"B" mode...what for?

There are times in this discussion (in regards to The Car Care Nut’s explanation) that it seems some have missed his reference to the research he put forth to get a correct answer to what B mode actually is. To that point, the explanation he provided was not simply his opinion. FWIW

 

The simplistic answer is that the car goes into warm up whenever the system had to spin up the engine. The oil is flowing, maybe engineers were concerned about condensation in the passages being picked up.

This behavior on hills has been reported by others.

 

Yes, as long as it doesn't have to spin up the engine in B Mode, something that D mode doesn't do while braking.

 

I did see that reference, and yet he still manages to muddle the explanation in a few places (or the people he asked may have muddled it to him):



B mode does not apply "maximum possible" regen braking as soon as your foot's off the go pedal. You can get much stronger regen braking (when there's room in the battery) by pressing the brake. Being off the go pedal in B mode just gives you a moderate amount. (It's also not as strong as what you get if you have cruise control set for a much lower speed than you're going.)

Being off the go pedal in D mode also gives you some. Both amounts are arbitrary choices made so the Prius will feel like a familiar car (otherwise, it would be natural for the car to just roll freely when you're off the pedal, like a Tesla). D mode is programmed to give you a certain amount of decel when you're off the pedal, and B is programmed to give you a certain amount more. Neither amount is "maximum possible"; they are just chosen to be about what you'd feel in a familiar car in D or in L.

He repeats the mistake in a few places,



like here, so it's more than just a one-off misspeak; somehow, during his research, he got that idea in his head.

As for how the transmission works while engine braking, this is both oversimplified and overcomplicated, depending on how you look at it:



Yes, MG1 is involved in spinning the engine for braking. But remember how MG1 accounts for only 30/108 of the engine torque when you're driving? Nothing about that is different when you're engine braking. The other 78/108 are coming mechanically through the gearing.

So in one sense, to say it's just MG1 spinning the engine is an oversimplification. But it's an overcomplication too, because it sounds like the transmission has to do some special thing that's not what it always does. Not so; once you know how it all works when the engine is powering the wheels, it's nothing but the same stuff backwards when the wheels power the engine. And that also makes it not really so all-fired different from other transmissions in other cars.



At this spot, he is introducing the engine-braking part by suggesting the regen to the battery (which he has taken, mistakenly, to be maxed out) is not enough to slow at the same rate a gasoline car would, so engine braking has to be used to make up the difference. He's missing that a lot of the time, even in B mode, you will get straight regen without engine braking, and yes indeedy, straight regen is enough to slow the car the expected amount.

My own check of engine braking in a Gen 3 liftback showed that you can't count on it for more than about 13 kW of slowing. From regen, on the other hand, you can easily see 27 kW if the battery is happy and ready to accept strong charging. (I don't know what the charge limit is on a Prime; it might even be higher than that.)

When the engine braking gets phased in, that's to substitute for some regen to avoid excess charging of the battery. Engine braking can be phased in whether you are in D or in B, but the car has a slightly greater preference for it in B, so that the battery won't be charged as aggressively.



Here he's explaining why B mode is counterproductive if you're trying to maximize efficiency, but by making it mostly about the engine braking, he's missing the main point:

Drivers attracted to B mode because they think it regenerates into the battery more, and that will improve their MPGs, are generally thinking that more regen into the battery is good. It really is not as good as just keeping the momentum you have whenever possible. Capturing it into the battery and reusing it later is better than wasting it, but you pay conversion loss in both directions on that deal. And B mode does more of this than D mode does when you back off the go pedal, and that's a hit on your efficiency whether it phases in any engine braking or not.

 

Until the engine kicks in, B and D behaves the same except B applies more Regen right off the bat than D. You can see it in Hybrid Assistant. The green Regen bar (hence the power given back to the battery) size is bigger in B mode than D mode when you let go the accelerator. You can apply the brakes as long as you want in D mode, the engine will never spin up. It will simply apply the physical brakes.

 

The ICE can be twirled up for engine braking in D mode too, when the battery gets full either while gliding down long shallow hills or while pressing the brake pedal. At least, it did so in both my Gen3s, though for me it was a rarely encountered corner case. For my routes, B mode engine twirling was vastly more common by a factor of at least 100x, maybe even 1000x.

I first noticed it in on a low speed road through a state or national Park in South Dakota or Colorado, and was quite surprised, then remembered it being described here a couple months earlier.

 

Yes, my Gen 3 can definitely also engine-twirl when braking in D. Also when descending on cruise control, whether in D or B. If that's something they took away for Gen 4 or 5, I don't know it.

In Gen 3 the car has the exact same bag of tricks in both modes, and the difference is just a bit of shift among its preferences toward or away from each trick.

 

Yes. To a point.

Toyota designed their hybrid system to behave like a tradition car. Take your foot off the gas while in D with a traditional car, it will coast, but there will be slight deceleration from the drag of the drive train and a little engine braking. The Prius software mimics this by applying a little regen braking when the foot is off the gas pedal.

Put the traditional car in a lower gear, and the amount of engine braking increases by spinning the engine faster. This is what B mode does. The software will spin the engine faster to get that increased deceleration. However, the big battery of the PHEV provides another option to get the same result. That is increase the regen amount when foot off the pedals. When the battery is near full charge, it goes to spinning up the engine.

Applying gentle braking in D will get the same level of regen as B in a Prime, but the option of using engine braking isn't there for the car. With a battery near full, the friction brakes will be used. As the car reads that as you wanting to eventually stop, not just keep the car from accelerating.

 

Unless that's something that changed between Gen 3 and later gens/Prime, I'm doubtful. See #88 and #89 for reports of Gen 3 using engine twirling in D when the brake pedal is applied.

 

I like the technical analysis in #86 and agree with everything except I have trouble with the comparisons to other cars and other transmission, because they do not reflect my experiences with either Tesla or typical Automatic transmissions that I've driven, but is more like many manual tranny cars I've driven.. blah blah blah. It's not that big a deal to me and hope it's not a big deal for you as well.
I'm really excited to learn more about how Gen 5 Prime is getting closer to a One Pedal Driving scenario and ( wishful thinking on my part ) if B will actually stop the Prime completely without friction braking at all ( however long it may take ).

 

I've never had the engine spin up in D while braking in my Gen4 Prime.

 

The car will use engine braking while applying the brake pedal?
It can do so while coasting to maintain a consistent deceleration rate, but I don't see a need for its use while the driver is actively braking the car.

 

I'm pretty sure I remember my Gen 3 doing it (though I have not rushed out just now to check it again), and you can see in #88 that fuzzy1 remembers the same thing.

The designers can build in a behavior or two even that we "don't see a need for". The capability is already there, so it's at most a few lines of code, or, more likely, a few lines they didn't have to write just to make braking in D behave differently.

 

My Gen3s were non-plugin, so were much more likely to fill up their tiny batteries via regeneration than would a Prime.

I'm remembering Plug-in and Prime owners reporting engine spin braking only after filling their batteries via wall charging near the top of a considerable hill, then descending said hill on regeneration. I can't think of any hills long enough to fill a plug-in battery on regeneration alone, absent serious battery degradation, though hot or cold temperatures might restrict battery current such that engine spinning becomes necessary.

I'm remembering it mostly from gliding down long shallow hills in D with foot-off-gas-pedal with very light regeneration producing 'synthetic engine drag' to mimic a traditional non-hybrid. In the back of my mind, I seem to remember it with light brake pedal application too, but it is a week too late to re-check. My Gen3 Prius's parking space has been taken over by a Rav4 Prime.

 

I don't bother with the B mode anymore. I just turn on the AC if going long down hills and if my passengers can tolerate the cold temps. For the small Prius HV battery it will get full so fast when going downhill that it will use the engine revving to try to slow the car down. Turning on the AC will allow the excess energy it gets from regen to slow down the car down. This will save your brakes a bit. Set it on lowest temps to get max regen from AC.

 

With the challenge of properly blending friction and regen brakes on the brake pedal, I'd think adding a change in engine braking at that time would have been something the designers avoided.

 

We can certainly try to armchair-outthink the designers in various ways, but as far as I know, the "properly blending" conversation is a conversation between the brake ECU and the HV control ECU. Brake ECU says "driver wants this much slowing", HV control ECU says "I can do this much", brake ECU says "thanks, I'll use friction for the rest".

As for how the HV control ECU ends up achieving the rate of slowing it agreed to provide, I think that's its own business, and the brake ECU doesn't try to micromanage how the HV control ECU will do that.

 

Nice technique. Do you turn the stereo volume up too? You can bleed off a few more watts that way.

G5 improves on this because it's got the heat pump, so you could alternate between maximum and minimum temperature, consuming lots of electricity for both cooling and heating.