Question about regenerative brake on the Prius

The regen is done by MG2 inside the HSD, not by the brakes. The brakes are standard friction disk brakes. They are actuated only when needed, however, by a complex master cylinder. It first uses regen but if that isn't enough (brake pedal is further depressed by driver) then it switches to a combination and then full friction braking.
In the event of a panic braking event (rapid depression of brake pedal) it is full friction braking, with "brake assist" as well.
The "power brake" booster is not vacuum operated, it is run hydraulically with an accumulator and boosted by an electric pump, backed up by a large capacitor (in case of 12V system failure).
The parking brake is a standard cable operated rear wheel only device.

 

Actually, you only need one wheel to lose contact with the road and you lose regen braking through the normal function of the differential.

There is a delay between losing regen braking and the Skid Control ECU engaging full friction braking. It is on the order of a half second delay for my car.

 

It's inside the transmission.

The Prius transmission has two electric motors, the big MG2 traction motor directly geared to the driveshaft that powers the front wheels, and the small MG1 counterbalance motor.

An electric motor can act both as a motor and as a generator (which we all remember doing as kids when we wire up a toy motor to a light bulb, and cause the bulb to light up when we spin the motor spindle with our fingers).

Same thing with the MG2 traction motor in the Prius PSD transmission-- It can draw power from the battery to get the car moving, and it can be used as a generator to convert the rotation of the wheels back into electricity to be stored in the battery (which is the regenerative braking that slows down the car).

How the Power Split Device transmission works: Toyota Prius - Power Split Device

The PSD is also used in the Nissan Altima Hybrid, Ford Fusion Hybrid, Ford Escape Hybrid, and Lexus hybrids. They also use the same MG2 regenerative braking process as the Prius.

 

When the wheels touch the road again, the sudden road-grip will apply A LOT of force on the drivetrain (and on the MG2 traction motor). The sudden regen load spike on the MG2 electric traction motor can actually burn it out.

That is why the Prius hybrid computer is programmed to cut out MG2 regen braking momentarily to protect it from getting burned out, and the reason why you feel the regen braking cut out for a split second before it resumes when hitting a rough spot in the road while braking.

You can press the pedal harder to bypass the regen braking and engage the hydraulic disc brakes though.

 

* To put the regenerative process simply, the Prius has two electric motors MG1 and MG2. For simplicity's sake, MG1 is used for starting the gas engine, and MG2 is used to augment the gas engine in providing acceleration for the car.

When electricity is supplied to an electric motor, it produces power, and when power is supplied to it, it produces electricity!

Thus, whenever you begin acceleration, you are providing electricity to MG2 from the Hybrid Battery Pack, and when MG2 receives electricity, it provides power to the Prius's wheels. And, conversely, when you apply brakes during decelleration, you provide power to MG2 from the Prius transmission (which now spins MG2 backwards) which mandates MG2 to produce electricity which is sent to the Hybrid Battery Pack, and the cycle continues.

This is a VERY simplistic explanaition of the process, but perhaps it will give you a basic understanding. If you want further details, there are members here much more knowledgable than I on this subject, and they will be glad to elaborate on this subject, all you need do is ask for help.

David (aka Blind Guy)

 

Actually MG2 is used for the regen braking, not MG1. MG2 is directly connected to the drive wheels. When you brake normally, the computer sends a signal to reverse MG2 from a motor into a generator. The by producing electricity using the momentum of the car, MG2 will slow down the vehicle. The more you step into the pedal the more electricity MG2 produces and the faster you slow down. Less pedal produces less electricity and less slow down. When you reach about 7MPH the system kicks out MG2 as a generator and switches it to a motor again, from then on you are using friction brakes. David is correct that hard force on the brake pedal at any time will instantly activate the brake pads and effectivly kick out regen from MG2.
And the problem with the brake issue, keeping from messing up your transaxle. Since MG2 is directly connected to your drive wheels, if the traction control, through the ABS computer, detects a great difference in wheel spin between left and right, it will kick out regen to not put too much stress on the PSD gears. Toyota has done this in a very conservative manner, this is the "problem" as reported. the software fix will lessen this sensitivity to wheel speed differences.

Wayne

 

I don't know how different the behavior is in the Gen3 (with or without the new software), but in my Gen2, I've experienced the momentary regen cut-out coming off a bumpy highway exit ramp at speeds as high as 45mph. Not an issue for me though, because I understand the phenomenon and know how to deal with it safely.

 

Yes, it isn't likely to happen at 90 mph because regen braking is limited by how much energy (current) can be accepted by the HV battery. At 90 mph, even light braking means that a lot of energy has to be dissipated; braking will be a mix of regen and friction braking.

In my experience, the momentary loss of regen braking occurs at speeds less than 30 mph where all of the braking is regen, a bump causes one of the tires to lose contact; regen is lost and it takes ~ 1/2 sec for friction braking to take over.

 

Keep in mind that regen braking power is limited by the battery current limit. Because braking power is the product of braking force times speed, the maximum regen brake force is inversely related to speed.

At 90 mph, this limit should be under 0.04 g (not counting electrical efficiency losses), which is nearly feather-light braking.

Any stronger braking force than this must go directly into friction brakes, bypassing the regen-to-friction transition suspected of causing this problem.

I didn't see this before posting ---

There is no speed where full-force braking can be regen. Even at the lower regen speed limit of 7 mph, theoretical full-force braking exceeds the battery current limit. But at 30 mph, maximum regen is still high enough to cover much ordinary (non-emergency) braking

 

I should clarify - when I'm braking at a little bit less than 30 mph, it is light braking where I am rolling up to a 4 way stop in my neighborhood. About 50 -60 feet before the stop, there is a sharp dip in the pavement. This is the only location where I consistently had the "cutout" - the wheel hits the dip, loses contact momentarily and then there is a ~ 1/2 sec delay before friction braking cuts in.

Since I replaced the OEM tires on the front, it isn't consistent anymore, plus I have learned where the dip is and I usually avoid it.

 

The second one.

 

I am not a car engineer.

But at 90 mph, the amount of time the tire slip on a slippery manhole cover would be 1/4 of the time when someone is going 20 mph. Any system would have a built in filter to filter out noise. Antilock brakes don't kick in if one tire moves just one inch more than the other tire. This would kick in everytime you turn the car then. Nobody likes antilock brakes that are trigger happy.

The slip probably happened so fast that it gets filtered out.

 

HTML:

These explanations are useful. In my experience, the brake slip in my 2005 does not happen when both front wheels hit a bump, but only when one does.

 

Does anyone know how much is saved by regen braking? Say regen braking is completely turned off. What sort of mileage would we get?

 

Much lower because the ICE would be forced to run more often to recharge the HV battery.