How does Eco mode really work?

[RobbW]

I understand the basic differences between the drive modes: D is regular drive mode with full power and minimal regen; Eco is reduced power output with nominal regen; B (Boost?) is regular full power like D but nearly maximum regen. However, I'd like to know more about how Eco mode really works.

Here's the reason why. I have been driving almost exclusively in Eco mode since I got my MiEV because it is kind of an automatic governor for my often lead foot. But, as I've been paying more attention to the "feel" of the car in the different drive modes, I've noticed something peculiar about the Eco mode. While D and B modes provide smooth full power during acceleration, accelerating in Eco mode is very obviously limited (unless the go pedal is punched for emergency acceleration). However, it doesn't "feel" like it is simply the power output being throttled back in Eco mode. To me, it feels like there is a significant drag on the vehicle when accelerating in Eco mode. Almost as if you are trying to accelerate with the parking brake slightly engaged.

What I'm wondering is if Eco mode actually engages the regenerative braking at the same time that it is driving forward, effectively limiting your speed of acceleration but also creating a little juice for the battery at the same time? In other words, Eco mode is the exact same drive mode as D, but with regen slightly engaged (the net effect of full power plus regen keeping your power gauge in the green while accelerating). There are a couple things that make me suspect this. The first is the drag effect I feel when accelerating. The other is the fact that this drag effect disappears immediately if I shift from Eco to D, and the acceleration and ride becomes very smooth. Vice versa, when I shift from D down to Eco mode. I feel a significant drag engage, and the car starts slowing down much quicker than if it was simply a reduction in power output. If Eco mode truly were just a reduction in power output, when I shift from D down to Eco, I would expect the deceleration to feel as smooth as if I had shifted into Neutral or were balancing the go pedal perfectly between the blue and green zones on the power gauge (effectively neutral). But instead, I feel what seems like a braking effect.

Anyone with advanced knowledge of the mechanics of our MiEV want to speculate?

 

[NeilBlanchard]

B = Brake is used for extended steep downhills when the car would go too fast otherwise. You will have already used much more energy getting up the hill, than you will regain with regen.

We have had this discussion several times already, but I think that more regen does not yield greater economy. Coasting is a far better use of the kinetic energy of the moving car, than regen. Accelerate less, coast more, and only use regen when you *need* to slow down.

 

[RobbW]

Thank you, Neil. Yes, I have read many threads regarding the debates between which drive mode provides for better economy or range. However, that is not what I am asking with this particular thread. I'm interested in the mechanics of how Eco mode actually works. My personal experience with driving the MiEV, although very brief, has caused me to wonder what is actually happening when driving in Eco mode versus the other modes. I thought this would be a significantly different enough conversation than the others to warrant its own thread. I apologize if this is not the case.

 

[alohart]

What I'm wondering is if Eco mode actually engages the regenerative braking at the same time that it is driving forward, effectively limiting your speed of acceleration but also creating a little juice for the battery at the same time?

There is only one motor-generator in the i-MiEV. It can't operate as both a motor and a generator simultaneously which is what it would need to do to both propel the car and charge the battery pack (regen). If it were able to do so, Eco mode would be anything but Eco because the most economical mode of operation is to minimize regen and friction braking.

AFAIK, there is only one cable connecting the motor-generator to the battery pack. Current flows in both directions through this cable, but obviously not simultaneously.

So I doubt that Eco is providing propulsion and regen simultaneously. You may be sensing the lower sensitivity of the accelerator pedal in Eco mode compared with D and B modes and the greater regen level compared with D mode.

 

[fjpod]

I would agree that ECO mode is unlikely to provide acceleration and regen at the same time. If it did, it wouldn't be ECOnomical. The only purpose of ECO mode is to help you avoid jack-rabbit starts which is a bigger drain on the traction battery than slower starts.

 

[RobbW]

Thanks, all. That's explanation enough for me. I'm usually super sensitive to any type of vibration, bump, rattle, clicking, etc. in my cars. I'm always afraid something is going wrong or breaking down. I guess the "drag" I was feeling in Eco mode was just the reduced go pedal sensitivity.

I understand that regen braking AND acceleration occurring at the same time would be highly inefficient. However, that begs me to ask another question that I've wondered about every since I first heard about production EVs. I'm sure this is going to be an extremely naive question for most of you EV old-timers, but I'm a newb and curious. Take our MiEV, for example. It is rear-wheel drive. Since the front wheels are not drive wheels and are merely free-spinning, why not have some sort of mini-generators/turbines attached to them that would create energy as the front wheels spin while you are driving? Sure, that would create some resistance that would normally lower the efficiency of the vehicle, but would the energy generated be more than enough to make up for that and then some?

I'm not proposing some type of pie-in-the-sky perpetual motion/energy machine. But I imagine that with front-wheel generators constantly creating energy as you drive combined with the rapid energy created from regen braking, you would be able to recoup much more of the energy used to drive the car than with regen braking alone. Does any of this make sense, or is it obvious that I have absolutely no idea what I'm talking about? I suspect the latter!

 

[HParkEV]

Thanks, all. That's explanation enough for me. I'm usually super sensitive to any type of vibration, bump, rattle, clicking, etc. in my cars. I'm always afraid something is going wrong or breaking down. I guess the "drag" I was feeling in Eco mode was just the reduced go pedal sensitivity.

I understand that regen braking AND acceleration occurring at the same time would be highly inefficient. However, that begs me to ask another question that I've wondered about every since I first heard about production EVs. I'm sure this is going to be an extremely naive question for most of you EV old-timers, but I'm a newb and curious. Take our MiEV, for example. It is rear-wheel drive. Since the front wheels are not drive wheels and are merely free-spinning, why not have some sort of mini-generators/turbines attached to them that would create energy as the front wheels spin while you are driving? Sure, that would create some resistance that would normally lower the efficiency of the vehicle, but would the energy generated be more than enough to make up for that and then some?

I'm not proposing some type of pie-in-the-sky perpetual motion/energy machine. But I imagine that with front-wheel generators constantly creating energy as you drive combined with the rapid energy created from regen braking, you would be able to recoup much more of the energy used to drive the car than with regen braking alone. Does any of this make sense, or is it obvious that I have absolutely no idea what I'm talking about? I suspect the latter!

Sorry RobbW, unfortunately the latter...
What you are actually proposing is 'pie in the sky perpetual motion' because you are implying that the energy you get out of the front 'mini-generators' is more than the energy you have to supply to spin these things. As soon as they are attached to the front wheels, they will put extra drag on that wheel, which will have to be overcome by the main motor using energy from the traction battery. Since no machine is 100% efficient, the energy your mini-generators will produce will be substantially less than the energy taken out of the traction battery to overcome the extra drag.
The generators at the front wheels would definitely be useful during braking, as all energy that is dissipated by the front friction brakes is completely lost to heat, while if we had regen brakes in the front, that would allow more energy to be recovered during the braking process, especially in cases where you're using more braking force than can be generated by the main motor regen mode. Most likely this would add too much cost and complexity to the car to be worthwhile, but I'm sure this is something that will be implemented in future EV's that would come with all wheel drive - that is, use a separate traction/regen motor for front wheels and a separate one for rear wheels, and use both for regen with the ability to properly set regen braking bias between front and rear axles. Seems to me like something that Tesla may be working on for the Model X.

 

[RobbW]

Let's discuss this further. I'm not sure that what I'm proposing IS perpetual motion. With the generators, or rather alternators, in the front wheels, I'm not trying to create enough energy to power the car and keep it going indefinitely. Just as the car is not trying to create more energy from the regenerative braking than is being used by the drive motors. The regen braking is merely trying to recoup SOME of the used power so that you don't have to plug in and recharge the car for nearly as long as you would without regen braking. Would the same concept work with alternators in the front wheels, kind of like regenerative driving instead of regenerative braking? The power generated by the spinning front wheels could be stored either in a separate battery or pumped back into the main traction battery, but not necessarily used to power the drive motors directly. Trying to power the drive motors directly would place too much load on the front-wheel alternators thereby making the drive motors work even harder to push the car forward to overcome the extra drag from the alternators. THAT is where these pie-in-the-sky perpetual energy ideas fail. However, if the energy created from the front-wheel alternators is simply being stored in a battery for later use, there isn't much load at all being placed on them, right? Sure, there will be a MINIMAL amount of friction/drag created by attaching alternators to the front wheels, but it shouldn't be too much. They should still spin rather easily, and the energy they create should be more than enough to offset the minimal drag they create. Again, not aiming to power the drive motors with the energy created by the front alternators. Just trying to produce a little "pick-me-up" power same as the regen braking.

 

[HParkEV]

Robb, you keep stating that the front wheel generators are producing 'minimal' drag - that is only the case when they are not producing any energy at all. As soon as they start generating energy (as in 'regen' mode on the main drive motor), then these generators start producing drag. If there will be any significant energy produced at any time, then there will be more than significant drag produced at the same time. Enough drag to make the traction motor draw significantly more power out of the traction battery than the small generators are putting back into the traction battery. There is no free lunch! The system you propose implies a 'free lunch', and if it was indeed the way things work, then EV's would have all kinds of extra wheels all over the place, generating some 'free energy'. There is very good reason why they don't: no free lunch - every system that will generate energy has to get that energy from somewhere, anything that is rolling (like the front wheel) is getting that energy from the kinetic energy of the car, which in turn comes from either the traction battery or from descending on a downwards slope. Either way, no free energy can be obtained from that wheel!

In any case, I think we're going way off the thread topic here, if you'd like to discuss some more ideas about making EV's more efficient, I think we need a new thread.

Martin

 

[RobbW]

Sorry, Martin. You're right. It was just musings in my head that I've wondered about for awhile. Since I'm not all that knowledgeable about how EV systems work, I figured I'd ask. My impression that alternators on the front wheels would produce only a minimal amount of drag was based purely on playing with science experiment kits when I was much younger. I remember the little electromagnetic motors/coils you could spin and they would produce enough energy to light up a little flashlight bulb. I recall those things spinning pretty easily and freely, especially if they were properly balanced and lubricated. I thought bigger versions on the front wheels would spin just as easily, albeit with slightly more friction due to larger magnets.

Anyway, I will abandon any additional musings in this particular subject and let the thread get back to the original topic. Which had pretty much already been answered, which is why I didn't think there'd be any issue with asking other marginally-related questions in a thread I started.

 

[JoeS]

RobbW, I don't think your initial question was fully answered, as the discussion sort of went OT. Here's my take on it:

Eco Mode is an attempt by Mitsubishi (and Nissan has a similar system in the Leaf) to induce drivers to drive more efficiently. Putting the shift lever into Eco mode closes a couple of switches which results in the following:

1. Activates an electro-mechanical constraint on the accelerator, making it physically harder to depress the pedal. This is simply a tactile feedback device meant to counteract a driver's tendency to leadfoot. Edit 7/30/13. This is incorrect, as there is only a gain change giving this perception (see next paragraph). Sorry.

2. Changes the gain of the controller's motor drive in response to the accelerator pedal position. Specifically, for a given pedal depressed position, you get more power in either D or B modes than you do in Eco Mode. My impression is that this is not linear, because at the upper end if you do indeed "floor it" in Eco, you WILL get full power (a good safety feature).

3. Increases the amount of regeneration over what you get in D mode, but less than what you get in B mode. What's nice about this is that when you want to slow down you can "downshift" through the modes and modulate you deceleration without ever tapping the brakes.

In the first two cases, increased efficiency (and thus improved range) comes from reducing peak power consumption, but resulting in simply getting up to speed slower than a leadfoot.

HTH

 

[RobbW]

3. Increases the amount of regeneration over what you get in D mode, but less than what you get in B mode. What's nice about this is that when you want to slow down you can "downshift" through the modes and modulate you deceleration without ever tapping the brakes.

In the first two cases, increased efficiency (and thus improved range) comes from reducing peak power consumption, but resulting in simply getting up to speed slower than a leadfoot.

Yes, #3 is a habit I practice all the time now with my new MiEV. I love being able to slow down to almost a full stop without ever touching the brakes. Takes proper attention and planning though.

As far as getting up to speed slower than a lead foot, that's fine with me. And better that way in the constant stop and go suburban traffic I drive in. Thanks.

 

[fjpod]

RobbW, I don't think your initial question was fully answered, as the discussion sort of went OT. Here's my take on it:

Eco Mode is an attempt by Mitsubishi (and Nissan has a similar system in the Leaf) to induce drivers to drive more efficiently. Putting the shift lever into Eco mode closes a couple of switches which results in the following:

1. Activates an electro-mechanical constraint on the accelerator, making it physically harder to depress the pedal. This is simply a tactile feedback device meant to counteract a driver's tendency to leadfoot.

2. Changes the gain of the controller's motor drive in response to the accelerator pedal position. Specifically, for a given pedal depressed position, you get more power in either D or B modes than you do in Eco Mode. My impression is that this is not linear, because at the upper end if you do indeed "floor it" in Eco, you WILL get full power (a good safety feature).

3. Increases the amount of regeneration over what you get in D mode, but less than what you get in B mode. What's nice about this is that when you want to slow down you can "downshift" through the modes and modulate you deceleration without ever tapping the brakes.

In the first two cases, increased efficiency (and thus improved range) comes from reducing peak power consumption, but resulting in simply getting up to speed slower than a leadfoot.

HTH

I agree with 1 and 2, but not 3

Eco and B mode do not give more regen than D. They give regen faster over a shorter distance. D mode gives regen slower over a longer distance. Assuming you have all the space in the world to stop, they will yield about the same amount of regen. If anything, less regen over a longer time is probably a better recharge than a faster regen over a short period.

And even if you don't have all the room in the world to stop, as long as you don't have to panic stop, you will get similar results from the regenerative brakes.

 

[JoeS]

fjpod, perhaps its semantics, and our definition of regeneration. I say that regeneration is measured by instantaneous current (or Watts, if you wish) going back into the battery.

For any given speed, with the foot off the go-pedal:

N gives zero regeneration, the primary retardation being air drag
D gives a small amount of regen and thus a small amount of retardation
Eco gives a larger amount of regen and thus a larger amount of retardation
B gives a large amount of regen and thus a large amount of retardation

Again, regeneration being measured as current or watts being stuffed into the battery. Take these watts and multiply them by the amount of time you're dwelling in regeneration and and you end up with kWh (energy) going back into the battery.

I agree with you that if, for example, you were going 60mph and came to a stop say 500 yards down the road, if we ignore wind resistance then it doesn't matter how quickly or slowly you got there using whichever mode: the amount of energy stuffed back into the battery would be the same.

The controllable regeneration of the iMiEV is especially appreciated by those of us living in the hills: we HAVE to slow down often, and having the ability to modulate this and put energy back into the battery pack is such an elegant way of doing it.

 

[Don]

Eco and B mode do not give more regen than D. They give regen faster over a shorter distance. D mode gives regen slower over a longer distance. Assuming you have all the space in the world to stop, they will yield about the same amount of regen. If anything, less regen over a longer time is probably a better recharge than a faster regen over a short period.

And even if you don't have all the room in the world to stop, as long as you don't have to panic stop, you will get similar results from the regenerative brakes.

I disagree . . . . but I'm open to being proved wrong, *if* we have some proof

Is this your personal opinion . . . . or do you have some printed info from Mitsubishi (or some other knowledgeable source) to back up what you're saying? I've spent the last couple hours Googling everything I can find about the car's regenerative braking system and I can't find one single FACT which backs up what you're claiming here . . . . and I sure don't get the impression what you're saying is correct by driving the car. If you have some factual information that what you're saying is correct, please post it here so we can all learn something

I did read where Mitsubishi says that by employing regenerative braking in city driving, the cars range can be increased by approximately 30%

Don

 

[fjpod]

Eco and B mode do not give more regen than D. They give regen faster over a shorter distance. D mode gives regen slower over a longer distance. Assuming you have all the space in the world to stop, they will yield about the same amount of regen. If anything, less regen over a longer time is probably a better recharge than a faster regen over a short period.

And even if you don't have all the room in the world to stop, as long as you don't have to panic stop, you will get similar results from the regenerative brakes.

I disagree . . . . but I'm open to being proved wrong, *if* we have some proof

Is this your personal opinion . . . . or do you have some printed info from Mitsubishi (or some other knowledgeable source) to back up what you're saying? I've spent the last couple hours Googling everything I can find about the car's regenerative braking system and I can't find one single FACT which backs up what you're claiming here . . . . and I sure don't get the impression what you're saying is correct by driving the car. If you have some factual information that what you're saying is correct, please post it here so we can all learn something

I did read where Mitsubishi says that by employing regenerative braking in city driving, the cars range can be increased by approximately 30%

Don

No, I don't have inside knowledge of any kind. I'm basing what I say on 4 things...
1. energy cannot be created or destroyed.
2. for every action there is an opposite and equal reaction.
3. I have spent many years on PriusChat where this issue has been discussed ad nauseum
4. Looking at the amp meter in front of me as I try out the various drive modes and braking.

 

[fjpod]

fjpod, perhaps its semantics, and our definition of regeneration. I say that regeneration is measured by instantaneous current (or Watts, if you wish) going back into the battery.

For any given speed, with the foot off the go-pedal:

N gives zero regeneration, the primary retardation being air drag
D gives a small amount of regen and thus a small amount of retardation
Eco gives a larger amount of regen and thus a larger amount of retardation
B gives a large amount of regen and thus a large amount of retardation

Again, regeneration being measured as current or watts being stuffed into the battery. Take these watts and multiply them by the amount of time you're dwelling in regeneration and and you end up with kWh (energy) going back into the battery.

I agree with you that if, for example, you were going 60mph and came to a stop say 500 yards down the road, if we ignore wind resistance then it doesn't matter how quickly or slowly you got there using whichever mode: the amount of energy stuffed back into the battery would be the same.

The controllable regeneration of the iMiEV is especially appreciated by those of us living in the hills: we HAVE to slow down often, and having the ability to modulate this and put energy back into the battery pack is such an elegant way of doing it.

Your definitions of D, E, B and N are correct but you leave out one thing...the time element. For example, and assuming an unimpeded stopping distance, the amount of time you spend slowing down in B is considerably less than in D, so even though you are generating more power in B in a given instant, you are doing it for less time. Net result...same total amount of power recouped.

The different driving modes are just for convenience...and you can accomplish the same thing by judicious use of the brake pedal.

IMHO, probably the best driving mode is Eco because it keeps you from jack-rabbit starts. Also IMHO, I think B mode is bad, unless you are doing a lot of downhill where you need to control your speed, because if you ease up on the go pedal just a little bit due to fatigue or distraction, the car slows down a lot...and then you have to use more energy to get back up to where you were.

Pick your poison... the different modes are just a convenience for the conditions under which you may be driving. There is no drive mode that will give you more energy or efficiency. energy cannot be created or destroyed. Better mpg all depends on the smartness of the driver and how he/she anticipates changing conditions.

 

[fjpod]

I would like to add...the imiev, or any EV, only has only one regenerative system. Why would MM, or any other car manufacturer, design and install three or four systems in one car, one being more efficient than the other? It only makes sense that MM installed the most efficient system they could build. The various drive modes are simply different applications of the same system to suit a driver's style.

 

[NeilBlanchard]

An important addendum is that aero drag is always happening; no matter what "mode" you are in. So, coasting has the smallest loss of the kinetic energy "stored" in the moving mass of the car. Regen increases the loss because the aero drag is still there (as is the rolling resistance) and the motor/charger/battery steps each increase the loss.

 

[alohart]

Also IMHO, I think B mode is bad, unless you are doing a lot of downhill where you need to control your speed, because if you ease up on the go pedal just a little bit due to fatigue or distraction, the car slows down a lot...and then you have to use more energy to get back up to where you were.

I think Eco mode is bad because it requires the use of the brake pedal more frequently than does B mode. If you press down on the brake pedal just a little bit too much due to fatigue or distraction, the friction brakes are applied which wastes energy.