A discussion of gliding and a comparison of methods: pedal control vs. neutral

[JimboK]

The CAN-View data capture and analysis fun continues, especially on a lazy Independence Day with nothing to do after eating too many burgers ....

As pulse and glide regulars know, the conventional method of triggering a glide is through go-pedal control -- completely lifting off and feathering back down until all arrows disappear on the MFD Energy monitor. It is fairly easy but takes a bit of practice to hit and hold.

My CAN-View observations of pedal position values have suggested that a pedal controlled (PC) glide can be executed with pedal position anywhere from 1-10%. Despite the MFD showing no arrows, there actually is a small amount of current flow out of the battery. But interestingly, there appears to be no relationship between pedal position and current flow until after the 10% threshold is exceeded:



This and my seat-of-the-pants sensations of drag suggest that the car no longer is gliding above 10% pedal position. Presumably the no-arrows condition disappears at that mark, but since my CV uses the MFD for its display, I can't monitor it and the Energy screen simultaneously to verify.

There is a relationship between speed and current flow with PC glides:




Some have suggested using neutral for glides. It's easy and requires no practice. After recording these PC glide data, and then observing on the display a fairly low amount and narrow range of current flow (seemingly less than 1.5 amps) with N glides, I have increasingly used neutral -- and, of course, I have recorded data:



This confirms that current flow moves within a more narrow range than with PC glides, at speeds below 30 MPH. Interestingly as speed approaches 30 it appears to creep up a bit, and then above 30 it's all over the map, even occasionally into the battery. Finally, what's most interesting to me is this blip:



This is a neutral glide that I let transition to warp neutral (i.e., >40 MPH) coming down a fairly steep bridge approach. It carries me across the bridge and then up a short incline to a toll booth, where the last data point is.

On this and other steep hills previously I've noticed a brief surge of positive current at speeds around the mid-30s, often spiking to well over 10 amps, along with a simultaneous sensation of increased drag as though I had let it slip into regenerative coasting. (It might have been over 10 amps here too but CAN-View captures data only every two seconds or so, and that's about the duration of the positive surge.) Then something I hadn't noticed before are the fluctuations of current flow in warp neutral being more erratic than most of those at slower speeds. I'd be curious to hear from any of the experts an explanation of these phenomena.

So the question became: "Which method produces less drag?" With the answer to that might come the answer to this one: "Which method is more efficient?" So I did a head-to-head test of the two methods.

The test consisted of four runs with each method on a four-lane road through a lightly traveled industrial park. The route is shown here. There was a net elevation decrease of approximately 36' from start to finish.

I aimed to begin each run at approximately 28 MPH. Each glide was allowed to continue to a defined ending point 0.6 mile from the starting point.

Outside air temperatures were in the low 90s, and winds were out of the WSW at around 15 MPH -- both as reported by Weather Underground's almanac information for my locale on test day.

Climate control was off to avoid the significant current draw from air conditioning. Both front windows and the right rear window were opened for driver comfort and ventilation for the hybrid battery.

Results, with charts for speeds and hybrid battery current flow:





As suspected, there clearly is more drag with neutral glides than with pedal-controlled glides.

The CAN-View data capture records battery SOC in half-percent increments. All glides began with SOC between 55.5% and 59%. SOC predictably dropped more with PC glides than with N glides: 1%-1.5% with each of the former and 0%-0.5% with each of the latter.

So it seems that with lower current flow, N-glides are more efficient, especially between 30 and 40 MPH. I'll hedge a bit by saying this conclusion is subject to correction, given that I don't understand everything going on under the hood as well as some. But it seems generally accepted that use of battery power and subsequent replenishment introduce inefficiencies from conversion losses, and better from an efficiency standpoint is to avoid the battery as much as possible.

I have incorporated these findings into my everyday driving, using N glides preferentially when safe, especially between 30 and 40 MPH. I use PC glides when preservation of momentum (e.g., heavier traffic) is more important than preservation of SOC, or when I want to quickly alternate between a glide or outright coasting (e.g., timing my approach to a red light).

It should be stated that using neutral on downgrades is illegal in many states. But states with such a law usually don't seem to have a similar prohibition on using neutral on level terrain or uphill, as suggested by a recent CleanMPG thread on the subject.

[Sheepdog]

wow. I havent read a disertation so closely since my last class in engineering school!

You make me desire can-view in my car. also a computer to run o'scope software!

If I read your charts right- you are actually conserving battery power by coasting in N instead of using the go peddle to go stealth? But you say the drag is more in N. This equates a significantly higher mpg you have found? I would think it would be less! and is their a loss in momentum dropping from N back into drive?

Fascinating..

[TonyPSchaefer]

Jimbo, you lying stack of crap!
There's no such thing as too many burgers!

Seriously though, that's a VERY impressive write-up. Thanks.

[andyprius]

Very nice job on the measurements, charts and interpretations. Not that I understand them! However each and everytime that I have practiced gliding in neutral ( 3 times maybe) my gasoline consumption was considerably worse. Which puzzles me. Apparently there is some sort of drag, although it is not apparent. I expected my MFD mileage to be worse, as something in the Electronic is not functioning. I also expected my calculated mileage to be impressibly better. But just the opposite. Do a real life check and run in neutral as much as possible, ( a full tank ) and let us know the results. Andy.

[JimboK]

Originally Posted by Sheepdog

If I read your charts right- you are actually conserving battery power by coasting in N instead of using the go peddle to go stealth? But you say the drag is more in N. This equates a significantly higher mpg you have found? I would think it would be less! and is their a loss in momentum dropping from N back into drive?

It might be better said that N glides use less battery power rather than conserve more. Hobbit has reported that current flow seen with PC glides in fact is indicating a small push. I suppose you could say a PC glide actually is a low-demand EV mode of sorts.

There can be a drop in momentum shifting back into D if the shift occurs with your foot off the pedal -- you have regenerative coasting until you begin the next pulse. But what's great for this purpose is that, while in neutral, the pedal is totally disassociated from the drive system. With practice you can learn approximately where to position the pedal for the upcoming glide, and you can have it in position well in advance of the shift. Then the ICE spins up almost immediately with the shift.

As for MPG results, I can't draw any firm conclusions yet. It seems intuitive they would improve. It is generally agreed that deliberate and sustained higher-demand EV mode operation hurts fuel economy. There is no reason in my mind that this "low-demand EV" would help it. The battery charge still has to be replenished from the ICE.

My fuel economy has improved recently, but that's to be expected anyway because of summer temperatures. I certainly haven't seen any dropoff. But I am reluctant to say, "Hey, look, my fuel economy is better because of [fill in the blank]" without a controlled test. There are so many variables that affect FE from trip to trip and from tank to tank -- weather chief among them. I'd like to test this with a series of runs on the same day in the same weather conditions, unimpeded and unhurried by traffic.

Instead of saying there is more drag with N glides (which some may interpret as a negative), my conclusion might be stated better that there is less battery assist.

Oh, and Tony ... I concede the point. About the burgers, that is.

[bwilson4web]

Many years ago, I read a report about combat aviation tactics and one was 'how to overtake an enemy plane.' The recommended practice is to firewall the throttle while nosing over to reduce lift drag and get a gravity assist. This gets the plane as rapidly as possible to the maximum speed and once the enemy plane is overtaken, climb as needed for the attack.

I haven't done the math, yet, but I would observe that very small power additions on a downgrade really increases the kinetic energy rapidly. But adding power on the up slope seems only to consume a lot of fuel for very little benefit. In fact, I tend to let my speed slowly bleed off as I try to keep the ICE in a peak efficiency range.

Bob Wilson

[DaveinOlyWA]

good stats, excellent charts, but i think a chart covering longer periods of time may be necessary. also, checking stats with 7 bars on battery verses 3 bars might be worth investigating.

i think we need to look at what happens when the Pri transitions to speeds under 40 mph... the spikes you see, i think have to do with the Pri's engine not turning maybe...occasional charges sent to the 12 volt battery which is my understanding only happens on a "as needed basis" to only charge when the lower threshold is met, and not trickle charged to maintain a small charge level range.

i dont coast a lot, but there is this one place that dips to a river in the middle of a small valley that i would coast in neutral down one side, using that momentum to get me about half way up the other side before slowing back down to the speed limit, and its simply too short of a range to tell what effect it has and i think your example is too short as well.

another thing to check, can you check valve timing to see how it changes when speeds transition? or even engine compression?... that last one might be tough

[9G-man]

I underatand and take advantage of the glide and stealth capabilities of this car every time I drive. (As should every Prius owner).
I've noticed, and others too have commented on, the almost uncanny ability this car has to glide extensively and maintain speed, and this often was attributed to superior aerodynamics and low rolling resistence.
In fact, I have felt a slight "acceleration" during a glide, and wondered where did that come from, what just freed-up, or became more efficient to allow that to happen.
I think JimboK has identified it. There must be some electric assist in there, and that what I've felt.

In the beginning, I tried N glides while I was getting my right foot trained for effectively manipulating the gas pedal. But, I dropped N glides in favor of PC glide and manipulation because of the easy, efficient and effective transition I can make into and out-of all the modes, linking glides with stealth, and back into glides, thus extending the gas-free momentum and travel.
You can't do that with N glides, because of the regen induced when reselecting D. To me it's simpler and re-enforces an overall more efficient driving technique to do it all with pedal control.

[taxachusetts]

This topic is very interesting if a bit confusing to a brand new Prius owner. Questions: When coasting, or gliding, and there are no colored arrows, is the HV battery charging or discharging? Is the ICE running? How does the no-arrows condition differ from the green- and yellow-arrow conditions?
My week old Prius is averaging 54.8 mpg. I'm looking forward to increasing that score.
Thanks,

Paul, Boston

[Neicy]

Hi Paul,

Great to have another local on PC. Welcome.

When coasting, you have green arrows to the battery (regeneration).

When gliding, there are no arrows at all (no battery use or regen)

When using battery only, there are yellow arrows going to the wheels.

When using the ICE (internal combustion engine), there are orange arrows going to the wheels and sometimes (yellow I think) back to the battery (charging).

John1701a's User Guide (see attachment)is a great resource for learning, as is hobbit's information:

Hobbit's techie-rants

I have spent days reading and rereading this info. Each time I grasp more than the last time. Be patient with yourself.