JoeS
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Re: Regen vs Neutral and speed for max range

Tue Oct 30, 2018 6:06 am

Hi Don, thanks for commenting.
Don wrote:...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).
Don wrote: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'.
Image

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.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
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Aerowhatt
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Re: Regen vs Neutral and speed for max range

Tue Oct 30, 2018 11:24 am

JoeS wrote: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
2014 cool silver ES, acquired new 4/2015 (42.7ah at ~26K miles)
2014 Labrador Black Pearl ES, acquired new 3/2016 (41.5ah at ~15k miles)

JoeS
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Re: Regen vs Neutral and speed for max range

Wed Oct 31, 2018 11:17 am

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.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
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Don
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Re: Regen vs Neutral and speed for max range

Wed Oct 31, 2018 12:03 pm

. . . . 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
2012 iMiEV SE Premium, White
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bradleydavidgood777
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Re: Regen vs Neutral and speed for max range

Thu Nov 01, 2018 7:44 am

JoeS wrote: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.
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JoeS
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Re: Regen vs Neutral and speed for max range

Thu Nov 01, 2018 9:23 am

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:
bradleydavidgood777 wrote:...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.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
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PV1
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Re: Regen vs Neutral and speed for max range

Thu Nov 01, 2018 3:18 pm

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.
:idea: :idea: :idea: :!: :!:

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jray3
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Re: Regen vs Neutral and speed for max range

Fri Nov 02, 2018 10:27 am

PV1 wrote: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!
2012 i-SE "MR BEAN" 94,000 miles
2000 Mazda Miata EV, 78 kW, 17 kWh
1983 Grumman Kurbwatt EV,170 kW, 32 kWh
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bradleydavidgood777
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Re: Regen vs Neutral and speed for max range

Sat Nov 03, 2018 3:21 pm

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.
2017 I-Miev

Aerowhatt
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Re: Regen vs Neutral and speed for max range

Sat Nov 03, 2018 4:57 pm

JoeS wrote:
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
2014 cool silver ES, acquired new 4/2015 (42.7ah at ~26K miles)
2014 Labrador Black Pearl ES, acquired new 3/2016 (41.5ah at ~15k miles)

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