Coasting question

Assuming it is legal,...
let's say you are on an open highway, and going down a fairly steep incline and do not need to accelerate to keep up speed, with no end in sight...would it be better to shift into neutral and coast rather than simply taking your foot off the accelerator which would engage regenerative braking?

Even though you would forego the regeneration, isn't it more efficient to take advantage of gravity and momentum rather than slowing the car by taking your foot off the accelerator?

I suppose another way of doing this would be to simply lighten up your foot and keep the Eco meter right above the charge zone but not quite into the Eco zone?

I think regen (b mode) should be used only if you need to slow down or stop. The energy you lose when you eventually accelerate again is greater than the energy you gain from regeneration. So if you're coasting downhill and the car doesn't exceed the speed limit, don't use b mode.

I agree with Tonymil,

I also don't think that keeping the needle between the two zones is as efficient as coasting, at least it doesn't feel that way to me.

Jenn

Hi fjpod, I was in a hurry and my post was abbreviated. Edited to add the following -
To answer your question: to maximize driving range it is always best to coast and keep the car out of regen.
Only use regen if you need to slow down.
You can coast by trying to balance the needle between the green and blue zones (I can't do that successfully without being unduly distracted) or simply pop the car into neutral.
Coasting was exhaustively covered here:
http://myimiev.com/forum/viewtopic.php?p=1210#p1210

I would keep in it N until I picked up too much speed at which time I would shift to D to regen until slowing down to speed limit and repeat this process which could possibly regain more regen over straight B mode (debatable) but would get you where you were going faster! Any better options?

For almost the two years and 22Kmiles I've owned our iMiEV, shifting into N and coasting has been in my standard bag of hypermiling tricks. As I've mentioned earlier, I find it impossible to balance the red power gauge needle in the "zero" consumption point. Not only that, but this "zero" point does not indicate zero power draw if the car has heat or a/c turned on, as it reflects regeneration to counteract those significant power consumers. What I recently discovered with CaniOn is that putting the car into N reduces the car's power draw by at least 0.1A, or around 35W, further helping our range even if by a tiny amount.

Once again, the iMiEV manual specifically warns against this practice, although I had been told that the reason for this is the fear of inadvertently shifting the car into R. The Mitsu manual also warns against towing the iMiEV with the rear wheels on the ground, presumably to avoid spinning the direct-coupled motor despite it not being energized.

While perusing the Australian Electric Vehicle Association iMiEV forum <http://forums.aeva.asn.au/mitsubishi-imiev_forum49.html> I came across this posting by acmotor:

"…putting the vehicle in neutral to cruise is technically bad for the controller when using a pmac motor since it is a passive generator and the controller must deal with the generated emf, with a full battery in particular this is an issue…"

Can anyone technically familiar with our iMiEV's ac motor and controller comment?

BTW, here's the link to the details of hypermiling employing coasting in N. http://myimiev.com/forum/viewtopic.php?p=1210#p1210

I used N in the past, but now i don't usually use it, because with canion i see that my cells became unbalanced at low SoC if I use it. Charging when I drive helps my cells to be balanced, and it's easier to get into turtle before the car stops.

Malm said:

I used N in the past, but now i don't usually use it, because with canion i see that my cells became unbalanced at low SoC if I use it. Charging when I drive helps my cells to be balanced, and it's easier to get into turtle before the car stops.

Click to expand...

Malm, I am having difficulty undestanding the logic in what you just wrote. How can drawing less current when coasting in neutral be deleterious to the battery, especially when you're at a low SoC? BTW, getting down to low SoC and turtle with a top-balanced battery pack (such as our iMiEV) certainly cannot be healthy for the cells, and drawing significant current in that region is even more unhealthy.

Back to my question: can anyone with motor/controller experience comment on the effects of freely-spinning our iMiEV motor?

“I suppose another way of doing this would be to simply lighten up your foot and keep the Eco meter right above the charge zone but not quite into the Eco zone?”

That is what I do. As Joe pointed out, that only works w/o the AC or Heater running since that’s also drawing current.

To reduce wear-n-tear on my shifter, I’ve been mimicking all the functions (N D ECO B) all in the B mode. And I can keep my red needle balanced between the two zones – for a long while—to mimic neutral. It’s all in the wrist --- I mean ankle. I haven’t had troubles doing it.

“I used N in the past, but now i don't usually use it, because with canion i see that my cells became unbalanced at low SoC if I use it. Charging when I drive helps my cells to be balanced, and it's easier to get into turtle before the car stops.”

Wow Malm. That is interesting! I wonder if that implies that just letting our batteries drain naturally, some batteries will drain faster than others; thus becoming unbalanced. But if we always shove some regenerative breaking in there, it will keep the batteries somewhat balanced while discharging.

-Barry

"Wow Malm. That is interesting! I wonder if that implies that just letting our batteries drain naturally, some batteries will drain faster than others; thus becoming unbalanced. But if we always shove some regenerative breaking in there, it will keep the batteries somewhat balanced while discharging."

You get my point.

Now I use N only when i know that I can use regenerative breaking for a while later, to re-balance the cells. I see it clearly at SoC below 30%.

I don't usually go under 30% of SoC, but when it happens I want to be sure that I can count on my turtle, and she maybe not be there when cells are unbalanced.

Another thing that I see is that peripheral cells, in the morning (when it is outside at night) have lower temperatures, and that unbalances the battery too. Voltage of the coldest cells tends to be lower.

As I showed, sometimes i can get 100% SoC after a full charge. This not happens often. Usually only goes to 95,5%. When I get 100% SoC, the 16 last as it was new. But when it gets to 60% and i stop for a while, when i start the car it will have just 55%. So, 5% disappears. This is symptom of 5% degradation. My i-MiEV has lost at least 5% of it's original capacity, that's for sure.

2 years and 8 months old is my i-MiEV, i think we can expect 20% after 5 years in a country like Portugal. On track, I suppose.

The canary keeps singing. Maybe, one of this days, i'll beat my personal record of longer distance with one charge, so, more then 201,4 km and if I want I can get 175 mRR, like i've done earlier.

BarryP said:

“I suppose another way of doing this would be to simply lighten up your foot and keep the Eco meter right above the charge zone but not quite into the Eco zone?”

That is what I do. As Joe pointed out, that only works w/o the AC or Heater running since that’s also drawing current.

To reduce wear-n-tear on my shifter, I’ve been mimicking all the functions (N D ECO B) all in the B mode. And I can keep my red needle balanced between the two zones – for a long while—to mimic neutral. It’s all in the wrist --- I mean ankle. I haven’t had troubles doing it.

Click to expand...

Same here

In 20 months and about 14,000 miles, I've driven 100% of it in B mode with no shifting

If I go uphill and the speed limit is 35 mph, I press the pedal just enough to maintain 35 up the hill. Down the other side, I balance the needle and let it coast, unless I get to going too fast, in which case I release the pedal to the proper position to maintain 35 mph with a little engine braking. It takes no 'thinking' on my part because this is exactly how I've driven all my manual transmission cars for my entire life. The engine and wheels are directly connected (same as the iMiEV) so when you release the pedal completely, you're engine braking and slowing at about the same rate as the iMiEV in the B mode

If shifting to Neutral really was a true Neutral, I might think about coasting that way, but since the wheels and engine are always connected, I just make it a habit to properly balance the pedal for what I need - Slight acceleration, slight deceleration, or 'coast'

Don

I could not find much difference between "D", "B" and "C" but each of them needs adapting the driver. Select yours and stay with it might gain you more than shifting.

I have chosen "B" because it is close to single pedal driving, almost no need for the breaks. We are living in the hills. Going down without regen I have to use the breaks or I wont make it around corners and roundabouts.

Very rarely in the flat lands I have tried "C" but it did gain me nothing.

Cheers
Peter and Karin

peterdambier said:

I have chosen "B" because it is close to single pedal driving, almost no need for the breaks

Click to expand...

Good point

I have a Kubota diesel garden tractor which has a hydrostatic transmission - Truly a 'one pedal' operation. One pedal for forward and reverse also controls speed in forward and reverse - The farther you press it the faster you go and you can almost instantly reverse your travel . . . . the rear wheels can be spinning in reverse while you're still going forward if you like :lol:

It also has a brake pedal but you don't really need it - A slight touch of reverse slows you down in a big hurry . . . . much faster than lifting your foot off the 'gas' and jabbing on the brake pedal. You do need the brake pedal though because the federal government says I must lock it before I can get off the tractor without the engine stopping

Driving the iMiEV is similar to me in that the one pedal can do it all, except for coming to a complete stop of course. If you maintain proper spacing and anticipate traffic lights ypu can go many, many miles even in suburban traffic and never need the brake pedal

Don

What I recently discovered with CaniOn is that putting the car into N reduces the car's power draw by at least 0.1A, or around 35W, further helping our range even if by a tiny amount.

Click to expand...

I've noticed driving beside a retaining wall around 20 mph, while in a drive mode, you can hear the inverter, but as soon as you shift into neutral, the inverter goes silent. This would explain the current drop. Neutral is truly zero current drive. This also opens up the possibility of wild voltages coming in from the motor, which is where the concern comes in, I guess?

Once again, here's the quote from the AEVA:

"…putting the vehicle in neutral to cruise is technically bad for the controller when using a pmac motor since it is a passive generator and the controller must deal with the generated emf, with a full battery in particular this is an issue…"

Click to expand...

Now, my Gen1 Honda Insight hybrid's ac motor is free-wheeling most of the time, and on the iMiEV if we hold the red needle at the zero-current position the motor is also spinning and doing nothing, so what's the difference if we put it into Neutral? As far as I know, putting it into Neutral does not open up a relay or switch to physically separate the motor from the controller or the controller from the battery, and the bi-directional controller inherently has to be designed to operate in these consumption/generation transitions.

I'll repeat my question seeking technical help: can anyone technically familiar with our iMiEV's ac motor and controller comment on the above quote?

interesting whether coasting in N could harm anything; I do it a lot. SWOT, consensus here was that shifting from R to D while still rolling backwards SLOWLY wouldn't cause damage, but I have managed to "crash" the inverter once through attempted speed-shifting from R to D while still rolling. I had to cycle the keyswitch for a restart. No other apparent ill effects, but I've more carefully coordinated the right foot with right hand ever since....

JoeS said:

Once again, here's the quote from the AEVA:

"…putting the vehicle in neutral to cruise is technically bad for the controller when using a pmac motor since it is a passive generator and the controller must deal with the generated emf, with a full battery in particular this is an issue…"

Click to expand...

Now, my Gen1 Honda Insight hybrid's ac motor is free-wheeling most of the time, and on the iMiEV if we hold the red needle at the zero-current position the motor is also spinning and doing nothing, so what's the difference if we put it into Neutral? As far as I know, putting it into Neutral does not open up a relay or switch to physically separate the motor from the controller or the controller from the battery, and the bi-directional controller inherently has to be designed to operate in these consumption/generation transitions.

I'll repeat my question seeking technical help: can anyone technically familiar with our iMiEV's ac motor and controller comment on the above quote?

Click to expand...

Sorry to bump this old thread. I was looking for the right thread to post something else and ran across this.

I think whoever was responsible for the quote is conflating some issues. A PMAC motor like in the "i" is always creating back EMF when in motion. Whether accelerating, decelerating, or throttled to zero torque there is always back EMF being produced if the motor is turning. As you pointed out JoeS, when throttling to zero torque there is no current (not from the inverter anyway) in the stator windings. Exactly the same as coasting in neutral at zero throttle. The electronic drive components have to be designed to handle the back EMF through a combination of blocking and damping it. In some systems the back EMF is used by the electronics to judge rotor speed quite accurately. There would likely be a problem if the car were coasting over it's top speed 82 MPH in neutral. Headroom is built in of course but the faster the rotor turns the more EMF voltage. So somewhere above 82mph you would exceed the controllers ability to protect itself from the back EMF. Where? . . . who knows . . . 10,000 RPM kinda gives me pause on mechanical grounds. So I'm not going to worry about the EMF. Since the controller must deal with back EMF anytime the car is moving the only way to avoid the wear and tear is to not drive it!

Car manuals don't want you coasting in neutral in general. Reason being that you have no acceleration readily available if you need it. For the "i" or a stick shift ICE there is the added adjustment of not having hold back when lifting out of the throttle so a different, if not reduced ability to slow down. That said I coast whenever conditions warrant it and traffic allows a little extra room for error.

For good counting sheep reading related to the subject see the following;
http://machinedesign.com/motorsdrives/whats-difference-between-ac-induction-permanent-magnet-and-servomotor-technologies

Aerowhatt

I think that if the controller had a problem with neutral coasting, we'd have seen it already.

That being said, there is a difference between simply being in a drive mode vs. neutral. There are parts of the controller that are active in a drive mode that aren't in neutral and park, regardless of motion. On my cars, I can hear some coil whine when stopped and in a drive mode (pushing hard enough on the brake pedal to shut creep off) that I can't hear in neutral. Whether or not that makes a difference, I don't know. I tend to think it is less stressful on the controller to simply coast in neutral rather than constantly floating between power and regen.

Now, coasting with an un-powered controller is probably bad, as there is the chance that any protection circuitry remains un-powered.

Aerowhatt, thanks for this, as well as the link. Long time coming. After 60K miles of driving the i-MiEV and invariably engaging Neutral at every opportunity, I'm now convinced that the motor controller is well-protected. Not worried about engaging Neutral above 80mph, as I'm usually not interested in hypermiling at that point. :roll: