Hacker News Reader

Comment by floren | Hacker News Reader

I'm a bit surprised to read that one-pedal driving actually saves energy; I've always assume that having once converted electricity to motion, it's far more efficient to just coast when you take your foot off the pedal than to convert it back to electricity. If I'm trying to drive my EV efficiently, I put it in D mode (behaves like a regular auto transmission, more or less, but regen on braking) and do my best to stay at the speed limit on uphill or level ground, coasting up a bit above the limit on downhills, and letting myself coast in to a red light instead of keeping my foot on the pedal until I'm close & then braking.

View on HN

Coasting isn't braking. If you want to coast just don't completely release the accelerator pedal, only up until the point where you're no longer accelerating or braking.

Every one-pedal-driving car I’ve ever tried has nearly no dead zone between acceleration and braking.

You don't need a dead zone. Just modulate the pedal to whatever level of acceleration you want. The car will either regen a little, use a little power or use next to no power if you're at the perfect inclination.

Energy from going downhill (assuming it's steep enough to overcome drag and rolling resistance) can go into either of two places: increased speed or charging your battery. The only difference compared to an ICE is that you only get the first option when going downhill (or wasting it into heat via the brakes).

How fast does the regenerative braking start? When I drive in an ICE with two pedals, when driving in an area with hazards, kids, etc, I frequently will take my foot off of the gas and hover it over the brake so that I'm able to respond quickly. I can do this because taking my foot off the gas while maintaining speed doesn't make an abrupt impact if I'm maintaining speed in a flat area.

I would still want to be able to respond equally quickly in an EV, but it seems like it would lurch quite a bit if I did the same in a one-pedal setup?

I don't know that I'd want a full toggle in an inaccessible spot to be able to coast. Do any EVs have like a paddle to coast/disable regenerative braking?

In a one-pedal setup if you jerk your foot off the pedal it'll immediately start slowing down significantly. In the majority of situations you will react significantly faster with one-pedal driving than without. The only exception is if you're hovering over the brake pedal. Not my style, but to each their own.

Sure, you react faster. With the wrong reaction in an emergency.

In a traditional (automatic, anyway) car, there is one way to brake. You brake lightly if you want to brake lightly, and you brake harder to stop faster.

In a one-pedal EV, you lighten up on the accelerator to slow down a bit, and you remove your foot entirely and put it somewhere else to stop faster.

I would love to see NTSB or a similar group do actual simulator studies to see how normal people react.

Do you have two feet?

Left-foot braking is unpopular outside of racing but only takes about a day to get used to. For the use case you're describing it's probably even less since you'd probably only use it if you need maximum braking in an emergency.

The real challenge is what happens when you have to react instinctively in an emergency. That's going to take more than a day.

I would recommend people who want it in their toolkit left-foot brake exclusively for a week and do it semi-regularly afterward. It took a day for it to stop feeling weird for me.

In my EV in single pedal taking my foot off the accelerator is already instantly like having heavy braking. By the time I'm touching the brake pedal even slightly it is like I'm stomping hard. Coupled with collision detection priming the brakes potentially before my mind registers the potential collision I'm not worried about not being able to hover over the brake like in my old car.

I think you should be worried. Taking your foot off is heavy in the sense that it slows you down at decent speed. It is not heavy in the sense of an emergency stop, nor is having your foot on the pedal ready to remove priming you to slam on the actual brakes.

I’ve never heard of collision detection priming the accelerator to apply the mechanical brakes when a foot is removed.

You've never heard of collision detection priming the accelerator because it primes the brakes not the accelerator when it detects a potential collision. Its unrelated to me lifting my foot.

And I've definitely experienced it happen. I've lightly touched the brake pedal with AEB warnings going off and it really slammed on the brakes.

Either way I imagine neither of us has any hard numbers to actually back up either driving style. I'm just pointing out there's no lag to regen braking and in many cars it can be pretty heavy braking.

My point is that, if a driver gets used to heavy-ish braking from the accelerator pedal, they might not tap the brake pedal on time in an emergency.

If you're relying on saving the time to take your foot from the gas to the brake in order to save the life of a child or your own life, then you should probably just slow down.

You're making a lot of assumptions about me and flipping them into a big false dichotomy.

It's actually entirely possible to both drive at a safe speed and even still want to be able to brake quickly when you see potential hazards on the road.

I think you're right.

It might be nicer to have a more gradual slope around the middle.

Sort of like how steering wheels have a little play for comfort.

Also, some people are TERRIBLE at modulating the accelerator pedal.

I was in the car with one of those people when newly trying an electric can and I got queasy pretty quickly.

Have you been in a Tesla? I have no problem maintaining an exact speed on a highway, or anywhere else, using one pedal driving.

It's so easy in the Model 3 I don't even think about it.

So what happens when you want to drive at a constant speed on the highway?

I haven't driven one but I assume the same thing happens to steering when you want to stay in your lane.

You'd use cruise control.

I'll say this as someone who's been driving for over 20 years, probably about 250,000 miles, and has owned an EV for just over 3 years...

Maintaining a constant speed on a flat highway isn't hard, but doing it on terrain that goes up and down a lot is much harder in an EV. Maybe it's because every car I've driven before was a manual, but I got really in tune with judging my speed based on my car's engine sound. The visual change going from 60 mph to 70 mph because you started going downhill is not nearly as obvious as the change in engine note as the RPMs rise.

Going through the hills on I-5 near the Oregon/California border, cruise control is an absolute MUST in an EV.

Coasting is braking using air and rolling resistance.

Coasting can be more efficient. For instance, on the hyundai ioniq 5/6 1 pedal is typically the most efficient mode for around town driving, though a skilled driver may be able to beat it on one of the lower regenerative modes but it its a good deal of cognitive load for marginal benefit. You can also enter into a "auto regenerative mode" when using adaptive cruise control on the highway this mode will mix in coasting and has the highest efficiency.

> but it its a good deal of cognitive load for marginal benefit

Also known as "fun".

Hypermiling in a gas car is my driving game.

True hypermilers are crazy, IMO. But most drivers can certainly learn from them.

When I drive an ICE, I drive like I don't have a brake pedal because it's an evil device that converts cash into heat and brake dust. The light ahead turned red? Take the foot off the gas. No sense maintaining speed if I'm just going to stop. Driving through a large dip? Let the car speed up as you go down so you'll give yourself momentum to climb out.

It blows my mind when I see someone driving up hill, notice they're going too fast, and they hit the brakes rather than just let gravity do the job.

My previous car was a Subaru BRZ, stickered at 24 mpg city, 30 mpg highway. I averaged 33 mpg. I didn't even drive slow. I just made sure that speed was never wasted unnecessarily.

Yes this is also Porsche's position,[0] which I believe is actually superior to the Tesla way. Of course the best option would be to let the driver choose between modes.

Letting off the pedal gives an easy way to explicitly command "torque sleep"[1] on the motor, which saves even more energy.

[0] https://cars.usnews.com/cars-trucks/features/porsche-says-no...

[1] https://teslaowner.wordpress.com/2015/10/19/7-0-torque-sleep...

I wonder if its more due to the fact if you're used to one pedal driving you're never touching the brake and thus practically never engaging the friction brakes.

My wife uses traditional driving in my EV, I use single pedal. While she's pretty good at keeping it in the regen-only braking zone, pressing just slightly too hard will end up having some percentage go to friction brakes. I just never touch the brake pedal, so I never engage the friction brakes.

This ought to be solvable by adding some haptic/force feedback through brake pedal? For instance have a distinct click at some point in the pressure curve that means "at max regen braking" and if you keep pushing there's a bit of resistance and then another slight click that means you're engaging the friction brakes? Placed close together in the pressure curve, but still distinct enough to get consistent results?

If might be more efficient to coast, but I bet regen braking is less efficient than using your brakes.

Then what would be the point of having it?

You can have coasting and regen braking with traditional two pedal driving. With Toyota's hybrids, for example, using the brake pedal engages regen and only engages the brake once you need more braking force than the regen can provide.

Depends on the car, mine (Polestar 2) uses regen braking regardless of whether one-pedal driving is selected. The consensus seems to be that coasting is slightly more efficient but barely enough to make a difference.

Converting kinetic energy back into something you can use later to accelerate is less efficient than dissipating it as heat you can never recover? I don't follow your math.

I’m not even sure what I meant. More sleep needed.

Edit: While coasting might be more efficient than braking, regen braking is probably more efficient than using brakes. (Which might explain the overall efficiency improvement when using one-pedal driving even if we assume coasting is more efficient than braking)

Reply on news.ycombinator.com