Why does the engine wind out when in B mode?

Discussion in 'Gen 2 Prius Technical Discussion' started by jerlands, May 15, 2019.

  1. fuzzy1

    fuzzy1 Senior Member

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    Don't guess or do a 'sounds like', put a tach on it (OBDII engine monitor, such as ScanGauge-II or Torque, or any other). Mine doesn't go above about 4600 RPM, somewhat below Gen3's redline. Your Gen2 has a slightly lower redline.

    The force is not extreme, it is significantly less than the acceleration torque when providing forward thrust.
     
  2. oldtechaa

    oldtechaa Member

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    I believe the redline of a Gen2 is 5k rpm, approx.

    Having the engine make a lot of noise is perfectly normal for compression braking. It's the exact same noise when you downshift in a conventional car, where the engine is being spun by your wheels, as opposed to vice versa.

    There is no regenerative braking in B mode. The motors are removed from the picture. It's purely the engine and friction brakes. It's thus inefficient and not recommended for normal braking.
     
  3. jerlands

    jerlands Junior Member

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    The force on my engine seems extreme to me. Anyway.. I'm not running the vehicle now but waiting for the battery to arrive Friday and will start removing tomorrow (weather permitting cause I'm working outside :) After that I'll be testing the new one to see how it behaves.
     
  4. jerlands

    jerlands Junior Member

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    This is like downshifting from 4th to 1st...
    @11:50 in this video the guy explains drive and brake modes..
     
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  5. fuzzy1

    fuzzy1 Senior Member

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    Mine is closer to shifting down from 5th or 6th to 3rd. Yes, both of my household's other cars are still manual transmission versions.

    I have inadvertently shifted lower and started letting the clutch out on prior MT cars, quickly re-applying the clutch when the error becomes blatantly obvious. Prius B mode is not like that.

    Both MG1 and MG2 are always involved in B mode, usually to spin up the ICE to create drag. And also to dump charge into the HV battery too, if it is not yet full. If MG1 & 2 are disabled, then the car is in Neutral and both battery charging and engine compression braking become impossible, the car turns into a glider.

    There is a low speed (<25 mph?), low drag (shallow hill descents) corner case where the ICE can remain stationary, and nearly all drag comes from MG1 charging the battery, slowly. MG2 spins as just the right speed to keep the ICE stationary. But when the battery gets full, it quits charging and MG2 changes speed to start driving the ICE at low RPM for some engine drag.

    Friction braking comes from the driver's foot only. The car doesn't apply them, neither in B or CC modes nor any other mode.
     
    #25 fuzzy1, May 16, 2019
    Last edited: May 16, 2019
  6. jerlands

    jerlands Junior Member

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    The car seems well enough engineered for them not to strain the engine but mine seems excessive. I'm thinking there is/was more current delivered to MG1 because of the poor state of my battery?
     
  7. ChapmanF

    ChapmanF Senior Member

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    No, there's nothing going on that's 'opposing' the engine force and direction; as described in #16, the kinetic energy from your descent is used to spin the engine in its normal direction of spin.

    The only 'opposing' going on is that the engine, which is not having any fuel injected at that time, so is not producing any power of its own, is actually pretty hard to spin. You could say its friction, as well as the work of uselessly pumping air through the cylinders, is 'opposing' the spin being applied to it by the transmission.

    And by attaching a tach, you will see that it is nowhere near 7,000, no matter what it sounds like to you. The engine is spec'd for around 5,000 in normal operation (that's its point of maximum power output, when it's producing power), and that sounds a mite whiny but it can do that all day without a sweat. The HV computer knows exactly how fast the engine is winding at every instant (and by every instant, I mean the computer is able, if there is a misfire, to tell you which cylinder, by tracking the rotational speed changes within a single revolution). It will never spin the engine to a speed you would need to be concerned about.

    For exactly that reason, you never really need a tach, unless you just find it interesting, or to reassure yourself that the car really does know what it's doing.
     
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  8. fuzzy1

    fuzzy1 Senior Member

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    The ECUs should be able to figure out how much current the battery can take. Battery charging is intentionally slowed if the battery is hot, and also tapered down as it approaches full. The ICE spins up before the battery fills, to absorb the difference.

    I do suggest putting on an OBDII engine monitor and monitoring the RPM. If the engine noise is excessive, I suspect it is more from an old worn engine than from excessive RPM for a good young engine. The B mode control seems unlikely to apply a reduced RPM to compensate for engine age and wear.
     
  9. jerlands

    jerlands Junior Member

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    Yes, I had that wrong...
    I simply doesn't seem normal to me the way the engine revs (very high) when it's dumping current... I don't know if this has anything to do with the state of the battery at the time (failing) or some other issue but I take it out and go into drive mode out of concern.
     
  10. jerlands

    jerlands Junior Member

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    I would think the ECU would slowly start dumping current to MG1 and continue gradually until it reaches whatever is demanded. Mine dumps everything it seems to MG1 and the engine suddenly revs high.
    I've listened to the engine, it is using oil and the valves sound a bit but the noise is from the high revs of the engine :)
     
  11. ChapmanF

    ChapmanF Senior Member

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    5k is quoted in the Gen 2 New Car Features manual as the point of best power output. The actual redline is not mentioned in the manual (6 to 7k might be a reasonable guess), because the control system will never take it there anyway; power output tapers off beyond the peak at 5k so there'd be no point.

    Watching a tach will be valuable for you.
     
  12. jerlands

    jerlands Junior Member

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    I shouldn't have the issue after Friday and then I'll be able to see how it behaves on the new battery. If the problem still exists I'll probably use Techstream and try and record just before I reach the dumping point...
     
  13. fuzzy1

    fuzzy1 Senior Member

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    The increase can seem to be sudden, especially if this B-mode spinup is new to you. It was only after watching many repeats that I noticed the battery tapering off near the top, and the RPM starting to ramp up.
    We really want to know what RPM is displayed. Despite very many readers here commenting on the loud noise and believing the RPMs were getting too high, and some even switching back to D and applying friction brakes to 'save the engine', nobody here with an actual tach display has reported seeing excessive RPMs actually indicated. And very many of us do use tach displays.
     
  14. Prodigyplace

    Prodigyplace Senior Member

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    MG1 is not involved in regenerative braking. It only charges the battery if needed when turned by the engine.
     
  15. oldtechaa

    oldtechaa Member

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    I definitely stand corrected. According to Hobbit's site, B may actually temporarily increase the regenerative braking current.
     
  16. jerlands

    jerlands Junior Member

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  17. jerlands

    jerlands Junior Member

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    I'm starting teardown today so the conditions from this battery aren't getting read... When I test the new one I'll be using Techstream and try and capture the RPM during the current dump.
     
  18. jerlands

    jerlands Junior Member

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    I made about 10 recordings since 4/24 when I first started playing with Techstream, and initially I was focused on the AC but all HV Battery recordings are taken when the battery was in a low state of charge. Blatantly though, difference in SOC is proudly showing around 46 and which I understood little about until not long ago.
     
  19. Skibob

    Skibob Senior Member

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    I think it sounds so much louder because of the lack of soundproofing in the Prius. In B mode MG2 acts as a generator, sending the current it produces to MG1. MG1 spins the engine using it as an engine brake. Just like when you downshift a regular car and take you foot off the gas going downhill.

    The OP doesn’t state it in this thread but his battery has a POA80 code. That might be why his engine speeds up so quickly. The car is trying to protect what little battery it has left, or not stress it so it explodes.

    POA80 and weird events (sped up on it&#039;s own.) | PriusChat
     
  20. jerlands

    jerlands Junior Member

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    POA80 occured like 6 months after I started noticing the way the engine winds out during the current dump. I went from driving on flat land to the hills :)
     
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