Question about B Mode

It saves your Brake linings. A free wheeling car can be dangerous on a long steep down hill run with no engine back pressure. I once owned a 1933 Chevy with free wheeling. Wow, was that scary with mechanical brakes and no engine.

Mr Ed

You can't tell when the brakes are actually in use in a Prius, since regenerative braking is used as much as possible. However, if regenerative braking is insufficient for the amount of braking being asked for, the mechanical brakes are engaged. B mode does something else: it gives the same effect that use of a lower gear in a standard automatic gearbox does, by spinning the engine and using engine braking. I live in an area where engine braking is virtually never needed, so I have only engaged B mode a couple of times, just to see what it does. I don't know if it engages the brakes, too--but I think not. It will definitely reduce the demand for braking.

It's only needed for long, steep downhill runs--the kind where a traffic sign "Use lower gear" probably appears.

Friction brakes on the front wheels are generally used only under two conditions:

1. When the vehicle is traveling under six miles per hour.
2. In panic stops.

The rear wheels, though, have only friction brakes, and no regenerative braking.

No more counter productive than downshifting an old-style transmission under similar circumstances. Saving the friction brakes is a good thing; you don't want to boil the brake fluid, which would otherwise be easy to do when coming down from Mount Hamilton or certain places in the Rockies.

"G-Man" ...

Here is a pretty good explanation of B Mode:

Perfect. I understand what it was and was for, but I was looking for the answer in that link. It throws energy away. I thought that maybe the drag induced was somehow turning the generators and producing energy as does braking. According to that article, no.

Thanks to everyone who replied.

G-Man

No, it does not regenerate electricity.

B mode is to get the added drag from the engine pumping air.

Yes, but are the rear brakes used when regenerative braking can handle the load, or are they, too, used only when etc.?

"G-Man" ha scritto nel messaggio news: snipped-for-privacy@news.supernews.com...

Consider what happens going on a long downhill: after some time the battery will get to 'Full' thanks to regenerative braking. At this point regenerative braking will stop and the mechanical brakes will do all the job. This is the right time to use the B Mode if you don't want to waste your discs and pads and if you don't want to boil the brake fluid, as Richard says. The B Mode will give a Prius the same effect of engine braking in standard cars. Right, this throws energy away but this is what inevitably happens once the battery is full. In my experience the length of time needed for a full battery is from 5 to 10 minutes, depending on how steep downhill I'm driving. This energy could be saved only if our Prius had batteries with bigger capacity: the feature the new Prius should have in 2009 version. Diako

...according to one who has obviously never driven a Prius under those conditions. I have. When the battery is as full as the battery can be the Prius ECU automatically starts "pumping air" to make up the minor drag which was being produced by regeneration.

Having been in that situation and wondering why the heck my ICE had started running going down a long steep grade, I tried B and found no difference between B and D in that situation. Then I thought to look at the other Info MFD screen and saw my battery was pegged at solid green.

By Diako's description if the battery filled then one would start going faster when the ECU halted regeneration. As if one had pressed on the throttle until all the arrows blanked on the energy flow animation. Toyota is smarter than that. I was shocked when my ICE started "running" but would have been really surprised had it started accelerating when the battery was full.

Not here.

There's no feed to the batteries from the wheels if the car is in B mode.

Oh boy... you must be deaf.

In these conditions (downhill, green batteries, no acceleration) the REV of the ICE is doubled in respect to D. I know: I have a Prius, a rev Counter and the Alpes. :D

if under 40 mph: Batteries < 80% (and not too hot, otherwise threshold lowers) In D RPM = 0 charge = +10 Amp. In B RPM = 1300 charge +20 Amp.

Batteries topped up: In D RPM = ~2000 rpm In B RPM = ~4500 rpm

One curiosity: a slight pressure on the accelerator while in B = RPM does down (and the car goes downhill faster)

I am curious, what is the source of this information? I would like to read up more on this, thanks. Tomes

The reason the Prius doesn't suddenly accelerate when it reduces regenerative braking is that as the regen resistance is reduced, the mechanical brakes increase their effect by an equal, offsetting amount. A similar thing happens every time you slow below 7 mph: at that speed regen braking cuts-out, but you don't feel the transition.

The same is true for the motors. If you start mov> There's no feed to the batteries from the wheels if the car is in B mode. >

There is still a certain amount of regen (depending upon SOC), though it is greatly reduced. This can be witnessed by the green arrows on the MFD. The regen will stop when the SOC gets too high, but B-mode by itself doesn't eliminate it.

B-mode is something I'm very familiar with, because I use it almost every day. On my ride home from work, I have a long, steep downgrade following by a significant upgrade, then a second, steeper downgrade. By the time I get to the bottom of the first downgrade my battery is usually all-green. After the upgrade I may have lost 1 green bar, so at the start of the second downgrade I shift into B-mode. I'm usually back to all-green about halfway down the second hill. At that point there is an audible increase in RPM, as more of the braking load is shifted to the ICE.

It's true that if you allow the SOC to get too high the car will start spinning the ICE to slow itself. The Prius does a good job of protecting the battery from over- or under-charge. But the purpose of B-mode is for when the driver can anticipate an overcharge state, and start dissipating some of the extra energy before hitting the upper charge limit.

Except for the rpm thing, any car would go downhill a bit faster if the accelerator had some pressure on it, even when a lower gear.

I had a 1971 Chevrolet once which automatically opened the throttle when the gas pedal wasn't being pressed at speed, so the car wouldn't suck gas through the carburetor. The result was that the car would run around at 30 mph without slowing down. If I applied a bit of brake or lower gear, it would slow down some and the open throttle would go finally close, permitting a slowdown to 2 or 3 mph, just like any gas car with auto trans.