Going downhill in b mode, gets fast quickly! Is this usual?

That's quite the slope if B doesn't slow you down.

Cruise control doesn't engage the friction brakes, just the regen braking from the motor. But cruise will not work in B.

 

What is significant about the 32 number ??

If the speed limit is 30, then you need to start down the hill at about 25 in B.......and/or let it go 35 and see if it won't level off about there.

But some manual braking likely will still be necessary.

 

How steep is the slope? What % grade?

No, cruise control (CC) won't use the brakes. But if the road is straight and even enough for CC, then go ahead and try it to hold speed steady. It will use the same battery regeneration and engine braking mechanisms that B mode uses, but applied in a bit different manner.

But if this doesn't hold back the car enough and you must still use the brake pedal, CC will disengage.

 

Recently i visited hill station, here one of the track is very steep and rough, on uphill it was ok but on downhill i engaged b mode but still speed was way fast due to steep, my question is how to control speed on such steep hills, i miss first gear of manual in these hills, please help

 

what is the downhill grade, how long and what speed limit?

 

Does your car NOT have brakes ??

If I remember correctly, pressing on the brake pedal increases the regen braking a bit more first before
applying the actual brakes.

P.S. Your car was not designed for "off road" use.

 

When B mode doesn't produce enough engine braking to adequately control speed, then you must use the friction brakes. There is no other choice.

At low speeds typical of downshifting to first gear in a manual transmission, the Prius just doesn't produce enough engine braking for a steep hill. It was designed more for typical Japanese and American paved roads, mostly smoother with somewhat higher speeds, where people used to downshift to third or second gear, not first.

On very long, steep, slow gravel roads, such as when I visited Pikes Peak in Colorado, the downhill drive required using the friction brakes, and stopping a couple times to let the brakes cool. I personally would liked to move a bit faster to allow better engine braking, but the car in front was downshifted to first gear, a good choice for that particular car, and there were no safe passing opportunities. I arrived at the mandatory brake check station with good brake temperature, but a pickup truck close behind me, that didn't do any downshift at all, was smoking badly and had to go sit out in the braking cooling-off parking zone.

About how much vertical descent did you encounter, over how much distance?

 

It was around 6km of steep downhill, it was very rough also, i tried reverse gear also, initially it slowed down car too much using engine braking but then car gained speed, ia using reverse gear ok for slowing down also can i apply accelerator with reverse gear to slow down.

 

If the car allows you to go into Reverse while moving forward down this hill, without overriding your choice and going into Neutral instead, then I believe it is too slow to get any significant engine braking in any mode.

For this situation, stick to friction pad braking with the brake pedal, and take braking cooling stops as needed. But do put the car into B mode, for the tiny amount of engine braking it can do, not D or R.

 

I've never tried to work out what would be the lowest road speed where the Prius could theoretically give you its maximum engine braking.

The most I've ever done on quantifying engine braking was in this post, in my Gen 3 liftback (1.8L engine), where I observed the car handling about 12.6 kW of input from gravity, with about 3.3 kW arriving at the engine mechanically through the transmission, 7.7 kW arriving there electrically (MG2 ➡ MG1), so the engine was soaking up about 11 kW. The remaining 1.6 kW were going elsewhere (electrical operation of the car, and miscellaneous losses).

The engine was being twirled around 4000 RPM then, at about 26 Nm torque. (That was the best hill I could find nearby. Indiana....) We know the Gen 3 will use its 2ZR engine at up to 5300 RPM, and if we assume a more or less constant 26 Nm needed to twirl it, at 5300 RPM it would be soaking up about 14.4 kW. That would be the maximum input power from gravity you could expect to handle with engine braking alone.

Now I wonder what's the lowest road speed where that amount of engine braking could be achieved. It'll kind of depend on the relationship of MG2's braking torque to RPM.

MG2's motoring torque is supposedly 207 Nm all the way down to 0 RPM. If it could supply that much braking torque down to, say, 700 MG2 RPM, then we could conceivably keep that much engine braking down to 10 km/h or so. But I'm leery of assuming MG2's torque curve when generating is the same as when motoring.

 

@Fuzzy beat me to it, but anyway: step on the brakes.

 

It has been a long time since my descent from Pikes Peak, so I don't remember all the details well. The road has since be paved, so average traffic speeds are probably a bit higher now.

At this point, I'm not even sure that the engine braking power matched the brake pedal regeneration, but that could also be an artifact of not being on watch for the transition from regen to friction. I mostly remember that well after the traction battery was filled, B mode alone wasn't providing enough braking to avoid running into the string of cars ahead, so periodic brake pumping was needed. But B wasn't spinning up the engine to anywhere near the 4600-4800 RPM it normally reaches on my regular highway-speed hills. I don't believe I was mistakenly in D, that produces even lower braking RPM.

I've never seen 5000 RPM in B mode, even at highway speeds when engine braking isn't enough to prevent speeds from creeping up higher. Mine seems to be capped at about 4800.

 

Car allowed me to put reverse as a stopped car using brake pedal and then engine gave me lot of noise and engine breaking was strong for a while but then it started to gain speed even i was still in reverse, i didn't applied accelerator as i was afraid of harm to car so when car gain speed i applied brake and put car in d then b but i wounder if i can move forward in reverse and apply throtile to slow it down without harm to car

 

The car is computer controlled, and is pretty good at knowing its limits and not hurting itself. As i explained in #25 (at one of your many other posts on the same subject!), the only power you can use for reverse is what's in the battery, which doesn't hold much; the engine cannot join in to contribute because the engine's contribution is always in the forward direction. So, as you saw, the car eventually becomes unable to do what you are trying to do.

 

I can’t believe this discussion! B mode is simply an equivalent to a downshift in a “normal” vehicle. It WON’T prevent the vehicle from gaining speed (it has absolutely no ability to overcome the laws of physics)! It will assist in slowing the vehicle down but it is totally incapable of overcoming gravity. That, my friends is why the Prius (and every other vehicle I know of) has mechanical brakes!
To navigate mountainous roadways, you downshift (or shift to B mode in a Prius) at a slower speed than you feel is the maximum safe speed, allow the vehicle to accelerate slightly above that “safe” (or, if you are intelligent enough to read, the posted speed), then apply the brakes hard enough to slow the vehicle below that said speed; allow it to accelerate (due to that strange phenomenon called “gravity”), slightly above the speed limit, apply the brakes again to slow it below the said speed. Repeat the above as many times as necessary until you have safely navigated the descent! If your brakes begin to overheat, bring the vehicle to a stop and let them cool! Then start the process again, going at a significantly slower speed until you stop overheating the brakes or reach level ground.
B Mode is not some “magic” device that overcomes the laws of physics.
Sorry if this post offends (not really), but for crying out loud, come on people!
And putting your car in reverse to slow it down while going forward???!!!!!!!! You have got to be kidding!!!!!!!!!!!!

 

Well, if we're talking about the laws of physics, those are the same ones that say gravity is giving you kinetic energy at a rate proportional to your speed of descent, and B mode is able to sink that energy at a rate of around 11 to 14 kW.

If you are gaining kinetic energy at less than the rate B mode is sinking it, B mode will be slowing you down.

If you're gaining kinetic energy at the same rate B mode is sinking it, you won't be slowing down, but you'll still be prevented from gaining speed.

If you're gaining kinetic energy faster than B mode can sink it, you'll be picking up speed (though more slowly than you would be without B mode).

All easily demonstrated. None of that is "overcoming the laws of physics". The laws of physics are why it works that way.

 

You've got a great grip on it, but you are just one point on a wide spectrum. Plenty of people simply never drive in the mountains and are completely unfamiliar with gravity defying bargaining driving techniques.

During a vacation in Hawaii, we were driving down the side of the volcano- a substantial grade. I've got our rented Tahoe down in PRND21 and all is well. Almost, anyway. The exception was that smell coming from the rented Wrangler up ahead, the one with the brake lights on the whole way...

Apparently it's a real problem there, flatlanders melting their brakes because they've never faced the problem before.

So... I understand your frustration.

 

Right. That’s why the poster didn’t slow down. B Mode is incapable of performing miracles, it wasn’t designed to. While some idiot is trying to decipher the kinetic energy equation for the situation, the best they can hope for is destruction of a well designed vehicle. The most likely outcome is serious injury or death.
Until Toyota comes up with a way to prevent idiots from driving their vehicles it is something we will have to live with.