The most misunderstood aspect of the Toyota hybrid synergy drive system

Whatever you have experienced in your own car and in your own driving conditions, we can't gainsay if we were not there.

On the other hand, this is a forum with a lot of us around who drive the same kind of car, and many of us recognize the sound of our engines revving, and many of us even use OBD-II devices to display a tach and see exactly when our engines are turning and at what RPM.

I have not driven a gen 4 or gen 5. Get that out of the way. In every Prius generation I have driven, the car will phase out regen braking as the battery nears 80% true state of charge, and the phase-in of engine braking is both audible and confirmed on a tachometer.

I just got back last month from a long road trip in my gen 3, where I would often be in D under cruise control on a descent long enough to fill the battery. (For situations like that, I could have selected B to charge the battery more gently but, being lazy, I often just left it in D and let the car do what it does.)

Just like clockwork, when the OBD-II state of charge reached 77%, the engine could be heard starting to ramp up, and by 80% SoC there was no further charging in progress, and the engine would be turning 4600 RPM or so.

Maybe it is true that for gen 4 or gen 5 Toyota decided to disable all engine braking except in B (though it strikes me as unlikely). And it certainly might be true that you, in your car, in your driving, have never seen a circumstance where engine braking gets phased in with D selected.

Just as none of us can insist on what you did or didn't see in your own car when we weren't there, you're in no position to tell the rest of us what we heard, and saw on our tachometers, in our own driving when you were not there.

There are two electric motors, so it's helpful to name them. The one you're thinking of that counterrotates when the car moves without the engine is MG1.

"Coasting in neutral" really isn't anything the drive train has to work very hard to "simulate"; it is what happens when the car simply does nothing electrically with MG1. Its rotor just freewheels then, and there you are. This was covered upthread.

 

So you are claiming that my aftermarket OBDII-port tachometer was lying to me when I did this at highway speed, and it never showed me less than 992 RPM? But in some descents, when the battery was nearing 'full', the engine RPM spun up 1000+, then even to 2000+ RPM? And in Cruise Control descents, it could hit 4600 RPM, just like Chapman's experience?

And now my RAV4 Prime, which has an actual Toyota tachometer function, and usually can stay at 0 RPM in the same conditions where my 2010-12 liftbacks could not, has occasionally spun up as high as ~2500 RPM while I was learning how to best operate this car. Is it lying too?

You appear unable to note or acknowledge that there are any functional differences between Toyota's regular hybrids vs plug-in hybrids. Or that there are differences between the generations. E.g. that a regular Gen1-3 Prius can't go 60 mph without the engine spinning (absent driver hi-jinx). Only Gen4-5 Prii (2016+) can do that, the older generations are limited to 40-something MPH. And plug-in Prii can go faster than their non-plug-in same-year cohorts.

And you haven't provided any documentation to show that previous documentation linked here, before you ever joined, was wrong.

You have nothing to lose by performing the test I requested in the other thread:

 

I haven't hooked anything up to my RAV4, yet. But I did have a ScanGauge-II hooked up to my 2010-2010 Prius Liftbacks for the entire time I owned them, one or the other from June 2009 to February 2024.

My RAV4 Prime should operate very very similar to the Prime that your profile claims to have, since they have the same basic systems, albeit different sizes and some different labeling and a few different features.

My 2010-2012 Prius Liftbacks behaved exactly as I have described, across my nearly 15 years of ownership and driving of them.

What do you use for an engine tachometer? Built-in, or aftermarket? Or none?

 

I am glad to have reached agreement on that point—at least for the non-plugin Prius. It's a little messy that Toyota did change what they call it in the 2023 Prime owner's manual. Now they've created a situation where for the same model year, 2023, if you download a Prius owner's manual it says this:



and yet if you download a Prius Prime owner's manual, it says this:



But this is the non-Prime, gen 5 Prius forum, so we can just leave that detail aside; as far as the non-Prime, ordinary Prius is concerned, there is no need to imagine that B has changed.

Still, there seems to be something significant missing from your post here.

You've summarized admirably by saying it's a good option on a long downslope; indeed that's what it's for. But where, in any of this response you've just written, do you address what, as recently as post #20, you said fuzzy1's "facts are wrong" about?

You were replying to a post where fuzzy1 corrected your belief that no engine braking happens in D. You insisted that fuzzy1 was mistaken and had not witnessed engine braking. I replied that it's easy to witness when a tachometer is connected. And you've replied here to my post, but somehow there is nothing in your post about whether engine braking happens in D.

In the other thread where you were also posting about B mode, you have also, time and again, contradicted other drivers who pointed out to you that engine braking also happens in D.

There was nearly a breakthrough in that thread when you wrote

and KMO laid it out:

And that's the key right there: the car makes what adjustments it must to preserve the road behavior the driver perceives, when the battery's ability to receive charge tops out.

Now, D and B already have different road behavior the driver perceives. When you lift off the pedal, D is programmed so you perceive about the same rolling resistance as in some other car in a cruising gear, while B is programmed so you feel stronger resistance, like some other car downshifted.

Notice that neither description locks the car into any single way of doing that. Even the owner's manual description above for B mode (in the non-Prime) says "engine braking or strong braking". If the battery isn't full yet, the car is free to use regen, or some regen and a bit of engine braking, even in B. As the battery fills, the car eventually transitions to all engine braking, and it does this transition so you perceive a consistent rolling resistance.

In D, the car also makes the same transition as the battery fills. Here also, it makes the transition so you perceive a consistent rolling resistance, and remember that in D that programmed level of resistance is lower than in B.

So, on some downgrades you may well find that D's programmed resistance isn't enough to hold your speed and B's programmed resistance is enough. On grades where that's true, it will be true regardless of the mix of regen and engine braking that either mode happens to be using.

A third combination fuzzy1 mentioned here is D with cruise control. It's a little different because the car is no longer targeting a consistent feeling of resistance, but a consistent speed, and it will resist harder, the further your actual speed is above target. I am not sure fuzzy1 is right that the resistance is "capped at the same maximum drag power as B". I've been known to approach some very winding roads by setting the cruise control at the lowest settable speed (25 MPH) and then driving at normal speed with my foot. Any time I lift off the pedal there is an immediate very strong resistance, and again, just as in plain D or B, the car will combine regen and engine braking as it sees fit. When the battery is full, this produces quite high RPM engine braking.

 

People ... Seems like we get into this every time a new generation comes out.

Drive mode "D": Standard drive mode. Encompasses ECO, NORMAL, SPORT/POWER ECT modes. You should always be in "D" when driving. Choose any of the three ECT modes for your style or preference of driving technique. The car maps throttle response differently in each of the modes. You have access to all available power regardless of ECT mode. In the 5th generation, regenerative braking is also increased in SPORT.

Drive mode "B": You should only be in "B" if you need more braking power when going down a long descent and want to stay off the brakes. You do not need to be in "B" if you don't want to, but depending on the generation Prius you drive, you may need to get on the brakes to slow your descent, EVEN in "B" mode. If you are driving the car in "B" when you should be in "D", you are driving it wrong.

In the generations previous to the 5th, you will gain speed on long downhill descents. In the 5th generation, the car detects that you are going downhill and will start to increase regenerative braking to slow your descent. This is more prevalent in SPORT ECT mode.

DON'T worry about the hybrid battery overcharging.
DON'T worry about the engine revving up high on long descents in "B".

Toyota engineers have thought of everything and every situation that the car can be in and have built in protections. You will never be able to overcharge the hybrid battery going down a long downhill descent. The engine will never blow up revving up in "B" mode during a long downhill descent.

HOWEVER, the only time you *Can* damage the hybrid battery is if you leave the car in NEUTRAL for too long and it drains the hybrid battery too low for the car to turn on. You'll have to have it towed to a dealership to get the battery recharged. So be careful taking the car into one of those pull-thru car washes. AND the engine will blow up if you neglect oil changes or the oil level is too low. Basically, do your regular maintenance and you'll be fine.

Finally, just drive the car!

 

Is it your position that every other driver who points out to you that engine braking also occurs in modes other than B has 'misunderstood' his or her tachometer, regen current and/or torque PIDs, and that you, gloriously alone on PriusChat, have learned something about how it all works that they all have missed?

 

Paul has a history of creating new threads and arguing with the people that post in his threads. The best response is to not post any replies in his threads. Maybe he will get tired of creating new threads if no one responds, and he will stop this nonsense.

 

For people who can recognize the sound of the engine revving, the evidence is freely available with no further effort. For those who doubt their ears, but have any of the simple OBD-II gauge devices that can show engine RPM, battery current and state of charge, engine and MG torques, and fuel injection status, the evidence is right there for the looking, and plenty of us who have corrected you in this and the other thread have looked and seen it, and nothing stops you doing the same.

PriusChat has existed since 2003. The car has had B mode that whole time, and threads discussing it with evidence go that far back. There are threads with graphs made from live datalogs that you can look at.

But again, I can walk right out my door here, start my car, go to the nearest hill, start descending it with a full battery at the top, and watch the engine RPM climb with the fuel injection status shown as off. And hear the engine sound rise in concordance with the displayed RPM. So can anybody else you've been arguing with here.

You are the only person here trying to say that isn't evidence of engine braking. So that leaves a pretty straightforward question: just what do you propose to say it is instead?

 

Excellent! We could be getting somewhere now.

You have a car. Somewhere near you, there must be a steep long downgrade.

If you don't have an OBD-II instrument that can display engine RPM and fuel cut status, battery state of charge and current, somewhere near you is one that you can beg, borrow, or buy. Plug it in. Put those parameters on screen. Datalog them, if it'll do that.

On your way to the top of your downgrade, set your cruise control for the minimum (here in US it's 25 MPH).

At the top, before starting your run, ensure there's no usable charge capacity in your battery. In a plain Prius, that's a true state of charge of 80%* (the instrument cluster bargraph shows full then, because it only shows the usable range). If you have a Prime, also make sure the EV capacity is full up. Use force-charge if necessary.

Once you know there is no battery capacity to fill, and your cruise was previously set for minimum speed, begin your roll in D and 'resume' the cruise control. Have your buddy videorecord everything that happens.

A little bit of battery may be used to get you up to speed at the start of your roll, and some regen may happen to fill that back up. Pay particular attention to what happens after that.

You will then have results that you can report in much the same way as this post here (scroll down to the "engine braking" part, the first half was about other stuff).

Having you do this is ideal, for two reasons:

1. You seem to hold 'anecdotal' to mean any result obtained by anyone but you (or even everyone but you), no matter how conscientiously collected, logged, and reported, so there would be no point in anyone else doing it for you anyway; and,
2. There isn't anyone else here who needs to see any more evidence that engine braking does happen in all modes, beyond what they see when they get in their cars and drive down big enough hills. You're the one claiming, against everyone else's evidence, that it doesn't happen, and this gives you the opportunity to present your evidence that it doesn't.

Now that would be interesting.


* Edit: 80% is the top of usable SoC for the NiMH batteries; if you have Li, the figure may be different. Someone else here probably knows what it is.

 

Please don't confuse the terminology. "B-mode" and "engine braking" don't mean the same thing. While they commonly occur together, either one can be present without the other.

We are not claiming that B-mode is self-engaging. Instead, we are claiming that engine braking is self-engaging. Though you can commonly force the later by engaging the former.

We have already described tests that you can perform to see it for yourself. But you haven't yet agreed to try any of them.

B-mode isn't just 'engine braking', it is 'stronger [engine and/or regenerative] braking than D mode'. Engine braking, per se, can be very weak, very strong, or anything in between, B-mode chooses the stronger version.

I cannot speak for Canadian law, but something of this sort is required by U.S. federal law. And likely by most other countries too.

Perhaps you were watching for the wrong thing?

DON'T look for the gear selector indicator to change from D to B. This won't happen, and we are not claiming that is does.

DO listen or watch for sounds or displays or other indications that engine RPM has risen when it is not providing any propulsion (downhill), when your foot is off the gas pedal. In your Gen3, that meant RPM higher than 992 RPM. In later generations, typically any non-zero RPM.

In usual D-mode, this engine braking will be much weaker than what you experience in B-mode. With Cruise Control engaged, it varies over the full range from weak to strong.