Why is e-pedal so detrimental to efficiency?

[Lucky]

Let me explain e-pedal in a simple way from my engineering perspective.

When you accelerate your car, you transform the "electric" energy from your battery into kinetic energy, energy of inertia at certain speed and this process isn't 100% efficient. This energy can be dissipated as tire and other friction losses and air drag so with no intentional deceleration (braking) you reach maximum possible distance. So with accelerating and then coasting, you travel the furthest. But roads are not straight lines so sometimes you need to decelerate. That is when a part of kinetic energy goes back to battery. Since this process also isn't 100% efficient, you get back less then you put in. E-pedal only translates one pedal to both, accelerator and brakes. Every time you release the pedal a bit it starts breaking and you loose a part of energy. So if your foot is restless then it's almost no coasting, always accelerating and braking and always loosing some energy.

Coasting is most energy efficient, if you have conditions that allows you to coast. If you need to press the brake, it regenerates anyway but only when you really want to decelerate. I use it when I can.

In ECO mode motor and inverter don't get any more efficient, it is just that the behavior (usually response) of the pedal is changed, usually it is more sluggish. But that works, because accelerating to certain speed slowly is more energy efficient then accelerating quickly. This is simply due to electrical losses not being linear to electrical current but squared. So for example if you accelerate from 0 to 60mph in 5 seconds you will loose 4x more energy to heat then if you do it in 10 seconds. Of course there are other factors involved, but I wanted to keep it simple.

I hope this clarifies why it is almost impossible for e-pedal to be as efficient as coasting.
English isn't my native language so I'm sorry for possible mistakes.

 

[ACDCPhilip]

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)?
free
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.

Perhaps I should have said no power is being generated rather than current flowing.

I do vaguely remember from physics lessons many moons ago that you can have watt less current, when the current flowing is 90 degrees out of phase with the voltage, resulting in zero power being dissipated, hence no retardation of the motor.
But surely if no power is being provided the voltage from the motor winding's will be at a maximum.
Are you saying that the free-wheeling diodes then protect the IGBT's from this higher than normal voltage ?

 

[Chad E. Mo]

Are you saying that the free-wheeling diodes then protect the IGBT's from this higher than normal voltage ?

https://en.wikipedia.org/wiki/Flyback_diode

 

[nlspace]

Diagram to show the diodes on IGBTs of 3-phase H-bridge motor driver,

 

[DaveinOlyWA]

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.

So you don't have LEAF Spy then because if you did, you would know current flows ALL the time including when in N mode.

 

[DaveinOlyWA]

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.

It is NOT the case and that fact was mentioned here by me a decade ago. How quickly we forget!

 

[DaveinOlyWA]

Perhaps I should have said no power is being generated rather than current flowing.

I do vaguely remember from physics lessons many moons ago that you can have watt less current, when the current flowing is 90 degrees out of phase with the voltage, resulting in zero power being dissipated, hence no retardation of the motor.
But surely if no power is being provided the voltage from the motor winding's will be at a maximum.
Are you saying that the free-wheeling diodes then protect the IGBT's from this higher than normal voltage ?

Your clarification lacks justification. No power is sent to the battery which is correct and you present that as "the" reason why N should be avoided but fail to mention how much power is wasted in E Pedal mode which...is considerable for all but the "expertise ladened" foot among us.

To succeed with E Pedal requires an insane (only insane because the knowledge of N is available) amount of dedication to a task that is essentially predicting the future action of the people you share the road with. Good luck with that!

The reality is "neutral" defined as a complete disconnect from the drivetrain is simply not a thing in an EV. It is simply drag you cannot detect.

There is also the well known concept that regen has velocity limitations which happens every time you come to a stop. Be it brief, but it is there. Its just small...

 

[BillAinCT]

To me all of this is sounding like there's a mindset that one cannot really coast in e-Pedal mode the same as a coast in N. I can feel very easily when I'm coasting in e-Pedal mode and when regen will just start (and it is before I see any blue bar in the dash (gen 2) regen indicator. I also know when I'm just going from coast to drive as I just see a hint of white in the power indicator. Inexplicably, the dash power indicator has more resolution that the energy usage screen on the Nav (is this in the Plus only?). So I feel like I'm coasting very well in e-Pedal mode. Or are we suggesting there's a different and better coast in N than in e-Pedal mode when one is between blue and white region?

Sure would be nice to LS to pick off the CAN message used to apply the friction brakes so we know if and when they are used. I have assumed as long as regen in the blue area and is not at the end or would go past the end it won't apply fiction brakes.

 

[BillAinCT]

There is also the well known concept that regen has velocity limitations which happens every time you come to a stop. Be it brief, but it is there. Its just small...

I find the small coast of 6-12" when regen ends and friction brakes are applied to be annoying. When I think I've regen stopped at a light perfectly the little extra distance added is sometimes closer to the car in front than I like.

 

[knightmb]

The reality is "neutral" defined as a complete disconnect from the drivetrain is simply not a thing in an EV. It is simply drag you cannot detect.

Very true and easy to test. Simply find a level surface, put the Leaf in neutral and try to push it yourself. You will feel the slight "jerks" as it rotates past the magnets. The Eddy currents that people worry about only has a heat effect on the wires and magnets. The whole system has thermal management, so as long as the Leaf is active and the cooling pumps & fans are working, no damage would happen with Neutral coasting, or even towing in theory.

To that point, even if you tow a Leaf in neutral with no cooling system active, everything should be find unless you are going to be towing at a very high speed for a very long time, then in theory, you could build up a dangerous amount of heat that might start to damage the motor. I would imagine it's easy enough to tow the Leaf with the front wheels off the ground if using a Tow truck, so again, very rare to damage the motor unless you are trying to do this on purpose. Like, towing the Leaf on all 4 wheels behind a Cyber-truck doing 100 MPH

 

[hradek]

Let me explain e-pedal in a simple way from my engineering perspective.

When you accelerate your car, you transform the "electric" energy from your battery into kinetic energy, energy of inertia at certain speed and this process isn't 100% efficient. This energy can be dissipated as tire and other friction losses and air drag so with no intentional deceleration (braking) you reach maximum possible distance. So with accelerating and then coasting, you travel the furthest. But roads are not straight lines so sometimes you need to decelerate. That is when a part of kinetic energy goes back to battery. Since this process also isn't 100% efficient, you get back less then you put in. E-pedal only translates one pedal to both, accelerator and brakes. Every time you release the pedal a bit it starts breaking and you loose a part of energy. So if your foot is restless then it's almost no coasting, always accelerating and braking and always loosing some energy.

Coasting is most energy efficient, if you have conditions that allows you to coast. If you need to press the brake, it regenerates anyway but only when you really want to decelerate. I use it when I can.

In ECO mode motor and inverter don't get any more efficient, it is just that the behavior (usually response) of the pedal is changed, usually it is more sluggish. But that works, because accelerating to certain speed slowly is more energy efficient then accelerating quickly. This is simply due to electrical losses not being linear to electrical current but squared. So for example if you accelerate from 0 to 60mph in 5 seconds you will loose 4x more energy to heat then if you do it in 10 seconds. Of course there are other factors involved, but I wanted to keep it simple.

I hope this clarifies why it is almost impossible for e-pedal to be as efficient as coasting.
English isn't my native language so I'm sorry for possible mistakes.

I think a rough guess of how much energy can be recovered from mild coasting is about 30%. The dashboard display can be set so you can see a relative recovery of energy. With E pedal you can still get good recovery as seen on the display but if you are maxing out the recovery than you are losing some recovery. The reason I like the e pedal feature is that it makes for a safer driver.https://www.nissan-global.com/EN/INNOVATION/TECHNOLOGY/ARCHIVE/E_PEDAL_STEP/