Hypermilers: Don't use Neutral to coast to a stop

[TickTock]

Warning: may be tedious for folks who don't obsess about putting every eV to the best possible use, feel free to move along[edit: added smiley] - this is a topic for the efficiency-focused crowd .

The reasoning why I say Neutral is not the most efficient mode to coast in is while the car is moving, the inverter draws 1-2kW of power even when in Neutral. The only way to get to zero energy in or out of the battery is to keep it in D or ECO and modulate the throttle. The inverter power can be treated as another rolling resistance. By staying in D and manually maintaining the battery draw at zero as many do, you are using kinetic energy to directly power the inverter as you slow rather than drawing it from the battery. Now, assuming constant average speed for the drive (i.e. constant time to get there) this ends up not mattering if you never come to a complete stop since the total energy for this component will be the same (~1.5kW times travel time). However, if this actually brings you to a stop sooner at any of the lights you encounter, you will consume that much less power from the battery.

Even if you don't expect to stop and are just burning off speed to give the yonder light time to turn back to green, you are better off burning off the speed sooner (as long as you don't use friction or regen) but lesser so 1) you end up with more speed(kinetic energy) remaining when you arrive at the light and 2) end up wasting less energy to air drag (since you slowed sooner). Directly applying kinetic energy to the inverter load instead of drawing from the battery helps you accomplish this, too.

There are cases where Neutral may not be sub-optimal (any case where drawing 1.5kW from the battery is exactly the right amount of energy to hold the desired speed), but I think these scenarios are less common and are still covered while in D or ECO.

 

[RegGuheert]

The reasoning why I say Neutral is not the most efficient mode to coast in is while the car is moving, the inverter draws 1-2kW of power even when in Neutral.

That seems like enough power that the car could accelerate on level ground.

I guess I'm a little skeptical since I believe this is the technique LEAFfan used to set his distance records. Can you tell us more about how you measured this?

 

[TickTock]

The reasoning why I say Neutral is not the most efficient mode to coast in is while the car is moving, the inverter draws 1-2kW of power even when in Neutral.

That seems like enough power that the car could accelerate on level ground.

I guess I'm a little skeptical since I believe this is the technique LEAFfan used to set his distance records. Can you tell us more about how you measured this?

This energy is getting used by the inverter. None of it makes it to the wheels.

 

[edatoakrun]

...I guess I'm a little skeptical since I believe this is the technique LEAFfan used to set his distance records...

I believe the decisive factor in "the technique LEAFfan used to set his distance records", was driving very slowly, taking over ten hours of driving time to cover 188 (?) miles, IIRC.

I don't recall anyone else has ever reported trying a maximum-range test using even half that much time.

Whether LEAFfan (or anyone else) could have gotten a few miles less (or more) driving exclusively in ECO, is, AFAIK, an untested proposition.

 

[TickTock]

Quite right. This is why I added the qualifier "assuming constant average speed for the drive" because there is little debate that the single best way to improve range is to slow down (by a large margin over all other techniques).

 

[palmermd]

where are you getting this 1500 watt consumption from? There is no way the vampire load on the pack is 1500 watt from the inverter. Have you ever stood next to a 1500 watt space heater? That is a lot of power going into heat. I suspect you are off by an order of magnitude. 150 watt seems more realistic.

 

[TickTock]

where are you getting this 1500 watt consumption from? There is no way the vampire load on the pack is 1500 watt from the inverter. Have you ever stood next to a 1500 watt space heater? That is a lot of power going into heat. I suspect you are off by an order of magnitude. 150 watt seems more realistic.

It has been confirmed from other sources (although the real number may be closer to 1kW...):
http://www.mynissanleaf.com/viewtopic.php?f=31&t=12692&start=30

 

[LEAFfan]

The problem for me with feathering the pedal instead of N is it is very difficult to hold it in neutral for any length of time and distance so it ends up actually using more energy than N. I will challenge anyone to do a feathering vs. N duel in the desert.

 

[TickTock]

I will challenge anyone to do a feathering vs. N duel in the desert.

Oh it's ON!
Seriously that would be a fun test. However, I will acknowledge that the stock LEAF doesn't come equipped to make it easy (too much lag and low resolution on the energy screen and it doesn't show this vampire load). With the original SOC meter (gidmeter), or any of the follow-on devices I think it is a lot easier.

 

[TonyWilliams]

Interesting catch in the high efficiency game. I will still recommend neutral, but now with a challenging caveat.

 

[mkjayakumar]

My empirical evidence suggests otherwise. I have had my best efficencies only by coasting in 'N' and shifting to 'D' if and when I want to slow down using the brakes (so that I will get some energy back thru regen instead of wasting as heat). Now it might be possible to simulate 'N' by feathering the pedal and keeping it in a position where it neither draws or generates energy, but it is *extremely* hard to do that for any reasonable length of time. For instance, when I exit the highway at 65mph, I have a fairly long access road (over a mile and half) going down a gentle slope, and I simply shift to 'N' and enjoy the free ride. Now try doing that in 'D' and with pedal in the right position for over a mile, simply not possible.

The good thing about Leaf is how easy and effortless it is to shift between 'D' and 'N' and back. The hockey puck design is simply too good for hypermilers.

 

[edatoakrun]

... I simply shift to 'N' and enjoy the free ride. Now try doing that in 'D' and with pedal in the right position for over a mile, simply not possible...

Sorry, but there is no such thing as a "free ride".

When you are "coasting", you are, in fact, expending the ascent and/or kinetic energy that you took out of your LEAF battery earlier, in order to attain speed and altitude.

When you are coasting at ~65 mph, you are expending a great deal of Wh just pushing air, Wh that could alternately be recovered for future use (assuming your battery is warm enough and at a low enough SOC) by regenerative braking.

Unless the grade is very close to that consistent with the coasting speed you desire, the "right position" for the A/D pedal is generally that that either applies or regenerates Wh as required, to give you the most efficient near-constant speed desired.

 

[palmermd]

When you are coasting at ~65 mph, you are expending a great deal of Wh just pushing air, Wh that could alternately be recovered for future use by regenerative braking.

this is false! Regenerative braking is in addition to the losses due to air. The air losses are still there when you regenerate.

 

[smkettner]

where are you getting this 1500 watt consumption from? There is no way the vampire load on the pack is 1500 watt from the inverter. Have you ever stood next to a 1500 watt space heater? That is a lot of power going into heat. I suspect you are off by an order of magnitude. 150 watt seems more realistic.

+1

I would like to understand how that was measured.

 

[edatoakrun]

When you are coasting at ~65 mph, you are expending a great deal of Wh just pushing air, Wh that could alternately be recovered for future use by regenerative braking.

this is false! Regenerative braking is in addition to the losses due to air. The air losses are still there when you regenerate.

No, not at the same level.

The Wh lost to air resistance are reduced as a function of the speed reduction due to regenerative braking.

And the total loss of Wh to air resistance is lowest, over a given distance, covered in a given period of time, if the speed is constant.

So the marginal loss of efficiency by applying or regenerating kW is secondary to air resistance, outside of a relatively small envelope of varying travel speed.

If the grade is ideal, go ahead and coast.

Just don't worry about either applying or withdrawing some kW with the A/D pedal, as required to maintain the most efficient speed, if the road grade is not perfectly oriented to do so by coasting, in N, D, or ECO.

 

[ttweed]

The one thing that is being ignored in this whole argument is that coasting in neutral is an inherently unsafe driving practice. If an unexpected emergency situation presents itself, the need to have immediate acceleration force to balance the car's handling during an avoidance maneuver or cornering on low-grip surfaces is compromised. I'm sorry, but no amount of "economy" is worth giving up full control of the car to me. In low speed situations it's probably no big deal, but IMHO, it's not worth gambling with for safety reasons. It's not a habit I want to develop to save a few kWhs or drive a few miles more.

TT

 

[EVDRIVER]

The one thing that is being ignored in this whole argument is that coasting in neutral is an inherently unsafe driving practice. If an unexpected emergency situation presents itself, the need to have immediate acceleration force to balance the car's handling during an avoidance maneuver or cornering on low-grip surfaces is compromised. I'm sorry, but no amount of "economy" is worth giving up full control of the car to me. In low speed situations it's probably no big deal, but IMHO, it's not worth gambling with for safety reasons. It's not a habit I want to develop to save a few kWhs or drive a few miles more.

TT

And a waste of time and energy for minimal gains. Plus HMs drive too slow, make people miss traffic lights and idle for longer. My guess is that when they get home they unplug everything in the house before sleeping. If you want to flush them out just flick a light switch on and off five times and they will come running. I'm all about efficiency but just drive the car already. :lol

 

[Cmahrle]

If an unexpected emergency situation presents itself, the need to have immediate acceleration force to balance the car's handling during an avoidance maneuver or cornering on low-grip surfaces is compromised.
TT

Agreed. But I don't think many people actually accelerate in these situations anymore. Seems the reaction of choice is to slam on the brakes.

 

[mkjayakumar]

A Neutral on an EV is not the same as driving in neutral on an ICE vehicle. When driving in 'N' in a Leaf the car feels no different from driving in 'D' and not on the accelerator (except the silly regen).

HM missing lights is all silly. When the other stupid ICE drivers keep accelerating (not even maintaining the speed, but increasing the speed) when the lights 100 feet in front has just turned red, I coast, time the lights and whiz right past them when it turns green. In back roads with lights and traffic, invariably I am ahead than others around me with minimal variation in speeds.

 

[ttweed]

If an unexpected emergency situation presents itself, the need to have immediate acceleration force to balance the car's handling during an avoidance maneuver or cornering on low-grip surfaces is compromised.
TT

Agreed. But I don't think many people actually accelerate in these situations anymore. Seems the reaction of choice is to slam on the brakes.

Which demonstrates how poorly educated most drivers are about basic vehicle dynamics. In an oversteer skid (rear end breaking loose) applying the brakes is the LAST thing you want to do. The proper correction is to countersteer and apply mild acceleration. Braking at such a moment will likely send the car into a spin. :roll:

TT