coasting in neutral questions

N is legally required in the US.

Standard No. 102; Transmission shift position sequence starter interlock, and transmission braking effect. - Federal Motor Carrier Safety Administration



Beyond that, it has little to recommend it, you can't regenerate electricity in N, so you have fewer braking options.
No charging of the HV battery happens in N, some folks car die in car washes.
N does not disengage any gears, unlike other transmissions, it just does not power the Motor/Generators.
Above 42* MPH the engine still has to run in N, on Gen 1 & 2 Prius you can damage M/G1 if you gain speed in N from under 42 MPH to over 42 MPH.
It is illegal to use N while moving in many jurisdictions.

May I ask why you think you want to use N?

* it will vary for the different Prius models, but I think all are somewhere in the low 40s.

 

True; the charge your battery is not receiving by coasting in N means that that lost energy is not there to assist with the next climb hence you will use more petrol. Also if you apply brakes you will cause excessive wear on them as they are rated to be used with braking contribution from regenerative breaking which will not happen if in N.

 

In a manual transmission, N allows the gears and engine to slow or stop as they are not still connected to the road. This can save gas. The HSD has no clutch, no different gears. It is a one speed that is always in gear with really complected software.

This is about a Gen 1, so so some details will differ for the c, but can give you a good idea about the Prius (and other Toyota and Fords) transmission.

Toyota Prius - Power Split Device

D you are really always in Drive
R is just drive with M/G2 spinning backwards
P is just drive with a pawl blocking the wheels from moving
N is just drive with no power to either motor.
B is drive with added engine braking.

 

Even if you are the safest driver ever, there is danger in damaging the engine if you coast too fast in neutral. You're better off coasting by applying slight pressure on the gas pedal until the HSI indicator is right on the first line or above it. It's easier to do in ECO mode. If you're below 43 mph, the engine will be off and should you need to slow down, just let go of the gas pedal. It's a lot safer for you, the engine, a whole lot easier to do.

 

I am a newb to most of these sorts of driving techniques, but am I correct in assuming with the c that if you simply try to keep the hsi bar above the blue regeneration region but as little into the green as possible, you are more or less getting the benefits of gliding in neutral anyway? When I have the second multimedia screen showing instantaneous mpg, it seems to go to infinite mpg during these glides (unless battery is depleted or engine is cold, in which case I just trust the car is doing what it needs to do)

 

Your assumptions are correct... note* just because it's over 100 mpgs doesn't mean it's infinite.

 

The Prius engine runs when the car is above a critical speed, 42 mph in Gen I and Gen II, 46 mph in Gen III and possibly the Prius c. If the car is above this speed when shifting into "N", the engine will continue to run and adjust its rpm to keep the gears in the transmission from over speeding. If you'll get a Prius aware scanner, you can actually watch the engine rpm increase when descending a steep hill and the speed builds up . . . part of the control laws.

Shifting into "N" can save energy in some operating regions . . . generally below the critical speed 42/46 mph. But the savings tend to weaken (not go away) at higher speeds. The increased drag from faster speeds can easily cut into any fuel savings. But at speeds under the critical 42/46 mph, it has the potential to make important savings.

The Prius c introduced a new transmission that has a second oil lubrication system. All of them have an engine driven, pump for the transmission oil. Not so much for operation as cooling and drag reduction. But the Prius c added a gear driven mechanism to raise oil up so it can remove heat from the motor stators. I read about this in the Toyota SAE paper.

The only known risk is over speeding the gears by starting at the top of a tall hill with the engine off and in "N". As the car descends and picks up speed with the engine stopped, there is a risk of over speeding MG1's rotor. In theory it might fracture from the excessive forces. However, two of us, one in a Gen I and the other in a Gen II have exceeded the 42 mph in "N" with our engines off and MG1 did not 'grenade.' Yes, it was exciting.

I typically drive both my Gen I and Gen III using cruise control and when bored, shifting into "N" when the energy flows make sense. But I don't recommend making a fetish of it.

Bob Wilson

 

I've been tempted to coast in neutral gear a few times. I'd rather not make it into a bad habit, so I just feather the pedal. It also helps me to practice keeping a steady foot on the pedal...or at least I think it does...

 

I agree:

Using cruise control and "N" can lead to:

Do you really want to be tempted by 41 hours of boredom?

Bob Wilson

 

That's what my iPod is for

 

indeed, i guess i was mixing metaphors. my car actually reports in L/100km, so it can display the equivalent of much higher than 100mpg.

my main question was if there is an additional advantage to shifting to N when the car can easily be coaxed into an a pretty fuel efficient state just by easing back on the throttle.

 

Yes, there are two approaches to explain it:

1. Math model - we have to use physics and chemistry, just high school level.
2. Practical, direct experience - you need to provide a nearby city and I'll provide driving instructions on how to replicate it.

Bob Wilson

 

Based on the numbers you posted I can't argue with results! In good conditions I've been able to get high 70s with gentle acceleration and coasting as much as possible. Does neutral just more thoroughly disengage whatever moving parts could be losing energy trough friction or something? In practice I'm content with the benefits staying in drive, but I do enjoy learning the technical aspects that might generally help me to drive smarter!

 

Like many Prius issues, if a beginner is trying to use N (or B) they are doing it wrong. There are edge cases where both N and B do help, but they are not common or for beginners.

 

I use N for three things:

1. If you shift to N while the engine is off, the engine will stay off. This is very handy to avoid having the engine turn on while you're in S1. There's nothing worse than having your engine fire up while you're in reverse.
2. At a stop light, I prefer to have no feet on the pedals. Personal preference.
3. When I want to coast with zero regen/assist, I shift to N after the ICE shuts off. You can get <10A by feathering the pedal, but N is easier and better, especially for long coasts.

 

I still remember I failed the driving test because of this. I put it in N (I missed stick shift) at stop light and no feet, she told me that's dangerous because if something happens you can't drive the car right away, it would take too much time to move my feet to push down the clutch to engage the first gear and then gas pedal. Automatic transmission is a little faster but still not a good idea.

 

There is no mechanical disengagement. If there is any energy savings, it is from removing the motor control overhead, or from how it changes your driving style.