B mode charges battery to 100%, then, if still engaged, is used to dump excessive energy via ICE engagement. ECU will NOT permit traction battery overcharge. As soon as it gets charged to full 100% and energy is being supplied, ICE turns on and uses oversupplied energy, to dispose of it.

 

Technically the ICE is not on (throttle is closed and no fuel), MG1 is receiving excess power from MG2 when the battery is full. It's the drag to turn the ICE via MG1 that's slowing things down. That's the main reason why even though I have 100% charge on my traction battery going downhill at 15mph on "B" mode, I didn't notice ICE kicking in.


Here's the technical explanation from toyota when in "B" mode;

Under deceleration or braking, HSD uses the car's kinetic energy to let the wheels turn the electric
motors and recover regenerative energy to recharge the battery.
DECELERATION, BRAKING: kinetic energy recuperation for battery charging
Under braking, kinetic energy that is normally lost as friction heat is now converted into electrical
energy. Electrical power is no longer supplied to MG1/MG2, but MG2 is powered by the wheels
and acts as generator to recharge the HV battery. Even when the accelerator pedal is lifted, deceleration energy is recovered in the battery to be reused later.
The HSD system has a special transmission setting labelled 'B' (for Brake) for engine braking on
hills.
The regenerative brakes in an HSD system absorb a significant amount of the normal braking
load, so the conventional brakes on HSD vehicles are undersized compared to brakes on a conventional car of similar mass. If the battery is full, the system switches to conventional compression
braking, drawing power from MG2 and shunting it to MG1, speeding the engine with throttle
closed and so slowing the vehicle.

 

The B mode is often misunderstood. The TCH ICE runs in Atkinson cycle mode. In Atkinson mode it provides little engine braking due to the late closing of the intake valves. Also you have no way of controling the transmission gear ratio in a TCH. The B mode reverts the engine to Otto cycle for more engine braking, shuts the fuel off, and forces the revs of the engine higher to provide more engine braking. In other words it is an engine brake mode. In engine brake mode all energy is wasted as heat.

Yes, the B mode also uses regenerative braking too, but so does cruise control if you are going down hill. And even better if you want maximum regenerative braking just put your foot on the brake.

Use of the B mode is a waste of energy and decreases fuel mileage. You are better to avoid it and just use your brake to regenerate. The only purpose it has is to waste energy on a long downhill after the battery is fully charged and you still want to go slower. Then it is a good way to burn off energy without heating up your brakes.

I doubt that you can harm the engine using the B mode, just your pocketbook!

 

Coming down the 17 mile mountain a few weeks ago. In the B-mode the battery charged to 100% in less than a mile. I was then dependent on the engine braking which revved the engine from 3800 to 4500 rpm varying due to some grades steeper then others. I did use the manual brakes to slow some on the 45 mph curves which seemed to work ok, even while in the brake mode.

Warning, I wouldn't depend on the 'manual brakes' for driving down a mountain with 10% or steeper grades. Once the battery reaches 100% the your regin don't seem to work. Then the next problem is that half way down the mountain your manual brakes can overheat and hardly work.

By using the B-mode on the steeper grades, the engine revs in the fuel cut-off mode to help slow the car. I kept my car in the B-mode most of the way down the mountain from Cloudcroft. The instant gas milage readout read 99.9 mpg nearly the whole trip down till we reached Alamogordo, where the car operated normally when I move the lever back to drive.

It should be ok to use the manual brakes if you have some straight or slow grades of 1/2 mile where you can allow the brakes to cool off before using them again.

Here is a blurry picture my wife took, go figure. This was taken over half way down the mountain. You can see all the 15 minute graph bars were at 60 mpg. The mpg readout on the dash was reading 99.99 mpg. So I doubt any gas was used till I started driving on level ground again. The part you mentioned about using gas in the brake mode is correct. With the engine revving at 4000 rpm and you touch the accelerator, you bet the injectors will start spaying in mucho gas and you will loose gas mileage big time.

DickNanCloudcroft015SC.jpg

 

I agree. The only useful purpose I have found for the B mode is to engine brake on a steep hill after you have fully charged the battery and have no more regenerative braking left, or you are simply going too fast with full regenerative braking.

Keep in mind that "manual" braking with the brake pedal fully uses regenerative braking before it starts to brake with the conventional disk brakes. The display may vary with the model, but on the 2012 XLE you get an eco mode display that gives you a good indication of regenerative braking. The idea is to use the brake pedal as much as you can without "pinning" the needle to the very bottom.

 

You are right, this reminds me of the straight highway east of Las Cruces, NM. When headed west 10 miles before the city you top pass that's 5700 feet. It's a straight 6-lane highway headed down. It's about 2 1/2 miles long so I tired it using my manual brakes on the '07 TCH. After pulsing the brakes about 1 1/2 miles the traction battery was full, almost no brakes so I moved he gear lever to Brake. The engine revved pretty high as I was at 60, slowing the car about 8 mph. I never tried the brakes as we were nearing the foothills were the highway was not a steep.

I do know the brakes worked rather good even with the 100% full traction battery while in the brake mode. This was the trip down from Cloudcroft I talked about earlier. It's interesting the things these hybrids can do.

.

 

One other plus for the B Mode. It keeps the gas engine running. This comes in handy on roads with repeating speed bumps. Without the B mode you need the gas engine to pull you over the speed bump and then you go the ev mode. This constant starting and stopping the gas engine after about 6 city blocks gets to be REALLY annoying!

SO it is good know how to force the gas engine mode when needed.

 

I seldom hear the engine or when it switched to the EV and back. Going slow I can if the windows are open due to the EV quietness. I do watch the scan gauge to see if it's at 0 rpm or the navigation reads 60 mpg. I drive slow like 20 mph at times to maximize the traction battery. If on a old paved road like the 2 miles to our house, I drive at 24 mph using the cruise. I would be heard to hear engine or EV with the road noise in that 2 mile stretch that seldom has any traffic.

 

It's more the start-stop, start-stop, start-stop, start-stop, start-stop, start-stop, start-stop, start-stop, start-stop, start-stop, with the noise and "running over the stick" feel. This can be eliminated with the B mode. (as described with the repeating speed bump example)

Normally it not a problem as you have mentioned rburt.

 

Seeing your from Colorado reminds me of a vacation my wife and I took from Dallas to your state. I had bought a '81 VW Rabbit diesel. It was broke in and got 55 driving in dallas and 60 to 65 highway depending on the winds. We drove to Manitou Springs for a few days, rode the cog train to pikes peak, then headed to Cripple Creek then on south to Leadville. We spent the night and headed south again I think it was to Salida.

I don't recall the fillips, but somehow I managed 60 mpg average while driving in some city's and lots of mountain highways. I figure what goes up has to come down. The VW was a 5 speed manual with overdrive. I spent lots of time coasting in neutral with the engine idling riding along at 70 to 80 miles per hour. The car took the long curves just fine as I had the tires aired up to about 4 pounds short of max psi. (when cold) I enjoyed that little car that seems roomy enough on the inside. I love this new '12 TCH hybrid much, much better. Don't have to worry with any diesel noise/vibration at idle or smell.