The most misunderstood aspect of the Toyota hybrid synergy drive system

I suppose semantically it’s a “gear”, not a “mode”, as TCCN says in the title. It’s on the gear shifter.

my sense, when I shift to B, charging reduces, and engine braking increases to slow the car more.

There’s a downhill we often do: if I leave shifter in D, and brake as needed, battery will indicate full to the gills by approximately halfway down.

Conversely, shifting to B at top of hill, it charges slower, less braking is needed, and battery will get to fully charged by base of hill, or even down one pip.

 

from what i've read, b mode operates differently in plugins than lift backs

 

It's kind of cute to think that only the driver (who's super smart) can make the drivetrain behave a certain way by pushing a lever to B mode that the poor dumb car couldn't figure out itself if necessary.

As if when's there's no regen to be had, the drivetrain's just going to throw its metaphorical hands up in confusion and be utterly unable to start the engine to provide any drag for D until someone helps it out by selecting B?



(Also trying to imagine an engineering meeting where someone is insisting "you can't start the engine in D mode!")

I'll agree that the amount of drag the drivetrain gives you is affected by B, and hence at what charge level engine braking will start, but the idea that D will just act like N when the battery's full because the car's too dumb to start engine braking itself at all without B is ludicrous.

We've already had people here asking for help in limiting their plug-in charge to stop the car automatically starting the engine for braking.

 

I read it all - a lot of waffle that no-one really disagrees with, all trying to distract from the fact that you are insisting contrary to common sense and evidence that the "ingenious idea" of engine braking - which we all agree the car does - can for some reason only happen with the manual "B" selection.

I can imagine you designing a system like that, but I can't imagine Toyota doing it; also plenty of users report engine starts for braking with a full battery in D mode, while no-one, including you, has observed D giving up and acting like N the way your system would.

 

This bit's maybe the biggest misconception:

Um, no, the engine could be running for all sorts of reasons. To generate heat for the cabin, to complete a warm-up cycle, or to stop an MG1 overspeed. It really is not universally true that "the engine is shut off descending a hill". Starting the engine to cause drag and bleed off energy is just another possible reason to have it running on top of all the other ones. And it can happen automatically like all the other ones.

The Prius really isn't as simplistic as you think it is.

 

Careful, you may trigger him into another rant! This video doesn't mention the plug-ins at all, so misses their expanded capabilities.

This video is decent, but even apart from the missing plug-in discussion, we've seen better items linked in years past. He doesn't mention the changed valve timing, or the very strong manifold vacuum that really turns it into a vacuum pump, not a real air compressor. The real engine drag here comes mostly from drawing the piston's intake stroke against that manifold vacuum, not from the following compression stroke. That compression stroke is completely offset by the following expansion stroke acting like a spring, though the greater expansion stroke may then add some additional drag itself.

He doesn't mention the non-plug-in low speed corner case where B is initially just regen-only, not spinning up the engine at all until the battery approaches full. He doesn't talk enough about how usually B-mode initially runs both battery charging and engine braking in parallel, and that when the battery nears full, the engine RPM sharply increases to keep total braking power about the same. And he doesn't describe why B-mode engine braking was so weak when I slowly descended the Pikes Peak road, causing me to need to ride the friction brake too.

He also doesn't mention that engine braking is sometimes used in other modes in other degrees, both regular D and also Cruise Control, when the battery fills.

It seems that Paul has abandoned the other thread, where there is an unfulfilled promise, and a request for him to repeat some tests I did, to see if he gets similar results.

 

That is what I said over in the previous thread, where you haven't yet replied.

 

What's kind of frustrating is that I remember another thread where somebody linked that exact same Car Care Nut video about B mode, and responding to it took a fair amount of time, but now I haven't been able to phrase a search that successfully pulls up the discussion from the last time around, so here we go again.

What he says in the video is quite right if you start with the top-line conclusion: use B mode only for its intended purpose: you're going down a hill.



Not that you hurt the car or anything if you use it at other times, but that you'll likely cost yourself MPGs while you're doing it. (The difference bisco alludes to between B mode in plug-ins and liftbacks may mean that it costs you less in a plug-in, because the battery has a lot more capacity to capture the energy from it.)

Once you get beyond the top line, though, there are things in the video that he doesn't get right.

For starters, he says B mode gives you full, 100%, regenerative braking when you lift off the go pedal. This doesn't seem to be just a careless slip, as he repeats it several times:

4:32 regenerative braking goes to maximum allowed

4:40 so every time you lift off the gas pedal it will apply full regenerative

5:12 in b mode is going to use it a hundred percent

9:35 yes we are applying full regenerative braking​

On the contrary, you can feel much stronger regen braking if you press on the brake pedal some. Just lifting off the go pedal, you do feel more resistance in B mode than in D, but it's a moderate amount. They just programmed the car to give an amount of resistance like what you'd feel in some other car if you downshifted, maybe from 5th to 4th or 4th to 3rd. It's all programming, so they could have picked any level they liked; they picked it to feel more or less familiar from other cars.

Next, he explains the role of engine braking in B mode, but he doesn't get the why quite right.

He says the engine braking is needed because the regen braking "is not enough to slow this car down at the same rate a normal gasoline car would slow down" (6:01). But remember, the car isn't applying the 100% regen he thinks it is; at most speeds, full regen in a Prius is enough to slow the car down way faster than the normal car slows when you lift off the pedal. So, that isn't the reason engine braking is used in B mode.

Engine braking is used in B mode as a way to have you feel the expected rate of slowing, but do less of it with actual regeneration. The reason for that is connected to the purpose of B mode in the first place, which is to be used on a long downhill where you know the battery will end up full anyway, so you might as well charge it more gradually and use more of the hill to do it, instead of getting it very full and hot early on and then needing to rely on engine braking the rest of the way down anyway.

Even in D mode, the car will trade regen off to engine braking as soon as the battery reaches full. In B mode, it just starts trading some of that off earlier, to keep the battery charge rate more gentle.

His explanation for how the engine braking happens also gets a little fuzzy. He starts with this comparison between the Prius drivetrain and a conventional car's:

6:53 in hybrids the engine goes through the planetary gear set it's not directly connected to the road unlike a conventional gasoline engine where the engine is connected to the torque converter to the transmission that's a direct connection to the road​

But he's making that out to be more of a difference than it is. Even the conventional transmission isn't always a direct connection: there are gear sets in there (often planetary ones, even, in automatics anyway) and things like clutches and brake bands, and when no clutch or band engages, there's no connection to the road, and it's called neutral,

The Prius transmission doesn't have clutches or brake bands, but it has MG1. MG1 connects to the one gear in the planetary that isn't connected to the engine or the wheels, so it's kind of a free agent. If it's allowed to spin freely, no power passes between engine and wheels. But whenever MG1 is put under electrical control, whether as a motor or as a generator, a mechanical power path between the engine and the wheels also exists.

So, when he says "it's gonna physically turn the engine with the force of mg1" (7:33), he's got the basic idea: with MG1 just freewheeling, nothing will spin the engine, so to make engine braking happen, the car has to take electrical control of MG1. At that point, MG1 will be counterrotating, and it doesn't matter whether the car makes it a generator to resist its counterrotation, or goes further and drives it as a motor in the + direction; either action will deliver torque to spin the engine for braking.

However, once that happens, it's a mistake to suggest all the torque to spin the engine is coming from MG1. In fact, MG1 is supplying only 30/108 of the torque that's now spinning the engine, and the other 78/108 is coming straight through the planetary gearset from the wheels. The transmission's not freewheeling anymore.

There have been a lot of other threads on PriusChat where an interested reader can go into more of those details. It would be great to find a single short video that cleared up all of the often-misunderstood aspects of B mode, but this video only gets part-way there.

Edit: at the time I started writing this, bisco's post #4 was the end of the thread! Anyway, I see fuzzy1 has covered the fact that the engine is doing more meaningful work as a vacuum pump than as a compressor, so I don't need to edit that part in now....

 

Yeah, there's this.

With regular hybrid, not plug-in, even if you don't select B gear (I'm gonna call it gear 'cus it's on the gear shift), the car will begin B-like behavior, when the displayed state of charge reaches maximum (experienced this just a few days back, coming down the final hill to Belcarra Park). Selecting B allows you to get a jump on this, say at the top of what you know to be a long downhill, something the car doesn't know.

Yeah he lost me there too: shifting to B when in a steep decline, I'll see the charging bar recede.

 

No, it is not regeneration into the battery. He is describing the case there the battery reaches full and regeneration must cease. But the 'synthetic drag' does not cease.

In this case, the CHG indicator goes to zero, and the ICE RPM increases (if already above the ICE auto-off threshhold speed, which is well below highway speed on our Gen3s), or spins up from 0 to above Idle (in later generations such as yours where the ICE can cease spinning all the way up to highway speed).


Many of us quit using the animated energy flow diagram long ago, because the HSI display (introduced in Gen3) is so much more useful. Just keep it up full time.