Is coasting in neutral a problem?

There is always the possibility of someone or something crossing your path while you're coasting in neutral and suddenly don't have the time to remember to shift the car correctly back into drive at the same time when you have to mash the accelerator.

 

So that you can speed up to hit them???

(j/k)

 

It is "generally accepted" that it is unsafe to have the car moving and be in neutral (or leaving the clutch depressed longer than needed to shift in a manual car). While I can't totally explain why this would be unsafe, it is not recommended in every car owners manual or mileage improvement book I've ever read (e.g. How to get more miles per gallon)

 

I have come to the conclusion for myself that it is perfectly safe to the 2004 hybrid system to coast in neutral although I no longer do it above about 42 mph. Much of my commute is on empty back roads where driving safety is not an issue. Since control of the power split device is by wire, I see no problem to the system by shifting back and forth between D and N. I welcome constructive discussion to the contrary.

YMMV

 

Hi Ray,

The owner's manual in Towing section says...
"Towing in this manner may be done only on hard-surfaced
roads for a short distance and at low speeds."
Japanese manual states the upper limit is 30km/h(18.75MPH).

Also, please note that the transaxle Oil Pump is driven by ICE, therefore
there maybe lack of lubrication in neutral.
CORRECTION: I think I was wrong about the lack of lubrication in neutral.

I think driving in neutral is not recommended.

Regards,
Ken@Japan

 

Thanks Ken,
I always injoy reading your posts. What do you see as the difference between coasting deadband in N or deadband with careful pedal control. Are both conditions controlled by the ECU based on inputs by the driver. How is the transaxle lubricated when Ice is off. When coasting in N, the ice will still run when required by engine temp. It doesn't seem as if N is different from feathering as far as the car is concerned but I'm still far from full comprehension of this wonderful system. Thanks for your time.

 

Hi Ray,

Sorry, but I've never tried to coast in neutral because I believe it is not designed for that.

The transaxle contains 3.8 liter of oil and the minimum lubrication is done without the oil pump. I think I should correct my previous post describing the lubrication is maybe a problem.

Anyway, I was told cornering in neutral is unstable than some power on wheels many years ago. I believe you won't do such risky cornering, however I still think driving in neutral is not recommended.

Regards,
Ken@Japan

PS. It looks QUOTE TAG is not working. Any suggestions?

 

Danny, also Evan,

Thank you for your suggestions.
Hope this post is OK using the QUOTE TAG.

Ken@Japan

 

in the Prius the PSD (CVT) is essentially a standard tranny with 3 moving gears that are constantly meshed. The sequence is all stopped, 2 moving and 1 stopped or all 3 moving. If the car is moving and you put it in neutral all it does is take the computer out of the circuit. As stated it's not a good idea to do that as it's possible to overspeed the main drive motor. This is a good url of a simulation of the Prius CVT
http://homepage.mac.com/inachan/prius/planet_e.html

 

The way I understand it, the motor would not be overspun. The generator is the part that is more vulnerable to that. Why do you think that the computer would no longer function as a part of the system while in neutral? I believe the computer is what is functioning to achieve neutral. Other than the towing warning,I see no warning from Toyota against coasting in neutral. It is likely that that warning refers to a condition in which the ECU is off. In that case, I believe the danger of towing above 42 or 71 depending on who you ask, would be inadvisable. I also think that there are factors that we are not yet considering. I wish the Toyota engineers could weigh in on this question, as it seems that currently, there are a lot of highly intelligent people who are just guessing, as am I. As far as that goes, the EV mod that some and perhaps I are doing, is as great a risk. Even though they have an EV button in Japan, doesn't mean that the system shipped to the US is programmed to safely allow it to be modded here. It probably is, but a risk nonetheless, until the Toyota engineer says,"Oh yeah, no problem" I have always suspected that Toyota left that button off the US model so that we couldn't use it to keep the engine off when it needs to run to keep emissions low. Since this could cause higher emissions under some circumstances, The button might have been left off in order to always meet california emissions standards. If you respond to the EV issue, let's split the thread, as I'd like to keep this one on the topic of neutral. Above, I mentioned 42 or 71. The generator was upgraded with higher production standards to be able to spin safely at 10,000 rpm which calculates under the old system to allow 71 mph but I recall that Toyota said that 42 is still the number. That would indicate that there are other factors that I am not aware of. Where is an engineer when you need one? Thank you all for your patience with this discussion and your input. I know that alot of minds are made up about this, but I would love to see the discussion continue.
regards,

 

with regenerative braking and charging, why would anyone want to coast?

after all, unless you coasting down a hill that isnt steep enough to maintain highway speed while in gear, is that your reason?

 

OK I'm going to throw some numbers at this. These are just representative guesses and were not arrived at scientifically. If I'm driving on a flat road with no wind at 40 mph and put the car in neutral I can coast for half a mile and still be going 33 mph. If I start at 40 on the same road and just take my foot off the pedal, in a half mile I'm down to 15 mph due to regeneration. If I take the energy I regenerated and use it to increase the speed of my vehicle I will only get back up to 20 mph before it is exhausted. The difference in my final speed represents the inherent losses in the conversion process. The coast can be achieved with "no arrow" pedal feathering but is harder to achieve and takes more focus. I realize my example has a hole in it but for this discussion it was close enough. The losses are substantial and the Prius will coast a very long distance on flat roads or gradual downhills. Happy Coasting
Ray

 

some where I read that the reason they recommend you don't coast in neutral is that the generator output is uncontrolled by the computer/inverter and there is the possibility of over maximum voltage going to the main drive motor as you return the gear selector to D. I can't prove it but I still wouldn't recommend doing it.

 

for anyonw who would like to void their warantee for savings amounting to fractions of a penny... i say be my guest...

 

In case anyone is wondering about this topic and how it is playing out, I thought I would update it at this time. My 2004 Prius now has 20,000 miles on the odometer. I have been using the coasting in neutral method for the life of the vehicle. I use neutral to coast at speeds up to 52 mph on back roads with no traffic, which makes up a large part of my commute. I have found no discernable problem caused by using this method and have never found any reason not to employ this method except for well intentioned fellow Prius owners who don't think it sounds safe. The max speed that I would employ this method would be less than 40 mph in the earlier version of the Prius, due to its lower max RPM of it's generator. There are no guarantees in life but I feel pretty safe with this one. I will come back here and update if a problem develops or I put another 20k on without incident.