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Another B-Mode Question...

Discussion in 'Gen 2 Prius Technical Discussion' started by ekpolk, Dec 20, 2010.

  1. ekpolk

    ekpolk What could possibly...

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    At the outset, if this has been discussed, I apologize. My missing a previous thread is a reflection of ineffective searching, not a lack of trying...

    Anyway, from 06-08, I drove an 04 Prius. In 08, I stumbled upon a great deal on a barely used 09 Camry hybrid, which I've been driving since (we still have the Prius in the fleet -- my son is "renting to own...".

    My question is this, and AFAIK, it applies to both the Prius and the TCH. I understand very well the concept of B-Mode (though it's largely academic here in the flat lands of Florida...). What I don't understand is the actual PHYSICAL IMPLEMENTATION of B-Mode. In other words, exactly what happens, physically, down there in the transaxle when I select B-Mode?

    And I do have a practical slant on this question. In the Camry only, when I select B-Mode, I occasionally get some shuddering (light but still very noticeable) as I decel to a stop. Happens from maybe 10 mph to zero. It never happens in the Prius, B-Mode or D. Any thoughts on this? Yes, I know, in Florida, I have no excuse for EVER engaging B-Mode... That said, please understand that if ever there was a cat that would be killed by curiosity, I am it. TIA.
     
    #1 ekpolk, Dec 20, 2010
  2. qbee42

    qbee42 My other car is a boat

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    Short answer: nothing happens mechanically in the transaxle when you shift to B mode.

    Long answer: When you shift to B mode, MG2 is used as a generator to make electricity. This is the same as ordinary regenerative braking. The difference comes with how this electricity is used. In regenerative braking, the generated electricity is used to charge the HV battery. In B mode it is used to spin the internal combustion engine (ICE) as a big air pump to dissipate the generated electricity. In other words, the regenerated energy from braking is discarded as heat through the process of moving air.

    The shudder you feel comes from the ICE stopping. It hasn't been running, but it has been spinning as a big air pump. When it stops it makes a shudder.

    For simplicity I have described B mode as a black or white process. It's not. In practice the car can use engine braking (the air pump thing) even when not in B mode, and B mode will put some power into the HV battery. The difference is that with B mode you tell your car to *immediately* start wasting power through engine braking, and not wait until the battery has reached a high limit. This is useful for descending mountain roads where the battery is rapidly charged to its limit.

    B mode also makes some subtle changes to the regeneration profile, but these are only of interest to an engineer or hypermiler.

    Tom
     
    #2 qbee42, Dec 20, 2010
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  3. ekpolk

    ekpolk What could possibly...

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    Two Eco
    Tom:

    Thank you very much. I'm not an engineer, and there are still plenty of things that I really don't know about the HSD system, but I get the concepts. But this was the last "basic" thing that I still hadn't figured out. Your explanation makes perfect sense. Again, thanks.
     
    #3 ekpolk, Dec 20, 2010
    1 person likes this.
  4. qbee42

    qbee42 My other car is a boat

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    The HSD is a very interesting beast. It all comes down to power paths through the system: how much power gets transfered which way. For low gear, if you want to call it that, most of the power goes through the electrical path: from the internal combustion engine (ICE) through the small motor-generator (MG1) to the big motor-generator (MG2) to the wheels. At speed a much higher percentage of the power goes through the mechanical path. In reverse, all of the power goes through the electrical path, plus it has to fight the mechanical forces.

    All of this happens by doing nothing more than varying the field currents to the MGs and applying power to the ICE when needed. It's truly an amazing system.

    Tom
     
    #4 qbee42, Dec 20, 2010
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