B Mode Mistake?

Shotgunning the exact same question in multiple places within the same forum site will NOT
increase your chances of getting useful replies.
Quite the contrary. It is annoying.

 

Can i put reverse gear and throtle to further slow it down in steep downhill

 

It depends on how fast you are going. The system (at least in the 2010–2015 that I have) will not allow shifting to R if the forward speed is above around 11 km/h. It will beep and shift to N instead.

Below that speed, it will allow shifting into R while moving forward, and yes, the go pedal will slow you faster.

 

Will it harm my car if i use reverse gear and apply accelerator to slow down my car while car is facing downhill. Offcourse i am applying reverse gear after slowing down my car using brake but as hill is steep it will gain speed once i release brake.

 

That might be hard to say. There is nothing intrinsically harmful about shifting a Prius to R while moving forward—unlike other cars, no gears will clash or anything—and yes, you can then use motor power to slow the forward speed and smoothly cross to reverse.

The idea of pouring sustained power into the electric motor to resist forward speed for a long period, that's just something I haven't tested. I can't think of anything obviously wrong with it, as long as the motor cooling is adequate for that case, but I would pause to think it might also be a mode of operation Toyota hasn't extensively tested either. They might just say that was why they put brakes on the car.

Note that, when driving forward, if you apply enough go pedal, or run low on battery power, the engine will contribute. In reverse, that would be no help, because the engine's mechanical torque contribution can only be forward, and 78/108 of it always reaches the final drive. So your slowing effect from using reverse would have to be limited to the amount of power you can get from the battery, and for the length of time you can get it.

 

I'm just going to put it out there that this is a stupid idea. As mentioned, it won't work above 11 km/h, and below that speed, it is not going to be practical in the way in which you'd be trying to use it.

Just put the car in B and use the brake pedal.

See also my reply in one of your other posts.

 

I have tried it, i stopped the car using brake and then applied reverse gear and then released brake as hill was steep car started moving forward but engine braking was much more than that in b mode but after a while car started gaining speed and engine breaking just vanished, i didn't press accelerator as i thought it may give harm to car as its moving forward and i am in reverse but i wounder wether it will harm car

 

The car is computer controlled, and is pretty good at knowing its limits and not hurting itself. As i explained in #25, 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.

 

what if you went down backwards while in drive?

 

No, Chap. The engine can drive one MG as a generator to supply power to the other for driving in reverse. The Toyota engineers were very clever so you can still go backwards with a depleted HV battery.

JeffD

 

No, Jeff. The engine can drive MG1 as a generator—30/108 of the engine torque always goes there—but the other 78/108 of the engine torque always goes straight through the PSD, and always in the forward direction.

In order to drive the car in reverse with the engine running, MG2 needs to be supplying enough torque to overcome the 78/108 of the engine torque pushing forward, plus the torque needed to move the car backward.

It can happen—the car is able to drive in reverse with the engine running—but it involves a lossy loop of power through the transaxle. MG1 is getting mechanical power from two sources: the engine, of course, and the reverse motion of the PSD ring, which adds into the forward motion of MG1. That second term can be viewed as a mechanical power flow from MG2 back to MG1, exactly balanced by the extra electrical power MG1 has to send to MG2 to make that happen: a loop. It's subtracted from the power MG2 can deliver to the wheels.

If things were perfectly lossless, the loop might just be an algebraic thing of theoretical interest and no practical concern, but the losses to the power circulating in that loop are real costs.

In my experiments on reversing some years ago, I never observed the car choosing to start the engine while reversing if it could be avoided. That's a contrast to what I wrote in one of my very first PriusChat posts, Prius out of mud | PriusChat, where I thought the engine was contributing to the reverse power. I specifically tested some time later, and with a warmed-up engine, I could not observe the car starting the engine to add any power in reverse. In my original out-of-mud case, the engine was running because of the warmup cycle, and the car was doing the needful in order to reverse despite the engine running, rather than using the engine to advantage in reversing.

With the warmed-up engine, IIRC, my observations were that giving enough go-pedal input in reverse would always stop the engine, even if it had been running. So it seemed that a better strategy for, say, backing out of a long steep driveway, would be to reverse on the battery, with occasional stops to force-charge.

I think I did those tests in my Gen 1, and it's possible a later generation could have changed the behavior. It comes down to comparing the losses in charging-then-using the battery to the losses in the parasitic power loop when reversing with the engine running. The behavior of my Gen 1 seemed to suggest Toyota had come down in favor of stopping the engine and using the battery.

 

Crash. If lucky, at grade level instead of over a cliff.

4 km is a very long way to back down a steep rough grade.

 

O

Understood but i wou

Understood but i wounder that if engine cannot contrite while in reverse then why there was svere engine braking initially when i put car to reverse (i am assuming it was engine braking as engine voice was too loud and speed was slow) but after some time that engine loud voice and braking just goes away and car started gaining speed due to gravity. I didn't applied throtle thinking it may harm to car. I applied break and applied d and then b and used friction brakes and continued downhill while in b and brake using. But was too afraid of brake heating, i also wana ask can i know wether brake i was using was regen or friction, is there any way to know so i can keep braking in regen portion while in b mode so to avoid risk of fry of friction brake

 

As you saw, my post #31 contained both theory and experiment. What you're reporting here might be able to add something to the knowledge we have from experiment, if you have a datalog of enough of the parameters simultaneously, such as RPM and power at both MGs and the engine. The use case you are thinking of is one few if any other members seem to have experimented with and been interested in, so the data gathering might have to be up to you. I would say in general that simply judging from the engine voice does not reveal enough to conclude what was going on.

As I think we've covered in one or more of your other threads, you might just be overcomplicating this. The friction brake capacity is simply far higher than the regen or engine brake capacity, and so using regen or engine braking to avoid using the friction brakes might be appealing but simply not always an option.

As we've also covered, the basic physics simply tells you that the work you're imposing on the brakes is the steepness of the descent, times the weight of the car, times the speed of the car. Just those three things.

Typically, as you drive down a descent, you don't really have the option of being on some different descent that is less steep, and you don't really want to stop and throw stuff out of the car to reduce weight. So the main thing in your control is your speed.

For most descents, you can drive slowly enough for the engine braking to hold your speed (though on a very steep descent that could mean single-digit speeds where the transmission's braking effect falls off). But that may be less of an option if there is other traffic that will hate you for driving that slowly. In that case, you may have to drive closer to the expected speed, and engine braking won't be enough, and you'll have to use the friction brakes also. But the basic lesson is, if you want the regen or engine braking to handle the job but they can't at your chosen speed ... choose a lower speed. (Of course, to get from the current speed to the lower speed, you will have to use the friction brakes.)

Even the friction brakes also have a limit to the work you can impose on them, determined by how fast they can shed heat. @fuzzy1 often posts a figure around 500 kW for the work in highway speed full-force braking, but that's much more than the brakes can shed. Sustained braking at that rate will overheat them quickly.

I don't think I've seen a figure for the braking rate they can sustain. But the answer to overheating friction brakes is the same as the answer for exceeding regen or engine brake capacity: choose a lower speed.

If you start to smell the friction brakes overheating, slow down to a much lower speed.

A study showed it slightly better to use the brakes intermittently than steadily: apply them, slow down below your target speed, release them completely to cool, repeat when you have exceeded your target speed again. In the moments while the brakes are released, engine braking will still help you: even when it isn't enough to hold your speed, it will slow down your gain of speed.

If you are doing all of that and the brakes still smell hot, you're just doing it all at too high a speed. Do it all at a lower speed.

Again, depending on other traffic, you might find the only speed that will work would annoy the other drivers.

If that happens, drive so you average the speed that will work—by alternating periods of driving at a traffic-friendly speed, and periods where you pull off and stop, and let the brakes cool.

People in other vehicles sometimes have to use all those techniques too. They're subject to the same physics you are.

 

I think you didn't understand what i am trying to ask, when we apply b mode, car starts engine braking and some more regen but there is still some room for more regen, i wana ask if i apply light break then regen will increase to a nax point, how i can make sure to press brake slightly continously instead of intermittently so only engage regen not friction, so i can fully utilize regen to max slow down without bothering friction brakes to work

 

you can't. because you are constrained by the speed limit you are imposing on yourself, you have to press the brake hard enough to reduce speed to that limit, and accept the consequences of whatever friction braking the car is programmed to use.
i would suggest that light braking from the start will use less friction than intermittent heavy braking, but you still have to be cautious about over heating the discs.

 

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My calculations ...

Braking 60 mph to a stop takes 120.3 feet at 1 g, or 133.7 feet at 0.9 g. One source gives a Gen3 Prius braking distance just barely under 120 feet, others give it a bit under 130 feet. This should be more about tire choice than brake rotor and pad capacity.

A basic 2012 Prius has a curb weight of 3042 lbs, plus 825 load capacity, for a total of 3867 lbs, or 1754 kg.

For 1 g braking, F=m*a, or 1754 kg * 9.81 m/sec^2 = 17,207 newtons.

60 mph = 26.82 m/s.

P = f * v = 17207 nt * 26.82 m/s = 461,500 watts = 461.5 kW.

So for my fully loaded 2012 Prius, 1 g 500 kW braking occurs at (500 kw / 461.5kw ) * (60 mph) = 65.0 mph.

If maximum braking is limited to 0.9 g, then 500 kW occurs at (1 / 0.9) * 65.0 = 72.2 mph. If this Prius can do a 60-0 braking distance of 128 feet as one source mentions, then 500 kW braking occurs at closer to 69 mph.

Of course, this power is momentary at full speed, declining proportionately as speed falls.

I don't know just how fast the brakes can shed heat. But presuming that neutral-gear braking down an infinitely long 7% grade at 60 mph will eventually overheat the brakes, would necessarily mean that these brakes shed heat at less than 32 kw, far far less than they can produce heat. I'm thinking that even a 5% grade might cause overheat on tall-enough grades (more than 2000 vertical feet), placing that shed rate to under 23 kW. And these don't account for how much of that power is actually dissipated by other means such as vehicle air drag and rolling resistance.

Did I get the calculations right? Any seat-of-the-pants feedback on brake overheat on actual grades? A Hawaiian member here replaced overheated brake parts twice from a 3000+ vertical feet descent in D gear and foot pedal braking, before learning that a certain few members here were wrong about it being completely automatic, so that he really did need B mode. But I don't know his speed and steepness.