B Mode--What is the Purpose?

I agree, this will be the 3rd time I have replied to a B mode thread in the last week. Toyota should just automate B mode and eliminate the confusion.

Here is a handy little set of questions to determine if you need/should use B. Everyone can print it out and tape it to their dash. You need to answer yes to ALL three questions before using B mode.

B Mode Reference Guide - Tape To Dash For Easy Reference
************************************************
Is your battery full?
Are you on a steep slope?
Do you have to break alot to maintain a proper speed?

If you answered NO to ANY one of the three questions you don't need B mode and are just decreasing your fuel economy.
************************************************


Hope that helps,
G

 

Again, the first one ( Is your battery full?) is not necessary.
The Prius will enter B mode if that happens, no need to worry.

I might as well add that when this happens the display still shows D mode engaged although it behaves exactly as B mode to protect the traction battery.

As soon as the battery looses some energy the Prius will enter regular D mode again without any intervention from the driver.

I have witness this myself once while descending the Swiss Alps with my 2006.

 

First, yes you can put it in B mode any time but it doesn't make my statement untrue. If you put it in at any other time than the circumstance above you are just wasting fuel.

Second, the rest of your statement is entirely NOT true at least on GII. When the battery is full it goes into what feels like neutral "not engine braking" causing the vehicle to gain speed at a pretty good clip. It is very noticeable if you have ever had it happen. It might be different on the GIII.

Edit: Maybe your 06 and the 08 I had act differently but the fact is mine did not enter engine braking when the battery was full. Could it be a difference in US vs Portuguese builds?
G

 

Jesus Christ, am I writing such a bad English today or what's happening here?

I'm not making 2 separate statements, it's one and only one!
I'm not saying I can put into B at any time, of course I can, what does have to do with this discussion at all???

The "rest of your statement" (as you call it) IS true, and like I said, it happened to me already on my second gen 2006 model. Are you calling me a liar here? I sure hope not...

IF your battery is full, and you're still descending a hill, with regeneration still ongoing, the car will BY ITSELF engage B and make the ICE run like crazy. Find a hill part on your country and test it yourself for crying out loud.

I could think this would be an European feature not set on American models, but I've seen this reported on YahooGroups Prius Technical Group from American owners as well, so there you go.

That's why I say you don't need to worry nor ask the first question. If the battery is full the Prius is clever enough already to protect the battery by entering B mode by itself.

<G>

 

Here you go, a testimony from Efusco I just found right here on PriusChat:

http://priuschat.com/forums/gen-ii-prius-technical-discussion/51795-automatic-b-mode-84-soc.html

Now go ahead and call him a liar too. You could use some cooling you know? Let me just add you to my ignore list and move on. What a character...

 

VERY VERY interesting!
You have found a new feature on the 2010, congrats! :rockon:

 

The car doesn't engage friction brakes (except to control vehicle stability, and even then I'm not sure) unless you press the brake pedal. The hydraulic braking system is controlled by the Skid Control ECU which reads the brake pedal sensors only. The braking from lifting off the accelerator pedal is engine/transmission-controlled only. This may be to emulate the feel of a conventional setup.

I initially misread you and thought you were claiming that the friction brakes weren't used at all! This led to some interesting calculations that I thought I'd still share.

Calculate kinetic energy at 70mph. The car is moving at 31.3 metres/sec, and weighs about 1400kg with a driver, so kinetic energy is 685,174 joules. The battery can absorb energy at about 20kW, which is a rate of 20,000 joules per second, so after one second the car's energy would be 665,174 J. The velocity is then 30.8 metres/sec, or 69.0mph.

Extrapolating, it would take 34 seconds to stop if this were the only force acting! Clearly the car decelerates much quicker than that.

Now, I haven't factored in wind resistance, which is about 12kW all on its own at 70mph, and tyre rolling resistance, but you can see that there must be a frictional braking component otherwise you wouldn't be able to slow the car down quickly.

At about 7 mph, there's only about 6,000J of energy (1.7Wh) left to recover, so the cutover to friction braking here doesn't lose much energy. I'm not convinced that the car can't actually do this with regen, I think it's just the cutover point to creep emulation, which also contributes to the dragging feel (compare with braking in N).

I've attached a spreadsheet you can play with: change the values in column I to tweak vehicle mass, starting speed, and deceleration rate. Cell K2 is just the speed in metres per second.

 

Excellent work Mike, although you are working with rough numbers, it serves well for demonstration. I'll give it a good look later on at home, I'm still at work, and posting like crazy on PriusChat is not what I get paid for LoL.

Also please consider that the regeneration efficiency is more or less the same (proportionally) to motoring efficiency. We know for a fact that at very low speeds the MG2 is not very efficient. The peak I seem to remember is at around 60 km/h.

So at around 12 km/h when the friction brakes enter (and it is the top creep speed) the low energy that exists is not only very little worth but also very hard to get since at those speeds MG2 regeneration efficiency is miserable.

Does this make sense? I think so.

 

I think that comment could apply to more than one person on this thread...

I've edited my contribution to this thread as they were wrong and served no purpose. I realize my mistake was thinking that the Prius used friction brakes in D-mode to supplement regenerative braking (battery full) and then switched to engine braking in B-mode. I now understand thanks to a very positive and friendly exchange that B-mode simply favours engine braking immediately.

 

Whoaaa look who is calling the kettle black ... Was there anything in my statement that indicated i was hot?

Well at any rate I am reporting my experience. When my 08 battery reaches full it has felt like being in neutral and the car has picked up speed. I suspect that the car is smarter than we are giving it credit for and my B mode reference guide will still help clarify when B should be used.

As most people that know me will atest most of my posts have touch of tongue and cheak.

G

 

Of course newbies will ask "Why does the car even have "B" mode if it causes so much confusion and discussion?"

You will find the answer to this very quickly when you descend your first steep and long downhill! The Prius is aerodynamically very slippery and the drive system very low loss. It takes off down hills!! Use "B" mode to control this.

I like to very lightly use the brakes on "shortish" downhills, but when descending a mountain pass I pretty much NEED "B" to control the car. It's entertaining to try to calculate if I need to engage "B" at the top or if it's better to lightly use the brakes until the battery is full (you will feel the car then loose a bit of braking if you are sensitive enough on the brake pedal) and -then- switch to "B". I suspect it makes little difference. Even in "B" the battery is very quickly at full green bars and regen ceases.

What is fun is when in "B" with a full battery, descending a steep hill that levels out for a few hundred metres and you press the accelerator pedal to keep your speed, the engine actually -slows down-. Very strange!! But correct as it causes engine braking to decrease.

 

"We beat this horse on a regular basis" Great word picture! :deadhorse:

Tom:

Thanks so much! A great, concise explanation. Very helpful.


Best wishes,

 

Although it will automatically start spinning the engine, it doesn't seem to be as aggressive as when B mode is manually selected.

For example when I come down from the Santa Cruz mountains without B mode the engine only goes up to about 3200RPM but if I engage B mode it will go as high as 4750RPM.

kevin

 

Good input Kevin, thanks, I was not aware of that difference.

I assume you can reproduce that behavior at will?

If that's the case, then it SEEMS that driving in D will spur the ICE just the right amount (minimum amount necessary) while on B it will turn at a higher minimum amount that is particular to B.

Could that be it? Interesting, I never noticed it. Again thanks Kevin.

 

First thanks. That was a clear decsription of the "b" mode.

If the Traction battery is not charged near full,say 10%. Would it not be better to use regenerative braking to absorb the cars kinetic energy rather that forcing the engine to pump air?

 

Yes, but when descending a mountain pass you face 10 or more miles of steep downhill. The battery, starting at 30% (it NEVER gets as low as 10% - no bars is 30% or so) it would fully recharge the battery in a mile or so. The car would then "take off", requiring friction brakes or "B", which helps a lot. It also helps a bit to open the windows, increasing the drag of the car, though it can get noisy doing that.

 

Regeneration will completely refill a maximally depleted traction battery in about 600 vertical feet of descent, according the someone else's calculation.

Gen3 generally doesn't deplete the traction battery when climbing, so it starts with less capacity to absorb regen. So when we begin a mountain descent of 2000 to 5000 vertical feet, we have a lot of gravitational (not kinetic) energy that must be thrown away.

 

I use the 'B' all the time, especially when I am coming up on a red light. I find it not only regenerates the battery but also gets me better mileage & less wear on the brakes. I also notice that Toyota dosen't use break lights when you use this selector as the por guy behind you has no idea your breaking. I just wish the 'B' used a different color so you didn't forget like I do to switch back to 'D'.