Electric motor assist frequency and duration questions

Discussion in 'Gen 2 Prius Fuel Economy' started by gptoyz, Feb 6, 2019.

  1. gptoyz

    gptoyz Junior Member

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    So around town, my electric motor can go easily 33 mph for long stretches or until the battery is completely depleted if I'm controlling the throttle.

    On expressway and freeway speeds (above 50) the ICE does 99% of the work, frequently deviating energy to recharging the battery. The electric engine spends brief (very brief) moments lending energy to the driving wheels periodically but won't assist for long periods of time.

    Actually rarely do the 2 motors help each other for long stretches of time. On my Prius C (I know much newer) but it would definitely help.

    I'm almost positive this is contributing to my poor gas mileage ~35 mpg.

    I'm a seasoned Prius driver but I don't try to hypermile. It does have a battery from batteries4prius.com.

    Service wise it has:
    cleaned denso injectors
    New ngk plugs
    Old coil packs
    New pcv
    New valve cover gasket
    All fluids changed

    Is this normal behavior of the electric motor?
    What can I look at or service in order to get HV motor to participate more at hwy speeds?





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    #1 gptoyz, Feb 6, 2019
    Last edited: Feb 6, 2019
  2. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    sorry, what was the actual question?
     
  3. gptoyz

    gptoyz Junior Member

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    Lol sorry I appended my questions to the bottom of my post.


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  4. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Sounds like mostly normal behavior.

    The electric motors are there to help you accelerate, and to harvest kinetic energy when you brake. The gas engine is there to maintain headway at whatever speed. The gas engine is configured to have excellent economy at steady RPM, but relatively little torque for acceleration authority. Thus the electric provides the real shove to get you going.

    If you try to drive all-electric, you're technically abusing the system, but only mildly. It can't be sustained very long unless you recharge the battery, and to do that you either run the engine or stay on the brake pedal without dropping below 6mph.

    The way the car is geared, the gas engine must turn at speeds above 37mph.* It doesn't necessarily burn fuel, but it must physically spin to keep the MG1 motor from going over redline according to the permanent gear ratio those two are locked at.

    So... speeds in the mid 30s are ideal for long electric glides, and that works out pretty well for saving fuel. At highway speed you pretty much need to burn gas.

    Your stated MPG is a bit low, you may want to check that out, but apart from that it sounds like the car is doing what it is supposed to.

    *it's 37mph for some, slightly higher for other model years. I don't have that detail in my head but it's always at a locked ratio to MG1.
     
  5. ChapmanF

    ChapmanF Senior Member

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    It's sometimes counterintuitive at first, but unless you have one of the plug-in Prius models, ultimately all the energy the car ever gives you goes in through the fuel tank filler hole.

    Driving on electric, you are using that energy over a longer and less efficient path. It either got into the battery by running the engine extra and sending charge back to the battery, or by running the engine to get you up to speed or up a hill, and then reclaiming that energy while you slow or descend, and putting that into the battery. The battery takes a cut for itself both when accepting your energy to store and when giving it back again.

    So when you're tooling down the road and the gas engine can handle the load without assistance, that's exactly what you want it to do.
     
  6. mpg_numbers_guy

    mpg_numbers_guy Junior Member

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    If your hybrid battery is weak, the gasoline motor will have to work harder and more often, and thus return noticeably lower fuel economy.

    Short trips, high speeds (65+ mph), and AC use all hurt fuel economy.
     
  7. gptoyz

    gptoyz Junior Member

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    I had a previous rehab prius, 2005 w/ ~220k miles on it and it was averaging ~46 mpg, of course that was in the summer.

    oh another tidbit of info, by my hand calculations I'm getting about 41 mpg based on miles driven and how many gallons I'm filling up, is the MFD that inaccurate?
     
  8. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    In Gen 1 and Gen 2, it is not possible to go above about 44 MPH without spinning the engine. (it may or may not use gas)

    In later transaxles, much higher speeds are possible without spinning the engine. There is a new gear, called the motor speed reduction gear, that allows the computers much more discretion in how to best generate power.
     
  9. gptoyz

    gptoyz Junior Member

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    not looking for higher EV speeds, I was curious if there was something I was missing that was causing my electric motor not to contribute more to HWY cruising mpg efficiency
     
  10. ChapmanF

    ChapmanF Senior Member

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    At the risk of rehashing #5, what you want for HWY cruising mpg efficiency is the least possible contribution from the battery: helping you with acceleration and deceleration, yes, but just staying out of the way during cruise. Which is pretty much what the car does. Anything else will be less efficient.

    (I said 'battery' there because there really isn't ever a time when you're moving the car and the electric motors aren't contributing; they're an indispensable part of the power path from the engine to the wheels, and without them, you have neutral. But the simplified dash display will show them 'not contributing' when they are not making a net credit or debit to the battery.)
     
  11. gptoyz

    gptoyz Junior Member

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    I guess the mfd is ridiculously off

    When I do hand calculations, I’m getting about ~44-50 mpg. I conservatively rounded down my range to the nearest miles and rounded up my gas to the nearest gallon.




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