Regen vs Neutral and speed for max range

[JoeS]

Hi Don, thanks for commenting.

...It's easy to see where using (our) neutral down a long grade limited by aero and friction to whatever speed that gets you is less efficient than regenning and going down the long grade even just 3 or 4 mph slower will have you with a fuller battery at the bottom, no question . . . . but, what have you proved??

The whole discussion started off as to which technique allows the i-MiEV to go further. On this particular Lake Tahoe trip it wasn't critical for me, but during that Route 66 trip by Rick there were a few spots in the mountains where it was touch-and-go for him to get to the next charging spot, which is when I brought it up again (and IIRC was admonished by Aerowhatt about the truck traffic).

That an iMiEV has a terrible Cd - We knew that already!...

Yes, and thus slowing down to, say, 35 mph regenning should offer a significant advantage over letting her roar along at whatever aero-limited speed a grade provides, vaporizing all that stored energy instead of stuffing it back into the battery. I'll keep looking for some uniform long downhills closer to home to see if there is something that will allow the car to coast without exceeding 70 mph and where I can (on an early Sunday morning in no traffic?) repeat the experiment holding the speed down. The one hill I found has a perfect grade, except it's too short.

Very early in my i-MiEV ownership (way before CaniOn) I recorded and plotted the fuel gauge bar on Highway 17 going up and over the Santa Cruz 'mountains'.

Allied to this, the Tesla Model S (but not the Model 3) has a wonderful predictive graph display that shows the SoC going back up on serious long downhills. I had taken some photos of the screen in Colorado and didn't realize that I hadn't posted them here - when I get home I'll dig them up.

 

[Aerowhatt]

So, let's revisit this without limiting constraints -

For any serious downhill, we have two competing conditions, and are asking which is the more efficient:

Coast in Neutral vs. limit car speed using Regen

1. One argument is that the car's electronics draws significant power when the motor is engaged, negating the power gained using regen. A quick check of quiescent power draw: when the car is stopped, foot on the brake, shifting from Neutral into Drive results in no change in the amount of current drawn by the entire i-MiEV system (about 1.2A as seen by CaniOn), with the car not moving.

2. If the car is moving and regenerating, it is indeed stuffing current into the battery and increasing the State of Charge of the battery, despite any losses it may have with the drivetrain engaged. OTOH, in Neutral, the car continues drawing its 1.2A.

3. At the bottom of a steep 20-mile downhill, if the car had been in Neutral the whole time (speed unchecked), the SoC would be unchanged (or maybe even lose a bit) compared to what it was at the top of the hill.

4. OTOH, going down that steep 20-mile grade and slowing down the car using Regen will result in a significant increase in SoC by the time the car gets to the bottom of the hill. Like I tell people, it's fun watching the fuel gauge go UP when going downhill!

Thus, I still contend, on downhills it is more efficient to regen than to coast.

JoeS,
It's amazing to me how much humans are invested in our beliefs. Even if facts show us those beliefs are flawed. Intractable beliefs that were toned down significantly (open to new information) would make the world a much better place overall!

I think you are missing the point and stating it at the same time. In your post you stated how you couldn't do what you wanted to because of traffic, which is always the case right? So does it even matter if something that you can't do because of safety and circumstances is more efficient or not?? And even if it does matter (to anyone) The hyper-miling thread has documented proof that using coasting as much as feasible is 20% more efficient on the test stretch of freeway. In that ~4 miles I was in and out of neutral a few times to slow or speed up so that I could match the traffic/speed limit/average speed of each comparative run.
I think I can safely say that I have sufficient experience making over 280 trips in my i Miev to the local mountains climbing at least 2600ft and ranging up to 5000ft in elevation on each trip depending on where we hike that day. Then descending that same elevation to get home. On the mountain "highway" NM 536 there are many switch backs with 20mph 180 degree curves. It makes no sense to shift into N there, the car runs away in just a few seconds. On N-14 on the other hand once a short hill is topped almost the whole way to I-40 can be rolled down in N to I-40 without speeding enough to get a ticket or slowing enough to be a nuisance. Doing this takes .5 to 1% off of the SOC. Riding the same stretch at the same speeds using Don's zero needle technique while in D results in 4.5 to 5% off the SOC. The power being used in "gear" at a stop is not comparable to the energy used to maintain a "zero torque" rotating field at speed. It's takes more power to have the "zero torque" field (rotating) existing at speed. How much more? I don't know and can not measure it with the cars reporting. To maintain a zero needle indication the 1.2 amps that you mentioned plus whatever is needed to maintain the active rotating field (no discernible acceleration or deceleration being provided) results in ~ 4% more SOC used for that same stretch of road. I've even tried doing that whole decent at 45mph in D getting some regen here and there. Instead of letting the car roll up to 58mph where it will in N. It still could not manage the lower power usage in N most of the way. Although it did do a little bit better than the zero needle experiments. I've tried these different techniques a number of times with comparable results. I haven't done a back to back comparison, like I did on the freeway stretch and I probably won't because in order to do it I would have to skip the hiking to save power in order to do that stretch 2 or 3 times in a row. It's just a fact in my mind and I think it should be in anyone's, considering the evidence on the hyper-miling thread. It would take some pretty specific (unrealistic) conditions to make the numbers go the other way. I am unable to imagine a real world situation where they would.

Aerowhatt

 

[JoeS]

Hi, sending this from Portland Oregon...

Aerowhatt, thanks so much for responding - I really do value your informed opinions.

As I see it there are three elements to this discussion -

a) Putting the car in Neutral vs. "zero-consumption" accelerator pedal position while in D or E or B
b) Going downhill in Neutral vs. continuous regeneration
c) Advisability of doing so

a) For very good technical reasons that Aerowhatt presented, having the car in Neutral with the motor free-wheeling is NOT the same as having the motor and its driving circuitry engaged. I agree, and the point (1) that I made in my example above was a curiosity I had noted and not a supporting argument, and I was remiss in even bringing this up. I'd be interested in seeing how we could graphically represent where the zero-power zero-regen spot (as seen on CaniOn) occurs (when in D) compared to the car coasting in Neutral, to add emphasis to Aerowhatt's argument.

b) All other constraints aside, arriving a the bottom of the hill while continuously regenerating will result in a higher battery State of Charge than coasting down the hill exclusively in Neutral; ergo, in ideal conditions, I contend that we can go further regenerating instead of coasting in N.

c) As far as the advisability of doing this I agree, that doing these regen machinations on an Interstate in traffic is simply too dangerous to employ, and thus it is not a viable alternative (remember, a few years ago I was rear-ended by a drunk driver going >>80mph while I was traveling 60mph on a 70mph Interstate in my Gen1 Honda Insight); however, on sparsely-populated country side roads with negligible traffic I contend this could indeed be a technique to employ if one really really needed to extend the car's range - the magnitude of this range improvement is simply a function of length and angle of the slope.

From a practical standpoint, out of habit, on virtually every downhill undulation with the i-MiEV I kick it into Neutral. On any significant downhill I continue to manually alternate between N-D-E-B with my foot off the accelerator, in order to keep the car indistinguishable from the flow of traffic around me. In this sense, I'd be willing to bet that Aerowhatt and I both drive the car very similarly.

Thank you all for suffering through these discussions with us.

 

[Don]

. . . . or, you can just put it in D, E or B and drive it like a normal car!

We have always used B. When traffic slows, we lift and try to blend in with the speed everyone else is moving, or slowing. Anticipating the need to slow and allowing regen to do it before I run up on the car in front of me has become a natural part of driving these cars for us. I find this has me touching the brake pedal less than using any other method and have always assumed that less friction braking automatically means less energy wasted and therefore, greater range. I'm very happy to hear Joe say using regen is the most efficient way to go down a hill, because that's how we've always done it

Constantly shifting from whatever gear to neutral and back is just a hassle I never embraced, partly because we're always in B and that makes the shift to neutral more complicated, maybe involving taking eyes off the road to make sure you don't go one notch too far and wind up in R!!!

Since we very seldom ever get anywhere near the max range of the car in our daily travels, putting it in B and forgetting about it has always served us well . . . . even if it turns out it's not the absolute least energy way to drive the car

Don

 

[bradleydavidgood777]

Hi, sending this from Portland Oregon...

b) All other constraints aside, arriving a the bottom of the hill while continuously regenerating will result in a higher battery State of Charge than coasting down the hill exclusively in Neutral; ergo, in ideal conditions, I contend that we can go further regenerating instead of coasting in N.

I don't think this is necessarily true depending on the hill and depending on whether braking is necessary. If it is a long hill and using N, I can go farther in N than by using any regen, because by using regen, I lose energy (about 50% or whatever). But by coasting, I go farther with that energy. I think that this is the most important point to realize.

I usually drive in D and find it very efficient. But when I need to go far and am pushing it, I use N on all long gradual downhill sections because there is very little loss of overall energy propelling you down the road.

The way I look at it is that every time you engage regen vs coasting in N, you lose 50% of energy that could have propelled you down the road. This is due to loss from the drivetrain parts and the efficiency of the regen components or whatever...everything that you engage is way less efficient than simply coasting farther like a soapbox derby racer.

 

[JoeS]

bradleydavidgood777, you need to be very careful when using numbers, especially after your previous criticism of using them (in a post which I had found very offensive and caused me to subsequently delete the pre-Tahoe trip data-gathering thread I had started).

For example, this statement is flat-out wrong, as demonstrated by a higher measured State of Charge at the bottom of the hill when using Regen instead of coasting in Neutral:

...If it is a long hill and using N, I can go farther in N than by using any regen, because by using regen, I lose energy (about 50% or whatever).

That said, you are re-introducing a variable which Aerowhatt also alluded to: Rollout at the bottom of the hill.

Remember, in my definition of measuring efficiency I took a snapshot at the top of the hill and a snapshot at the bottom of the hill. Yes, the SoC at that instant is demonstratively (measured) higher than at the top of the hill for the car that is regenning, but now we should perhaps discuss what happens after that instant of distance, because the two vehicles are presumably moving at different velocities. To do that, I'd like to suggest a number of different scenarios:

1) There is a stop sign at the bottom of the hill
2) The roadway turns into an instant climb
3) The roadway flattens out and both vehicles resume travel at a defined speed.

I need to stop here and get back on the road, but I'll leave this discussion open to further quantitative analysis with the reminder that this discussion was all spurred on by my trying to capture the energy otherwise dissipated by the faster-traveling car and stuff that energy back into the battery.

 

[PV1]

I think what's getting lost in the conversation is the steepness of the hill (grade). Bradley's post seems to relate to slight downgrades where the i-MiEV can maintain or very slightly gain speed while coasting, and therefore regen has almost no excess energy to recover. In this case, best to coast as having the inverter active will use up most of the energy being recovered. The final approach to my road has about a half mile of a slight downgrade where coasting maintains speed and leaving the car in a drive mode burns energy. Obviously regen is used to slow and make the turn, I coast just for the section I need to maintain speed.

If the car is going to gain more than 5 MPH, then regen has something worth recovering, in which case using regen to maintain speed will extend range. The hill before the final approach to my road is a perfect example of this. It is steep enough where the car can start at the top at 40 MPH and be doing over 60 MPH at the bottom by coasting (50 MPH speed limit). Here, I regen to limit speed gain, and this is more efficient than rolling out the speed on the level. The i-MiEV is too light and has too much drag to really take advantage of this. Remember, aero drag increases exponentially in relation to speed.

 

[jray3]

I regen to limit speed gain, and this is more efficient than rolling out the speed on the level. The i-MiEV is too light and has too much drag to really take advantage of this. Remember, aero drag increases exponentially in relation to speed.

Absolutely correct-, PV1. From my mountain highway EV driving experience, I'm convinced that the ideal situation would be to use regen to limit downhill speed (all the way down to 35-45 mph if possible). The car's light weight and high drag prevent it from coasting much over 70 mph even on a steep downhill grade, and it doesn't coast very far on the level before needing some power to avoid becoming a semi truck's speed bump...

In short, regen is the best way to limit or reduce speed, and momentum should be retained whenever possible. If it comes down to choosing whether to speed or regen, regen is better and safer in most circumstances. If coasting can carry you through at lower speeds, roll on!

 

[bradleydavidgood777]

If you are offended by some opinion that I offered, please let me know by private message. I don't know what you are talking about. I thought we all could have whatever opinion that we wanted as long as we were tactful. I don't remember being offensive or what you are talking about.

About numbers, I was being careful, that's why I said "or whatever". Because it doesn't matter what the number is to my point. My point is that is my opinion is that it is not necessarily true that regen will get you farther down the road than coasting on all hills. I think PV1's post explains it.

 

[Aerowhatt]

b) All other constraints aside, arriving a the bottom of the hill while continuously regenerating will result in a higher battery State of Charge than coasting down the hill exclusively in Neutral; ergo, in ideal conditions, I contend that we can go further regenerating instead of coasting in N.

I don't understand the intractable attachment to this idea. It simply cannot be quantitatively shown in practice. I have done more data collecting for the suppositions brought to the discussion than anyone and have not been able to beat coasting (N) when appropriate, compared to remaining in a drive gear. Copied above is a supposition based on opinion, there are no facts, no data, to back it up. The facts are that regen is around 60% better than friction braking for range extension. Coasting in N is practically free. If you don't need to speed up, or slow, down it is THE best choice for extending range. I didn't set out to prove my supposition by collecting the data. I did so to find out for myself which was my best choice for the driving that I was/am doing. When I get to the higher altitude hiking trails that we go to, the RR is 1 to 4 miles upon arrival . . . and . . . I'm 34 miles from home!! My only Dog in this fight is getting home each and every time without running out of juice! Without regen to slow the roll (instead of friction brakes) on the steep mountain road, I wouldn't have a prayer (so it has very great value). Without using N where it makes sense I would need to pray every time. That (quantitatively documented) 20% efficiency advantage over remaining in gear for ~40% of the drive home gets the job done nicely with no praying needed.

Aerowhatt

 

[JoeS]

b) All other constraints aside, arriving a the bottom of the hill while continuously regenerating will result in a higher battery State of Charge than coasting down the hill exclusively in Neutral; ergo, in ideal conditions, I contend that we can go further regenerating instead of coasting in N.

I don't understand the intractable attachment to this idea. It simply cannot be quantitatively shown in practice...

Um, I had proposed that the way we would compare these two techniques was by reading the State of Charge at the bottom of the hill compared to SoC at the top of the hill. I submit that, all other things being equal, the vehicle with the higher State of Charge at the bottom of the hill will subsequently travel further.

Aerowhatt -
I take it you don't agree with this?
Would CaniOn Wh/km or mi/kWh over that distance be preferable?
What other measurable definition would you like to propose?

I see that in your subsequent discussion that you are including regeneration as part of your downhill driving to slow the car down (instead of simply aerodynamic drag). That is a change in the ground rules and is not the case we have on the table. If that's what you meant by "It simply cannot be quantitatively shown in practice...", then we might as well fall back and redefine our ground rules.

I thought that our basic bone of contention is energy loss due to aerodynamic drag for the higher-speed vehicle vs. regenerating inefficiency with drivetrain engaged for the lower-speed vehicle.

 

[Aerowhatt]

I'm providing a proposed conclusion born of extensive data collection which all matches up well.

http://myimiev.com/forum/viewtopic.php?f=28&t=287&p=36475#p36475

You are putting forth a supposition based on your convictions only. I invite you to take it to the street and find this mythical hill which will give you the data that you wish to see. Bring back the screen shots for peer review and then there are grounds for a discussion. A discussion, which from my experience sussing out the most efficient way to drive the car in variable real world conditions. Would be purely academic since nobody seems to know of a road/hill that will get the result that you seek without risking safety, or a moving violation.
Personally I think you do a disservice to less tech savvy owners/drivers of the car persisting in presumably attempting to undermine the veracity of facts supported with accurate documented data.
Is anybody else regularly taking drives in this car that are 68 to 70 miles and have an elevation change spanning 5,000 ft (5,000ft up and 5000ft down) on a single charge and arriving back at the point of origin with range to spare?? The car has only seen turtle once when changing weather conditions and a long hike provided a moderate headwind for both directions of the trip.

Aerowhatt

 

[JoeS]

Aerowhatt, I am saddened by the vehemence in your remarks because, like the man in the movie said, "what we have here is a failure to communicate".

Here's where we agree: on any downhill, be it a 5000-ft mountain drop or the backside of a highway overpass, judiciously alternating between N and D (or even E and B to avoid going too fast) will result in optimizing the car's distance traveled/charge.

You and I both do this, and we'll see who's first to wear out the shift lever.

On downhills, neither of us advocates leaving the car in "gear" continuously, be it D or E or B, and attempting to "feather" the gauge at the "zero-energy" point. I personally find it impossible to find and hold that "zero-energy" spot (somewhere between green and blue on our power meter).

You have aptly demonstrated this by showing the CaniOn data for the five-mile stretch, the metric of interest being miles/kWh, showing the differences among D-N, D, E, and B. I note that your average speed on these downhills is around 60mph, which is what I also usually hold the car to unless surrounding traffic dictates otherwise.

Once again, we are in complete agreement on the coasting technique, each of us having espoused and advocated it in numerous posts on this website for years.

So, what's all the fuss about?

What I've unsuccessfully been trying to convey is that we have an even more subtle subset within this advocacy discussion.

Specifically, on a steep downhill (say, >6% grade) where we can alternate between N and the other settings to maintain a constant speed during this downhill, all I'm saying is that we will get a better result by maintaining a lower speed. By "better result" I mean a lower Wh/mi or Wh/km or higher mi/kWh or km/kWh for that particular stretch of road.

My basis for this argument is that aerodynamic drag at the lower speed will be lower, thus allowing us to stuff a little more energy back in the battery at the lower speed than at the higher speed (since the car has a higher retarding force at the higher speed).

In an attempt to demonstrate this I tried to set up an extreme case whereby one car merely coasts while the other regens continuously and I thought that coasting would show no change in SoC whereas regenning would show an increase in the SoC at the bottom of the hill. Notably, as shown by your arguments and my own recent experience, this scenario is proving to be unrealistic - I haven't found a hill long enough and deserted enough around here to demonstrate this. I am sorry that this extreme example attempt on my part became misconstrued.

So, where are we?

Aerowhatt has shown data comparing the N-D (and E and B?) technique vs. leaving the car in D or B or E, at the same constant speed.

I would like to do the same, but using the N-D-E-B technique comparing two different constant speeds: say, 70mph vs. 50mph.

In real life, during the last 2-1/2 years and 60,000 miles that I've criss-crossed this country and made numerous West Coast trips in the Tesla I have often had the opportunity when facing a 6%, 7%, or even 9% downhill to select the cruise control speed at which to go down those grades and have actually slowed down a few times when I was running a bit low. Did it do any good? - I dunno. Hmmm, now that I think about it, I could do a controlled test in the Tesla even though it has a much lower Cd than the i-MiEV and the result might just be measurable, and it just might work... now, all I have to do is convince my wife that after we've gone down the hill that we're going to go back up and do it all over again! :roll:

To close this, once again, Aerowhatt - you and I are in violent agreement that coasting is superior to leaving the car "in gear". I'm simply trying to say that going downhill slower is ever-so-slightly better.

 

[PV1]

Joe, thanks for clearing up your talking points. After reading through the last few pages, it's been tough for me to even follow what was disputed. It will be interesting to see results of regenning down a hill at different speeds. Be careful out there.

 

[Don]

It's pretty obvious to me that if you coast down the hill in N and the aero drag limits your speed to 75, that regenning just a little bit in D and using that to limit your speed to 70 will have you with more energy in the pack at the bottom of the hill than coasting. Instead of wasting energy to aero drag and using the car's quiescent 1.2 amps (or whatever it is) you're getting something back from regen and you're getting to the bottom of the hill at roughly the same time . . . . and maybe even avoiding a speeding ticket

It is nice to have a cruise control (like my Volt and your Tesla have) which makes this an 'automatic' exercise

Am I missing something? Why do we even need to try to 'prove' this??

Don

 

[Aerowhatt]

I'm sorry if my conviction level hit you wrong Joes. I get frustrated with supposition being held up against fact. For people who are not tech savvy, really can't see the difference. I appreciate your clarification of your viewpoint on the most efficient way to drive. Enclosed is a dizzying amount of information on this topic. I'm posting it because My wife asked me about my frustration and I told her that I was using our mountain trips as an example. She said I would be surprised if anyone believes that we take those drives. I wouldn't believe it if I wasn't right there with you and I drive the i miev every day too.

So hear we go. the following is screen shots of canion and GPS data of the drive to and from the 10K trial head in the Sandia mountains and back from our home.

View attachment 10k Trail head up only.png

OK so that's the way up. canion screens at the house and at the trail head follow:

View attachment canion one.png

View attachment canion two.png

As we can see the trip so far has covered 35.9 miles and used 87.5% of the useable battery capacity. Notice the RR = 1

 

[Aerowhatt]

The trip down and back home:

View attachment 10k trail head down only .png

View attachment canion three.png

Interesting is the screen below which was grabbed at the bottom of the steep mountain road:

View attachment canion four.png

It shows an SOC of 24.5% compared to the 12.5% at the top After 11miles and 3000ft of altitude descended 12% of SOC has been gained maximizing regen all the way and no friction braking whatsoever. This same stretch going up took the SOC from 54.5% at the base to 12.5% reducing it by 42%.

So 42% SOC to climb and 12% returned on the way down 12/42 = 28.6% of the power to climb was returned by the regen on the same mountain road. As you can see the speed on this stretch of road is quite low, average is certainly < 30mph. So regen is critical and also relatively less effective than we would like, at the same time. I point this out because I think it shows that regen is inefficient enough that it would take a pretty big difference in speed to do better with regen than coasting at a faster speed. It does go against "common sense" or expectations. If there is any advantage to going 30 mph in gear down a hill that will support a coasting speed of 60 mph, it would be small at best. One or two percent SOC increase for several miles of hill. The problem is that there are no hills where 30 mph or 60 mph on the same road are permissible, let alone safe. So what is the point of discussing it and muddying the waters of definitive range extending advice for others?? That's what is frustrating me. I'll make this my last word on the thread unless someone has questions, related to what I have posted.

Aerowhatt

 

[jray3]

Excellent. That was quite a trip.

 

[Don]

So 42% SOC to climb and 12% returned on the way down 12/42 = 28.6% of the power to climb was returned by the regen on the same mountain road. As you can see the speed on this stretch of road is quite low, average is certainly < 30mph. So regen is critical and also relatively less effective than we would like, at the same time. I point this out because I think it shows that regen is inefficient enough that it would take a pretty big difference in speed to do better with regen than coasting at a faster speed. It does go against "common sense" or expectations. If there is any advantage to going 30 mph in gear down a hill that will support a coasting speed of 60 mph, it would be small at best.

So . . . . now I'm completely lost . . . . but I get lost easy, so no worries there

You went up the hill and used X amount. You regenerated going down the hill at about the same speed and got a little less than 1/3rd of the amount of energy you used climbing the hill back by regenning. I understand that part and agree the return is certainly less than we would like . . . . but it is still something - Better than coasting and getting nothing, no??

But, where I get lost is . . . . you suggest that not regenning and coasting down the hill at twice the speed in N is going to be more or less just as efficient? You're not going to have that 1/3rd amount of energy you used going up the hill back in your pack . . . . are you? You'll get to the bottom sooner, but with less energy left to resume the drive home . . . . or, am I completely missing the point of this exercise?

Don

 

[Aerowhatt]

You went up the hill and used X amount. You regenerated going down the hill at about the same speed and got a little less than 1/3rd of the amount of energy you used climbing the hill back by regenning. I understand that part and agree the return is certainly less than we would like . . . . but it is still something - Better than coasting and getting nothing, no??

But, where I get lost is . . . . you suggest that not regenning and coasting down the hill at twice the speed in N is going to be more or less just as efficient? You're not going to have that 1/3rd amount of energy you used going up the hill back in your pack . . . . are you? You'll get to the bottom sooner, but with less energy left to resume the drive home . . . . or, am I completely missing the point of this exercise?

Don

OK so the point is that this grade starts out at -13.5 % I actually pull out of the parking area in N to avoid using any power. You must put the car in a drive gear within 150 ft or you will be going to fast for the first switchback corner. On a grade like that if all I can get from regen is a tad more than 1% SOC per mile then a lesser grade (one that you could coast down without exceeding 80 mph) is going to yeild much less power per mile. According to the math 6% grade would yield less than half of the SOC gain per mile. It's not worth talking about it, is the point. Regen when you need to for speed control otherwise coast coast coast.

Aerowhatt