[mary]

Hi - this happened once to my sister, who was driving while I was on vacation, and once (I think) to me. When it happened to me, I literally thought I'd just blinked, it was so quick and apparently minor - but I'm posting to see if we need to worry. The situation-
After driving some miles at normal speeds (40 to 60 mph), you go into a parking lot and slow down. As you move into a parking space, the prius dies on you and the check engine light comes on. This happened to my sister two weeks ago; the only thing she noticed that was at all unusual was that the car was in "B" rather than "D" at the time. It started up normally and has been fine since. What could have caused this? could it have anything to do with a hesitation I somethimes sense in the motor when shifting between the gas and electric motors? Or is it just a freak occurence that we needn't worry about? If it occurs again, I'm certainly going to call the dealer; this was a demo 2003 which I bought just 6 weeks ago. TIA!

 

[john1701a]

> thing she noticed that was at all unusual was that the car was in "B" rather than "D" at the time

"B" = Engine Braking mode

You shouldn't ever use this mode unless you really need it, like when driving down a the side of a mountain. When engaged, you are forcing the engine to spin whenever the Prius is moving. This reduces the amount of strain you need to put on the brakes, which is really helpful for keeping brakes from overheating on a long, steep decline.

You may have discovered a shortcoming of "B" (besides the fact that it reduces MPG and works the engine more than needed).

If the Prius never stalls when in "D" (which should be 99.999% of the time), don't worry about it.

 

[john1701a]

Also note that traditional cars don't even offer a "B" mode.

So the fact that Prius offers it is pure gravy, a special hybrid-only feature that doesn't cost anything extra to include.

 

[mary]

thanks!

I guess I won't worry, then. We both found you really have to look when shifting; if you're not careful, you can get it into "B" rather than "D" really easily. In future I will certainly be careful!

So the hesitation when shifting from engine to engine is normal? I ask because, as I said, this was a demo model with a lot of previous drivers and I didn't have a good experience buying it (though I love the car).

Also, I hadn't realized that "B" was so different from "1" and "2" gears in a normal car. Well, I'm still learning! Thanks again-

 

[GreyMud]

You're not saying that it shifted itself into 'B', are you? I may be tired, but I read the thread twice and still can't tell for sure. I'm thinking the 'B' stands for 'Bad', but they wouldn't design a bad gear in it, would they? ;^]

Best to make sure you ask the stupid questions up front, it saves a lot of time in diagnosis.

 

[mary]

Not at all-

What I said precisely is that it slips easily from "D" into "B" when shifting from reverse, for example - and I usually back out of parking spaces. It's also very easy to get into "N" rather than "R" - the shift lever moves extremely easily - but when you're in "N", you know it! When you've gone from reverse to braking, you don't necessarily know it because you can drive forward at slow speed - normally exactly what you're doing at such a time. So I've just got to be sure to check the monitor every time, because I can't always tell by feel what gear I'm in. I guess you could compare the lever to an extremely sensitive keyboard - I'm used to a much stiffer one. Hope this is clear now. I'm just glad I don't have a serious problem!

 

[chris]

Also note that traditional cars don't even offer a "B" mode.

So the fact that Prius offers it is pure gravy, a special hybrid-only feature that doesn't cost anything extra to include.

I have recently gotten very fond of B mode and use it on off-ramps, coming up to lights,
mild slopes, etc, in fact most places where I would previously have braked, now I get into
B mode if there is little traffic around. The trick (and the fun) is in going back into D
at the right speed so that neither brake nor accelerator use is required for the subsequent
section of road.

Has Toyota ever cautioned against use (or excessive use) of B mode? If not, I dont see
why it should not be a viable (or evern preferable) alternative to normal braking.
Preferable because you get fairly heavy braking without any guesses about brakepad use.
I hope the 2004 model has B, and made it easier to switch out of B, without slipping into N.

Edited by Pete...BBCode must be enabled for formating to work

 

[mrv]

"B" gear vs. brakes

Also note that traditional cars don't even offer a "B" mode.

So the fact that Prius offers it is pure gravy, a special hybrid-only feature that doesn't cost anything extra to include.

I have recently gotten very fond of B mode and use it on off-ramps, coming up to lights,
mild slopes, etc, in fact most places where I would previously have braked, now I get into
B mode if there is little traffic around. The trick (and the fun) is in going back into D
at the right speed so that neither brake nor accelerator use is required for the subsequent
section of road.

Has Toyota ever cautioned against use (or excessive use) of B mode? If not, I dont see
why it should not be a viable (or evern preferable) alternative to normal braking.
Preferable because you get fairly heavy braking without any guesses about brakepad use.
I hope the 2004 model has B, and made it easier to switch out of B, without slipping into N.

Well, Toyota does caution in the Owner's
Manual that you'll get lower MPG driving around
in "B" as opposed to the proper "D"
mode.

Also remember - using the brake pedal
to slow down causes your rear brake
lights to come on - signalling to
traffic around you that you're slowing
down. If you use the "B" mode, then
there's no brake lights. Use caution,
as having no brake lights could cause
an accident (someone behind you hitting
you) or you could be pulled over by
police for safety reasons...

 

[frenchie]

The difference between "B" and "D" is rather simple to understand.

In "B" you are using the thermal engine to brake, more or less like with a manual gearbox or modern automatic gearboxes. It is true that the brake lights don't illuminate, but it is just the same with the above example, and is not dangerous because it does not produce a brutal slowing down. And of course, the energy the engine dissipates can not be recovered; that's the reason why toyota says you get lower MPG.

If you release pressure on the accelerator pedal while in "D" the electric motor produces some electricity that is stored in the battery. You can see the green arrow on the multi-display, and can also notice the difference if you try the "N" position a few seconds : there IS some braking in "D" even if you don't press the brake pedal, that does not exist in "N". But the Prius is even more subtle : if you press gently on the brakes, you will ALSO use the electric motor to slow down and not the brake pads.

Therefore, I think the key for secure driving and great MPG is to make minimum use of "B" position (except in very steep slopes) and to anticipate your braking, because you will make regenerative braking more efficient (and the pads will not wear out so quickly...).

Now some basic physics. Acceleration and braking are equivalent in physics ; we use only one word (acceleration) for both. To get some acceleration level you need some power (expressed in kilowatts), and the same for braking. Therefore the limiting factor for efficient regenerative braking is the maximum electric power that can be directed to the battery (the electric motor has a higher power output in the Prius, and hence is not the limiting factor), because the excess power will be wasted in wearing out the brake pads. According to Toyota this point has been improved in the new Prius, and that's one main reason for better mileage... but also for better performance because if you raise maximum input power, you raise maximum output power too! Isn't that great ?

 

[chris]

Firstly, thanks for the responses that pointed out the safety issue with B mode braking.
Would'nt it be appropriate for the brake lights to come on during B mode unless the
accelarator is pressed, i.e. whenever the car is slowing.

However, regarding the regen braking in B mode and the lower gas mileage, I am
confused by seemingly contrary opinions. (Note that I was using B only to decelerate,
then going back into D)

For instance,

And of course, the energy the engine dissipates can not be recovered; that's the reason why toyota says you get lower MPG.

But here is snippet from Toyota press
http://www.pressroom.com.au/pressroom/s ... kit.htm#11

The Prius gear shift pattern includes a B setting, to maximise the utilization of regenerative braking.

Page 7 of the following article
http://www.inrets.fr/ur/lte/publication ... elec99.PDF
appears to speak directly to the difference between regen braking in D and B modes,
using tables and graphs, unfortunately my French is not adequate to translate.
Perhaps Frenchie could have a go...

Edited by Pete...BBCode must be enabled for Formating/Quoting to work

 

[frenchie]

Well, I must admit this is rather confusing indeed... but fortunately the contradiction is only apparent.

Page 7 of the following article
http://www.inrets.fr/ur/lte/publication ... elec99.PDF
appears to speak directly to the difference between regen braking in D and B modes,
using tables and graphs, unfortunately my French is not adequate to translate.
Perhaps Frenchie could have a go...

I've got the paper version at home and have read it many times :wink: ! (although I think it is not an excellent piece of work...)
First, I want to emphasize this study was made with the first japanese version of the Prius, and some important differences might exist with the current one. For instance, at the bottom of page 7 this sentance :

Aucune r�up�ation d'�ergie suppl�entaire n'intervient lors d'une pression sur la p�ale de frein.

which translates into "There is no extra energy recovery when the brake pedal is pressed", is certainly WRONG for the current Prius : gently pressing the brake pedal makes the generator spin (you can hear it if you listen carefully) and produce some electricity.

But anyway, there is another point. In this paper they compare, as a function of speed, the current intensity drained into the battery both in "D" mode and in "B" mode, but WITHOUT pressing on the brake pedal. In this case, it is true that the current intensity is larger in "B", and that's the reason why Toyota says :

The Prius gear shift pattern includes a B setting, to maximise the utilization of regenerative braking.

But of course braking is more intense in "B" than in "D", and therefore the comparison is not relevant. As you decelerate more slowly in "D", you need more time to slow down to a given speed. Since the electric charge is the product of the intensity by the time, you can recharge by the same amount with half the current, if it takes twice the time... Now if you want to produce the same deceleration in "D", you have to press a little on the brake pedal and this will produce more electricity, since the Prius brakes are "smart" and don't use only the brake pads. I don't know any study comparing the amount of electricity generated in "B" and "D" modes FOR THE SAME DECELERATION, but since in "B" there is some energy dissipated through the engine I would say "D" mode is more efficient than "B" mode, and that explains why the owner's manual tells you not to use the latter very often if you want to maximize MPG...

Finally, concerning the brake lights, I had a car with manual transmission before the Prius and I can tell you I could get a higher deceleration level using engine braking than with the "B" mode, so I think this is not really an issue. But in Europe some cars are introducing variable intensity brake lights (depending on the deceleration level)... maybe a good idea to take into account not only the action on the left pedal ?

 

[citicar1976]

Deceleration Light

... But in Europe some cars are introducing variable intensity brake lights (depending on the deceleration level)... maybe a good idea to take into account not only the action on the left pedal ?

If memory serves me right, there was a movement in the late 1960's or early 1970's, to have a yellow deceleration light mounted on the rear of American cars. It would have been similar to the third brake light that is currently on all cars, but would have come on only when the car was coasting and the brake pedal was not depressed. Supposedly, a study was done which found that the light was very effective at reducing rear end accidents.

 

[johnson487682]

OT: Variable intensity brake lights

But in Europe some cars are introducing variable intensity brake lights (depending on the deceleration level)...

When I started seeing cars on the road with LED bars for center brake lights, I thought this would be a cool thing to do: if you just tap the brakes lightly, only a few LEDs in the center would illuminate, but as you press the brake pedal harder, more LEDs would illuminate (towards the edges of the LED bar), until at full pedal all LEDs were lit.

I think this would be easier for other drivers to understand than brake lights that just get brighter as you press down. The problem with varying the intensity is that every manufacturer's brake lights have a different base intensity. When I see two dim red lights in front of me, I can't be sure if the driver just turned on his headlights, or if he's braking and his center bulb is burned out.

Adjusting the lit width of an LED bar gives positive indication of brake action at all times, and it intuitively indicates increased levels of braking.

Douglas (2002 Silver, Wisconsin)

 

[citicar1976]

Re: OT: Variable intensity brake lights

Adjusting the lit width of an LED bar gives positive indication of brake action at all times, and it intuitively indicates increased levels of braking.

I think that this would be very easy to do with the Prius since it already has a computer for the braking system. The brake ECU could send a signal to the third brake light telling it how many LEDs to light. It would just require a programing change and the addition of a small control module to the brake light.

 

[Joel]

My understanding of the "B" mode was similar to "engine braking" in a manual transmission car, where the engine is actually slowing the vehicle down. Main difference is that in the Prius, the brakes are engaging slightly to simulate this effect.

It doesn't seem bad, just different if that's the driving style you prefer.

 

[frenchie]

My understanding of the "B" mode was similar to "engine braking" in a manual transmission car, where the engine is actually slowing the vehicle down. Main difference is that in the Prius, the brakes are engaging slightly to simulate this effect.

No ! There is no "simulation" of engine braking in the "B" mode, but REAL engine braking, plus regenerative braking thanks to the generator. And the brakes are not engaging at all if you don't press the pedal.