"B" Range...What's Really Going On...

Below 7 MPH* there is no regen, B does nothing.
Below 24 MPH, B can slow the car without engine braking.
Above 24 MPH, engine brsking in B will reduce MPG compared to D.

* Gen 2, other Prius will vary slightly, plug ins will vary a lot.

 

Forum info is nice, but here what the manuals say. Once from an '07 and one from a '13 PiP. Both say that fuel efficiency is reduced in "B."

 

Having read Jimbo's many posts here since before that forum even existed, I must say he understands B mode -- at least regarding the Prius, not Camry -- better than the author of that link.

You can read numerous better descriptions of B-mode here on PriusChat. Even if you ignore the ones Jimbo has written himself.

Here are some technical errors in that ToyotaNation link:

"Main difference between Hybrid ICE engine braking, and conventional ICE one is that fuel is not supplied and ..."
This is not a difference for modern conventional ICEs, they also don't supply fuel. This modernization happened before the Prius ever existed.

"Second very important difference is that, because TCH - Toyota Camry Hybrid - does not have conventional transmission - B mode can be used for normal acceleration."
Sorry, conventional transmissions can also used downshifted gears for acceleration. In fact, this is often desirable.

"2. Though it is still argued in this forum, B mode recharges hybrid battery faster, than ... or from regenerative braking."
Sorry, this is outright false, at least with regard to Prius. Regular regenerative braking with the standard brake pedal can easily reach the battery's maximum charge rate, and does it without dumping any power or energy into ICE engine braking. B-mode cannot boost this battery limitation. If the ICE is spinning at all (which it usually is, that sub-25mph ICE-less B-mode is not common), B-mode will throw away some energy, making it forever unavailable to regenerate into the battery.

"I use B mode for sharper, sportier cornering. Instead of using brakes before corner entry, I kick lever into B mode. It gives just enough braking to enter the corner faster than in D mode"
This line is simply about sportiness, not efficiency. D-mode and regular brake will achieve the same, though may require left foot braking to achieve the same quickness as he gets in B-mode.

"6. It is said - and I'll check it out winter time - that B mode permits much better stability on snow and ice."
Sorry, but there is no difference here, other than driver style. I'd like to see his update after actually getting some winter experience.

 

I would add special emphasis to the word 'can', because this particular mode is not an automatic 'will'. The great majority of the time I'm using B-mode, I don't get this special case.

 

Regarding B-mode, I must challenge this. If you are praying due to low gas, B-mode will continue to be wasteful. At the very best, it will not extend your tank's ultimate range. Typically, the waste-gate effect will actually shorten that range.

 

Fuel efficiency reduces when we floor the accelerator or keep it down in any gear and quite possibly the B mores than D. Nevertheless, it has a special value mysteriously omitted from the manuals, so maybe you might like to play with it at your leisure.

 

Regen is resulting in EV power hence no use of gas.

 

The Prius has some features that were designed into it deliberately to emulate the way conventional cars feel. The most obvious example is the forward creep when you are stopped and take your foot off the brake. There's no inherent reason a hybrid powertrain would do that, but other cars always have, so the Prius is programmed to do it too. In conventional cars, it's a direct consequence of the way the engine and transmission work. In a Prius, it's just a behavior programmed into an ECU, but it feels the same.

Another example (and the connection to this thread) is the way you feel resistance, a bit of active slowing, when you take your foot off the go pedal (even without using the brake at all). Again, no reason for a hybrid to do this, but other cars do, and the Prius is programmed to mimic it. The amount of resistance is just a number programmed into the ECU. Other cars feel more aggressively slowing when you shift to lower gears, so the Prius mimics that too. The number programmed into the ECU for B mode is larger than the one for D.

That's what people are referring to when they say B mode regenerates more. Not that it collects more energy regeneratively when you're braking, but that it collects more in that mimicking state when you just have your foot off the go pedal.

But there's another effect of B mode that partly counteracts that: B mode is programmed to be more willing to dump energy to the engine instead of sending it to the battery. None of this is all or nothing; if the battery has lots of available capacity and isn't too warm, B mode will still put some juice into it, and even D mode will dump to the engine on a long downhill if the battery gets chock full. But, comparatively, B mode is programmed to have more of a bias toward dumping; it will start earlier, and dump more. That's what makes it useful to keep the brakes and battery cool on a long descent.

As for conserving energy, it's not as useful as you might think. The idea rests only on the increased active-slowing mimicry when your foot's off the go pedal, which means in that fairly small fraction of driving behavior it will recover a bit more energy to the battery (if there is plenty of capacity, and the battery is cool, and the B mode bias toward dumping the energy isn't coming into play).

In the grand scheme, that's not really an energy-conservation win: really the pedal response has become touchier, you have less 'glide' leeway between powering the car forward, and the extra-aggressive B mode slowing when you let your foot up. You have a harder job to drive the car efficiently. Sure, you capture some of the energy, but don't forget you pay the conversion-losses piper twice on every joule of energy you sap from forward motion to put in the battery, and then put back into motion later.

-Chap

 

Regen is slowing the car down.

It's better to keep the momentum in the kinetic energy of the car rather than wasting it in a double traverse from mechanical to electrical power, to chemical energy and back to mechanical.

kevin

 

Let us frame a limited example. In the mph range appropriate for EV speed. How I chose to handle my C was to do two major things. Stop gas burning at low speed and stop gas burning at high speed. (This car is no longer mine but enjoyed a lifetime average of 54mpg.)

To stop gas burning at low speed my battery had to be well charged and not under an extraneous strain. How did I charge the battery. Of course with standard Regen. Brake and brake some optimally more. But there is traffic flow. What do I do? I put it in B and let the system Regen at its own rate.

And driving without gas was only so to get to an open station. What sense would measuring fuel efficiency have when I am avoiding the use of gas at all?

The efficiency of the gas I would burn could be lesser because what gas I burn isn't meant to be burned.

I think people have their minds made up. So be it.

 

I am absolutely satisfied slowing the car in Regen braking.

 

This was related to the statement that 'B' mode would be useful when you are running out of fuel. I maintain that it would make things worse. It is better to keep the car rolling.

kevin

 

In your focus I agree but it, in real experience, was not one you could consider smooth driving. Accelerate and roll in B accelerate and roll in B. If I had kept it in D the energy would have more depleted. Using brake is too demanding a dip of momentum while using B one has a tighter control on momentum.

It was not an experiment.

 

All that regenerated EV originates from gasoline. But every conversion cycle involves some efficiency losses skimmed off the top, the more conversions, the more the loss.

I must challenge this. In my nearly 8 years of Prius experience, the D version of this is more efficient. My ScanGauge-II reveals significant losses in B.

Plus, within D mode, the 'glide' version is more efficient than the 'coast' version.

 

At the risk of sounding trite, challenge accepted. Your experience respectfully and duly noted. I will start like this:
D is for emphasis on fuel use.
B is for emphasis on Battery use.

I shall return. Phones need charging too. Lol.

 

For most of us, that last line reads:

B is for emphasis on preventing overheated brakes, and secondarily on reducing battery heating when regenerating. Both are issues only on long mountain descents.

If you are to demonstrate any better MPG by using B mode, I would guess it to be most likely in careful application of this:

This mode could be more purely regenerative than light use of the brake pedal, as the later may apply a bit of friction pad brake with regen to better control the feel of its transitions.

But to use B mode this way, the engine must already be auto-stopped. If it is still spinning, the ECUs will disable auto-stop and force it to continue spinning, wither with compression braking or idling with fuel. Both spin methods waste energy, cutting overall trip MPG.

 

No, D stand for Driving, that thing you do every day.
B stands for engine Braking, that thing you do down long, steep, downhills.

If you are not descending at least 600 vertical feet in a non plug in Prius, you never want B. It will just lower MPG and endanger your car from other drivers, who can't see you brake.

US law requires Toyota to provide a gear with increased engine braking than D. You are never required to use it. 49 CFR 571.102 - Standard No. 102; Transmission shift position sequence, starter interlock, and transmission braking effect. | US Law | LII / Legal Information Institute Section S 3.1.2

 

unfortunately, we'll never know.

 

I only use B as I am coming to a red light. Very little, but I'm good at moderating my battery. I always go EV in our neighborhood with the A/C off and windows down too.


iPhone ?

 

Both B mode and EV mode are reducing your MPG. Windows down may improve MPG versus A/C below 30 MPH.