Yanquetino
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"D/B" on shifter?

Tue May 11, 2010 8:22 am

I am intrigued by this photo of the Leaf's shifter:

Image

It clearly shows a mode labeled "D/B". The "D" obviously stands for "Drive." As for the "B", I am guessing it might stand for "Braking," i.e., the ability to "downshift" when descending steep hills. At least this is what "B" stands for on a Prius, as well as on the Mitsubishi iMiEV:

Image

I understand that in the iMiEV, its "B" mode increases the regenerative braking, so I am guessing that would likely be the case in the Leaf as well.

Am I guessing correctly? If so, how does one select "B"? By pulling the knob down a second time? And then how does one resume normal "D" mode? By pulling it down yet again? In other words, do the "D" and "B" modes toggle back-and-forth between "normal" and "strong" regenerative braking?

Also, when the "B" mode is engaged, do drivers see this on the dashboard display? (I have only seen "N" for "Neutral" in photos, but assume that the letter must change when shifting, right?)

I hope that I am correct in my interpretation of "B", as I believe that U.S. regulations mandate that a car must have a way of "downshifting" for steep declines --lest drivers "ride" their brakes.

Thanks!

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garygid
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Re: "D/B" on shifter?

Tue May 11, 2010 8:44 am

Good question.

Perhaps in the D/B mode one has both Driving and regen Braking, variable Driving with the "gas" pedal, and variable Braking (regen) with the first part of the brake pedal?

And, to be legal it must have a mode labeled "B" (auto-B mode, invoked when pressing the brake pedal), which also happens to be the "auto-D" mode (touching the accelerator)?
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JasonT
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Re: "D/B" on shifter?

Tue May 11, 2010 8:55 am

Yanquetino wrote:I am intrigued by this photo of the Leaf's shifter:
I understand that in the iMiEV, its "B" mode increases the regenerative braking, so I am guessing that would likely be the case in the Leaf as well.

That has been the assumption, and it is a reasonable one. As to how it is engaged, though, I have not seen anything on that yet. I have tried to look at photos of the dash and of this section as well looking for an additional switch that would engage the "brake" mode, and have so far failed to discover the activation method.

For now, your toggle theory is the best one I've seen. Since there is only one indicator light below the shifter, I would imagine that the main dash would indicate the change between D and B mode.

mitch672
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Re: "D/B" on shifter?

Tue May 11, 2010 9:14 am

Since there is no real "transmission" in the Leaf, why would anyone assume there is a difference between "Drive" and "Brake" (Engine Brake)? If you take your foot off the acelerator, you are automaticlly engaging the engine (motor in this case) "brake" because you are not putting energy into the motor... There will be drag on the motor, and it will slow down and do some regen, and why they show it as just 1 position. No distinction needs to be made, if you are not acelerating or staying the same speed (foot at the same position on the accelrator), you are "braking" (B). This is without touching the actual brake pedal...
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JasonT
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Re: "D/B" on shifter?

Tue May 11, 2010 10:19 am

mitch672 wrote:Since there is no real "transmission" in the Leaf, why would anyone assume there is a difference between "Drive" and "Brake" (Engine Brake)?

If this works like some of the other EV vehicles work, it would change the software to a more aggressive engine braking profile. This can be useful in hilly areas, or even just someone that wants to take more advantage of braking regen.

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garygid
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Re: "D/B" on shifter?

Tue May 11, 2010 11:22 am

The B-mode, if it exists as a distinct mode, will undoubtedly increase the regen (motor) braking, to aid going down hills.

My Prius does that, but even the increased regen was not very "strong", not at all like downshifting a typical ICE vehicle. Perhaps more like not downshifting an ICE vehicle. On a steeper hill (behind Laguna Beach), I had to use the Prius' mechanical brakes "all the way" down.
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daniel
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Re: "D/B" on shifter?

Tue May 18, 2010 4:32 pm

There is a very important distinction between a pure EV and a hybrid, such as the Prius: Both can use regenerative braking to slow the car, and both can use a "B" mode to increase that braking. The difference comes when the battery is full and there is no place to send the electricity. The Prius still has an internal combustion engine and it can (and does!) use engine compression braking when the battery is full and "B" mode is engaged.

Contrary to what Mitch states above, there is no automatic drag on the motor when you lift your foot off the accelerator. An electric motor is freewheeling when no contacts are applied. It can be configured (by the car's computer) to act as a generator, and that applies drag, but that also produces electricity, and that electricity must go somewhere! As long as the battery is not full, it can go into the battery and all is well. We are even recovering energy. But once the battery is full we have a serious problem: Energy does not disappear! Engine compression braking turns that energy into heat (by compressing air in the cylinders) and dumps that heat through the exhaust system. But an EV does not have an engine.

A pure EV must have some way to dump the energy of slowing the car if the battery is full. The situation will be rare, because the large battery will usually have room to absorb the energy of regen braking under normal conditions. But if you live on top of a hill and you fully charge the battery and then start down a long, steep hill, you run the risk of overheating the friction brakes. (Under normal circumstances, a conventional car converts kinetic energy into heat in the brake linings, and that heat is dissipated in the time between braking events.)

The problem could be solved by dumping excess electricity (from regen braking after the battery is full) into the resistive heater, and then venting excess unwanted heat to the outside.

My question is, does the Leaf do this, or does it rely only on friction braking for long descents after the battery is full???

I'd be very interested in an answer from Nissan on this.
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Re: "D/B" on shifter?

Tue May 18, 2010 5:19 pm

I'd like to see some calculations - but my guess is that even if you practically start on top of a hill - a BEV battery will have space for regen. Most BEVs will not top off the battery & thus will have some space left.
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Re: "D/B" on shifter?

Tue May 18, 2010 5:35 pm

The brakes on the Leaf will be larger than required so braking with a full pack will be adequate. When an EV is fully charged the regen can be significantly reduced as it is directly related to the load on the motor and most will not allow higher currents to a fully charged pack but more importantly the pack will not supply the high load as it is full, this happened often on my EV as I live at the top of a hill and the regen was extremely low when fully charged, it was even software restricted below the load of the pack for protection form overcharge. There is a way to shunt power to a resistive load for regen loading or in those cases where the pack is fully charged the regen will be significantly reduced or cut off. The best option is to move to the bottom of a hill but I'm sure this will not be an issue for the Leaf:)

mitch672
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Re: "D/B" on shifter?

Tue May 18, 2010 5:36 pm

numbers such as discharge down to %10 SOC and charge to %90 SOC have been bandied about (using %80 of the usable capacity of the battery pack), the very rare condition of the battery pack being at %90 SOC at the start of a long descent would be very rare indeed, unless you live at the top of a large mountain or hill, that would be the only time it could even be an issue. I'd bet that Nissan just reverts to friction braking in those rare conditions, you can't count on running the resistive heater, it could be the middle of summer, and unless they made a way to dump that heat outside, it would could get very hot in the passenger compartment. Of course you could run the air conditioner and turn on the lights at that point, so I guess there is always something they could do, such as run the AC/Lights in the summer/warmer weather, and run the resistive heater in the winter/colder weather... I wonder if they have even considered this as a possible condition, being how truly rare it would be? I suppose they could also have a special "resistive load" under the hood, that would be air (or liquid) cooled via the front vent, it would only be used in this special situation, kind of a "load bank" for excess energy.
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