B Lever on Dashboard

[dwreed3rd]

Thanks qbee42

[archae86]

If your foot is not on the brake pedal, the battery is not warm, and the state of charge is not too high, then in b mode you are slowing down quite a bit quicker. And that is a combination of extra engine drag and of higher regenerative current. On my favorite hill, which I descend at about 50 mph with a battery that is cool because of recent startup, the b mode regenerative current is about 30 amperes compared to about 20 amperes for d mode.

If you are on a long downhill which in your experience or judgment is going to lead to a full state of charge at the bottom in either case, then using b mode to get more of the drag from the engine and less of a drag from the combination of regenerative and friction brakes spares your friction brakes and your electrical system without actually wasting any potentially available regenerative capture energy at the end.

The entire range of state of charge used in the Prius is only equivalent to a surprisingly small altitude change when reckoned as the potential energy of the automobile. So hills that are much less impressive than coming down from an Alpine mountain pass meet this condition. Almost any hill that requires steady application of the foot brake and has a total descent of more than 800 feet or so of altitude is quite possibly a candidate.

If there is lots of downhill still to go and you are steadily using the foot brake, there is no virtue in waiting to start use of B mode until all green shows.

Regeneration is limited, even with a cool battery in moderate state of charge, to something like 90 to 100 Amperes. Using B mode on steeper hills you descend at a fair rate of speed can spare the friction brakes some wear--the same maximum regeneration rate is still available. I do not need to do this on a 4% grade at 50 mph, but suspect that a 6% grade at the same speed with even a few hundred feet of total descent would be a reasonable place for this option. A Scangauge monitoring traction battery current is very helpful in judging this. While at 25 mph 90 Amperes is pretty strong slowing which is easy to stay under, at 50 mph on a downhill it is really easy to get past it into friction braking if you slow for traffic conflict or a stop.

[Bob64]

Originally Posted by qbee42

This is not correct. First, it won't prevent the ICE from shutting off, and second, why would you not want the ICE to shut off? The air conditioning in a Prius is driven by an electric motor - it works just fine when the ICE is off.

Tom

Actually... I have read somewhere that this might be true. If the engine is running and your in B, it won't turn off. I'll have to test this out today on my commute to see if it is true or not. I'll post back with the results of my test.

[htmlspinnr]

Originally Posted by Bob64

Actually... I have read somewhere that this might be true. If the engine is running and your in B, it won't turn off. I'll have to test this out today on my commute to see if it is true or not. I'll post back with the results of my test.

Anecdotally, regardless of whether when it will shut off, the ICE will run *longer* when the vehicle is stopped when "B" mode is selected. I've noted this from personal experience in both Prius (previous and current gen).

[dwreed3rd]

Thanks archae86:
There's a numer of stretches of several miles each on my wifes drive to work, best route for time, distance and traffic. Speed limit is 35mph, well enforced, mostly residentiaal, up and down multiple fairly long hills, probably at least 6% grades, several hundred yards, some stop and go traffic. We shift into 2nd in our 2000 Avalon XLS, sometimes even 1st in downhill traffic, to avoid riding the brakes and having the tranny continously shifting up and down, especially trying to avoid those annoying times when you go to give it a little gas just as it decides it needs to shift. I figured these stretches would be ideal to experiment with "B" mode.

[Fraser]

Originally Posted by qbee42

This question comes up a lot on this forum. Some of the information in this thread is correct, some not. Here is the real story: ...
Tom

I understand the comparison to jake brakes. I have considered the B mode more like downshifting on a grade to increase engine compression. Both allow a safer descent and less reliance on physical brakes. The result, of course, appears to be the same. But you raise a question in my mind about the use of friction brakes in normal driving, not on grades. When I approach a red light or stop sign and apply the brake pedal, am I actuating the physical disks/drums or is a form of regenerative braking/B mode being used to slow the forward motion, as a sort of stop-by-wire?

[Bob64]

To compare: B mode spins the engine in addition to regular "D" Mode, which wastes energy, but also increases regeneration by a bit... Keep in mind that you can achieve the exact same regeneration "increase" by pressingly lightly on the brakes in "D" mode.

Regular "D" mode avoids engine spin, and regenerates as much as it can until it hits below 7mph, which it WILL transition to friction brakes.

[Bob64]

Originally Posted by htmlspinnr

Anecdotally, regardless of whether when it will shut off, the ICE will run *longer* when the vehicle is stopped when "B" mode is selected. I've noted this from personal experience in both Prius (previous and current gen).

Ah, I remember where I read it from now:
http://www.techno-fandom.org/~hobbit/cars/b-mode.html

If the engine is running and you come to a standstill while in "B" mode, the
engine *stays* running -- just idling. The reason for this is not really
known, but it is a way to continue warming the engine when it's cold out and
you're stopped in traffic.

[Danny Hamilton]

Originally Posted by Fraser

. . . .When I approach a red light or stop sign and apply the brake pedal, am I actuating the physical disks/drums or is a form of regenerative braking/B mode being used to slow the forward motion, as a sort of stop-by-wire?

Yes. . . ., and it depends.

In a panic stop where you suddenly slam on the brake pedal, the friction brakes are immediately applied, and nearly all the braking is done by generating heat via friction with the pads on the disks/drums. There is still a bit of regen going on at the same time, but not much.

In a gentle stop on the older model Prius, there is a gradual change with most of the braking done regenerative at higher speeds, and the pads being applied more firmly as the vehicle slows down. It isn't a linear transition, I've seen a graph of it around here somewhere and there is a definite curve to the percent friction/regen as a function of speed.

In the more modern Prius (2004+), from what I've read around here, it is nearly all regen until you get down around 7 MPH at which point there is a sudden transition to almost all disks/drums. During especially gradual braking on an especially smooth road you can feel this transition. Some describe it as feeling like the brakes "release" momentarily, others have described it as feeling like the brakes suddenly grab a bit.

If you press the brake pedal hard enough your braking desires eventually exceed the ability of the regen system. At this point the regen system is assisted with the disks/drums to provide the additional braking force required.

Now in the more modern (2004+) Prius, if you traverse a sizeable bump, hole, or very rough patch of road (such as a railroad crossing) while braking, the Traction Control/Anti-lock brake system may cut off the regen braking and switch over to disks/drums just like in a panic stop.

In the unlikely event of a failure of the braking control system, solenoids that are typically held closed fall open and provide a direct hydraulic path between the brake pedal and the disks/drums to revert to traditional friction braking.

The brake fluid pressure is maintained by an electric pump. In the unlikely event of an accident or other emergency situation whereby the battery is unable to provide the power needed to run the pump to regain brake pressure, there is a bank of capacitors that are maintained at a constant full charge. These the pump can draw power from these capacitors for long enough to bring the vehicle to a stop.

Somewhere around here there was a document posted that presented much of this in a great graphical form. If I find it, I'll post the link (or perhaps someone who knows where it is will post the link before I get to it.

[hybred tom]

please no trash in this forum