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.