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2020 Corolla Hybrid
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Discussion Starter #1 (Edited)
Has this been solidly settled, i.e. Using "B" to decelerate - on flat road - if only to preserve the brakes.

example where I shift into B: exiting the freeway on short off-ramps.

Is this a bad practice ?
Does it do any harm ?
Does it do any good ?

Thanks
 

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B-mode explained

B-mode made clearer:nerd:

There is a lot of mystery about the "B" shift selector position in the Prius. The official name for it -- Engine Braking -- should provide a first hint as to what it is really for. Page 137 in the '04 owners' manual spells it out fairly clearly, in fact. But somehow a whole body of mythology about "more regeneration" and extra battery-charge magically appearing from nowhere has sprung up since, and really needs to be permanently debunked.

In the smallest possible nutshell, "B" mode is designed to WASTE some energy that the car cannot recover and store. But to fully understand "B" mode one must also take numerous other running conditions into account -- speed, temperature, battery state of charge [SOC], brake pedal pressure, etc.
A preliminary to this was originally posted at Priuschat on Sep 20 2005, but was subsequently lost in the Great Database Fire.
In recreating the information, several other posts and bits of mail were pulled together and rewritten.
This is long and detailed and hopefully not too technical, but tries to cover everything known and verified about "B" mode. It is also worth reading Graham Davies' explanation, at Questions about B Mode
A few basics must be understood up front:
The job of "B" mode is to help stop the car, not to save or recover energy. In fact, it largely throws energy away.
The difference betwen "D" and "B" only appears during decelerative coasting [no pedals pressed at all] or actual braking.
When accelerating or just maintaining speed, "D" and "B" produce the same behavior.
All shift positions, with the exception of "P", are simply electrical states within the control computers. In "P", a locking pawl engages gear teeth to lock the front wheels against turning, but that's the ONLY mechanical change.
Regenerative braking really has nothing to do with the physical wheel brakes on the car -- the regenerative and friction systems certainly cooperate closely, but they are separate systems. "B" mode has nothing to do with the physical hydraulic brakes at all, other than that the two systems can combine forces to help slow the car. Nonetheless, hydraulic braking, regeneration, and "B" mode are really all parts of the overall braking functionality.
A good overview of the braking systems in both the '04+ and earlier "classic" Prius is in the technician-training material on hybrids available from Toyota's Techinfo site, in section "T072 Chapter 6 Brake system" and in the file ileaf/techtrg/techtpdf/techtrg/techtrg/cours/section6.pdf ... which is mirrored temporarily at http://techno-fandom.org/~hobbit/cars/cours-section6.pdf for reference. It does not really explain "B" mode well either, and in fact this document may be one of the sources of confusion -- where it says selecting "B" ... will maximize regenerative efficiency and is useful for controlling speeds downhill. In "B" mode about 30% of the energy is recovered. This is very misleading, because what it does not say is that in normal "D" mode with proper braking technique, considerably more than 30% of stopping energy is recovered! I think they meant "30% of the possible energy recovery" that would otherwise happen in "D" mode, but something got lost in translation.
Going back to the original definition of "B" mode -- it takes energy to turn a car engine that is otherwise not running. Friction in bearings and pistons and cams can be significant, as anyone who tries to turn an engine by hand is aware. But more significant is that it takes energy to pump air through the engine -- without fuel and spark, an engine is really nothing more than a big air compressor. The amount of energy needed to keep all that turning and pumping is what causes the "engine drag" felt in conventional cars when the accelerator is released. And if the transmission is geared down, the drag effect is more pronounced because the engine must then spin [i.e. be pulled around against air and mechanical resistance] that much faster.
In the Prius, however, under most normal conditions when the accelerator is released the engine shuts down entirely and leaves the car in electric-only mode. This would provide no engine-drag, so the Prius fakes a drag feeling electrically by having the main electric motor-generator [MG2] generate some current and thus present an energy load against the turning wheels. Above 41 mph, the engine does still spin, but it is not fed any fuel or spark and the valve-timing is changed so that the air-pumping loss through the engine is minimized. Electrical energy is still drawn from the wheels by MG2 regardless of the car's speed -- and the most useful place to send that energy is to store it in the battery. This is a regenerative stopping force, but in "D" with no brake pedal applied it is a fairly weak force -- not one you could really consider as "braking". It is not enough to prevent the car going faster and faster down an appreciable hill, for example.
Regeneration current into the battery in this state is between 10 and 20 amps [out of a possible 100] depending somewhat on speed -- at 200 battery volts that's still 2000 or 3000 watts or more, which is quite a bit of energy from just gently resisting the car coasting along! Gently applying the brake pedal increases that regeneration current, up to a maximum of 100 amps -- 20 kilowatts -- and in the '04 and up Prius, does not use the physical friction brakes at all until they are needed. That's like the motion of your car powering four electric dryers at once, and it's all going into the battery pack! But even 20 kilowatts cannot always provide enough stopping power, especially at higher speeds, so anything over and above that must be supplied by the friction brakes. The rest of that energy then gets wasted as heat in the rotors and pads. But what if you're driving down a long, steep mountainside? Maybe that 20 kilowatts of battery-charging energy is enough to hold your speed back -- for a while. But eventually the battery gets full -- actually, to the 80% charged limit enforced by the computers -- and to protect the system, charging current is eventually reduced to zero, and the only thing now holding the car back from disaster is the hydraulic brake system -- which is now rapidly getting hotter and hotter and reaching its own limits on how well it can continue stopping the car. Brake fade, when the parts cannot absorb or dissipate any more energy, is a very real problem on mountain roads. Enter "B" mode. As in, "trucks use lower gear". By forcing the wheels to spin the engine and pump air, a good deal of that energy can be turned to heating the air going through the engine instead of heating the brake parts. Since fresh air is always coming into the engine, having it leave as much warmer air provides a convenient place to dump excess energy.
In a conventional car the wheels push the engine around through the transmission, but the Prius needs to help that process out a little bit by actually having its combination of electric motors spin the engine. In this case, the valve-timing in the Prius engine is advanced to increase the amount of air taken in and the suction against the throttle flap -- which uses much more energy than the coasting-in-"D" scenario above. Either way, stopping power now comes from a combination of things and the burden on the friction brakes is greatly reduced, allowing the hill to be descended safely.
"B" mode also increases regeneration current to 30 - 40 amps with no feet on the pedals, so the part about "more regeneration" is somewhat true. That is one of several mechanisms used to increase the "drag" feeling. That level also varies with the car's speed. However, the car's movement is often supplying much more energy than that, so what isn't captured in the battery is wasted by flailing the engine around. This is not more efficient usage -- it is almost always better to gently brake in "D" for maximum energy recapture, if you have room ahead to do it. This is one of the common misconceptions about "B" mode -- it does not create more energy from nowhere, despite how much it may feel like traditional "gearing down" and using the brakes less.
In fact, for those times when the rolling car has too much energy for the battery, "B" helps get rid of it. In addition, using the brake pedal while in "B" mode behaves exactly the same as in "D" -- if there's any capacity left in the battery, the system tries to regenerate up to the same limit of 100 amps, above which the friction brakes are brought in to help -- the only difference in "B" that the engine is also spinning away against air pressure. Again, the hydraulic brake system does not care if you're in "D" or "B" -- it just supplies what the rest of the systems cannot. The only time the physical brakes are used by preference is during a panic stop, when the pedal is suddenly slammed down. The system senses this fast rate of change and immediately brings in the hydraulic brakes for faster and safer stopping with all four wheels. "B" mode makes no difference there, either.
And of course all regeneration quits at less than 6 or 7 mph, when the motors aren't turning fast enough to provide useful power -- the physical brakes handle the last part of stopping. Many people can feel a sort of braking "sag" at the transition, although Toyota has managed to make that fairly smooth and seamless.
If one thing must be understood here, it is the distinction between the BRAKES and the total BRAKING SYSTEM. The hydraulic friction brakes in each wheel cannot supply energy -- they can only waste it, throwing it away as heat to the air around them. Parts of the braking SYSTEM -- that one could consider as including the driveline and electric motors, the hybrid and braking computers, the battery -- can work together to recover energy and direct it around to where it needs to go. But when someone naively says "the brakes charge the battery", that's really rather wrong.
Now with all of that said, there are a few funny quirks and factoids to know about "B" mode, none of which really help increase fuel efficiency but are interesting to know about regardless.
In general, the amount of extra resistance given by "B" mode is sort of staged upward depending on the car's speed and how charged the battery is. Some of these conditions can be utilized in entertaining ways. Under 20 mph, if the engine is not already running and your foot comes off the accelerator, B mode simply regenerates reasonably heavily [30A or so] into the battery. This drops off around 12 mph to a lower current, and is then similar to being in D until regen capability kicks out entirely around 7 mph. So between 19 and maybe 10 miles per hour, you can use "B" to slow down in an energy-productive way, and essentially drive around in electric-only mode with one pedal -- but be careful to not do something the person behind you doesn't expect without showing brake lights!
As soon as you crest 20 mph, however, the engine begins spinning -- to enable the system to dissipate more energy at the higher speed. If the engine is running and you come to a standstill while in "B" mode, the engine stays running -- just idling. The reason for this is not really known, but it is a way to continue warming the engine when it's cold out and you're stopped in traffic.
Driving around in "B" during warmup also tends to charge the battery a little faster, since electric-only mode is avoided, but again at the expense of burning more fuel to do it. Engine start/stop transitions are avoided. Sometimes this state feels more surefooted and responsive in snow and other tricky conditions.
People who have autocrossed the Prius have recommended staying in "B" for better and quicker control -- having the driveline "fall on its face" the instant your foot comes off the accelerator pedal may be desireable behavior at times. This may feel familiar to some EV drivers, where regeneration control all comes from releasing the go-pedal in controllers without integrated braking features.
Fuel usage in "B" is somewhat mitigated by the fact that when decelerating above some nominal speed, somewhere around 17 mph, no fuel is sent to the engine and it just spins "dry". It's still wasting energy and slowing the car, but there's no reason to throw away gas along with that. This is sometimes called "fuel-starve" mode, and is also used in some conventional cars during high-speed coasting conditions.
It is difficult to tell when that 100 amp battery-charge limit is exceeded without extra instrumentation. When the battery pack is cold, that limit is actually lower -- down around 50 amps, until cabin heat begins to circulate through the battery pack ventilation ducts and the pack self- heats a little from being used. The system is quite good at protecting the battery against things like overcurrents, and sometimes that gets in the way. Slowing over bumps often confuses the regen mechanism, which can sometimes be felt by the seat of the pants as that same little braking "sag" right after the bump. The system has given up on regen at that point, and is now only collecting the "coasting" baseline 10 amps of battery current, and using the physical brakes almost entirely to stop you. Recovery from this situation appears to be time-based, so your best bet at that point is to slap it down into "B" for the duration of that stop since while you may spin off some energy in the engine, regen current will be a bit heavier than in your now pathological D-but-confused braking state and you might recover a tiny bit more energy.
But don't get into the habit of using "B" to slow down unless you really need it -- that's sometimes hard to get used to if you come from ingrained years of "gearing down" in conventional cars.
Many strange things happen when the battery pack gets up to "eight green bars" full level. The hybrid system begins doing several things to pull a little energy back out of the pack -- the engine will tend to spin in "D" mode even at low-speed, low-demand conditions, in fact just about in the same way as "B" mode does when the charge state is more normal. If "B" mode is selected in the full state during coasting, then the engine really screams and even more energy is pumped away. So while all the energy of a long descent cannot be captured, speed can be controlled in some interesting ways by creative shifting between "D" and "B" even after the pack is topped out.
When the car is stopped, the engine may randomly start and stop several times -- the theory is that the system is ridding itself of excess battery charge to get it back within safe limits. You only really get 600 watt-hours full range to play with, which isn't a whole lot. Still, the car really goes out of its way to make sure there's plenty of stopping-power reserve on tap if the driver needs it. And there's no question that larger battery packs would give a much wider range of energy-recovery -- possibly enough to hold an entire mountain descent's worth. Some of the extra-battery experimenters have successfully done that, in fact. It is said that the early Japanese "hypermilers" used B mode to gain fuel efficiency. There is no advantage to be achieved by this with the current generation of Prius, but in the earliest Japanese models and the "Classic" imported to the US for 2000-2003 the regenerative braking system is a bit more crude and brings in the physical brakes much earlier in the game even when they're not necessarily needed. With higher-power-capable motors and the reworked "by wire" brake system in the '04 and since, this is no longer relevant since little or no pressure is sent to the wheel brakes until the system has extracted as much regeneration as possible. However, those early and somewhat vague stories could be another source of myth and misinformation. Help clear up some of the confusion about B mode. Tell other owners [and dealers!] who don't necessarily read this stuff why "B" is NOT saving them any gas.
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http://techno-fandom.org/~hobbit/cars/b-mode.html


This is about the best article available to explain B-mode. I also find it rather contradictory, as it states that B-mode is not efficient way to charge the battery - then gives multiple examples of how it actually does charge it faster. I read this article several times and come to feel that author either has some prejudice against this mode or/and never actually drove a hybrid and used B mode for extended time. hence, author simply does not have real life feel of it and take more scholastic, biased stance.
 

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In nutshell, OP, if you ever drove a manual car ans know how and when to use engine braking - you do same with B mode. When I had hybrids, my hand will toss it into that mode very frequently.
 

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Discussion Starter #4
haha hi ukrkoz

Thanks for the info you've provided on this topic in all these years.

The very first search result I looked at has your posting
so you've been doing this shift-to-B-to-decelerate for quite a while.
Irregardless of MPG gain or loss I trust you never had mechanical issue traceable to this practice ?

Yes I had MT before.
 

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Read manual. It says, you can drive in D or B position. In old Prius manual it said to not drive in B for extended times as "certain gears may get damaged" without getting into detail of what extended time is.
If you ever drove Tesla, driving in B mode is pretty much like driving Tesla.
 

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Unlike previous years, you have:
Electronically controlled Continuously Variable Transmission (ECVT) with sequential shift mode
What is, to my understanding, fake gear shifting, as there is no gears in PSD. I think, some complained that "transmission is not working as it does not shift" so they came up with this thing here.
Look at the picture:
301607


Shift into "manual" mode and then, shift all the way down to (-). That should work just like B mode discussed.
 

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Unlike previous years, you have:
Electronically controlled Continuously Variable Transmission (ECVT) with sequential shift mode
What is, to my understanding, fake gear shifting, as there is no gears in PSD. I think, some complained that "transmission is not working as it does not shift" so they came up with this thing here.
Look at the picture:
View attachment 301607

Shift into "manual" mode and then, shift all the way down to (-). That should work just like B mode discussed.

Yes, mine looks exactly like that. Thanks for the info.
 

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PLEASE, try it and repost results here. I am very curious.
On the surface, it looks exactly like the same transmissions from before, only with fake shiftable mode.
I'd appreciate thta.
 

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PLEASE, try it and repost results here. I am very curious.
On the surface, it looks exactly like the same transmissions from before, only with fake shiftable mode.
I'd appreciate thta.
It looks like using sequential shift mode allowed me to engine brake. Thanks for the info.
 

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Great! I really appreciate the follow up. I was wondering, what the heck happened to it.
It's very useful mode.
 

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Since 'B' mode is automatic on the 2018+ Camry, what's the advantage of forcing 'B' mode rather then just letting it automatically enter 'B' mode when going downhill, wanting to slow down (using the brake pedal), and the battery is completely full?

It's not like it's any less wear and tear on the brakes (assuming the algorithm used to decide whether to regenerate or use the friction brakes is intelligent enough).
 

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Is it? Never heard of it being automatic. Means nothing, they might have changed it.
I used B mode pretty much all the time. As I told Pujuma, if you know how to use engine braking on standard shift car, you'll like using B mode. There is much more to its application, then just decelerating down the hill.
 
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