Help With Regenerative Braking

Heavier regenerative braking does indeed recover more energy for the reasons listed above. However, and this is a big however, there are a lot of variables so the max point varies dramatically from one braking cycle to the next. Battery temperature and State Of Charge (SOC) play a big part.

Aerodynamic and frictional loses are why you need gas to move a car. Otherwise you could expend some energy to get the car up to speed, coast to your destination, and then get the energy back when you slow down. So yes, aerodynamic and frictional losses are important. The key to stopping efficiency is to pay for these losses with house money. You already know you have to stop. You know you will have to pay for the frictional losses, so why not pay for them with the kinetic energy stored in the moving vehicle? You have to dissipate that energy to stop, so use it to move the car. You can also recapture some of that energy through regenerative braking, but it's not as efficient as coasting.

Yes. You can install instrumentation such as Scan Gauge.

Tom

 

Have 3 Pri's now and don't see any difference in what reverses the flow of juice back into the Battery, Gen 2 vs. Gen.3. Eco bar & extra regen icons are misleading. Lift your foot and the juice flows back into battery. Hitting brake pedal applies pads to rotors. There is no seceret spinning roter/motor that engages when you apply brakes that I could find to generate more juice to battery. That would be a big help on the rear wheels and that would/could be a sourse of extra power. Hitting brake will guanteree that you will not enter the hyper glide mode that maximizes mileage. Does the Gen 3 electric only reverse itself when brake is applied? I see juice flowing to Batt every time I lift foot on traditional Pri screen. Extra Regen icons and regen bar are misleading to me.

 

Please read this.
After you've done it - you will have less questions - I am sure.
There is only 1 generator and only 1 motor. They are located in the engine bay, serially to the ICE (internal combustion engine), to the right hand side (when looking the car frontally). They are located, before the differential.

 

You ask good questions -- no apology necessary. Part of the confusion comes from the simplified diagrams used by Toyota.

There are two Motor-Generators in the Prius, called MG1 and MG2. They are both located inside of the transaxle.

MG1 is smaller and is used to start the Internal Combustion Engine (ICE), as a generator, and as a means to control the speed of the ICE.

MG2 is larger and is used to power the car and for regenerative braking. It is attached directly (through a chain or gears) to the drivetrain. MG2 is what moves your Prius when you pull away from a stop, and it is what slows it down when you brake.

The amount of braking force possible through regeneration is limited by the capacity of MG2 and the ability of the battery to absorb the regenerated energy.

Energy is the product of force times distance. At high speed you move much farther in a given amount of time, so you generate much more energy for the same amount of force. Because of this it only takes a small braking force at high speed to saturate regeneration. At low speeds it takes a lot more braking to get to saturation.

Once the HV battery gets to its high limit, the Prius will no longer attempt to charge it. At this point MG2 can still provide braking force, but the regenerated energy cannot be stored in the HV battery. To discard this excess energy, the Prius uses MG1 to spin the ICE as a big air pump, wasting energy. This is much like a "jake brake" on trucks. You can manually force this mode of operation by invoking B mode with the shifter.

Tom

 

I guess it's possible. 100K without regen seems like a heck of a lot. Do you do a lot of freeway driving? Light touch with the brakes?

 

You need to see this as well:

The 3rd gen works essentially the same, but the overall electric part is smaller (10-20% in volume, albeit being more powerful and efficient).

If you could point me which link does not work I will fix it and will let you know here so you can view it.

 

My understanding from what I have read on this site is,

when you push the brake pedal on a 2010 Prius, the brake shoes do not try to grab the brake rotor unless you push hard on the brake pedal or until the car is moving at 12 mph or less (or some slow speed, pls correct me if anyone knows the exact speed).

Then why does Toyota have that "B" mode on the shifter?

I use the "B" mode, but worry people behind me don't know I'm slowing down unless the brake lights turn on when in the "B" mode (again, pls let me know if they do). Wouldn't it be safer to have drivers only use the brake pedal so people behind them know they are slowing the car?

On a normal car, you down shift to prevent overheating your brakes when going down a hill. Riding the brakes on a Prius shouldn't hurt anything if the shoes don't touch the rotors like you say when gently pushing on the brake pedal.

 

The friction brakes engage below 7 mph, (coincidentally the point at which regen voltage is lower than the battery voltage).

B Mode is using during mountainous or hilly descents. As a baseline, in B-Mode, only 30% of "D-mode" regen is used to charge the battery. The other 70% goes to run the engine as an air pump. Once the battery is full, 100% of energy is used to spin the engine as an air pump (this would also otherwise occur in D-mode, just the battery gets full faster).

You don't normally use B-mode in regular driving, since it wastes energy during coasting (foot off of pedals). I understand sometimes you want to slow down without showing brake lights, then occasional use is OK.

And yes, it is not exactly safe to use B-mode for the purpose of slowing down instead of braking, in normal driving. It's like when I don't appreciate it when a person with a stick shift downshifts to slow down, when I'm driving behind them. There's nothing wrong with flashing your lights.

But it is actually safer to use B-mode instead of riding your brakes during a long descent (which in a Prius doesn't wear out your brake pads) . If you have your brake lights on continuously during a descent, the person behind you won't know when you are actually making an emergency stop.

 

This is wrong, unless I misunderstand your point. Pressing the brake does not automatically apply friction brakes, other than making them ready. Pressing the brake causes MG2 to work as a generator, slowing the car and recharging the HV battery. Pressing harder causes more regeneration, up until one of the electrical limits are reached.

There are three situations where friction brakes are used instead of regeneration:

1) Loss of traction, forcing the use of ABS.

2) Panic stop.

3) Very low speed, where regeneration is not effective.

Tom

 

Normal regenerative braking uses MG2 as a big generator, slowing the car and making electricity to recharge the HV battery. Unfortunately, the HV battery isn't very large, and on a long steep descent it is possible to charge it up to the high limit. At that point there is no place to put the regenerated electricity. To get around this problem the Prius needs a method to discard this excess electricity. The method it uses is to send power to MG1 which then spins the ICE as a big air pump, dissipating the excess energy. This is similar to dynamic braking on diesel-electric locomotives, where they dissipate braking energy by powering large electric heaters.

Spinning the ICE only takes up so much power. If you need more braking than that and the battery is already maxed, all the Prius can do is use the friction brakes. This is where B mode comes into play. If you know in advance that you will max out the battery on a long downhill, shifting to B mode starts engine braking without waiting for a full battery. This will forestall the point where the HV battery maxes and friction braking begins. B mode initiates engine braking, whether it is needed or not.

For normal drivers, there is only one reason to use B mode: long steep downhills that will saturate the HV battery. If you don't have those, you don't need to use B mode. In fact you shouldn't use B mode if it's not needed, because all you are doing is wasting energy.

As an aside, engine braking produces a very funny effect on a steep downhill. As you ride the brake down the hill the engine screams like a small turbojet. This is the air pump effect dissipating energy. Lift your foot a bit on the brake and the engine gets quieter, as less engine braking is needed. It's very counterintuitive to have the engine slow down as you speed up.

Tom

 

Based on my observation using B is not really useful. If you have a really long downhill, you can re-gen brake as much as the HV battery will let you without using the brakes - so if the downhill is long and not so steep that full regen is not strong enough to keep speed at bay, you can stay in D.

When HV battery is full, the ICE starts becoming a pump, even when in D. So still no need to use B.

I think B is really only necessary when you are going down a true steep downhill and you know that re-gen braking is not going to be enough to keep speed at bay.
Other than that, using B is not necessary and the Prius will take care of itself after the HV Battery is full.

PS: the HV battery doesn't hold much, but I still have had very *few* chances to fill it up to the point that the ICE needs to pump air wasting energy. It seems to me, at least where I drive, that getting a full HV is *really* difficult...and even when you go up a hill, the battery will still be used to provide extra torque via MG2 to the wheels - so whatever extra energy generated from the ICE will be used immediately and even topped off from the battery to go uphill - so not really to charge fully the battery.

The only way I have found so far to *fully* charge the battery is driving down a very *long* downhill (like in Austria from Brennerpass to Innsbruck on the highway).

 

More specifically, B mode saves wear on the friction brakes, but only on steep downhills. The only time I ever use B mode is on rock climbing trips out west.

Tom

 

My recent cars have never been my only wheels, and rarely suffered congested traffic. And when new, the nonhybrids received comparatively few city miles, the short miles went mostly to the older car. Combined with an older form of what hypermilers now call 'DWB' (driving without brakes), average brake use was very light.

But the spouse, also using DWB but experiencing far more traffic congestion and more city miles, beat 100k miles on the original pads too.

And with over 800k miles of manual transmission distance between us, neither of us has ever replaced a clutch.

 

Well, pre-Prius, I considered myself an average driver, not practicing DWB (Prius mellowed me out even more ). My manual car had to have its brake pads replaced somewhere in the 60k range IIRC, and I thought that was pretty good. I didn't do much engine braking. Almost all of those miles were in the D.C. area with heavy congestion being routine.

So, you both sound above average to me. Must be the DWB.

 

There is only one problem, at least here in Germany weather, about "never" braking using the pads - the brake rotors get rusty because they get wet and never dry up...
I hope I will not have to change the brake rotors sooner rather than later...

 

What's wrong with a nice protective layer of rust on the rotors? Keeps 'em from rusting all the way through, right?

 

The problem is when the rust is *inside* the rotors where the inlets for the air are...no way to remove it...