B Mode and Brake Wear

If you had a No, I would have voted no.

I would use B anytime I would use L in a more normal automatic. It helps keep you slowed down descending steep hills without over heating the friction brakes, I use B at the top of any hill that will fill the HV Battery.

It will lower my MPGs, but that is a small cost to avoid brake failure.

Other than that, I can't think of a time I do not want the car behind me to know I am braking, so I do not use B.

1. While there maybe some additional transmission or engine wear, I doubt it is measurable.
2. When it is rotating the engine, B mode is less effective than D mode and regenerative braking. Below 25 MPH, it may not rotate the engine, then it would be equal to D mode.

 

Don't worry about it. Brakes in a Prius will last 100k + miles.

 

The Prius (PiP included) does not have a transmission or torque converter. MG1 and the Power Split Device and a lot of sophisticated software make it all work without any gear shifting.

 

B turns the ICE into an air pump and it's job is to waste energy. I would use it when coming down a long grade (1 or 2,000 foot drop) on a very hot day. I once overheated the HV battery and had it shut down on me on such a grade. I now know what it's like to run a Prius on the ICE only. Very poor acceleration.

 

When you touch your brakes you regenerate until the brakes are pressed (?)75%, the brake pads don't do a thing unless you panic stop or press them about 75% of their capability
When you "B", your ice spins and you get some regeneration but with use of the MFD, you can see that the regeneration via "B" is insignificant, wheres the regeneration via the brakes is massive.

Err, as JumboPalmer points out using "B" slows you without turning on the brake light, and I enjoy it's use, but...
The brakes used lightly regenerate much more efficiently and and will give better mileage,ymmv

 

I'm pretty sure that I'll end up replacing the original brakes when the rotors become too rusty--not because of wear.
Of course there's nothing in the way of spectacular hills around here anyhow.
If I use my brakes less I'll probably end up having to replace them sooner

 

The regeneration limit is much lower than that, due to the maximum battery recharge rate. It corresponds to light to moderate braking, depending on speed.

The battery limit is 36 horsepower. Full braking at highway speed can exceed 600 horsepower, being dumped as heat into the brake rotors and pads.

 

In my wife's 2008 (former mine) with 117.000 miles, the brake pads look to be as new as ever and never had brake dust collected on the rims. During the NY State motor vehicle yearly inspections, the guy who does the job is always asking me if I had the brakes done a couple of months back.

 

You don't have a Yes, but its negligible choice. There is some tiny immeasurable wear on the ICE. It's like adding a drop of water to your bath. It won't make a lick of a difference but technically, that drop increased the volume of your bath.

You still shouldn't use B except on downhill situations anyway because it wastes gas. If you need to slow down faster, press the brakes but keep the HSI in regen. That's your best answer. You don't put negligible wear on the ICE and you don't put wear on your friction brakes. Plus better fuel economy. Triple win.

 

The brake pads will last forever. I do run the car in neutral coming into my garage and press the brakes lightly just to clean the disks. Using your brakes normally in D does not hurt a thing and of course is recommended for more regen.

 

So here's a little more technical information on regen braking in a Prius. The HV battery can only charge at so much electrical current. When you exceed that, or possibly when you're about to exceed that, the Prius will add in friction brakes to augment the regen (but it will still regen as well so long as you meet the conditions required to use regen braking, which I'll explain a bit about).

I don't know what the maximum charge rate is for PiP because I don't have one to play with, but in the Gen 2 Prius, the charge rate peaks out at about 100 amps at the maximum I've ever witnessed, at 201.5 volts, or roughly 20 kW/h. Since the PiP uses a Li-ion battery of about 3.5 times the capacity, it should be able to accept a charge at a higher current rate, but I have no idea what that is, and it is still possible that the inverter limits regen to the same 100-ish amps of regen braking. One limiting factor is that MG2, which is the drive motor and also the motor used for regen braking, is a 50 kW/h motor, so no matter what, you won't ever be exceeding that (about 250 amps assuming the PiP battery runs at the same voltage).

Another factor is that the higher the regen current, the less efficient the charging of the HV battery, so if you want to maximize mileage/efficiency, then you want to brake at a lower rate than the maximum. In my Gen 2, I find that if I brake at more than 60 amps, I'm mostly wasting the energy because I wind up with significantly less charge level in my HV by the time I'm stopped. I do best in most cases between about 30-40 amps, and although on level ground or slight downhill, I can get even more regen charge at 20 amps, the braking rate is just too low to be useful if you're not just playing around and have time to kill. I find for my driving, 40-60 amps is my sweet spot.

60 amps is a fairly moderate braking rate at lower speeds, such as 25 MPH or less, but if you're on an interstate going 75 MPH, 60 amps of regen braking will hardly even be felt (but you will see your speedometer very steadily shedding speed). Regen braking current translates into basically work done per unit of time, and regen braking at 75 MPH is doing a LOT more work per unit of time because you're travelling much further per second than at say 20 MPH, which is why using regen braking exclusively at high speeds takes paying attention to the road well ahead of time and a careful foot on the brake pedal. Using a ScanGauge II, if I display my HV battery current, I can brake from interstate speed to a stop entirely under regen, put the car in park immediately after coming to a stop, get out of the car, and touch my brake rotors with my bare hands and not burn my fingers (seriously, DON'T try this if you're not ABSOLUTELY CERTAIN of what you're doing or you could severely burn yourself! Brake rotors get EXTREMELY HOT when they're used, even at low speeds, and metal transfers heat very quickly).

There are some conditions under which your braking will not use any regen at all, or which will disable regen for about 10 seconds (and then only if you let off the pedal completely and reapply the brake pedal). Here are the ones I'm aware of in my Gen 2:

• Pressing the pedal too quickly-- the Prius tries to anticipate an emergency braking maneuver, and will instantly disable regen if it thinks you're braking in an emergency. It has nothing to do with the pedal pressure, it has to do with how quickly the pedal moves down when you apply pressure-- i.e., a quick jab vs. smooth, steady application.
• Hitting the brake pedal while cruise control is engaged. This is because you begin applying the brake pedal while the throttle is still being applied, and the Prius brake interlock disables regen while you're applying throttle because it's electrically impossible to do both at once. So when the throttle is being applied, the Prius has already switched the braking system over to the hydraulic brakes. This is a safety feature which prevents the kind of runaway acceleration that was in the news a few years ago (which was not caused by a glitch in the system, but a physical obstruction of the gas pedal on the floor). In the event that the computer does continue to apply the throttle after removing your foot from the gas pedal, the hydraulic friction brakes are already pre-engaged, and are strong enough to overpower the gas engine and electric motor so you can still slow the vehicle. This means that to use regen braking with the cruise control set, you have to either tap the brake pedal and let off and reapply the brakes, or flip the cancel switch on the cruise control (or turn it off) before applying the brakes, otherwise you're using the friction brakes.
• Like the cruise control above, if you have one foot applying gas and step on the brake pedal with your other foot, you're using the friction brakes, and no regen will occur until you let off the throttle completely, and then let off the brake pedal completely and reapply the brakes without applying any throttle.
• If the ABS system detects a wheel locking up under braking, regen will be disabled for approximately 10 seconds, and will not resume unless you let off the brake pedal completely after the timeout is up and reapply the brakes. This one is pretty much impossible to overcome because if you're braking, you're probably doing it because you need to, and the need will have passed before the timeout is complete. The only way to get back some of the lost regen in this situation is to shift into B mode, as this will add an extra 10-20 amps of regen on top of the 10 amps or so that the system still allows after it detects wheel lockup.
• Sometimes, certain irregularities on the road surface can "trick" the system into thinking a wheel has locked up, which triggers the above timeout. I find that it takes VERY little of a road irregularity to trigger this, and I also find that the higher your tire pressure, the more likely it is to happen. It will happen over speed bumps, pot holes, and in some cases, railroad crossings or expansion joints or uneven cracks across the road surface. In the Gen 2 Prius, you can feel when this happens sometimes, because if the bump isn't interfering with your "seat of the pants" sense, it causes a little "hitch" in the transition phase between regen braking and friction braking, because it's impossible to anticipate precisely when and to what extent the transition between regen and friction will be. So you'll feel as if the brakes have just "quit" for a tiny fraction of a second while the system figures out the proper balance during the transition. Some people say it feels as if the car is "lurching forward" during this transition.

These are the regen "requirements" that have to be met that I can think of off the top of my head in order for regen braking to function.

As others have said, braking in D mode will offer more regen charge than braking in B mode for a given rate of deceleration. Just letting off the gas pedal in B mode will slow the car more quickly than letting off the gas pedal in D mode, which gives the impression that slowing down in B mode will charge the battery more than in D mode. It won't and it can't. The maximum regen current is based on what the HV battery's maximum charge rate, and this charge rate doesn't change from D mode to B mode-- it's always the same (it does change based on battery operating parameters, such as battery temperature, SOC etc., so the maximum regen current is much lower if the battery is overheated or if it's very cold than if it's at its prime operating temperature). B mode is "engine Braking mode", which is designed to function much the same as shifting into a lower gear in a car with a conventional manual or automatic transmission-- that's because the Prius doesn't have any gears it can shift between, so it simulates this electro-mechanically.

I'm pretty sure that you aren't "wasting gas" in B mode, because under most circumstances, I believe the computer cuts the fuel flow to the ICE when slowing down in B mode (when the gas pedal is not applied at all) just the same as if you're in D mode and let off the gas at speeds over 43 MPH. It may still burn a tiny amount of gas under some circumstances, such as if the ICE hasn't warmed up and the system isn't operating in "stage 4", which is its full hybrid mode.

What happens in B mode is that the hybrid control computer applies a braking current to MG1, which is the motor which starts the ICE, which forces the ICE to spin because of the way that MG1, the ICE, and MG2 are physically connected to each other. If you've seen how the system actually works, you'll see how that happens and what it's doing mechanically, but it isn't really important. What's really happening is that in B mode, energy is being drawn from MG2 for regen braking, and that energy is divided between the HV battery, and MG1. If the HV battery is full, then ALL of the regen energy generated by MG2 is diverted to MG1 (up to a certain maximum, which varies by speed, after which B mode cannot brake the car anymore), which causes the ICE to spin at an even higher RPM. People who've used B mode while going down a mountain may have noticed the sudden increase in RPM shortly after the HV battery indicates 8 bars, or full charge. When "coasting" down using B mode, the hybrid computer also sends a signal to the ICE computer to adjust the ICE valve timing and engine compression (that's what the Atkinson cycle allows for) to maximize air pumping effectiveness so that the ICE provides the maximum possible braking force (by using up the maximum possible energy that MG1 can provide against it without exceeding the operating threshold of either one) through the HSD system.

The only time using B mode will reduce friction brake wear without wasting energy is when the HV battery is completely full, when the amount of braking force required exceeds the maximum regen current that the HV battery can accept, or in those situations where regen has been temporarily "disabled" due to the reasons listed above. Otherwise, B mode is just wasting some of the kinetic energy that you would otherwise be able recapture through regen braking.

I hope that helps, and isn't too much information!

 

Great explanation! However I don't understand the dimension kW/h. Since you are multiplying amps times volts perhaps you mean kWh/h, that is, a charging rate of 100 amps at 200 volts which would produce 20 kWh in 1 hour. Or, perhaps, you just mean kW.

 

I think he means kW.

 

I probably misused the term there and kW is the correct one. I'm not an engineer so I sometimes get little details like that mixed up when I'm typing up a post. Sorry about that.

 

In my 2012 Prius Plug-in, I drove the freeway in cruise control and pressed the brake as I started down an off ramp. By the time I had stopped after continual braking, the car said I had added .4 miles of EV range. Perhaps this is an improvement in the regeneration system over the older Prius reported above.

 

That is entirely possible. I was surprised to find that my 2008 didn't work that way, because seems like a pretty common sense thing to do.

 

In EV mode, PiP will not use the engine as air pump to slow down the car. PiP has bigger battery so it can regen brake more and harder. I have tried dropping into B gear at higher speed (40+ mph) and ICE did not kick in. In HV mode it definitely will.