Does it make sense to shift to neutral during driving?

[dominicsavio]

Folks,
I'm wondering on a technicality (to increase fuel efficiency) and need some input:

When driving downhill (or even coasting a modest speeds for that matter), does it make sense to shift to Neutral to take FULL advantage of gravity (or momentum)?

When the car is in Drive, it is still partially engaged and will not run completely free under the effect of gravity (or momentum). However, when you shift to Neutral (while going downhill or even coasting towards a stop), you can go further quicker because of the 'free flow'.

I've tried this with conventional manual shifts and it works just fine.

I'm guessing that for the Prius this needs to be weighed against losing regenrative braking or battery recharching (becase shifting to Neutral disengages the motor/battery)?

Thanks!

[efusco]

1) It's probably illegal to do so.
2) It's potentially unsafe to do so under some conditions (see #1)
3) Yes, it makes sense if you're in a glide below 41mph. It's the lowest energy use/waste state you can be in ASSUMING the ICE is not spinning when you go into neutral.
4) Going into neutral when you are already going over 41mph does not make sense b/c it will force gas burn to keep the ICE spinning.
5) Most of us recommend learning how to glide and hyperstealth glide instead of using neutral.
6) If you do go into an ICE-off neutral below 41mph then coast down hill to a speed above about 62mph you may exceed the 10,000 rpm rating of MG1 and could damage it resulting in costly repairs.

[dominicsavio]

Thanks efusco. In terms of gliding:
1) Whats the most efficient way to start from a stop/slow roll? I've seen your document recommend depressing the gas pedal 1/3 way to "rev" up speed but is this most efficient (vs. slowing to a speed by initially using solely the battery?)
2) When driving at a constant speed of 40-50 mph (which is probably the zone of most efficiency), is it better to do so under cruise control? I've done this a couple of times and my mpg monitor does not cross the 40 or 50 mark
3) what is hyperstealth gliding? Is it working the gas pedal so your ICE is off and your battery is driving the car?

Thanks!

[qbee42]

Remember that N in the Prius does not disconnect anything, unlike a normal car. All of the pieces and parts are always engaged, and there isn't any way around it. What N does do is keep the control system from doing regeneration, powering the vehicle, or changing the state of the ICE. It could help, but it's better to feather the accelerator pedal.

Tom

[xsmatt81]

no.

it's pointless in the prius. Besides you don't get regen gains in N

just keep it in D

on regular cars with an auto it can help FE, but it's illegal in most states

if you want to shift gears, get a car with a stick

[JimboK]

Originally Posted by dominicsavio

Thanks efusco. In terms of gliding:
1) Whats the most efficient way to start from a stop/slow roll? I've seen your document recommend depressing the gas pedal 1/3 way to "rev" up speed but is this most efficient (vs. slowing to a speed by initially using solely the battery?)
2) When driving at a constant speed of 40-50 mph (which is probably the zone of most efficiency), is it better to do so under cruise control? I've done this a couple of times and my mpg monitor does not cross the 40 or 50 mark
3) what is hyperstealth gliding? Is it working the gas pedal so your ICE is off and your battery is driving the car?

Thanks!

1) Evan might have a different answer, but I "goose" the go-pedal just enough to get the ICE to light, then back off enough to allow modest acceleration. I want to minimize battery use as much as possible. By "modest" acceleration, I suggest keeping the instantaneous MPG at least half the vehicle speed. This keeps the ICE within efficient operating ranges.

2) Actually, at steady speeds the car operates more efficiently at speeds below 41 MPH (most efficiently at 20, in fact) where the ICE will shut down completely when not needed for propulsion. But assuming driving conditions mandate 41+, I avoid CC except on level terrain. Because of its goal of holding a precise speed, it pushes the ICE into inefficiently high ranges on uphills. See this for more.

3) Hyperstealth is another name for "warp stealth," which is reviewed here in great detail.

Originally Posted by xsmatt81

no.

it's pointless in the prius. Besides you don't get regen gains in N

just keep it in D

on regular cars with an auto it can help FE, but it's illegal in most states

if you want to shift gears, get a car with a stick

I disagree. First, in states where it is illegal, it generally is illegal only on downgrades. No state outlaws it on level terrain or upgrades that I know of.

Second, as Evan suggests, it is an alternative technique for gliding and, IMHO, a good one. (For the OP and others unfamiliar with gliding, do a good search on "pulse and glide.") True, there is no regeneration, but gliding is not done to regenerate. In fact, it deliberately avoids regeneration and enables better fuel economy by maximizing use of available kinetic energy. Using neutral is easier, draws less off the battery, and provides at least the same fuel economy as (and possibly better than) feathering the pedal. See this (rather long and somewhat technical) link for more.

To Dominicsavio, I wouldn't normally bombard a newbie with links so detailed, but you seem more interested in driving technique than many. Take your time, study these and the many good threads on P&G, practice, and have patience. I still am fine-tuning my technique after three years with the car. The results pay off: MPG in the low to mid-60s as of now, and often 80+ (with occasional trips above 90) in warmer weather.

[Rokeby]

This is a tricky question as no one answer is applicable in all cases.
Evan gives a good summary of the many little considerations:

Originally Posted by efusco

1) It's probably illegal to do so.
2) It's potentially unsafe to do so under some conditions (see #1)
3) Yes, it makes sense if you're in a glide below 41mph. It's the lowest
energy use/waste state you can be in ASSUMING the ICE is not spinning
when you go into neutral.
4) Going into neutral when you are already going over 41mph does not
make sense b/c it will force gas burn to keep the ICE spinning.
5) Most of us recommend learning how to glide and hyperstealth glide
instead of using neutral.
6) If you do go into an ICE-off neutral below 41mph then coast down hill to a
speed above about 62mph you may exceed the 10,000 rpm rating of MG1
and could damage it resulting in costly repairs.

and other folks have added more tangential considerations.

[Edit: And JimboK's post has some real meat in it to be slowly digested.]

In my experience, in general the answer is no, the energy savings are very
small.

That said, in some very specific case it might be worth considering. And that
case is probably imbedded in a larger situation that you've been able to
analyze over many repetitions.

Let's look more closely at Evan's point #3 above:

"3) Yes, it makes sense if you're in a glide below 41mph. It's the lowest
energy use/waste state you can be in ASSUMING the ICE is not spinning
when you go into neutral."

Gliding in neutral uses about 1/3 the energy from the HV battery that a "no
arrows glide" does. So if you're in a situation where maintaining high SOC is
really important, and the distance to be covered is longish, an ICE-OFF N
glide could be helpful.

I've got just this kind of situation in my daily, 28.8 mi round trip commute.
I have learned that how I handle a 1/2 mi. section at the end of the in-bound
morning run will make a 5 MPG difference at the end of the day. And getting
that 5 MPG involves a long ICE-OFF N glide at 25-30 MPH to keep the SOC as
high as possible. (I have a ScanGauge, EV switch, and grill blocking.)

You see, if when I leave the parking garage heading home I have a high 6
bar, or better yet 7 bar SOC, I can EV through a 1/4 mile run of stop signs,
traffic lights, and low speed right angle turns and have the SOC at 4 bars.

[Edit: And air temps are above 32 deg F, as EV mode is unavailable to me
at lower temps. Yes, there are many things that can nix this whole approach.]

I know that once the ICE kicks on anywhere in the run, it will continue to run
when stopped at least until the colant temps are above 130 deg F.

I will admit that in the morning it is a very delicate dance indeed, sometimes
letting the ICE run when I could "no arrows glide." But as best as I can
arrange it, that ICE-ON time keeps the temps up, saves or slightly add to
SOC, keeps me moving with traffic, and reduces MPGs by only ~0.2.

But this intentional ICE use plus the SOC saving ICE-OFF N glide sets me up
for the 5.0 MPG gain later on.

In my daily driving, this is the only time where I've found an ICE-OFF glide is
helpful.

You'll have to look carefully at your daily driving conditions to discover if
there is some different situation that might work for you.

Hope this helps.

[qbee42]

These are good answers, and no one answer to this question will work for every driver. The different answers basically come down to three different viewpoints:

1) Legal - As pointed out, there are some legal issues related to N, almost all of which are carryovers from the days of manual transmissions, where a driver could lose control of a vehicle not in gear.

2) Efficiency - Pulse and glide is an efficient way to move a Prius for hypermiling. With N you can P&G with less fuss.

3) Need - There isn't any need to pulse and glide, or shift to N, or use any other hypermiling technique for ordinary driving. If you are just driving your Prius for simple transportation, then you don't need to worry about fancy techniques. Just drive it and it will do pretty well on mileage.

Tom

[EZW1]

Why would you shift into neutral and risk all kinds of problems when if you back off the pedal to the sweet spot you'll find the power mode go into its own 'neutral' evidenced on the MFD as no power from the ICE nor the battery (everything grey).

[JimboK]

Originally Posted by EZW1

Why would you shift into neutral and risk all kinds of problems when if you back off the pedal to the sweet spot you'll find the power mode go into its own 'neutral' evidenced on the MFD as no power from the ICE nor the battery (everything grey).

I present my "why" in the link in my previous post. As it shows, there actually is more battery power consumed during a pedal-feathered glide.

What are these "all kinds of problems?" There are only two easily-managed risks that have been raised: the illegality of it on downgrades and the (theoretical) risk of over-revving MG1 that Evan mentions. The latter would occur in conditions that most drivers will never see, even if they routinely use neutral. It has to be invoked deliberately and then (and only then) it will happen in very limited circumstances.