Why is e-pedal so detrimental to efficiency?

[fester]

I tried e-pedal a bit when I first got my Leaf, it was novel but didn't find a use for it until I had both knees replaced this summer. When returning to driving a couple of weeks after the right knee was done, I found it very useful because lifting my foot from accelerator to brake was a bit of a major OUCH. Back to regular driving style now, but it sure was handy .

 

[GerryAZ]

Any amount of regen results in considerable losses...

An EV's motor is at best 80% efficient at converting electrical energy to mechanical output at the drive shaft. Then, there are some losses in the gear box too.

When regen kicks in, the EV motor becomes a generator, converting mechanical energy at the drive shaft back to electrical energy stored in the pack. However, most EV motors aren't as efficient at acting as a generator vs a motor.

In a perfect system, regen recovered energy would be the power originally drawn from the pack times the square of the motor's efficiency.

So, assuming perfectly symmetrical efficiency (unlikely), if 10 kWh of energy is drawn from the pack to accelerate a vehicle from 0 to 30 mph, then regen would at best put back 10 * 0.64 = 6.4 kWh back into the pack when decelerating from 30 mph to 0 mph.

But, EV motor's efficiency isn't linear at all speed either. Then there are the gearbox losses to consider...

I would guess that if you can get 50% regen efficiency average (so 70% motor and gearbox efficiency combined), that's pretty good.

I've had conversations with people who swear that they can go up a hill and come back down again and end their trip with "almost the same pack SOC". It's laughable that they think that's even close to possible.

Note that with cruise control, the car will do regen on downhill sections or when slowing down with adaptive cruise. If you want higher efficiency, you need to learn to coast in neutral at the right times. Doing that can definitely beat cruise control on longer trips, unless the highway is dead flat and you never change speed. Coasting downhill is super efficient, unless you go so fast that increased air drag cancels any gains.

WRONG----Reduction gear losses are only a few percent. Synchronous motors (like the LEAF uses) are mid 90's percent efficient when either delivering power to the gears or generating power. Since there are small losses, the regeneration from going downhill will not quite match the energy to go up the hill. The difference is not large if the speed is kept the same going both ways.

E-pedal is generally less efficient than using conventional cruise control in B-mode because e-pedal applies friction brakes a lot.

 

[alozzy]

From a Motortrend article:

(EV) losses can be broken down into approximately 10 percent of the source energy from the grid lost in the charging process, 18 percent lost to the drivetrain motor components, up to 4 percent lost to auxiliary components, and another 3 percent lost solely from powertrain cooling and other vehicle systems.

I think you're drinking the Nissan kool-aid with respect to efficiency - peak efficiency maybe, but I am skeptical that average efficiency is as high as you state.

The precise efficiency aside, the losses happen in both directions so, assuming bidirectional efficiency is identical, it's like I said - the efficiency factor is the square of the losses.

Are you arguing that regen is more efficient than coasting? My main point was that coasting, at the right times, is going to beat regen...

 

[GAXIX]

E-pedal is generally less efficient than using conventional cruise control in B-mode because e-pedal applies friction brakes a lot.

I was under the impression that the E-pedal only used friction brakes at the very end of braking, once you've reached a very low speed, in order to bring the car to a complete stop and then keep it from creeping.

At what other times does the e-pedal use friction brakes? During max regen?

 

[knightmb]

E-pedal is generally less efficient than using conventional cruise control in B-mode because e-pedal applies friction brakes a lot.

I was under the impression that the E-pedal only used friction brakes at the very end of braking, once you've reached a very low speed, in order to bring the car to a complete stop and then keep it from creeping.

At what other times does the e-pedal use friction brakes? During max regen?

E-pedal doesn't actually use full regen until your speed is under some threshold. Try it out on the Interstate doing 70 mph and just let go of the accelerator and watch LeafSpy, it limits regen to the same amount as B mode until your speed is low enough to engage a higher regen (and then your brake lights come on too). I'm not sure of the speed, maybe under 50 mph you get full regen? I think it was that way on purpose because full regen at Interstate speeds just mean someone will crash into the back of you every-time.

 

[bojoho]

Gerry Is right. E pedal is surprisingly heavy on the friction brakes....at least my 2018 sv is. There is a downhill section I frequently drive on ~30mph and B mode will often Regen 15-20kW. On E pedal it nearly always switches to full friction brakes down the hill and I get 0 Regen for most of the hill.

 

[k9spud]

If you want higher efficiency, you need to learn to coast in neutral at the right times. Doing that can definitely beat cruise control on longer trips, unless the highway is dead flat and you never change speed.

It took me a while to get the hang of using neutral, but wow, you're right. This coasting in neutral can make for some very impressive efficiency gains. I managed a max of 7 mi/kWh on my normal 19 mile one-way trip (slightly downhill direction, maxxed 5 mi/kWh on the uphill direction), never got that good before.

I see why Nissan made the shifter a joystick now -- it makes this feel a lot like driving a manual stick. Only, I'm not shifting up in gears from a start like in an ICE -- the predominant activity is downshifting to N when I can see I'm going to need to stop way ahead, then D, B, and finally braking to a final stop (if I get that far, sometimes the light turns green and I just get back to D or B and go).

Now, regarding accelerating from a stop or staying at constant speed... In my ICE car, I get the best efficiency by just ever so slightly pressing the accelerator pedal by the minimum amount to get the car to barely accelerate or just barely maintain speed. I'm finding that strategy doesn't pan out in the Leaf, and I think this graph below explains why:

At low torque (barely accelerating/maintaining speed), you end up in the purple 85% efficient zone along the bottom of the graph, at all speeds. If you push the accelerator a little harder so the motor is under load, that's when you can get up into the 95% efficiency zone.

I wonder if cruise control keeps the motor in the 95% efficiency zone (while operating at a constant speed) by applying small pulses of alternating acceleration/coasting? That could explain why it does so much better than trying to maintain constant speed manually with the accelerator pedal.

 

[BobLerwill]

If I may add my bit,
I have read through all the replies and I think some have made useful observations about how much "hidden" application of friction brakes is going on when you use ePedal, but that does not fully explain either why this happens or why it ruins efficiency.
International vehicle safety regulation places limitations on regenerative braking and require that is some circumstances friction brakes should also be used. This is for two reasons.
Firstly because on vehicles like the Leaf where there are only two driven wheels, regenerative braking can only act on two of the wheels. There is a danger that if too much braking goes on, those two wheels could lose grip and cause loss of control. Simple regenerative braking is therefore limited to low rates of deceleration. Ten years ago, this limit was 0.1G. (Not sure what it is now, but that was the regime under which the Leaf was designed.)
Secondly because the driver expects consistent results from their brakes, so if braking is brought about by lifting your foot of the accelerator pedal, that should also be consistent. Unfortunately, raw regenerative braking does not give consistent results. Most importantly, if your battery is fully charged, there will be no regenerative braking! You cannot get rid of the kinetic energy of your car by using it to charge up your battery if your battery is already full! Leaf drivers who have stopped using ePedal and instead use the "B" mode will be well aware that there is little or no regenerative braking when the battery is full. The dashboard efficiency display that shows how much power you are using and how much regenerative charging is going on also shows in a dimmer blue how much regenerative power is possible. With 100% charge this is barely visible. It does not rise to the maximum until charge has dropped to about 85%.
If the dashboard display is proportional and the maximum power drain is taken to be 110kW, then the maximum recharge is about 50kW. Presumably the Leaf electronics have been designed with a maximum recharge rate of 50kW, and this puts a limit on how much regenerative braking you can get. basic physics should tell you that the faster you are going, the less deceleration you will get from this 50kW. On the other hand, as the car speeds drops to zero, the motor will stop acting as a generator and not be able to bring the car to an absolute halt. These simple engineering truths also result in challenges to the drivers expectations of the regenerative braking systems.
The consequence is that the epedal system, which provides braking above the 0.1G limit, must also apply friction brakes. The brakes are applied when you take your foot off the pedal when the battery is full. The brakes are applied when you are travelling fast and simple regeneration will not give you the expected deceleration. The brakes are applied to bring you to a complete halt. (This last situation is the only one Nissan seems to ackowledge.)
I believe the reason why ePedal so eats into your mileage is because people who use it are not aware of just how often it causes friction brakes to be applied. They think they are getting all that energy back, but the truth is that it is going into heat in the brake pads.
There is some truth in the observation that regenerative braking does not recover quite all of the energy, but it is not bad. Modern motors and drive trains are on average around 90%. The electronics needs to be about 95% (or it will cook). The battery itself has an efficiency: not all the charge put in will be available to take out, but lithium cells are pretty good on this. Taken all together I would expect around 80% efficiency from regeneration. You will get better mileage if you never have to slow down at all, but we live in a real world where some deceleration is necessary. Personally i switch to "D" mode on the motorway to try and keep regeneration to a minimum and bear the loss if i have to use friction brakes in the odd tricky situation, but I have no firm evidence it makes much difference.
One pedal driving is fun. I understand why people like it, but if you are keen to increase your range I suggest you try turning it off and using "B" mode.
(The only other thing I would suggest is that if you charge at home and like me you live at the top of a hill, avoid charging to 100%, Otherwise whatever setting you use, every time you leave home, that electricity you paid for will be going straight to the brakes.)

 

[Drew21]

I'm one of the folks who will swear to you that our 2014 Leaf can make the mostly uphill 2.5 mile drive into our small town, using 3-4% on the battery gauge and then regain that charge on the downhill home. I've done it dozens of times over our two years of ownership.

I recognize that I don't gain back the full charge, but I can consistently get to the same percentage on the battery gauge on the dash (e.g., start at 70%, park at the grocery store in town with 66 or 67% on the gauge, and then pull into the garage at home back at 70%).

Having said that, on longer drives I use N quite a bit to extend range. I recall watching a Dala video where he created a function in his Leaf to turn regen off completely. I wouldn't mind having a button for that, in addition to a paddle that allowed the amount of regen to be fine-tuned as in the Bolt.

Barring at that, I have a pretty good mental map of the uphills and downhills in our local area and a sense of the downhills that are best driven in N, D, or B mode to maximize range.

 

[Snakebitten]

I just want to thank the interior design engineer for both locating the E-pedal button where it is, and for the momentary type switch that was used.

I use it as a brake hold at traffic stops and already have the procedure committed to subconscious muscle memory.
I rarely actually drive in E-pedal mode unless I'm forced to go to the city and get caught up in stop&go. It's awesome for that miserable driving circumstance.

I love that smooth woosh you get when accelerating from a dead stop and then release the E-pedal. It's such a nice transition.

 

[Kenny]

Being a life long hyper-miler, I quickly discovered after purchasing my new '18 SL that e-pedal was not conducive to this endeavour. While I consider myself to be an expert on modulating the throttle to obtain the optimal amount of coasting/regenerating, I found this to be virtually impossible to do with e-pedal engaged due to the unpredictability of when the friction brakes would apply.

 

[Kenny]

I just want to thank the interior design engineer for both locating the E-pedal button where it is, and for the momentary type switch that was used.

I use it as a brake hold at traffic stops and already have the procedure committed to subconscious muscle memory.
I rarely actually drive in E-pedal mode unless I'm forced to go to the city and get caught up in stop&go. It's awesome for that miserable driving circumstance.

I love that smooth woosh you get when accelerating from a dead stop and then release the E-pedal. It's such a nice transition.

I do the exact same thing. My wife's Toyota has a brake hold feature that engages as soon as the vehicle comes to a full stop. I wish my Leaf had that feature.

 

[DaveinOlyWA]

I understand that you lose efficiency every time you engage the friction brake; however, according to Nissan, e-pedal will (supposedly) only engage the friction brake near the end to bring you to a complete stop. When driving, I do my best to regulate the accelerator that I coast whenever possible.

I much prefer the convenience and experience of e-pedal; however, last night I decided to try driving without it, in D mode after reading other people's reported efficiencies of around 4mi/kwh or more. I've always struggled to consistently reach 4, and typically get 3.5 or below. Last night though, a short trip resulted in an average of 4.7! I was shocked! Outside temp was about 55F, and I didn't run any climate control. Similar conditions with e-pedal usually result in at least an entire mile less per kwh, which seems insane.

Maybe I'm just not as good at coasting as I thought?

That is correct, you are actually a typical human who is actually quite bad at coasting. Most of us fool ourselves into thinking we are good due to short momentary bouts of excellence but we are fighting gravity which never sleeps and is unrelentless. We don't stand a chance. So the higher the regen profile, the greater the penalty for even the tiniest of miscalculations.

Now; driving styles are highly personal so you will see a full book on the why you should or why you shouldn't but the reality is for best efficiency, there is no one drive mode that works.

The only hard fast rule is ECO 100%.

The other fast rule is each drive mode has its purpose and excels if used properly. Efficiency is determined by how closely you adhere to that ideology.

This includes D, B and N. If you truly think you have mastered coasting, shift to N and see how close you actually are.

 

[Philip Veale]

I do the exact same thing. My wife's Toyota has a brake hold feature that engages as soon as the vehicle comes to a full stop. I wish my Leaf had that feature.

My VW Golf has auto hold, and I sorely miss this function on the Leaf.

 

[DougWantsALeaf]

Dave

I do wish that the Leaf had a true coast function (without doing the manual neutral move).

Personally, i use D on the highway, and B around town. I stopped using epedal a couple years ago as that let me explicitly decide when to engage the friction brakes. I used to manually neutral sometimes but it's kind of a pain, so try very hard to keep the power bar to 0, which I think still uses 1-2kWh, but is generally negligible in driving at any speed. It does still surprise me how easy it for perception of speed to cause me.to leave the 0 power position. The Kona paddles allow for much easier pwr mgmt I found when renting.

 

[DaveinOlyWA]

Dave

I do wish that the Leaf had a true coast function (without doing the manual neutral move).

Personally, i use D on the highway, and B around town. I stopped using epedal a couple years ago as that let me explicitly decide when to engage the friction brakes. I used to manually neutral sometimes but it's kind of a pain, so try very hard to keep the power bar to 0, which I think still uses 1-2kWh, but is generally negligible in driving at any speed. It does still surprise me how easy it for perception of speed to cause me.to leave the 0 power position. The Kona paddles allow for much easier pwr mgmt I found when renting.

I primarily use D with ECO on all the time. B is used for speed control on slopes, traffic, stop lights, etc.

So why did I move away from higher regen?

Flaky drivers. I found it nearly impossible to predict what other people will do making speed management a very tough task to perform well.

The best efficiency is achieved by maintaining velocity as much as possible, NEVER using friction brakes and using regen as little as possible.

I drive generally monitoring traffic flow several blocks ahead. When the unexpected happens nearer to me, my first reaction which is actually a reflex is to take foot off electron feed. In high regen, I am losing velocity rapidly, even if just for a few seconds.

So D lowers that loss in case my deceleration wasn't needed or needed as much.

I also use N EXTENSIVELY. This is where the efficiency really kicks in but also the hardest to judge. A win is shifting to N 4 blocks from the red light with several cars already there or in front of me at say 40 mph and coasting thru the light at 30 mph.

I know that even if the light turns green right away, I won't have a clear path so N maintains most of my velocity as the LEAF coasts very well. If need be, I will shift to D then to B rolling up to the light generally at speeds well under 10 mph.

Right now with decent weather, I am well over 6 miles/kwh and its 80% in town driving. I would probably be better but have a bad habit of zipping thru yellow lights

 

[knightmb]

I do the exact same thing. My wife's Toyota has a brake hold feature that engages as soon as the vehicle comes to a full stop. I wish my Leaf had that feature.

I does it have that, both my 2013, and 2020 do this, so does my wife's 2018. Unless the inclination meter is having issues, the Leaf should do exactly that? Unless you mean on a flat surface?

[edit] Never-mind, got confused and thought this was about stopping on a hill...

 

[nlspace]

Right now with decent weather, I am well over 6 miles/kwh and its 80% in town driving.

That is impressive

 

[ACDCPhilip]

I've always been reluctant to use N when the car is rolling, although I can see the definite efficiency gains.
The reason being that as far as I understand, the drive motor is a permanent magnet type, with the drive coils used as a generator when regen is being used.
When the selector is in N, there is no current flowing from the motor coils, but they will still have a voltage being generated across them going to the IGBT's in the inverter ready to be used for regen.
If no current is flowing, the voltage across the coils will be at a high voltage depending on the speed of the motor, & that voltage is still being presented to the IGBT's that will have a certain breakdown voltage, believe i've seen a value of 1200v mentioned on data sheets as the breakdown voltage.
The thing is, I have no idea what voltage is being put out by the motor coils.
Obviously the op has been doing this a long time with no ill effects.
Wondered if anyone had tested what sort of voltage the motor coils were putting out at high speed when in N.

 

[nlspace]

When the selector is in N, there is no current flowing from the motor coils,

How do you know that this is the case?

The motor is called an AC synchronous with internal permanent magnets.

i would suggest that the inverter is always engaged and the resolver is always active and the rotor magnetic field vector is always known for synchronous commutation regardless of the "gearshift position". This is demonstrated by the smooth and seamless shifting in and out of N while the car is moving/coasting.

When the motor is spinning while coasting there can indeed be current flowing from the motor windings--if it is routed thru the free-wheeling diodes of the IGBTs.

But who is to say that the inverter is not still engaged (it is) and capable to energize the windings and switch the IGBTs to route the back-emf currents in a 'neutral' manner, neither creating regen drag or using any significant pack energy (free-wheeling)?

This is a neutral timing of the IGBTs in synch with the rotor, whereas motoring involves a slight "advance" of the timing angle, and regen involves a slight "retard" of the timing.