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.