this helps.
[/b]

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.