The 'B' Factor

You are correct it does not improve mileage. From what I understand "B" does not help charge the battery. Using your brakes does recharge the battery. So use your brakes when coming up to a curve and recapture that energy. On long grades when your battery is topped off then use the "B".

 

This would shock me. If the battery is not at full charge, then using B *should* recharge the battery using kinetic energy... and slowing you down. If the batter is full, you just get engine braking. However, since the battery is so small, the generator is so small, the losses are relatively high... you just won't see any great mileage gain, though it should be there as a fraction of a percent.

But then again, I've been wrong before!

 

"B" mode only spins up the ICE for extra drag so that energy is just being lost by spinning the ICE. "B" is only for long downhill descents like coming down the mountain.

 

Futhermore, I believe that the Prius doesn't have a true engine braking. It simulates engine braking by essentially applying the regenerative brakes. In fact when you let up the accelerator you also get a lighter version of this simulated engine braking. The Prius also has a bit of simulated engine creep as well so if you're on a level road and lift off the brakes the electric motor will drive just a little.

So it's my understanding that the "B" mode should be no different than light pressing of the brakes. So the only advantage of the "B" mode is that it's a little easier on your brake foot, if you're on a long and steep downhill. In normal driving using the "B" mode to do your braking is just a nuisance versus just applying light pressure on the brakes. Either technique should regenerate electricity just fine.

 

Benighted is correct, the "B" mode simply spins the ICE for more drag and is only intended for long downhill grades.

If you are in "D" coasting down a hill and all battery bars are illuminated, the system will spin the ICE just to spill some energy. What "B" does is allow you to decide to use engine drag before all the bars are illuminated.

However, to every rule there is an exception. If you are coasting to a stop and are below 20 MPH, the "B" mode will increase the regenerative braking, but not spin the ICE. Several people have reporting using this successfully along with pulse and glide. Just remember to put the car back into "D" after you come to a stop, otherwise, the next time you take your foot off the accelerator, the ICE will spin and the drag during coasting will be much higher than normal.

Personally, I tried "B" a couple of times to see how it worked, and I haven't used it since.

 

Well, color me shocked! Why the hell would they do this! If you don't have a full charge, the slowing should come from regeneration as much as possible. Yikes. Not in a million years would I have guessed it worked this way. Is this actually explained somewhere official, or was it discovered through tial and error by drivers? Most Prius drivers I've talked to are pretty confident that they're charging the battery in B. That's certainly what happens in the Toyota Rav4EV. It is called B there as well.

 

You *are* charging the battery in "B" ... and you also are in "D".
Please search for the NUMEROUS prior posts about "B mode" or
"shift to B" or "driving in B" and the like -- the info is there.
.
Somebody should write up the definitive guide one of these days..
there's a lot of subtlety in exactly what B does depending on
numerous input conditions.
.
_H*

 

Yeah well... I HAVE searched on "B mode" and I've read all the arguing and theories and contradicting information that is slowly cropping up here as well. Until I actually get my own car, I have NO idea who the hell to believe.

From what I can tell after reading SO many posts that go one way and then the other, is that light braking MAY generate more than B. B does some generation but also bleeds off kinetic energy with actual engine braking.

Honestly, I have NOT heard the definitive answer on this. If you have it, please let us know what it is, and I'll put it in my FAQ. Searching all over these forums for B mode doesn't answer the question - it just extends the drama and pain!

I see NO reason why B mode should't provide MAXIMUM regeneration... but I'm pretty confident that it does not. But that's about all I'm confident about on the subject at this point.

 

As far as I know, B mode is explicitly for "engine braking". That is, you might be descending down a steep and prolonged incline, such that you can't use your brake pedal without overheating your brakes.
In a normal vehicle you can shift down into a lower gear and use 'engine braking'.
In the Prius you shift into B to accomplish the same thing. If you're shifted into B with your foot off the throttle, the Prius will attempt to decelerate the vehicle as much as possible without using the brake pads. It can use both regen and engine braking to accomplish this, but it can't use the brake pads unless the driver presses the brake pedal.

 

"Maximum regeneration" would be 100A into the battery, which I'm
sure isn't what you want simply by moving a lever.
.
My old *large* rundown on B from last September seems to have died
in the PC crash, but fortunately I still have a copy. I'll dig
it up and rework it a little.
.
_H*

 

Darrel,

Dont' think in the EV mindset here. The wee l'il prius battery get's "full" awfully fast on a good slope. B is for use in long downgrades and when the battery is at full SOC the engine braking will completely take over. Moving at 60 mph on a 6% downgrade will charge the battery in a heartbeat. That was my experience coming down off of the continental divide on my first trip up into the mountains in the Prius.

 

If you are going below 20 MPH, engaging B mode does NOT spin the ICE.

It uses the maximum available regenerative braking to slow the car.

Japanese Hypermilers swear by it, and I find it handy at times in stop and slow traffic.

Also, as an added benefit, you can use B to force an immediate ICE shut off at a stop light or sign if the engine is warm enough.

Sometimes when you come to a stop, it can take 5 seconds or more for the ICE to decide to shut off. Depending on the situation, this can get you honked at

If you know you're going to stop, engage B right before stopping, then go to Neutral, then D, and the ICE should shut off immediately. I find that one handy too.

 

Tempus, you're fooling yourself. That simply ain't the way it
works. What's happening is that as you stop and spend the next
5 seconds wanking around with the shifter, the normal stage-4
timeout has occurred. If the system is ready to do it, and you
simply stop in "D" and wait the same amount of time, you should
see the "jerky shutdown" that tells you you're in full hybrid mode.
.
In other news, I found my old rant about B mode and have put
them back together into a longer document. Please look at
http://techno-fandom.org/~hobbit/cars/b-mode.html
for a prelim. Critiques welcome.
.
_H*

 

Hi All! I just picked up my new Barcelona Red yesterday. The salesman told me to put it in B mode. He said most people just leave it there. I drove that way all 120 miles home. I hope I didn't screw something up....I got 44 mpg for my first trip. Please tell me how exactly do I drive this car. Am I trying to keep the green mpg markings around 40 or 100 or what? Thanks

 

After 10s of thousands of miles and thousands of stops, I'm afraid I must disagree with you.

I can fairly easily tell the difference between one second (the time it takes me to stop the engine my way), and up to 10 seconds (the time it can take for a normal shutdown).

It's not that hard to distinguish.

It's much more discernable when the temperature is colder if you're inclined to actually find out 'how it works'.

 

Basically just drive it. Avoid speeding up and slowing down as much as possible. Read the "driving tips" section of the owner's manual.

Put it in "B" only as an alternative to riding the brakes when going down a long hill (on a steep hill you may still have to ride the brakes, but "B" will reduce the wear on the brakes.) Using "B" in any other situation is for fanatics and leads to religious arguments.

If you feel benevolent humanitarian impulses call the salesanimal and ask him to read the "hybrid transaxle" section of the owner's manual (this starts on p. 135 of the 2004 edition). It states explicitly that "B" should not be used for extended normal driving because it can reduce fuel economy.

 

Hell, I'd LOVE to have 100A of regen. Of course then I'd want more battery capacity... and then a plug and then... In the Rav I've done over 200A and I often wish I had more!

Yes, 100A would be a pretty big jerk (bigger even than the driver?) but when I say "max regen" I mean the max that the car does normally - like when the brake is pressed.

So... from what I can tell here, there's still quite a bit of controversy on so many of these aspects. I'll go read your blub now. Thanks for putting it back together.

 

I'm happy to report that I don't ever see that happening.

Yeah, I know the battery is a cute little thing, and that the "generator" in this case is somewhat small as well. I'd just think that using B would first do all the generating that it can, and only THEN would it blend in engine air pump. On the Rav, of course you get pure regen in either of the regen settings, and when using the brakes you get *mostly* regen (in the 90% area) with the brake pads just being seated against the rotors. If you stop without shoving the brake pedal hard (in other words, you are not "panic stopping" - you end up with almost all regen (and the associated losses of course), and very little energy bled off to friction. Oops... starting to ramble... back to the vehicle at hand....

Still plowing through Hobbit's piece. Maybe one day this will all be clear to me *and* logical. At that point I'll help spread the word so we can teach the world about B mode!

 

I almost hope this isn't a real post.

First of all, on the assumption it is real: congratulations on your car! We both got Barcelonas on the same day. And welcome to PC. Where are you from?

The reason I am suspicious is that not even the most undertrained salescritter should ever tell a Prius buyer to drive in "B" all the time. Dealerships are OBLIGATED to give customers at least a rudimentary "training" in the most important elements of driving their new cars. Not to do so can be dangerous.

Several posters above mistakenly say that "B" is only for engine braking on long downhill coasts. It is also to provide such braking on short, but steep grades, too. The only time I ever use B, for example, is when driving in San Francisco.

Duck, you need to take a breath and read your owners' manual before broadly asking "please tell me how exactly do I drive this car". There's enough to say here just addressing the "small things". Then read our FAQ and information base. Duck, don't worry about trying to hit 100 with the fuel consumption indicator. Worry first about how to start the car, shut it off and use its controls. The fun part comes AFTER that.