3,500 foot decent = crazy HOT brakes and Burning Smells

This is incorrect. By employing engine braking early, B delays filling the HV Battery. You will have regen further down the hill.

This is why I do not recommend B if the hill is short, you lose regeneration opportunity. If B worked as you imagine, you would use B on every hill.

(Below 24 MPH it is possible that B may well only use regeneration, so for very slow descents your explanation may be true. Aside from school zones, I do not spend any time below 24 MPH)

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Toyota is required by law to have a N position, you are never required to use it and I can find very few examples where it is wise. (There are car washes where the car is towed through the wash)

 

There is nothing imaginary about it. It is how a power splitting device works. The engine revving is a matter of mg1 doing work to charge the HV bat adding it's current to mg2. You can watch the current in MG1/2 and MGR on the SUV hybrids. You can see how they add together and how it affects the HV bat amps. In "B" total regen increases in decel without brake pedal, adds more with brake pedal.

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And the additional issue with "N" is that in order for mg1 to charge the HV bat with the engine idling, the wheels MUST be held still by the parking pawl, or by the brake pedal in other gear positions.

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You will see additional amps in B position right down to about 2 or 3 mph.

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As far as why the car wash is OK....That is a very low speed and for short duration.

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You don't know what your talking about. I've been putting my car in neutral for 9 years to clean the brakes. The rotors look really good.
It doesn't harm anything.

 

You are completely leaving out the engine compression braking, which is the primary reason for using 'B' on Gen2 and Gen3. [I can't speak for your Gen4, just in case it does things different than earlier generations. This thread is in the Gen2 forum.]

Not in my Gen3s. 'B' is clearly intended for multi-thousand-foot descents, and I have used it for that purpose well over a hundred times.

Nope. In 'B' mode, MG1 is not charging the HV battery at all, only MG2 is doing that. MG1 diverts some of MG2's power output away from the battery, and towards engine drag instead.

Maximum total regen is no different in 'B' mode vs brake pedal vs cruise control. All modes are limited by the same battery restriction.

7 mph is the lower limit in a Gen3. I believe Gen2 is similar. If Gen4 can go lower, then Toyota has improved something.

 

Well. I'm pretty sure you must be right about all of that.

 

Consider a simple energy balance. When a car descends a hill, it loses potential energy based on the product of its weight and the vertical drop. This energy changes into either kinetic energy (more speed), electrical energy (more charge in the battery) or heat (lots of heat somewhere). If the speed is held constant during the descent, once the battery is full there is nothing left but to create lots of heat. A locomotive uses giant resistors and fans in the roof to burn off such excess energy. A Prius uses mechanical brakes to burn the excess energy (turn it into heat). It has to go somewhere. It's as simple as that. Of course you could let it go into increased speed....

 

You forgot another outlet for that energy: air heating via engine compression braking. That is the main purpose of 'B' mode.

 

Hi Tony. When the engine turns mg1 (that is, applies torque to mg1 in the same direction that mg1 is rotating) then mg1 is generating and as you say contributing to charging the battery.

When however mg1 turns the engine (that is, mg1 applies torque to the engine in the same direction that it is rotating), then mg1 is acting as a motor and consuming power from the battery. The latter is what is happening when the engine revs up (without any accelerator input and most importantly, without any fuel injection) as you are descending in "B" mode. (And also to a lesser extent in "D" mode when the battery is fully charged.)

Yes this is correct, however be careful as it's easy to come to the wrong conclusion from this fact.

When you release the accelerator pedal and let it coast under only simulated "engine brake", then as you say, the Prius will generate more current if you're in "B" mode compared to "D". Under these conditions however, "B" mode will also slow the Prius much more rapidly, and that is the catch - all other things aren't equal. If you applied a small amount of brake pedal in "D" mode so that the deceleration was the same in "D" as it was in "B", then "D" mode actually generates more energy (due to it not having the extra losses associated with turning the engine without fuel injection).

 

I have driven my Gen 2 180,000 miles in the Colorado Rockies and this is my experience as well. I check the brakes at every tire rotation and still have just under 50%. I use "B" mode quite a bit and find it very effective.

 

The engine takes a LOT of energy to rotate an engine. This is what mg1 pushes against to generate ADDITIONAL current to MG2 during decel. MG2/can only generate energy down to about 10 mph depending on the year, MG1 generates energy in order to maintain near peak torque and horsepower depending on the power or economy demands of themonitor/ecu. During acceleration the engine pushes against a calculated "slip" through the planetary gear set, the excess power generated during this slip is put on the HV bus, to be used by MG2 or recycled to the battery. During decel in "B" more energy is generated by both MG1 and MG2, MG2s speed is linked proportionally to the final drive/differential/wheels/internal gear of the planetary gearset, while MG1 pushes against the ENGINE (planetary carrier) and the sun gear.
the power between the engine and MG1 comes together at some proportion of speed (dynamic rpm of the ring gear/reduction shaft/MG2) whether MG1 is being used to control rpm under engine load, or pushing against the engine and the ring gear to generate current during B driving. Wrapping your head around what is happening during the many blended modes of operation isn't easy. You can explain 75% easily, the last bit is rather hard to explain. Toyota courses 071/072/074 Includes information for the THS system starting with GEN1.

Look, it comes down to this, referring to the original mountain decel: If your (not even close to accurate) battery on the hybrid monitor screen is displaying all battery bars (blue to green on gen 1/2 always green on gen 3), THERE IS NO ROOM LEFT IN THE BATTERY, YOUR REGEN IS MOST LIKELY OUT TO LUNCH, LEAVING THE UNDERSIZED HYDRAULIC FRICTION BRAKES TO DO ALL THE DIRTY WORK. (sorry about the caps, not for yelling, for emphasis).

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B mode generates more current at the same given speed, to a higher rate of decel, slowing the car more, but yes, given a Lithium Battery instead of a NiMh one, that extra generation will convert to a charged HV bat better, recovering more of the gasoline energy you wasted during acceleration.

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The B mode, with mg1 pushing against the engine, is the only way to generate more current at low speeds. While some may be wasted, more is recovered at lower speeds vs. Mg2 in "D"

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To further explain, MG1 operates in both reverse and forward rotation during driving.

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You are right. I neglected engine braking which is an important effect. In a conventional car, it is the main energy dissipation means. If engine braking cannot provide enough reverse torque at the wheels, friction braking is necessary to keep speed under control (after the battery is full). This is what I experienced on a long downgrade going south on I-24 out of Monteagle TN. Toward the bottom of the grade, it smelled bad. I'm just saying that a car is designed for most needs, but you can find situations that exceed its energy handling capacity. Then you need to limit the energy demand by other means such as beginning the descent at a slow speed to allow some acceleration to occur or use the brakes only intermittently to slow down and then allow cooling time or have everyone open their doors for more drag (just kidding) or head for a truck turnoff lane (very extreme) or add a drag chute (not currently a Prius option).

 

But again you are failing to recognise that once the HV battery is at the nominal "full" mark, re-gen braking is replaced by engine braking (very noticeable by the signature high engine rpm sound), so no, the hydraulic brakes are not left to "do all the dirty work". Whether this is enough to hold the speed is another question, and you may need to engage the hydraulic brakes from time to time. If you need to hold on the brake for any length of time, then it may also require stopping for a braking break.

 

Keep an eye on rear brake adjustment (North America only, I hear). If you never use your parking brake, they might be out of adjustment.

 

Engine braking is an afterthought. Merely a result of squeezing regen out at low rpm. Otherwise, there would be a clutch in the transmission to hold the planetary carrier. You'd get more regen that way, but also it is necessary to watch the speed of the planetary gearset and MG. MG1 can spin to about 13000 rpm in both directions.

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No, it subtracts current from MG2's output. Go back and recheck the basic laws of energy conservation.

Sorry, but using all caps for emphasis does not make it any less wrong.

No, engine braking is the primary purpose of 'B' mode. A lower 'gear' for engine braking is a federal requirement, and is an essential means of keeping a vehicle under control during mountain descents in many portions of North America west of the Mississippi.

 

Yes. In the northwest where your decent from 5000 feet can take hours, when you have periods of flat or valleys between peaks. Ive driven those roads too. Not for a continuous descent from 6200 down to a 1400 in 7.5 miles. This is what I am talking about. Gen 1,2,3, doesn't matter your regen is out to lunch before the second mile. It is about a 12% grade for the entire ride down. If you have all eight bars, you have little or no regen left. No engine braking, just friction brakes. The laws of conservation always apply to the right circumstances. mg2 cannot generate much power below about 15 mph. It cannot deliver current because it doesn't spin fast enough to, but mg1 can IF it has something to push against, the engine. Remember, there is No mechanical connection that can provide torque between the engine and wheels. Mg1 and the planetary gearset does that. MG2 can push the car with the proper current supply, but MG1 cannot. The engine cannot. If you lose battery or supply bus integrity, a Prius will not move.
If the shifter B was there just to satisfy FMVSS requirements it would be labelled L or , or even S like cars with automatic transmissions. And again, if your engine is worn out, or your battery full or if any portion of the hybrid system has lost integrity, B has less or no effect on deceleration. That alone is enough for it not to meet FMVSS standards. If a cell in an HV battery module has shorted, or is low in capacity that will cause it as well. The only way mg1 can take power from mg2 during decel is below the threshold rpm for mg2 to produce amps. Even at 60,70,80, the current input from mg1 and mg2 is enormous in B. The power factor losses at a given speed for both mg1 and mg2 are huge as well, this is the main reason gen1 cars didn't get great fuel economy on the highway.

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We have many 2000 foot descents that take 5 to 8 minutes at 60 mph, not hours. B is extremely useful for those.

Are you talking specifically about Mt. Washington with a very low speed road? If so, then yes, low speed is a major problem for 'B' on that particular road due to the low RPM of MG2. For another example, see past discussions of the descent from Pikes Peak, 14000 feet down to 7500 feet. There is even a mandatory brake checkpoint part way down, where an attendant with an IR thermometer checks every vehicle and diverts problem cases in a 30 minute cooling off zone. Nothing unique to hybrids. When I was there, the offender was an F150 pickup with Florida plates.

But for highways at 40 to 60 mph, 'B' mode engine braking is very effective, though limited by the engine's smaller displacement than common non-hybrids.

 

tony2ltr, what is your source of the information upon which you are basing your arguments?

Is it just observation and conclusion, or have you had some top-secret training at the Toyota factory?

 

The problem appears to be limited to long steep low-speed grades, say those exceeding 10% and over 2000' drop. It's not just the Prius that requires knowledge of braking system limitations. I've even had to stop and let my bicycle rims cool down on descents like that. We have the one in HI in the OP, Mt Washington, Pikes Peak, and I can add the short stretch from Summerhaven, AZ to the summit of Mt Lemmon and Flagstaff Rd in Boulder, CO.

Fuzzy1, I didn't know Pikes Peak had the mandatory check point. I've never been up there by car. (I'd rather hike two beautiful days than pay the toll.)

 

We were traveling and didn't have two whole days available for that item. And hadn't been in CO long enough to be fully acclimated to the altitude (home is at 500 feet), so it was worthwhile for us to pay the toll.

But recent snowfall had closed the road above Devil's Playground, so we still had to hoof it up the final 1200 vertical feet if we wanted to see the top. It was very much worth the effort.