What is the purpose of N, neutral?

<div class='quotetop'>QUOTE(RolfS @ Sep 12 2006, 02:56 AM) [snapback]318200[/snapback]</div>

If the above were true I would think this is a VERY serious design flaw. One would think the Prius would be "smart" enough to detect such a situation and avoid an overspeed of the MG's. The Prius is smart enough to know when Park or Reverse is allowable but not for neutral?

An earlier poster indicated he has a long down hill run to work where the car does not decelerate when in neutral but does when left in Drive. I suspect thousands of other Prius drivers have (or encountered) the same situation and utilize neutral for that purpose (I always did with manual shift cars, either clutch in or neutral position).

Is warning against never to use neutral at highway speeds in the manual? If this problem exists then at the very least a warning on the MFD should come on when neutral is selected at a high speed and a warning should be prominently placed in the manual.

Could one of the Prius techs verify this? Now I wonder how many MG1's have been irreparably damaged (have any been ruined??) by something as innocuous as shifting into neutral on a downhill run??

Rick
#4 2006

 

There's no warning about neutral at high speed in the manual, and I've never heard of any problems. It's one of those folk wisdom things that people pick up on the Interweb.

There is a warning against towing significant distances in neutral, but that could be mainly to prevent doing further damage to the transaxle of an already-disabled vehicle. Oh, and on safety grounds - the MGs will generate electricity as you tow, so if they're not isolated (eg if the HV cables leading to the transaxle are damaged), this could lead to a fire. But if the car is undamaged, this is not an issue.

There are a variety of known rev limits quoted for MG1 and MG2 (6500/10000 for MG1, 6000 for MG2), which can be used to infer various "speed limits", but those are the rev limits observed in normal powered operation. None of us know what the actual mechanical limits are when the electrical system is disabled, as in Neutral.

People saying you shouldn't switch to N at high speed are on even shakier ground than those saying you shouldn't inflate your tyres to more than 35/33. At least the latter can find justification in the manual.

 

<div class='quotetop'>QUOTE(KMO @ Sep 12 2006, 09:25 AM) [snapback]318272[/snapback]</div>

The fact that the limits are not widely known does not mean that they do not exist. Mechanically, they must exist; therefore, shift into N above 42 MPH at your peril.

 

<div class='quotetop'>QUOTE(IsrAmeriPrius @ Sep 12 2006, 12:13 AM) [snapback]318139[/snapback]</div>

Yes...I tried this a couple of times just to see if it were possible...my car wouldn't go from D to N .....

terri

I agree it's not to use for coasting down hills....

 

Then why no warning, ever, from Toyota? The actual safe mechanical limit of an isolated electrical motor generator is likely to be significantly higher than what it can usefully achieve when controlled.

Anyway, if you want to state numbers, that 42MPH number is a bogus one dating back to the old Prius, corresponding to 6500rpm on MG1. But MG1 on the current Prius can reach 10000rpm in normal operation, so why not 65MPH?

My current deduction is that Toyota do not see switching into Neutral as a problem mechanically. Hence the lack of manual warnings, or any sort of interlocks in the car. Do you really think it wouldn't have occured to them to protect the car, if it needed protecting? The topic comes up about once a month on a public Prius owners' site; the design engineers must have considered it at least once since 1995

Agonising about N above 42 makes as much sense as turning the AC off and sweating when the bars go pink. Er, guess what, the engineers thought of that too.

 

<div class='quotetop'>QUOTE(richard schumacher @ Sep 13 2006, 12:00 AM) [snapback]318292[/snapback]</div>

No need to worry about it on such condition. Shifting into N above 42 mph causes the ICE idling with fuel.
The dangerous condition is when you are on a downhill, you're in stealth mode (ICE off) and you're exceeding the 42 mph barrier, the ICE won't spin. :angry:

Ken@Japan

 

Previously I wrote:

"Your argument is sensible and attractive except that it presumes that Toyota engineers and document writers were perfect and that they perfectly anticipated every contingency, whereas we know that they are not and did not.

It would be good for someone who knows to comment. Until then I will continue to foolishly reject the many benefits and pleasures which accrue from shifting to N while at speed :_>"


I take it all back! Per the manual, the *default* behaviour is to shift to N whenever a variety of stupid shift maneuvers are attempted while moving, so surely it must be non-damaging to do so deliberately. Toyota may not have anticipated everything but apparently they anticipated this.

 

Don't know what you're missing. The thrill of the breeze, the spinning of the isolated synchronous AC motor-generators, the frisson of excitement as the ICE remains dangerously motionless, coasting helter-skelter down the mountain. :blink:

(Nips off for a sensible and attractive cold shower)

 

<div class='quotetop'>QUOTE(kingofgix @ Sep 11 2006, 01:47 PM) [snapback]317854[/snapback]</div>

That's why people learn to glide, not coast (assuming it's 40 mph or less). When in neutral you can't recharge the batteries, including by regenerative braking. If you're going above 41, a glide is harder but not impossible. And a little pressure on the <strike>gas pedal</strike> accelerator would still reduce your speed reduction, even if you don't get to the no-arrows screen.

I used to use neutral quite a bit in an ICE-only car (stick shift). I would only do it when I could read the road ahead and nobody was tailgating me. If something should happen (a deer jump out, say) I would brake, not accelerate, so it didn't seem too dangerous to me. But I was smart enough to use it carefully, state laws can't make that assumption. And it looks like you can use N to coast up to a stoplight in California, just not downhill.

The only case I could come up with where acceleration would be needed, is if you needed to switch lanes to avoid debris or something, but there was somebody coming up fast in the other lane, so a strong acceleration would get both of you past the debris in time, without them skidding to avoid rear-ending you.

Fortunately the Prius is a better vehicle and using N is no longer a benefit to saving gas while driving. That should be reserved for moving the Prius by anything other than its own power, IMO.

 

Cranking along at 85mph, shift to N... what happens?

Case under the hood explodes?

Doesn't shift since the car knows this is bad news?

Or, the system shifts to N and everything is fine...

Do we have any volunteers?

 

<div class='quotetop'>QUOTE(KMO @ Sep 12 2006, 04:13 AM) [snapback]318214[/snapback]</div>

Good question. As I said, it was my understanding, so where I heard that may have been incorrect. However, the engine can run while in neutral, no? If the ICE is running when you select "N," does the ICE shut down? And if you were driving at high highway speed, the ICE would be running, even if you don't see it on the energy monitor, correct?

I have never paid attention to whether or not the ICE runs in neutral as I could probably count on one hand the number of times I have used that mode for anything.

 

<div class='quotetop'>QUOTE(Curtis SAC @ Sep 12 2006, 09:21 PM) [snapback]318544[/snapback]</div>

The engine can continue running if it's already running - it'll just idle. But to actually start it, just like any engine, it needs a push from outside. The Prius doesn't have a starter motor for this job, or indeed a hand-crank. It instead spins the engine up with a push from MG1 (and probably a balancing push from MG2). In neutral, this isn't available.

And yes, if you were going much above 40 when you entered N, the engine would be already running, so no real problem. You won't see it on the energy monitor. I seem to recall hearing that the engine will continue to run even if you drop below 40 in N; possibly because the car normally uses MG1 and MG2 to bring it to a controlled halt.

 

<div class='quotetop'>QUOTE(Curtis SAC @ Sep 12 2006, 02:11 AM) [snapback]318201[/snapback]</div>

What I see in my Camry, which may not be identical to the Prius, is that it does not have some of the bad characteristics described above.

First of all, going to N does not cause the engine to either start or stop. If it was running, it will stay running, same as a normal car. If it was stopped, it will stay stopped, same as a normal car. You do not lose your brakes when in N, same as a normal car. So at less than 42 MPH, you can coast with the engine on or off in N, depending on what it was doing when you went to N.

The computer is still working and monitoring things. If your vehicle speed gets too high (about 43 or maybe 44 in my car), the engine will start while in N if it was off, in order to protect things from overspeeding.

While I have not tried it, I suspect that if you sit stopped in N for a considerable period of time with a load on the system, such as air conditioning, I think the engine would start and run as needed to maintain battery charge. This would be a normal thing to do in bad stop and go traffic situations.

You don't want to coast in a 'normal' car with an auto transmission because the oil cooling stops working when you are not in a gear. The Saturn is a noticable exception, it also has a rear drive line oil pump, so can be towed wth all four wheels on the ground (unless they have recently changed it).

The issue with regard to the legality is interesting, and I think depends on how the law for any given state is written and interpreted. Technically, you cannot take a Prius or a Camry Hybrid out of gear. Going to neutral is a bit like stepping on the clutch in a manual transmission. You remove the drive power of the ICE, but it is available at a moments notice; you don't have to shift any physical transmission gears to regain control.

I think that neutral is much like gliding, and I don't see how it can be harmful. You cannot stop any of the gears from spinning at the speed they want, and there are none that are taken out of service that would cause a loss of oiling to my knowledge.

 

<div class='quotetop'>QUOTE(Curtis SAC @ Sep 12 2006, 04:11 PM) [snapback]318201[/snapback]</div>

In IG-ON mode, the HV battery is disconnected.
There is no way to supply power to MG1 to spin the ICE.

Ken@Japan

<div class='quotetop'>QUOTE(ChilliPepper @ Sep 13 2006, 04:15 PM) [snapback]318823[/snapback]</div>

My Prius experience is I was coasting in N at 40 mph on downhil, exceeded 42 mph, nothing happened, ICE was still 0 rpm then I shifted in D at 47 mph and the ICE started.

Ken@Japan

 

<div class='quotetop'>QUOTE(daronspicher @ Sep 13 2006, 03:21 AM) [snapback]318472[/snapback]</div>

My Prius experience is I was at 50 mph coasting on a slight downhill with ICE spinning without any fuel and no arrow to/from battery, then I shifted to N.
The ICE was still running, but fuel was consumed for idling.

Ken@Japan

 

<div class='quotetop'>QUOTE(kingofgix @ Sep 11 2006, 11:47 AM) [snapback]317854[/snapback]</div>

I have a similar hill on my commute. However, I leave the car in drive and use cruise control to set an appropriate speed. The car tends to cruise on battery power when needed, gas as it sees fit, and if I have to brake, or reduce my speed, I know that my HV battery will get charged via the regenerative braking system. If I were to be in neutral, I'd lose all of those benefits.

 

<div class='quotetop'>QUOTE(viking31 @ Sep 12 2006, 07:14 AM) [snapback]318269[/snapback]</div>

Several things:

I think it's a given to most people that you shouldn't travel at highway speeds in neutral for extended periods of time. I seem to recall being taught that as part of driver's education, but that was some 25 years ago, so my memory may be a wee bit fuzzy on that point.

I'm thinking that the rev-limit on MG1 and MG2 is not a mechanical safety issue, but an electrical one: excessive current being produced/consumed when they are spun up beyond their limits. While in neutral, in theory, no current is flowing to/from MG1 or MG2. They are just spinning freely. But as soon as you shift back into drive, you may temoprarily get an over-current situation while the computer adjusts the speed/direction of MG1 so that it is not being over-revved. If this happens often enough, I can easily see the windings of MG1 becoming damaged.

Now, as to whether this is a serious design flaw remains to be seen. It depends upon if the engineer thought that no one in their right mind would ever drive down the road at highway speeds on a regular basis in neutral, thus they don't have to worry about MG1 being over-rev'd and having a potential over-current situation on a regular basis Or perhaps the engineer designed MG1 with a safety margin such that the above situation wouldn't cause permanent harm if it only happened ocassionally.

There's only so much an engineer can do to design around the stupidity of the average driver. First of all, they can't anticipate every boneheaded move a driver might make. And some boneheaded moves are so unlikely to come up that it's not worth the time, energy or money to design around a problem that has the ever so slight chance of happening. And even then, you may not be able to design a solution. Case in point: Someone had taken their manual transmission car out on the highway. They were going at a fairly fast clip when they had to downshift from 5th to 4th. Only they didn't end up in 4th, they ended up in 2nd. Boom, engine is instantly way over-rev'd, pistons are suddenly traveling at much higher velocity than designed for and end up exiting the engine case at high speed. A rev-limiter wouldn't help since it wasn't a matter of too much gas being applied. Instead it was a case of the wheels of the car itself, via the transmission, that was driving the engine past it's limit. What can an engineer due to design around this situation? Pretty much pray that someone won't attempt such a boneheaded move.

 

I did a little research on this neutral issue and found some accurate information in what happens when one shifts into neutral. As some of you suspected the system takes care of itself so you won’t get it into trouble. However it also means that “N†does not do exactly what you some of you think it should. The following has been extracted from Toyota’s new car features document for the 2004 Prius. So my original post on the subject was not accurate. I hope it is not a problem quoting this small section:

Clutch-Less System

A clutch-less system has been adopted to mechanically link the front wheels and MG2 via gears and a chain. To disengage the motive force in the neutral position, the shift position sensor outputs an N position signal to turn OFF all the power transistors in the inverter (which connects MG1 and MG2). As a result, the operation of MG1 and MG2 shuts down, thus rendering the motive force at the wheels to zero. In this state, even if MG1 is rotated by the engine or MG2 is rotated by the drive wheels, no generation of electricity occurs because both MG1 and MG2 are inactive. As a result, the SOC (state of charge) of the HV battery decreases as the shift position remains in the “N†position.


Shut Down Control

Generally, MG1 and MG2 are shut down when the shift position is in the “N†position. This is because MG1and MG2 must be stopped electrically as a means of shutting down the motive force, since MG2 is mechanically joined to the front wheels. However, the shut down function is canceled under the following exceptions:

 During driving, if the brake pedal is depressed and a wheel lock up, the ABS with EBD is activated. After rotation of the wheel. Even if the shift position is in the “N†position at this time, the shut down function is canceled to allow the wheel to rotate. After the wheel rotation has been restarted, the system resumes its shut down function.

 When the vehicle is driven in the “D†or “B†position and the brake pedal is depressed, the regenerative brake operates. At this time, as the driver moves the shift position to the “N†position, the brake hydraulic pressure increases while the request torque of the regenerative brake decreases gradually so as not to create a sluggish brake feel. After this, the system effects the shut down function.

 When MG1 and MG2 operate at higher speed than the specified level, the shut down function is canceled.

 

Nice one Rolf. That was in NCF, was it? Don't remember seeing it.

Not sure I understand what this is trying to say. Why would the MGs need to do anything after the ABS kicks in? Is it saying it wants a bit of an MG2 push to start the wheel turning again as rapidly as possible after a lock-up? I suppose ABS systems might be engineered to expect the wheels to be powered, which raises a question about what a normal car does if the ABS kicks in in neutral.

Cute - makes sense to attempt a smooth transition from regen to real braking.

And this is the clincher. So the system does have safeguards, and will effectively come out of its unpowered state if necessary to protect MG1 and MG2. But what are the "specified" speeds here, I wonder... Someone with a CANview should investigate this. It'll be tough to get MG2 overspeed in neutral, but accelerating downhill in neutral starting at 40mph would tell you about MG1. Personally, I wouldn't expect anything to happen until over 60mph (~10000rpm).

 

Burning fuel and getting no benefit from it. Why are we shifting into N? :_>