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Do AWD Toyota hybrids use rear motor regenerative braking?

Discussion in 'Knowledge Base Articles Discussion' started by RGeB, Oct 3, 2022.

  1. RGeB

    RGeB Member

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    The short answer is No (at least in my experience with a gen5 1999 rav4 AWD hybrid).

    But green arrows appear on your dash display, and move from the rear axle to the battery in the picture, so it must do regenerative braking ... right? Wrong. That display could be programmed to react to anything, such as absence of accelerator pressure. I think it indicates some potential regeneration from coasting, but certainly not regenerative braking. To add to the confusion, NCF seems to show neither from the rear.
    Deceleration.jpg

    For a motor/generator to generate, there must be torque in the opposite direction to (axle) rotation. If you use your favourite OBD tool to look at Toyota PIDs, you will see that both front and rear motors show slight negative torque while coasting, so this may be the signal for the in-car display.

    There are also PIDs for front- and rear- motor regenerative braking (when you use the brake pedal, not coasting). In both OBDLink and Techstream, the front displays ca 3x -ve motor torque, but the ratio varies, so it evidently uses a separately located sensor. I have never seen any result but 0 for requested or delivered regenerative braking by the rear motor.

    Moreover, rear motor negative torque does not change when braking, so this car probably uses slight rear motor regeneration when coasting (little electricity recovered), but lacks rear motor regenerative braking. On long downhills, the traction battery fills anyhow.

    This example 1 second interval log shows (among other things):
    1. Regenerative cooperation is nearly, but not quite, the same as stop light operation. Both seem to respond (by slightly different algorithms) to signals from the brake stroke sensor and regulator pressure sensor (and dynamic cruise control not shown here).

    2. Greater signal amplification is needed to see negative torque from the rear motor than from the front motor, and rear motor negative torque is not increased during braking.

    3. Deceleration can involve any combination of friction braking, regenerative braking and engine braking that the car computers decide is appropriate to the inputs received.
    Log.jpg

    I have seen improved regenerative braking advertised for an AWD bZ4X BEV, but my guess is that what I have shown for a rav4 likely applies to AWD prii and other Toyota HVs.
     
  2. Trollbait

    Trollbait It's a D&D thing

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    I know that the rear motor on the Prius is an induction type. With an asynchronous AWD system, this means better efficiency when it isn't in use, as there is no magnet braking going on then. Tesla does the same with their front motors. Without permanent magnets, the system will have to supply power to the induction motor to get the magnetic field needed for regenerative braking. With the Prius rear motor being just 5kW, the potential recovered energy simply isn't worth it. Without any measured braking being seen in other hybrids, Toyota likely uses induction for all the rear motors.

    It appears both motors on the AWD bZ4X are permanent magnet. The front motor also has about half the braking potential of the motor in the FWD model. Chances are that the rear motor is used for braking. Improved depends upon how they are defining it. The near same system power between the two means the same potential braking force. The AWD will have braking on the rear, which might mean better control of the car under some conditions.

    edit: Looks like the rear motors of most hybrids use permanent magnets.
    https://www.toyota-club.net/files/faq/19-10-10_faq_e-four_4wd_eng.htm
    Maybe Toyota decided not to use them in the hybrids because of the regen limits imposed by the battery size and a cost benefit analysis. Developing the software to use both at the time wouldn't be free
    As for the BEV, Subaru had more say with the AWD to influence design.
     
    #2 Trollbait, Oct 3, 2022
    Last edited: Oct 3, 2022
  3. RGeB

    RGeB Member

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    Thanks Trollbait, very interesting. Yes, the 1999 rav4 AWD HV (AXAH54) uses the Q610 rear transaxle with "Electric motor - 4NM (40 kW / 120 Nm), with permanent magnets". Most folk believe the in-car display and think they are getting regen braking from the rear motor, but they are wrong. Like you, I don't know for sure why Toyota designed it this way. In non-hilly driving there is enough battery capacity to benefit from rear regen braking; and the software does seem to do rear regen coasting. As you say, it may be harder to design safety into rear regen braking.