The most misunderstood aspect of the Toyota hybrid synergy drive system

if not, do it again with an instrument that can show the engine RPM, fuel cut status, battery state of charge, and current. Use the cruise control as described in #38. Datalog this and/or capture it on video showing your preparation, your roll down the hill, and the absence of engine braking.

At this point, you are the one owing the forum evidence of your contrarian claim.

 

I also won't accept a negative result based on ear alone, without instrumentation. I have enough relatives with significant enough hearing impairments to know that not everyone can hear it.

I must note that your dispute with our statements is also anecdotal, without any proof or evidence. While many of us have seen documentary evidence posted here, going back long long before you (or even I) joined this forum. And experienced it first hand, both without and with instrumentation.

 

Given B mode is just a momentary switch contact to the hybrid system, it’s hard to comprehend how engine braking could be anything other than completely computer controlled.

Just had automatic engine braking in D happen today on a pair of steep mountain grades in Colorado. Never used B and frankly never do. The car quickly filled the battery using normal light brake controlled regen. The hybrid system automatically switched to no fuel engine braking and reved higher if I braked slightly harder.

I also tried cruise control and the system automatically maintained the set speed without operating the hybrid brakes. In both cases the unfueled engine reved abnormally high.

There are more interesting technical explanations on what actually happens to achieve engine braking but I would recommend gaining a thorough knowledge of the Toyota e-cvt system first. Spoiler alert, Brake mode is less efficient than D mode by design. I will say certain “deal” assumptions are wrong.

 

You're gonna feel it more than hear it in a 2024. Like I said in my post, if you go down a downhill, the car will detect this and increase regenerative braking as well as a bit of engine braking to slow your descent. It is more noticeable in SPORT but will also happen in the other ECT modes.

It should not be consider as automatically engaging "B" mode by the car. "B" mode can only be engaged physically with the shifter.

 

It doesn't. This operation has no connection to posted speed limits.

You may find some hills and speeds where even set up like this, speed will still gradually creep up, beyond the cruise setting. Prius' engine braking power is limited by its small size, and on a couple of my regular hills, the Gen3 1.8L engine just wasn't big enough to hold steady at the posted speed limit. Though it could hold steady at lower speeds, varying with the slope angle.

In these cases, if you disengage Cruise Control, the much weaker D-mode engine braking will cause vehicle speed to run away much faster.

Don't prematurely throw this test away! just because you may misinterpret what regen is doing.

Initially, you are almost certain to see some regen indicated on your CHG gauge, because the battery is very unlikely to be completely 'regen full'. You will have to give it some downhill distance to ensure the battery is completely full (by battery ECU management standards). But you should very soon see the regen amount shrinking, as the battery approaches full and more braking power is shifted away from regen and to engine braking.

As Chapman wrote [emphasis added]:

On my Gen3 Liftback Prii, I had to descend several hundred vertical feet to get the battery full enough to completely cease regen. The larger Prime batteries very likely need to descend much farther to get the same result. From other sources, I also understand that force-charging leaves a significant gap between its limit, and the regen limit. Absent a plug-in full charge at the crest of the hill (such as at my regular ski hill), you will need to allow some time and distance for the battery to reach regen-full. But seeing a shrinking regen quantity will be an indicator that you are nearing that full battery condition.

 

Did you ever watch the regen CHG indicator on the HSI display of your Gen3 Prius? Did you not seem them combined?

Earlier, Chapman (or was it Mendel?) explained why combining engine- and regen-braking during hard B-mode drag makes sense. It reduces both battery heat and engine stress, compared to doing just regen-only first, followed by just engine-braking-only. With non-Prime Prii, there are lots and lots of hills tall enough for this is to be the best practice.

Again, did you ever watch the regen CHG indicator on the HSI display of your Gen3 Prius? Mine always showed significant amounts of regen happening, at the same time as the engine-braking whine, for a significant distance down my hills. After a certain point, the regen indicator would shrink and the engine would whine much louder, more of a roar, with the tachometer showing increased RPM. It still took a bit more distance before the shrinking regen indicator completely vanished.

Save the separate regen only method, followed by engine braking, for the shorter hills where the regen indicator won't completely vanish if you start B-mode too early.

Fuel economy tests are not performed on long steep hills, they are done on level roads. Or 'level' dynomometers. And when the hill is longer than the battery can absorb, all the excess energy is lost anyway, regardless of how you share / split the braking.

If you want to improve fuel economy in your Prime, then use up enough of your battery capacity before arriving at the crest of the hill, so that the battery can regen all the way down the hill, with no engine and friction pad braking at all. But in the other thread, you were quite averse to this practice.

 

Have you read pages 260-261 of your 2024 Prime Owner's Manual? This appears the same as the same-numbered pages that @HacksawMark presented for his 2023 Prime, at post #64 of the other thread, provided as "hard evidence" disputing one of your related claims.

You didn't show acceptance then, and continuing writing as if you still don't accept it.

Are you dismissing your Toyota Prius Prime Owner's Manual as "anecdotal"? If so, it seems there is nothing we can present here that you will accept.

 

When the dash is showing you a "full battery", it's not actually a "full" battery. What is considered "full" for a lithium ion battery is 90% and what the car shows you when you have full bars on the dash isn't completely full. When I had my 2017, I was on a road trip to Monterrey and going down the mountains on the 152, my battery went to completely full. It started flashing and engine braking was high.

I think your misunderstanding is that you are listening for the high revs coming from the engine to determine that engine braking is in fact occurring. There can be light engine braking with the standard amount of engine noise.

Your gas tank can still be shown as being "full" even after driving 100-150 miles before it starts to move. Same deal as the battery SoC.

If this is your first Prius, rest assured it happens.

 

It isn't his first. Or apparently even second.

 

While at the same time, of course, plenty of people can. It's perfectly commonplace for passengers in my car, people with no technical background and no interest in this topic, to say "hey, why's your car sound like that going down this hill?" And I can show them what's happening on the ScanGauge on the spot, if they're interested in the answer.

That's part of what makes "where's the hard evidence?" so funny. Whether deliberately collected with instruments, or casually with ears, it freely rolls in with day-to-day use of the car.

You betcha. If X is a claim, then ¬X is certainly also a claim. Whether you state it as above or as the complement "Engine braking can occur in no situation but B mode", it's a claim either way. It's a bit late for you to pose now as just disinterestedly trying to verify or refute some claim you've happened to hear. You have spent multiple pages in at least two threads actively making the "no situation but B" claim, obdurately, in the face of multiple others who both know better and have directed you to about as much hard evidence as could possibly be shown you short of being in the car with you in person.

You have solved the evidence problem by, it seems, adapting the word 'anecdote' to mean any form of evidence, no matter how collected, how quantitative, how reproducible, or how carefully reported, that you didn't happen to gather yourself. This allows you to say things like

or

rather disingenuously, because the material has been posted, repeatedly, only for you to trot out the word 'anecdotes'.

Leaving popularity aside, what you face at the moment is a bit of a credibility hurdle. Some of it is not of your own making, and some of it is.

The part that's not your doing is just the usual credibility hurdle that any person faces who repeats a widely-familiar experiment but wants to report a different result. You're hardly the first or last person to be in that position and it isn't a hopeless one, but nobody escapes facing questions in that situation about just how they prepared, conducted, and interpreted their experiment, and you can't let that surprise you. What makes the best impression is when you come well prepared for those questions.

When I offered in post #38 concrete suggestions for your materials and methods, it may have struck you as just more work to do, but believe it or not, the purpose was to help you run a better experiment, which would strengthen the position you're now in. Proper instrumentation would allow you to put hard criteria behind your statement "engine braking did not engage", and using cruise set at the minimum allowable setting maximizes the contrast between a positive and negative result, making your result as definitive as possible, whichever result you get.

If you had taken those suggestions, you would have conducted an experiment broadly comparable to the one I conducted last year. That would've created the possibility of comparing the two results, perhaps even learning what key factor accounted for your result being different from mine. Always important, when getting an unfamiliar result from a familiar experiment.

Granted, in #38, I suggested you collect only engine RPM, fuel-cut status, battery state of charge and current, which would be enough for your purpose, but was not all the data my experiment collected. Because my purpose last year wasn't to show whether engine braking happened or not—it came up in a conversation among people who already knew that it does, but wanted to learn its power limits—I collected additional data sufficient to quantify the power. I also began the run at 70% state of charge rather than 80%, specifically to observe the car's gradual transition from all-regen (below 77.2%) to all-engine (at 80%), and be able to compare the braking power being achieved by regen alone (13.45 kW) just before the transition to that from engine braking alone (12.60 kW) just after. (Those were the numbers seen on my particular run down my particular hill.)

The data needed for that experiment were battery state of charge, voltage, and current, engine RPM, MG1 RPM, MG2 RPM*, MG1 torque, and MG2 torque. The math can be followed in the post. I used gear ratios specific to gen 3, so anyone replicating the experiment in a different gen would have to look the right ratios up.

So my experiment wasn't conducted expressly to find out if engine braking happened, but of course if there had been no engine braking, it would have been hard to measure 12.60 kW of it.

But you could have collected only the data I suggested collecting, and while you wouldn't end up able to measure the engine braking power, you'd have been well positioned to support your conclusion about whether it happened or not. Instead, you chose to conduct your experiment using none of those suggestions, which leaves you with not much more than your assertion, "engine braking did not engage", to present from a familiar experiment that others have conducted more rigorously.

This post appeared while I composed, and I am glad to see that you may conduct another test. I'll again encourage you to do it as documentably and quantifiably as possible. This will do nothing but strengthen your position, if indeed you have another negative result to report.

As to what you expect to see at the transition when battery capacity is exhausted, perhaps again you might review my experiment from last year as a guide to what you might expect, or at least be prepared, to observe. In a gen 3 Prius, in D using cruise, the transition is gradual, from all regen and no engine below 77.2% state of charge, to all engine and no regen at 80% charge. Those charge thresholds are for a gen 3 with NiMH and might, I don't know, be different if you're in a gen 5 Prime. Allowing for possibly-different thresholds, I would expect the basic pattern of a gradual transition to remain.


* Edit: it's not strictly necessary to collect all three of engine, MG1, and MG2 RPM, given that if you have any two you know the third. But the ECU has all of them to log, so that's easy.

 

It's been a very long time since anyone has needed a tach of that sort. Engine RPM is available right over the OBD-II port in US cars 1996 or newer and other cars from shortly thereafter, including every Prius, and so are all the other PIDs I mentioned above. And the engine RPM PID is accurate regardless of ignition or fuel pump status.

You can get engine RPM from either the ECM or the power management control ECU (in gen 3, anyway; it's mirrored there). All the other PIDs I referred to come from the power management control ECU anyway, so it can be convenient one-stop shopping for an experiment like this.

That's a slightly different question from whether anybody reading here will be able to know what you think you know. Every little bit you can do to show that you take the credibility question seriously will help.

You have already been presented again and again with information more reproducible and more quantitative than anything you have brought to the table, all of which you have waved away with 'anecdotal' or the like. On that same standard, you can hardly be surprised if some "it didn't happen, take my word for it" from you just hangs in the air a little.