The most misunderstood aspect of the Toyota hybrid synergy drive system

My 2024 is much quieter and smoother in spinning up for light engine braking than were my 2010 and 2012, despite having a larger engine. I need the tach to be certain.

That is so 20th Century. Those of us quoting engine-braking RPMs here for various operating conditions, have used the 21st Century method, which as Chap noted, was available in all U.S.-market cars a handful of years before the 21st Century actually arrived.

Most people don't have to prove themself to themself.

 

Ah, now THAT's an interesting nugget.

I had hypothesised that maybe B did something that D could not, in that engine braking strength was determined by the D/B selection.

In normal use, we know regen could substitute for brake action on a pedal press, and regen or engine braking could subsitute for simulated transmission drag.

What I didn't know was whether engine braking could substitute for brake action like regen can. Maybe it only engine braked up to the transmission setting, and the brake pedal could only affect brake pads if regen was unavailable, so B was significant to give you more engine braking, and could help your brake pads.

But if the engine revs up to give more engine drag when you press the brake pedal in D, then that suggests the whole "braking+drag" system is fully integrated, and it seems B really doesn't do anything D can't. (Apart from the legal safety aspect of "what if something happens to the driver and they release the pedals").

 

Couple of thoughts:

@Paul Gregory has a plug-in, which may change things.

I’m puzzled about The Car Care Nut’s emphatic assertion, that shifting to B gear invokes full regen.

 

I'm sure plug-in does change things, but it's largely a knock-on effect. The plug-in has far more regen ability generally, and is far less likely to be anywhere near real top-of-SoC. So given the same basic control algorithm of "start engine braking when insufficient regen available", the chances of either D or B needing to start engine braking to satisfy a drag requirement is far less.

On an earlier generation, B may well have almost always required engine braking to meet the drag requirement - or B may been even beyond full regen and always required it (back in Japanese G1?). Thus clearly associating B with engine braking. But B drag is now firmly less than full regen in a plug-in, so the chance of it starting engine braking is slim, not much higher than D. The main thing associated with engine braking will be a massive downhill, not which drag you're asking for.

 

i'm puzzled about many things the nut says

 

That's the key insight right there; as a number of older threads have put it, the car knows all the same tricks in all modes, and B changes its preferences slightly.

In a non-plugin, B is an adjustment to two algorithm parameters.

The first is simply how much rolling resistance you feel when off the go pedal. The algorithm could generate any level of resistance, including pure coast, but that parameter has been chosen to feel familiar like other cars. In D it is chosen to feel like other cars in a cruising gear, and in B it is chosen to feel like other cars in a downshifted gear.

The other parameter that B changes is the car's eagerness to use engine braking. In D, the car strongly prefers regen braking, and does not even begin to blend in engine braking until 77.2% SoC, ending up with nothing but engine braking at 80% SoC. It is simple to demonstrate this.

B changes this parameter (in a non-plugin) so that engine braking supplies a larger fraction of the total resistance earlier, so that the charging stress on the battery is reduced. The only time you would want to do that is on a long downhill where the battery will reach full charge either way, and in B it can get there more gently. And this is the only purpose for which Toyota says to use B. Car Care Nut gets that point right.

I notice that Paul has also denied that this parameter change happens. (In fairness, Paul seems to drive a Prime, and Prime is different on this point. The parameter change in a non-Prime is simple to demonstrate.)

Paul's argument denying the reality of this parameter change seems to boil down to:

which doesn't make the point he thinks it does. Yes, blending in engine braking does impact fuel efficiency (also noise), which is exactly why the non-Prime owner's manual advises B only for "steep downward slopes etc." and says "To improve fuel efficiency and reduce noise, set the shift position in D for normal driving."




That language remains in the Prime owners' manual, but a major change is that the battery has been made large enough to accommodate realistic Earth downhills with no need to limit its charge rate. Far from it: in the Prime owners' manual (and only there), Toyota added a section saying B activates "Regeneration Boost". That's a quite significant difference between non-Prime and Prime.

Paul has also denied that the "Regeneration Boost" language appears in the Prime owners' manual, even responding this way when HacksawMark posted PDF from the owners' manual showing it. (Both I and, I think, fuzzy1 have independently downloaded the Prime owners' manual from TIS and confirmed HacksawMark did not fake the PDF or make anything up.)

Yes, that's weird. The rolling resistance programmed in for B, which was chosen to feel like a mildly downshifted car, is clearly but a fraction of the regen the car is capable of, as anyone can demonstrate in seconds. It almost seems like a phrase Car Care Nut came up with a bit unthinkingly, like while he was thinking more about other parts of his script, but then once he had come up with it he ended up quadrupling-down on it in the video, maybe still without stopping to think about it much.

It doesn't seem to be a critical detail in his overall video; most of what he says doesn't stand or fall over whether he said "full regen" there when he should have said "more regen". Maybe the only point that does is his idea that the engine braking has to be added because full regen just can't be enough. And that is the other point where he's wrong; B doesn't add engine braking because regen can't be enough; it adds engine braking to avoid (in non-Prime) having to use that much regen.

 

Do you believe this is this really a separate parameter, rather than the intersection point of "desired drag" with "regen available per SoC" moving?

B wants more transmission drag, and hence would need to start engine braking earlier as regen ability diminishes smoothly with increased SoC.

If D just starts engine braking at 77.2%, what's B doing at 77.2%? Is the total amount of regen the same? Engine braking will be a larger fraction of the total resistance if the total resistance is higher while regen remains the same.

(Still trying to absorb your linked post to see if it answers that - not figured it out yet...)

 

Yes, I do. I predict you could show it with a test such as this:

Find a downslope, find a speed at which B just holds your speed steady on the way down, and also go down that slope in D with cruise set to hold that same speed. (Starting the runs at the same SoC, of course.)

Or start a run in D with cruise set for that speed, and switch to B partway down.

You have controlled the car to supply the same total rolling resistance on both runs. Observe the battery charge power. I predict you will (in a non-Prime) see it lower on the B run, or drop a bit when you switch to B. That would confirm that the car has changed the regen/engine balance, in favor of gentler battery charging in B.

I haven't, however, formally done that and recorded the numbers, so if you wanted to, it could be a contribution.

I also predict you don't see the same change (and maybe even the opposite change: "regeneration boost") if you do this in a Prime, which would further confirm the two parameters are distinct, because the first parameter (perceived total roll resistance) is of course still going to change the same way, non-Prime or Prime.

 

I'd love to, but your grumbles about Indiana's topology made me laugh. This is entirely theoretical for me.

As Wikipedia says:

"Next to the Gulf of Bothnia the landscape of Finland is extremely flat with height differences no larger than 50 m. This region called the Ostrobothnian Plain extends inland about 100 km and constitute the largest plain in the Nordic countries."