High speed in N (and ICE @ 0 RPM)

Is Neutral mode really neutral ?

Hello from a French Prius driver.

I also use Neutral and use a modified version of PriusCanMonitor (Thanks to all programmers who made it and modified it) and I was sometimes surprised reading battery-amperage in this mode.

This is an example 100% in neutral : duration 1min18sec, max speed 65 km/h, amperage from +4 Amp to -7 Amp. Ice off.



Over the green zone the amp change from + to - very quickly.

Then the question is: is really Neutral mode neutral for Igbt ?

:wave:

 

Hi Dave,

I think one reason for Toyota's advice about not allowing the front wheels to contact the ground while the car is towed is that in the event of an accident (which would be one reason why the car is towed) who knows what damage is present and what state and gear the car is really in (i.e., is the car IG-ON or READY? Is the car in P, R, N, or D?)

If you're not sure what gear and state the car is in, then you can't be sure whether high voltage is being generated by MG2 while the wheels are rotating. Toyota usually provides ultra-conservative advice for liability reasons...

You may recall seeing Bob Wilson's story about how he towed his 2001 (with all four wheels on the ground) over a long distance successfully. This is relatively safe with Classic since the gearshift mechanism is mechanical, so once the shift lever is set to N it will stay there.

However this is unsafe with 2G (or 3G) with its electronic shift mechanism. Imagine a 2G owner who has the bright idea to copy Bob. He sets the car in N and starts off, merrily towing the car down the road at 55 mph. Meanwhile, the traction battery is at two bars and is not being charged by the gasoline engine. The 12V battery is also weak. After some time passes, the 12V bus voltage sags. The car switches to IG-OFF. Will the transaxle move automatically from N to P? Usually that will happen when the car is stopped, who knows for sure whether this will happen when the car is moving, if 12V power is weak. Now the parking pawl engages and causes the transaxle to blow up...

 

oh, ok. so if towing, make sure that Pri is on and plugged into 12 Volt power?? (just asking. my cars work fine and i have no plans to tow for any reason and if i did i would request flatbed service anyway)

now, i have coasted in neutral SEVERAL times thinking it had no adverse issues in anyway, but have to admit i have never exceeded probably 45 mph tops

 

I towed my 05 prius maybe 50 ft? when it was crashed by turning on the ignition and shifting into neutral, immediately disconnecting the 12 volt battery, it was stuck in neutral after that.

The brakes worked, just not as effective.

 

As for knowing whether high voltages are being generated in the MGs, recall that the Prius uses hybrid permanent magnet MGs. Because the MGs have permanent magnets, any time an MG spins it generates a voltage. There is nothing the control system can do to change this, so if you spin an MG at high speed, you will generate a high voltage.

Tom

 

Yes Patrick, I was being sloppy. It was along time ago that I read that, and I did not have the exact info at hand (it was on my other old laptop and i was too lazy to look it up again). I forgot that it was just under ABS system being activated to prevent wheel lockup.

I don't know what everyone is reading in their instrumentation, I was just going by Toyota specifications. Being a former software developer (just retired) I sort of doubt that it would not occur to their developers that that could be a problem. If we could figure it out that over-revving MG1 while in neutral could be a problem, I'm sure their developers could also. Since it is just a software state, not mechanical, it would sure make sense for them to just change the state of the software to take it out of neutral (internally) when needed to protect the mechanical components.

Rolf

 

I have often driven down our local slope ( about 8000 feet ) the Sierra Nevada,, from Reno to Sacto. I constantly change from D to B to N, back to D and then Brake, depending on if I want the car to speedup, slowdown or just glide moderately. Often I'll transfer the car to N when the HV is totally charged ( to the 80% point) But fully green. That is so the engine does not run in trying to dissipate a unneeded charge. I can get total green charges on this run 4 times and approach 70mpg. I try to keep the max speed down, but do reach 90MPH from time to time. Since other drivers are doing the same thing, we pose no danger to each other. IF however I find myself sticking out as the fastest car I'll throw the gear shift to its D then B mode. I have been doing this for about 5 years now with no apparent harm to anything. Car runs as great as ever. And no tickets. To allay anyones thoughts as to my saneness I normall drive 55-60 MPH. It would be nice to know some definitive answers on MG1, MG2, computer etc.:cheer2:

 

It's a safety matter. The driver, who ought to have the best grasp of the situation, is on the scene, and the designers are not. They have to assume that the driver commanded "N" for some reason, and therefore it might be dangerous to undo that even if it appears that the car is about to be damaged.

 

I wonder if the car records the fact that it was opearated above a certain speed in neutral. If MG1 could be damaged or destroyed by engaging neutral above a certain speed it would make sense from Toyota's point of view.

 

To review:
1. Whenever the front wheels turn MG1 also turns if the ICE is off.
2. Whenever MG1 turns it produces voltage. If "disconnected" by the IGBTs there is no current generated and therefore no power produced so it's classed as "off". But also, because there is no load, the voltage can be rather high. Which is why the Toyota engineers are worried about towing with the front wheels on the ground. The voltage -might- exceed the ratings of the IGBTs or their ability to resist it. In the latter case destruction is immediate and final but not usually entertaining.
3. ALL engineering designs have a "safety margin" built in. Exceeding the design published limits and relying on the "safety margin" routinely is ignoring the "Laws of Murphy", and will eventually result in costs. For the Italians here, this is not a cost from government litigation, it's a physical reality cost. Such as destruction of mechanical bits. Lots of expensive mechanical bits.

If you enjoy gambling, continue as before. It's even more exciting as the odds are NOT in your favour.

 

Planetaire -- very good question, and you're in luck because
a discussion came up about this over on CleanMPG a little while
back. About midway through this thread page, someone else observes
similar current blips, and I dug back into my notes and did a
little more observation and offered what I think is the reason.
Basically the motor driver IGBTs are shut down per the GSDN
and MSDN leads but the boost converter is not, and keeps the
motor-side rail voltage "out of the way" of the PM-generated
envelope as needed.
.
_H*

 

IIRC, ORNL did a motor study on the HSD, and the peak voltage was somewhere slightly under 1kV at no load. This is not too high, perfectly reasonable for a robust IGBT. Although dumping that back into the 500V bus might be entertaining.. (12181.pdf,fig3.6)

Hobbit, thanks for providing a quick way to access this document!

The behavior of MG1 is detailed in Table 3.5, it looks like they screwed up that RPM figure though, instead of 1krpm, they say 100rpm, but the graph says 100rpm... AARGH.
Assuming that the line continues straight, it is possible to assume MG1 generates 50ish volts for every 1000RPM, so, 10krpm=10x50=500V, not THAT high!

 

Yeah, but MG2 rises a lot higher ... look at the hack chart
in my "VH" page, that theoretically gets well over 500V at
highway speeds if you believe the ORNL chart...
.
_H*

 

Thanks, hobbit.

Then I join a 4 second zoom always in neutral. It is the special moment where amperage change suddenly from +4.5 to -9A. This is done in 0,05 sec.

(On this graph each point is separated by 0,01sec)

Acceleration
During this 0,05sec MG2 Rpm goes from 1885 to 1900 rpm. The cars's speed increase too, and the driver feels this small acceleration. You can see that it's a real acceleration.

Regen
I draw a red arrow where my Prius is using regen-braking, while I don't tell her to do such a job.


Voltage
MG2 Rpm is 1885 when amp fall suddenly.
If I look on your site (*) MG2 Peak voltage is about 280v.
MG1 rpm is 1885*2,6=4900. His peak voltage is about 235v.

This is not a very high voltage. My pack voltage was 220V (and goes down to 217 when amperage was -9A)

-Is the reason the actual voltage too high or
-Is it to avoid high voltage at a higher speed ? Or
-Is it simply to stop regen-braking, what the driver doesn't want ?
-Or ??

This was 4 sec of my Prius life.

(*) Sorry I may not insert Url. I follow you link on Cleanmpg, then a other that point on Priuschat to "A discussion of gliding and a comparison of methods: pedal control vs. neutral", from Jimbok. Look at the "bd beach approach and crossing" graph !