Help With Regenerative Braking

As I understand it, your brake pads aren't engaged until much later in the braking cycle. Up until then, it's all "simulated" by the torque into the generator. Friction isn't slowing you down, the generator is. Your pedal is only telling the system how much energy the generator should take in.

That is, unless you give it hard brakes, in which case the friction comes in and slows you down (as in a hard braking situation).

 

You are making a false assumption that pressing the brake pedal always causes friction braking. It doesn't.

The braking system on the Prius is normally by wire. When you press the brake pedal, all you are doing is telling the computer to slow the car. Initially it does this by regeneration. Only if regeneration is impossible or insufficient does it resort to friction braking.

Tom

 

­In a tr­aditional braking system, brake pads produce friction with the brake rotors to slow or stop the vehicle. Additional friction is produced between the slowed wheels and the surface of the road. This friction is what turns the car's kinetic energy into heat. With regenerative brakes, on the other hand, the system that drives the vehicle does the majority of the braking. When the driver steps on the brake pedal of an electric or hybrid vehicle, these types of brakes put the vehicle's electric motor into reverse mode, causing it to run backwards, thus slowing the car's wheels. While running backwards, the motor also acts as an electric generator, producing electricity that's then fed into the vehicle's batteries. These types of brakes work better at certain speeds than at others. In fact, they're most effective in stop-and-go driving situations. However, hybrids and fully electric cars also have friction brakes, as a kind of back-up system in situations where regenerative braking simply won't supply enough stopping power. In these instances, it’s important for drivers to be aware of the fact that the brake pedal might respond differently to pressure. The pedal will sometimes depress farther towards the floor than it normally does and this sensation can cause momentary panic in drivers.

 

The OP might find the 1st 2 hybrid articles at AUTOMOTIVE TECHNICAL ARTICLES along w/the hybrid16 brake system.pdf insightful.

However, these were written before the 3rd gen and there have been significant changes to the design of the braking system and power split device on the 3rd gen.

 

One thing is as your speed steadily declines, you have to press the pedal down further in order to maximize regenerative braking.

I dont' know the exact reason, but partly it is because the battery can only accept 21 kW of power. So the stopping force with regenerative brakes varies inversely with the speed you are traveling at to stay in this power limit.

Most people like to brake in a "linear" fashion, that is with a constant deceleration, or where the pedal position is constant. If done in this fashion, the Prius will use friction brakes at higher speeds.

It takes a lot of getting used to, in order to maximize regen from a high speed with limited distance. You are not slowing down as quickly as you want at first, then you have to decelerate more quickly at the end.

I've learned or gotten used to how much distance I need to stop from about 45 mph using regen alone, which is the typical high speed for me during city driving. I guesstimate it is about 70 yards. Technically it should take more than 100 yards, but there are many other losses involved, including friction brakes always taking over below 7 mph. But in this way I can glide up to that point and then start regen.

 

It's because kinetic energy varies with the square of your speed; E = 0.5*m*v^2. So making the reasonable assumption that your car's mass is constant, changing from 11 mph to 10 mph takes the v^2 factor from 121 to 100 - a difference of 21. But changing from 61 mph to 60 mph takes that same factor from 3721 to 3600 - a difference of 121. That's as much kinetic energy as the whole car has at 11 mph, but crammed into the difference between 60 and 61 mph!

Since power (watts) is a measurement of energy divided by time, to keep the power at it's peak as you're charging the battery, you have to increase the rate of deceleration to match the decrease in kinetic energy as the square of your velocity declines. Or whatever.

 

First a minor correction. The unit kW is an measurement of power, not energy. To get the "total energy recovered" you need to multiply by time, which is most commonly expressed as kWh, or thousands of watt hours.

As for your question, when slowing from speed A to speed B, the same amount of energy is converted no matter how the car slows down. This is simple conservation of energy. What happens to that energy is the issue.

If you slow down by lifting your foot to the point where there is no thrust and no regenerative braking, then all of the energy is dissipated by friction: aerodynamic drag and rolling friction. In this case none of it gets back into the battery.

In the second case you press on the brake and force regenerative braking. This causes the car to slow more quickly than it did in the first case, thereby reducing aerodynamic drag and rolling friction. This difference in frictional loss goes into the battery, minus whatever electrical and chemical losses there are in the battery and charger.

Tom

 

I like to use two methods of max regen braking from higher speeds

Initial Regen, Medium Speed Glide, Max Regen to Stop
Useful if you think the light will change to green. You've hedged, and you've only partially slowed down. But you've left yourself a lot of buffer to come to a complete stop if the light stays red. Also you are gliding at a more reasonable speed so traffic behind you is not as annoyed. This will probably recapture as much energy into the battery as possible.

High Speed Glide, Full Regen To Stop
This is useful sometimes to avoid disrupting traffic, if you know the light ahead is going to be red for a while anyways. Sometimes the person behind you needs to get into the left turn or right turn lanes, this speeds things along. It is a bit more lossy, since your average speed will be higher.

 

I understood your intention. I described the endpoints, i.e., regenerative braking verses coasting. As for efficiency, coasting beats light regenerative braking which beats heavy regenerative braking which beats friction braking. It's pretty simple.

Tom

 

why are you refusing to read posts that explained it to you?

when you press brakes, first thing that happens is increased regen braking.... pads are usually applied only at later stage.

 

As a new Prius owner, this thread helps me to understand how regenerative braking works. So, going the next step, does this mean that the brake pads on a Prius should last longer, since it's doing less friction braking than on a normal car?

 

Yes, PCers have reported over 100,000 miles on the original pads.

 

Yes

 

Yep. Another example at CTV British Columbia - Hybrids prove very reliable - CTV News (which we already have a thread on).

250K km == 155,342 miles

 

Even non-PCers have reported over 100,000 miles on the original brake pads on non-hybrids. That includes two of the last three non-hybrids in my household.

Could that mean that I'll never have to replace the original pads on my hybrid?

 

Tom/qbee's response was good, but I'd add a few things to it...

If you're comparing light regenerative braking to heavy regenerative braking from the same speed, the heavy regenerative braking will put more energy back into the battery, for several reasons. First, you're not going as far, and you spend less time moving, so both your aerodynamic and your rolling resistances are less. Second, some people have discovered that there is a fixed overhead in the regeneration process (as well as a proportional amount of overhead); when you are in light regeneration, a larger percentage of your energy is wasted by this overhead.

That said, the light regenerative example is probably more fuel efficient, because you went further while you were slowing down. So, assume you're going 60 mph and discover that you have to stop. In order from most efficient to least efficient, you can:
1. Glide immediately (no regen, no power), and just come to a rest whenever the car's momentum runs out. This puts nothing into the battery, but you'll go the furthest distance. Of course, your distance is typically limited by whatever is stopped in front of you, so this isn't a very realistic option. Also, the traffic behind you will be annoyed.
2. Regen fairly heavily (near the maximum of the CHG bar, but not so much as to get any friction braking) until you're at a speed that you can glide to a stop with no additional regen (eg the same as #1, but starting at a lower speed). This reduces your aerodynamic drag, which is proportional to the square of your speed, as soon as possible, and also has the lowest overhead for regen.
3. Regen fairly lightly the whole way to a stop. This has more aerodynamic drag and more regen overhead than #2.
4. Coast (not glide, so there's light regen from not touching the accelerator) until late, then regen fairly heavily. This has higher aerodynamic losses than #2 and #3 because you're moving faster for more time. It does have somewhat lower regen overhead than #3, but I doubt that it makes up for the higher aerodynamic losses.
5. Keep your speed up and regen heavily at the last moment. Now you're using more gas than you were for the first 4!
6. Completely forget about stopping, and slam into whatever stopped object is in front of you. A lot of fuel is wasted in this case to ship new parts for your car.

These all assume that you have to stop. If you're approaching a light that turns green when you're partway there, #3 or #4 might be more efficient than #2, if #2 ends up slowing you down more than you would have needed to.