B Mode--What is the Purpose?

I still have not been able to figure out the purpose or the B mode.

Can someone please explain it to me, as well as tell me when/why I would want to use it?

Thanks everyone! :welcome:

Welcome aboard! We have another thread going on the B shift sellection. All it does is shift down one gear. It would be used to slow the car going down a hill. I used it twice to find out what it was and vowed never to shif to B again unless I'm in a race and need a lower gear. :welcome:

Think of what you do in a regular car by placing the vehicle in a lower gear when on a long down grade in order to avoid excessive use and overheating of the brakes. The B mode stands for "engine brake mode." Due to Prius technology, it's not normal engine braking, but achieves much of the same thing as far as minimizing the need to step on the brakes. You would probably never want to use the B mode for normal driving, and it does not save fuel in a Prius.

I assume you meant "shift"?

The queston is would the B mode downshift to help you go UP a hill?

That was a real typo. I have fixed it!!

Thank you!

The purpose is to slow down the car using the gas engine to intake and compress air; basically to waste energy.

It is best used in a very long downhills situations to minimize brake pad fading when the HV battery would become too full to absorb any more regen-braking.

It's not really a gear, so no.

There is no downshift or upshift in Prius. Prius only has one gear and it is always the right gear. The only thing that shift is between gas or electric. Low gear is the electric motor and high gear is the low torque Atkinson cycle gas engine. How the computer blends the two power source creates the effect of a CVT. It is named eCVT because it is an electric transmission rather than a belt-cone mechanical one.

Engine braking or compression braking is achieved by one of the electric motor turning the gas engine (pumping air in and out through a small hole) while the second electric motor regen-brake the car. All this was done while charging the HV battery.

B mode only behaves differently if you let go the accelerator; it slows down the car faster. If you are accelerating, it is no different than the "D" mode.

[EDIT] Removed this post as its information was partly wrong and contributed nothing to the available knowledge.

I thought it was the electric motor switched into a charge mode that accounted for the braking effect. If so, it would not have any impact on mileage. It does reduce your disk brake pad wear.

I use it and seems to work well, especially for long downhill sections of highway. I am also used to driving a manual transmission where down-shifting is more common. In my other Toyota vehicles, I turn-off the over drive to achieve a similar purpose (only here it probably does impact mileage).

My question is then whether B mode would also be useful while in reverse, such as backing up on a steep hill. Would the extra resistance from the ICE help prevent rolling when starting out from a parking space or a stop on a hill (either in forward or reverse)?

I can't believe that so many years after the Prius was introduced there are still people who believe this is how B mode works!

What is said above is ALL WRONG folks! You will NOT charge your battery more when in B mode unless when going really slow bellow about 35 km/h.

Please read this very complete and detailed work from Hobbit on B mode:

http://www.techno-fandom.org/~hobbit/cars/b-mode.html

Please make sure you know what you are talking about before spreading these myths...

Although I'm fairly certain that "B" mode is intended to protect your brakes while going down long hills, I've also heard that "B" mode will not allow the gasoline engine to turn off while in stop-start situations (might be a myth since I haven't yet tested this for myself).

While on vacation to Los Angeles last year, I rented a Prius and immediately ran into some of the worst stop/go traffic (mostly stop) that you can imagine for about an hour or so (it turned out that there was a large truck on fire about five miles ahead that was causing the delay). The battery eventually went down to 2 bars and then the gasoline engine began to cycle on and off, with a corresponding shudder each time it cycled. Touch the brake to stop and the engine would shut off. Touch the gas to go and the engine would immediately start back up. After a while the near constant shuddering as it endlessly repeated this process started to become annoying. I wanted the car to just stay on and charge the battery for a while and then run off the electric for a while.

Perhaps "B" mode would have allowed me to force this scenario.

[EDIT] Removed this post as its information was partly wrong and was only fueling controversy with some forum members.

This is a common misconception. We beat this horse on a regular basis. It seems every time we get a new crop of Prius drivers, the topic comes up again. For those unwilling to read hobbit's excellent write-up on B mode, here is a very brief, but accurate description:

B-Mode: What it does and how it works:

1) The Prius has normal friction brakes like any car. It also can do regenerative braking by using one of the electric motors, MG2, as a generator. MG2 slows down the car by making electricity and storing it in the HV battery. This saves wear on the friction brakes and improves mileage by recovering ("regenerating") some energy.

2) Regenerative braking makes electricity. For this to work, the electricity needs to have a place to go. Normally this is the HV battery, where it is stored for later use. However, the HV battery has a finite capacity, and it will eventually fill up, leaving no more room for regeneration. When this happens, the Prius throws away braking energy by spinning the internal combustion engine (ICE) as an air pump. MG2 continues to regenerate, but the electricity from MG2 is sent to the smaller electric motor-generator (MG1), which is used to spin the ICE without fuel. In this configuration, the ICE is just an air pump, turning braking energy into heat to get rid of it. This mode is no more efficient than friction brakes, but it saves the brake pads and is not susceptible to fade. This type of braking is often called "engine braking" and is similar to down-shifting a manual and taking your foot off the gas. However, it does have some subtle advantages relating to variable engine speed and zero fuel consumption.

3) This engine-braking mode continues indefinitely as long as it is needed, regenerating electricity and using it to spin the engine as a dead weight. As a side note, the ICE can spin quite fast in this mode and often alarms new owners. It also produces the rather counter-intuitive action of slowing when you press on the gas. In other words, imagine going down a steep hill with engine braking. The ICE is spinning madly under the hood, going "BZZZZZZZ" making a fast buzzing sort of sound. The hill shallows a bit and you need a little gas to keep up with traffic. You step on the gas and the ICE slows down: "bzzzzzzz". That's not right! Ah, but it is. The ICE is throwing away energy. The faster it goes, the harder it brakes. Add some gas, you need less braking, so therefor less ICE.

4) There are a few special cases where friction brakes supplement or replace regenerative braking:

• At low speed, somewhere around seven mph, the speed of MG2 becomes too low for effective regeneration. At this point the Prius gives up and uses the friction brakes for the last little bit of stopping.
• In a panic stop you need to stop as fast as possible using all four wheels and the ABS. If you mash the brakes the Prius assumes this is a panic stop situation and immediately goes to full friction braking.
• Loss of traction. If the front wheels (the driving wheels) lose traction, the Prius senses this and switches to friction braking. This can happen on slippery roads or over bumps and chuck holes.
• When regenerative braking isn't enough. If the Prius is using full regenerative braking and you still call for more, friction brakes are engaged to supplement the regenerative braking system.

5) So what does B mode do? B mode tells the Prius to switch to engine-braking right now, and not wait for the battery to fill. Why would you want to do this? You would do this to save the friction brakes, mostly relating to the last exception above. If you are going down a very long and very steep hill, the HV battery will fill to its high limit, causing engine-braking to start. If the hill is too steep, the friction brakes will also engage, causing wear on the brakes and potentially brake fade on a long descent. In this situation, engaging B mode at the top of the hill starts the process of discarding energy, which postpones or eliminates the need for friction brakes. Some drivers also use B mode for comfort on long descents. With B mode you can often control speed with only the accelerator pedal instead of shifting back and forth to the brake. It's a matter of driving style, but it can be a nice way to go down a mountain pass.

As a final note, there are some subtle parameter changes when using B mode. Engine-braking appears to be a bit more aggressive, and the regeneration curve may be slightly different, but these are subtle points beyond the scope of this simplified explanation.

Tom

Switching to N would keep the engine running if it was running. Unfortunately N also prevents battery charging, so that would defeat part of your objective. A better idea is to let the Prius do its thing. If you have EV, you can use it to minimize start-stops in heavy traffic.

Tom

Well, I'm sorry to say you have some reading problem my friend.
You were wrong, and you are even so more wrong in choosing quotes out of context. This is not how you will prove your point.

This implicitly implies that if you were not in B mode this behaviour would not happen. This is not true.

Again wrong. Even in D mode, if you reach the point where your battery is full, the Prius will engage the ICE as if in B mode even without you having to select B. The car will not in any circumstance use the brakes for you.

And by the way, it's BRAKE not BREAK, and English is not even my native language... Once again you're wrong.

Funny you choose this sentence from Hobbit's article, but deliberately choose to hide that in normal D operation you can recover up to 50% of the inertia energy. You are still trying to hide your assumptions. Why? Oh heck, I don't really care.

Funny you take this sentence from Hobbit, and choose to ommit the immediately following sentences that reveal this apparent advantage as pure waste:

You most clearly implied that by using B you would help brake the car with the motor regeneration. It's the general idea of your post that is wrong, not loose sentences.

And taking isolated phrases out of context omitting fundamental parts of the main text is not a nice thing to do, it does not make you look good you know...

Anyway, the link to Hobbit's excelent work is there, anyone can read it and learn, that's all that matters to me. I'm out of here, fire at will if you like.

Never saw this so clearly explained.
Congrats Tom, you really excelled here!

Now this one I'm 99.9% certain it ain't true. The Prius never engages the friction brakes. If the regenerative breaking, D or B is not enough, you will gain speed, and you'll have to act on the brakes yourself.

Can you prove me wrong here please?