B versus D

Someone more knowledgeable will soon chime in, but basically the B mode is engine braking and not regenerative braking. You should switch over to B mode on a long descent when your traction battery is full.

 

Yes, B still spins up the ICE for some engine compression braking, which pushing the brake pedal does not.

 

1) In the US, all automatics are required to have a gear that provides more engine braking than D. So Toyota had to include B. (They are too honest to call it L, as the gearing does not change)

2) If you live somewhere where there is a mountain descent of more than a 1000 feet, then you should consider using B from the summit. It will help keep both the friction brakes and the HV Battery cooler than D. If you boil the brake fluid, it is BAD. Overheating the battery tends to shorten battery life, which is only expensive.

I found the roar of the engine counter-intuitive, the harder you press the brakes, the louder it gets.

There is nowhere near you in TX you need to worry about this, but if you are on the AR- OK state line there are some roads for B.
Talimena National Scenic Byway Home - Talimena National Scenic Byway
The fall foliage is almost at it's peak, go try your B gear.

Guadalupe Peak is 3000 feet above the surrounding desert, but there is no road up it. Guadalupe Peak - Wikipedia, the free encyclopedia

 

Great question about the B mode. I had a good chance to try it in my Gen4, and I forgot all about it. Rookie!
I rode the brakes for about the last 3 miles, down a long sweeping freeway road. I was just cruising along, enjoying the scenery, I happened to look at the speedometer and it showed 94! Man, that car is pretty smooth because I didn't even realize I was going that fast. (and the road is new so it was quiet in the cabin.) I was afraid I was going to burn up the brakes.

 

I can always buy more gas. For me, safety is more important. My car took longer to slow down than I was comfortable with. The brakes didn't seem to be very effective, and took a long time to slow down. (

But of course, I was stupid for not paying more attention, and allowing the car to get up to that high speed before trying to brake on a downhill.

 

Use B when you feel the need to "downshift." The transaxle lacks gears, hence downshift in quotes; but the same concept. Downshift to have the engine provide resistance from gaining speed quickly, and reducing the need for more braking, thereby reducing your btakes from overheating b/c they will be doing less work due to the contribution of engine resistance in slowing speed gain that would otherwise occur rapidly in D mode.

B does provides regen, albeit a little.

Over the course of the driving life, D vs B will likely be negligible, unless you drive in B on flat roads.

 

People, READ the Fahrvergnügen Owners Manual!
There should be a charge for questions that are answered in the owners manual.

Page 206 for the 2015 Prius Owners Manual. D vs B, clearly answered.

Big picture:
1) D = normal driving, maximum Regen, maximum fuel efficiency.
2) B = acts as a "downshift," some Regen. B is for downhill to help slow the car.

D:

* provides considerably MORE regenerative braking.
* ≥30% regen under normal application of braking force.
* does nothing to slow the car down on steep decents.
​

B:

* provides considerably LESS regenerative braking.
* ≤30% regen under normal application of braking force.
* SLOWS the car down on steep decents.​

Info Source:
http://techno-fandom.org/~hobbit/cars/b-mode.html
http://techno-fandom.org/~hobbit/cars/cours-section6.pdf

 

It sort of brakes without the tell-tale red-lights at the back coming on, which is sometimes useful for giving tail-gaters a hint (laundry or otherwise) to "BACK-OFF!!!!"

 

I'd disagree with most of that, sorry. As Jimbo said, reserve B use for the occasional very long descents, otherwise leave it in D.

 

In this sense, it is no different than downshifting in any other car.

 

Apologies not necessary.

Downshift is a concept most understand in a big general sense. It's useful application eludes many, esp those who ask what is "B"?

As I said in post #9:

Downshift to have the engine provide resistance from gaining speed quickly, and reducing the need for more braking, thereby reducing your btakes from overheating b/c they will be doing less work due to the contribution of engine resistance in slowing speed gain that would otherwise occur rapidly in D mode.​

This should be inferred as to mean downhill. My bad for not stating it. But Jimbo eluded to it in his post #4.

Between his post #4, my post #9,11,& 12, if people don't understand "B" acts as a downshift, most useful for steep/long descents, they are lost.

Can't fix stupid. Can fix ignorance, as they have just been enlightened, and likely understand now. Or maybe wishful thinking on my part.

 

Then only one thing left to clarify. My assumption, downhill in B mode foot off the gas the engine will be cycling "dry" with fuel injectors off. Or nearly so. Engine acting as an air pump.

 

That is my belief too; car is super smart firing up the ICE as needed. Just like in D: flat-ish road, gentle acceleration, you can be all electric. Press gas hard, ICE fires up.

Have not found information stating this unequivocally.

One can drive in B, on flat roads, indefinitely. MPG will suffer for sure. Car does feels like it has better acceleration when in B though.

 

"D" is the equivalent of "high gear" light engine braking, whereby much of the braking energy is recouped to charge the battery, and the engine doesn't actually do much braking until the battery is full.

"B" is the equivalent of "low gear" heavy engine braking, whereby only a small part of the braking energy is regenerated to charge the battery, and the engine wastes energy by braking before the battery is full.

Exception: At very low speeds engine braking is disengaged, and there is no additional energy wasted on engine braking in "B".


When in "D" and when the battery becomes full, recharging stops and engine braking automatically takes over. Then, the engine braking is soft when coasting with feet off the pedals, but when you press the brake pedal, the engine braking increases and the engine revs up like in "B". So, in effect, the car automatically runs in "B" when the battery is full and the brake pedal is pressed. The difference is that in "D" you need to press the brake pedal slightly to get the same amount of braking as you get with your foot off the pedal in "B".

 

Yes, that is true when going down a steep hill. In other cases B mode will use gas to idle the ICE where it would otherwise be shut off completely in D. This is based on data from my ScanGauge.

I tend to use B at the same times I lock other vehicles out of overdrive, and this includes following other traffic at about 40 mph through rolling hills, particularly around the 46 mph speed where the ICE spins up anyways. For whatever reason I find it particularly aggravating to have the ICE turn off as I crest a hill, turn back on as I gain speed coasting down the hill, turn back off as I hit the brakes at the bottom because I'm catching up to the car in front, then turn back on to go up the next hill. IMHO, once the engine spins up at 46 mph, it really should stay spinning down to about 35mph or so. Anyhow, the extra drag from B (or locking out overdrive) is useful when it avoids the need to ride the brake pedal down every slight downgrade, or when people up ahead slow unexpectedly because "squirrel".

One other observation: Because B mode does use gas to idle the engine, it prevents the coolant temp from dropping drastically when driving in cold weather. So when I do get into town a couple miles later and am driving at 20 mph, the ICE can shut off and stay off. I don't see the point in allowing the engine to shut off coasting downhill at 40 mph if it's just going to start up later at a dead stop to provide heat.

In practice (or at least my practice), B mode doesn't use noticeably more gas than D mode, cuts down on the number of times the ICE cycles on and off, and makes for more stable coolant temperatures. In some ways, I suspect B mode is easier on the ICE and battery than D mode when driving in rolling hills at 40-50 mph, particularly when repeatedly going back and forth over 46 mph.

 

Correct, but the ICE is driven by MG1, so you will use electricity and therefore loose some mileage.

 

I can't see why you would have worse milage provided B mode is being used as intended by Toyota (steep descents).

As I understand it.

Maximum energy reclaim is already occurring, within the limits of battery charge rate and state of charge, and the problem is to dissipate the surplus energy. Without engine braking this energy would be dissipated by the disc brakes. With engine braking, to the extent possible, the surplus energy is dissipated by spinning the engine without fuel. This is accomplished by the joint action of (a) the mechanical transmission path from the wheels and (b) the electrical transmission path from MG2 to MG1.

No is gasoline is used, no battery power is used and no energy that could be stored in the battery is used to spin the engine. Hence, use of B mode as intended will have no effect on mileage. What it appears that B mode will do is extend brake life and improve life safety in extreme conditions by reducing the risk of overheated brakes.