[chuchorifico]

I just had the opportunity to drive an 04 Prius (rented from Zipcar) and loved it. I couldn't figure out what the "B' means on the shift lever. BTW, the manual wasn't in the car.

I'm sure you all know what the "B" is.

Thanks for all the good info.

 

[Brian]

Engine Braking.
Uses the engine to slow the car, designed for long hills, ect...

 

[chuchorifico]

Many Thanks

Thanks

 

[ehurtley]

It's the functional equivalent of the 'L' gear in most other automatic transmission cars. It uses the gas engine to slow the car down rather than having you hit the brakes. It also isn't as powerful as engine braking in a conventional car, though, since when you 'coast' in the Prius, it uses the electric motor 'in reverse' to recharge the battery. The 'B' mode is really useful only in cases where you are going to be going down a slight, but long, incline, and you know that the battery would be charged to full by the bottom. 'B' stresses the engine system slightly less by using engine braking early, rather than waiting until the battery is full, then using HEAVY engine braking (which isn't so good for the engine,) or resorting to using the physical brakes (which it isn't good to 'ride' in any car.)

 

[DuncanNixon2004]

"b"

Truckers that have been in the car call it a "jake brake".

 

[Canadian Prius]

I never use the 'B' position. It seems that it throws away the useful energy that would otherwise go into the battery. I know that others here have said it is used only when you anticipate the battery will be full by the end of the hill - but that is very hard to know in most cases. Quite often the battery does not charge as much as you expect. For example, going down the same hill every day I get as many as three and a half 'regeneration cars' and as few as two.

My preference is to use about 20% pressure on the brakes when I want to control the speed from increasing down a hill. I am pretty sure that less than about 35% braking pressure will only result in extra charging to the battery, and not in use of the friction brakes.

Also, I believe the battery guage shows only from 40% to 80% if you see red bars to full green bars. This means the battery can go on charging 20% more even when it shows full.

 

[frenchie]

I never use the 'B' position. It seems that it throws away the useful energy that would otherwise go into the battery.

Not really. It's just a little less efficient at it.

Also, I believe the battery guage shows only from 40% to 80% if you see red bars to full green bars. This means the battery can go on charging 20% more even when it shows full.

No. Although at "full" according to the multi-display you might be not really at maximum state of charge, the battery will never be really full, because the system protects it against overcharging. When the maximum state of charge is reached (around 80%) excess energy is "wasted" through engine spinning without fuel. You can notice the engine noise when this happens (but it's very rare).

 

[john1701a]

"B" is quite handy on very slippery roads. It provides a graceful slowing without any risk whatsoever of the wheels locking, since the engine is providing the deceleration rather than the brakes.

You can even shift into in on-the-fly too. In fact, it's pretty easy to do without having to look down either. Just blindly paw the dashboard. The shifter is in exactly the right position for that.

 

[Canadian Prius]

"B" is quite handy on very slippery roads. It provides a graceful slowing without any risk whatsoever of the wheels locking, since the engine is providing the deceleration rather than the brakes.

If you have the ABS brakes, they are computer controlled not to lock, but if you were on a really slippery surface, and put it in 'B', there would be nothing to stop the front wheels from skidding. This is not likely, though, because the deceleration is so mild with 'B' that you would have to have almost no friction on the wheels to skid.

 

[john1701a]

Sorry, "lock" was clearly the wrong word.

What I actually meant was "loss of momentum".

Applied pressure helps to keep the wheels on the road. That is significantly reduced by braking hard enough to trigger ABS. By using "B" the odds of remaining within the safer threshold are higher.

 

[Canadian Prius]

Applied pressure helps to keep the wheels on the road. That is significantly reduced by braking hard enough to trigger ABS. By using "B" the odds of remaining within the safer threshold are higher.

Agreed. These are very fine points.

 

[RSnyder]

. . . If you have the ABS brakes, they are computer controlled not to lock, but if you were on a really slippery surface, and put it in 'B', there would be nothing to stop the front wheels from skidding. . . .

I haven't verified, but I'm pretty sure that just like in D, the Prius will not apply electric generator drag at or under 7 MPH in B.

 

[Canadian Prius]

I haven't verified, but I'm pretty sure that just like in D, the Prius will not apply electric generator drag at or under 7 MPH in B.

I think 'B' is engine braking, isn't it? It may be both generator drag and engine braking. But yes, I think it would be at low speeds like that where it could be more of a factor in loss of traction, if it ever was at all.

 

[RSnyder]

. . . I think 'B' is engine braking, isn't it? It may be both generator drag and engine braking. But yes, I think it would be at low speeds like that where it could be more of a factor in loss of traction, if it ever was at all.

In B mode, the engine does not apply a torque to the wheels. The larger motor/generator, acting as a generator, applies a strong torque to the wheels in the reverse direction. Since it is generating more electric power than the battery can accept, the rest is dumped into the smaller motor/generator which acts as a motor to spin the engine forward. The engine is not supplied with fuel and it's throttle and intake valves are set to offer the largest amount of "pumping loss" air resistance as possible.

This is called simulated engine braking because it feels to the driver almost exactly like putting the car in low gear and getting off the accelerator pedal.

 

[Canadian Prius]

In B mode, the engine does not apply a torque to the wheels. The larger motor/generator, acting as a generator, applies a strong torque to the wheels in the reverse direction. Since it is generating more electric power than the battery can accept, the rest is dumped into the smaller motor/generator which acts as a motor to spin the engine forward. The engine is not supplied with fuel and it's throttle and intake valves are set to offer the largest amount of "pumping loss" air resistance as possible.

So you're saying that 'B' mode gives the maximum amount of charging to the battery, i.e. even more than it can accept. If that's true, then it probably is the best way to control speed down a hill, since there is sure to be no wear on the friction brakes. Do you have a reference for this info?

 

[hyperion]

Of course brakes are probably the most inexpensive and simplest parts on a Prius to replace.

 

[faqsman]

The dealer said the same thing. It charges the battery the best and he uses
it on long freeway exit ramps.

I have never made it to the end of the ramp after selecting B ..

That really slows the car down in a big hurry!

 

[Guest]

Sorry, that is not correct.

I've seen several owner studies now that have proven recharging is reduced by using "B", not increased.

You are better off just using the brakes as Toyota intended.

 

[RSnyder]

In B mode, the engine does not apply a torque to the wheels. The larger motor/generator, acting as a generator, applies a strong torque to the wheels in the reverse direction. Since it is generating more electric power than the battery can accept, the rest is dumped into the smaller motor/generator which acts as a motor to spin the engine forward. The engine is not supplied with fuel and it's throttle and intake valves are set to offer the largest amount of "pumping loss" air resistance as possible.

So you're saying that 'B' mode gives the maximum amount of charging to the battery, i.e. even more than it can accept. If that's true, then it probably is the best way to control speed down a hill, since there is sure to be no wear on the friction brakes. Do you have a reference for this info?

I may have to backpedal a bit on this and chalk it up to "dramatic effect". In the 2001 Prius (the only one I've measured), it is possible to see 60 amp charging current for short periods of time while braking hard. It's not all that easy to maintain any charging current though as regen cuts out pretty easily at the slightest provocation from the anti-lock brakes. In B mode, it will sustain 40 amp charging for long periods if you're going down a sufficient hill. I've not seen B mode go over 40 amps.

 

[Canadian Prius]

I may have to backpedal a bit on this and chalk it up to "dramatic effect". In the 2001 Prius (the only one I've measured), it is possible to see 60 amp charging current for short periods of time while braking hard. It's not all that easy to maintain any charging current though as regen cuts out pretty easily at the slightest provocation from the anti-lock brakes. In B mode, it will sustain 40 amp charging for long periods if you're going down a sufficient hill. I've not seen B mode go over 40 amps.

Thanks for the reference. So far what I had seen was only that 'B' mode caused engine braking friction to slow the car - which didn't make any sense to me. i.e. with all that development, why throw away power with friction and wear on the engine? The way you have described it makes much more sense. Now I am more likely to use it as a controlled higher level of charge on long down grades.

You say it charges at about 40 amps. How many amps can the battery absorb?