The Art & Science of One-Pedal Driving (an electric car)

The YouTube video version is now up: The Art & Science of One-Pedal Speed Control

 

     The most revolutionary aspect of modern electric vehicles, from a driver’s perspective, is almost certainly one-pedal driving.

    One-pedal driving is simply the control of both acceleration and deceleration—speed— with a single pedal.  In the past this has been done with two pedals—the brake pedal, controlling the friction brakes, and the “gas” pedal, controlling the internal combustion engine.  Two completely separate systems—one to make the car go faster, and the other to make it go slower.  

    So how do you get the car to go faster, slower, to stop, with the one pedal?  How do you as the driver make it work?  It’s really simple—much simpler than driving two-pedal.  Push the pedal in to speed up, and let it out to slow.  Mind you, usually not all the way in, and not all the way out—unless you want to slow down quickly or when coming to a complete stop.

   This illustration shows that the more you push down on the pedal, the more power you're sending to the motor.  The more you lift off, more of the car’s kinetic energy is used to spin the motor (as a generator), thereby slowing the car.  The motor/generator converts that power to electricity and sends it back to the battery.

  Unlike a fossil car’s gas and brake pedals, the action of the pedal is not determined by its position alone. The car’s computer also factors in the car’s speed, and whether you're pushing down or letting up.  So if you're stopped and just begin to press down on the pedal, the car will accelerate.   But if you're already driving and you pull pack to that same position, the car will configure the motor to decelerate. 

    But this is okay, because electric drivetrains are so responsive.  You’ll discover a light touch on the pedal almost gives you mind control.  Subtle movement with your foot—often just a slight change in pressure—gives you an immediate, precise speed change.  After you get used to it, you almost just have to think it. 

 Okay.  So somewhere in the middle of the pedal travel there will be a point where power is neither flowing from the battery to the motor or back from the motor into the battery—neither acceleration nor deceleration.

    If you hold the pedal in this position the car will “coast,” although not precisely that because you will still be controlling of the car’s speed.  It’s not like sitting in your little red wagon flying down a hill—to no uncertain fate.

    Pulling back further on the pedal starts to reverse the magnetic forces in the motor, creating drag that slows the car.  The more you lift up on the pedal, the stronger the drag.

    This drag is widely called “regen”—short for regenerative braking—but more precisely regen can be described as magnetic braking, and this magnetic braking takes place inside the motor (or motors)—completely separate from the regular friction brakes.  You control the amount of regen with the pedal—from zero all the way up to the maximum the car maker has decided to give you.

    But don’t overthink this control mechanism—just drive the car intuitively.  The pedal controls the speed—push down some to go faster, let up some to go slower, or put your foot at a spot that maintains a constant speed.  It’s seamless; there’s no shifting your foot from one pedal to another to inform you when you go from acceleration to deceleration.  Because the motor is always directly connected to the wheels, you won’t be able to tell where the transition is—and there’s no reason to know.

    Slowing is controlled in the same way as accelerating—with the motors—same pedal, same foot, same motion.

    One-pedal driving means keeping your foot on the pedal all the time, except when you need to make an emergency or sudden stop, or when you're sitting at a red light.  But when you’re driving, keep your foot on the pedal all the time, as you would when driving a fossil car on a highway.  

    Say, you’re driving a fossil down the highway, and want to increase your speed a little—you would ease down on the pedal.  If you want to reduce speed a little—you don’t use the brakes—you just ease out a bit on the pedal.  That’s exactly how one-pedal driving works.

    You wouldn’t stomp the pedal to the floor to increase speed a little, and neither should you pull your foot completely off the pedal if you just want to gradually slow.  But until you lose your old fossil habits, you just might lift full off the pedal when you just want to coast or slow a little.

    So, while learning to drive one-pedal, you have to be aware of your long-engrained fossil habits.  You don’t drive an EV one-pedal at all the same as you would drive a fossil car.  It’ll take a couple of weeks of daily driving one-pedal before you can break old habits.  And the longer you drive one-pedal, the more natural it will become.

    One-pedal is nice for straight line driving, but particularly advantageous when going around a turn.  With two-pedal, you’re shifting your foot back and forth.  

    With one-pedal, you control the speed continuously and smoothly through the corner—approach, turn, accelerate back up to speed.  So you’ll get through the turn much faster.  No lags or lurches. Your foot never leaves the pedal.   

    Remember, driving an EV with one pedal is all in one mode.  When you drive a fossil, your mind is managing three modes of driving, even if you are not consciously aware of that.  
1) Pushing down on the gas pedal,
2) Foot off both pedals (coasting with some engine drag—so, no control over the car), and
3) Pushing down on the brake pedal.

    In just about every EV, the motor is always connected to the wheels, even if you put it into “neutral”, which is an electrical disconnect, not mechanical.  If the car is moving, the motor will be turning.  Technically, you don’t “coast,” because you are always actively controlling the speed with your foot, and the motor is never disengaged from the wheels.   

    By the way, coasting is not more efficient than driving in full regen mode.  If you disable full regen or turn it to low, you won’t recuperate energy when you need to slow.  With regen turned low, you’ll likely convert that energy to waste heat by using the friction brakes.  

    On the other hand, with full regen enabled, you can “coast” to your heart’s content by keeping your foot part-way down on the pedal, until you need to slow—reducing pressure on the pedal—when you’ll send energy back to the battery.  Using regen to the fullest, you can recover 50-70% of energy expended, depending on how you drive.  With friction braking, it’s all lost.  

    So, if you have your magnetic braking on a strong setting (like the “B” mode in some EVs, and “standard” in Teslas), that doesn’t mean magnetic braking will be strong whenever you engage regen by lifting off some on the pedal.  You very often will be driving with little or no power either going to the motor or being generated by the motor—your foot will be holding the pedal part-way down.

    With one-pedal, there is just one mode.  Don’t think in the fossil terms of accelerate, coast, and brake.  The one pedal controls your speed, including how fast the speed changes (up or down).  Period.  

    The “deep dive” part of this video is about how the technology works.  You won’t need to understand the technical bits just to drive one-pedal, but if you're going to teach one-pedal driving to someone else—and you will, it really helps to have been exposed to the underlying technology.  

    Fossil cars have a gas pedal and a brake pedal.  Pushing on the brake pedal creates hydraulic pressure, which forces a brake pad against the brake rotor, generating heat and a lot of toxic black dust.  You can explain it by showing the friction brakes to people—it’s physical.  But with an electric car’s regen, it’s not so easy because those magnetic brakes are invisible.  

    Electric cars operate by using magnetic fields—inside the motor, so we need to talk about that for a bit.  

    Magnets.

    Obviously electricity powers the car, but it’s the magnetic fields inside the motors—created by the electricity—that make the car go, and slow it down.  It’s all done with magnets.

    Magnets are everywhere.  We couldn’t live without them.  In fact, our planet is one big magnet, and without its protective magnetic field, we’d all die from cosmic radiation.  

    And then there’s the lowly refrigerator magnet.  As you bring it toward the refrigerator, it pulls your hand forward (like accelerating a car), and it resists as you pull it away (like slowing a car).  These are invisible forces.

    The refrigerator magnet is what’s called a permanent magnet. 

    Magnets have north and south poles.  North poles are repelled by north poles, but are attracted to south poles (opposites attract).  The closer they are, the stronger the attraction, or repulsion.  It’s these attraction and repulsive forces that make electric motors work, both to make the car go and to make it slow.

    A common nail is not magnetic, but can temporarily be made into a magnet by wrapping a coil of wire around it and running an electric current through that coil—turning the nail into an electromagnet.  

    The important thing here is that if you reverse the direction of the electric current flowing through the wire, the magnetic pole will change—north becomes south and south becomes north.  If you keep reversing the direction of the current, you get alternating current—AC—what your house has.  Electric motors work because of this alternating current and electromagnets.

    There are all sorts of motor variations, but let’s consider a simple design that uses permanent magnets in the core part of the motor—the part that spins (the rotor), and electro-magnets in the stationary outside part of the motor (the stator).  This diagram is simplified and generic; EV motors and their electronic controls are quite complex.

    In this example, the blue permanent magnets around the rotor’s periphery maintain their polarity (that is, the north pole is always north, and south is always south).

    The north poles of the permanent magnets are pulled toward the south poles of the electromagnets in the outside ring (and pushed by the stator’s magnets north poles behind it). But the alternating current that powers EV motors constantly reverses the stator’s magnetic fields—north to south to north, so the magnetic poles never catch up with their opposite.

    The rotor’s magnets are pulled along by these constantly changing magnetic fields in the stator.   

    Now, as your foot eases the pedal back, power from the battery to the motor is gradually reduced until it’s nothing.  

    The stator’s electromagnets are deactivated.  No power from the battery to the motor and vice versa.  You could call that coasting.

    At this point your foot is still holding the pedal part-way down.  As you pull your foot further back, the motor’s electronics  gradually shift the stator’s rotating magnetic poles to lag behind the rotor’s opposite permanent magnet poles, so the magnetic forces are now pulling against the direction of shaft rotation, working to slow the motor—and the car. 

   When this magnetic braking is activated, an electric current is induced in the wires of the stator’s electromagnets, and that alternating current flows out of the motor, is converted to direct current, and charges the battery.  The same basic principle is used in wireless cell phone chargers.

    And of course it’s all invisible.  Just remember the refrigerator magnets.

    Unfortunately, knowing how to drive one-pedal, and having an idea how it works, is not enough.  You’re going to have to know how to talk about it, when you teach others, and answer questions.  You have to know the lingo, and, well, that can get a little murky. 

     The big bugaboo is the term “regen”—everybody uses it but very few can explain it very well.  As I mentioned earlier, it’s short for “regenerative braking.”  When you let up on the pedal, the EV slows—hence “braking”.  The drag on the car comes from the wheels turning the motor (rather than the motor turning the wheels).  When the wheels turn the motor, the motor’s magnetic fields slow the car and charge the battery. 

    Instead of regenerating the battery,  I like to just say magnetic braking simply charges the battery.  

    The term “brake energy recuperation” is another term that’s used for the same thing—that is, “regen” or magnetic braking.
 
    You are indeed recovering energy (recuperation) — just not “brake” energy, but rather either kinetic energy or potential energy, or both. 

    Kinetic energy is the energy the car has by way of its motion, and potential energy is the energy your car has relative to the planet’s gravitational field, or, in other words, up a hill.

    Just know that energy recuperation is a side benefit of one-pedal driving, and that increases your car’s efficiency by charging your battery instead of wasting energy as heat (what friction brakes do).  

    Before you start one-pedal driving, you configure your EV for it.  For my Tesla Model Y, configure driving settings for regenerative braking to "Standard", and stopping mode to "Hold." "Creep" simulates what a fossil car with an automatic transmission does, and "Roll" simulates what a manual transmission fossil car does when the car is in neutral or the clutch is depressed.  "Hold" will bring the car to a full stop when the pedal is fully released, and keep the car from rolling, even if you are on a hill.  

    Other car makers do it differently (often with a "B" mode), but most still allow drivers the choice of sticking with the familiarity of two-pedal driving, or advancing to one-pedal, or something close to that.  

    For a few EVs, one-pedal is the only way.  For example, in the BMW i3.  The maximum regen power is set—you can’t adjust it.  But that’s the maximum value.   You can of course control the strength of “regen” from zero to maximum, even in the i3.  You do it with the pedal—with your foot.

    Some EVs don’t allow one-pedal at all—magnetic braking is controlled only with the brake pedal, which blends in the conventional friction braking—preserving tradition, that is—nostalgia, a sentimental longing for the familiar past.  

    In any case, I would strongly recommend that you buy an electric car that gives you the choice to drive one-pedal.   You can try both and decide for yourself.  After you’ve been driving one-pedal for awhile, having to shift your foot over to the brake pedal becomes very annoying!

    So, a couple of final thoughts:

    First, on two-wheel drive versus four-wheel drive EVs, and one-pedal.

    Slowing down is important.  More important than accelerating.  When driving one-pedal, when more than 95% of braking is magnetic, you should know that only the wheels directly connected to a motor are capable of reducing your speed.  

    In other words, front-wheel drive EVs only brake with the two front wheels, while rear-wheel drive EVs only brake using the less-effective rear wheels.  

    If you want to drive one-pedal, and want more effective, balanced four-wheel braking, you should get a full-time AWD electric vehicle—one motor in front and a second motor driving the rear wheels.  Four tires all working to slow the car, instead of just two.

    As a bonus, AWD gives you better handling, even tire wear, and better traction in slippery conditions.

    Fossil cars slow exclusively with their all-wheel friction brakes, so having all-wheel drive plays no role in their braking.  But would you buy a fossil car that only had brakes on two wheels?  I’m guessing you’d think that was a bad idea.

    That said, there are a number of good EVs with two-wheel drive, and those will work fine, especially if you drive conservatively (and you always will have your friction brakes).  It’s certainly vastly better than driving a fossil.  Just be aware there are limitations with two-wheel drive EVs.

    Second, it’s important to have a good mind-set to driving one-pedal.  You heard me say magnetic brakes can be hard to explain because they’re invisible.  Well, actually, that’s because there are no magnetic brakes.  So when you drive one-pedal, you are really not braking per se.  Magnetic forces in the motors control the speed, and you control the motors with the pedal.  

    So, when talking about one-pedal, the words “braking” and “brakes” are simply metaphors used to help people who still have a fossil car mindset—like talking about the “gas” pedal.  Electric cars are fundamentally different than fossil cars.  They don’t work the same, and you don’t drive them the same.

    With one-pedal, your foot tells the car what speed you want to go; after awhile, you won’t think about how the car is doing that at any particular moment.

    You also heard me say that fossils have three modes, while one-pedal EVs have just one.  The motors control the speed.  They control acceleration and deceleration—speed—the exact same way—with magnetic forces.  There are no brakes.  Once you realize this, when you master one-pedal, when you internalize it, you’ll achieve a Zen-like enlightenment.  And I’d never lie to you about something like that.