Highest speed to shift from D to B

First, B-mode isn't the same as regenerative braking.

The main goal of B-mode is to waste energy by pumping air with the engine, to avoid overheating the friction brakes. Regenerative braking (electic generator used to recharge the batteries) will still happen in B-mode, but you will waste part of your kinetic energy. Useful only for longs descents, to spare your friction brakes.

See the article on the subject from hobbit.

In general, the Prius will not do stupid things even if you try to. For example, you cannot swith to Park or Reverse when the car is rolling forward (all you will get is a beep). Also, you cannot rev the engine faster than it is designed for, etc.

To answer your question, you can switch to B-mode at any time/speed (from D-mode). As soon as you engage it, engine pumping will begin, to waste energy and slow the car down (slowly) (unless the gas pedal is engaged, in which case B-mode will do nothing more that D).

Be aware that B-mode will generate a counter intuitive feeling: the more you push brakes, the faster the engine will turn; if you press the gas pedal, the engine rpm will instantly go down (the faster the engine pumps air, the more energy it can waste).

B-mode has nothing to do with unintended acceleration. If the gas pedal is floored/stuck and you engage B-mode, nothing will happen (it only slow the car down when you release the gas pedal). If it ever happen, go to neutral (push the lever to N for a second). The engine will throttle down, even if the gas pedal is floored. If even this isn't working, turn off the car by pushing the Power button for 3 seconds. All power will be cut off: you will lose power steering, eventually power brakes, etc. You will need to stop the car and put in Park in order to turn it back on as usual.

You can also simply apply the brakes in such a situation: the brakes will overide the gas pedal (even if it is floored), throttle down the engine and brake the car. Be sure to understand that if the gas pedal is still floored, as soon as you release the brake pedal, the car would suddently accelerate forward (logical!).

 

B is not regenerative braking. It is to brake with the gas engine.

You can do it at any speed except below 15 MPH. If you switch to B below 15 MPH, it will regen brake to recharge the battery (gas engine shutdown).

 

I think there is a maximum speed at which B mode can be engaged. I was recently going downhill on a freeway at maybe 80 mph & needed to slow down a bit. I shifted to B, and the car beeped and went to neutral. Tried it again, same result. So I shifted back to drive and used the brake. I've use B mode regularly below 45 mph, but there does appear to be some upper limit.

BTW, 80 was just the general flow of traffic - I was being passed on both sides and wanted out of the slot car race. Can someone tell me what the practical speed limit is on an open Los Angeles freeway? Is there a number, or is it just "a little slower than the other guy"?

 

Heck ... it'd be a waste ... but you can go to 'B' at 100. You can go to 'B' pulling hills with the accelerator floored. The prius HSD is built so that even most dumb things won't wreck it necessarily ... but watch you MPG's plummet. As USB stated above, regen and 'B' are not synonymous unless you're under 20mph. It aint a jake brake.

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There is no upper limit. Something else was going on in your situation, whether your shifter is having problems or simple human error.

Tom

 

Everyone has written something about B mode, so you should more enlightened by now.
The fear of having an unintended acceleration I cannot on earth explain that. Tv shares specialists and US auto makers probably

 

Do note that even when "off" Prius brakes will still work with power assist. The brake boost pump will run when off. Power steering won't but with a car as light as the Prius and tires that small you shouldn't need it.

Oh, and I just got back from the west coast. I go over some pretty wild mountain pass', one with 18% grades. "B" works just fine at any speed. The driver sometimes messes up when shifting though (paying more attention to getting around those tight 30 km/hr corners ).

 

I'm not following you here. When the car is OFF who needs brakes or power steering? Or by "OFF" did you mean, "when not in B"? The brakes and power steering don't care whether it's in "B" or not.

[D'ohh!! See below.]

 

Meaning that if you have a theoretical car with theoretical unintended acceleration and you decide to turn off the car while it moves (like really turning it off, while you continue to move in neutral), you will still have brake pressure left, to drive the power brakes. Once this pressure reserve is exhausted (by using the power brakes), you will still have regular unassisted brakes.

 

No, I mean if you turn the car off while moving, the brake hydraulic booster pump still works. It works even when the car is off. So you can't "drain the pressure" while braking with the car off.

You DO notice the brake hydraulic booster pump run when you step on the brakes while the car is sitting and off, for example, when you get in to start it? You step on the brake pedal to put it in "ready", but before you press "start" the brake booster pump runs (brrrp).

 

stupid question, what is not mentioned is if this can be used safely within the hybrid system to slow the car on flat land. EG the car in front of me is getting too close to fast and if I use conventional brake Ill decelerate to much and loss some of the momentum I have, but if i tap into B for a "hot second" I can still keep the forward inertia going and only loss the 1-3 mpg I need with out sacrificing my momentum.

I have been told this is bad and I should not do this, to only use the B when I go down a mountain road, well I dont have a mountain around me and I dont plan on finding one. Is this "bad" should I not do this? I want to keep this car around for a while. I am a prii Noob, just about 6 mo. into ownership (gen 3 package 3 2011), or 12,000 miles which ever one you count as more. I drive it every day and try to change the minds of the prius haters, I used to be one till I test drove one out of pity for it and fell in love with it. funny how things go isn't it. thanks in advance, Im sure I will never find this thread again to see if anyone answered. cheers- Drew V.

 

I'm sorry, but that just doesn't make any sense at all. No matter which method you use to slow down you will lose the same amount of inertia (if you shed the same amount of speed). Your best option is just to slow early and gradually using light braking in "D" mode.

 

As Uart indicated, this makes no sense. The loss of speed / inertia / momentum is the same no matter how it is accomplished. But the standard brakes will save the most energy through regeneration and storage into the battery.

BTW, if you only need to dump 1-3 mph in a 'hot second', you are following much too close.

Note also that the standard brakes are much more controllable and adjustable than a B 'downshift'. And in case the gradually slowing car in front suddenly becomes a very rapidly slowing car, your foot will already be on the brake, shortening reaction time. I speak from first-hand experience a few days ago, in an incident that produced police documentation that my Prius can outbrake a certain VW Passat. Unfortunately, my Prius rear bumper now contains the bumper / grille / license frame imprint of the Ford that was in front of that VW. Fortunately, my rear bumper is in far better shape than the Ford's rear end, which in turn is in much better shape than the VW's nose.

But my Prius' nose is still clean. So is my driving record. And so is the record of the Ford driver. And this didn't become a 4- or 5-car chain reaction.

 

I don't think he means the same speed. I believe he's trying to avoid excessive braking with his foot by using the increased no-foot braking in B-mode, which he considers to be more predictable.

But yes. Hypermiling rules 1 and 2 (Awareness and buffering) means it shouldn't happen and if it is happening it's an emergency and you need just to be concerned about braking at that moment.

 

If I brake with my foot, my brake lights come on and other drivers are informed I am slowing down.

If I use B, I get the chance to be rear ended, as I failed to warn other drivers.

How can this be a good thing?

 

Unintended acceleration has been proved to be a load of fud, other than when people get non standard floor mats trapped under the accelerator.

If this happens you can stick the car in Neutral by selecting N and holding it there for 3 seconds or above 10 mph you can select R reverse and the car will jump into Neutral straight away. Don't do this below 10 mph or the car will engage R reverse!

 

You can do this without hurting your car, but it doesn't make any sense. Slowing down is slowing down; you can't keep your momentum and slow down at the same time. This is basic Newtonian physics.

You really want to use the normal brakes in this situation for two reasons: 1) More of the lost momentum (energy) is stored in the battery through regenerative braking, and 2) Brake lights. Some other aggressive driver may be riding your bumper just as you were riding the bumper ahead (forcing you into this "hot second" braking operation) - B mode does not show brake lights.

As a further point, if you discover that your "hot second" is more of a "hot millisecond", having your foot already on the brake may make a difference.

Tom

 

there is a different felling from the car if i use conventional braking to try to keep distance (not tailgating lead of MORE than 4 seconds), but if I drop into B then when I go back into d the car does not feel like it has lost as much, I can regain quicker constant speed and better mpg on the indicator, without tapping my brakes. as is around here people avoid the prii like the plague. I used to.. then I fell in love with one and have noticed this phenomena ever since. However if anyone has ever traveled on 495 around D.C. then they know how crazy it gets. thanks for your responses at least I know I am not harming my car and to only use this technique when no one is behind me.