Is there a legitimate use for charge mode in the Prime?

You're obviously not thinking straight. On a long downslope like the type I described, your hybrid battery is likely to reach full charge from regeneration. At that point, there's nothing holding back your car on the slope, and it tends to move faster and faster, and could reach dangerous and or illegal speeds. Regen has no place to put the energy, the battery is at full charge. There's simply no way to dissipate the energy gained from coasting downhill, except friction braking, which will destroy the brakes on such a long hill. The only option left at that point is engine braking, which is what the "B" is for. It's an engine brake that works by compressing air and releasing energy as heat. It's the same way an engine brake works on an 18-wheeler. Truckers don't want to burn out their brakes on long hills either.

 

Concerning a regular non-plugin Prius:

Yes. Lightly.

For stronger braking. D and B use the same tools, but in different mixtures and degrees.

The first time my 2010 Prius began engine braking in D mode, I was startled, not having selected or needed B. But then quickly remembered long-ago experts here describing it, so then understood what was happening. It was on a long, very shallow, slow descent in a wildlife viewing area of a National or State Park, below the ICE auto-off speed.

I'm still learning about that expanded capabilities of the Primes, and the differences between the two Primes.

As a comparative newcomer, you have missed many discussions about this.

In the regular non-plug-in Prius, on downhills long enough to fill the battery regardless of strategy, many of us prefer to engage B right at the top and use regen and engine braking in parallel, for as long as regen is available. This lets engine braking run at medium (quiet) speed for a while, delaying the high-RPM screaming banshee / vacuum cleaner / jet engine howl as long as practical. And applies less stress to both battery and engine.

The only good reason to delay engaging B is for shorter hills where the battery may not completely fill if B is engaged early.

In the larger-battery Primes, it seems that engine braking ought to be comparatively rare, once the driver learns how to manage battery charge level. Done right, the Prime - PHEV batteries should never fill up from regeneration alone. Referring back to the base post of this thread and earlier replies, if the battery fills up from downhill regeneration after the driver used Charge mode in preparation for the uphill climb, then they used Charge mode too long, wasting gasoline. If it fills from regeneration after climbing fully or mostly in HV, then they wasted gasoline by not reverting to EV soon enough.

Or for cases where they plugged in near the top, they took in more charge than they could use, so some gets thrown away, wasted.

 

Sounds to me like a made up story.

 

As previously pointed out, Prius B mode engine braking works the same as downshifting in regular passenger cars, NOT the same as 18-wheeler 'Jake Brakes'.

Jake Brakes apply compression relief by opening the exhaust valve at the top of the compression stroke, before the expansion stroke. While the Prius engine does use adjustable valve timing, it does not short-circuit the expansion stroke. The working principles are not the same.

 

It is most definitely not a placebo in a regular non-plug-in Prius, it really is needed in some situations. I described two specific cases of overheated brake in an earlier post. I've been down a fair number of even greater descents.

A downshift "gear" is a legal requirement. Though modern brakes do seem to go farther without overheating failures than old-era brakes could.

The Primes have greater capabilities, so if operated appropriately, should do better than the non-plugins.

 

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Wow, he's nearly there. I can hear the brain cells lining up.

Because a "low gear" that can be used downhill to increase drag with the pedal released is legally required in some places, and drivers expect it.

The answer is basically the same as "if it doesn't disconnect the gear train, why is there a N position?"

The N and B positions provide the required user interface functions of neutral and high no-accelerator drag.

There is no direct association between the engine braking and the B position.

They are somewhat associated in that you will tend to use both going down a long hill, but you can use B without getting engine braking and get engine braking without using B.

Of course it's hard for you to figure. You are impervious to the substance of what anyone else is saying if it goes against the preconceptions in your alleged brain. We saw that on the thread where you had the preconception that the battery you refused to look at is AGM.

Oh dear, spotted this bit. I guess you didn't manage to read what I wrote after all.

Trying again.... No it won't, because the driving behaviour as experienced by the driver doesn't significantly change when the battery is full. It will start using engine braking to provide the same resistance. Even if you don't select B.

Yes, if Toyota hadn't done that, and had chosen to just stop regeneration, then the Prius would have kept picking up speed once the battery was full, but fortunately they don't have you working on their design team, so they were able to figure out that would be a bad idea (as well as probably illegal), and engage engine braking automatically.

 

Are there any G1 (ex-)owners who recall how B worked in the beginning? I believe in the past "B" may have prompted engine braking more than it does now. But maybe that's just about battery capacity.

I think the HEV algorithm may still contain something like:

* If charge capacity > 8 green bars and accelerator released, perform engine braking
* If charge capacity >= 7 green bars and B mode and accelerator released, perform engine braking

So on HEVs it may tend to noticeably start engine braking downhill earlier in B mode. The logic being "if I am full, suggesting I might be going downhill, and the driver has selected B, further suggesting that, let's start add some extra drag now, even if I could regen a bit more, to pace myself".

That behaviour was probably noticeable enough to make people think B automatically started engine braking - not noticing the "only if nearly full" condition on that. HEVs are often near to the top of their charge range if they've been cruising.

(This is similar to thinking that the defroster automatically starts the engine - actually it's "only if air temperature is cold", but people only tend to try it when it's cold.)

But in a plug-in, you're far less likely to be close to the top of your charge capacity, because the car tries to get below 80% as soon as possible, and it looks at the top of real capacity for limiting regen - not trying to constrain charge to your current "HEV band", so even if B would prompt engine braking a bit earlier, that's very rarely going to kick in. If you're not going downhill, you'd have to have just come off the charger. Even just off the charger, I think the "100%" for that is still kind of treated as a "6 blue bars" state with a little bit of HEV regen reserve still left above it - I've not spotted significantly less regen indicated just off the charger - so engine braking still wouldn't kick in.

The car is full of automatic controls - it's silly to think that there's a basic user interface element that would always or only start engine braking. There's no basic user interface element that is directly associated with even starting the engine. (We had a few pages on a thread the other day with a user trying to figure out how to force it). So why would there be one directly associated with engine braking? Even the "neutral" that disengages the electrical system isn't 100% - it has speed exceptions.

 

It's just basic laws of physics. You can't break them. When you are descending a long hill, your hybrid battery can easily hit full charge. Using friction brakes on a long hill is a bad idea, so your only option is for the engine to produce drag, the B setting does that. I pointed it out in the manual, but someone said it was the wrong manual. It was not.
I give up.
You can't fix stupid.

 

KMO's reply reminds of another point, which he partially covered, I'll expand.

When descending a constant slope in an regular Prius, D-coasting, D w/CC, and B all use the same regen and engine braking tools, but in different mixes and levels, depending on circumstances.

Generally, as each one of them transitions between all-regen, mixed regen + engine braking, and all-engine-braking, they adjust the levels of each element so that the combined total drag remains approximately constant. The 'seat feel' doesn't noticeably change when the mixture changes.

D mode gives the lightest drag, so can easily "run away" to ever higher speed.

B mode gives the strongest drag, so needs a much steeper slope before it will "run away" to ever-increasing speed (which happens one of my regular hills).

D w/CC drag falls anywhere in between, as needed to maintain constant speed, though capped at the same maximum drag power as B.

In a regular non-plug-in Prius, I have experience B-mode working in all-regen without any engine braking, though it seemed like an infrequent lower-speed corner case. But now in my RAV4 Prime (under a different label than B), all-regen without any engine braking seems to be its primary operation, at all speeds.

 

I personally like to use B when driving more curvy and hilly (or downhill) terrain. Similar to one-pedal drive (despite far from it) - having stronger regen braking makes it much easier to accelerate and decelerate rapidly without changing the pedal (brake/gas) all the time. Its basically more fun and sporty for me.

Otherwise I use DRCC most of the time anyway so the B mode is not really helpfull or even annoying.

 

But I bet that the change in visible behaviour is not explicit in the algorithms. In both systems you probably have the same condition that it has to be close to "full" (let's guess within 0.2kWh) for B to prompt engine braking when D wouldn't. If it's lower, it won't happen in B, and if it's above "full", it will happen in D too. But the Prime is going to far less often be within 0.2kWh of full. The SoC bands of the core HEV operating logic don't scale up with the battery capacity.

So there is a grain of truth to all this - B mode might make braking happen when D wouldn't - at certain states of charge. That state is far less likely in the plug-in. But the plug-in retains that text with the now-excessive emphasis on the engine braking from the basic non-plugin manual.

It's not totally wrong for the manual to say of B:

"Applying engine braking or strong braking when the accelerator pedal has been released on steep downward slopes etc"

Although I can see it's potentially misleading, as engine braking isn't mentioned anywhere else (neither is strong braking).

The error is to jump from there to concluding:

* B always does engine braking (did you miss the "or"?)
* Engine braking can only occur if you're in B (do you think "strong braking" can only occur if you're in B?)
* It only happens on steep downward slopes (not seen that misconception yet, but you could it read it like that)

 

If Toyota had made engine braking automatic, why would they give you the choice by giving you the B position on the shifter?
Regeneration is the best way to control runaway freewheeling when it's available, but it only works when the hybrid battery is capable of taking a charge. When regeneration is impossible due to a full battery, the only reasonable choice is engine braking.

If you never encounter a 20 mile downslope, good for you, but they do exist. In my other car, which is a non-hybrid, I can downshift to increase engine drag, thus saving my brakes from overheating and burning out. I can't do that with my Prius. The engine is stopped. That means no engine drag. So I use the B just like it says in the manual.

I understand that some drivers would rather use friction brakes. Some truckers do that too, and that's why there are runaway lanes constructed along downslopes. Over-use of brakes on a long downslope can result in the total loss of brakes when they burn out, and the vehicle can speed out of control. That's the scenario Toyota had in mind when they designed the B engine brake.

Not usually a problem in a conventional non-hybrid car. You can downshift and apply intermittent brakes, and it usually turns out OK. I choose not to abuse my brakes by using them to keep my Prius from speeding out of control.

Someone said I should have drained my hybrid battery first. How ridiculous is that? On the long slopes I'm talking about, that would be impossible. When the hybrid battery reaches full charge, there is simply no regen braking anymore, and no, the engine braking does not kick in automatically. I might want to ride the brakes all the rest of the way down, but that would be stupid and wasteful of brake pads, plus it might be quite dangerous if the brakes burn out completely. That's what the engine brake is for. The manual recommends using B to save the brakes, which is a simple, basic idea that should make sense to anyone. Instead, we find many posters here going against the idea for some reason, and quite adamantly so. A real headscratcher.

 

This is an example of the of the "begging the question" fallacy in its true sense. "An argument's premises assume the truth of the conclusion".

Which is why Paul never understands any explanations given - always unable to shake the assumption of the truth of his conclusions.

B is not a choice of engine braking or not, so its existence is not evidence of engine braking not being automatic.

At most B is a hint to use engine braking earlier when approaching full SoC.

Another argument in the same form by someone with the misconception that regen only happens in B: "If Toyota had made regen automatic, why would they give you the choice by giving you the B position on the shifter?"

And you've observed this, have you? Your Prius just acts as if it's in Neutral - there you are going down this hill, and you try shifting between N and D and see no difference?

 

I've observed the same thing you've observed. When the hybrid battery can take a charge, the regen gauge shows a bit of regen drag when I let off the accelerator. That's not engine braking, that's simply regen. It happens without you touching the brake.

Under normal driving, that's what happens; you let off the accelerator, and the car begins to slow. It looks like the engine braking we're use to in a non-hybrid. But it's not engine braking, it's just regen.

If you've ever driven down a long slope of many miles, it's a different situation. Regen has topped up your battery, so the regen drag is gone. There's simply no place to store the excess energy gained from descending the hill. The engine does not kick in; it has stopped, so you are freewheeling down the hill like a runaway truck. Applying your friction brakes can work for a bit, but if you keep using them for too long, they start to burn up. After that, you might have a crash.

This is the scenario the B is for. You may never have it and thank your lucky stars, but there are places where it can happen. That's when you need to get real and understand what the B is for. Ignore the idiots who say it's for some type of regen braking; it's not. It's an engine brake. that might be the only thing that can save you on a long downslope.

 

You didn't answer the question. You've observed this, have you? I don't believe you have.

I'm currently taking this with the level of belief I have in your incorrect determination that your battery you hadn't looked at was AGM.

 

BTW, we do have reports over on this thread of someone who wanted to limit charging to 95% to stop their engine automatically starting when starting a journey going downhill...

Limit charging to 95%??? | PriusChat

So there's someone who HAS observed the engine automatically starting in a downhill scenario and being unable to prevent it.

You're saying yours behaves differently?

 

Read the previous post. I answered you directly. When you let off the accelerator, the car seems to slow like it does from engine drag in a conventional vehicle. That is not engine braking, it's regen. If you watch your CHG meter, it shows a bit of regen happening when you let off the gas. This is not engine braking, it is regen braking. There is no engine braking when you are decelerating, because your engine is stopped.

 

No you didn't - you answered something I hadn't asked about normal non-full-battery regen, and then went off begging the question again by assuming drag can only come from regen in D.

I'm asking about your claim that there is no engine braking and hence no drag in D once the battery is full.

Have you ever observed this happen?

 

Another way to look at it. From a user interface point of view, there are 5 possible demands for drag:

N, D, B1, B2, B3. Lets say that's 0,1,2,3,4 in some scale.

When the battery is empty enough, regen can supply any of those drags up to 4.
When the battery is nearly full, it might be able to regen enough provide drag 1, but not 3. So at that point shifting between D and B might cause engine braking to start.
When the battery is totally full, it will not be able to provide any regen drag, so it will have to activate engine braking even in D to get the correct drag level 1.

There is a "saving the brakes" component here - this is a "not from the brakes" drag request for any of D to B3, so by making it, you are putting the onus on the hybrid system to take up the drag. And if it is unable to provide your request by regen, it will then perform engine braking.

And it is possible that when full B will give you MORE engine braking - as you are requesting a higher drag level. I think maybe that point got missed when countering the incorrect claim that D will just "run away" as if neutral.

So maybe the correct answer is somewhere between "no engine braking in D" and "B doesn't do anything D doesn't".