It actually does the opposite of what you'd think. It's designed for times when the motor/generator does not provide enough a braking force, for instance on steep declines. Putting it in B mode causes the wheels to spin the ICE to slow the car, I believe with the ICE off so you get it's full inertia. As a result you get LESS regen than if you kept it in drive.

Pacific, you may want to think about the way you use this. If you apply the brakes lightly you'll get full regen, assuming that you don't need the hydraulic brakes. However, using the B setting like you do actually cuts down on the regen.

 

From what I can tell, there are very few instances that you would want to use the B setting. When you first apply the brakes the brake lights do not come on. There is regeneration happening, and the lights are off. then if you press harder the regeneration gets higher and the brake lights come on. It is not untill you press fairly well on the pedel do the brakes actually start to apply.

When using the B setting, you are throwing away all that extra "free" energy, and even wearing the engine prematurly.

One intersting thing I noticed today while washing the car, the front brake rotors seem to be slightly rusty on the pad path, but the rear brakes are nice and shiny. I wonder if the rear brakes are applied to balance the motors' regenerative load, so when stopping, the front brakes might not be applied, but the rears are.

 

agforever writes:One intersting thing I noticed today while washing the car, the front brake rotors seem to be slightly rusty on the pad path, but the rear brakes are nice and shiny. I wonder if the rear brakes are applied to balance the motors' regenerative load, so when stopping, the front brakes might not be applied, but the rears are.

The last time I drove 200 miles and came home and i could touch all four brake rotors, and after 1800 miles they still have the machining crosshatches on them. it seems to me that there is not much mechanical brakeing going on and brake pads should last 100K miles. on a normal car you could not touch the rotors after a five mile drive.

 

I don't think regen is his concern based on his description.


I'd have to say that in my mind, using "B" means I've failed as a hybrid driver. Toyota might as well have labeled that shifter setting as "ED" for Energy Dump, because that is what it's designed to do, bleed off energy to the environment. This is normally exactly opposite what you want to do when driving as you want to use the least amount of energy possible in forward motion and recover as much energy as possible slowing.

I sometimes downshift my Sienna going down steep hills as I know I can't recover that energy, but on the TCH "B" is always going to be my last option, perhaps on a very long down hill where there is just too much energy to recover (ie regen is topped up and now riding the mechanical brakes).

 

Using B when diving into a corner - there I'm sure his purpose is not to regen, but for performance. However, in the first paragraph he states

"It would be nice to be able to coast all the way down to a stop all the time, but that's just not acceptable in city driving. So, I'll kick in B when I need to decelerate to match speed with the car in front of me. Just a flick in and out works most of the time."

In that case it's not done for performance reasons and there's no reason to shift to B as opposed to lightly tapping the brakes.

As an aside, if I were interested in real spirited driving I don't think I would have chosen this car. It's a great car, but it's not built for performance like that.

 

I am pretty sure that 1) the brake lights come on when the brake pedal is pressed beyond a very light touch, and 2) regen does occur when in B mode, although some of the energy that might otherwise be available for regen gets used instead forcing higher ICE RPMs, and 3) it is safe and not abusive to the car to switch in and out of B mode any time, although it is not always smart.

Keep in mind that the regen can only use so much current (to keep the batteries from overheating during charge), and for so long (until the batteries are fully charged - although "fully charged" is never max capacity of the cells in order to prolong the life of the cells). With more current than allowed, or any current beyond full charge, all of the remaining current is dumped as waste heat from the electrical power control unit (PCU - it has its own radiator!), and/or in the case of B mode, as waste heat in the engine.

In order to turn the ICE faster, a force is necessary, and that turning force comes from the wheels. The PSD (power split device) directs some of the wheel force to the ICE, and some to MG1 (Motor/Generator 1), and some is siphoned off by MG2 before it gets to the PSD. In order for B mode to force more of the energy into the ICE, MG1 has to be slowed down. However, making MG1 slow down also causes MG1 to generate MORE current, so regen is still occuring in B mode.

To get a bit more technical, the Power Control Unit sets the voltage on each motor. The speed of each motor determines the zero current voltage of that motor. If the PCU voltage is higher than the motor-speed voltage, then the motor is driven. However, if the PCU voltage is lower than the motor-speed voltage, then the motor acts as a generator. The bigger the difference in the voltage, the greater the current consumed or generated by the motor. (Just ignore the three phase control that is also needed, and assume positive and negative DC voltages to see how the system works.) The only thing B mode does is cause the PCU to lower the voltage on MG1 to make it turn slower, which makes the ICE turn faster. The regen power is determined by the voltage times the current, and while more current is going to be generated in B mode, it is at a lower voltage, and there is less total regen power (as the rest of the power is turning the ICE).

For most situations where B mode is useful, such as keeping a reasonable speed down long hills without needing to use the brake pedal, the max charge current is already reached, and regen will occur just a completly and as quickly as without B mode, but more heat will be dumped through the ICE radiator instead of through the PCU radiator. Then again, there is some minimal additional wear on the ICE, which should be insignificant compared to the normal wear of the ICE when not in B or EV mode.

And you thought the HSD was complicated! (I'll let someone else give a lesson on how the PSD regulates the voltage and phasing of the 600V AC motors with 244V DC batteries!!!)

-- Alan

 

Alan,
I agree with what you said, but my point was that by using B when you don't really need to will cause more wear to the engine. The engine is turning when it might not have been turning at all. Also if you are not at max battery capacity, and have not maxed out the voltage limit on recharge, even you agree that B will waste more energy than just regular braking.

Also, I have watched the third brake light in the rear view mirror (I can see when it lights up as some light spills into view), and the regen starts well before the lights come on. There is a fine line between regen with brakes applied and no lights to regen with brakes applied with lights on. I also understand that regen starts even without the brakes applied.

There is a good use of the B position, but adjusting speed in regular traffic is not one of them. I would even say that adjusting speed coimg up on a curve is not one of them in this car. In a non-hybrid car I would use engine braking all the time for those purposes, but that is a habit that I personally feel should not be transfered to a Hybrid.

 

B seems to give quite a bit of regen. Using it going down a mountainy area was the only time I ever got the battery to light up solid.

 

I'm guessing you still get a bit of regen, just not as much. Downhill you get a bit, but I bet if you used it on flat services you'd see a big difference.

 

If the ICE is not turning at all, then you are already under 44 MPH (some claim 42, and that is probably the max EV speed for the Prius, but I think my TCH has gone a solid 44 MPH in EV for more than a block!). At low speeds, unless you are on a steep hill, B mode is probably not smart.

If the ICE is already turning, then turning a little faster is virtually irrelevant as far as additional wear is concerned. By irrelevant, I mean that additional wear are to parts that typically never wear out anyways. Even stress on piston rings are insignificant when there is no compression forces acting on them, as is the case in B mode with no throttle. The valves are held open, there is no compression; only the water and oil pump are seeing fairly normal loads, which are minimally greater than they see at idle (non-B and non-EV mode with no throttle typically has the ICE at idle). Unless B mode is used for a great deal of driving, say 10% or more of the car's total milage, then I would think the additional ICE wear due to B mode is insignificant (much less than 1%). I understand that this claim of less than 1/10 wear of the ICE in B mode vs. normal non-hybrid ICE running is just guesswork on my part, but I think I have a reasonable basis for making this estimate.

More guesswork on my part, but I think anything more than light braking, that is, typical braking for most drivers on the road, from speeds above 50 MPH, will generate far more current that the batteries are capable of absorbing. Typical NiMH cells can be safely charged at 4C, but I believe Toyota limits charge to 2C. 4C results is a fully depleted cell being complete recharged in a little over 15 minutes, and 2C is 30 minutes. Since less than half the total capacity of the cells are allowed to be used (to prolong life), a zero SOC reading to a full SOC reading cannot occur faster than 15 minutes. I have seen nearly an 1/8 SOC reading increase in about a minute with just coasting, which has to be close to max charge rate. That would indicate that most of the braking energy for average or strong braking is not going to the batteries, but is being dumped (well, some is used to charge the lead-acid battery, and power the A/C and other equipment which otherwise would be getting energy from a non-hybrid ICE alternator). More importantly, when B mode is really indicated, while descending long hills, letting the ICE dissipate some of the excess heat is probably very good for the PCU, allowing it to stay cooler.

I think we are mostly in agreement, that B mode is best not used every time you might want to downshift a sports car with a manual tranny. I think it should be used for any prolonged braking where there is no need to keep the brake lights on. I think we are disagreeing about whether B mode adds any significant engine wear, or reduces the SOC at any point in the drive. On the other hand, if you have your A/C on max cool, high beam headlights on, CD at max volume, low state of charge on the lead acid battery, etc., then you might need the full current that B mode reduces in order to maintain max charge rate to the batteries.

Seems to me to come on with about the same amount of pedal depression as with my Sienna. The Sienna has a travel adjustment on the brake light switch, but I haven't checked to see if there even is a separate switch for the TCH brake lights. It is reasonable that may be no separate brake light switch in the TCH.

Every time I have used B mode, I have a full, or nearly full SOC before I am leaving B mode! I would guess there are few good reasons to use B mode that would result in less than the max possible charge rate to the batteries, whether actually going into B mode or not.

-- Alan