Regenerative braking in a hybrid

hybrid-repair
brakes

#1

When exactly does regenerative braking kick in in a hybrid? and how does it interact with the normal brakes?



I assume under heavy braking, the disk brakes are the only thing working. Also when you just take your foot off the gas and coast, regenerative braking is operating. But about in between?



Does the regenerative braking kick in with a light pressure on the brake, for example?


#2

Does the regenerative braking kick in with a light pressure on the brake, for example?

I’ve never owned one, but based on the prius that a co-worker rented a few weeks ago that does seem to be the case. The brakes feel very strange under light pressure; definitely test dive one of these things before you seriously consider buying one, they have plenty of “different” features. As my friend said after driving it for a week, “At least this saved me a trip to the dealer.”


#3

My question is one of intellectual curiosity.


#4

Maybe there is some info on the toyota website about how the system works.


#5

Regenerative braking is done using electromagnetism.

Those of us who own riding mowers know that the same electrical component serves as both the starter and the generator. In one case the component is a motor to turn the engine, then afterwards it uses the engine’s power to make electricity and recharge the battery.

Regenerative braking uses the traction motors in a similar double role. Step on the “gas” and the motors propel the car. Step on the brake and the drive wheels turn the motor which now acts as a generator. The magnetic resistance of the motor/generator acts as a braking force on the wheels.

Neither conventional braking nor regenerative braking operate when you coast; that would destroy the vehicle’s ability to achieve high mpg. We can assume that the design engineers have specified just the right foot pressure for conventional and/or regenerative braking to kick in, with a comfortable balance between the two.


#6

“We can assume that the design engineers have specified just the right foot pressure for conventional and/or regenerative braking to kick in, with a comfortable balance between the two.”

You would assume that, until you actually drove a prius. It feels very strange.


#7

If you take you foot off the gas at highway speed, the momentum of the car will turn the electric motor, which them becomes a generator and charges up the battery. It would be really dumb not to use this energy freely available. The term regenerative braking is really confusing, since you still need your normal brakes to actually stop. Going down a mountain, the normal engine drag is replaced by the generator drag, and your brakes will do the rest of the braking.

Electric locomotives have done this for years; an electric train slowinjhg down pumps juice back into the wires. The concept is nothing new. Diesel locomotives generate electric power on slowing dwon and it is dissipated as heat on the roof of the unit, saving the brakes.


#8

According to the silly little display, it does charge the battery while coasting. However, it also seems to have some kind of “two stage” braking where the first little bit of “mushy” peddle movement seems to engage regenerative braking, additional pressure engages the real brakes (just like a real car). Just for fun, drive on of these things sometime, they are pretty strange.


#9

In a Prius, regenerative braking occurs at light to medium pedal pressure. Mechanical braking is secondary. For this reason, Prius brakes almost never wear out. More info at

www.priuschat.com


#10

That sounds correct based on how it feels. They need to fix the pedal feel, it does not inspire confidence.


#11

thank you for the replies.

Summary: Regenerative braking kicks in when you take your foot off the gas and coast. This has the same effect as engine braking on a standard car, but puts some energy back into the battery. Probably the regenerative braking is only lightly engaged at that point.

And if you push on the brake lightly, regenerative braking becomes more engaged. As you increase the pressure, at some point the regenerative braking is fully engaged, and further pressure engages the normal disc brakes.

Bill


#12

I doubt if there’s a “more engaged” situation with regenerative braking…it’s either on or off. I think what the posters above are saying that coasting or light braking turns on the regenerative braking, and hard braking (or at some measured brake pedal pressure) the real brakes kick in.


#13

.


#14

I’m sure they control the amount of regenerative braking by varying the current limit available for recharge. More recharge current would mean more back EMF, lower current less back EMF.

Here’s a totally made up example. Let’s say the car is running along at 50 MPH down a slight hill with foot off the accelerator. The traction motor/generator is supplying 5 Amps of recharge current. The driver puts slight pressure on the brake pedal, the electronics allow more recharge current, say 10 Amps to go into the battery charging. This slows the car down some more. Now there will be some upper limit to how much charge current the batteries can take, and if they are already fully charged that current will be small, so there has to be some pretty tricky logic keeping track of all that and apportioning back EMF with hydraulic brakes depending on conditions.

If someone has a better explanation, or more accurate information, please correct me.


#15

regenerative braking is easy to control, by varying the amount of current fed back to the battery. This changes the load, thus the braking effect. Maximum braking would occur when the motor (now being used as a generator) is feeding all of its power to the battery.


#16

Didn’t you use to play for the Celtics?