[moocatdog]

The last stretch of my drive home is a downhill slope a couple miles long. It isn't particularly steep but the speed limit is only 25, so you have to ride the brakes to stay legal. Tonight I thought I'd drop the car into "B" to see what it felt like and to check if it would noticeably boost the charge in my battery. The car slowed significantly, but as I was coasting down the hill either the engine or the VCT (it happened quickly, so I couldn't tell which) started "racing," as though I was pressing the accelerator. It was quite loud and disconcerting, so I immediately popped the car back into "D." The noise quieted right down and the car seemed fine, so I continued home. I have used "B" mode only once before and that was at about 70mph on a much longer, steeper grade and I didn't notice anything like this happening. Can anyone tell me what was going on? Was this normal? Any advice will be greatly appreciated.
Thanks,
Moo

 

[efusco]

Did you note the battery charge status Moo? Someone over at the YG-2004 board mentioned a similar response in B mode when his battery was full--almost like things got wound up he said. I suspect it's some built in safety feature to prevent battery overload, but as far as I remember no one could say for certain what happened.
--evan

 

[moocatdog]

Did you note the battery charge status Moo? Someone over at the YG-2004 board mentioned a similar response in B mode when his battery was full--almost like things got wound up he said. I suspect it's some built in safety feature to prevent battery overload, but as far as I remember no one could say for certain what happened.
--evan

Evan,
I did notice my battery charge and it was not completely full, but nearly--only one bar was empty. I hope it is some kind of safety feature and not a malfunction.
Moo

 

[RSnyder]

B mode, aka engine braking mode, simulates the effects of putting a conventional automatic in 2 or 1 without any gas. It opposes the forward motion of the Prius with the larger motor/generator (MG2). This generates electric power which is sent to the battery to the extent that it can accept it. The rest of the electric power is sent to the smaller motor/generator (MG1) to spin the engine (without gas). As the battery becomes less able to accept energy, more must be dumped into the engine (while MG1 has additional capacity) or MG2 must reduce its braking effect.

A good rule of thumb with the Prius is to ignore engine sounds. They don't typically track the behavior of other cars. Once you've become more enlightened to the energy flows of the Prius you can once again listen to the engine and have it make sense. Engine sound correlates closely with the amount of power the engine is providing (or absorbing in the case of B mode). Engine sound is only very loosely correlated to vehicle speed and only moderately correlated to total power use. In other cars engine sound is highly correlated to vehicle speed and total power use.

 

[moocatdog]

B mode, aka engine braking mode, simulates the effects of putting a conventional automatic in 2 or 1 without any gas. It opposes the forward motion of the Prius with the larger motor/generator (MG2). This generates electric power which is sent to the battery to the extent that it can accept it. The rest of the electric power is sent to the smaller motor/generator (MG1) to spin the engine (without gas). As the battery becomes less able to accept energy, more must be dumped into the engine (while MG1 has additional capacity) or MG2 must reduce its braking effect.

Robert,
Thanks for yor reply. I understood the theory behind the engine braking mode, but had no practical experience with the process. If I understand you correctly, you are surmising that the sound I heard was the normal result of the engine spinning. If this is so, does this mean the system was increasing the engine spin in order to "dump" excess energy, which wasn't needed due to the battery being close to fully charged?

A good rule of thumb with the Prius is to ignore engine sounds. They don't typically track the behavior of other cars. Once you've become more enlightened to the energy flows of the Prius you can once again listen to the engine and have it make sense. Engine sound correlates closely with the amount of power the engine is providing (or absorbing in the case of B mode). Engine sound is only very loosely correlated to vehicle speed and only moderately correlated to total power use. In other cars engine sound is highly correlated to vehicle speed and total power use.

I've been trying to re-learn engine sounds with the Prius and have made positive steps. Most of the sounds and sensations I've experienced so far are relatively subtle and seemingly benign. This one was a little too loud and dramatic for me to dismiss as harmless without further investigation.
I've yet to find any description or warnings about this in the owner's handbook. If anyone has a link that leads to information about this particular "B" mode operation please share it here.
Thanks again,
Moo

 

[ken1784]

If anyone has a link that leads to information about this particular "B" mode operation please share it here.

There is a Japanese Q&A page answered by Toyota Chief Engineer Masao Inoue.
http://gazoo.com/nvis2/prius/qa/qa00.asp?NO=7

Translation....
Q: What is "B" used for?
A: "B" means a "brake". When you release an accelerator, there is a slowdown effect like engine brake by normal car.
This raised engine revolution compulsorily by the power car runs, and has been obtained by friction produced in that case. Although braking power is stable and a smooth slowdown can be performed, since there is little energy regeneration effect, the slowdown by the brake pedal is more effective for energy-saving operation.

 

[RSnyder]

Robert,
Thanks for yor reply. I understood the theory behind the engine braking mode, but had no practical experience with the process. If I understand you correctly, you are surmising that the sound I heard was the normal result of the engine spinning. If this is so, does this mean the system was increasing the engine spin in order to "dump" excess energy, which wasn't needed due to the battery being close to fully charged?

. . .

Thanks again,
Moo

Yep, you got it.

 

[patrickg]

Robert, I sure hope the "B" mode works the way you describe it. Your description is the best and most lucid I have heard and I have asked several folks who thought they new, including Toyota employees.

Thanks for laying it out.

Pat

 

[ken1784]

Please note that the "B" mode creates very limited regeneration no matter how battery is charged.
Use brake pedal if you would like to get more energy-saving as Masao Inoe said.

 

[patrickg]

Ken, Oh yes indeed! I read the Japanese gentleman's comments 3 times. I am fully on board as regards the use of the B position. It was sort of counterintuitive but it works like they designed it not how we guess. I have an aftermarket exhaust brake on a diesel truck for long donwgrades and expect the B to give the same general brake saving function. Not being optimized for regeneration is a separate issue. Most of us should be able to be trained to apply partial braking via the brake pedal to get regeneration when desired.

Tell me Ken, is there any way to really know when the friction brakes are actually engaged at speeds where regeneration is effective? I recognize when the likelihood is increased but is there a definite indication. The older model seems to "let you know" by feel but the '04 integrates the two braking systems more smoothly. If I knew for sure how much brake I could use without adding in friction brakes, regeneration would be a more efficient energy recycler.

Thanks for your consideration,

Pat

 

[ken1784]

Tell me Ken, is there any way to really know when the friction brakes are actually engaged at speeds where regeneration is effective?

Hi Pat,
I'm not sure, but the use of hydraulic brake is very last seconds before stopping.
http://www.toyota.co.jp/en/tech/environ ... tem_06.gif
Maybe, you can attach some pressure switch onto the brake fluid line like traditional cars had for lighting stop lamps.

Ken

 

[patrickg]

Maybe, you can attach some pressure switch onto the brake fluid line like traditional cars had for lighting stop lamps.

Ken

Ken, You must be pretty bright, over 30, or both! It has been a while (in my limited experience) since cars have had pressure switch activated brake lights. I think switches on the brake pedal are THE WAY anymore.

You are, of course, right on. An indicator light on the dash or an audio annunciator wired to a pressure switch in the hydraulic line to the brakes would be just the ticket. That way if you were looking to use regenerative braking pretty much to its max capability without wasting energy with friction brakes, you would be notified when you "crossed the line" into friction braking.

The transition into friction braking is so much smoother in the '04 it could go unnoticed. This annuciator system would give you useful feedback.

I hold an opinion, not proved or disproved, that you'd get better economy if you maintained normal speed closer to the point at which you want to stop (traffic light or stop sign or ...) and then used regenerative braking fairly vigourously to do most of the stopping. The idea being not to "waste" your energy of motion in letting it "bleed off" to friction/drag and instead to recover it with regenerative braking. My surmise is that the additiional energy consumed to maintain speed for just a little while longer as you approach your stop is significantly less that what you save in stopping with regeneration.

Thanks again for reminding me that the NEW solution I needed was not really so new.

Pat

 

[mrv]

brake kit

Tell me Ken, is there any way to really know when the friction brakes are actually engaged at speeds where regeneration is effective?

Hi Pat,
I'm not sure, but the use of hydraulic brake is very last seconds before stopping.
http://www.toyota.co.jp/en/tech/environ ... tem_06.gif
Maybe, you can attach some pressure switch onto the brake fluid line like traditional cars had for lighting stop lamps.

Ken

for the classic (2001-2003) Prius,
CoastalETech already has a brake
indicator that tells you when the
hydrolic brakes are being applied.

http://www.coastaletech.com/brake.htm

They just got their 2004, so give
them time to come up with items for
it. If you're really interested,
you may want to let them know that
you'd like a similar kit for your
2004 so it'll be on their list of
mods to make.

 

[ken1784]

Ken, You must be pretty bright, over 30, or both! It has been a while (in my limited experience) since cars have had pressure switch activated brake lights. I think switches on the brake pedal are THE WAY anymore.

Yah! I'm over 40.
I believe the pressure switch solution IS very affordable.

for the classic (2001-2003) Prius,
CoastalETech already has a brake
indicator that tells you when the
hydrolic brakes are being applied.

I'm not sure how the Braking Indicator from CoastalETech works,
but I'm doubtful because...
1. The classic Prius always engages the hydraulic braking.
http://www.toyota.co.jp/en/tech/environ ... tem_05.gif
2. There is a motor sound pumping the brake pressure accumulator even if car stops.(I only know on 2004 Prius case.)

Is there actual Braking Indicator user here?
How are you satisfied using that?

Thanks,
Ken

 

[howgus]

Why does B mode not behave same as brake pedal

Why doesn't B mode behave same as brake pedal WRT regeneration when battery charge allows it, then shift to the use of the engine for friction braking if batteries fully charged? I much prefer to brake by a simple shift of mode instead of holding my foot in the air. Is there a good engineering reason for this? I can't think of one. The brakes and accelerator are both fly by wire when the brakes aren't in mechanical mode.

I usually drive a stick diesel and really enjoy using the engine for initial braking instead of wearing out the brakes and my leg.