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'D'rive vs 'B'raking mode - improving performance?

53K views 100 replies 40 participants last post by  VW TECHNICIAN 
#1 ·
It's been my understanding that the best place to take advantage of 'D' mode is on highways, where there can be substantial benefit from gliding. Similarly, regenerative 'B' mode is best for local traffic, especially stop and go.

I've recently begun to follow the infotainment data (especially miles/kWh) and was surprised that I'm getting better performance with 'D' in both environments. I live in a city, and pretty much every corner has a stop sign: in 'B' mode, I'm getting 2-2.4 (mind you, I'm never doing a fast start from stop, and rarely exceeding 30)! However, in 'D' mode this stop and go traffic gets me 2.8-3.2 -- still nothing to be pleased with, but consistently better.

I'm wondering how accurate this infotainment data is -- maybe I'm just getting bad data? But I'm also a bit concerned that I'll do a series of 5 mile trips, and each one takes off 7-10.

Now I know that these are guess-o-meter (GOM) estimates, but they never fall the other way (that is, drive 10 miles and only lose 5 miles). What advice do folks have for measuring performance more accurately, and optimizing the use of B and D modes?
 
#4 ·
I agree. Historically I've never liked paying twice for momentum.

However around town I simply like the "auto-braking" of 'B' mode. I've gotten so used to it in just over a month that when I drive my wife's SUV I have to consciously tell myself to brake earlier.
So for me I really don't care if there's actually a performance hit. Like appetites, there's always another nightly charge opportunity coming along. ;) [~credit Seinfeld]
 
#3 · (Edited)
From reports so far the infotainment data is very accurate. According to VW - D is designed to be more efficient and it makes sense:
Here is a good article about Brake or coast? Intelligent regenerative braking on the Volkswagen ID.4
The amount of benefit of D will vary depending on how much you use coasting, and how much the system learns your route and observes the other cars and then applys its smart regen just where needed, instead of every time you take your foot off the pedal that you get with B mode.
 
#7 ·
I personally only plan on using “B” mode for mountain driving or on winding roads where you can use the motor to decelerate quickly. It can definitely be a benefit on the downhills. When I ultimately go back to the office I’ll test which mode I like better for my commute, but I think even in stop and go D mode is ultimately better. If it was true one-pedal driving, though, it might be a different story, but until they add that as a feature I’m ok with D.
 
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#9 ·
In D mode you get the benefits of both coasting and regen. If you're brake gently enough with the pedal it's using regen for the braking power until you hit a certain g amount, at which point the physical brakes kick in. B mode gives you the more aggressive regen braking just letting off the accelerator, but in doing so you lose the coasting. Ask any hypermiler how important coasting is to maximizing fuel economy.
 
#32 ·
Coasting [as in 'D' mode] is certainly important in the hypermiling scene and I do use it when on the highway, but in my case mostly to not undergo the harder effective braking of 'B' mode. Again, around town I just like the feel of the 'auto-braking' afforded by 'B' mode, and have no hyper-mileage aspirations. I kind of make a game of lifting off the accelerator at just the right point to sufficiently brake and then use the 3 mph roll to final-close on the stopped vehicle ahead.
 
#13 ·
I think you get more going down because its always further going down the hill than going up (Grandpa told us that so it must be true)
 
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#14 ·
Cruise control is more efficient than coasting, coasting is more efficient than regen, and regen is more efficient than friction brakes. But comparing coasting with regen is a category error. Coasting won't bring you to a stop in a reasonable amount of time.

The benefit of a full-strength B-Mode (Which the ID.4 doesn't have) is you can learn how much space you need to stop using regen only, and maximize your regeneration when you do actually need to stop. The way I drive involves very little true coasting. It also gives me one pedal for all of the motor functions, and the other pedal for all of the friction brake functions. If I need to maintain constant speed on a highway, I turn on cruise control (and CC will manage speed efficiently). If I need to stop frequently, I want to maximize regen - and thus I want to have all the regen available to me in B-Mode.

In D I have to guess whether I'm braking too much for regen and engaging friction brakes.
In B, I can brake completely on regen. If I want to coast for a short period I can feather the pedal to a neutral position quite easily.

In the end, both methods can be quite efficient, especially with smart software (and sometimes, D mode is way safer, like when the roads are frozen). I want the option to use either mode, depending on how I want to drive for the day. I absolutely want B-mode to have the option of giving me full regen. And I absolutely think creep should be able to be disabled in any mode.
 
#15 ·
Cruise control is more efficient than coasting...
I think this is debatable as a blanket statement.

The ID.4 cruise is pretty smooth, but let's talk in generalities.

Cruise can't sense merging vehicles like a human can and open a gap. It can approach a slow moving vehicle too rapidly then have to bleed speed quickly. It can sense an open gap then accelerate into it too quickly to get back to target speed. It can't see approaching inclines although this is more of a problem driving ICE, not as affected with instant torque EVs.

My point being a driver who is "on point" can probably easily "out-efficient" cruise control. But there are a lot of drivers who aren't driving with the constant attention necessary, and there's also variability in ACC systems.

I just think it's hard to make that generalization.
 
#17 ·
Driving in D mode, the car seems to do more than just coast. It feels like it’s maintaining momentum (without cruise control on). Maybe that’s the low drag coefficient I’m feeling. It’s very impressive.

What really bugs me is when I come to a stop at an incline using minimal pressure on the brake pedal, after a few seconds holding the brake at that same pressure I used to come to a complete stop, the car starts to roll back. I then have to quickly slam on the brake pedal to stop. I thought this car has an anti-roll feature. Why do I have to press the pedal harder than is required to come to a complete stop in order to avoid rolling?
 
#18 ·
It has hill hold, but that will only prevent the car from rolling for a short period of time and then the brakes release. This is similar to how hill hold works in most cars. What you’re experiencing is that it takes less braking force to stop the vehicle when you’re driving up hill (even a slight incline), so it take less force to stop the car than it does to hold the car still because gravity is helping you stop the car. I experience the same thing in MINI which is a manual transmission.
 
#19 ·
I have 3, actually 4, hill holds on my i3.

If I roll to a stop on an incline (no brake), the electronics will create enough resistance in the drive motor that the car holds, won't roll back. "Ready" appears on the driver's display to indicate the car is in gear. This state consumes electricity.

If I step on the brake, this engages the hydraulic brakes and releases the electric hold mentioned above. But when I release the brake pedal, an inclination sensor registers I'm susceptible to rolling back, and electrically engages a solenoid that locks brake pressure while my foot transitions from the brake pedal and onto the accelerator. This solenoid holds for about 1 second; at that point, it releases, and the vehicle begins to roll back before the motor catches it. It won't continue to roll backward.

Lastly, I can set the parking brake, just like most gas cars with electric parking brakes. The parking brake will remain engaged and hold the car until I press on the accelerator, at which point it will automatically release.
 
#21 ·
Maybe there is a more appropriate thread elsewhere, but does anyone know if you can still use the parking brake in ID.4 like an emergency brake? I've been wondering about this, since it's combined with the Park "gear" in the same button...what happens if you try to hold down that button while in motion? Does ID.4 have a parking pawl?
 
#22 ·
It doesn't have a parking pawl (thank goodness!). VW states that specifically in the documents linked in the NHTSA thread, that they were able to dispense with it.

The parking park gives me fits in the i3 and was the one part that's failed me, standing me for a short time.

Good question on emergency braking. Can the Park button be held down while driving? I'd better crack open the owner's manual.
 
#26 ·
Just interested in your performance? As I mentioned, I'm getting 2-2.4 M/kWh in 'B' mode. This is within city limits, driving very conservatively, easy starts from stop signs, staying under 35MPH (flat roads, and in moderate temperatures at the moment). This is where I thought 'B' would excel, but it is underperforming 'D'.
 
#28 ·
This reminds me of an incident as a young driver, I learned on an automatic but my first car was a manual. After acclimating to the ways of the clutch pedal, I was driving my mother's Toyota Corona, accelerating up a freeway on-ramp, and instinctively went to upshift. Of course, instead of stomping the clutch pedal, it was the brake, and the car lurched to a nearly immediate stop. Thankfully nobody was behind me.

I did something similar as a new EV driver, on the freeway, lifting my foot off the accelerator to either stretch my ankle or just to coast — having an effect similar to stepping on my brake in the middle of a free-flowing freeway. This was in the i3; I don't think the ID.4 is nearly as drastic. Regardless, it's a mistake you make once and instantly learn from.
 
#31 ·
This reminds me of an incident as a young driver, I learned on an automatic but my first car was a manual. After acclimating to the ways of the clutch pedal, I was driving my mother's Toyota Corona, accelerating up a freeway on-ramp, and instinctively went to upshift. Of course, instead of stomping the clutch pedal, it was the brake, and the car lurched to a nearly immediate stop. Thankfully nobody was behind me.
I was raised in an all-manual household. As soon as I started reading your post, I knew exactly where it was going. I have lost count how many rentals I have slammed on the brake accelerating on an on-ramp!
 
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#29 ·
I've fully adapted to B mode on daily driving and find it very infrequent to use D mode. On the highway I'll frequently turn on ACC until we're in traffic or coming out of the highway, in which case I frequently switch back to B mode. I've found D mode nicest in areas where there is a slight incline down or flat roads with plenty of breaks between stoppages. Even then, I'll sometimes use ACC if there's enough traffic that allows for it. Now that I've driven in B mode enough, it actually feels a little odd not to stop when I release the pedal and the cruising functionality can be a bit jarring.

In the city, I've been using mostly nonaggressive driving techniques and been getting upwards of 4 mi/kwh on good days. There are occasions when I dip low, but most of my inefficient driving is in the highway, where running ACC on up-and-down roads at high speeds can leave me at 3 mi/kwh and sometimes less in colder conditions.
 
#36 ·
Probably a combination of factors. I live in downtown Baltimore, so it's a lot of 1 block accelerations and stops. But I was also experimenting to see how much my driving style and driving mode (D or B) made a difference. Turns out I wasn't able to detect a discernible difference, I rarely get over 3.0 KwH.
 
#37 ·
I'm on my second week driving the ID.4 Pro S. In the city, I've been using B mostly driving as close to one pedal as possible with soft acceleration. No need wasting energy stop light to stop light. Plus anticipating traffic keeping decent space from the vehicle ahead. I found that a light foot on the brake pedal also increased regen performance slightly too. I'm getting acclimated to the ID.4 and learning more each trip. Now getting getting an average of 4+ .1-.3 mi/kWh in town. I prefer D on the highway but haven't driven enough to hyper-mile or master it yet. My first time at highway speed with ACC on was an embarrassing 2.7 mi/kWh. I'm much better at efficiency without cruise control in any vehicle. But I really fell in love with the Adaptive Cruise Control as I spend most of my time in the slow lane. So 5 miles over down the hill and 5-7 miles under going up. Slow lane of course.
 
#45 ·
wow lots of great discussions here. my 3 weeks experience in D vs B mode is this... for higher speed travel on freeways for example, i'll use D mode. If traffic slows down to where it's similar to city driving, i'll switch it to B mode. and of course while in city driving, i'm in B mode most of the time. Not sure which is more efficient but it as far as responsiveness goes, D mode seemed slightly quick to respond to accelerator.
 
#47 ·
B mode for driving in city, small highway 55mph with turns, stop signs, traffic, etc.. for me. D mode for freeway non-stop driving. This seems to work out best for me, but of course I'm always ready to press break pedal when I need to. Never get too comfortable driving...that is the moment you have the highest chance of an accident in an abnormal situation. Always be ready for an idiot on the road to do something stupid...which is an everyday event for me
 
#48 ·
It's been my understanding that the best place to take advantage of 'D' mode is on highways, where there can be substantial benefit from gliding. Similarly, regenerative 'B' mode is best for local traffic, especially stop and go.

I've recently begun to follow the infotainment data (especially miles/kWh) and was surprised that I'm getting better performance with 'D' in both environments. I live in a city, and pretty much every corner has a stop sign: in 'B' mode, I'm getting 2-2.4 (mind you, I'm never doing a fast start from stop, and rarely exceeding 30)! However, in 'D' mode this stop and go traffic gets me 2.8-3.2 -- still nothing to be pleased with, but consistently better.

I'm wondering how accurate this infotainment data is -- maybe I'm just getting bad data? But I'm also a bit concerned that I'll do a series of 5 mile trips, and each one takes off 7-10.

Now I know that these are guess-o-meter (GOM) estimates, but they never fall the other way (that is, drive 10 miles and only lose 5 miles). What advice do folks have for measuring performance more accurately, and optimizing the use of B and D modes?
I find that One Pedal Driving is safer and easier.
 
#49 · (Edited)
I've been thinking about D vs. B and have some thoughts to try out on you all.

I'm coming from a 2007 Fit, so I've been enjoying the novelty of using B so far in my first couple weeks of ID.4 ownership; however, I think there's a strong case for D being better mostly because frictional losses (aero, rolling resistance, bearings, drive shaft couplings, etc.) are unavoidably 100% efficient in doing their job of slowing down the car, whereas there is some waste in using the charging of the batteries as the mechanism for reducing speed. Whatever heating occurs due to resistance in the electrical circuit (battery internal resistance being the largest) while using your motor/generator as a motor or a generator is just as wasted as brake heating is wasted in ICE cars. Heat is wasted energy, with the small caveat that the car might have to use battery power get the batteries to minimum temperature when it is cold outside, so for a small fraction of the drive the resistive heating might not be totally wasted energy within the battery pack itself.

Whether your are in D or B, the first 0.3g of the brake pedal is all regen in the ID.4, right? If your driving habits often put you beyond 0.3g of braking, then B might be a good idea for you because it forces slow-down sooner thereby preventing deep braking. But if you have a habit of lifting your foot off the accelerator in anticipation of slowing, and therefore can stay out of the >0.3g braking zone while using D, you can take full advantage of the perfectly-efficient frictional forces slowing the car and avoid the inefficiencies of using battery charging to do so.

Granted, there's a second reason that B might help as a habit-changer. Speed kills as far as EV efficiency goes due to aero losses going as the cube of velocity*, so if using B instead of D causes you to spend a larger portion of your drive under ~50 mph than you would have using D--in other words, if using B slows you down and delays your arrival times--then B would tend to improve your miles/kWh.

(* There's a second reason speed kills in EV car efficiency that isn't talked about as much and that is because almost all EVs have one speed transmissions. While electric motors are efficient over a much wider range of RPMs than ICEs, efficiency does start dropping off at the 10K+ RPMs our electric motors reach at highway speeds.)

In a nutshell, B only might win if its feedback cues are what it takes to change your driving habits keeping you out of deep braking and below ~50 mph more often. Otherwise, it is more efficient to stay in D, anticipate the need to slow, and let frictional losses do their job and reduce the amount of current flow (in either direction) happening in your batteries. Less current flow through the battery cells means less heat energy loss, and as an added bonus it means fewer micro-charging cycles on your battery perhaps extending battery life.

Am I wrong?

Edit: I just found a link to this article on another thread here that expands on all the points I was trying to make: https://cleantechnica.com/2020/12/15/regenerative-braking-vs-coasting-volkswagen-id-4-gets-it-right/

I vote for D.
 
#52 · (Edited)
Interesting, I believe that article and your points above miss a major factor: we're not robots.

When any of us drives 60 MPH, a plot of our speed wouldn't produce a precisely straight line indicating 60 all the way. It would be a wavy line, representing that we slow down a little, we speed up a little. Not talking "flow of traffic" variations, just being human.

Under true one pedal driving, for every unintended dip in speed, there a corresponding regen activation. For bit of regen, there's a proportional waste of energy since regen-to-battery is not 100% efficient, nor is battery-to-motor as we lightly, unintentionally correct that dip in speed.

Under coasting mode, these micro regen cycles never take place, so that component of waste is removed from the calculation. Any "waste" during the coasting phase is due to the same friction and aerodynamic forces that the vehicle is experiencing any time it's moving.

BTW even though cruise control IS a "robot," it's an imperfect robot functioning in the physical world, and also can't draw a perfectly straight speed plot, either. It won't have the same wild swings in speed as might an inattentive human driver. I wonder if there's a regen filter under Travel Assist or even under normal B mode driving to limit its use, and ignore very small deviations?
 
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