B Mode--What is the Purpose?

Gets better gas mileage: wrong, in D mode regeneration captures the braking energy above 7 MPH unless it is a panic stop.

Less wear on the brakes: Wrong, the friction brakes only normally work below 7 MPH, which is true for B as well.

I can't imagine how being slammed into from behind is a good thing, but to each his own.

* Spelling corrected, unless you meant the car would break, it won't.

 

Agreed. I go up and down hills big enough to fill the battery periodically, and it's generally been my experience that:
1. Until the battery is full, B doesn't really run the engine very fast, preferring to use regen.
2. Once the battery is full, both B and D will rev the engine very high when you press on the brake pedal.

The only difference is that B has a higher amount of drag when you are just coasting.

So the only time you need it is if you don't want to press on the brake pedal to give yourself moderate and continuous drag. Otherwise, D will perform fairly similarly to B when it needs to. However, under normal battery charge levels, B will still spin the engine (often even when you're gliding, or trying to), so you lose efficiency there.

YMMV, as some of these behaviors change according to your car's speed, battery temperature, etc. But this is what I've observed in my area.

 

I have only had one opportunity to use B mode but because of a car less than one car length behind me, I did not try it because I didn't know if it would reduce the speed too quickly and the idiot would run into me.

I was in the Black Hills in South Dakota. It was a very long several mile downgrade. I noticed that only one bar to go for a full battery. Right at the point the battery showed full the car started going faster. Until that point just staying off the gas keep the car at 45 mph which was the speed limit on that very curvy road.

I keep taping the brakes for the fellow to quit riding my bumper. Even when I slowed to 40 in a passing area, he stayed on my rear.

So I never got a chance to try B. I live in flat Illinois and have no need.

I do wonder how abrupt the change would be from slowing gaining momentum and dropping into B on a long down grade.

 

It is no more abrupt than downshifting a nonhybrid transmission.

Anyone who runs into you because of it, is either already in extreme violation of following distance laws, or is not paying any attention to the road and traffic.

Unfortunately, the later covers a lot of drivers. For those who saw the Rachel Beckwith story on NBC this evening, they glossed over the cause of her death (local to my area): a semi-truck driver not paying attention to the traffic jam and large mass of brake lights ahead. Two local bicyclists were also killed this week by drivers not paying attention. Both riders were in their designated lanes.

 

I agree 100% with Jimbo.

B mode means less pressing the brake when approaching bends or when trying to maintain a safe speed down hill.
Also I find that using B mode the harvesting of kinetic energy is boosted resulting in a faster charge of the drive battery.
Futhermore I do get better MPG when using B mode on up / down hill routes.
Also when the drive battery is full, B mode just seems to drop a gear to slow you down causing high revs.
Its cool to approach bends in the road at 30MPH and allow the additional B mode drag slow you down to a safe speed to take the bend without pressing the brake. IMO this results in a more cautious drive by anticipating the braking points and harvesting energy gradually while carrying momentum into the bend.

 

I think you're mis-reading him: in his post above he doesn't seem to like B-Mode.

I'm inclined to agree (with Jimbo): if you need to slow down, just pressing the brakes is nomally your best bet: it uses regen (charging the battery), alerts drivers behind that you're slowing, and so on.

The main reason for B-Mode is to provide engine braking in cases where the battery is already fully charged and the car has started relying on friction brakes to slow, say on a long downhill run.

 

B mode does not increase the harvesting of kinetic energy. B mode decreases the harvesting of kinetic energy by deliberately burning off energy as pumping losses in the ICE. The ultimate limit of regeneration is the same for both D mode and B mode. At best, B mode can sometimes equal the efficiency of D mode, but in general it is less efficient due to the deliberate use of the engine for engine braking.

Better mileage using B mode is an indication that your mileage calculation methods are flawed.

Tom

 

Or in cases where (without it) your braking demand exceeds the max. regen current allowed, say in a steep downhill slope with winding road.

 

I like B-mode from the top of any hill that will fill the HV battery (600 vertical feet?) My goal with B-mode is to keep the friction brakes as cool as possible, so the brake fluid never boils. Running both the regen and air pump engine braking reduces the need for friction braking, Starting the air pump early means the regen braking will last farther down the hill.

Once I am off that hill and did not die in a fiery crash, I go back to using D mode exclusively for maximum efficiency.

I would use B mode anywhere I would use L in a normal automatic transmission, many owners would be less confused if Toyota had labeled it L, but they were too honest, it does not actually lower any gearing.

 

This says it all in three short paragraphs.

Tom

 

B mode video explanation!

‪Paul Guzyk of Boulder Hybrid Conversions Test Drives Upgrad‬‏ - YouTube

 

I'm a fairly new Prius owner and found switching to "B" when I'm about to descend a long, curvy hill helps tremendously. I do not have to ride the break and it slows me down going back and forth around the curves. The road flattens and comes to a stop sign, so I keep it in "B" right up to the stop (and usually never even have to use my break pad until the stop). Loving the "B" mode...