How to use "B" gear?

If you called it L, people would use it to climb steep hills and complain that it didn't work.

 

When I use brake mode when the traction battery is full, the engine seems to rev very high so I tend not to use it when the traction battery is full. Is it ok to use the brake mode when the engine revs high like this?

 

Pretty much. Here's how I understand its operation in normal "D-mode." At speeds below 42mph (again speaking from a Gen 2 perspective, Gen 3 is similar) under low power demand conditions (such as a long down hill run) with SOC near nominal the ICE will be completely off with rpm=0. No rpms means no air pumping through the engine, means no engine braking. Above 42 mph the ICE will spin to balance out the rpms in the PSD. However, under similar low power demand / nominal SOC conditions, the fuel injectors are turned off so there is no fuel being consumed and the valve timing is adjusted so that pumping losses are nearly zero. Net effect is once again essentially no engine braking. This allows for maximal regenerative braking efficiency under normal conditions, and/or maximal high speed ev drive efficiency in whats commonly referred to as "warp stealth" mode.

On a long steep downhill you have two options. Do nothing and stay in D-mode, or manually switch to B-mode. In the first case you use the brake pedal to control speed, with the hydraulic brakes being used for additional slowing power once regen is at its max output power (~20kW). Once the battery has been charged to max (~80% SOC) the controller will suspend regenerative braking and automatically invoke a condition very like B-mode to use engine braking to supplement the hydraulic brakes. In the second case the manually selected B-mode will immediately begin using engine braking to help slow the vehicle (or control speed) and will use a moderate amount of regenerative braking as well as long as the battery SOC is below full. In this mode you can use the gas pedal to reduce this combined braking, or the brake pedal to add additional hydraulic braking. Either way you should end up at the bottom of a sufficiently long steep downhill with a full battery, invoking B-mode manually just makes it easier to control downhill speed with less use of hydraulic braking overall, saving wear and/or reducing brake pad heating / fade.

Rob

 

That's how it's intended to work. Surplus energy that would otherwise have to be dissipated with friction brakes is dissipated by spinning the engine with the fuel supply cut off. Valve timing could be adjusted to increase the drag but I'm not sure if it does this or not.

 

"In this case, the valve-timing in the Prius engine is advanced to increase the amount of air taken in and the suction against the throttle flap -- which uses much more energy than the coasting-in-"D" scenario above. Either way, stopping power now comes from a combination of things and the burden on the friction brakes is greatly reduced, allowing the hill to be descended safely." - hobbit
http://techno-fandom.org/~hobbit/cars/b-mode.html


Top of the second column, I think.

 

I use B mode on steep multithousand vertical foot highway descents so the brakes don't overheat. A frequent one I travel is about 7%, for about 6 miles, 2000 vertical feet, a hill that killed numerous runaway truckers until the state improved the escape options and figured out how to better alert the unwary drivers at the top.

A regular Prius fills up the battery quite early on that hill, so without the extra compression of B mode, the brakes will get quite hot. The plug-in models (PiP, Leaf, etc.) should be able to use regen all the way down.

Descending Pikes Peak, an F250 with Florida plates came down that way not far behind me. At the mandatory brake check point (and voluntary gift shop), it was ordered into the mandatory 30 minute cooling off area. It smelled absolutely horrid, with that typical burning brakes smell.

 

I don't think that the Prius changes valve timing in B mode in quite the same way as a Jake Brake does on a diesel; I think it's relying mostly on friction and pumping losses.
With a Jake Brake, a diesel engine goes through the regular compression stroke, and right when the air is compressed, it opens the exhaust valve. This lets the pressurized air out suddenly, and keeps it from helping to push the piston back out - instead, the energy put into compression is lost. The sudden exhausting of pressurized air is what makes Jake Brakes so loud.

In the Prius, to the best of my knowledge, the valves are operating in some fairly normal-ish fashion, as far as I know. (i.e. possibly adjusted for best braking, but still something that might be used while driving normally.) Instead, the computer tells MG2 to act as a generator, and produce electricity to slow the vehicle. Some of this electricity is sent to the battery, and saved for later. The rest of it is used by MG1 to spin the ICE up to near the (software-enforced) redline. Running the engine at that speed maximizes the friction and pumping losses, but doesn't quite pack the same punch (or noise) that changing to a Jake Brake's valve timing would.

 

That might be true for the regular Prius. With the PiP I need the full 7000 ft decent (Donner Summit to Sacramento Valley) that also includes some short uphills, to get a full charge for the battery.

 

In any case it isn't a Jacobs Brake (a separate device added to throttle-less engines). It is just the average ordinary braking that all gasoline/throttled engines have. Nothing extraordinary as the Toyota salesguy tried to claim. NOTE:

I admit I have a bit negative opinion of Toyota salesguys. The first guy I met refused to let me testdrive the Prius on the highway. The second guy LIED and said he had a base Prius but when I looked at the car, it was obviously the higher level (with vinyl seats). The third guy claimed he had a manual shift Prius, but it was actually a used Civic. ("What? It's still a hybrid and that's what you said you were looking for." No. I said prius. Way to waate my time donkey.)

Terrible terrible service. Anyway: That's what I get angry of some salesguy claiming Prius has a Jacobs Brake. He's just an example of theidiocy I had to deal with on my shopping experience.

 

Unless you are deaccelerating, there is no downside to B, it is only a braking strategy. One hint, you cannot engage cruise control in B.

 

There is no struggle when the ICE is providing propulsion, because there is no difference. It isn't the same as a downshift on a traditional car.

As Jimbo said, except for cruise control being locked out, the differences are only when decelerating. The speed threshold for shutting off the ICE for glides is sharply reduced, engine compression braking is increased, and battery charge rate is increased. But don't mistake the later as getting more energy into the battery, because more total energy is thrown away outside the battery.

 

But that descent takes more than 70 miles, an average slope of only 1.9%. Most of that descent energy is being burned off as air drag and rolling friction, not being sent into the battery.

On a 7% grade, I'm betting the battery gets filled up much quicker, with much less elevation change. On such hills, can you get much over 2500 vertical feet before the battery fills? Can you get enough pure regen braking to control speed without going into B mode?

 

Since its not one grade there really is no answer to your questions. Some of the 7% grades are a couple of miles, that does a nice job of charging, then there is some flat or even some uphill. It pretty much takes the whole 35 miles with various ups an downs to fully charge the battery from a low (23%SOC) start. I've never used B in the PiP, I did a few times in my previous regular Prius. I'll try some B on my next trip with a few hills but that probably won't prove much. I don't expect to go over Donner Summit in the near future.

 

Not being a local, I've traveled that route very few times, not enough to memorize much. Neglecting very short portions, is any of it 7%? Plotting an elevation profile on toporoute.com, I don't spot anything steeper than 4.5%.

 

I believe there are some 7%, maybe 6%, but I'm not sure. Sometimes I can't remember why I walked down the hall to the kitchen

 

Over the years I've read numerous posts questioning what "B" is...and how it should be used.

Just trust me when I say that salesmen have given even worse answers than that.

 

I used the B mode on two long, steep downgrades today. I didn't have to brake. It was like turning the overdrive off on a regular car. Battery regeneration was more aggressive too. I noticed both the EV and ECO lights were out.