I Finally Understand Hybrid Regenerative Braking

Discussion in 'Gen 3 Prius Technical Discussion' started by twittel, Dec 23, 2010.

  1. twittel

    twittel Senior Member

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    I've read lots of your good thread discussion on hybrid regenerative brakes, but as a Prius newbie, I'm still baffled by the how it works. While web researching, I came across this article that explained everything in lay terms. Some of you have probably seen this article; some of you could probably write this article. It's about 5 easy read pages. I'm not smart enough to know how to hot link this, but I hope some of the newbies benefit from this article:

    HowStuffWorks "How Regenerative Braking Works"

    Happy Motoring,

    Tom

    Here's a short excerpt:
    "...­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.

    In the following pages, we'll take a more detailed look at how a regenerative braking system works, and we'll discuss reasons why regenerative braking is more efficient than a typical friction brake system..."
     
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  2. Cpl Bloggins

    Cpl Bloggins New Member

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    A link to a YouTube video I found helpful to understand the hybrid transmission, reminds me of autoshop in high school.

     
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  3. Mark57

    Mark57 2021 Tesla Model 3 LR AWD

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  4. mite66

    mite66 Junior Member

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    Wow, the Toyota guys are smart :rockon:

    I won't ever see my car the same way :D

    Thanks Cpl
     
  5. Canard

    Canard Member

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    Excellent explanation of the setup!

    I've noticed a quirk on decelerating from say, 80 km/h or above - when coming down, the engine of course has already shut down. I can see this as the 0-10 L/100 km gauge next to the speedometer has fallen off the bottom... but when decelerating and passing through about 68 km/h, there is a discernable blip on the consumption monitor - the engine starts briefly, then stops about 1 second later. Any idea what it's doing?

    -Iain
     
  6. uglymuddah

    uglymuddah Junior Member

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    What you can't see without a scanguage or similar device is that the ICE doesn't shut off at 80 km/h but at 70 km/h. In fact at 80km/h your ICE is still going at 1000rpm. This to protect your MG1 from spinning too fast. At 70 km/h the ICE revs up to 1200 rpm's and then goes to 0 rpms. This is what the consumption monitor shows. This last rev-up allows the ICE to turn off smoother.
     
  7. Blue Prius Texan

    Blue Prius Texan Crazy? Nah...just humorous!

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    So, does this mean that "riding the brakes" will not make your brakes need replacement sooner, and/or that it saves you gas if you ride your brakes?
     
  8. macman408

    macman408 Electron Guidance Counselor

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    Yes, your brakes won't need to be replaced quite so soon, as long as you don't fill the CHG bar on the HSI. But don't ride your brakes just to fill the battery. If you're riding your brakes, then that means you accelerated too much. If you're not willing to accelerate less (or it's a really big hill), then braking moderately will recover some (but by no means all) of the excess kinetic energy.
     
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  9. MrPrezident

    MrPrezident New Member

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    What I want to know is what is the mechanism for determining normal breaking vs regenerative breaking? I hope that normal breaking is controlled mechanically from the break pedal and not by the ECU. If not, what happens if the ECU fails? If it is controlled mechanically via the break pedal, then how does this mechanism get overridden for regenerative breaking?
     
  10. Britprius

    Britprius Senior Member

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    In the explanation about how the brakes work it says the motors are reversed, this is not true. The motors are driven in the same direction but are not supplied with current and when an electric motor is driven by an external source "the wheels" it makes an electric current witch is used to charge the battery through a regulator controlled by the foot brake (more brake higher current to the battery) using the energy slows the electric motor "now a generator" witch is connected to the wheels via transmission and so slows the vehicle.
     
  11. MrPrezident

    MrPrezident New Member

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    You are basically saying the same thing as they are. They didn't say that MG2 is physically rotating in the opposite direction of the wheels during regenerative breaking. What they said was that it goes into "reverse mode," causing it to "run backwards," meaning that its functionality is reversed (not reverse rotation) and is acting as a generator instead of a motor. Thus the back emf slows down the motor-generator/wheels.
     
  12. Tideland Prius

    Tideland Prius Moderator of the North
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    The brakes are controlled electronically with a mechanical backup in case of an electronic failure.
     
  13. MrPrezident

    MrPrezident New Member

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    Thanks Tideland. How does the mechanical backup work?
     
  14. Tideland Prius

    Tideland Prius Moderator of the North
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    Haven't really looked into it but I figured no different than how a regular Toyota brake system works. Toyota has kept their brake-by-wire systems strictly to hybrids for now. Other companies are already using them on standard models (notably MB)
     
  15. MrPrezident

    MrPrezident New Member

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    Hmm.. does anyone know how the mechanical backup system works? I'm having trouble finding the right info on this.
     
  16. jk450

    jk450 New Member

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    Start here: http://techinfo.toyota.com It's a lot more reliable than websites such as How Stuff Works.
     
  17. spiff72

    spiff72 Member

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    I'm sure this would have been shared here in the past, but I thought this was a cool interactive demo of the way that the ICE, MG2, and MG1 interact.

    Toyota Prius - Power Split Device
     
  18. cit1991

    cit1991 New Member

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    Here's my understanding of the brake system:

    There is a hydraulic master cylinder, ABS pump and valves like a normal system. However there is no assist booster (the "power" in power brakes) and it does not work like a normal system.

    Under normal operation, you press the pedal, and the master cylinder pistons move forward, pressurizing the brake fluid. But rather than that fluid acting directly on the calipers (normal car), it is blocked from acting on the calipers by valves. It's pressure acts only on a pressure sensor. That sensor tells the computer how much braking you'd like. The computer looks at the speed, HV battery charge, etc and decides how much regen braking is possible, and how much is needed by the friction brakes (if any).

    If friction brakes are needed, the ABS system applies pressurized fluid to the individual calipers at whatever pressure is needed. There are two valves for each caliper, one to let in pressurized fluid, and another to let it back out to the reservoir.

    It will also do ABS and traction control with the pressure to the calipers. The pressurized fluid comes from an ABS pump and accumulator. That's the pump noise you hear when you open the driver's door...the ABS pump charging the accumulator so the system's ready.

    If there's a detected failure, the charge and discharge valves all close (they are fail-closed), and the master cylinder-blocking valves open (they are fail open). Now the master cylinder pistons act directly on the fluid to the calipers in the traditional pedal-piston-tubes-calipers manner. There are still redundant circuits...two tandem master cylinder pistons, one for the front brakes, and one for the rear.

    Of course, in fallback mechanical-mode, there is no booster, so you basically don't have power assist, but that's the same situation you'd have if the booster failed in a conventional system. I.e., you'll have to push harder.

    So, in normal mode, you don't actually control the brakes like you do in a conventional system. You ask for braking, and the computer applies the friction brakes using the ABS system. Of course, like most Prius things, it's all integrated. There is no "separate" ABS system (pump and valves). It's all one system, and is used for multiple purposes.

    In true Japanese attention to detail, there's even a pedal-stroke-simulator device (spring and piston) that allows the pedal to travel forward based on how hard you push..so the brake pedal feels normal. Otherwise, the pedal feel would be very stiff.

    It's really brilliant when you think about it. The car would have ABS (pump, accumulator, valves) anyway. They eliminate the booster (usually vacuum, which would be a problem for no ICE operation). They add the blocking valves and the stroke-simulator. Regen is fully integrated and initiated by the brake pedal (more natural than regen from a gas pedal signal). Very little extra hardware is needed (beyond what would be there anyway), and the regen function is integrated seamlessly.

    This system also enables the panic-stop mode (if it senses a rapid gas-pedal liftoff then brake-pedal apply). Then it applies more-than-normal friction braking. It also enables hill-start assist. If you press hard on the pedal when stopped, it leaves pressure to the calipers until it sees a gas pedal signal increase. Then it releases it. You can do these cool things with only software if the computer has full-authority control over the pressure to the calipers. You can also use the pump to power-bleed the brakes. Via Techstream, you ask the pump to send fluid to the calipers continuously. You just have to be sure to keep the reservoir full.
     
  19. Canard

    Canard Member

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    Thank you, thank you, thank you! This has been on my mind for the last several months and I've been scratching my head to imagine how it worsks. Brilliant.

    -Iain
     
  20. DumbMike

    DumbMike Active Member

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    That was a terrific video. But, is there a third grade version?

    Actually, this video is hard to find because it's "hidden" in a braking thread. Maybe somebody can move it to its own thread so that others can find it easily. I see that the "thank yous" are many.

    Mike
     
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