modes

I think those buttons were introduced with 3rd gen, 2010 model year?

Don't worry, you're not missing much. I just leave it in the aforementioned "normal" mode, the other modes don't really do much, mainly change gas pedal feel.

 

You can use 'B' mode to avoid overuse of the brake pedal when descending Siskiyou Pass. Otherwise, don't use B as it will cut MPG, and has no benefit on flat roads.

As Mendel mentioned, the other modes didn't appear until 2010.

 

Correct, use B where you would normally use L (someone at Toyota was too honest to call it L as it is not a Lower gear, just engine Braking) going down long, steep hills.

 

Criteria for using B mode is long downhill grades, ie: don't use it as a substitute for day-to-day braking. It's only needed where there's a likelihood of the hybrid battery becoming fully charged by regenerative braking, and the car switching to all friction brake, and a lot more downgrade still ahead of you.

Like coming down a ski hill, for example.

 

great post, all the best on your trip!

 

On my 2008 spirit (UK), the EV button is on the lower dash just under the steering wheel - next to the intelligent assist parking button.
Whether it's the same on US models I don't know.
It would show you on the diagram in the manual.

 

The UK 2008 Prius TSpirit does indeed come with an EV button . However, bizarrely, this option was not included in the US equivalent Touring models . Just like they have heated side mirrors on Swedish Gen 2 Prii, and we don't get that in the damp and cold UK

 

I frequently make drives with ~7000-ft elevation changes, and I rarely, if ever engage B even on those drives. I suppose every Prius will be unique, but it usually takes stretches of a bout 15 miles and a few thousand feet in elevation change in order for my battery to become so charged that it needs to engage B mode. Your car should automatically apply engine braking if the HV battery is in danger of becoming overcharged, so the only reason for applying B mode I can think of is not HV battery protection, but rather speed control on descents.

For instance, there is a hill on my drive where the speed limit is 55mph, but coasting will take me to 90mph+ if I don't press my brakes. Engaging B mode early on the hill will keep me in a reasonable speed range. However, rather than mess with that I just tap the brake pedal every now and then to keep my speed in check. No B mode for me.

 

^^ 'B' mode is to protect the mechanical brakes, not the traction battery.

The need depends on the slope too, not just the absolute elevation change. 2000 feet in 15 miles is just 2.5%, that is not bad and highway speed air drag can scrub much of that energy. I occasionally drop that elevation in 7 miles, much steeper, and my battery fills up in the first two miles even with B engaged.

Earlier this year, a new member from Hawaii overheated and seriously damaged his brakes twice before learning here that B mode is required on his road. The weaker automatic engine braking was not enough to protect his mechanical brakes on his 4000+ foot descent.

A couple days ago, I went through some National Park Service volunteer training under our new overseer. As part of the program, we get use of an official government truck while on duty, donated by another office that doesn't usually work the same days we do. Coming back to HQ is a 2600 foot descent in 11 miles, a 4.5% average slope at an average speed under 25 mph, so very little energy is scrubbed to air drag. We are required to downshift to protect the brakes. Excessive brake wear compared to the other staff, or failure to report overheated brakes -- these will be discovered by the park's mechanics -- may result in loss of the vehicle from this program.

 

Question, then. If B has nothing to do with the SOC, why does it automatically engage on long downhills even when I am not using the brakes? Honest question. I just assumed it had to do with the SOC (as well as saving the brakes) because mine automatically engages.

And fyi, when I say mine automatically engages, it doesn't actually go to B on the dash display. It is just that the car is obviously using engine braking to slow the vehicle's descent.

 

B mode does not automatically engage. Period. If it doesn't show B in the dashboard, you are not in it.

When gliding with foot off the gas, the car produces a light synthetic drag from regeneration. Normally this energy is dumped into the battery. When that is full, the engine is spun up to low-moderate rpm to burn it off. But this drag is much weaker than B mode drag.

I did some comparison testing on a hill earlier this year. After the battery filled, B mode held the car close to speed limit by spinning the engine up to 4600 rpm. Switching to D mode dropped the engine to 2000 rpm, and the car began speeding up significantly due to insufficient drag.

In full B mode drag at high rpm, the engine screams like a banshee, or a turbine jet engine, or a giant vacuum cleaner. Some readers here have actually switched out of B because they thought the engine was on a path to self destruction.

 

Yup on that last paragraph. It takes will power to stay in B on a steep down grade, sometimes I chicken out.

 

Perhaps I'm mistaken, but when my car automatically engine brakes on extended declines, there is no RPM difference between D and B. I've never done it while monitoring the RPMs via software, but the D engine braking is enough to prevent me from accelerating when at about 75 on a 7% decline. There is no discernible difference between D and B at that point for me. It goes full banshee in either mode.

 

Could be different, second gen vs third?

Per Fuzzy1, when I'm coming down our local ski hill on a steep grade, there is a definite change going between B and D. I can see the rpm jump, then down (with ScanGuage), and even not looking you can feel/hear what's going on. Go into B and it really holds you back, and the revs get high, sometimes alarmingly so.

 

I don't mean to be argumentative, but your perception that there isn't a difference, doesn't mean there isn't a difference.
I just default to the rather obvious thought that Toyota wouldn't bother to give us a "B" option, if the whole action was handled automatically anyway. Ergo...there has to be a difference, even if you aren't noticing it or it isn't noticeable to you.

 

Interesting. Anybody have any idea what type of overuse it takes to overheat the brakes on a Gen 2? My trek doesn't appear to have threatened to cause any damage, but I've been through a ton of vertical ascents/descents that are pretty much the most severe in the US (pass through the Sierra Nevadas, Lake Tahoe, the Mogollon Rim), so I'd imagine I'm constantly at risk.

I get what you're saying, but I disagree with the notion that they wouldn't give you manual control over an automatic feature (especially when the action serves more than one purpose, such as to protect the regen brakes and to limit the SOC). Auto manufacturers do that all the time.

However, after reading up a bit more on B mode and the automatic discharge I'm experiencing, they do appear to be separate functions (although they have very similar effects).

 

Which roads around Tahoe? I will say that I-80 and Highway 50 are not in the 'most severe' class. While I've been on some others, it hasn't been enough to remember in detail.

The most severe I have been on was Pikes Peak road in Colorado. Prevailing traffic was too slow to get the engine spun up for adequate braking. Some other cars were geared down to the right speed for themselves, but it wasn't right for my Prius, and passing was not an option.

 

Gotta say I've never seen the car cease to charge, at least the display says it is.

Our previous Civic Hybrid reached that threshold often, and the transition was definite, bordered on dangerous: the column of green blips indicating charging would suddenly diminish, then disappear, and you immediately needed to increase braking pressure to maintain status quo.

 

Pikes Peak is a doozy, yes. That's probably the most severely graded road I've driven (not in my Prius though). 7,700 to 14,100 in about 20 miles. That's about 6% average, if my math is correct, and there are sections that have got to be around 10%. Haven't been there in forever, but that's a beautiful place. That type of elevation changes the game as well. I can tell my lungs are starving for air at anything much about 10,000, so I can't image how hard it is for a little Prius to chug up a mountain at 14,000!

As for Tahoe, it was SR431 that I recently drove in my Prius. It's not Pikes, but when I said "pretty much the most severe," I meant only to say that there aren't many places in the continental US where you're going to go from ~4,500 to ~9,000 and back to ~6,200 within 24 miles (and about 7 or 8 miles of that is relatively flat, so the steep stuff is actually fairly steep by any standards).