Recently was in smoky mts. Attempting to use “B” mode for Eng braking going down hills and engine would rev extremely high, as if thought the system was cutting charging out of the circuit, possibly? But why such high Eng braking rpm? I wouldn’t allow it to do this so slipped into neutral and used brakes. Seems odd. 2015 with 160K miles. I believe it’s been doing this since new, but rarely do I see this as live in Indiana. Techy stuff and want to understand it. Thx
That's exactly what it is supposed to do. B mode tells the hybrid system to apply extra drag to slow the car down for safe mountain descent without overheating the brakes. The hybrid system prefers to solve this problem by generating electricity and storing it in the battery, but that isn't always possible. When the battery is either too full, too hot or both, there's no way to brake the car electrically. In response, the hybrid system changes the effective gear ratio to use the engine as a compression brake. The fuel injectors are off, no gas is being used. It just spins the engine like an air compressor to safely draw off the car's kinetic energy. The RPM is high because the engine is small- same as when you were going up the hill. It isn't the most pleasing thing to have to listen to, but the car can sustain that all day long without damage. By contrast, you can permanently ruin the brakes in just a few minutes on a fast mountain descent.
I use 'B' mode when coming off the interstate on the off-ramps to slow from 80+ mph to a full stop, without being so hard on my brakes. I enjoy watching the battery charge climb. Sometimes it can give me an additional .4 to .5 of a mile of EV driving. Some of our roads do the rollercoaster thing. A mile of down slope then a mile of up slope. On all of those down slopes I use 'B'.
That indicates a need for recalibration of driver perceptions. If you connect an OBD-II device like a ScanGauge or one of the phone apps, and watch the engine RPM, you will see that the sound you are calling "extremely high" revving really isn't. The Gen 3 engine is rated for its peak power output at 5300 RPM. There is surely a redline for the engine, somewhere above that, but it's not even published because the engine is computer controlled and never goes above 5300 anyway. In engine braking, I've never seen it higher than 4900. The computer knows what it's doing. Once you learn what those sound like, connected to what you see on the tach, you can recalibrate your sense of what sounds "extremely high", and relax and let the car do what it's built to do.
Normal. The car will actually default to that type of braking when the charge gets too high around 75-80% soc. Sometimes if im feeling frisky/picky i will just go ahead and put the car in neutral and that will defaut the car to mechanical braking only instead. I did that back when i knew my car was an oil burner and didn't want the extra revs. Did it make an improvement on oil consumption? I don't actually know lol.
You aren't using mechanical brakes coming off the freeway at that speed, you're using regenerative brakes. Basically the electric motor is generating electricity to charge your battery pack when you use the brakes unless you're below 6mph or doing hard emergency braking. So if you want better MPG don't bother with B mode.
Yea that's been debated for years on here and no one has posted a clearly defined chart... My best proof that it's almost all regenerative brakes is that I have 280K miles on original brakes and I've met lots of others who have had similar experiences.
?? Should be 100% regenerative in "D", with the obvious exceptions (battery hot, battery charge high, speed too low, braking too hard, etc.). "B" provides a regenerative/friction/"engine braking" mix in a proportion that varies with conditions.
Missed one obvious exception: higher speed. Because regen is limited by the power the battery can handle, and the power is wheel rpm ✕ regen torque, when you are at highway speeds it takes very little regen torque to reach the power limit. So yeah, this is the obvious exception of "braking too hard", but the faster you're going, the less braking that is. And friction comes in to take up the rest. The more your speed comes down while you are braking, the less limiting the battery power limit is, and the more of the job regen can handle. That's why Toyota has regularly published graphs, from Gen 1 all the way up to this one for a 2020 Prime, showing regen's contribution increasing initially while the car slows (that concave curve upward), then steadily being most of the braking force, and then becoming less significant again when the car has slowed below where the regen is effective. There may be others on PriusChat who'll debate it till the cows come home, but the basic physics agrees with what the folks who built the car say.
All very true, but I thought I covered that possibility not only with my "braking too hard" exception, but also with "etc." At every speed, there's a limit, above which is "too hard," regardless of how the limit varies with speed. I rarely exceed that limit due to speed.
And my point for still not achieving metal on metal in my original brakes on my 280K miles Prius is that I like to drive slow and driving above 70mph is something I rarely do... It's the number one rule of avoiding car accidents too. The slower you're driving the more options you have for avoiding an accident. And yes sometimes the faster you accelerate the more likely you can avoid an accident, but that's an exception, not the rule.
Pulse and glide makes your brakes last a really long time as well as boosting MPG... There's a reason why one of the main tips for how to pulse and glide is "drive as though your car doesn't have brakes."
Some effects can be difficult to accurately condense into a single statistic, and this is one of them. The plots Chapman posted provide a clearer understanding of the contributions of the two mechanisms than memorizing '100% in D with exceptions', especially because the exceptions occur so frequently. When exiting the freeway, the speeds are simply too high for the vehicle to sustain regen continuously and come to a stop within a reasonable distance. One might, of course, begin demanding a small, constant amount of braking torque a couple of miles out, and that might do it, but that is certainly not how I imagine most people brake, and might even be seen as discourteous on a busy road with only one exit lane.
Neutral is probably the worst choice here. If B hurts your ears, D is still better than N. The engine will still spin up well above idle, but will probably be about half or less of the B-mode RPM. It should be noted that you have a Prime, which has a more generous B-EV mode lacking from the OP's regular Liftback. As a frequent user of B-mode engine braking on steep hills for fourteen years, I have never noticed 5000+ RPM, but finally spotted a brief 4992 RPM last week. My hills more typically produce 4600-4800. From common highway speeds, it takes a fairly long exit ramp to avoid including mechanical brakes in the mix. Most drivers won't achieve it without careful retraining. On some ramps, nobody will achieve it. See Chapman's discussion of the basic physics and Prius-specific limits. That simply isn't proof of the claim. I can take it only as a likely indicator that you have managed to push at least 2/3rds of your braking to regen instead of friction, based on my household's history of typically about 100k miles on factory brakes with non-hybrids. If/when you get 1,000,000 miles on the factory brakes, then I will admit that you are doing at least 90% of your braking with regen instead of mechanical friction. At common highway speeds, "too hard" for pure regeneration will be in the range of 0.03-0.05 g, which to very many people is very light braking. Not even moderate braking.
hello and I understand that is what it “normally” does. I am saying the battery completely topped off, the engine revs extremely high…unlike when say the battery is not quite topped. It is a different sound. Like it is reving way way above any normal limits than I am used to hearing. If I were in B mode and also using brakes a little, coming down off mountain say at 45 mph, the engine would very very slowly come back down but stay at a noticeably high idle. Recon this is the nature of the beast, but I would say the Eng rpm sound at least 8k, at which I would never let it stay at that.
Yes, it sounds quite different. Various descriptions here are that it sounds like a giant vacuum clear, or jet engine, or screaming banshee. But many of us have various aftermarket ODBII-port tachometer displays, and have verified that it does not exceed or even reach the maximum RPM listed in Prius datasheets. 4600 - 4900 RPM seems to be the normal upper range. I have never ever seen it reach 5000 RPM, though last week did spot a very brief 4992 RPM, which quickly returned to just under 4900. I use this mode frequently, including every time I visit my (now late) father's home, two B-mode descents away from my home. This mode is entirely under computer control, and those computers never spin it up to dangerous levels in any mode, including when producing power. In engine braking mode, there is no mechanical path that can override the computers and go faster. Plus, without any fuel to produce combustion, the cylinders are seeing much less pressure than if you were using engine power to climb a mountain road at the same RPMs. Yes, using some mechanical braking will reduce the engine RPM. But that is simply not necessary to protect the engine, the ECUs don't allow it to overspin.
If you borrow a scan tool and watch the RPM when that happens and it's anything above 5k, let us know.
