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

[Neicy]

Originally Posted by 9G-man

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.

I have felt that too, and I was on an incline at those times, so it scared me a little (actually a lot the first time) since I didn't expect it and didn't understand what I was feeling. I felt kinda like the car was taking off on me unexpectedly. I was expecting to slow on the incline for a left turn, and as a result needed to brake a little. But this is a perfect explanation.

[taxachusetts]

Hi, Neicy,

Thanks for the reply. I understand the normal conditions (when the colored arrows are present), but I can't figure out what the operating conditions are when there are no colored arrows.

Thanks again,

Paul

[Rokeby]

JimboK,

Thanks for posting this info. It is an excellent entry point for considering
what happens in the HSD when gliding. To be honest, I'm not sure that I
understand what you're saying -- no big surprise as I'm only just conversant
with the intricacies of the Hybrid Synergy Drive (HSD).

I've considered what you've said for a whole day. My hope was that other
really smart posters would come up and offer other perspectives. Well, they
haven't, and that is the reason that I'm about to reveal my ignorance before
God and the whole world.

As gross simplifications, what I understand you to say is, there is an outflow
of energy from the HV battery when you are in a no-arrows-glide -- about
1.5% of SOC. There is also an outflow of energy from the HV battery when
you are in a neutral glide -- about 0.5% of SOC, or 1/3 of the no-arrows-
glide flow.

I understand you to say that in both the no-arrows-glide and neutral glide
the energy from the HV battery goes to providing some small amount of
forward thrust to the rear wheels. [Edit: should read "front wheels"]

"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."

This is counter to my "seat of the pants" experience. When I have shifted
into neutral directly from a no-arrows-glide state, it feels like the car picks
up speed. Kind of like when you're skiing and pass from hard packed snow
to glare ice. But your speed vs time graph for the two states contradicts this
-- and for now I'm believing that "numbers don't lie," and neither are you.

I guess I gotta get my butt checked -- OMG, not another finger wave!!! -- or
at least get my OEM, used-but good, butt-accelerometer re-calibrated.

[grand total]

Originally Posted by Rokeby

I understand you to say that in both the no-arrows-glide and neutral glide
the energy from the HV battery goes to providing some small amount of
forward thrust to the rear wheels.

That would be front wheels.

[Rokeby]

Originally Posted by grand total

That would be front wheels.

Oops, you're entirely correct. It continually amazes me how much stuff is
crammed under the hood of this car.

Note to self: Pay attention.

[donee]

Hi All,

I did the side road drive home yesterday using the Neutral Glide (<41 mph) and a few Warp Neutrals (< 40 mph, glide, engine off, then Neutral down a hill up to as much as 45 mph) yesterday. And the Battery SOC stayed up nicely, where it usually drops down to 3 or 4 for this commute. Mileage was less than I had hopped for, average stayed level. I got caught by some out-of-character lights, that usually go green at different times than they did, however.

Today, I did the highway route. And did one Neutral Glide in a 45 mph construction zone. After putting the car back into Drive, I noticed it would not go into coast. I eventually figured it had to do with getting up over 35 mph, which took about 1/2 a mile due to the slow traffic. After that, I stopped doing the neutral glides, unless I knew I was going to be coming to a stop long enough to kill the engine. Mileage was good, pulled up the average by .5 mpg. And SOC was up a bar from the usual on that route.

The 35 mph speed requirement for gliding never happens unless the car is quite cool. Temp yesterday was 85 F with a quartering 25 mph wind and sunny, versus about 83F today with a rear quartering 15 mph intermitant wind and clouds. Is it possible the 35 mph glide status is delatched by the neutral glide? Then the last part of the Stage 3B stuff has to recure? This neutral glide was after doing a 10 mile 53 mph SHM cruise, then a 5 to 10 minute slow and go , including a long ramp descent, with lots of short regenerative brake stabs. The lady in front of me in the Audi was driving wierd. Battery temp was 37 C when this happened.

[JimboK]

Originally Posted by Rokeby

I've considered what you've said for a whole day. My hope was that other really smart posters would come up and offer other perspectives.

Me too! I’m a relatively uneducated (in technology, anyway) data geek. I’m on the lookout for those more knowledgeable than I to tell me exactly what some of these data mean, and to correct any inaccurate conclusions.

Originally Posted by Rokeby

As gross simplifications, what I understand you to say is, there is an outflow of energy from the HV battery when you are in a no-arrows-glide -- about
1.5% of SOC. There is also an outflow of energy from the HV battery when you are in a neutral glide -- about 0.5% of SOC, or 1/3 of the no-arrows-glide flow.

You are correct in concept, and probably in the ballpark with 1/3, though I’d be reluctant to pin it down with a precise number. Incremental SOC changes obviously are pretty small. It would be nice to see SOC readings for this purpose at least down to a tenth of a percent. But the CAN-View data capture reports it only in half-percent increments, so there is considerable opportunity for rounding errors.

After reading your post, I did some additional math on my data. The average SOC loss for N glides was 22% (relative percent, not absolute) of that for PC glides: 0.25% per run vs. 1.125%. Then I averaged current flow and approximate time for each run and figured the average amp-hours used. N glides used 39% of the current of PC glides: 0.091 Ah per run vs. 0.035. But with that calculation comes another opportunity for error. Current flow of course is continuous, whereas CV’s data capture essentially does data sampling. And the sampling interval actually is slightly less than two seconds and seems to vary somewhat. I use two seconds (and state that they are approximate time intervals) for convenience and to keep the math simple.

Originally Posted by Rokeby

I understand you to say that in both the no-arrows-glide and neutral glide the energy from the HV battery goes to providing some small amount of
forward thrust to the rear wheels. [Edit: should read "front wheels"]

I don’t consider battery energy to be providing any push during a neutral glide. The car’s “basal metabolic rate,” so to speak, is close to 1A. I generally run with the radio on, and I did so during these tests, so that adds a few mA. There’s not much further to go to reach the average 1.3A of the N glide runs. EDIT: I am also powering my laptop via a plug-in inverter during my tests, so there's a few more mA.

On the other hand, as I mentioned earlier, Hobbit states that there is some push with PC glides. The CV data stream also includes battery voltage, so I calculated average battery power for each glide to be 286W for N glides and 820W for PC glides. After converting the difference to horsepower we could say that PC glides provide approximately 0.7 hp extra thrust.

Thanks, by the way, for giving me the excuse to run these numbers!

Originally Posted by Rokeby

"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."

This is counter to my "seat of the pants" experience. When I have shifted into neutral directly from a no-arrows-glide state, it feels like the car picks up speed. Kind of like when you're skiing and pass from hard packed snow to glare ice. But your speed vs time graph for the two states contradicts this -- and for now I'm believing that "numbers don't lie," and neither are you.

I guess I gotta get my butt checked -- OMG, not another finger wave!!! -- or at least get my OEM, used-but good, butt-accelerometer re-calibrated.

Try this shift mid-glide a few times on a steep hill, and I think your butt check results will be revised.

Originally Posted by donee

Hi All,

I did the side road drive home yesterday using the Neutral Glide (<41 mph) and a few Warp Neutrals (< 40 mph, glide, engine off, then Neutral down a hill up to as much as 45 mph) yesterday. And the Battery SOC stayed up nicely, where it usually drops down to 3 or 4 for this commute. Mileage was less than I had hopped for, average stayed level. I got caught by some out-of-character lights, that usually go green at different times than they did, however.

Today, I did the highway route. And did one Neutral Glide in a 45 mph construction zone. After putting the car back into Drive, I noticed it would not go into coast. I eventually figured it had to do with getting up over 35 mph, which took about 1/2 a mile due to the slow traffic. After that, I stopped doing the neutral glides, unless I knew I was going to be coming to a stop long enough to kill the engine. Mileage was good, pulled up the average by .5 mpg. And SOC was up a bar from the usual on that route.

The 35 mph speed requirement for gliding never happens unless the car is quite cool. Temp yesterday was 85 F with a quartering 25 mph wind and sunny, versus about 83F today with a rear quartering 15 mph intermitant wind and clouds. Is it possible the 35 mph glide status is delatched by the neutral glide? Then the last part of the Stage 3B stuff has to recure? This neutral glide was after doing a 10 mile 53 mph SHM cruise, then a 5 to 10 minute slow and go , including a long ramp descent, with lots of short regenerative brake stabs. The lady in front of me in the Audi was driving wierd. Battery temp was 37 C when this happened.

OK, I’m a little puzzled. I’m not sure what you’re saying about not going “into coast” after shifting into D. And what is the “35 mph speed requirement for gliding” you’re referring to? Since you mention S3b, maybe it’s referring to the speed threshold between S3a and S3b. Finally, what is the “delatching” you’re referring to?

Anyway, my tests were all done with car in S4. For S3, as I’m sure you know, you can’t glide below 34 MPH without either an EV switch or beginning the glide above that speed.

[donee]

Hi JimboK,

Excuse my imprecise terminology. That reference to "coast" was meant to say "glide".

In cooler weather, well after S4 has been entered (at least once) the car will need to get to 35 mph to enter a glide. Trying to enter a glide below 35 mph does not work.

My guess is possibly the neutral glide might be resetting to 3B from 4, if something in the drive train is not above some temperature. Most likely the transmission. Or maybe it has to do with time between transmission lubrications (engine operations)?

"Latching" is an engineering term. When some set of conditions occurs, some systems will set a new operrating parameter, which cannot be undone, ie latched. The most common form of a latch is a relay with a push button to operate the coil, but also contacts of the relay are wired to short out the pushbutton, once the relay is operated. Thus releasing the pushbutton does not deactivate the coil. The relay is said to be "latched". Usually, there is a normally closed pushbutton in series with the relay contacts. Pushing it delatches the relay. On industrial equipment, its common to see a START green pushbutton, and a RED stop pushbutton. This is how the operation of the machine is effected.

Another similar situation is a vehicle rear window defroster. It will operate after a pushbutton is pressed, but will turn off if the car is turned off, and will not restart if the car is turned back on. This is to avoid draining the vehicle battery, and again this operational logic is that of a latched relay.

[JimboK]

Originally Posted by donee

....

OK, it's starting to sink in. I read on a "See Spot run" level, so it takes awhile sometimes.

I do routinely experience a short episode of the car refusing to glide after entering S4, but it has nothing to do with my glide method -- nor, I think, whether I'm gliding at all, but I can't say that for sure because it's so infrequent that I'm not doing some semblance of P&G at those speeds. Anyway, according to Hobbit, it's from the secondary cooling loop opening to completely heat what remains in the thermos. (I hope I've paraphrased accurately.) But it's not the behavior you describe, which I've never noticed. Instead it refuses to glide (actually more typical of S2) but it resolves spontaneously after a minute or so.

Having said that, I'll keep an eye out for what you describe, especially as weather starts to cool in a couple of months. It's been only recently and after gathering these data that I've been using N glides with any regularity.

[JimboK]

Originally Posted by JimboK

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.

OK, it's done, at least with one long-ish run with each method:






The test was performed on this route. The route is mostly four lanes, 45 MPH, lightly traveled, in the vicinity of some industry stuck in the middle of an otherwise rural area. It is relatively flat with some very gentle hills. Net elevation change from end to end is less than 15'.

Two continuous round trips, a total of 12.4 miles each, were made for each method, beginning and ending at the northern end. That end has a stoplight, so time spent awaiting the light after the first round trip is highlighted on the charts. The neutral glide test was performed first.

Weather was mostly sunny, temperature around 90F, light winds out of the SW. I used mobile broadband to stay continuously connected to Weather Underground, monitoring real-time temperature and wind readings at the closest weather station less than a mile away. Temperature increased by one degree and wind speed by one MPH between starting and ending the tests; wind direction remained constant. Modeling performed with the Prius MPG Simulator suggests that these changes would mostly offset each other insofar as their effect on fuel economy is concerned, so they were considered insignificant.

As before, I kept climate control off to eliminate its effect on battery discharge. All windows were opened for comfort and hybrid battery ventilation.

Hybrid battery temperature, incidentally, held fairly steady. For the N glide test all modules remained at 100F throughout. For the PC glide test, the coolest module stayed at 100F while the hottest ranged from 100F to 102F.

The pulse range was 15-40 MPH, other than (obviously) leaving or approaching the ends of the test route. A bit of on-the-fly judgment was required, along with an occasional rapid pedal adjustment, in executing pulses approaching the ends, trying to time the speed and distance of those pulses to minimize the need to brake. Braking (predictably) still was required at both ends, but the charts suggest I succeeded in keeping it to a minimum.

Because of the previously-discussed issue of state of charge (SOC), I attempted to begin each test with an identical SOC. Obviously, however, the tests were not likely to conclude with the same SOC, and the charts demonstrate that to be the case. In fact, one of the more significant data findings (to me) is the steady rise in SOC during the N glide test as compared to the regular undulations seen during the PC glide test. So obviously the charge replenishment seen with the PC test pulses was enough only to offset the discharge during glides -- this despite those pulses obviously producing more current flow into the battery than the N test pulses.

To adjust for the effect of greater battery discharge during PC glides, I included a period of force-charging at the end of the PC glide test to bring the charge to the same level as where it ended with the N glide test. (Force-charging is done, with the car in “D,” by depressing the brake [to keep the car stationary] and accelerator simultaneously, the latter to force the ICE to light and rev enough to generate enough current to charge the battery.) Here are the various SOC readings via the CV display and data capture:

SOC reading	N glide	PC glide
Beginning, display	56%	56%
Beginning, recorded data	56.5%	56.0%
Ending, display, before force-charge	60%	55%
Ending, recorded data, before force-charge	60.5%	55.5%
Ending, display, after force-charge	NA	60%
Ending, recorded data, after force-charge	NA	60%

The charts show raw ending MPG figures for both tests and the force charge-adjusted MPG for the PC test. With the force-charge adjustment, the N glide test shows modestly better fuel economy.

Some limitations to the test: Even though the distance was identical and weather conditions nearly so, minor variations in technique and weather could have been confounders. This includes the variations in number of P&G cycles – not surprising, given the greater speed decay and shorter duration of N glides – and the aforementioned adjustments of pulse speed and length approaching the ends of the course. Also, there could have been transient wind speed and direction changes not reflected in the local weather station's recordings.

So the two methods appear at least comparable for fuel economy, with a possible slight edge in favor of using neutral to glide. With the test limitations and the relatively minor adjusted difference, I still am reluctant to state that unequivocally. A longer test on a closed course with no stops would be ideal, though probably impractical and maybe even impossible in my area.