[JimJeffers]

When I "downshift" to "B" on my 08 HH it almost sounds like the engine is revving way up. But the chart of the screen shows the engine still off. Can I shift to "B" at any speed? Is it safe to use for a long downhill mountain pass?

 

[Tideland Prius]

When I "downshift" to "B" on my 08 HH it almost sounds like the engine is revving way up. But the chart of the screen shows the engine still off. Can I shift to "B" at any speed? Is it safe to use for a long downhill mountain pass?

"B" = engine brake. It's the equivalent of you shifting to "2" or "L" in a regular automatic.

Yes the engine will rev. The faster you go, the higher the rev (my Prius sounds like a small plane coming in for landing)

By "off" it means it's not using any fuel which is true when you engine brake. You're just pumping the cylinders and using that resistance to slow you down.

It helps on long downhill mountain passes cause you don't end up cooking your brakes.

 

[Revo291]

When you say "it's the equivalent of you shifting to "2" or "L" in a regular automatic", does that mean that it acts like "2" or "L" in say, snow, or mud, or uphill, for extra traction and torque?
I always wondered what it did, I have experienced the same behavior and sounds as the original poster, I use it on nasty offramps and for downhill braking, but I always wondered if it would do any good as a "Low Gear". Thank you!

 

[ukrkoz]

B-mode explained


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.
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.
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.
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. ?
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.

Yours
Ukrkoz

 

[ukrkoz]

does that mean that it acts like "2" or "L" in say, snow, or mud, or uphill, for extra traction and torque?

NO. It means, it acts just like D normal driving, the very moment gas pedal is touched. Owner manual actually says that you can drive in D or B.