[tryoty]

for info on how the regen braking really works. I know I read somewhere that the actaul brakes aren't used until 7mph or something like that, unless a hard braking stop is called for.

Under normal braking, what percentage of the braking force comes from regen versus brake pads?

Also, there seems to be some confusion regarding the "B" setting. I sent an email to my salesman correcting his assertation that you should use "B" to increase regen. He just sent one back saying he contacted his regional guy and found that using "B" DOES increase regen. I also found an article on USA Today's site saying "B" increases regen.

Everything I've found here and on john1701's site say to avoid "B".

Who's right?

 

[RSnyder]

for info on how the regen braking really works. I know I read somewhere that the actaul brakes aren't used until 7mph or something like that, unless a hard braking stop is called for.

Under normal braking, what percentage of the braking force comes from regen versus brake pads?

Also, there seems to be some confusion regarding the "B" setting. I sent an email to my salesman correcting his assertation that you should use "B" to increase regen. He just sent one back saying he contacted his regional guy and found that using "B" DOES increase regen. I also found an article on USA Today's site saying "B" increases regen.

Everything I've found here and on john1701's site say to avoid "B".

Who's right?

Everybody?

Under most situations use of B will reduce overall MPG. This is noted by Toyota in their manual and has been confirmed by many drivers. The "B" setting does indeed increase regeneration, but it also uses some of that generated electricity to spin the gasoline engine. In "D" mode, under most circumstances, all the generated electricity is used to charge the HV battery.

The main difficulty is that no matter how hard the electric motor spins the engine, it never puts gasoline back into the tank.

As to regen vs. brake pads, the 2004-05 model Prius does try to do all braking over 7 MPH by regen rather than with the brake pads. But this is strictly limited by the amount of charging the battery system is willing to accept. Any additional requested braking is done with the brake pads. There's also the ABS issue. In the 2001-03 model Prius, the ABS would cut out regen at the slightest provocation (unevenness in the road surface). I think they improved this for the 2004-05 model, but at some point this does still kick in. It will also apply the rear wheel brake pads (no regen for them) if needed to maintain vehicle stability.

Hope this helps.

 

[Sanny]

...The main difficulty is that no matter how hard the electric motor spins the engine, it never puts gasoline back into the tank. ...

Clearly, this was an oversight on Toyota's part. Sure, NOW it's obvious, but during the design phase, it must have been overlooked.

Perhaps in the NEXT generation Prius that's something we can look forward to?

Well, that and a bagel toaster and coffeemaker in the dash, standard.

 

[RSnyder]

...The main difficulty is that no matter how hard the electric motor spins the engine, it never puts gasoline back into the tank. ...

Clearly, this was an oversight on Toyota's part. Sure, NOW it's obvious, but during the design phase, it must have been overlooked. . .

I think they forgot to set the motor to spin the engine backward

 

[jeromep]

Well, this thread should be filed under stupid dealer comments.

B mode is there mostly because the U.S. government requires there to be a low gear in all vehicles for the purposes of engine braking. I find that when I'm descending a long hill, which isn't that often, that B mode is very helpful. It does what it is advertised to do, it acts like a lower gear in an automatic transmission, but is achieved by running the ICE like a Jacob's break like on a large truck. The ICE, instead of providing power turns into a compressor and spins and compresses air to provide resistance.

Certainly, you wouldn't want to do your everyday driving in B mode, however it should be used when you want additional breaking assistance and when going down long hills where you want to retain control of the vehicle without riding the break.

 

[LisaMurray]

The only time I've ever downshifted a car, and thus the only time I'll probably use the B shift, is to slow down the car on icy roads. I know there are probably more efficient ways of doing this, but I have an ingrained fear of stepping on the brakes when on ice, particularly if I'm going downhill. I just feel safer using the downshift method. In fact, if the Prius didn't have this option, it might have been a deal breaker for me.

 

[Hobbit]

For what is probably the closest, most in-depth rundown
on the brakes themselves, dig up U.S. patent
number 6164734 at http://www.uspto.gov. Doesn't really
cover regen in that much depth but explains how the
braking system itself works.
_
In the overall system, by and large, your braking request
via the pedal sends a voltage to the braking / skid-
control ECU, and everything is pretty much brake-by-wire from
there on via regenerative control and a rack of 8 apply/release
solenoids for sending pressure to the wheel brakes. Very
gentle braking seems to try staying mostly in the realm of regen
first, and bring in physical braking only if you indicate
it's needed with more brake pressure. Thus, beginning a
gentle deceleration path earlier in the game is way better than
slamming 'em on at the last second. If you listen carefully, you can
hear "inverter whine" at higher speed, and the motor system sort of
"growling" at lower speed as energy gets generated/regulated back into
the battery.
_
However, there's a limit to how fast you can send
energy back into the battery, and trying to burn off all of
a car's kinetic energy at once can easily exceed that. Again, slow
and early braking. The guy riding your ass can simply
learn from what you do -- don't worry about him, especially
when you're being way more predictable than he is.
_
Regen stops and the system falls back to physical braking around
5 or 6 mph, when the motor cannot spin fast enough to
produce a usable charging voltage anymore. You'll probably
feel a transition there, and have to adjust your foot. One
gets used to it fairly quickly. You also have the option
of kicking into Neutral if you want your stop to be
smoother, and then back into Drive to take off again.
Nothing mechanical really happens when you do that; you're
just telling the computers what to do.
_
Sorry if this still doesn't explain things fully, but I
don't have good references ready at hand.
_
_H*

 

[ehurtley]

The only time I've ever downshifted a car, and thus the only time I'll probably use the B shift, is to slow down the car on icy roads. I know there are probably more efficient ways of doing this, but I have an ingrained fear of stepping on the brakes when on ice, particularly if I'm going downhill. I just feel safer using the downshift method. In fact, if the Prius didn't have this option, it might have been a deal breaker for me.

Anti-lock brakes are significantly safer on ice than engine braking. With engine braking, the ABS system makes no attempt to control the spin of the wheels. So you can (and I have) end up with locked wheels under engine braking. With ABS, the system tries pretty darned hard to prevent the wheels from locking.

Think about it, what you are wanting on ice is to keep traction on the tires. The sticking point is that while slowing down, you have to create friction with the road while decelerating. Wether you use brakes, or engine braking, you're still slowing down the wheels. With engine braking, you're doing it by slowing down the axle in the transmission. With brakes, you're doing it at the wheels themselves by applying direct friction to the brake rotors/discs. In non-ABS cars, the engine braking way is perceived to be 'safer' because the engine won't cause the axles to come to a complete stop suddenly. With brakes, you could accidentally hit the brakes too hard, and lock up the wheels. This will cause you to skid, having lost all moving traction. This can still happen with engine braking. In fact, 'hydroplaning' on water is the same effect. No matter what's going on with controlling wheel speed (accelerating, decelerating with engine braking, coasting, or hard braking,) you can still lose traction.

Insisting on engine braking as a 'safer' way to slow down is silly. It's no safer on a non-ABS car than careful braking. And on an ABS car, it's less safe, since the ABS system won't kick in properly. (Traction control probably would work, though.)

 

[richard schumacher]

"B" does increase regen, which is exactly why it is *less* efficient than using "D". Regeneration does not save (recover) a lot of energy because regeneration is not very efficient; the more you do, the more energy gets turned into heat. It's better for fuel economy to coast as much as possible rather than regenerate. Regen does recover more energy than friction braking, which turns all the energy into heat.

The only sensible reason to use "B" is to reduce the use of the friction brakes, for example in a long session of close stop-and-go driving or when going down a long hill. (And personally I'd not use it in stop-and-go because the car will decelerate strongly, as though it were braking, but without the brake lights going on; this increases the chances that the car behind will rear-end you.)

 

[KTPhil]

"(And personally I'd not use it in stop-and-go because the car will decelerate strongly, as though it were braking, but without the brake lights going on; this increases the chances that the car behind will rear-end you.)"

While true in theory, I wonder if it happens in real life. I have two reasons for saying this. First, it is identical to a stick shift in that there will be no brake lights until very near a full stop. Do stick shift cars have a statistically higher rate of rear-enders? I don't know, but I've never ead that. Second, if the front driver is riding his brakes in stop/slow-n-go driving (as is often the case,especially on a downhill stretch), the brake light are already on, and the following driver must watch the actual speed, same as if there were no brake lights.

For this reason, I use B in such traffic without fear of getting rear-ended. I've been bumped in three other cars in 30 years, two in auto tranny cars, one with a stick, for what it's worth. In all cases, the drivers who hit me were distracted and not paying any attention at all.

 

[LisaMurray]

The only time I've ever downshifted a car, and thus the only time I'll probably use the B shift, is to slow down the car on icy roads. I know there are probably more efficient ways of doing this, but I have an ingrained fear of stepping on the brakes when on ice, particularly if I'm going downhill. I just feel safer using the downshift method. In fact, if the Prius didn't have this option, it might have been a deal breaker for me.

Anti-lock brakes are significantly safer on ice than engine braking. With engine braking, the ABS system makes no attempt to control the spin of the wheels. So you can (and I have) end up with locked wheels under engine braking. With ABS, the system tries pretty darned hard to prevent the wheels from locking.

Think about it, what you are wanting on ice is to keep traction on the tires. The sticking point is that while slowing down, you have to create friction with the road while decelerating. Wether you use brakes, or engine braking, you're still slowing down the wheels. With engine braking, you're doing it by slowing down the axle in the transmission. With brakes, you're doing it at the wheels themselves by applying direct friction to the brake rotors/discs. In non-ABS cars, the engine braking way is perceived to be 'safer' because the engine won't cause the axles to come to a complete stop suddenly. With brakes, you could accidentally hit the brakes too hard, and lock up the wheels. This will cause you to skid, having lost all moving traction. This can still happen with engine braking. In fact, 'hydroplaning' on water is the same effect. No matter what's going on with controlling wheel speed (accelerating, decelerating with engine braking, coasting, or hard braking,) you can still lose traction.

Insisting on engine braking as a 'safer' way to slow down is silly. It's no safer on a non-ABS car than careful braking. And on an ABS car, it's less safe, since the ABS system won't kick in properly. (Traction control probably would work, though.)

I know, I know. This is one of those stupid things that I picked up years ago, probably from my dad teaching me to drive in an empty parking lot. Your logic makes perfect sense, and I'm trying, I really am! Old habits.....