Two questions about them thar hills...

It can't downshift because there's only one gear- nothing else to change to. The prius transmission combines torque from the gas engine and the motor-generator (MG1) in a variable arrangement. But it's a zero-sum game: When the wheels are rolling, if the engine does not turn, then MG1 must turn instead. If MG1 does not rotate, then the engine must rotate, and so on.

So... you're rolling down a hill. Therefore some combination of engine and MG1 must be spinning. If you're going down hill, then the best use of either is to slow down the car, or at least prevent runaway acceleration. MG1 can go into generator mode and convert that kinetic energy into electricity, which is normally awesome. But you can't keep that up forever- eventually the battery will be full, and there's no place to put that electricity. OK, no problem... spin the gas engine instead. It will rev high, but it can sit there and absorb kinetic energy all day long and not have a problem.

There's probably a better explanation out there somewhere, but essentially B mode tells the car that you are likely to overcharge the battery before you get to the bottom of the hill- please use the engine to absorb as much energy as possible. Note that the engine isn't burning any fuel, it's just being spun by the momentum of the car. It winds up pumping air, which provides a lot of the braking effect. It can do a lot of that for a long time without any ill effects whatsoever. Much easier than what it normally does.

Remember that the car will still control the speed of MG1 and the engine to keep either one from going over redline. If your transmission isn't broken, you'd be going well over 100mph before over-revving either.

Nothing practical, anyway. It's just buzzy and annoying. Reducing your overall speed will cut the noise but at some point you'll become a hazard to traffic.

 

It is startling the first time you use B. If you think of it as a Jake Brake, it is remarkably quiet.
In B, (and in D once the battery 'fills') the computers use Engine Braking to assist Regenerative Braking, and Friction Braking.

So if you use B from the top of a long down hill, you minimize heat in the friction brakes, and heat in the Battery, so your Prius is safer and cooler. While the computers will not let B spin the engine past redline, it is true that the harder you brake, the louder it gets. You are moving more and more air though the engine, without fuel, and past a closed throttle.

B mode still charges the battery, just more slowly, so the regenerative braking last farther down the hill.

Engine braking - Wikipedia

 

I consider B gear same as low gear (1st) in a non-hybrid automatic car and never use it unless starting uphill on icee/snow covered roads...or I want to out-jump the car stopped next to me at a traffic light...

SM-S327VL ?

 

The owner's manual does not consider it that way. It gives you no benefit, in fact, starting uphill, or out-jumping anybody at a traffic light.

Instead of L for "low" in a conventional car, it's called B for "braking", to remind you that it's only provided for one purpose, to change the way the car manages the potential energy of long downhills.

The OP was using it as intended. The sound just becomes familiar once you understand what it's doing. The computer is not going to let the engine rev any higher than it comfortably can.

 

While ChapmanF is right, there IS one subtle difference between B and D when not braking; you can't use Cruise Control in B. That will not help at lights, nothing will help at lights but your right foot.

 

Plain and simply put:
While it sounds bad it really is NOT.
That is what it is designed to do.

Actually cruise control in use will automatically invoke B mode if needed.
And a CVT transmission actually does "shift gears" but just not in the conventional sense.

So......you did good. Don't worry about it.

 

Well, it varies the input-to-output ratio. Continuously, even. Which is the same kind of thing gear-shifting transmissions do by shifting gears, only those transmissions aren't continuously variable, because they've only got so many gears to shift.

I understand "shift gears" was used in #7 in a metaphorical sense. But sometimes people early in learning about the car are still working out literal meanings, and not tackling poetry just yet.

 

Your Prius is always in D.
To emulate P, a rod is stuck in the gears. Toyota calls it a pawl.
To emulate R, spin Motor/Generator 2 backwards.
To emulate N, do not power M/G 1 or M/G 2.
To emulate L, add engine braking. Someone at Toyota was too honest to call it L, but you use B just like L, when going down hill.

No gears ever shift, no gear ratios ever change. Emulates a CVT.

 

That's pretty much how P works in ALL transmissions.

And I quite assure you that it is a real CVT and the gear ratio DOES change.......for the gas engine at least.

 

Yeah, I don't get the persistence of this "emulates a CVT" business ... it's like after seeing one way to build a continuously variable transmission, we look at any other way to build one and say "oh, that's an emulation of the first way I saw."

Show me a box with a shaft going in and a shaft coming out, and it transmits power through while continuously varying the shaft RPM ratio, and I'll show you a CVT.

 

It has been quite a while since I drove an automatic, 1971 Ford Torino Station Wagon, but I think the idle changes in P versus D; as P is not trying to push the torque converter. In a Prius, the idle does not change as the engine is still trying to push, just failing.

With the exception of the pawl in P, nothing changes in the 'transmission' except the RPMs of M/G 1, M/G 2, and the engine. Nothing else is varying, it is all software to emulate a CVT.

 

There it is again! It isn't all software, there are permanent magnets, big coils of wire, giant honkin' transistors, and software telling them when to switch on and off, all to produce (not "emulate") a CVT.

There's a shaft the engine delivers power into. There's an output shaft ending up at the differential and the front wheels. Power sure enough gets transmitted between those two shafts, while they rotate at speeds in a ratio that is continuously variable.

Build anything that does that and you've built a CVT, regardless of what the details look like inside.

I'm not sure it's as different as you think. That 1971 Ford surely had no clutch between the engine and the torque converter. The parking pawl was downstream of the torque converter (it wouldn't work if it were upstream; a torque converter isn't a solid connection). What happened whenever you idled in P was the engine was gently turning the input blades of the torque converter, the fluid was slipping around them, and the output shaft wasn't moving, because of the pawl. (Edit: of course the Ford transmission also had clutches/bands downstream of the torque converter that were able to release, so the torque converter output side probably was turning some, and that was turning some of the input gears in the transmission but not the output ones.)

More or less the same thing happens in the Prius, except the thing that's being spun around not doing much work is MG1, and there isn't any clutch or band that mechanically released.

 

I think you need a good nights sleep.
Nothing you are saying here makes any sense.

With a "conventional" engine, it is disconnected from the drive wheels in P......and a parking pawl engages.

With a CVT, the transmission is put in "neutral" however that works with a given design and a pawl engages.

There is an actual CVT transmission present. There is no software emulation.

This is NOT the same design used in a diesel locomotive with a traction electric drive.

 

And neither happens in a Prius, you are still in D, just with a pawl engaged. If you were in N with a pawl, your Prius could not charge the battery in P.

 

Jimbo has a point there; when sitting in P and the ECU has decided to take some charge, that means MG1 is doing real work, meaning real torque is needed to spin it. Because the PSD delivers torque in fixed proportions, that is (as always) 28% of the torque the engine is producing, and the other 72% is holding MG2 and the parking mechanism snugly, but motionless, against the pawl.

Because power is torque ✕ rpm, and MG1 is turning and MG2 is at 0 rpm, all of the engine power at that moment is going to MG1, and none of it to MG2 or the wheels, and the pawl is playing its part in making that true.

 

Deleted by author.

 

What is a PSD ?

And since there really IS a CVT transmission, are you saying that MG1 is driven through the transmission ??
And if you are applying 78% of the torque against a "pawl", it must be a really STRONG piece.

 

11: 45 to 15 minutes

 

The Power Split Device is the name for the planetary gear whose sun gear connects to MG1, planet carrier to the engine output shaft, and ring gear to MG2. It and MG1 are both parts of the CVT.

Yup. Definitely strong enough to withstand 72% of the engine torque that's produced while sitting in P and charging (which is close to idle speed and with only somewhat more throttle opening than a true unloaded idle).