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Torque observations on long hwy trip

Discussion in 'Gen 3 Prius Technical Discussion' started by Former Member 68813, Dec 13, 2014.

  1. Former Member 68813

    Former Member 68813 Senior Member

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    I did a longer hwy trip yesterday and hooked up ELM327/torque. Observing the MG1 RPM and MG2 torque gave me better understanding how the hybrid transmission works.

    At the speeds I traveled (70-76 MPH, mostly with cruise control) and on a flat surface, MG2 works like a generator and takes several lbs*ft (typically 3-5) of torque from the tranny output to give energy to MG1 that turns several thou negative RPM (up to -3,000) allowing to slow down engine RPM into the more efficient range (about 2,000). The loss of efficiency in transmission is offset by increased efficiency in engine. At the same time the values fluctuate at lot with ECU trying to balance that in real time.

    If one goes moderate uphill, the situation reverses and MG2 works like a motor, outputting several lbs*ft getting the energy from MG1 that works like a generator and turning as fast as up to 5,000 RPM (positive spin). That increases the engine RPM up to about 3,000.

    Only on a mild uphill the MG1 would slow down to near zero and same with MG2 torque. That appears to be the most efficient mode for the transmission.

    I could not find out what the MG1 torque meant. It fluctuated around -20 and made no sense to me. Maybe erroneous value/formula in Torque app?

    I also looked at the HV battery current in those conditions. I was relived to see that no major current is taken from battery on uphills like it was reported in gen2.

    But, here is the shocker. The battery current flows all the time from or to battery even in smooth driving at constant speed. About 0-5 Amps and fluctuates a lot. Battery is used as a buffer for the MG1/MG2 balancing act that is not very smooth and instead oscillates near the target value. For that reason, the battery was mildly warm (~30C), even on a cold day near 0C and under 20C cabin temp.

    This shows that battery works and wears out even in hwy driving, but obviously not as much as in city driving when battery temp goes to 40C in no time in bad traffic.

    Anyone had similar or different results to discuss?
     
    #1 Former Member 68813, Dec 13, 2014
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  2. kc410

    kc410 Active Member

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    In the middle of a long trip, using CC at 70-75mph I have watched the battery charge/discharge "dance" around the 0-5A level continuously. The batt would stay at the 60% state-of-charge forever. This is on a completely flat/level interstate & after 1-2 hours of the same speed. I did not notice the batt temp rising much, if any, above ambient, but I was not looking for that specifically. The batt fan may have been running at level 1 & I would have considered that normal.
    Its like the ICE is being operated at a constant torque/rpm & the batt/electric-motor is being used for the constant minor speed control corrections. (using ELM327/torque)
     
    #2 kc410, Dec 13, 2014
  3. CR94

    CR94 Senior Member

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    That seems perfectly reasonable, considering the system is designed to keep the engine at its most efficient speed-torque combination as power demand varies, which means relatively constant torque while speed varies. The torque on MG1 is a fixed percentage of engine torque.
     
    #3 CR94, Dec 13, 2014
  4. Former Member 68813

    Former Member 68813 Senior Member

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    Really? Even when MG1 RPM is zero?
     
    #4 Former Member 68813, Dec 14, 2014
  5. vincent1449p

    vincent1449p Active Member

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    Together with MG1 RPM, the MG1 torque will tell you it is acting as a Motor or Generator.

    Relationship of RPM & Torque to Mortor_Generator.JPG

    For e.g. if a motor generator is rotating in the forward (+) direction, and it applies minus torque, it will generate electricity (producing electrical power). Alternately, if the motor generator is rotating in the reverse (-) direction and it applies minus torque, it will act as a drive source (consuming electrical power).

    Even when the car is not moving, as long as the car is in READY mode, the battery current still flows all the time. It has to supply current to the DC-DC Converter for all the electrical accessories and also to the A/C Compressor if you turn on your A/C.

    Vincent
     
    #5 vincent1449p, Dec 14, 2014
  6. Former Member 68813

    Former Member 68813 Senior Member

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    Exactly, I saw both - and + MG1 rotation and was driving, so I should see both - and + MG1 torque. Instead, the Torque app formula only showed - MG1 torque.
     
    #6 Former Member 68813, Dec 14, 2014
  7. CR94

    CR94 Senior Member

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    Yes, as long as the engine is producing power at more-or-less constant torque. As you've noted, the direction of MG1's rotation may change in order to change the effective ratio of engine speed to wheel speed, or may go to zero, but torque on MG1 remains at the same fixed proportion to engine torque.
     
    #7 CR94, Dec 14, 2014
  8. CR94

    CR94 Senior Member

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    Why? As far as I know, torque on MG1 reverses only when the engine is being started, being spun with injectors off to avoid overspeeding MG1, or similarly being "pushed" in "B" mode. You can't reverse torque on MG1 without also reversing torque on the engine, which is what's happening in those circumstances.
     
    #8 CR94, Dec 14, 2014
    Last edited: Dec 14, 2014
  9. vincent1449p

    vincent1449p Active Member

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    I captured my c using Techstream, MG1 also shows negative most of the time. It only shows positive briefly at certain combinations of vehicle speed (MG2 rpm) and engine rpm.

    MG1 RPM and Torque.JPG

    It agrees with your observations so I don't think there is anything wrong with the formula.

    Vincent
     
    #9 vincent1449p, Dec 14, 2014
    Former Member 68813 likes this.
  10. Former Member 68813

    Former Member 68813 Senior Member

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    #10 Former Member 68813, Dec 14, 2014
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