peterdambier
Posts: 284
Joined: Thu Oct 04, 2012 3:26 am
Location: Bergstrasse, Germany

Re: Regen vs Neutral and speed for max range

Mon Jul 15, 2013 3:28 pm

We just come from visiting Mount Katzenbuckel. From the city of Eberbach it is a climb of some 500 meters part of the road is 8% steep. It took us some 4 bars up and we got one bar back going down.

http://en.wikipedia.org/wiki/Katzenbuckel

http://en.wikipedia.org/wiki/Eberbach_(Baden)

Keeping my feet from touching anything I let "Snowwhite" decide for herself how fast to get down. Most of the time it was 44 kilometers per hour, not the top of the regen scale but it still felt like that kilowatt came back about the same speed as 16A, 230V of charging. Hitting the break would get me more regen. I was driving in "B" for "Berg" or mountain.

I have seen one bar coming back, time and again but never more than one bar. Mount Katzenbuckel has been my longest run downstairs yet.
Peter and Karin Dambier, DL2FBA, www.piraten-fraktion-bergstrasse.de

Aerowhatt
Posts: 306
Joined: Wed Jan 28, 2015 1:52 pm
Location: Albuquerque NM

Re: Regen vs Neutral and speed for max range

Sat Apr 29, 2017 12:01 pm

I'm bumping this old thread rather than muddy up Rkarl's route 66 journey thread. But this is inspired by the brief discussion over there about how to descend a hill most efficiently.

As a preface I drive to the mountains here four to six times a month. And by mountains I don't mean big hills like most of the Appalachians. We are talking the southern Rockies here.

I live at 4956 Ft elevation. Sandia crest (Elevation 10,600 Ft.) is 32 road miles away, just under half of it freeway. That's 32 miles with 5,600 ft in elevation gain. Most of my trips are not to the crest but I have driven there and back twice. So these tens of trips are more like ~28 miles one way and 4,200 to 4.900 ft in elevation gain. Having made so many trips on this route and experimenting with different range stretching strategies. I can unequivocally say that Neutral is far more efficient than Regen. The only place I leave it in "gear" (downhill) is on some stretches that are so steep that it takes B mode with slight brake pedal to keep speed down below 30 for 20 mph switchback turns. Other sections not as steep I coast along at the posted speed allowing it to go to ~5 over the limit and drop it in to D (with no pedal input) to slow down, then back to N as needed.

I-40 on the way back really illustrates the difference though. Coming out of Tijeras Canyon into the city is a long steady grade. In N the Imiev will coast at 60 to 70 mph on this grade depending on the winds. Do the same section in D at the same speed and Canion is showing a 12 to 18 amp continuous power consumption. With the inverter on (in D) it has to create a rotating field to match the motor speed. Even though there is virtually no torque being produced. In N the inverter is off, so only accessory power consumption. Changing from N to D at high speed is seamless. The unit has a nice entry ramp and it feels like a light brake application.

I'm hoping Rick has enough time and the inclination to take the drive with me while he is passing through again. I don't think most people believe that you can do a 64 mile round trip with 5,600ft elevation change at mostly the posted speed limits on a single charge. Heck, I could barely believe it after the first try. My wife believes it because she has gone with me dozens of times (56 miles and 4,900 ft elevation change). A little semi-truck surfing on the freeway leg up to NM-14 and lots of neutral usage, plus a very steady accelerator foot goes a long way in making this possible and repeatable on a regular basis.

Aerowhatt
2014 cool silver ES, acquired new 4/2015
2014 Labrador Black Pearl ES, acquired new 3/2016

JoeS
Site Moderator
Posts: 3070
Joined: Thu Dec 15, 2011 5:47 am
Location: Los Altos Hills, California

Re: Regen vs Neutral and speed for max range

Sat Apr 29, 2017 2:19 pm

Aerowhatt, delighted you reopened this thread, as it has been on my backburner as a topic to pursue further.

As you may recall from a couple of my early threads, I am strong proponent of coasting in neutral - in fact, I used to brag that maybe a quarter of my i-MiEV driving is in N and that I'd probably be the first to wear out our shift lever.

My N experience is a carryover from my Gen1 Honda Insight hypermiling days, whereby kicking the car into neutral and shutting off the fuel injectors to stop the engine was just one of the tricks up my sleeve which gave me a lifetime 77mpg on that car.

Now, enter the i-MiEV: almost all of the conventional hypermiling tricks are directly transferrable from the ICE vehicle to the BEV. There is, however, one terrific advantage that the BEV has over the ICE: regeneration, which can result in actually having the fuel gauge go UP on a downhill run. Putting the car into Neutral and coasting the same distance will get us to the bottom of the hill faster, but what about our State of Charge?

Sorry, but starting from the top of the hill with, say, 8 bars and coasting down the hill in Neutral and arriving at the bottom with 8 bars gives less total trip range than arriving at the bottom of the hill with, for example, 10 bars achieved by regeneration.

As an aside and to satisfy my curiosity I just took a drive to verify what the quiescent power draw is. Coasting in Neutral, my CaniOn showed:

50% SoC
343V
1.3A (it first showed 1.5A, but settled down on 1.3A
Aircon/heater OFF
Nav radio ON
Fan on LOWEST setting
The 1.3A quiescent power draw was independent of vehicle speed.

We do have one more factor to take into account: altitude.

On a steep downhill we can regen to limit the speed to anything we wish. One of the ideas behind regenning to keep the speed down is to capture the energy that would otherwise be lost to aerodynamic drag. At sea level, this is significant; however, at 10,000ft altitude the numbers change noticeably and I'd have to go look up the air density and drag to see what the numbers are. In any case, regenning at a higher speed at altitude would probably give us the same net effect as doing it at a lower speed at sea level.

So, in conclusion, I contend that going downhill slower being slowed by regeneration will result in a better range than coasting down the hill in Neutral with the speed simply limited by aerodynamic drag.

Aerowhatt, I just re-read what you wrote and I think we may be in violent agreement as we are both proponents of coasting in Neutral, but we may be talking about apples vs. oranges.
Aerowhatt wrote:I-40 on the way back really illustrates the difference though. Coming out of Tijeras Canyon into the city is a long steady grade. In N the Imiev will coast at 60 to 70 mph on this grade depending on the winds. Do the same section in D at the same speed and Canion is showing a 12 to 18 amp continuous power consumption. With the inverter on (in D) it has to create a rotating field to match the motor speed. Even though there is virtually no torque being produced. In N the inverter is off, so only accessory power consumption. Changing from N to D at high speed is seamless. The unit has a nice entry ramp and it feels like a light brake application.
You are referring to a situation that is a shallow downhill, and I completely agree that kicking the car into neutral in this situation is preferable to leaving it in D if you want to maintain the same speed. I spend a lot of my driving doing just that while maintaining a high speed, with no one around me realizing that I'm coasting.

What I was referring to was a conscious sacrifice in vehicle speed in order to further maximize range. Once again, arriving at the bottom of the hill with a higher State of Charge (more bars on the fuel gauge than at the top of the hill) is, by definition, extending the driving range of the car.
EVs: 2 Wht/Blu SE Prem., '13 Tesla S85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV

Aerowhatt
Posts: 306
Joined: Wed Jan 28, 2015 1:52 pm
Location: Albuquerque NM

Re: Regen vs Neutral and speed for max range

Sat Apr 29, 2017 6:30 pm

JoeS wrote:What I was referring to was a conscious sacrifice in vehicle speed in order to further maximize range. Once again, arriving at the bottom of the hill with a higher State of Charge (more bars on the fuel gauge than at the top of the hill) is, by definition, extending the driving range of the car.


I get what you are saying. And, it does make sense on it's face. Pretty sure I disagree overall though. Unless one is on a steep enough grade that the car just runs away in (N).

I went for a drive today too. We had east face snow last night and this morning. I wanted to see how deep and how wet it was to be able to gauge conditions for later in the week. On the stretch down I-40 from NM-14 which is not steep enough to need to slow (unless traffic slows). In (D) maintaining speed I'm showing 16 to 20 on instantaneous mi/kWh - - shifting to (N) those numbers change to 178 to 184 mi/kWh! I do have the option of taking old 66 from NM-14 to the edge of the city. Since old 66 and I-40 connect at both ends of this stretch, elevation change is the same either way. Still time after time I end up using less SOC between the two points mostly coasting down I-40 at 60 to 75mph in (N) moving with traffic. Than I use (D)riving down route 66 at 35 and 40 mph. I've used route 66 often when they are doing bridge work on west bound I-40 so this is not a one off analysis. So according to this stretch of descent slowing is at a slight disadvantage (net power used) to letting it roll in (N). Is that always going to be the case? I don't know but in my experience it is. And as I said this is a drive I have analyzed dozens of times.

Even Coming down from the crest (10,600 ft) to the bottom of the part of the road (~7,400ft) where full time, sometimes maximum regen is required to stay slow enough to be able to negotiate the road about 2.3 kWh are generated ( two to three bars depending on how much of the top bar showing was left when starting down) in a 3,200 ft descent!. Aero not much of a factor since speeds on this section range from 20 to 35mph. Regen is great and the car couldn't do what I ask of it without it. But using (N) more and regen only to keep in posted speeds I get back with significantly more power left.

Today I started at 100% at 4956ft elevation. Truck surfed the freeway to NM-14 (got lucky and was behind a heavily laden truck which slowed to as low as 52 mph on parts of the climb). Wet roads from there on up to 7,600ft elevation. Slushy snow about 3 inches worth left for the last couple of miles up 536. Return trip much the same except the freeway traffic was moving at 70+ on a wet roadway (West side dries slower). I and a couple of other cars were loosely immersed in 7 or 8 semi's. Have to move with traffic in that situation (much of it in (N)). Arrived back home with 29.5 SOC and 4 bars showing. 53 miles total with 2,600 ft in elevation gain on only the top 70% of SOC used. I guess I just don't see how I could slow to 35 and gain 2 bars, or even 1 in ~1000 ft elevation gentler slope descent (than the steeps near the peak)? What you are saying makes sense logically, but it doesn't hold up in my experimenting and resulting data.

Aerowhatt
Last edited by Aerowhatt on Sat Apr 29, 2017 9:40 pm, edited 3 times in total.
2014 cool silver ES, acquired new 4/2015
2014 Labrador Black Pearl ES, acquired new 3/2016

JoeS
Site Moderator
Posts: 3070
Joined: Thu Dec 15, 2011 5:47 am
Location: Los Altos Hills, California

Re: Regen vs Neutral and speed for max range

Sat Apr 29, 2017 8:28 pm

Aerowhatt, seems to me your driving style and mine are almost identical. I like the expression "truck surfing"! :D

The excessive slowing down and regen is primarily applicable to steep grades, say 6% or higher. Below that, there would be a lot of fiddling between D and N to keep the speed up. Don't forget, at higher speeds the air resistance is robbing the car of its energy, so we want to minimize that and stuff as much as we can back into the battery.

I think we're probably picking nits, as on longer trips we drive efficiently yet not obsessively, knowing in almost all cases that we have no problem making our destination and thus almost never need to employ this rather extreme technique.

The scenario of regenerating heavily and limiting the speed to <35mph is basically an ace-up-the-sleeve when faced with, for example, the situation Rick had today on his Route 66 Amboy to Goffs leg where he arrived with no bars and RR=3.

Image

Looking at the graph in the middle of that leg on that downward slope I would have been really working hard to maximize the regeneration and not letting the car pick up much speed (it's so shallow that I think a lot of fiddling between N and D would be required just to keep the speed up), especially knowing that I was facing another uphill. Even though its a rare situation for most of us, knowing ahead of time that it's going to be really close enables us to focus on our full bag of tricks, with this being just one of them.
EVs: 2 Wht/Blu SE Prem., '13 Tesla S85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV

PV1
Site Moderator
Posts: 2792
Joined: Fri Apr 27, 2012 4:22 pm
Location: Pittsburgh, PA
Contact: Website

Re: Regen vs Neutral and speed for max range

Sun Apr 30, 2017 4:59 am

A heavy truck only going down to 52 MPH when climbing? That's not even a posted grade up here in PA. We have hills, especially on I-68 (I know, technically Maryland) where trucks can barely keep 20 MPH when climbing. ;)

On my yearly trip to 7 Springs, there is a steep climb out of the town of Connellsville. It is 3-4 miles practically straight uphill. Not using regen coming down would destroy the drive motor as the car could easily exceed 100 MPH. So, obviously can't just let the car go in Neutral. To maintain the posted speed of 35 MPH, I use no-pedal regen in both B and ECO. This can easily net me a bar gain.

However, just yesterday I hypermiled to the local mall after being rained out of an event in Pittsburgh. The last 2 miles before the mall consist of a quick uphill climb then a rolling descent. I practically coasted the whole way from the top of the hill to the mall, and in total I drove almost 10 miles on the 4th bar from the bottom using this technique.

Moral of the story, it seems that both Aerowhatt and Joe are correct. Slight grades, coasting seems to work better, using slight spurts of regen to control speed (bouncing between N and D), but on steep descents, pull your speed down a bit and use regen. The i-MiEV isn't heavy enough to effectively ride out the momentum of a high speed descent (although it's fun to try).
"Bear" - 2012 Diamond White Pearl ES with QC - 2/21/2013
Solar-powered since 10/10/2013

"Koorz" - 2012 Cool Silver Metallic ES with QC - 1/5/2015

Driving electric since 2-21-2013.
3 years with the i-MiEV and loving every mile!

Aerowhatt
Posts: 306
Joined: Wed Jan 28, 2015 1:52 pm
Location: Albuquerque NM

Re: Regen vs Neutral and speed for max range

Sun Apr 30, 2017 6:19 am

PV1 wrote:A heavy truck only going down to 52 MPH when climbing? That's not even a posted grade up here in PA. We have hills, especially on I-68 (I know, technically Maryland) where trucks can barely keep 20 MPH when climbing. ;)


That's really my point, which seems to be falling on deaf ears, mainly. The I-40 grade is not steep enough to ever see a truck climbing at less than 40 mph. It's a very similar grade to the downhill part of Ricks leg of his trip which JoeS did such a good job of documenting the data on. Such a grade is not steep enough to gain anything by slowing down to 35 and trying to get some regen. I have not been able to do better (energy per mile) than mostly coasting at freeway speeds, compared to slowing down (35 mph) and taking route 66 for the same elevation descent. My original post was about the similar grade Rick had ahead of him. I know and still feel that he was given the wrong advice. So I put my two cents worth in, which I wouldn't do unless I knew it was correct. Counter intuitive perhaps, but demonstrably correct.

Is not the benchmark of extending range mi/kWh?

16 to 20 mi/kWh - in (D) maintaining constant speed downgrade.

178 to 184 mi/kWh - in (N) maintaining the same constant speed downgrade.

Same grade, same day, same speed, both states maintained for more than 1 minute each. A ten fold increase in the benchmark value for how far one can travel on a kWh - - by coasting in (N)! Doing some quick math based on my data, I could have done Ricks challenging 57 mile leg of his trip with two bars likely remaining. Most assuredly one bar. Sure there is 6 times the aero drag at the higher speed. The point is that however counter intuitive it may be. The car cannot effectively recapture that energy. So you waste it going a safe speed for the freeway or you fail to recapture it going an unsafe speed for the freeway. I stand by my data and experience, which says he wasn't given good advice for the circumstances. Enough said :|
2014 cool silver ES, acquired new 4/2015
2014 Labrador Black Pearl ES, acquired new 3/2016

JoeS
Site Moderator
Posts: 3070
Joined: Thu Dec 15, 2011 5:47 am
Location: Los Altos Hills, California

Re: Regen vs Neutral and speed for max range

Sun Apr 30, 2017 12:21 pm

Aerowhatt, I appreciate your detailed argument, supported by the data.

Inasmuch as both you and I drive very similarly, kicking the car into N on downhills and utilizing regen to slow the car down to match traffic or speed limits, I think we're pretty much on the same page.

The discussion on this thread centers about maximizing the range of the i-MiEV, with the specific issue being the use of regeneration vs. coasting in Neutral.

Let's see where we agree:

1. Agree that slowing down inordinately on a freeway to regen is way too dangerous to be practiced. Separate discussion, and sorry if someone thought I was advocating that.
2. Agree that at any fixed speed on a downhill that putting the car into Neutral consumes less energy than keeping it in Drive if having to apply power to keep the speed constant. Put another way, if a downhill grade results in the car holding a steady 65mph in Neutral, then if one were to put the car into D and then hold the accelerator such that the car goes 65mph then the car's instantaneous power consumption would be far greater than when it was is Neutral (and you provided the numbers to back that up)

Can we agree on the following scenario(?):

Let's say we have a steep ten-mile downhill run. If we start off at the top of the hill with 8 bars (50%SoC). If we put the car into Neutral and coast all the way down, we will still have 8 bars at the bottom of the hill; however, if we utilize Regen all the way down and arrive at the bottom of the hill with 10 bars, have we not just significantly increased the range of the car, allowing us to drive perhaps ten miles further because of the gain of two bars?

Let's perhaps bound the discussion:

1. We are only talking about maximizing range.
2. Vehicle speed (either average or max) is not a constraint
3. We can apply regen using D, Eco, and B, or zero regen using N, however we wish

I submit that the primary controlling variable is slope of the downhill.
The other variable is vehicle speed, recognizing that the i-MiEV's poor Cd means significant retardation as speed increases, but that we are free to go as fast or slow as we wish (ignoring traffic and safety constraints).

Perhaps we should iron out the ground rules before we continue?

Finally, the following graph is the best one I recall and is taken from this post that depicts the i-miEV range on level ground:

Image

Since we'll be talking numbers, perhaps going forward we should include both metric and archaic to cover our entire audience? :)
EVs: 2 Wht/Blu SE Prem., '13 Tesla S85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV

Aerowhatt
Posts: 306
Joined: Wed Jan 28, 2015 1:52 pm
Location: Albuquerque NM

Re: Regen vs Neutral and speed for max range

Sun Apr 30, 2017 4:07 pm

JoeS wrote:Can we agree on the following scenario(?):

Let's say we have a steep ten-mile downhill run. If we start off at the top of the hill with 8 bars (50%SoC). If we put the car into Neutral and coast all the way down, we will still have 8 bars at the bottom of the hill; however, if we utilize Regen all the way down and arrive at the bottom of the hill with 10 bars, have we not just significantly increased the range of the car, allowing us to drive perhaps ten miles further because of the gain of two bars?


Well . . . no we can't agree on this. If one can coast down a grade without needing to brake to slow to the speed limit (even if it is 65 mph) then using regen to slow to 30 will not yield any significant regen total at the bottom (if any). It takes a much steeper grade to make it worthwhile (one which is too steep to even think about trying to coast down). A big part of the reason is shown in my side by side (N) and (D) comparison which is compelling to say the least. The power used in (D) is not used to maintain speed at all, that is proven by the steady state coasting speed in (N). The power used is to create and maintain a rotating field in the motor allowing it to "freewheel" at that RPM with negligible torque in either direction.

Ok, so you take that and slow it down to 30mph on the same grade which will support the steady state (N) 65 mph coasting. In (D) with the accelerator depressed enough to maintain that 30 mph. Power is coming from regen yes and almost all of it is being used up creating the rotating field that makes the regen possible. In fact for my local grade I could not do as well in (D) at 35mph as I could in (N) at 65 mph (total power consumption) for the grade. No matter what I tried. The only possible way to do better would be to coast up to 65 in (N) and drop it into (D) and without pedal input regen down to ~25mph. Back to (N) and coast back up to 50 - 65 mph, rinse and repeat. Not really practical but it might yield a small net battery SOC gain.

The only grade that one can gain two bars worth of regen on has to be steep enough that the steady state coasting would normalize well above 80-90 mph! This type of grade requires no discussion, you either use regen to slow, damage your drive system, and/or crash.

Ancillary factors work against your position as well. If one takes twice as long to get there. They use the background system amp draw 1.2 to 2.3 amps (depending on what is on) for twice as long.

JoeS wrote:Let's perhaps bound the discussion:

1. We are only talking about maximizing range.
2. Vehicle speed (either average or max) is not a constraint
3. We can apply regen using D, Eco, and B, or zero regen using N, however we wish

I submit that the primary controlling variable is slope of the downhill.
The other variable is vehicle speed, recognizing that the i-MiEV's poor Cd means significant retardation as speed increases, but that we are free to go as fast or slow as we wish (ignoring traffic and safety constraints).


The primary variable is slope, or grade, I can agree with that!

I'm not going to venture in to the nuances of any non real world case studies. I don't need to. In the real world (N) coasting down a grade at the posted speed has a ten fold advantage over not coasting. If one needs to slow down to keep from speeding - regen as needed, of course ;) . Coasting (N) at speed also beats slowing to 35 - 40 and maintaining it (D) on the very same grade.

A 3,200 ft (elevation) descent of a very steep grade, maximum regen available being used to control the speed at 20 to 35mph. Only yields 2.3 kWh back into the battery or ~ 0.72kWh per 1000ft of elevation descent.

My (N) 65 mph coast has an elevation descent of ~700ft. Enough potential energy to regen 0.525kWh if (only if) it were less than half as many miles between beginning and end points, so that it would be steep enough to roughly match the average grade of the 3,200ft descent above. There are not 2 bars worth of energy to harvest at the efficiency of the regen system on the Imeiv! The existing (dare I say common) freeway grade does not allow for the harvesting of what is there because of the power needed to generate the rotating field that allows regen to happen . There in lies the rub, the alternative to coasting (for range maximizing) that you propose is not physically or electro-mechanically possible. It's a "non comparison" due to the facts.

Aerowhatt
2014 cool silver ES, acquired new 4/2015
2014 Labrador Black Pearl ES, acquired new 3/2016

JoeS
Site Moderator
Posts: 3070
Joined: Thu Dec 15, 2011 5:47 am
Location: Los Altos Hills, California

Re: Regen vs Neutral and speed for max range

Sun Apr 30, 2017 9:17 pm

Aerowhatt, I can tell this is going to be a protracted discussion, and I appreciate your presenting the math which I need to stare at a bit. One of the variables compounding the problem is the non-linear relationship of aerodynamic drag with speed.

In trying to find an example that might accompany this discussion, I went back to a trip I made five years ago soon after I bought the car (and before CaniOn) that might be of some use - it briefly shows the fuel gauge going up by one bar on a fairly short stretch of highway in the Santa Cruz mountains. There was nothing special about this trip, as it had no serious hypermiling associated with it nor was I trying to maximize regen as regen was simply used to slow the car down to keep from exceeding the 50mph speed limit in the hills. The datataking was simply me reading the pertinent i-MiEV gauges and Garmin GPS readings into a voice recorder. FWIW, here it is (I just added the GPS Visualizer graph and you need to click on the link to see the primary pdf graph):
http://myimiev.com/forum/viewtopic.php?f=28&t=269&p=993#p993

I'll be taking your comments into consideration and also try to take some CaniOn readings to see if I can develop a scenario that will quantitatively further develop this topic. For now, we are at an impasse. :cry:

Although the ball is in my court, everyone is welcome to join in.

To be continued...

[All this Marching for Science and Climate Change over the last two weekends has me motivated to play with the numbers!] :geek:
EVs: 2 Wht/Blu SE Prem., '13 Tesla S85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV

Return to “Driving the iMiev - Range - Energy Efficiency - Operating Cost”