Braking Mode <B-Mode> Useless?

I use it for one particular long downhill 35 mph road, where I will get up to 50 if I'm not riding the brakes, otherwise.

On that stretch, if I throw it in B right away, and leave it, I will slow down to 30 by the bottom.

Think of B not as a lower gear - it is more related to engine-compression braking, aka: Jake Brakes, like semis use when having to slow down on long downhills. It works over the long distance, not for such a short decline.

 

B mode works well for its intended use. Unfortunately, your gravel road doesn't fall into that category. For a small descent like that, use your brakes and let normal regeneration do the job.

Having to press the brake pedal is not a sign that B mode does not work. B mode does increase the amount of regenerative braking applied to simulate engine drag, but its most important feature is to throw away energy by using engine braking. Pressing the brake pedal tells the car to brake harder. In B mode it spins the engine faster to waste more energy.

I believe you are confusing pushing the brake pedal with using friction brakes. The two are not intimately linked. Pushing the brake pedal tells the Prius to apply some sort of braking. How it does it is up to the conditions, and whether you have selected B mode.

For a short descent like yours, you want to recover as much energy as possible. Press the brakes and let regeneration do its job.

As for your lower gear question, the Prius does not have shiftable gears.

Tom

 

I find "B" works very well. I only use it on mountain descents, which is what it's intended for. It works better at higher speeds (40 MPH and higher). It works better than the anti-Prius (2001 Nissan Pathfinder LE AWD) did in second gear (automatic).

Do be careful when you use it, as it only slows the front wheels. On loose surface or slippery surface roads that -could- cause you problems.

 

B works very well. We have taken several trips to the mountains and it really helps on the down hills. It ups the regenerative braking until the battery is full, then as soon as the battery is full it revs the motor more. On long descents you know exactly when the battery gets full because the engine suddenly revs very high, sounds like redline. On more gradual hills it is great for maintaining speed on the way down, as opposed to accelerating. At very slow speed it doesn't have a great effect, but at normal driving speeds the effect is significant.

 

That is a slope in excess of 40%. That is steeper than the pitch of my roof.

I have doubts that braking is even possible on ball bearingsXXXXXXXgravel at that pitch.

 

The Prius transaxle doesnt have "gears" in the sense of a traditional automatic trans (1st gear, 2nd gear etc).

Its a continuously variable Transmission and the gear ratio is varied by MG1 exerting negative slip. Its really a very elegant design, few moving parts and very simple (mechanically) compared to a traditional automatic trans.

I find B mode very helpful on long descents in the mountainous area's near where I live. It is not real useful otherwise.

Eric

 

I often descend a steep mountain which B mode works well on to preserve my brake pads, but I wish I could use up my batteries better on the upside first, in order to refill them on the downside without reaching full battery halfway down, and gain some effficiency while at it. Anybody know how to do that?

 

I have only used it passing Wheeling WV in that 5% grade 5 mile section of interstate highway.

 

Ahh, a "B" mode question opportunity.

It's said that if the shift lever is in "B" mode and you press on the accelerator the car overrides "B" mode and there's no harm done to the "B" mode mechanism.
I notice that the "B" mode works from 1mph to whatever speed the car is capable of.
Under 22mph the ICE doesn't spin but "B" mode does slow the car, over 22mph the ICE does spin(scangauge2 rpm setting), so what's slowing the car under 22mph?
Regeneration of heat into the battery you say, but how do I know that when pressing on the accelerator the regeneration isn't still working, charging the battery?
How can I be certain that when the shift lever is in "B" mode and I'm going 15mph using the accelerator (going uphill) that the regenerative system isn't still causing drag?
Or more to the point does the scanguage have a reading that shows the regeneration effects?

 

Hi ursle, you're very much thinking about regen braking in the wrong way and worrying about things that aren't an issue. First off regen braking does not convert "heat into battery", it converts the cars rolling energy (kinetic energy) into electricity and hence battery charge.

As for worrying about whether the extra "regen" of B mode really does go away when you're on the go pedal, you need to stop thinking of regen as being a different thing to the normal electrical "go" power - they're in fact just different aspects of the very same thing. What I'm trying to say is that "regen" and (electrical) "go" are underlying the very same thing, so they CANT fight each other.

Try thinking of it like this. With a pure electric car you wouldn't actually need separate accelerator and brake pedals. You could just use a single lever, kind of like a joystick, that just used lever-forward to provide "go" and lever-back to provide regenerative braking. That is, the electrical output of the motor/generator really can be thought of as pretty much a continuum from the +ive input power of "go" to the -ive input power of regenerative braking. So if for example you put the lever in center position there would be no motor torque in either direction and the car would just coast.

Now a lever control like that would be a bit radical for a car so they basically just use the accelerator pedal to provide the "lever forward" part and the brake pedal to provide the "lever back" part. Now they could have designed it so that with the foot on neither the accelerator or the brake that this would be the equivalent of the hypothetical "lever" center position, but since people are used to having some engine brake from a regular car they make it so that this condition gives a small amount of regeneration (to mimic the engine braking of a normal car). Think of it like the "lever" analogy where when you release the hypothetical lever it doesn't go back exactly to the center position but instead it rests just slightly in the reverse direction so that it gives a small amount of regeneration. Essentially all that the "B" mode is doing is altering this "center position" slightly so that it rests just a little more in the reverse direction and therefore commands the motor/generator to regenerate a little more strongly. Notice that in this case when you move the lever forward (that is, taking control and moving it from it's resting position) it automatically turns off the regeneration - because there really was no separate regeneration control anyway, it was just a different set point on the SAME control as the go pedal.

Wow that was a lot more long winded than I thought it would be to explain such a simple thing. Anyway, if it doesn't make sense then read it again (and repeat if necessary), because I cant explain it any better.

 

Uart - That is the best explanation of how regenerative braking works that I've seen so far. You have made it entirely understandable to the layman. Thank you!

I also now understand why, on my 2009 Prius, I can get the equivalent of "stealth mode" at slower speeds by keeping my foot just slightly on the accelerator so that neither propulsion nor regen is seen on the display, ie: no lines between the electric motor and the wheels. That is the neutral position to which you refer, just slightly on the go pedal.

Dave

 

Scanguage can read the current going in or out of the battery. The X-guage Bta that can be manually programmed into Scanguage will show you what you want to continuously monitor the HV battery current.

The programming (see Scanguage manual) for Bta is:
TXD 07E321CE RXF 056186CE0000 RXD 3810 MTH 0001000AF333

 

Something funny about B mode; notice that while in B mode, if you take your foot off the gas the engine RPM actually increases, and if you very lightly put your foot on the gas the engine RPM decreases. This is even more noticeable when the battery is fully charged and it uses 100% engine braking (no regen because the battery is full).